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JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 12
PAGES 1891–1919
1999
The Piedras Grandes–Soncor Eruptions,
Lascar Volcano, Chile; Evolution of a Zoned
Magma Chamber in the Central Andean
Upper Crust
S. J. MATTHEWS1, R. S. J. SPARKS1∗ AND M. C. GARDEWEG2
1
DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF BRISTOL, WILLS MEMORIAL BUILDING, QUEENS ROAD,
BRISTOL BS8 1RJ, UK
2
SERVICIO NACIONAL DE GEOLOGIA Y MINERIA, AVENIDA SANTA MARIA 0104, CASILLA 10465, SANTIAGO, CHILE
RECEIVED MAY 1, 1998; REVISED TYPESCRIPT ACCEPTED JUNE 7, 1999
The Soncor zoned magma chamber developed in the upper crust at
about 6 km depth as a result of repeated influx of hydrous mafic
magmas. The magmas in the chamber evolved by open-system
fractionation with magma mixing being important in the less evolved
magmas. Repeated influxes of hydrous mafic magmas resulted in
convective stirring and addition of heat, volatiles and mafic components to the chamber. This produced complex histories of individual
crystals and heterogeneous character of phenocrysts in individual
samples. Halogen contents of amphibole, biotite, apatite and glass
inclusions, S contents of glass inclusions, stabilization of anhydrite
in the silicic magmas, and mass balance calculations imply major
transfer of volatile components from the hydrous mafic magmas into
the interior of the zoned chamber in the form of a co-magmatic
fluid phase.
Stage II of Lascar Volcano, Chile, involved development of an
andesite to dacite volcanic complex and associated hypabyssal
porphyry intrusions above the main magma chamber. The system
culminated in development of a large zoned magma chamber that
erupted in the large-magnitude (8 km3) Soncor explosive eruption
at 26·5 ka, forming a Plinian pumice deposit and ignimbrite. Ventderived lithic clasts in the Soncor deposits sample the pre-existing
Stage II complex. The Piedras Grandes hornblende andesite unit
represents a pre-Soncor dome complex. The andesite consists
of a heterogeneous phenocryst assemblage of plagioclase–
amphibole–orthopyroxene–oxides and minor biotite, clinopyroxene,
quartz, apatite, anhydrite and olivine together with commingled
basaltic andesite inclusions and streaks. Temperature estimates from
zoned orthopyroxenes and Fe–Ti oxides and disequilibrium between
phenocrysts indicate an origin by remobilization and remelting of
an igneous protolith by influx of hydrous mafic magmas so that the
andesite is a mixture of partial melt, restite crystals, mafic components
and phenocrysts. More silicic Stage II rocks are also interpreted as
partial melts with entrained restite. The zoned Soncor chamber
contained dacite (67 wt % SiO2) to silicic andesite (61 wt %
SiO2) crystal-rich magmas with an assemblage of plagioclase–
orthopyroxene–clinopyroxene–oxides with minor biotite, amphibole,
apatite, zircon, anhydrite, pyrrhotite and olivine. Hornblende-rich
mafic andesite pumice from late flow units in the ignimbrite provides
evidence for influxes of hydrous mafic magmas at the base of the
chamber at sufficient depths to stabilize amphibole. The hydrous
mafic magmas are interpreted to have evolved in the lower crust by
high-pressure fractionation with some lower-crustal contamination.
The Soncor ignimbrite and its associated Plinian pumice
fall deposit were formed by the largest magnitude explosive eruption of Lascar Volcano, Chile (23°22′S,
67°44′W). This eruption took place at 26·5 ka and ejected
a minimum of 8 km3 of magma (Gardeweg et al., 1998).
The Soncor deposits are heterogeneous, with juvenile
∗Corresponding author. e-mail: [email protected]
 Oxford University Press 1999
KEY WORDS:
andesite; Andes; magma mixing; compositional zoning
INTRODUCTION
JOURNAL OF PETROLOGY
VOLUME 40
ejecta ranging from dacite (67·6% SiO2) to basaltic andesite (56·1% SiO2). The Soncor deposits are complexly
zoned. The initial Plinian deposit is predominantly composed of silicic andesite and dacite pumice, and there is
an upper zone of more diverse pumice including banded
pumice and more mafic compositions. The following
ignimbrite is mostly composed of dacite and silicic andesite pumice, but late flow units contain mafic andesite
scoria and heterogeneous porphyritic amphibole-rich
pumices.
The evolution of the magmatic system to generate the
zoned magma chamber sampled in the Soncor eruption
has been investigated by study of pre-Soncor volcanic
rocks, including remnants of the pre-existing volcanic
complex and lithic clasts within the Soncor deposits that
were vent derived. A sequence of block-and-ash flow
deposits and flood deposits, termed the Piedras Grandes
unit, consists of hornblende andesite (60·1–63·9% SiO2)
with minor basaltic andesite streaks and inclusions (56
wt % SiO2). This unit represents a dome complex, which
developed before the climactic Soncor eruption. Ventderived lithic blocks included in the Soncor deposits
range in composition from basaltic andesite (56% SiO2) to
dacite porphyry (68% SiO2). These blocks are commonly
prismatically jointed, implying high pre-eruption temperatures. Their textures and compositions indicate that
they originate from pre-Soncor hypabyssal intrusions and
dome complexes.
This paper documents the petrology and geochemistry
of the Soncor deposits, included lithic blocks from the
pre-eruption complex, and the Piedras Grandes units, and
demonstrates their petrogenetic coherence. The study
provides insights into how large zoned magma bodies
are generated within the upper crust of the Central
Andes. The Piedras Grandes magmas are interpreted in
terms of remobilization and reheating of a solidified or
semisolid andesitic source region by influxes of basaltic
andesite. This solid protolith represents the pre-existing
high-level Lascar magma chamber, which cooled and
crystallized following a long hiatus in activity. We present
a model for the development of the Soncor magmas from
the Piedras Grandes magma by progressive mobilization
of the protolith and interactions between mafic magmas
and the products of partial melting of the protolith,
culminating in the development of a compositional stratified magma chamber in the upper crust.
Evolution of Lascar Volcano
Lascar Volcano, Chile (23°22′S, 67°44′W, 5580 m) is
the most active volcano of the Andean Central Volcanic
Zone. Lascar is a stratocone elongated along a WSW–
ENE trending lineament. The evolution of the volcano
was described by Gardeweg et al. (1998), who recognized
NUMBER 12
DECEMBER 1999
four stages in its history. A brief synopsis of this evolution
is given as a context for the development of the Piedras
Grandes and Soncor magma system.
Stage I began <43 ka ago with the formation of a
stratocone on the eastern end of the present complex,
and the eruption of two-pyroxene andesite lavas (55–
64·7% SiO2) and coarse-grained pyroclastic flow deposits.
The Stage I products are preserved as the remnants of
a steep andesite stratocone and as flank lavas and
pyroclastic rocks. Dacite xenoliths in various stages of
melting and disaggregation are observed in andesite
ejecta from younger pyroclastic flow deposits of Stage
I. The Stage I products were significantly eroded before
Stage II.
Stage II involved the development of a silicic andesite
dome complex to the west of the Stage I stratocone.
Associated block-and-ash flow deposits and deposits from
a large flood event are preserved to the west of the
volcano. This unit, known as the Piedras Grandes unit,
preceded the Soncor eruption. Vent-derived lithic clasts
in the Soncor deposits also indicate the existence of
domes and shallow-level intrusive rocks of rhyodacite to
basaltic andesite composition in the Stage II structure.
The Soncor eruption produced an initial Plinian deposit
and a coarse-grained pumice- and lithic-rich non-welded
ignimbrite, which extends up to 27 km from Lascar
(Calder et al., 1999). Accelerating mass spectrometer
(AMS) radiocarbon analysis yields an age of 26·45 ±
0·5 ka for the eruption.
Stage III built an andesitic to dacitic stratocone over
the source area of the Soncor eruption. Stage IV began
with a major basaltic andesite to andesitic pyroclastic
flow eruption and formation of a small (1·5 km diameter)
summit caldera at 9·2 ka. Gardeweg et al. (1998) interpreted this pyroclastic flow deposit as the last products
of Stage III. However, the compositions of this eruption
are much more similar to the Stage IV lavas and ejecta
and there is a long period of dormancy of Lascar (at
least 6 ka) between Stage III products and this deposit.
Stage IV included an andesitic lava flow at 7·1 ka and
the formation of three deep collapse craters in remnants
of the Stage I edifice. Present eruptive activity is centred
on the westernmost crater of Stage IV. Since 1984,
activity has consisted of lava dome extrusion, explosive
eruptions and vigorous degassing, culminating in the
19–20 April 1993 explosive eruption, which involved 0·1
km3 of andesitic magma in the form of pumice flow
and tephra fall deposits (Gardeweg & Medina, 1994;
Matthews et al., 1997).
The petrology and geochemistry of Lascar volcanic
rocks were described by Matthews et al. (1994b), who
attributed the petrological and geochemical features of
Lascar magmas to combined fractional crystallization
and magma mixing as a result of periodic replenishment
and reheating of a steadily evolving magma chamber by
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MATTHEWS et al.
PIEDRAS GRANDES–SONCOR ERUPTIONS, LASCAR VOLCANO
basaltic andesite influxes. Two types of shallow-level
(subvolcanic) magma chambers were distinguished. First,
andesitic magma chambers received frequent mafic
magma influxes and underwent efficient magma mixing
and vigorous convection. The products of these chambers
are two-pyroxene andesite lavas and pyroclastic flow
deposits with limited petrological and compositional variability but marked disequilibrium textures. Second, more
evolved, dacitic, volatile-rich magma chambers were remobilized by a mafic influx following a long hiatus. This
second group, characterized by the Piedras Grandes and
Soncor magmas, is the subject of this paper.
THE PIEDRAS GRANDES AND
SONCOR DEPOSITS
The Piedras Grandes unit consists of monomict hornblende andesite block-and-ash flow deposits related to
dome-collapse events. A more distal facies, occurring to
10 km distance on the western flanks, consists of thin
(<1 m) fluvial sands and gravels with large (up to 15 m)
boulders with chilled, glassy selvages and radial cooling
joints. This facies was interpreted by Gardeweg et al.
(1998) as a jokulhlaup deposit, produced as a result of
flash-melting of a summit glacier by the growing dome
complex.
The Soncor eruption produced a minimum of 15 km3
of pyroclastic deposits [8 km3 dense rock equivalent
(DRE)]. The initial Plinian pumice fall deposit extends
to the ESE of the volcano, with a thickness of 18 m on
the southern flank and 2 m at a distance of 20 km.
The ignimbrite is non-welded and coarse grained, with
abundant lapilli- to block-sized lithic clasts and pumices.
There are also pumice-rich facies and proximal coarse
lithic-rich lag breccias. The ignimbrite was emplaced
dominantly in quebradas and valleys to the W, NE and
SE of the volcano. Vent-derived lithic clast types include
Stage I lavas, Piedras Grandes hornblende andesite, a
dacitic porphyry and hypabyssal andesitic intrusive rocks.
Prismatic jointing in some of the lithic types indicates
that they were at high temperatures at the time of
the eruption. The high lithic content in the ignimbrite
(average of ~50 wt %) is attributed to the destruction of
the pre-existing Stage II volcanic complex, deep explosive
cratering into the intrusive interior of the volcano, and
erosion during pyroclastic flow emplacement. Criteria
for distinguishing vent-derived lithic clasts from those
entrained in the pyroclastic flows have been given by
Calder et al. (1999). Vent-derived lithologies in the Plinian
deposit and proximal co-ignimbrite lag breccias are readily distinguished from lithic clasts eroded from the ground.
The Soncor deposits exhibit both large-scale compositional zoning and strong compositional heterogeneity
within individual units. The initial Plinian deposit is
largely composed of white, compositionally heterogeneous, two-pyroxene andesitic to dacitic pumice (62·4–
67·3% SiO2). The uppermost part of the Plinian deposit
contains a similar variety of pumice compositions, ranging
from dacitic pumice (66·5% SiO2) to darker, more silicapoor silicic andesitic pumice (61·1% SiO2), and compositionally banded pumice is prominent. The immediately overlying ignimbrite flow units only contain
white dacitic pumice. However, the ignimbrite, which
consists of numerous flow units, is compositionally zoned.
Late flow units contain a diverse assemblage of pumice
compositions ranging from dacite to mafic andesite (56%
SiO2) and with compositionally banded pumice. There
are also amphibole-rich, juvenile, less-vesicular crystalrich pumice clasts. The pattern can be described as
double zoning (Smith, 1979; Hildreth, 1981). The Plinian
deposit and ignimbrite are each normally zoned. A
compositional reversal at the boundary between the
Plinian deposit and ignimbrite has previously been described only in the Cape Riva deposit, Santorini (Druitt,
1985).
PETROLOGY
The Piedras Grandes unit
The Piedras Grandes unit consists of dominantly hornblende silicic andesite (60·1–64·0% SiO2) with minor
basaltic andesite bands and inclusions (up to 5 cm in
diameter). Individual blocks show multiple, parallel, basaltic andesite bands 5–10 cm across. The andesite is
poorly vesicular and porphyritic (42–47 vol. % phenocrysts; Table 1; Fig. 1). Phenocrysts are anhedral to
euhedral. Plagioclase is the dominant phenocryst phase.
Amphibole is the main ferromagnesian phase and ranges
from fresh euhedral crystals to those with thin reaction
rims of pyroxenes, plagioclase and Fe–Ti oxides (Fig. 1)
to complete pseudomorphs. Minor phenocrysts of orthopyroxene, augite, Fe–Ti oxides and accessory apatite
are present. Rare biotite crystals are commonly pseudomorphed by a fine-grained aggregate of pyroxenes,
plagioclase and Fe–Ti oxides. Olivine crystals occur
sparsely and are partially or wholly pseudomorphed
by a vermicular orthopyroxene–magnetite symplectite.
Anhydrite and scarce Fe–Cu sulphide and anhydrite
inclusions occur in Fe–Ti oxide minerals. The groundmass consists of microphenocrysts of plagioclase, pyroxenes and Fe–Ti oxides set in a matrix of rhyolitic glass
(Fig. 1).
The mafic component is represented by a moderately
phyric (12 vol. % crystals; Table 1) basaltic andesite band
(LAS451a). The main phenocryst phases are plagioclase,
olivine with Cr-spinel inclusions and augite with minor
orthopyroxene, Fe–Ti oxides and accessory apatite. Occasional biotite and amphibole crystals are partially to
1893
JOURNAL OF PETROLOGY
VOLUME 40
Table 1: Modal analyses of samples from
the Piedras Grandes flow deposit, quoted on
a vesicle-free basis
Sample:
LAS451a LAS451b
Description:
Basaltic
andesite andesite
Matrix
LAS61
LAS62
Hornblende Hornblende Hornblende
andesite
andesite
88·3
67·4
64·6
63·0
Plagioclase
6·6
25·5
29·0
27·5
Orthopyroxene
0·5
0·9
0·6
1·4
Clinopyroxene
1·9
0·4
1·5
Olivine
1·8
0·5
Hornblende
Biotite
Pseudomorphs
Fe–Ti oxides
trace
0·4
0·2
trace
—
0·6
—
2·1
—
3·8
1·4
1·1
0·8
1·0
—
5·3
—
0·8
1·0
Quartz
trace
—
—
—
Apatite
trace
trace
trace
trace
Total
100·0
100·1
100·0
100·0
15·3
7·3
18·4
26·8
% Vesicularity
NUMBER 12
DECEMBER 1999
completely pseudomorphed as in the host andesite. The
matrix is a fine-grained groundmass of plagioclase, pyroxenes and Fe–Ti oxides.
Plagioclase phenocrysts in the andesite are euhedral,
up to 2 mm in length (Fig. 1) and range from An30 to
An71 with both normal and reverse zoned crystals (Fig. 2).
In the basaltic andesite the compositional range of most
phenocrysts is An38–52, although one crystal with a calcic
core (An85) was found. Pyroxene phenocrysts in the
andesite are subhedral to euhedral and up to 1 mm in
length. Orthopyroxene phenocrysts in four samples of
andesite have a restricted compositional range (mg-number = 0·65–0·69; Fig. 3) show no zoning of mg-number,
apart from two crystals with a thin (10 lm) more magnesian overgrowth (mg-number = 0·72) and a magnesian
inclusion (mg-number = 0·81) in an amphibole phenocryst. One sample of the andesite contains orthopyroxene
with a wider compositional range (mg-number = 0·68–
0·81) with both normal and reverse zonation. Orthopyroxene in a basaltic andesite band has a
compositional range similar to that in the host andesite
(mg-number = 0·67–0·70). Augite phenocrysts in the
andesite (mg-number = 0·75–0·82), and a basaltic andesite band (mg-number = 0·78–0·82) are normally zoned.
Olivine phenocrysts in the basaltic andesite are normally
zoned, with core compositions of Fo85–87 and rim compositions of Fo79–80.
Fig. 1. Photomicrograph (crossed polars) of Piedras Grandes andesite, showing subhedral to euhedral, zoned plagioclase phenocrysts, both fresh
and slightly reacted hornblende phenocrysts, and small, rounded orthopyroxene phenocrysts, in a glassy matrix containing plagioclase,
orthopyroxene and hornblende microphenocrysts. Polars slightly uncrossed to show matrix texture. Field of view on long axis ~2 mm.
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MATTHEWS et al.
PIEDRAS GRANDES–SONCOR ERUPTIONS, LASCAR VOLCANO
Fig. 2. Plagioclase compositions in Piedras Grandes and Soncor rock types and Lascar Stage II biotite porphyries. ‘Xenolith’ in a porphyry
sample denotes coarse groundmass alkali feldspar between quartz xenocrysts.
Amphibole phenocrysts (up to 2 mm in length) fall
mainly in the fields of tschermakite and tschermakitic
hornblende (Leake, 1968). The amphibole phenocrysts
show a wide range of mg-number from 2·9 to 3·5 in
individual samples. Amphibole phenocrysts in the basaltic
andesite are relatively Al poor and Si rich with a restricted
range of mg-number (Fig. 4). Occasional amphibole crystals similar in composition to those in the andesite are
interpreted as xenocrysts from the andesite. Biotite crystals in both the andesitic rocks and the basaltic andesite
band are similar in composition and have a wide compositional range (Fig. 4).
The Soncor ejecta
The main juvenile components are two-pyroxene andesite
and dacite pumice (61·0–67·6% SiO2). These pumice
clasts contain phenocrysts (28–44 vol. %, calculated vesicle free; Table 2) of plagioclase, orthopyroxene, augite
and Fe–Ti oxides (see Fig. 6a, below) in a highly vesicular
(24–68 vol. % vesicles) high-silica rhyolite glass matrix
(76–77% SiO2). Some samples contain minor amphibole
phenocrysts and amphibole pseudomorphs consisting of
plagioclase, pyroxene and Fe–Ti oxides. Completely fresh
euhedral amphibole and coarse-grained pseudomorphs
can occur in the same thin section. Sparse biotite is
present in a few samples. Occasional rounded quartz,
and olivine with reaction coronae of orthopyroxene–
magnetite symplectite are present. Apatite is common as
phenocrysts and as inclusions in phenocrysts. Rare zircon,
anhydrite and pyrrhotite are present as inclusions in
Fe–Ti oxides (not in the same individual crystals).
Hornblende andesite pumice clasts (58·0–63·8% SiO2)
from late ignimbrite flow units contain phenocrysts
(32–39 vol. %, calculated vesicle free) of plagioclase,
amphibole, orthopyroxene, clinopyroxene, Fe–Ti oxides
and minor biotite (see Fig. 6b, below) in a low-silica
rhyolitic glass matrix (71–74% SiO2). One sample
(LAS191) has a high-silica rhyolitic glass matrix (76–78%
SiO2). Crystal clots, involving various combinations of
amphibole, plagioclase and orthopyroxene, are abundant.
Also present are accessory apatite microphenocrysts, rare
pyrrhotite inclusions in amphibole phenocrysts, and rare
olivine crystals with orthopyroxene–magnetite reaction
coronae. Some of these pumice clasts are heterogeneous
because they vary locally from hornblende-rich, pyroxene-poor areas to areas with no hornblende and two
pyroxenes similar in appearance to the main Soncor twopyroxene pumice.
Basaltic andesite scoria (56·2% SiO2) contains phenocrysts of olivine, augite, plagioclase, orthopyroxene and
magnetite in an andesitic groundmass consisting of plagioclase, pyroxenes and magnetite, and rare ilmenite.
Olivine crystals are rimmed with a fine overgrowth of
augite, orthopyroxene, sub-calcic augite and pigeonite,
and contain inclusions of Cr-spinel and andesitic glass.
Augite crystals contain inclusions of orthopyroxene,
pigeonite, magnetite and dacite glass.
Two varieties of glassy lithic clasts are interpreted
as juvenile components. Glassy agglutinate clasts are
interpreted as early welded pyroclastic facies of the Soncor
eruption disrupted by later pyroclastic flows. They are
dense, poorly vesicular, crystal-rich (40–50% crystals)
hyalocrystalline silicic andesite. They contain phenocrysts
of plagioclase, orthopyroxene, augite, amphibole and
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NUMBER 12
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Fig. 3. Pyroxene compositions in Piedras Grandes andesite, Soncor ejecta (basaltic andesite, two-pyroxene pumice and hornblende pumice)
and included rock types (hypabyssal lithic clasts, agglutinate and vitrophyre clasts).
Fe–Ti oxides in a brown glassy andesitic matrix. They
commonly contain lithic clasts of Stage I andesitic lavas,
medium- to coarse-grained granitic rocks, rhyodacite
porphyry and skarn xenoliths, as well as rounded and
embayed quartz crystals. The matrix is heterogeneous
with interbanded streaks of pale and dark brown glass.
Dacite vitrophyres (65·7–67·7% SiO2) are pale cream to
pink, poorly vesicular welded rocks consisting of glassy
fiamme and slightly flattened dacitic pumice. Phenocrysts
and microphenocrysts of plagioclase, two-pyroxenes, amphibole and Fe–Ti oxides are contained in a cloudy,
oxidized rhyolitic glass matrix.
Plagioclase phenocrysts in Soncor ejecta have large
compositional ranges (An32–84; Fig. 2) with complex zonation. The most calcic compositions (An75–84) are found
in phenocryst cores, which are commonly rounded and
resorbed or sieve-textured. Oscillatory-zoned, more sodic
overgrowths are common. A basaltic andesite scoria
contains calcic cores (An75–80) with more sodic (An61–64)
rims and microphenocrysts. Orthopyroxene and augite
in all pumice types typically have very large compositional
ranges (Fig. 3) with both normal and reverse zonation (mgnumber opx = 0·65–0·84, mg-number cpx = 0·63–0·86).
The entire compositional range can sometimes be observed in a single thin section. In contrast, the hornblende
andesite pumices mostly lack augite and contain orthopyroxene with more restricted compositional ranges
(mg-number = 0·66–0·75). Basaltic andesite scoria contains magnesian augite and orthopyroxene (mg-number
opx = 0·76–0·79, mg-number cpx = 0·70–0·81) and
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PIEDRAS GRANDES–SONCOR ERUPTIONS, LASCAR VOLCANO
Fig. 4. Amphibole compositions from Piedras Grandes and Soncor
(primitive and included) samples. Axis labels in formula units, calculated
according to Leake (1968). Element covariations are chosen that most
effectively discriminate between different rock types.
normally zoned olivine (Fo75–86). Amphibole phenocrysts
in Soncor ejecta have a lower Fe3+/Fe2+ ratio, lower Al
and higher Ti and Si than Piedras Grandes amphiboles,
and more restricted range of mg-number (Fig. 4). One
hornblende andesite pumice (LAS191) contains two compositional groups of amphiboles, one of which has lower
Al and higher Si than the other group. Biotite phenocrysts
in two-pyroxene pumice and agglutinates lie within the
range of Piedras Grandes biotite compositions (Fig. 5).
Biotite phenocrysts in one hornblende-rich pumice clast
(LAS36-1) have relatively high Ti and low Al.
Vent-derived lithic clasts
Vent-derived lithic clasts are divided into two main rock
types: prismatic-jointed blocks and porphyries. These
lithologies are interpreted to originate from the preSoncor Stage II volcanic complex and still hot subvolcanic
intrusive bodies. They, together with the Piedras Grandes
unit, provide information about the early stages of evolution of the magma system that eventually led to the zoned
chamber, which was sampled by the Soncor eruption.
Prismatic-jointed blocks include both juvenile material
and fragments of the pre-existing volcanic edifice. Two
main varieties have been recognized.
Pale to dark green medium-grained basaltic andesites
(56·1–56·8% SiO2) are porphyritic or holocrystalline
rocks with hypabyssal textures and containing phenocrysts of plagioclase, pyroxenes and Fe–Ti oxides (Fig. 6c).
The matrix is an intersertal intergrowth of plagioclase,
augite, orthopyroxene, Fe–Ti oxides, quartz and minor
apatite. Euhedral plagioclase phenocrysts (up to 2 mm
across) have large cores and oscillatory-zoned overgrowths. An andesitic prismatic-jointed block contains
plagioclase with a restricted compositional range (An40–50).
Some crystals have sieve-textured cores and growth zones.
Pyroxene phenocrysts (mg-number opx = 0·64–0·81, mgnumber cpx = 0·74–0·86; Fig. 3) are up to 1 mm in length
and euhedral. Orthopyroxene–magnetite intergrowths
in crystal clots of orthopyroxene and plagioclase are
interpreted as pseudomorphed olivine.
Pale two-pyroxene dacites (64·9–65·5% SiO2) contain
phenocrysts of plagioclase, pyroxenes, amphibole pseudomorphs, Fe–Ti oxides and occasional biotite in a matrix
of colourless glass with microphenocrysts of plagioclase,
pyroxenes and Fe–Ti oxides. Plagioclase phenocrysts (up
to 4 mm in length) are subhedral to euhedral with large
cores and oscillatory-zoned overgrowths. Occasional
rounded, embayed and sieve-textured plagioclase crystals
also occur. Pyroxene phenocrysts (mg-number opx =
0·64–0·81, mg-number cpx = 0·74–0·86; Fig. 3) are up
to 2 mm in length and subhedral to euhedral. Biotite
phenocrysts (up to 2 mm across) are rounded and embayed with a thin rim of Fe–Ti oxides. The mineral
assemblage, mineral chemistries and textures are similar
to Soncor pumice, and this rock type could represent an
early dome of the Soncor eruption.
The porphyries are dense non-vesicular to poorly vesicular andesitic to dacitic rocks. They are divided into
three petrographically distinct types.
(1) Type A porphyries are pale grey, fine-grained
microporphyritic dacites. Microphenocrysts (48 vol. %)
and sparse (<1 mm) phenocrysts of plagioclase, orthopyroxene, clinopyroxene, Fe–Ti oxides and pseudomorphed amphibole and olivine are present in a pale
brown rhyolitic glass.
(2) Type B porphyries commonly occur as inclusions
in type A porphyries and in agglutinates. Type B dacite
porphyries (65·1–67·0% SiO2) contain phenocrysts of
plagioclase, biotite, amphibole, pyroxenes, quartz and
Fe–Ti oxides, and microphenocrysts of apatite and zircon
(Fig. 6d). The matrix consists of a fine-grained rhyolitic
(73% SiO2) groundmass of plagioclase, pyroxenes and
Fe–Ti oxides. Plagioclase phenocrysts are up to 5 mm
1897
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 12
DECEMBER 1999
Table 2: Modal analyses of samples from the Soncor flow deposit, quoted on a vesicle-free basis
u
u
u
Sample:
LA124
LAS29-2
LAS149
LAS150
LAS151
LAS152
LAS72
Description:
Basaltic
Andesite
2-pyx
2-pyx
2-pyx
2-pyx
2-pyx
andesite
scoria
pumice
pumice
pumice
pumice
pumice
Matrix
u
80·0
71·3
72·0
69·1
55·7
59·2
67·9
Plagioclase
9·8
19·0
17·5
21·9
32·4
23·0
22·6
Orthopyroxene
0·3
4·7
6·9
3·9
7·2
6·0
3·7
Clinopyroxene
5·1
3·3
2·3
2·8
2·9
4·4
Hornblende
—
Biotite
—
Olivine
Pseudomorphs
Fe–Ti oxides
Quartz
Apatite
Total
% Vesicularity
4·2
—
0·7
0·6
—
—
Acc.
—
—
—
1·9
Acc.
—
—
—
—
4·0
—
1·0
1·4
—
3·7
—
—
1·0
0·3
0·3
—
1·0
1·0
—
0·9
—
2·1
—
—
—
—
—
—
—
—
Acc.
Acc.
Acc.
Acc.
Acc.
Acc.
100·1
99·9
99·7
99·7
99·9
99·7
9·8
41·9
52
61
65
68
100·0
24·4
Sample:
LAS57
LAS58e
SM95/38
SM95/39
LAS47
SM95/41
SM95/26
Description:
Hornblende
Hornblende
Andesitic
Andesitic
Green
Green
Pale
pumice
pumice
agglutinate
agglutinate
PJB
PJB
andesite
Matrix
61·4
68·3
58·9
48·4
—
50·1
69·5
Plagioclase
20·7
19·2
29·3
39·3
72·9
39·3
23·2
3·4
4·9
5·6
6·9
4·4
2·2
4·6
3·0
14·9
1·5
0·4
0·6
—
0·2
Orthopyroxene
Clinopyroxene
Hornblende
Biotite
7·4
—
6·1
—
—
8·0
—
Acc.
—
—
—
Olivine
1·7
—
—
—
—
—
Pseudomorphs
1·3
—
Fe–Ti oxides
1·4
Quartz
—
Apatite
Acc.
Total
100·0
61·6
% Vesicularity
0·9
—
0·1
1·0
1·7
1·8
0·3
—
—
Acc.
Acc.
100·0
100·0
100·0
100·0
100·0
100·0
11·0
1·1
1·0
0·5
0·4
0·0
SM95/27
SM95/47
SM95/28
SM95/29
Pale
Type A
Type B
Type B
dacite
Porphyry
Porphyry
porphyry
Matrix
68·7
51·8
61·2
50·5
Plagioclase
21·9
42·5
27·7
36·5
Orthopyroxene
1·8
2·7
0·6
2·8
Clinopyroxene
5·2
0·9
Acc.
—
Biotite
—
—
Olivine
—
—
—
1·5
—
2·0
1·2
—
Pseudomorphs
0·6
1·1
8·3
Fe–Ti oxides
1·3
1·2
0·9
Quartz
0·5
1·9
—
3·1
1·8
—
Acc.
—
Acc.
Acc.
Apatite
Acc.
Acc.
Total
100·0
100·2
99·9
100·1
2·6
0·5
3·2
0·8
% Vesicularity
2·0
1·0
Acc.
Description:
0·6
0·5
4·0
Acc.
Sample:
Hornblende
4·7
0·1
—
Acc.
0·2
—
—
2·0
—
u, vesicularity (and thus corrected modal composition) from density measurements by water displacement method.
1898
MATTHEWS et al.
PIEDRAS GRANDES–SONCOR ERUPTIONS, LASCAR VOLCANO
phenocrysts of amphibole, orthopyroxene, augite, plagioclase and Fe–Ti oxides in a medium-grained groundmass of plagioclase, alkali feldspar, quartz, pyroxenes
and Fe–Ti oxides. Amphibole phenocrysts are euhedral,
up to 4 mm long and partially pseudomorphed. The
amphibole compositions are compositionally similar to
those in the Piedras Grandes magmas. Subhedral plagioclase phenocrysts are up to 3 mm across with antiperthitic
texture. Pyroxenes are subhedral to euhedral, up to 1 mm
across and commonly inclusion rich.
GEOTHERMOMETRY AND OXYGEN
BAROMETRY
Methods
Fig. 5. Biotite compositions from Piedras Grandes and Soncor (primitive and included) samples. Plots of Ti, Fe and Al against Mg (formula
units calculated to 22 oxygens assuming all Fe as Fe2+).
across. Some crystals have sieve-textured cores. A wide
range of plagioclase compositions (An19–83) is observed
with sodic compositions occurring in phenocryst rims
and groundmass grains. Amphibole phenocrysts are up
to 3 mm in length and are often pseudomorphed. Amphibole phenocrysts have distinctive high Si and low Al
and Ti in comparison with other Soncor and Piedras
Grandes amphibole phenocrysts (Fig. 5). Anhedral biotite
crystals are up to 3 mm in length and are rimmed with
Fe–Ti oxides. Biotite crystals in these rocks have higher
Mg and Ti and lower Fe than in other Soncor and Piedras
Grandes samples (Fig. 5). Subhedral quartz crystals are
up to 5 mm in length.
(3) Micro-granodiorite porphyries (one analysis; 62·5%
SiO2) are medium-grained intrusive rocks containing
One-pyroxene temperatures were calculated from individual orthopyroxene and clinopyroxene analyses using
the ‘QUILF’ program (Frost & Lindsley, 1992; Lindsley
& Frost, 1992). This method projects the pyroxene compositions onto the pyroxene quadrilateral and calculates
temperatures according to the method of Lindsley (1983).
Pressure dependence is slight (+2°C/kbar) and a pressure
of 3 kbar was assumed. Aluminous pyroxenes (opx Al2O3
> 2% and cpx Al2O3 > 3 wt %) yielded variable and
often very high temperatures, and were assumed to
represent disequilibrium compositions produced under
rapid-quench conditions (e.g. Schiffman & Lofgren, 1982;
Ohnenstetter & Brown, 1992). Errors in the temperature
calculations related to uncertainties in the probe analyses
are estimated at ±10°C from repeated analyses of the
same area of a crystal. The most important source of
error is calibration of Si, which affects calculated Fe3+/
Fe2+ ratios. Analyses of orthopyroxenes in experimental
studies of the hornblende andesite of Montserrat (Barclay
et al., 1998) produced calculated temperatures within 5°C
of the run temperatures. However, Murphy et al. (2000)
estimated an error of ±20°C in temperatures calculated
for Soufriere Hills andesite, which is the product of
0·05–0·1 wt % errors in microprobe Ca analysis. We
therefore estimate a maximum error of ±20°C in our
calculations.
Temperature and oxygen fugacity were calculated from
average titanomagnetite–ilmenite compositional pairs
using the QUILF program (Andersen, 1993), which uses
the model of Frost & Lindsley (1992) and Lindsley &
Frost (1992). This method gave good agreement with
glass inclusion melting temperatures in Lascar rocks
(Matthews, 1994). At least 10 titanomagnetite and ilmenite crystals were analysed for each sample. Magnetite
and ilmenite were checked for equilibrium using Mg–Mn
partitioning (Bacon & Hirschmann, 1988), and only
assemblages with equilibrium compositions, low compositional variability and absence of exsolution lamellae
were used. This limited the calculations to rapidly cooled,
1899
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 12
DECEMBER 1999
Fig. 6. Photomicrographs. (a) Soncor two-pyroxene pumice (crossed polars), showing small plagioclase, orthopyroxene, clinopyroxene and
magnetite phenocrysts in a vesicular glassy matrix. The central grey orthopyroxene has a clinopyroxene overgrowth. (b) Soncor hornblende
andesite pumice (crossed polars), showing hornblende, orthopyroxene and plagioclase phenocrysts in a vesicular glassy matrix. (c) Basaltic andesite
prismatic-jointed block (crossed polars) with flow-aligned plagioclase, orthopyroxene, corroded clinopyroxene and Fe–Ti oxides. In (a) and (b)
the polars are slightly uncrossed to show matrix glass detail. (d) Type B porphyry (plane-polarized light), with corroded, oxidized biotite, sievetextured plagioclase, and subhedral orthopyroxene phenocrysts in a medium-grained groundmass of plagioclase, pyroxenes and Fe–Ti oxides;
Field of view on long axis of all photographs ~2 mm.
usually dacitic rocks and very slowly cooled, re-equilibrated shallow intrusive rocks. For olivines with Crspinel inclusions in the basaltic andesite scoria, oxygen
fugacities were calculated using the method of Ballhaus
et al. (1990) and the temperature was calculated from
augite one-pyroxene thermometry.
Results
The Piedras Grandes andesite displays a wide range of
calculated temperatures (740°C to over 1060°C) despite
the restricted range of pyroxene mg-number (Figs 7 and
8). Clinopyroxenes from a basaltic andesite band give
temperatures of 1130–1220°C. Many individual orthopyroxenes have low-temperature cores and higher-temperature rims, with apparent temperature contrasts of
up to 150°C. In contrast, the Soncor two-pyroxene
pumices have a more restricted range of orthopyroxene
temperatures (mostly 900–1000°C) over a wide range of
mg-number (Fig. 8). Clinopyroxene temperatures fall in
a similar range with a few high- and low-temperature
outliers (Fig. 9). Orthopyroxene temperatures increase
with increasing mg-number in Soncor pumice, albeit with
considerable scatter (Fig. 8). Hornblende pumice samples
overlap the Soncor field, but one sample (LAS191) gave
lower and more varied temperatures (760–900°C) with
a few outliers with lower temperature and lower mgnumber. Core-to-rim zoning trends in Soncor pumice
are both up- and down-temperature. The type A and B
biotite porphyries gave a wide range of temperatures
(740–950°C), up-temperature core-to-rim zoning trends
and restricted mg-number, features similar to results from
Piedras Grandes samples (Fig. 7).
The Piedras Grandes andesite, Soncor juvenile ejecta
and vent-derived lithic clasts from the Soncor deposits
contain equilibrium Fe–Ti oxide assemblages with low
compositional variability. Three samples of Piedras
Grandes andesite yielded temperatures of 905–925°C
(Table 3). A basaltic andesite band recorded a slightly
lower temperature (875°C). Soncor two-pyroxene pum-
1900
MATTHEWS et al.
PIEDRAS GRANDES–SONCOR ERUPTIONS, LASCAR VOLCANO
Fig. 7. Core-to-rim variations in temperature and mg-number for
orthopyroxenes in Piedras Grandes andesite and basaltic andesite,
and Soncor Type B porphyry lithic clasts, illustrating the similar
compositional ranges and heating trends observed in orthopyroxenes
in these rocks (marked by arrows connecting cores to rims).
Fig. 8. Temperature–mg-number variations of orthopyroxene in Piedras Grandes and Soncor (primitive and included) ejecta. Data (upper
diagram) and interpretation (lower diagram).
GEOCHEMISTRY
ices, hornblende pumices and a vitrophyre sample yielded
temperatures in the range 860–940°C. Two andesitic
scoria samples recorded a temperature of 900°C. Lower
temperatures were calculated for an andesitic prismaticjointed block (748°C) and two Type B biotite porphyry
samples (740–766°C), and are interpreted as (probably
sub-solidus) closure temperatures for magnetite–ilmenite
equilibrium.
Calculated oxygen fugacities of the Piedras Grandes
and Soncor andesites and dacites range from FMQ
(fayalite–magnetite–quartz) + 2 to + 2·9 (Table 3).
High oxygen fugacity is supported by the ubiquitous
presence of anhydrite in Piedras Grandes andesite,
Soncor hornblende and two-pyroxene pumices, and vitrophyres (e.g. Carroll & Rutherford, 1987; Matthews et al.,
1994a). Basaltic andesite samples range from FMQ +
1·1 to + 1·2, which is consistent with the interpretation
of Matthews et al. (1994a) that oxygen fugacity increases
with falling temperature in Lascar magmas.
Piedras Grandes and Soncor products are medium- to
high-K basaltic andesites, andesites and dacites. They
are compositionally similar to other Lascar and Pleistocene–Recent Central Andean magmas (e.g. Davidson et
al., 1990; Feeley et al., 1993; Feeley & Davidson, 1994;
Matthews et al., 1994b). Major and trace element analyses
are presented in Tables 4 and 5 and illustrated in Figs
10 and 11. Analytical methods are listed in the Appendix.
Representative electron microprobe analyses of glass matrix and glass inclusions in phenocrysts in Soncor basaltic
andesite, hornblende pumice and two-pyroxene pumice
samples are presented in Table 6.
Major element geochemistry
Piedras Grandes and Soncor products show typical Central Andean variations of decreasing TiO2, Al2O3, Fe2O3∗,
MgO and CaO, and increasing K2O with increasing
SiO2 (Fig. 10). Na2O contents are highly variable, and
significant scatter in TiO2, Al2O3 and P2O5 contents is
1901
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 12
DECEMBER 1999
Table 3: Temperature and oxygen fugacity of
Lascar ejecta from Fe–Ti oxide equilibria
Sample
Temperature (°C)
log f O2
dFMQ
Piedras Grandes
Hornblende andesite
SM93/10
910
−9·92
+2·66
SM93/22
905
−9·89
+2·78
LAS451B
925
−10·03
+2·29
875
−11·55
+1·68
LA103
900
−10·21
+2·55
LAS36-1
899
−10·74
+2·04
Basaltic andesite
LAS451A
Soncor
Andesite
Two-pyroxene pumice
LA122
920
−10·21
+2·20
LA155
906
−9·97
+2·69
LAS191
860
−10·6
+2·91
Hornblende andesite pumice
Fig. 9. Temperature–mg-number variations of clinopyroxene in Piedras
Grandes and Soncor (primitive and included) ejecta. Data (upper
diagram) and interpretation (lower diagram).
observed. Samples of Soncor two-pyroxene pumice with
low Na2O (Fig. 10) were hydrothermally altered after
emplacement, and have cloudy, altered glass and contain
large quantities of iron sulphide, silica and various sulphate minerals decorating the inner surfaces of vesicles.
Inflections in the MgO and CaO trends are observed at
about SiO2 = 60–62%. The majority of Piedras Grandes
hornblende andesite analyses are close to this compositional boundary, which separates Lascar basaltic
andesite and andesitic compositions from more evolved
banded pumice, two-pyroxene pumice and biotite porphyries.
Trace elements
Selected trace elements are plotted against SiO2 in Fig. 11.
Rb and Ba increase with increasing SiO2, although Ba
shows increasing scatter in more evolved compositions.
V and Co decrease strongly with increasing SiO2. Basaltic
andesite magmas have highly variable Cr (48–230 ppm),
Ni (17–112 ppm), V (130–190 ppm), Sr (440–606 ppm)
and Zr (124–150 ppm), and in the case of V, Zr and Sr
this variability extends to more silicic compositions. Cr
and Ni are low for more silicic magmas (>60–61% SiO2).
Sr and Zr decrease markedly for SiO2 contents higher
LAS29-1
940
−9·48
+2·58
LAS57
898
−10·11
+2·69
LAS58e
899
−10·21
+2·57
LAS58f
861
−10·85
+2·64
887
−10·52
+2·48
748
−12·88
+3·05
SM95/28
740
−13·65
+2·47
SM95/29
766
−12·15
+3·36
Vitrophyre
SM93/44
PJB
LAS47
Type B porphyry
than 60–62%. Lascar magmas have light rare earth
element (LREE)-enriched patterns when normalized to
chondrite, and Eu anomalies are weak or absent
(Matthews et al., 1996).
Geochemical zonation and heterogeneity
The Soncor Plinian fallout deposit and ignimbrite show
compositional heterogeneity and a weak but complex
zoning pattern (Fig. 12). The main part of the Plinian
deposit, as exposed on the south flank of Lascar, consists
of white andesitic to dacitic two-pyroxene pumice with
variable SiO2 (62·5–66·5%). The uppermost 2 m of this
deposit in the most proximal exposure also contains
1902
MATTHEWS et al.
PIEDRAS GRANDES–SONCOR ERUPTIONS, LASCAR VOLCANO
Table 4: Whole-rock major element analyses of Piedras Grandes and Soncor samples
Rock type: Piedras
Piedras
Piedras
Piedras
Piedras
Piedras
Piedras
Piedras
Basaltic
Andesite Andesite Andesite Andesite
Grandes Grandes Grandes Grandes Grandes Grandes Grandes Grandes andesite
Sample:
SM93/22 SM93/22 GLA-101 GLA-201 LAS61
LAS62
LAS451a LAS451b LA124
TU-69
LAS29
LAS79
LAS79
60·99
SiO2
63·21
63·95
60·07
61·58
60·87
61·44
57·58
63·88
56·19
61·40
61·79
59·16
TiO2
0·59
0·63
0·65
0·90
0·70
0·69
0·81
0·64
0·99
0·76
0·72
0·67
0·77
Al2O3
15·56
15·52
16·74
16·31
16·50
16·23
16·35
15·76
16·52
15·49
15·96
16·18
15·87
Fe2O3
n/a
n/a
3·69
3·33
n/a
n/a
n/a
n/a
3·34
3·02
n/a
3·78
n/a
FeO
n/a
n/a
2·66
2·22
n/a
n/a
n/a
n/a
4·06
2·73
n/a
2·89
n/a
(Fe2O3∗)
5·18
5·06
6·64
5·79
5·96
5·80
7·39
4·91
7·85
6·05
5·89
6·99
6·03
MnO
0·09
0·09
0·07
0·07
0·09
0·09
0·12
0·08
0·11
0·10
0·11
0·08
0·10
MgO
2·92
2·93
3·32
2·90
3·10
2·96
5·56
2·38
5·81
3·32
3·35
4·16
3·47
CaO
4·67
4·76
5·36
5·47
5·51
5·33
7·22
4·64
6·92
5·39
5·44
5·92
5·46
Na2O
3·64
3·45
3·61
3·42
3·29
3·33
3·09
3·51
3·64
3·80
3·76
3·66
3·52
K 2O
2·43
2·52
2·16
1·75
2·11
2·26
1·60
2·72
1·39
2·17
2·06
1·92
2·00
0·20
0·25
0·22
0·22
0·21
0·90
1·48
P 2 O5
H 2 O+
0·19
n/a
LOI
—
Total
98·48
H 2 O–
0·21
0·19
n/a
0·60
0·19
0·22
1·00
1·50
—
—
99·69
99·52
99·67
n/a
n/a
n/a
0·22
n/a
0·25
0·20
n/a
n/a
n/a
1·26
1·28
0·15
0·85
99·61
99·64
100·07
99·58
n/a
n/a
n/a
n/a
—
n/a
1·20
0·22
n/a
—
—
—
100·13
99·88
99·30
99·83
99·76
1·31
0·13
0·00
0·11
n/a
n/a
Rock type: Andesite Andesite Banded
Banded
2-Pyx
2-Pyx
2-Pyx
2-Pyx
2-Pyx
2-Pyx
2-Pyx
2-Pyx
2-Pyx
pumice
pumice
pumice
pumice
pumice
pumice
pumice
pumice
pumice
pumice
pumice
Sample:
LAS95a
GLA-123 TU-85
TU-86
LA121
LA122
LA147
TU-65
TU-66
TU-87
LAS5
LAS7
LAS35-1
64·05
SiO2
61·31
59·51
62·23
62·92
64·94
63·88
64·23
63·87
63·14
64·08
64·01
62·47
TiO2
0·75
0·76
0·68
0·67
0·57
0·64
0·59
0·66
0·68
0·62
0·64
0·70
0·63
Al2O3
16·00
16·58
15·74
15·70
15·32
15·71
15·40
15·85
16·08
15·21
15·62
15·79
15·35
Fe2O3
n/a
3·60
2·57
3·23
2·00
2·31
n/a
1·97
2·00
2·13
n/a
n/a
n/a
FeO
n/a
2·54
2·96
2·14
2·35
2·34
n/a
2·80
3·02
2·72
n/a
n/a
n/a
(Fe2O3∗)
5·77
6·42
5·86
5·61
4·61
4·91
4·91
5·08
5·35
5·15
5·00
5·56
4·86
MnO
0·09
0·07
0·12
0·09
0·08
0·08
0·08
0·09
0·09
0·09
0·08
0·10
0·07
MgO
3·26
3·52
2·98
2·72
2·39
2·59
2·59
2·54
2·60
2·37
2·59
3·11
2·74
CaO
5·39
5·67
4·78
4·48
3·76
4·49
4·54
4·70
4·71
4·24
4·62
5·03
3·46
Na2O
3·54
3·77
3·63
3·73
3·64
3·45
3·17
3·72
3·56
3·64
3·07
3·28
3·35
K 2O
2·03
1·90
2·19
2·33
2·64
2·40
2·52
2·49
2·34
2·58
2·39
2·21
2·47
0·19
0·18
0·18
0·19
1·18
1·62
1·56
P 2 O5
H 2 O+
LOI
0·20
n/a
1·48
0·18
0·28
0·20
0·17
0·19
1·44
1·60
1·56
2·44
1·97
—
—
—
Total
99·82
99·54
99·76
99·77
100·31
—
100·05
H 2 O–
n/a
n/a
0·00
0·00
0·14
0·23
n/a
—
1·33
99·55
n/a
abundant banded pumice and dark andesitic pumice (61
wt % SiO2) as well as white pumice, and exhibits a wide
compositional range (61·0–67·3% SiO2). The ignimbrite
can be divided into two parts. The ‘main’ flow units
consist of lithic-rich lag breccias and pumice-rich
ignimbrite. ‘Late’ flow units are pumice- and scoria-rich
distal flow units and the uppermost flow units on the SE
—
—
—
100·05
100·02
99·43
0·00
0·00
0·00
0·18
n/a
0·19
n/a
0·18
n/a
1·51
1·23
2·71
99·70
99·68
99·86
n/a
n/a
n/a
flanks. The ‘main’ flow units contain mostly andesitic to
dacitic two-pyroxene pumice types (62·5–67·6% SiO2).
The ‘late’ flow units contain more silica-poor white
pumice, compositionally banded pumice and andesitic
scoria (59·2–65·0% SiO2), hornblende andesite pumice
types (58·0–63·8% SiO2) and basaltic andesite scoria
(56% SiO2). A crude double zonation is indicated, with
1903
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 12
DECEMBER 1999
Table 4: continued
Rock type: 2-Pyx
pumice
2-Pyx
2-Pyx
2-Pyx
2-Pyx
2-Pyx
2-Pyx
2-Pyx
2-Pyx
2-Pyx
2-Pyx
2-Pyx
2-Pyx
pumice
pumice
pumice
pumice
pumice
pumice
pumice
pumice
pumice
pumice
pumice
pumice
LAS69b
LAS72
LAS78b
LAS145b LAS145c LAS145d LAS145e LAS148
LAS149
Sample:
LAS35-3 LAS50
LAS53
LAS69a
SiO2
67·64
62·01
62·46
63·72
64·98
63·99
60·60
61·14
66·58
61·02
63·41
67·31
TiO2
0·49
0·72
0·64
0·61
0·60
0·65
0·74
0·75
0·50
0·75
0·63
0·49
62·45
0·70
Al2O3
14·51
15·51
15·97
15·09
15·36
15·40
16·22
16·00
14·62
15·74
15·21
14·45
15·57
Fe2O3
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
FeO
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
(Fe2O3∗)
3·75
5·80
5·14
4·97
4·78
4·96
5·89
5·85
4·04
5·89
4·85
3·68
5·33
MnO
0·07
0·10
0·12
0·09
0·07
0·10
0·09
0·09
0·08
0·08
0·08
0·07
0·08
MgO
1·85
3·21
2·87
2·79
2·53
2·57
3·44
3·25
2·23
3·44
2·72
1·96
2·99
CaO
3·10
5·13
4·92
4·28
4·16
4·71
5·55
5·45
3·77
5·46
4·43
3·40
4·87
Na2O
3·22
3·27
3·03
2·85
2·70
3·37
3·39
3·43
3·43
3·36
3·34
3·42
3·61
K 2O
3·11
2·22
2·44
2·47
2·56
2·46
1·99
2·06
2·89
2·02
2·50
3·01
2·26
P 2 O5
H 2 O+
LOI
0·15
n/a
0·24
n/a
0·32
n/a
0·18
n/a
0·17
n/a
0·20
0·22
n/a
n/a
0·21
n/a
0·14
n/a
0·20
n/a
0·19
n/a
0·14
n/a
0·21
n/a
1·83
1·45
1·90
2·75
1·84
1·21
1·63
1·80
1·73
1·74
2·33
1·86
1·63
Total
99·72
99·66
99·82
99·79
99·75
99·62
99·76
100·02
100·00
99·70
99·69
99·79
99·69
H 2 O–
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
Rock type: 2-Pyx
n/a
2-Pyx
2-Pyx
2-Pyx
2-Pyx
2-Pyx
Hornbl
Hornbl
Hornbl
Hornbl
Hornbl
Hornbl
Hornbl
pumice
pumice
pumice
pumice
pumice
pumice
pumice
pumice
pumice
pumice
pumice
pumice
pumice
Sample:
LAS150
LAS151
LAS152
LAS153
LAS190
GLA-208 LAS30
LAS57
LAS58e
LAS58f
LAS108
LAS109
LAS110
59·08
SiO2
63·51
63·37
63·02
66·56
65·02
65·57
59·17
58·34
58·05
59·86
59·17
59·02
TiO2
0·65
0·64
0·64
0·51
0·50
0·71
0·83
0·79
0·86
0·80
0·82
0·84
0·79
Al2O3
15·28
15·23
15·21
14·48
14·28
15·40
16·03
15·90
16·17
15·82
16·24
16·17
16·38
Fe2O3
n/a
n/a
n/a
n/a
n/a
1·97
n/a
n/a
n/a
n/a
n/a
n/a
n/a
FeO
n/a
n/a
n/a
n/a
n/a
2·09
n/a
n/a
n/a
n/a
n/a
n/a
n/a
(Fe2O3∗)
5·13
4·99
5·02
3·91
3·90
4·29
6·83
6·64
7·03
6·54
6·65
6·74
6·75
MnO
0·10
0·08
0·09
0·07
0·07
0·05
0·10
0·09
0·09
0·09
0·10
0·08
0·10
MgO
2·73
2·85
2·80
2·05
2·12
2·44
4·21
4·23
4·49
3·98
4·11
4·26
4·15
CaO
4·64
4·74
4·54
3·64
4·45
4·32
6·14
6·44
6·54
5·90
6·18
6·06
5·99
Na2O
3·38
3·44
3·32
3·39
3·41
3·41
3·30
3·22
3·21
3·24
3·27
3·24
3·28
K 2O
2·41
2·39
2·41
2·85
2·86
2·05
1·73
1·80
1·68
1·98
1·75
1·79
1·64
P 2 O5
H 2 O+
LOI
0·18
n/a
0·18
n/a
0·19
n/a
0·14
n/a
0·15
n/a
0·16
0·23
0·06
n/a
—
0·21
n/a
0·23
n/a
0·22
n/a
0·25
n/a
0·23
n/a
0·23
n/a
1·81
1·81
2·36
2·40
3·05
1·23
1·94
1·30
1·36
1·28
1·44
1·21
Total
99·82
99·72
99·59
100·00
99·80
98·23
99·79
99·59
99·66
99·79
99·81
99·88
99·62
H 2 O–
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
two cycles in which more mafic magma is erupted at the
end of a cycle.
Glass compositions
Glass inclusions in olivine and clinopyroxene in a basaltic
andesite (LA124) define a continuous compositional
trend. Inclusions in olivines range from 56 to 62% SiO2
n/a
n/a
and inclusions in clinopyroxenes range from 63 to 67%
SiO2. This trend shows variable TiO2 and Na2O, decreasing Al2O3, MgO and CaO, and increasing K2O and
P2O5. FeO∗ has a flat but variable trend to 63% SiO2
then decreases sharply. The more silicic glass inclusions
coexist with titanomagnetite.
Hornblende pumice samples contain low-silica rhyolitic
1904
MATTHEWS et al.
PIEDRAS GRANDES–SONCOR ERUPTIONS, LASCAR VOLCANO
Rock type: Hornbl
Hornbl
Vitro-
Vitro-
Green
pumice
pumice
phyre
phyre
PJB
Green
Sample:
LAS156
LAS191
SM93/44 SM93/44 GLA-125 LAS47
Andesite Dacite
PJB
Type B
Type B
Dacite
Dacite
porphyry porphyry
95/026
95/027
Green
PJB
95/028
95/029
95/030
95/041
95/042
SiO2
63·84
62·39
67·66
65·68
56·81
56·13
62·36
65·54
67·04
65·14
65·46
64·90
TiO2
0·65
0·69
0·54
0·54
0·88
0·91
0·68
0·63
0·57
0·66
0·65
0·65
56·67
0·91
Al2O3
16·04
16·03
14·72
14·95
16·58
16·78
16·68
15·03
15·66
15·91
15·15
16·14
17·23
Fe2O3
n/a
n/a
n/a
n/a
3·08
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
FeO
n/a
n/a
n/a
n/a
4·09
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
(Fe2O3∗)
5·32
5·55
4·53
4·51
7·62
7·94
5·58
4·53
4·18
5·02
4·73
4·76
7·86
MnO
0·09
0·09
0·09
0·08
0·10
0·14
0·10
0·08
0·08
0·09
0·09
0·08
0·14
MgO
2·72
2·72
2·39
2·54
5·31
5·71
3·10
2·51
2·03
2·54
2·62
1·81
5·17
CaO
4·93
5·07
3·77
4·10
7·17
7·08
5·57
4·28
3·93
4·69
4·40
4·42
7·73
Na2O
3·42
3·27
3·64
3·54
3·73
3·63
3·88
3·61
3·82
3·65
3·56
3·80
3·45
K 2O
2·45
2·35
2·68
2·70
1·65
1·28
2·22
2·65
2·93
2·55
2·68
2·47
1·37
P 2 O5
H 2 O+
LOI
0·18
n/a
0·20
n/a
0·10
1·36
Total
99·73
99·72
H 2 O–
n/a
n/a
Rock type: Andesite Type B
0·17
n/a
—
100·19
0·28
0·16
n/a
0·21
0·18
0·25
0·24
n/a
n/a
0·17
n/a
0·17
n/a
0·18
n/a
0·18
n/a
0·24
n/a
0·23
n/a
—
−0·15
−0·14
0·92
−0·04
−0·11
0·91
0·70
−0·23
99·86
99·79
99·70
100·26
99·96
100·36
100·32
100·42
99·95
100·52
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
1·06
Grano-
porphyry diorite
Sample:
95/043
95/049
LAS101
SiO2
58·86
67·18
TiO2
0·88
0·56
62·47
0·70
Al2O3
17·11
15·35
16·43
Fe2O3
n/a
n/a
n/a
FeO
n/a
n/a
n/a
(Fe2O3∗)
7·21
4·27
5·77
MnO
0·11
0·08
0·10
MgO
4·00
2·10
2·90
CaO
6·56
4·23
5·29
Na2O
3·43
3·64
3·44
K 2O
1·69
2·48
2·28
P 2 O5
0·28
0·17
0·22
H 2 O+
n/a
n/a
LOI
−0·18
−0·08
0·10
99·94
100·00
99·70
Total
H 2 O–
n/a
n/a
n/a
n/a
n/a, not analysed.
glass, both as matrix and as inclusions in amphibole
phenocrysts. Glass matrices and inclusions in phenocryst
phases in two-pyroxene pumices are more evolved (76–
77% SiO2). Hornblende pumice sample LAS191 has
matrix glass (and inclusions in minerals) similar in composition to that of the two-pyroxene pumices. There is
no continuous compositional trend for all Soncor glasses.
Glasses in hornblende and two-pyroxene pumices have
separate compositional fields, both from each other and
from the compositional trend of glass inclusions from
basaltic andesite scoria. These low-silica rhyolitic glasses
have relatively low TiO2 and P2O5.
1905
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 12
DECEMBER 1999
Table 5: Whole-rock trace element analyses of Piedras Grandes and Soncor samples
Rock type: Piedras Piedras Piedras Piedras Piedras Piedras
Piedras
Basaltic Andesite Andesite Andesite Andesite Andesite
Grandes Grandes Grandes Grandes Grandes Grandes Grandes andesite
Sample:
SM93/22 GLA-101 GLA-201 LAS61
LAS62
LAS 451a LAS 451b LA124
TU-69
LAS29
LAS79
LAS95a GLA-123
Ba
Ce
Co
Cr
Cs
Cu
Ga
La
Nb
Nd
Ni
Pb
Rb
Sc
Sm
Sr
Th
U
V
Y
Zn
Zr
Cl
F
430
400
600
39
41
25
41
30
11
10
10
36
19
12
73
15
14
78
451
444
38
23
156
8
41
21
20
7
13
58
13
43
19
595
52
12
22
15
23
20
26
10
21
9
18
88
9
387
45
470
16
41
2
50
17
39
6
34
230
61
25
9
30
20
15
77
36
9
30
15
14
78
6
546
16
2
90
23
58
161
326
984
1
537
15
2
84
21
52
166
431
1089
445
8
3
157
24
73
135
53
1094
439
9
5
73
20
53
162
142
1162
571
4
460
446
191
17
88
150
469
135
129
16
65
2-Pyx
pumice
LA122
2-Pyx
pumice
LA147
2-Pyx
pumice
TU-65
2-Pyx
pumice
TU-66
2-Pyx
pumice
TU-87
2-Pyx
pumice
LAS5
2-Pyx
pumice
LAS7
459
520
470
480
469
414
494
497
468
45
45
38
18
42
4
28
15
47
5
26
10
53
4
30
12
36
13
26
16
46
6
26
33
9
25
17
16
88
23
10
29
29
11
77
18
11
27
22
12
90
25
11
23
17
13
117
24
9
23
21
19
79
4
430
16
4
78
16
41
144
542
679
5
443
14
5
96
17
52
143
509
679
1
352
12
4
85
20
44
141
1235
783
5
307
16
7
59
19
31
120
673
435
2
466
10
5
92
17
55
160
400
627
11
436
575
585
108
17
61
144
19
83
164
128
19
74
159
Rock type: Banded Banded 2-Pyx
pumice pumice pumice
Sample:
TU-85
TU-86
LA121
Ba
Ce
Co
Cr
Cs
Cu
Ga
La
Nb
Nd
Ni
Pb
Rb
Sc
Sm
Sr
Th
U
V
Y
Zn
Zr
Cl
F
451
440
440
481
50
467
54
55
50
36
33
19
18
24
11
22
22
10
97
10
33
18
26
10
23
23
8
90
12
32
16
81
31
13
83
470
440
381
12
416
12
130
120
73
65
95
19
39
132
1042
100
18
63
144
2190
32
32
12
10
21
10
25
19
20
91
9
407
15
4
79
19
50
137
414
557
19
20
19
9
23
98
6
34
19
25
14
95
27
13
92
23
16
100
420
450
440
109
115
110
68
72
66
1906
425
429
423
23
21
60
8
44
16
53
6
42
18
10
31
30
9
70
25
9
23
36
14
72
5
480
8
1
109
20
58
149
536
775
8
464
13
4
108
18
58
146
508
1001
42
12
36
8
77
47
370
55
44
12
33
12
70
13
70
602
140
16
82
169
2-Pyx
2-Pyx
2-Pyx
pumice pumice pumice
LAS35-1 LAS35-3 LAS50
MATTHEWS et al.
PIEDRAS GRANDES–SONCOR ERUPTIONS, LASCAR VOLCANO
Rock type: 2-Pyx
pumice
Sample:
LAS53
2-Pyx
2-Pyx
2-Pyx
pumice pumice pumice
LAS69a LAS69b LAS72
2-Pyx
2-Pyx
pumice pumice
LAS78b LAS145b
2-Pyx
pumice
LAS145c
Ba
Ce
Co
Cr
Cs
Cu
Ga
La
Nb
Nd
Ni
Pb
Rb
Sc
Sm
Sr
Th
U
V
Y
Zn
Zr
Cl
F
538
470
465
478
423
429
492
418
459
499
481
472
453
15
39
7
50
15
45
7
22
16
46
8
28
12
44
9
25
17
46
2
48
20
49
1
42
11
41
8
28
21
62
2
51
12
42
7
42
13
38
4
27
14
44
7
37
16
48
10
37
15
54
8
34
40
8
28
23
20
88
25
9
25
28
19
91
26
9
30
21
16
95
30
8
35
23
19
93
23
9
20
25
8
69
25
9
26
33
15
73
19
12
31
21
17
108
23
10
22
25
17
73
23
9
22
23
16
90
17
11
28
21
18
117
27
10
29
32
14
84
24
9
29
30
17
89
18
9
21
29
18
89
1
446
13
4
78
21
61
147
418
810
1
401
17
4
79
18
47
134
714
757
1
400
15
3
76
18
47
133
543
539
5
422
15
3
79
19
50
140
327
618
5
489
12
1
98
16
51
137
598
827
2
466
14
2
109
19
61
150
563
679
5
342
18
3
60
20
41
119
588
627
3
470
10
4
105
19
61
147
584
1036
4
386
16
7
81
20
44
132
590
600
1
319
17
6
58
19
37
122
477
557
2
441
12
4
91
17
59
150
458
871
6
415
14
7
87
19
52
139
461
417
3
420
14
3
85
16
53
136
422
740
Rock type: 2-Pyx
2-Pyx
2-Pyx
2-Pyx
Hornbl
pumice pumice pumice pumice pumice
Sample:
LAS152 LAS153 LAS190 GLA-208 LAS57
Hornbl
pumice
LAS30
Hornbl
pumice
LAS58e
Hornbl
pumice
LAS58f
Hornbl Hornbl Hornbl Hornbl Hornbl
pumice pumice pumice pumice pumice
LAS108 LAS109 LAS110 LAS156 LAS191
Ba
Ce
Co
Cr
Cs
Cu
Ga
La
Nb
Nd
Ni
Pb
Rb
Sc
Sm
Sr
Th
U
V
Y
Zn
Zr
Cl
F
399
399
378
401
394
404
404
556
476
24
62
2
30
25
72
9
58
25
72
5
41
23
71
1
26
16
73
5
40
23
67
4
50
19
66
1
41
15
37
10
33
18
50
13
22
23
7
28
38
14
59
24
8
22
36
11
63
23
7
21
44
13
56
21
8
31
41
15
70
12
9
28
39
12
62
22
8
26
36
13
61
24
7
21
37
10
61
24
8
19
15
17
95
25
8
23
24
18
84
7
469
10
3
125
21
60
124
466
888
1
494
12
2
119
18
49
127
782
818
4
525
9
3
121
20
64
129
465
783
9
480
17
3
111
18
61
142
520
844
1
516
12
3
124
19
55
128
544
888
10
494
10
1
118
19
53
135
411
888
10
535
12
1
114
19
62
125
382
871
4
449
13
4
77
30
48
155
81
932
1
530
13
3
86
22
56
179
384
426
472
467
475
17
53
7
36
11
35
8
40
9
44
8
28
25
9
24
32
16
89
27
12
22
23
23
103
27
11
19
21
19
106
9
402
13
4
82
39
51
134
499
975
1
323
13
5
60
20
36
116
431
740
1
353
14
4
60
19
41
126
453
897
650
41
24
10
23
17
100
422
102
17
69
134
1907
2-Pyx
2-Pyx
2-Pyx
2-Pyx
2-Pyx
2-Pyx
pumice pumice pumice pumice pumice pumice
LAS145d LAS145e LAS148 LAS149 LAS150 LAS151
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 12
DECEMBER 1999
Table 5: continued
Rock type: Vitrophyre
Sample:
93/44
Green
Green
PJB
PJB
GLA-125 LAS47
Ba
Ce
Co
Cr
Cs
Cu
Ga
La
Nb
Nd
Ni
Pb
Rb
Sc
Sm
Sr
Th
U
V
Y
Zn
Zr
Cl
F
370
368
208
25
84
60
10
112
6
53
15
5
29
35
13
41
440
17
22
12
32
Andesite Dacite
606
95
17
53
167
16
78
150
2
550
8
2
130
17
63
124
129
1097
Type B
porphyry
95/029
Dacite
Dacite
95/030
95/041
Green
PJB
95/042
Andesite Type B Granoporphyry diorite
95/043
95/49
LAS101
95/026
95/027
519
48
12
31
5
17
21
22
8
17
13
7
74
13
505
50
7
30
7
16
18
28
11
20
13
15
83
9
648
49
8
18
6
11
21
32
9
15
5
21
103
10
558
55
12
27
6
16
20
26
11
22
8
14
95
9
486
44
12
35
5
12
18
28
9
16
13
18
81
7
553
66
7
16
16
16
21
32
12
25
2
17
83
6
383
34
21
48
28
28
24
16
7
15
17
19
38
19
465
65
19
45
27
27
22
24
10
24
15
0
64
15
533
54
10
22
15
15
19
31
9
17
3
18
82
9
514
5
3
88
18
54
144
94
1133
330
12
4
79
20
48
117
200
1131
381
12
5
64
21
48
162
59
1146
410
7
3
88
22
56
164
46
1164
336
6
3
75
18
50
129
174
1129
411
10
5
73
21
57
194
129
1164
549
4
2
152
21
77
124
28
1124
567
12
2
123
26
92
170
50
1125
373
14
3
66
16
54
140
43
1091
9
348
Type B
porphyry
95/28
VOLATILE ELEMENTS
Chlorine and fluorine
In Soncor two-pyroxene and hornblende pumice samples
apatites have approximately constant F/Cl ratio in most
individual samples (Fig. 13a), but ratios differ between
samples. Halogen contents increase towards apatite rims.
Apatites from one sample of the Piedras Grandes andesite
(SM93/10) show a trend of variable F at constant Cl
content whereas another sample (SM93/20) shows no
coherence (Fig. 13b). For most Soncor hornblende andesite pumices Cl is low in amphibole, with the exception of
sample LAS191, which has relatively Cl-rich amphibole
(Fig. 14a). This high-Cl trend is continued to higher mgnumber and lower Cl by amphibole in a Type B porphyry.
For both high- and low-Cl groups, Cl increases slightly
with decreasing mg-number. Amphiboles in the Piedras
Grandes andesite show low Cl in comparison with most
Soncor ejecta (Fig. 14b). Individual samples of Piedras
Grandes andesite contain hornblende phenocrysts covering a large range in mg-number, in contrast to Soncor
samples, which show more restricted hornblende compositions in individual samples (Fig. 14). Biotite phenocrysts in Soncor two-pyroxene pumice have higher Cl
contents than biotite in Piedras Grandes andesite and
520
15
32
1
13
24
7
22
15
19
94
1
486
14
5
81
21
48
163
63
862
basaltic andesite (Fig. 15). Biotite in the Type B porphyries
extends from the magnesian end of the Piedras Grandes
range towards high-Mg, Cl-rich compositions. Glass inclusions in phenocrysts in Soncor hornblende and twopyroxene pumices contain 0·10–0·16 wt % chlorine
(Fig. 16a). Chlorine contents of matrix glasses are lower.
Glass inclusions in olivine and clinopyroxene phenocrysts
in the Soncor basaltic andesite scoria LA124 have more
variable Cl (0·12–0·34 wt %).
Sulphur
Glass inclusions in phenocrysts in Soncor pumices
(Fig. 16b) are relatively sulphur poor (<500 ppm). However, there is a negative correlation with glass SiO2,
indicating a melt composition control on sulphur solubility. Glass inclusions in olivine and clinopyroxene
phenocrysts in the Soncor basaltic andesite show a strong
decrease in S from 5000 ppm at 55% SiO2 to <1000
ppm at 65% SiO2.
PETROGENESIS
In this section we discuss the evolution of the Piedras
Grandes–Soncor system based on the assumption that
1908
MATTHEWS et al.
PIEDRAS GRANDES–SONCOR ERUPTIONS, LASCAR VOLCANO
Table 6: Representative glass analyses from Soncor ejecta
Sample:
Analysis:
Host:
SiO2
TiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K 2O
P 2 O5
Cl
F
S
Total
LA124
LA124
LA124
LA124
G2
G3
G6
G14
OLIVINE OLIVINE OLIVINE CPYX
55·28
1·05
22·66
5·18
0·00
1·66
2·40
3·28
3·17
0·39
0·12
—
0·36
95·55
57·12
1·54
22·71
4·72
0·14
1·61
2·19
3·60
3·24
0·51
0·20
—
0·44
98·02
LA124
G15
CPYX
LA124
G20
CPYX
LAS30
LAS30
LAS30
LAS57
G52
G51
G50
G13
MATRIX MATRIX MATRIX AMPH
LAS57
G2
AMPH
LAS57
LAS57
G8
G7
MATRIX MATRIX
58·97
1·13
19·01
4·45
0·12
0·90
4·73
3·78
3·24
0·43
0·11
—
0·16
97·03
63·64
1·94
13·92
5·40
0·00
0·62
1·04
3·05
3·39
1·09
0·19
—
0·00
94·28
65·13
2·01
15·77
4·32
0·00
0·73
0·59
3·81
3·58
0·85
0·15
—
0·02
96·96
67·87
2·11
13·43
3·09
0·06
0·47
1·45
2·85
3·80
1·09
0·13
—
0·03
96·38
69·89
0·44
15·11
2·44
0·12
0·70
2·38
3·70
3·02
0·14
0·10
0·12
0·03
98·18
69·94
0·40
15·06
1·72
0·04
0·68
2·47
3·06
2·97
0·09
0·12
0·00
0·02
96·56
71·25
0·47
15·30
2·65
0·08
0·68
2·34
3·14
3·01
0·19
0·11
0·26
0·02
99·50
69·56
0·46
14·70
0·89
0·00
0·00
2·16
3·05
3·15
0·11
0·14
0·00
0·04
94·25
70·47
0·38
14·44
1·97
0·06
0·17
1·82
3·25
3·17
0·09
0·12
0·19
0·02
96·13
70·63
0·44
14·86
2·16
0·15
0·56
2·41
3·07
3·15
0·11
0·10
0·00
0·02
97·65
71·95
0·38
14·89
2·29
0·09
0·64
2·43
3·49
3·44
0·11
0·13
0·07
0·03
99·95
Normalized to 100% volatile free
SiO2
58·14
58·66
60·94
TiO2
1·10
1·58
1·17
Al2O3
23·84
23·32
19·65
FeO
5·44
4·84
4·60
MnO
0·00
0·14
0·13
MgO
1·75
1·65
0·93
CaO
2·53
2·25
4·88
Na2O
3·45
3·70
3·91
K 2O
3·34
3·33
3·34
P 2 O5
0·41
0·53
0·45
Total
100·00
100·00
100·00
67·63
2·06
14·80
5·74
0·00
0·66
1·10
3·24
3·60
1·16
100·00
67·30
2·07
16·29
4·47
0·00
0·75
0·61
3·93
3·70
0·88
100·00
70·54
2·19
13·95
3·21
0·06
0·49
1·51
2·96
3·95
1·13
100·00
71·37
0·45
15·43
2·49
0·12
0·71
2·43
3·78
3·08
0·14
100·00
72·54
0·42
15·61
1·78
0·04
0·70
2·56
3·17
3·08
0·09
100·00
71·89
0·47
15·44
2·68
0·08
0·69
2·36
3·17
3·03
0·19
100·00
73·94
0·48
15·62
0·95
0·00
0·01
2·30
3·24
3·35
0·11
100·00
73·56
0·39
15·07
2·06
0·07
0·17
1·89
3·39
3·31
0·09
100·00
72·42
0·45
15·24
2·21
0·15
0·57
2·48
3·14
3·23
0·11
100·00
72·15
0·38
14·93
2·30
0·10
0·64
2·44
3·50
3·45
0·11
100·00
Sample:
Analysis:
Host:
LAS72
G5
OPYX
LAS72
G6
OPYX
LAS72
G2
MATRIX
LAS191
G12
AMPH
LAS191
G3
AMPH
LAS191
G7
OPYX
LAS191
G8
OPYX
74·15
0·24
12·69
1·19
0·07
0·16
1·14
2·76
4·78
0·03
0·11
0·19
0·00
97·51
76·92
0·23
12·32
1·40
0·00
0·09
1·07
2·62
4·49
0·00
0·14
0·00
0·00
99·27
74·49
0·19
13·34
1·38
0·01
0·19
1·27
2·92
4·52
0·05
0·14
0·00
0·01
98·51
74·85
0·24
13·26
1·19
0·00
0·19
1·27
2·99
4·49
0·03
0·13
0·16
0·00
98·79
76·23
0·21
13·46
0·77
0·00
0·04
1·11
3·74
3·65
0·05
0·13
0·00
0·00
99·41
75·58
0·23
12·29
1·56
0·10
0·23
1·00
2·79
4·62
0·03
0·14
0·03
0·01
98·61
75·53
0·30
12·31
1·25
0·00
0·26
1·02
3·29
4·60
0·03
0·09
0·23
0·00
98·92
Normalized to 100% volatile free
SiO2
76·28
77·59
75·72
TiO2
0·25
0·23
0·20
Al2O3
13·05
12·43
13·56
FeO∗
1·22
1·41
1·40
MnO
0·08
0·00
0·02
MgO
0·17
0·09
0·19
CaO
1·17
1·08
1·29
Na2O
2·83
2·64
2·97
K 2O
4·92
4·53
4·60
P 2 O5
0·03
0·00
0·05
Total
100·00
100·00
100·00
75·99
0·24
13·46
1·20
0·00
0·19
1·29
3·03
4·56
0·03
100·00
76·79
0·21
13·56
0·78
0·00
0·04
1·12
3·77
3·68
0·05
100·00
76·78
0·24
12·49
1·59
0·11
0·23
1·01
2·83
4·70
0·03
100·00
76·61
0·31
12·48
1·27
0·00
0·27
1·03
3·34
4·66
0·03
100·00
SiO2
TiO2
Al2O3
FeO∗
MnO
MgO
CaO
Na2O
K 2O
P 2 O5
Cl
F
S
Total
LA124, basaltic andesite scoria; LAS30, hornblende andesite pumice; LAS57, hornblende andesite pumice; LAS191, hornblende andesite pumice; LAS72, two-pyroxene silicic andesite pumice.
1909
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 12
DECEMBER 1999
Fig. 10. Harker diagrams of major element variations for Piedras Grandes and Soncor ejecta.
the Piedras Grandes and Soncor vent-derived lithic assemblage preserves information on the development of
an evolving intermediate to silicic magma system, which
culminated in the formation of the zoned Soncor magma
chamber.
Origin of mafic magmas
The variability of many major and trace elements in the
basaltic andesite samples from the Piedras Grandes and
Soncor eruptions implies that these magmas were modified before emplacement into the shallow magma system.
Variations in Al2O3, Na2O, Cr, Ni and Sr with little
change in SiO2 imply fractionation processes involving
olivine, clinopyroxene and spinel. The abundance of
these early phenocryst phases in the Soncor basaltic
andesite sample LA124 is reflected in high Cr, Ni and
V compared with other Lascar basaltic andesite samples.
Some magmas became depleted in Cr and Ni by fractionation of olivine, clinopyroxene and Cr-spinel, whereas
others (like LA124) entrained crystals of these phases and
were thus enriched in these elements. The geochemical
variations do not show any features indicative of plagioclase fractionation, which is consistent with evolution
of basaltic andesite magmas by fractionation at high
pressure. These features suggest that the basaltic andesite
magmas were fractionated in the lower crust.
There is, however, poor correlation between Cr and
Ni and other major and trace elements, indicating that
fractionation and accumulation of olivine, clinopyroxene
and spinel were not the only important processes. The
LREE-enriched and relative heavy rare earth element
(HREE)-depleted patterns of Lascar magmas implicate
garnet (Kay et al., 1991). A number of models have been
proposed for the ‘baseline’ compositional and isotopic
variability of Central Andean magmas, involving MASH
(melting, assimilation, storage and homogenization) type
processes (e.g. Hildreth & Moorbath, 1988; Rapp &
Watson, 1995) in garnet-bearing lower crust. There is
also evidence for modification of the mafic magma in
the shallow magma chamber. For example, Mg-poor,
low- to moderate-temperature (770–900°C) orthopyroxene phenocrysts and hornblende crystals identical
to those in the andesite occur in the basaltic andesite
band LAS451a of the Piedras Grandes unit. These phenocrysts are compositionally and thermally similar to the
phenocrysts in the host andesite and are interpreted as
crystals that have mixed into the basaltic andesite from
the host magma.
1910
MATTHEWS et al.
PIEDRAS GRANDES–SONCOR ERUPTIONS, LASCAR VOLCANO
Fig. 11. Trace element variations as function of SiO2 for Piedras Grandes and Soncor ejecta.
Pre-Soncor magma evolution
The pre-Soncor Stage II volcano consisted of porphyritic
hornblende andesites of the Piedras Grandes unit and
porphyritic biotite dacite porphyries as sampled by the
Soncor eruption. We interpret the petrological and geochemical data as the consequence of basaltic andesite
magmas being emplaced into a high-temperature highly
crystalline magma body that was remobilized by the
Fig. 12. Whole-rock SiO2 through the Soncor Plinian and later pyroclastic flow deposit showing complex compositional variation. The
vertical axis represents relative stratigraphic position.
supply of heat and to some extent mingled with the
basaltic andesite.
Orthopyroxene crystals in the andesites of the Piedras
Grandes unit and in the dacite porphyries have lowtemperature (650–750°C) cores and show rising coreto-rim temperatures. We interpret the low-temperature
orthopyroxene cores as restite crystals and the core-to-rim
trends of rising crystallization temperature as evidence of
reheating and partial melting in their origin. In the case
of the Piedras Grandes andesite the presence of basaltic
andesite bands and inclusions provides direct evidence
that influx of hotter mafic magmas provided the heat
input. Several features are consistent with hybridism
between the andesite magma and the mafic magmas.
Rounded, sieve-textured, anorthite-rich cores are observed in plagioclase crystals, commonly with more sodic,
oscillatory zoned overgrowths. These cores have similar
compositions to phenocrysts in the basaltic andesite.
Rounded and embayed quartz grains with inclusions and
fracture-fills of rhyolitic groundmass together with reacted
olivine crystals in the andesite are illustrative of the
disequilibrium hybrid character of the andesite. The
amount of hybridism is, however, limited. The proportion
of crystals that might originate from the basaltic andesite
end member is low. The geochemical variations in the
1911
JOURNAL OF PETROLOGY
VOLUME 40
Fig. 13. F and Cl contents of (a) Soncor and (b) Piedras Grandes
apatites. Formula units recalculated to 6(P + S + Si).
andesite do not fall on linear trends between more evolved
dacitic rocks and the basaltic andesites as in other systems
where hybridism can be demonstrated to be more significant (e.g. Clynne, 1999). The remobilization of highly
crystalline low-temperature magma bodies by influx of
mafic magmas with associated hybridism has been deduced in many other orogenic intermediate magmatic
systems from similar kinds of evidence (e.g. Heiken &
Eichelberger, 1980; Clynne, 1999; Murphy et al., 2000).
Fe–Ti oxides in Piedras Grandes rocks give consistent
pre-eruption equilibration temperatures of 905–925°C,
which are much higher than the temperatures given by
the orthopyroxene cores but lower than temperatures
indicated by some orthopyroxene rims and clinopyroxene
crystals in the basaltic andesite inclusions. It is proposed
that large temperature variations occurred in the Piedras
Grandes andesite, but that thermal homogenization and
equilibration occurred before eruption.
To discuss the formation of the Piedras Grandes andesite magmas and the dacite porphyries it is necessary to
define the terms used. ‘Partial melting’ is used here to
indicate the formation of a partial melt by heating of a
solid rock or partially molten rock that is too crystalline
NUMBER 12
DECEMBER 1999
Fig. 14. Cl contents of (a) Soncor and (b) Piedras Grandes amphiboles,
plotted against Mg [formula units calculated according to Leake (1968)].
The individual ranges of amphibole compositions in Soncor hornblende
andesite pumices are clearly visible, as well as the relatively high Cl
contents of amphiboles in Type B porphyries and the hornblende
andesite LAS191. Piedras Grandes amphiboles have large compositional
ranges in all samples.
Fig. 15. Cl contents of biotites in Soncor and Piedras Grandes samples,
plotted against Mg (formula units calculated to 22 oxygens, assuming
all Fe as Fe2+). The rapid decrease in biotite Cl in the biotite porphyry
samples, trending towards Piedras Grandes andesite biotite compositions, is clear. Soncor biotites have relatively high Cl contents.
to behave as a magma. ‘Mobilization’ is defined here as
the formation of magma by partial melting. This magma
consists not only of the partial melt, but also of entrained,
unmelted, restite crystals. As recognized by Chappell &
White (1992), segregation of partial melt from restite
1912
MATTHEWS et al.
PIEDRAS GRANDES–SONCOR ERUPTIONS, LASCAR VOLCANO
Fig. 16. (a) Cl contents of Soncor glass inclusions, showing a flat but
scattered trend. The relatively large error bars on analyses of LA124
glass inclusions are due to analysis by microprobe using energydispersive spectrometry (EDS), as opposed to wavelength-dispersive
spectrometry (WDS) for other samples. (b) S contents of Soncor glass
inclusions, illustrating the very S-rich basaltic andesite glasses of Soncor
olivine-rich scoria, and the rapid decrease in S with melt evolution.
crystals produces a magma of more evolved composition
than the source rock. Complete mobilization produces a
magma of the same composition as the source rock. The
protolith could be crustal rocks unconnected genetically to
the mafic magmas that provide the heat for mobilization.
However, they could also be plutonic or cumulate rocks
formed by previous batches of magma of the same
character and origin. In this sense, distinguishing restite
crystals from cumulate crystals is a matter of semantics.
Situations in which previous magma batches have partly
consolidated and are then remobilized and partially
melted by later batches of magma in the same system
are likely to be very common.
We suggest that a solidified or largely solidified intrusion
of andesite composition in the upper crust was activated
at the beginning of Lascar Stage II by influx of basaltic
andesite magma. This intrusion may represent the Lascar
Stage I andesitic magma chamber, which cooled and solidified over a long period (thousands of years) of inactivity
before the initiation of Stage II activity. The overall geochemical affinities of the Piedras Grandes andesite with
the earlier Stage I andesites (Matthews et al., 1994b) are
consistent with the ultimate origin of these magmas by
fractional crystallization of basalt. Partial melting must
have occurred at depths of at least 5 km to stabilize hornblende (Rutherford & Hill, 1993; Barclay et al., 1998). The
orthopyroxene core temperatures (650–750°C) of the Piedras Grandes andesite and dacite porphyries indicate that
this intrusion was close to its solidus temperature. However, it is possible that these temperatures are simply the
final closure temperatures of the pyroxenes, preserved during cooling of the andesite intrusion, and that the actual
temperature was even lower before remelting. The andesite rock was mobilized by mafic magmas with addition of
heat and some mass. The dacite porphyries are interpreted
as representing low-percentage partial melts with some
entrained restite crystals. The Piedras Grandes andesite is
interpreted to represent later complete mobilization of the
intrusive protolith. Limited hybridism by the mixing of
basaltic andesite with the mobilized andesite generated the
geochemical trends in magmas more mafic than 61–62%
SiO2, and explains in part the inflections in geochemical
trends at these compositions. This interpretation of some
orogenic andesites to rhyolite sequences as the products of
partial melting is increasingly supported by accumulated
evidence, as exemplified by Mount St Helens (Blundy &
Gardner, 1997), the andesite of Montserrat (Murphy et al.,
1998, 2000), Fish Canyon (Lipman et al., 1997) and granites
of the Lachlan Fold Belt (e.g. Chappell & Stephens, 1988;
Chappell & White, 1992; King et al., 1997).
The petrological features of the Piedras Grandes andesites can be interpreted in terms of fluid dynamical models
of crustal melting (Huppert & Sparks, 1988). In mobilization of a solid or partially molten rock by mafic
magma intrusions, melting occurs in the boundary layer
region, but cooling and crystallization take place in
the interior as its volume increases. The wall-rocks are
variably heated both in space and time, and crystallization
in the convecting interior occurs simultaneously with
partial melting at the chamber margins. Parcels of magma
with similar composition but with very different temperature and crystallization histories are progressively
mixed in by convection to produce a fairly homogeneous
magma, but with a highly heterogeneous crystal assemblage. The large spread of orthopyroxene temperatures and hornblende compositions in individual
samples reflects this diverse thermal history. The more
restricted Fe–Ti oxide temperatures are interpreted as the
final temperature attained at the end of the remobilization
process just before extrusion.
The heterogeneous Soncor chamber
The Soncor magma chamber contained a wide range of
bulk magma compositions. The melt phase in Soncor
two-pyroxene pumice clasts is always rhyolitic and so
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JOURNAL OF PETROLOGY
VOLUME 40
bulk magma compositional variation is largely a function
of crystallinity. We interpret the bulk compositional
Soncor ejecta in terms of fractional crystallization, but
with significant disruption in the chamber as a result of
convective stirring related to injection episodes of hydrous
mafic magma at the base of the chamber.
The most mafic two-pyroxene pumice clasts have compositions comparable with the Piedras Grandes andesites.
We interpret the Soncor two-pyroxene andesite and
dacite magmas as products of fractionation of the observed phenocryst assemblage from an andesitic parent.
The origin of the parent could be the melt composition
formed by complete melting of the same source region
that supplied the Piedras Grandes andesites, or more
likely by formation of a hybrid between fractionated
derivatives of the injected basaltic andesite magma and
partial melts of the protolith. When mafic magmas are
emplaced into a crustal magma system and exchange
heat, magmas of intermediate composition are generated
both by fractionation of the cooling mafic magma and
by reheating of the pre-existing protolith (Huppert &
Sparks, 1988). The exact contributions of partial melting
and fractionation of cooling mafic magma are difficult
to demonstrate conclusively, particularly if, as is thought
to be the case at Lascar, the protolith and mafic magma
are of the same magmatic lineage. We envisage the
formation of higher-temperature andesitic magmas as a
logical development of the process of magma chamber
reheating by influx of mafic magma that initiated with
the Piedras Grandes eruptions.
The phenocrysts and temperature estimates indicate
that the Soncor magma chamber had a complex opensystem history. The interpretation of its evolution is aided
by reference to Zr and Sr variations (Fig. 17). In individual
samples, orthopyroxene phenocrysts show a wide range
of mg-numbers and calculated temperatures (mostly
900–1000°C). In some samples reverse and normally
zoned orthopyroxenes suggest mingling of different
magma parcels in the chamber. In most cases there
is evidence for admixture of a small basaltic andesite
component in these pumices, as evidenced by the occurrence of relict magnesian olivine crystals (replaced by
magnetite–orthopyroxene symplectite), calcic plagioclase
phenocrysts and some high-temperature (>1000°C) magnesian pyroxene crystals. These features are interpreted
as evidence for influx of higher-temperature, more mafic
magma, which stirred together parts of the fractionating
chamber.
The origin of the hornblende pumice is now considered.
These ejecta come from the last flow units of the ignimbrite and are assumed to be derived from the deeper
levels of the chamber. Most of these samples have mafic
andesite bulk compositions (58% SiO2) and one possible
interpretation is that they represent a melt composition
from which the whole Soncor zoned system was derived.
NUMBER 12
DECEMBER 1999
Fig. 17. Interpretation of Sr and Zr patterns in Soncor magmas
on the basis of a zoned magma chamber. Variable mafic magma
compositions are interpreted to have mixed with an intermediate
composition equivalent to the Piedras Grandes andesite. More evolved
compositions are interpreted as products of fractional crystallization of
this intermediate composition.
However, these samples contain abundant hornblende–
plagioclase–orthopyroxene crystal clots as well as individual phenocrysts, suggesting that their mafic
compositions can be partly attributed to admixture of
cumulate materials. Observation of inhomogeneous pumice clasts containing both two-pyroxene and hornblenderich domains indicates a hybrid relationship between the
magmas.
Although the hornblende pumice might contain cumulate components, it cannot represent the cumulate
equivalent of the main fractionated chamber, as such a
cumulate would have to be relatively Zr rich rather than
Zr poor (Fig. 17). A more probable explanation of the
hornblende pumices is that they represent crystallization
products of earlier episodes of hydrous basaltic andesite
magma invading into the base of the Soncor chamber.
This is supported by their low-silica rhyolite matrix glass
compositions as opposed to the high-silica rhyolitic matrix
glasses of the two-pyroxene pumices.
Hornblende was clearly stable in the lower parts of
the Soncor magma chamber. The similar orthopyroxene
temperature ranges (and magnetite–ilmenite temperatures) of the hornblende and two-pyroxene pumices
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MATTHEWS et al.
PIEDRAS GRANDES–SONCOR ERUPTIONS, LASCAR VOLCANO
suggest that hornblende stabilization was due to water
pressure (e.g. Allen, 1978; Rutherford & Hill, 1993), and
possibly also the influence of the somewhat more mafic
compositions of these magmas. Recent experimental data
on similar rock compositions imply that the chamber
base would have to be at about 120–150 MPa (Gardner
et al., 1995; Barclay et al., 1998), which would place the
chamber in the upper crust at about 5–6 km depth. The
occurrence of minor amounts of hornblende in some
two-pyroxene pumices provides further support for a
protracted open-system mixing history in the chamber.
In the same samples hornblende ranges from coarsegrained plag–pyx–oxide pseudomorphs, to crystals with
reaction rims, to perfectly fresh euhedral hornblende.
The hornblendes have the same composition as in the
hornblende pumices. We interpret the pseudomorphed
hornblendes as the result of older periods of mixing of
magma from deeper in the chamber, where hornblende
was stable, into the upper parts, where it was unstable.
The fresh hornblende may either be the consequence of
the last episode of mixing associated with the eruption
or it may be due to evolution of the temperatures and
water pressure to a point where hornblende became
marginally stable.
There is no direct evidence that the eruption of the
Soncor magma chamber was triggered by a final magma
injection event. Just before the eruption the Soncor
magma had stabilized, and we take the Fe–Ti oxide
temperatures as indicative of a thermal stratification in
the chamber (860–940°C). The basaltic andesite scoria
and pumice in the late flow units are interpreted as new
hot magma emplaced at the base of the chamber. The
petrology of the olivine basaltic andesite scoria implies
slow cooling, which indicates that it had an extended
residence time in the Soncor magma chamber. At least
some of the heterogeneity in the ejecta can be associated
with the convective disruption of the chamber and the
conduit flow processes in the eruption. A complex magma
chamber morphology is required for both hot basaltic
andesite and hornblende andesite to exist at the base of
the magma chamber. In one part, basaltic andesite was
mixing with two-pyroxene andesite and dacite. In another
part, hornblende andesite underlay the two-pyroxene
magmas (Fig. 18).
Interpretation of volatile behaviour
Chlorine and sulphur in the Piedras Grandes and Soncor
magma chambers have complex histories. Low Cl contents of amphiboles and biotites in the Piedras Grandes
andesites indicate low f HCl. Apatite trends of variable
F at low Cl are attributed to open-system degassing of
Cl during slow ascent of the Piedras Grandes andesite
to the surface. Cl contents of amphibole and biotite
in the Soncor two-pyroxene- and amphibole-bearing
magmas indicate much higher f HCl than in the Piedras
Grandes andesite. Poor homogenization occurred in the
Soncor magma chamber, leading to differences in f HCl
in different areas. Some glass inclusions from the basaltic
andesite scoria are very Cl rich. We interpret these data
to indicate that the basaltic andesite magma was the
main source of Cl in the magma chamber. The approximately flat trend of Cl for glass inclusions with
>65% SiO2 in the Soncor pumices suggests probable
melt saturation in a Cl-bearing volatile phase. We propose
that Cl was steadily being degassed into a co-magmatic
vapour phase during crystallization of this magma; this
supports the interpretation that volatiles are transferred
from mafic magmas into the overlying zoned chamber
during replenishments. The trends of roughly constant
F/Cl in apatite crystals in different Soncor pumice
samples are consistent with an external buffer of halogen
fugacity. We suggest that this buffer is the implied comagmatic volatile phase.
Lascar is a sulphur-rich volcano, as indicated by the
occurrence of anhydrite and by the large fluxes of SO2
from the volcano (Andres et al., 1991; Matthews et al.,
1994a, 1994b). The low solubility of sulphur in rhyolitic
melts, as illustrated by analyses of sulphur in glass inclusions in Soncor ejecta, excludes the more evolved
magmas as the major source for the sulphur. The olivine
basaltic andesite (LA124) provides evidence that the
basaltic magmas are the source, with melt inclusions in
olivine containing up to 5000 ppm S. Crystallization of
this magma caused exsolution of dissolved sulphur into
a co-magmatic vapour phase, causing a rapid fall in melt
S to <1000 ppm. The presence of anhydrite in the
evolved Soncor magmas implies transfer of sulphur from
invading mafic magmas in a free volatile phase (e.g.
Andres et al., 1991; Matthews et al., 1994a) and requires
both high f O2 and high f S2 (Carroll & Rutherford, 1987).
Transfer of sulphur into the zoned chamber resulted in
oxidation of S and dissolution into the melt as sulphate
to stabilize anhydrite. The high f O2 of Soncor and Piedras
Grandes magmas resulted in oxidation of dissolved sulphur in the basaltic andesite to SO2. Melt S in the Soncor
magma chamber was dissolved dominantly as sulphate
(Matthews et al., 1998). Buffering by a co-magmatic Srich vapour was invoked by Matthews et al. (1994a) as
the cause of the high f O2.
The source of some of the Cl and all of the S is
attributed to influxes of volatile-rich basaltic andesite
magma, which exsolved a large proportion of its dissolved
gas during quenching or slow crystallization after ponding
at the base of the zoned Soncor magma chamber.
Water contents
The above discussion indicates that volatile-rich mafic
magmas have been a major source of Cl and S during
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JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 12
DECEMBER 1999
Fig. 18. Model for Piedras Grandes and Soncor magma chamber evolution. (a) Generation of biotite porphyries by remelting of andesitic
protopluton, following injection of hot basaltic andesite magma. Basaltic hypabyssal rocks included in Soncor ejecta are products of slow cooling
of this primitive magma at shallow depth. Green PJBs, green prismatically jointed rocks. (b) Total remobilization of protolith produces the
Piedras Grandes hornblende andesite. Mafic inclusions are entrained by convection. (c) One possible configuration of the Soncor zoned magma
chamber, with basaltic andesite, hornblende andesite and mixed magmas at the base and two-pyroxene dacite in the upper part. Fractional
crystallization is dominant in the two-pyroxene dacite.
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MATTHEWS et al.
PIEDRAS GRANDES–SONCOR ERUPTIONS, LASCAR VOLCANO
the evolution of the system. Water, however, is the major
volatile species, and the transfer of water from invading
hydrous basaltic magmas during partial melting or evolution of the Soncor zoned system is implied. Indeed,
partial melting of largely solidified plutonic rocks cannot
produce volatile-rich silicic magmas, as the volatile contents of the constituent minerals are too low. The requirement for substantial amounts of water to be
transferred can be illustrated as follows.
The Piedras Grandes and Soncor magmas are interpreted to originate from remobilization of intrusive
source rocks of andesite composition by invading mafic
magmas. There is insufficient water in such rocks to
stabilize the formation of hydrous phenocrysts in the
resultant magmas. For example, let us consider a granodiorite porphyry sample as a possible protolith composition; magmatic water contents can be modelled
during partial melting, assuming no loss of volatiles by
degassing, or gain by addition from the injected and
quenched mafic magma. The granodiorite porphyry has
a low intrinsic water content, which is contained in
amphibole. The amphibole content of this rock is 18 wt
%, which yields a maximum total water content of 0·46
wt % (assuming 2·5 wt % H2O in amphibole). If this
composition is partially melted by 40%, a melt with 0·94
wt % H2O could be formed. In both cases the water
content is far too low for the stabilization of biotite and
amphibole, and far too low in the case of the Soncor ejecta
to account for a large-magnitude explosive eruption.
Stabilization of amphibole in the Piedras Grandes and
Soncor magmas requires water contents of about 4–5%
(Gardner et al., 1995; Barclay et al., 1998). Low totals in
glass inclusions from basaltic andesite and hornblende
andesite ejecta (Table 6) are consistent with such water
contents. Most of the Soncor ejecta are anhydrous, as a
result of pressures being too low for stabilization of
amphibole, but biotite is stable. Modelling of major
Plinian eruptions requires water contents of at least 4%
(Gardner et al., 1996; Melnik, 2000). These considerations
imply transfer of volatiles as well as heat during influx
of mafic magmas into evolved magma systems in the
crust.
source with admixed restite. These magmas, together
with some more mafic andesite magmas, were erupted
or intruded at shallow level to form the Stage II
volcanic complex. Volatile element concentrations in
glasses and minerals imply that the invading basalt
must have been a major source of volatiles and that
crystallization took place under conditions of opensystem degassing.
As further heat, volatiles and mass were supplied
from invading mafic magma the shallow system became
hotter and a substantial zoned magma body developed.
The transition from amphibole-bearing to clinopyroxene-bearing magmas indicates that the focus of
the magma system became shallower, although the
base of the chamber was at sufficiently high water
pressures to stabilize hornblende. Comparison with
experimental studies on similar compositions indicates
that the base of the chamber was at depths of 5–6 km.
Fractionation in an open magma chamber periodically
invaded by more mafic magma resulted in a zoned
chamber (Fig. 18c). Hornblende andesite magma was
present in the lower part of the magma chamber, and
is considered to represent the crystallization of older
invasions of hydrous mafic magmas. These invasions
repeatedly stirred up the chamber, adding heat, volatiles
and some mass from invading mafic magmas, but
without destroying the overall zonation caused by
layered convection (Sparks et al., 1984).
Additions of mafic magma must be a major source
for the volatile components in the zoned chamber.
Zoning of halogens in minerals and evolution at
constant Cl/F ratio imply that crystallization occurred
in a closed system with respect to volatiles, although
the data are also consistent with periodic additions of
volatiles from the mafic magma. Sulphur was derived
from the invading mafic magmas, and the oxidizing
conditions stabilized anhydrite in the evolved magmas.
Convective stirring during replenishment events and
conduit flow during the eruption has mixed together
the various parts of the chamber. The eruption of
components from the base of the chamber suggests
that most of the chamber was erupted, although
considerably more basaltic andesite is implied at the
base of the chamber than is observed in the ejecta.
CONCLUSIONS
Following a long hiatus in activity after Stage I of Lascar,
a highly crystalline pluton of andesite composition was
remobilized by intrusion of basaltic andesite magma
(Fig. 18a). Partial melting of the protolith was initiated
by addition of heat and volatiles from the mafic
magma. Mobilization of the protolith and some limited
mixing with the basaltic andesite produced the Piedras
Grandes andesite (Fig. 18b). Biotite dacite porphyries
can also be interpreted as partial melts of this plutonic
ACKNOWLEDGEMENTS
This project was supported by the Servicio Nacional de
Geologia y Mineria, Chile, the Antofagasta regional
government, NERC Grant GR3/9047 and an NERC
Professorship to R.S.J.S. Reviews of the paper by Charlie
Bacon, Tod Feeley and Shaun de Silva improved the
manuscript. We are grateful to the Fuerza Aerea de
Chile for the use of a helicopter in 1994, to Sergio
1917
JOURNAL OF PETROLOGY
VOLUME 40
Manquez and Sergio Palma for their invaluable assistance
in the field, and to the people of Talabre for their
hospitality.
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APPENDIX: ANALYTICAL
TECHNIQUES
Electron microprobe analyses were perfomed using a
JEOL-733 Superprobe equipped with Link Systems AN10000 EDS analyser at Birkbeck College, London, and
a JEOL JXA-8600 Superprobe equipped with LIF, PET
and TAP crystals, processed by LINK systems Specta
WDS analyser at the University of Bristol. Whole-rock
major and trace element analyses were performed on glass
beads and pressed powder pellets by X-ray fluorescence
(XRF) spectrometry at Holloway College, London, and
the University of Nottingham, and by atomic absorption
spectrometry at the laboratories of the Servicio Nacional
de Geologia y Mineria, Chile.
1919