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Journal of GEOLOGY, Series B, No 28/2006, p.27-34
MESO - CENOZOIC ACIDIC-POTASSIC DYKE ROCKS
IN THE KON TUM BLOCK MARGIN AND THEIR ORIGIN
VŨ VĂN VẤN, TRẦN TRỌNG HÒA, TRẦN TUẤN ANH, TRẦN HỒNG LAM
Institute of Geological Sciences, VAST, 84 Chùa Láng, Đống Đa, Hà Nội
Abstract: K-high calc-alkaline acidic small intrusions and dykes in the Kon Tum
Block margin belong to 2 major stages of magmatism: Late Paleozoic - Early
Mesozoic and Late Mesozoic ones, related to Indosinian and Pacific (Yanshanian)
orogenies. The rocks of the first stage are distributed mainly in the southeast of the
Trường Sơn Belt, whilst the second stage - in the Đà Lạt segment of the East-Asia
volcano-plutonic belt. These formations are associated spatially with potassic and
ultrapotassic lamprophyres. Their chemical composition implies that they are
products of subduction-related magmatism, however between two stages (Indosinian
and Pacific ones), some differences have been recorded.
D
ykes and small intrusions associated
with major igneous activities are
important objects not only in
magmatic evolution, but also in regional
geodynamic interpretations. Their presence
sometimes is good indications of endogenic
metallogeny, especially gold mineralization
associated with such vein rocks [2]. The role
of porphyritic granites in Cu-Mo-(Au) are
mentioned by different investigations.
However, in the field, the investigation on
dyke rocks attracted less attention than major
phase rocks, and were briefly described at
the end of each study.
In the marginal area of the Kon Tum
Uplift, acidic-subalkaline dykes are
widespread, belonging to two major igneous
stages: Late Paleozoic - Early Mesozoic
(mainly Permian-Triassic) and Late
Mesozoic ones [6]. In these both stages,
subalkaline dykes generally occurred at last,
reflecting the termination of Indosinian and
Yanshanian Orogenies [4, 6]. In places,
subalkaline rocks are abundant and they
exist in single dykes, sometimes dykecluster, like Măng Xim in the northern
margin of Kon Tum Block, Ia Mơ at the
south, Phan Rang and Trảng Sim at the
southeast. These dykes are investigated
under mapping projects at 1:200.000 scale
[4]. Our detailed studies in recent years
combined with other author’s results allow
to give better interpretation on the origin and
formation of the subalkaline dykes at Măng
Xim, Ninh Trữ, Đèo Cậu, Vũng Tàu, Trảng
Sim, as well as their geodynamic
implications related to Late Paleozoic - Early
Mesozoic and Late Mesozoic subductions at
the southeastern margin of the Indochina
Block, and some new interpretations are
given.
I. GEOLOGICAL SETTING
1. Măng Xim dyke rocks
These rocks have been investigated along
the Nước Tan stream, Sơn Bao Commune,
Sơn Trà District. They consist of dykes of
some to ~200 m in thickness, with the
composition mainly of porphyritic quartz
syenite and aplitic granosyenite, penetrating
metamophic rocks of the Tắc Pỏ and Khâm
Đức (PR) formations, and causing the
feldspathization of biotite-sillimanite gneiss
and quartz-feldspar schists.
Pinkish to grayish coarse-grained
porphyritic quartz syenite is very common in
the area. The major phenocrystal phase
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includes K-feldspar. Chromatic minerals are
actinolitized pyroxene and chloritized
biotite. Accessory minerals include sphene,
apatite, and orthite. Secondary minerals are
sericite, chlorite, epidote and calcite.
Aplitic granosyenite occurs in smaller
dykes that have lighter colour than quartz
syenite. It is pinkish equigranular finegrained rock, consisting mainly of Kfeldspar, plagioclase, quartz. Chromatic
minerals are biotite and amphibole;
pyroxene has been met only in the core of
amphibole. Accessory minerals include only
apatite, and secondary minerals are kaolinite
and sericite.
Chromatic minerals of Măng Xim dyke
rocks are relatively different, with highMgO (15.5÷19.2 wt.%), high-titanium (4÷5
wt.%) and low-iron (10.5÷13.78 wt.%)
biotite and low-alumina (Al2O3 = 0.2 - 1
wt.%) augite-diopside.
2. Ninh Trữ dykes
These dykes crop out at Khánh Hải
Commune, Phan Rang Province. The rocks
are greenish-grey with pinkish K-feldspar
phenocrysts, sometime plagioclase occurs
with 2÷3 mm up to 1÷1.5 cm in size. The
dykes are 8 m thick, penetrating pinkish
porphyritic granite, dipping vertically with
sharp contacts.
Rock-forming mineral composition: Q =
20÷24 vol.%; Pl = ~25 vol.%; K.Fsp =
40÷45 vol.% with A-La twinning; chromatic
minerals are scarce biotite. Accessory
minerals include zircons. Secondary mineral
are mainly sericite, chlorite, kaolinite and
epidote.
3. Đèo Cậu and Krông Pha dykes
They are not only subalkaline-acidic
dykes, but also basic varieties forming
bimodal dyke-cluster system. According to
mapping works in 2004 and 2005, these
dykes intruded into coarse- to mediumgrained
porphyritic
hornblende-biotite
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granite. The K-Ar and Ar-Ar ages of these
rocks are 91÷126 Ma (Huỳnh Trung et al.,
1979, 1985). The relationship between these
dykes can be determined as follows:
+ Microgranosyenite dykes, which are
the earliest formations, intruding into
hornblende-biotite granite.
+ Microgabbro-diabase dykes, which are
intermediary formations, cross-cutting both
hornblende-biotite
granite
and
microgranosyenite dykes.
+ Diabase dykes, which is the last
formation intersecting all previous dykes.
The chemical correlation of microgabbro
- diabase and basalts of adjacent areas (Buôn
Ma Thuột, Đà Lạt, Xuân Lộc) show
similarities, reflecting that they were derived
from a close source.
Porphyrites at Đèo Cậu show unclear
relationship with surrounded gabbro-diabase
and microgranosyenite. There are two
assumptions about their origin: 1)
porphyrite, formed by material exchange
between
mafic
(microgabbro-diabase)
magma
and
previous
crystallized
microsyenite; and 2) porphyrite, product of
trachyandesite (Đèo Bảo Lộc - Nha Trang
Complex) that was developed around Đèo
Cậu area. Based on the rock composition,
the second assumption is probably more
reasonable.
4. Phan Rang vein rocks
These rocks crop out on the way from
Phan Rang to Cà Ná. They are various in
width, from 1 to some metres, extending on
hundred metres. They are pinkish porphyritic
rocks with K-Fsp phenocrysts. Rockforming minerals include mainly K-Fsp,
plagioclase, quartz. Chromatic minerals are
biotite, sometimes amphibole. Accessory
minerals: zircon and ore minerals. Secondary
minerals: sericite, kaolinite and small
amount of epidote.
5. Tà Năng
granosyenite
porphyritic
granite
-
Beside acidic volcanites and associated
tuff, there still are some small intrusions
(1:25.000 geological map). The most
significant is the Tà Năng massif. It
actually consists of two small bodies of
~3.5 and ~0.4 km2 in width. Their rock
composition consists of porphyritic
granosyenite and subalkaline porphyritic
granite. These bodies intruded and caused
the metamorphism of sandstone, siltstone
and shale of the La Ngà Formation. In the
Tà Năng gold mine, porphyritic granite
veins intersected the La Ngà Formation or
inside ore bodies (Nguyễn Văn Mai,
1993).
The rocks are grey porphyritic
medium-grained
granite.
Under
microscope, the phenocrystal phase
reaches up to 50-60 vol.%, consisting
mainly of K-Fsp (25 - 30 vol.%), quartz
(1-2 vol.%) and biotite (1 vol.%). The
groundmass (40-50 vol.%) consists of
microcrystals
that
have
mineral
composition similar to phenocrysts.
Accessory minerals are apatite and
epidote.
6. Vũng Tầu porphyritic subalkaline
granites
This formation consists of small
intrusions
and
dykes
that
are
homogeneous in composition. The rock is
pinkish-grey, of porphyritic texture. The
phenocrysts reach up to 60-65 vol.%,
consisting of quartz (10÷20 vol.%), K-Fsp
(30÷35 vol.%), plagioclase (20÷25%).
The groundmass consists of microcrystals
that have mineral composition similar to
phenocrysts.
II. GEOCHEMISTRY OF DYKE ROCKS
1. Măng Xim alkaline dykes
These dykes are characterized by SiO2
(60÷70wt.%), relatively high in alkalinity
(total alkali = 9.07÷10.61 wt.%), that
belong to K-high calc-alkaline series
(K2O/Na2O = 1.3÷2.3) (Fig. 1). They are
relatively high in TiO2 (0.32÷0.76 wt.%),
MgO (3.2÷3.75 wt.%) and iron (Fe2O3 =
2.65÷6.55 wt.%). The Al2O3/(Na2O +
K2O + CaO) ratio varies from 0.5 to 1.1,
similar to that of A-granite (Kent C.
Condie, 1988). Their aluminium content
is much higher than that of dykes in Ninh
Trữ, Phan Rang and Đèo Cậu areas (Fig.
2). Along with the increase of SiO2 from
60 wt.% to 70 wt.%, Fe2O3 (~6÷2 wt.%),
CaO (3.08÷0.76 wt.%) and magnesium
are reduced (3.2÷0.6 wt.%) (Table 1).
The REE content of the study rocks
represents high Rb (422÷477 ppm), La
(89÷90 ppm), Ce (180÷183 ppm), Gd
(10.1 ppm), Sm (17.5÷17.7 ppm),
especially U (10.3÷10.9 ppm) and Th (79
ppm); medium Nb (10÷18 ppm) and Ta
(1.33÷1.36 ppm) with correlative ratios of
La/Yb = 39÷40, Ce/Yb = 79,3÷80,9,
La/Lu = 306÷310, La/Nd = 0,9÷1, La/Ce
= 0,5, and La/Sm = 5.
On the REE plot (Fig. 6), the Măng
Xim rocks are rich in LREE in comparing
to HREE, with steep slope from La to Lu
and slight negative anomaly of Eu. The
rocks have relatively high DEu
(0.64÷0.72). It reveals that the medium
crustal contamination might contribute to
the rock formation.
The multi-element plot (Fig. 5) show
steep pattern with negative anomalies of
Nb and Ta, reflecting that they are not of
intraplate origin as considered in previous
studies [4, 7]. Their geochemical
significances (Fig. 3-6) and Ar-Ar age
dating of Măng Xim alkaline dykes (225238 Ma, [4]) imply that they are postcollisional products of a contaminated
mantle domain in the Permo-Triassic
subduction zone.
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Bảng
30
2. Ninh Trữ dyke
These rocks show varied SiO2
(70.68÷76.6 wt.%), high alkalinity (total
alkali = 7.8÷8.5 wt.%), subalkaline type,
minor alkaline type and dominant Kalkaline type (K2O/Na2O = 1÷2). The
Al2O3/(Na2O + K2O + CaO) ratio varies
from 1.4 to 1.5, belonging to
peraluminous series (Fig. 2). On the
correlation diagram, the Ninh Trữ dyke
rocks are of K-high calc-alkaline series.
Along with the increase of SiO2, the MgO
and CaO content decreases (0.6÷0.17
wt.% and 1.07÷0.24 wt.%, respectively).
The multi-element diagram shows
negative anomalies of Nb, Ta and Ba
(Fig. 5). In the REE plot, LREE
significantly exceed HREE content,
representing by Ce/Yb (68÷188) and
La/Lu (35÷98), with sharply negative
anomaly of europium (Fig. 6).
On the correlation diagrams the Ninh Trữ
rocks fall in post-orogenic granite field
(POG, Fig. 5, 6).
3. Đèo Cậu rocks
They include microgranosyenite and
porphyrite (Fig. 1), with oscillating SiO2
(63.82÷78.78 wt.%, in average 72.86 wt.%),
of medium- to high- alumina series (Fig. 2),
and belong to K-high calc-alkaline series
(K2O/Na2O ratio around 2.6).
4. Phan Rang dyke
These rocks are dacite to rhyolite in
composition, with SiO2 = 63.82÷78.78
wt.%, in average 72.86 wt.% [4] (Fig. 1),
belonging to K-high calc-alkaline series
(K2O/Na2O ~ 2.6), of medium- to highalumina series (Fig. 2); their Shand index
is predominantly >1.
Similar to Ninh Trữ and Măng Xim
rocks, the negative anomaly of Nb, Ta
and Ba in the multi-element plot is
recorded (Fig. 5) along with steep slope of
LREE, flat in HREE in the REE
distribution pattern (Fig. 6) and rather
deep Eu trough. These features are typical
of subduction related to magma having
some degree of contamination.
The study on the Rb-Sr, Nd-Sm and Pb
isotopic ratios of the Phan Rang dykes [4]
shows that on the correlation diagram of
87
Sr/86Sri - 143Nd/144Ndi , they may be
derived from a mantle source similar to
Bulk Silicate Earth (BSE), implying that
they are derived from a less enriched
source in the depth. The correlation of
(206Pb/204Pb) and (207Pb/204Pb) ratios
shows that they are similar to EMII that
may be derived from a lower crustal
environment. These characteristics imply
that the Phan Rang dyke rocks are
probably derived from an enriched upper
mantle source with some degree of crustal
contamination. According to Hall (1966),
dyke rocks normally are latest products of
coeval volcanic or extrusive activities.
The Phan Rang dyke rocks are probably
latest products of the Định Quán - Đèo Cả
igneous Complex [10].
5. Tà Năng granosyenite and subalkaline
porphyritic granite
The Tà Năng rocks are of alkaline acidic
composition, with SiO2 = 71.37÷72.24 wt.%,
(total alkali = 9.5÷10 wt.%), potassium-rich
(K2O/Na2O>3), belonging to K-high calcalkaline series, with medium to high alumina
(Fig.2), and Shand index varying from 1.2 to
1.4. The rocks are rich in Sn, Cu, Pb, and
Ag. In porphyritic granite, average content
of these elements is: Sn =13.7 ppm; Ag = 4.6
ppm; Cu = 3.5 ppm; Pb = 3.6 ppm, whilst
that of granosyenite is Sn = 18.2 ppm; Cu =
3.3 ppm; Mo = 4.4 ppm; Pb = 3.6 ppm.
Their trace element content (Fig. 5)
presents a slope from Cs to Lu with negative
anomaly of Nb, Ta, and Ba, along and high
LREE, medium HREE with significant Eu
negative anomaly (Fig. 6).
31
Figure 1. Position of vein rocks on the SiO2(Na2O+K2O) diagram (Cox et al, 1979)
Figure 2. Position of vein rocks on the ANKACN diagram (Maniar and Piccoli, 1989)
Figure 3. Position of vein rocks on the SiO2Al2O3 diagram (Maniar and Piccoli, 1989)
Figure 4. Position of vein rocks on the R1-R2
diagram (Batchelor and Bowden, 1985)
Figure 5. Rare and trace element diagram
of vein rocks
Figure 6. Rare earth element diagram
of vein rocks
Legend: see Fig.1.
32
On the tectonic discrimination diagrams,
the Tà Năng rocks fall into syn-collision and
post-collision fields (Fig. 3, 4).
The studied geochemical characteristics
imply that the Tà Năng rocks do not belong
to intraplate magmatism, but last
differentiated products of Late Mesozoic Khigh calc-alkaline magmatism.
6. Trảng Sim dykes
The Trảng Sim rocks are highest in SiO2
(up to 77%), of medium alkalinity (~ 8.4%),
potassium rich (K2O/Na2O>2), alumina high
(Al2O3/Na2O+K2O+CaO=1.4), whilst the
Vũng Tàu dyke has lower SiO2, but higher in
total alkali and medium in alumina (Fig. 2).
Characteristics of trace elements of Vũng
Tàu and Trảng Sim are relatively identical
(Fig. 6) with negative anomalies of Nb, Ta,
Ba and high content of LREE in comparison
to HREE, and sharp Eu negative anomaly
(Fig. 5, 6). On the correlation plot, they
belong to post-collision magmatism (Fig. 3).
The study on Late Mesozoic magmatism
and associated dykes of South Central Việt
Nam imply that the continental margin was
uplifted with compressional regime like in
Andes, and in late Cretaceous the small
intrusions and dykes are more common.
However, it should be emphasized that
although the size of these dyke rocks are
small but they are developed widely, crosscutting older continental crust. Their trace
element and REE significances imply that
they are crystallized from a similar
magmatic source that are poor in Tb, Ta, Eu
and rich in U, Th.
II. CONCLUSIONS
On the basis of the above mentioned study
results, the following conclusions are made:
1. Small intrusions and K-high calcalkaline acidic dykes of the Kon Tum Block
margin belong to 2 major stages: Late
Paleozoic - Early Mesozoic and Late
Mesozoic ones, corresponding to the
Indosinian and Pacific (Yanshanian)
orogenies. The rocks of the first stage are
mainly distributed in the southeast of the
Trường Sơn Belt, whilst the second stage –
in the Đà Lạt segments of the East-Asia
volcano-plutonic belt. These formations are
associated spatially with potassic and
ultrapotassic lamprophyres.
2. The chemical composition of small
intrusions and K-high calc-alkaline acidic
dykes implies that they are products of
subduction-related magmatism, however
between two above stages some differences
have been recorded.
3. Generally, small intrusions and K-high
calc-alkaline acidic dykes are derived from a
mantle - originated source, that previously
was contaminated by crustal materials. Their
presence is indications of post-collisional
extension on arcs of Indosinian and Pacific
Orogenies on the southeastern margin of the
Indochina Block.
The paper is supported by the National
Research Project DTDL-2003/07 and the
project 708.406 of the Basic Research
Program.
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Figure 1. Position of vein rocks on the SiO2 - (Na2O+K2O) diagram
(Cox et al, 1979)
Figure 2. Position of vein rocks on the ANK - CAN diagram
(Maniar and Piccoli, 1989)
Legend:
Măng Xim vein rocks
Phan Rang vein rocks
Đèo Cậu vein rocks
Vũng Tàu vein rocks
Tà Năng vein rocks
Ninh Trữ vein rocks
Trảng Sim vein rocks
Figure 3. Position of vein rocks on the SiO2 - Al2O3 diagram
(Maniar and Piccoli, 1989)
Figure 4. Position of vein rocks on the R1 – R2 diagram
Batchelor and Bowden, 1985
Figure 5. Rare and trace element diagram of vein rocks
Figure 6. Rare earth element diagram of vein rocks
Legend: see Fig. 1
35