Download reconnaissance field study of the sarmiento ophiolite with emphasis

UNIVER SIDAD DE CONCEPCIÓN
DEPARTAMENTO DE CIENCIAS DE LA TIERRA
10° CONGRESO GEOLÓGICO CHILENO 2003
RECONNAISSANCE FIELD STUDY OF THE SARMIENTO
OPHIOLITE WITH EMPHASIS IN THE PETROLOGICAL
MEANING OF LEUCOCRATIC DIKES AT PENÍNSULA TARABA
CALDERÓN, M.1, HERVÉ, F.1, FILDANI, A.2, CORDANI, U.3, HERRERA, C.1,
RAPALINI, A.4 & PIQUER, J.1
1
Departamento de Geología, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile
([email protected]; [email protected]; [email protected])
2
Deparment
of
GES,
Build.
320,
Stanford
University,
Stanford,
CA
94305-2115
([email protected])
3
Centro de Pesquisas Geocronológicas, Universidade de São Paulo, Brasil ([email protected])
4
Instituo de Geofísica Daniel Valencio (INGEODAV), Dpto. Cs. Geológicas, FCEN, UBA, Pabellón II, Ciudad
Universitaria, 1428 Buenos Aires, Argentina ([email protected])
INTRODUCTION
Since 1970s to mid-1980s several geoscientists have widely studied the Late-Jurassic to Early
Cretaceous “Rocas Verdes” basin of southern South America, interpreted as the mafic oceanfloor remnants of an ensialic marginal back-arc basin, developed along the evolving
convergent plate boundary of southwestern Gondwanaland (Dalziel et al., 1974; Stern et al.,
1976; Elthon & Stern, 1978; de Wit & Stern, 1978; Saunders et al., 1979; Stern, 1979; de Wit
& Stern, 1981; Allen, 1982; Stern et al., 1992; Mukasa & Dalziel, 1996). Considered as a
well-documented analogue example of an Archean greenstone belt (e.g. Tarney et al., 1976;
Stern & de Wit, 1997), the Rocas Verdes basin consistS of a group of en-echelon and
elongated ophiolitic units with discontinuous exposures along 1000 km (Fig. 1a; e.g. Stern,
1979).
The Rocas Verdes basin represents the northern edge of the Late Jurassic proto-Weddell sea
(e.g. Grunow, 1993), developed within a continental crust and separating two continental
blocks over a diffuse zone dominated by the interaction between mafic magmas and
continental rocks during the formation of the marginal back-arc basin (e.g. Stern & de Wit,
1997). In this model, an active calc-alkaline volcanic arc, founded on the rifted sliver of
continental crust, was separated from the adjacent continent (e.g. Stern, 1979; Saunders et al.,
1979). The earliest intrusions of the South Patagonian batholith represent the roots of such
subduction-related volcanic arc (e.g. Stern & Stroup, 1982). Subaquatic ash-deposits
intercalated within deep-sea turbidites that overlie conformably the pillow basalts of the mafic
complexes, and volcanic detritus from greywackes deposited in the basin, are considered
evidence of volcanic activity during basin formation (Katz & Watters, 1965; Dott et al.,
Todas las contribuciones fueron proporcionados directamente por los autores y su contenido es de su exclusiva
responsabilidad.
Figure 1. (a) Location of studied area (Sarmiento ophiolite). (b) Geological units at the
Sarmiento ophiolite (after Allen, 1982). Location of samples of interest.
1977). The marginal-basin closure, uplift and deformation began at mid-Cretaceous times, as
a result of the flattening of subduction angle due either to ridge subduction or a global
increase in spreading and plate tectonic convergence rates (e.g. Dott, 1977; Stern, 1991).
The main ophiolitic units of the Rocas Verdes basin, called Sarmiento and Tortuga
complexes, located north and south respectively (Fig. 1a), show contrasting petrological and
chemical features suggesting that the Sarmiento complex represent a less developed stage of
evolution than the Tortuga complex (Stern, 1979). The former contains intermediate
icelandites and silicic dikes and lavas which are conspicuously absent in the Tortuga complex
(Stern, 1979). In this contribution some field considerations about the mafic and felsic
igneous-rocks are given, as well as supplementary whole-rock geochemical data, with
relevance in the petrological significance of the leucocratic rocks of the Sarmiento complex.
GEOLOGICAL BACKGROUND OF THE SARMIENTO OPHIOLITE
The Sarmiento ophiolite (Fig. 1b) is flanked on both sides by gabbro bodies and dike swarms
that intrude both the pre-Jurassic continetal basement (Staines Metamorphic Complex), and a
Middle-Late Jurassic silicic volcanic unit (Tobífera formation), the last formed during the
early stage of continental rifting (Bruhn et al., 1978). Acid volcanism of Tobífera Formation
was erupted in a volcano-rift basin, coeval with the deposition of fossiliferous shales of the
Zapata formation (Allen, 1982).
The pseudostratigraphy of the Sarmiento complex (e.g. Stern, 1979) consists of a deeper zone
of coarse-grained gabbros and massive diabase, and an upper level of plagiogranite, all
comprising a minimum of 1 km of thickness. Over the plagiogranite level (in sub-horizontal
igneous contact) is the sheeted dike unit (300 m thick), which is intruded by fine grained grey
to white dikes of similar composition to the silicic plutonic rocks, which are extensively cross
cut by mafic dikes. Over the sheeted dike unit occurs a 2 km thick succesion conformed by
pillow lavas, pillowed dikes, primary explosive tuffs, pillow breccias and the extrusive unit of
water-lain pillow lavas. Most of these lavas and dikes are ferro-basalts and basalts (Stern,
1979). Ultramafic rocks are not exposed, and geochemical considerations suggest that a large
body of magnesian cumulate gabbros is also unexposed (Stern, 1979). In the Sarmiento
ophiolite a large relative volume of extrusives to extensional dikes exist compared to the
Tortuga complex, suggesting that in the former, ie. in the narrow northen extreme of the
Rocas Verdes basin (~25 km; de Wit, 1977), extension was slow relative to magma supply
(Stern, 1979).
Zircon fractions from fine-grained plagiogranites, interpreted to be cogenetic with the mafic
rocks, yield lower concordia intercept U-Pb ages of 140 ± 0.7 Ma (Fiordo Lolos) and 137 ±
0.6 Ma (Fiordo Encuentros), considered as formation ages for the northen portion of igneous
floor of the Rocas Verdes basin (Stern et al., 1992). Lower concordia intercept ages of 147 ±
10 Ma, from coarse-grained trondhjemites within the gabbro unit (Fiordo Lolos), reflect
inherited zircon components (probably of Proterozoic age), and therefore interpreted as
remobilized fragments of contry rocks entrapped within the essentially mantle-derived rocks
of the ophiollite complex (Stern et al., 1992; see below).
According to Elthon & Stern (1978), the hidrothermal and non-deformative ocean-floor
metamorphism develops secondary mineral assemblages in a steep metamorphic gradient
consisting of: Zeolite facies (zeolites and palagonitized glass ± smectites ± calcite ± quartz ±
sulfides ± titanite ± albite); Greenschist facies (chlorite, epidote, sodic plagioclase, titanite ±
quartz ±calcite ± biotite ± sulfides); Lower Actinolite facies (green tremolite-actinolite, clacic
plagioclase, titanite ± biotite± calcite; Upper Actinolite facies (brown-blocky and green
tremolite-actinolite, calcic-plagioclase, titanomagnetite ± ilmenite ± biotite. The
metamorphism in the deeper level of the complex, as well as being less extensive, was more
nearly isochemical than at higher pseudostratigraphic levels, involving large scale migration
of only K2O and Rb (Stern, 1979).
FIELD AND PETROGRAPHIC CONSIDERATIONS
PENÍNSULA TARABA
A detailed field-work at the NW-SE striking Seno Profundo (Fig. 1b), carried out over the
pillow lavas unit with subordinate intercalations of radiolarian cherts and shales (according to
Allen, 1982), reveal interesting information about non-described porphyritic quartzfeldespatic rocks with coarse-gneissic texture (Fig. 2a,b). Most of the observed lithologies
show a variable and partitioned penetrative cleavage (S1), which strikes N10-20W and dips
80-90W. The felsic rocks are composed mainly of plagioclase and minor quartz phenocrysts
within a microgranophyric and dinamically recrystallized groundmass, with thin aggregates of
secondary chlorite in the sense of S1 (sample ST02-03). Accessory minerals are titanite,
epidote and opaques minerals. Although the original texture of these rocks is partly oblitered
due to the dynamic recristallization (deformation lamellaes and undolse extintion in quartz,
strain shadows and domino microstructures; Fig. 2c,d), the intrusive contact with pillow
basalts and/or fine-grained holocrystalline basic or intermediate rocks (NS/steep dips to the
West) allow to consider them as shallow depth intrusives.
At the northen edge of the Península Taraba (mapped as the Tobífera formation by Allen,
1982), several porphyritic quartz-feldespatic dikes, with miarolitic cavities, intrude massive
and pillow basalts with NS strike and steep dips to the west. These leucocratic rocks have
embayed quartz phenocrysts and a microgranophyric and spherulitic texture in the
groundmass. The texture of these rocks indicate hypabisal or sub-volcanic depths during
crystallization. The pillow lavas show a quenching plumose texture with relict brownishclinopyroxenes in the groundmass and abundant chloritized pseudomorphs of olivine
phenocrysts (sample OW99-62).
The field similarities between both zones of the Península Taraba and the sub-parallel
intrusions of acid rocks within the subaquatic sequences give a layered “jail-dress”
appearance to the western segment of the peninsula (Fig.1b). To the east of the Península
Taraba, fossiliferous shales of the Zapata formation rest conformably over altered olivine-rich
basalts. Both units are in contact along an east verging reverse fault, concordant to the
western limb of the anticlinal fold of the sedimentary rocks (Fig.1b; e.g. Allen, 1982).
FIORDO ENCUENTROS
Across the Fiordo Encuentros the sheeted dikes and the gabbro units crop out from west to the
east (Fig.1b). At the northen shore of the Fiordo Encuentros a large N-S elongated
plagiogranite body crops out (Allen, 1982). Allotromorphic fine-grained plagiogranites,
comprising plagioclase, quartz, epidote and accesory minerals, occurs within the amphibolebearing gabbros as narrow bands and dikes of meters in width. These rocks show
micrographic textures and partially saussuritized plagioclase.
Within the sheeted dike unit and without a clear temporal contact-relation tabular and
irregular granophyric rocks occur (also in the Fiordo Lolos). The felsic rocks are of the same
lithology of the previouosly described granophyre phase of the trondhjemites in this area,
which are considered as country rocks xenoliths after the intrusion of the mafic dikes of the
ophiolitic complex (e.g. Saunders et al., 1979).
Figure 2. (a) Mafic and felsic rocks at Seno Profundo (Península Taraba). The sample ST0203 came from the leucocrstic rock. (b) Intrusive contact of leucocratic rocks at Seno
Profundo. (c) and (d) Microphotographs of leucocratic rocks showing strain shadowds,
anastomossed foliation and domino microstructures.
GEOCHEMISTRY
Lithologies of the Sarmiento ophiolite complex are geochemically well documented in wholerock major and trace elements (Stern, 1979; Saunders et al., 1979). This allows to compare
with the chemical data of selected rocks from the ophiolite that crops out around the
Península Taraba. Moreover rock-samples from the Tobífera formation and others from the
sedimentary rocks of the Zapata formation were analysed.
Previous petrogentic considerations about leucocratic rocks are presented below. The
plagiogranite pluton of Fiordo Encuentros is cut by mafic dikes, indicating that the
plagiogranites are contemporaneous with the mafic activity which produced the Sarmiento
complex (Stern et al., 1992). Trondhjemites and granophyres of the same area are interpreted
as remnants of remobilized country rocks, probably from the Tobífera formation, engulfed in
the mafic rocks of the ophiolite (Saunders et al., 1979; Stern et al., 1992). The granophyre
phase of tronhjemite may have formed by concentration of partial melts arrived from the
precursor trondhjemite material (Stern et al., 1992).
Table 1. Major and trace element composition of analysed rocks
Sample
FIL ST0201 FIL FO0096 FIL OW9962 FIL ST0212 FIL ST0203 FILCAN9950 FIL FO00100 FIL FO00107 FILCAN9949 FIL OW9958 FIL FO0018 FIL 3/10 5
Basalt
Basalt
Basalt
Hypabisal Granophyre Tob;ifera? Foliated Tuff
Pillowed
Sandstone Slate
lutite
Lutite
andesite
rhyolite
Major elements wy%
SiO2
49.32
TiO2
1.55
Al2O3
19.05
FeO*
10.83
MnO
0.15
MgO
8.80
CaO
5.90
Na2O
3.83
K2O
0.25
P2O5
0.34
LOI
5.16
50.29
0.91
15.84
7.15
0.29
6.22
14.04
4.33
0.14
0.14
7.30
49.04
1.53
15.91
11.32
0.34
10.47
4.17
4.22
0.81
0.21
4.32
59.61
2.03
14.59
10.13
0.24
4.69
3.63
3.58
1.23
0.34
3.18
75.08
0.28
12.00
2.51
0.03
2.41
0.43
4.08
2.26
0.05
1.70
78.77
0.06
12.63
0.28
0.00
0.07
0.52
6.85
0.22
0.01
0.28
80.63
0.05
10.69
0.76
0.01
0.53
0.33
2.77
4.05
0.01
0.83
74.20
0.94
9.37
6.11
0.12
1.05
2.59
4.59
0.09
0.27
1.27
70.72
0.61
15.05
3.75
0.04
1.63
2.17
4.05
1.91
0.13
1.65
67.28
0.66
16.28
5.00
0.07
2.37
1.60
1.91
4.20
0.18
3.07
65.43
0.68
16.91
4.65
0.06
2.10
2.32
3.26
3.29
0.23
2.31
77.21
0.25
13.08
4.02
0.25
1.75
0.38
0.51
2.43
0.04
3.04
Suma
99.35
98.02
100.06
99.13
99.41
99.84
99.33
100.05
99.55
98.93
99.92
55
0
3
189
77
19
234
226
42
46
30
62
164
3
4
103
104
32
269
284
103
49
5
257
170
4
8
213
162
37
19
327
6
46
13
117
841
14
8
73
164
20
6
25
7
12
0
41
25
18
11
79
88
37
3
5
9
9
2
1
700
14
9
34
84
24
3
6
4
8
0
18
46
1
5
36
101
28
1
58
4
20
11
33
489
13
10
318
246
23
31
62
12
10
2
22
789
14
12
48
145
30
42
99
16
12
23
90
784
13
12
415
171
30
60
103
23
22
12
65
562
14
12
85
127
38
1
30
13
3
8
123
0
24
16
2
1
1168.2
5473.1
614.4
0.2
3
5
20
22
17
6850.4
9363.6
934.1
0.3
17
31
15
17
3
10190.4
12138.3
1494.6
0.5
44
63
9
60
15
18899.4
1663.1
233.1
0.5
18
52
14
10
0
1834.6
355.8
43.9
0.7
13
40
9
107
5
33629.7
312.3
43.7
0.6
8
26
9
2
4
751.1
5673.4
1202.7
0.3
49
79
16
79
2
15826.8
3637.3
553.5
0.7
57
66
21
171
15
34974.3
3962.4
775.2
0.8
43
74
21
134
4
27569.9
4102.6
1000.5
0.8
26
54
13
94
22
20161.2
1524.0
183.3
0.8
100.02
Trace elemnts (ppm)
Ba
179
Th
4
Nb
11
Sr
109
Zr
212
Y
52
Cr
173
V
219
Ni
57
Sc
48
Cu
41
Zn
106
Hf
La
25
Ce
50
Ga
20
Rb
2
Pb
1
K
2072.1
Ti
9290.6
P
1490.9
Ta*
0.7
FeO* as total iron
Ta * = Nb/10
K, Ti and P calculated in anhidrous base.
FIL ST0201
Spider E-MORB
100.00
FIL ST0212
FIL ST0201
Spider N-MORB
1000.00
FIL ST0212
FIL OW9962
FIL FO0096
FIL OW9962
100.00
Sample/N-MORB
Muestra/E-MORB
10.00
1.00
FIL FO0096
10.00
1.00
0.10
0.10
0.01
Sr
K
Rb
Ba
Th
Ta
Nb
La
Ce
P
Zr
Hf
Sm
Eu
Ti
SpiderE-MORB
LAVAS
1000.00
Sr
K
Rb
Ba
Th
Ta
Nb
La
Ce
P
Zr
Hf
Sm
Eu
Ti
Y
Sc
Cr
Spider N-MORB
LAVAS
1000.00
100.00
100.00
Sample/N-MORB
Sample/E-MORB
0.01
Y
10.00
1.00
0.10
10.00
1.00
0.10
0.01
Sr
K
Rb
Ba
Th
Ta
Nb
La
Ce
P
Zr
Hf
Sm
Eu
Ti
Y
0.01
Sr
K
Rb
Ba
Th
Ta
Nb
La
Ce
P
Zr
Hf
Sm
Eu
Ti
Y
Sc
Cr
Figure 3. Multielement variation diagram normalized to E-MORB and N-MORB according to
Sun & McDonough (1989). Composition of lavas from Saunders et al. (1979).
High field strength elements (HFSE; Nb through Cr in Fig.3) are believed not to be mobilized
by ocean-floor metamorphic processes (e.g. Stern & Elthon, 1979). Most analyses of basaltic
lavas have SiO2 content around 50 wt%. The olivine-rich pillow basalt (or metabasalt) from
the northen edge of the Península Taraba (sample OW99-62), shows an alkaline affinity (with
major elements), differing to most of the other basaltic lavas and dikes which have tholeiitic
affinity. The enriched HFSE concentration of basaltic lavas shows a flat pattern when
normalized to E-MORB (according to Sun & McDonough, 1989), generally with normalized
values greater than 1 (Figs.3a,b,c,d). Although most basalts show slightly negative anomaly in
Nb and Ta (with NbN, TaN > 1) and high contents of incompatible elements as K, Rb, Ba and
Th, relative to a N-MORB source composition, it is not possible to constrain the influence of
a subduction zone component in the petrogenesis of the basaltic rocks. Contrastingly, the
sample OW99-62 and others from Saunders et al. (1979) do not show a negative anomaly in
Ta and Nb.
ST02-03
Zapata an
fo
FIL ST0203
M63
1000.00
100.00
10.00
1.00
0.10
BaRbThKNbLaCeSrP ZrTiY
PA24D
PA37D
PA37B
ST02-03 a
D3
1000.00
100.00
10.00
1.00
0.10
BaRbThKNbLaCeSrP ZrTiY
PA28A
PA28W
PA23J
1000.00
100.00
10.00
1.00
0.10
BaRbThKNbLaCeSrP ZrTiY
ST02-03
a
rocks
fromT
FIL ST0203
A25
Rocks/Primitive Mantle
Rocks/Primitive Mantle
PA25E
1000.00
100.00
10.00
1.00
0.10
BaRbThK NbLaCeSrP ZrTiY
Rocks/Primitive Mantle
Rocks/Primitive Mantle
PA24C
FIL FO00100
FILCAN995
FIL
ST02-03
and grana
FIL FIL
ST0203
FILCAN994
3/10
FIL5FIL
OW
The granophyric rock from the Seno Profundo (sample ST02-03) shows strong similarities to
the tronhdjemites and granophyres from the Fiordos Lolos and Encuentros (multielement
diagram normalized to the primitive mantle of Taylor & McLennan 1985; Figs.4a,b,c,d).
Noticeable is the chemical similarity between these rocks and the lutites from the Zapata
formation. In the same figure, note that plagiogranites have strongly depleted concentrations
of Ba, Rb and K. The samples from the Tobífera formation show a wide but similar
compositional pattern than the granophyre.
Figure 4. Multielement variation diagram normalized to the Primitive Mantle of Taylor &
McLenan (1985). Plagiogranites and trondhjemites analyses from Saunders et al. (1979).
CLINOPYROXENE CHEMISTRY
Relict skeletal clinopyroxene grains, found in the groundmass of basalts from the northen
edge of the Península Taraba (sample OW99-62) are diopside and augite in composition
(according to Morimoto et al., 1988). Ti, Cr, Na and Ca contents in tectonic discrimination
diagrams indicate that the liquid from which this basalt crystallized has alkaline affinities, and
others with subalkaline chemistry plot within the field of anorogenic basalts (Figs.5a,b;
according to Leterrier et al., 1982). This supports the alkaline affinity of the analysed basalt as
whole rock major element chemistry suggested.
Table 2. Clinopyroxene composition (sample OW99-62)
OW62PX1
OW62PX2
OW62PX4
OW62PX5
OW62PX6
OW62PX7
OW62PX8
OW62PX9
Major oxides wt%
SiO2
51.10
TiO2
0.82
Al2O3
3.29
Cr2O3
0.38
FeO
8.91
MnO
0.23
MgO
16.27
CaO
19.66
Na2O
0.24
K2O
0.00
NiO
0.01
BaO
0.03
H2O
0.00
Sum
100.94
50.69
0.95
3.35
0.21
9.25
0.26
15.91
19.32
0.26
0.01
0.05
0.04
0.00
100.30
46.31
2.55
6.23
0.13
11.76
0.26
11.06
21.47
0.33
0.00
0.05
0.12
0.00
100.28
46.13
2.70
5.20
0.21
13.57
0.36
11.13
20.31
0.37
0.01
0.01
0.11
0.00
100.10
45.98
2.76
5.69
0.21
12.02
0.26
11.46
20.69
0.33
0.01
0.04
0.09
0.45
100.00
45.58
2.87
6.89
0.19
11.72
0.24
11.49
20.65
0.36
0.02
0.00
0.10
0.00
100.09
46.80
2.31
6.19
0.16
10.70
0.26
12.08
21.39
0.30
0.01
0.04
0.07
0.00
100.31
46.52
2.22
5.77
0.13
11.86
0.31
11.49
21.16
0.37
0.02
0.00
0.07
0.07
100.00
45.96
2.90
5.78
0.19
12.13
0.27
11.39
20.82
0.33
0.01
0.01
0.08
0.13
100.00
46.95
2.07
5.19
0.21
12.17
0.26
11.80
20.41
0.35
0.02
0.03
0.09
0.46
100.00
46.32
2.53
5.73
0.11
12.59
0.26
11.56
20.41
0.40
0.00
0.00
0.10
0.00
100.00
1.87
0.03
0.15
0.01
0.29
0.01
0.87
0.76
0.02
0.00
4
1.75
0.07
0.28
0.00
0.37
0.01
0.62
0.87
0.02
0.00
4
1.75
0.08
0.23
0.01
0.43
0.01
0.63
0.83
0.03
0.00
4
1.70
0.08
0.25
0.01
0.37
0.01
0.63
0.82
0.02
0.00
4
1.72
0.08
0.31
0.01
0.37
0.01
0.65
0.84
0.03
0.00
4
1.76
0.07
0.27
0.00
0.34
0.01
0.68
0.86
0.02
0.00
4
1.75
0.06
0.26
0.00
0.37
0.01
0.64
0.85
0.03
0.00
4
1.73
0.08
0.26
0.01
0.38
0.01
0.64
0.84
0.02
0.00
4
1.73
0.06
0.23
0.01
0.38
0.01
0.65
0.81
0.02
0.00
4
1.75
0.07
0.26
0.00
0.40
0.01
0.65
0.83
0.03
0.00
4
Si
Ti
Al
Cr
Fe
Mn
Mg
Ca
Na
K
Sum
1.87
0.02
0.14
0.01
0.27
0.01
0.89
0.77
0.02
0.00
4
OW62PX10 OW62PX11 OW62PX12
Cationic content calculated for 6 oxigens pfu and 4 cations
0.12
Ti
(a)
Ti + Cr
(b)
0.1
0.15
0.08
Alkaline basal
0.1
0.06
Anorogenic basalt
0.04
0.05
Subalkaline
basalts
0.02
Orogenic basalts
Ca + Na
0
0.5
0.7
0.9
1.1
Ca
0
0.5
0.6
0.7
0.8
0.9
1
Figure 5. (a) and (b) Tectonic discrimination diagrams for clinopyroxenes from the ground
mass of mafic rocks according to Leterrier et al. (1982).
DISCUSSION
Whole rock and mineral chemistry indicate both, an alkaline and enriched sub-oceanic mantle
source without the influence of subduction components for the basalts at Península Taraba,
and the E-MORB affinity of most of basaltic lavas and dikes. It is important to consider that
the enriched mid-ocean ridge basalts (E-MORB) occur throughout the ocean basins far from
plume influence (Langmuir et al., 2003). The slight negative anomalies in Ta and Nb of some
basaltic rocks, suggest a transitional changes in the chemistry of the source of basaltic rocks.
Allen (1982) considered that the northen edge of the Península Taraba is composed by
rhyolite flows, volcanic breccias ans lappilli tuffs of the Tobífera formation, and mafic pillow
lavas intercalated within the sequence, considering that both volcanic members were
deposited in a submarine enviroment. However, few meters to the north of the Península
Taraba, trondhjemites from Isla Young (Fig.1b), are interpreted as xenoliths of older silicic
rocks, cut by numerous mafic dikes and intruded by gabbros (De Wit & Stern, 1981).
Contrastingly, field and petrographic observations from the northen edge of the Península
Taraba indicate that felsic dikes (trondhjemites?) intrude a sequence of subaquatic and
primitive basaltic lavas. Considering the dynamic recrystallization of the felsic dikes at Seno
Profundo, the intrusion of these rocks occured after the active the ocean-floor generation and
before the closure of the marginal basin. Although, between the felsic dikes and the
trodhjemites and granophyres from the Fiordo Lolos and Encuentros contrasting contact
relations exist at the outcrop scale, the mineralogical and chemical similarities suggest a
common origin among them, and probably a common source rock from which they were
generated. Plagiogranites fall out of this discussion because they are interpreted as
fractionated liquids derived from the differentiation of basic magmas within the spreading
center (e.g. Stern, 1979).
The similar HFSE chemistry among the felsic dike, trondhjemites and shales from the Zapata
formation, make hidrous anatexis of the shales as an alternative origin for the felsic rocks.
The partial melting of metasediments could be triggered by the yuxtaposition of hot and dry
mafic rocks during the asymmetrical spreading ridge collapse, probably at the initial stages of
the marginal basin closure.
This and other hypothesis about the petrogenesis of leucocratic and the basaltic magmatism
will be tested soon with isotope geochemistry and geochronology. The misinterpretation of
the petrogenesis of leucocratic rocks in the Samiento complex could lead to erroneous
geochronologic interpretations. Therefore, previous age determinations must be considered as
minimum ages for the ophiolitic rocks generation.
AKNOWLEDGEMENTS
This study forms part of the PhD Thesis of the first author, financed by the “Beca de Apoyo
Para la Realización de Tesis Doctoral (Conicyt)” and is also supported financially by the
Fondecyt 1010412 and 7010412 grants to F.H. Whole rock chemical analyses forms part of
the PhD thesis of A. Fildani. Conrado Alvarez and his crew transported us in the yatch Foam
to the studied area.
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