geochemistry and provenance of pre

TOMO 1 - Análisis de Cuencas
GEOCHEMISTRY AND PROVENANCE OF PRE-LATE
CRETACEOUS UNITS IN ARGENTINE TIERRA DEL FUEGO:
PRELIMINARY RESULTS
Cerredo, M. E.1, Remesal, M. B.1 and Tassone, A.2
1
Dpto. de Ciencias Geológicas. FCEyN. UBA-CONICET. Ciudad Universitaria. Pabellón 2.
Entrepiso. 1428-Ciudad autónoma de Buenos Aires. E-mail [email protected]
2
INGEODAV. Dpto. de Ciencias Geológicas. FCEyN. UBA-CONICET. Ciudad
Universitaria. Pabellón 2. Entrepiso. 1428-Ciudad autónoma de Buenos Aires.
INTRODUCTION
The pre-Late Cretaceous units of Argentine Tierra del Fuego are represented by the restricted
outcrops of the Basement Complex (BC), the Upper Jurassic siliceous volcanic/volcaniclastic Lemaire
Formation (LF) and two Lower Cretaceous sedimentary units: Yahgan (YF) and Beauvoir Formations.
These last three units form NW-SE belts which cut across the Isla Grande de Tierra del Fuego (Fig.
1).
Figure 1: Geological sketch of Argentine Tierra del Fuego
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XI CONGRESO GEOLOGICO CHILENO
The BC (made up of very low- to low-grade metamorphic rocks in Argentina) was interpreted
as the Late Paleozoic to Early Mesozoic accretionary wedge formed along the western Panthalassic
margin of Gondwanaland later metamorphosed and uplifted prior to the Late Jurassic (Dalziel 1982).
LF comprises the acid volcanic and interlayered volcaniclastic rocks evolved in the extensional tectonic
regime which led to the formation of the Rocas Verdes back-arc Basin (RVB). The Lower Cretaceous
units represent the infilling of the RVB with detritus source areas located either in the Southern
magmatic arc -YF- or in the northern margin of the basin –Beauvoir Fm.- (Olivero and Martinioni,
2001).
GEOCHEMISTRY
Studied samples of the BC come from the limited ouctcrops of Bahía Ensenada and Lapataia at
the SW (Fig. 1), LF samples belong to the fine-grained sedimentary facies of B.Ensenada and
outcrops of the central NW-SE belt, YF rocks correspond to the lowermost facies of fine-grained
turbidites (Olivero and Martinioni, 1996) and the Beauvoir Fm. rocks (LKM) were sampled in
central Tierra del Fuego to the SE of Lago Fagnano (Figure 1). Based on major and trace element
data preliminary results are presented.
Th/U vs. U diagram (Mc Lennan et al., 1993) provides a tool for estimation the weathering of
sedimentary rocks, given that there is a tendency for an elevation of Th/U ratios above upper crustal
values of 3.5-4 as a result of alteration. Only the group of LKM (Fig. 2) displays a slight weathering
trend, the remainder show no evidence of weathering with Th/U ratios below the canonical upper
crustal value of 3.8 (McLennan et al., 1990).
COMPOSITION
The Zr/Ti vs Nb/Y diagram (Winchester and Floyd, 1977) provides a broad estimation of the
composition of (meta)sedimentary rocks. The LKM from central Tierra del Fuego display very
limited Nb/Y and Zr/Ti variations and plot within the dacite/rhyodacite field. The LF rocks show
larger spread within the same compositional field. The YF and BC samples, in turn, scatter from the
andesite to dacite/rhyodacite fields (Fig. 3).
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TOMO 1 - Análisis de Cuencas
Figure 2: Th/U vs. Th after McLennan et al. (1993)
displaying low degrees of weathering. YF= Yahgan Fm.;
BC=Basement Complex; LF= Lemaire Fm.; KLM= Lower
Cretaceous Mudstones.
Figure 3: Zr/Ti vs. Nb/Y plot to indicate the general
composition of the (meta)sedimentary rocks.
Abbreviations as in Fig. 2
RARE EARTH ELEMENTS
Samples show mainly a typical upper continental crust composition regarding REE. When
normalized to PAAS (Post-Archean Average Australian shale after Nance and Taylor, 1976; Fig. 4)
average values for each unit display smooth negative Eu anomaly (0.87 for LF samples), negligible
positive Eu anomaly (1.06 for LKM) or moderate positive Eu anomalies both for YF (1.3) and BC
(1.29) rocks.
Figure 4: Average REE patterns normalized to PAAS.
Abbreviations as in Fig. 2
Figure 5: Th/Sc vs. Zr/Sc after McLennan et al. (1990).
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PROVENANCE
HFSE (i.e. Th, Sc, Zr) and REE are useful for provenance studies as they are insoluble and
generally immobile under surface conditions, therefore they preserve characteristics of the source
rocks (Taylor and McLennan, 1985, Bathia and Crook, 1986, McLennan et al., 1993). The Th/Sc
vs. Zr/Sc diagram (Fig. 5) allows inferring two main compositional sources for the studied units. A
more evolved one represented by LKM and LF with Th/Sc ratios close to the upper crustal average,
and a less fractionated source for YF and BC, characterized by lower Th/Sc ratios. No significant
crustal recycling may be interpreted for any of the studied units.
The influence of a less fractionated source for YF and BC might be also interpreted from their
low Th/Sc (Fig. 5) and Th/U ratios (Fig. 2). On the basis of available chemical data it is not possible
to distinguish distinct provenance components for YF and BC. Both units share some common
interelemental ratios (i.e.: La/Th>2.8, low Th/Sc, low LaN/SmN, smooth Eu/Eu*CN) which suggest
mafic sources in the provenance. Moreover, there is a striking overlapping in the average REE
patterns of both units (Fig. 4).
LKM and LF show higher Th/Sc (0.5-0.9), LaN/SmN, lower La/Th (1.8-2.8) and more
pronounced Eu/Eu*CN which point to a more felsic provenance, but higher Th/U of LKM (4.4-4.7)
suggests a slight degree of weathering in comparison with LF which displays lower Th/U ratios (2.83.3) pointing to some mafic component in the provenance.
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