Detrital zircon age patterns and provenance of the metamorphic complexes of southern Chile
Introduction
The western edge of the South American plate south of 34°S is mainly composed of low grade metasedimentary rocks, interpreted by Hervé (1988) as a Late Palaeozoic subduction complex and intruded by the Mesozoic-Cenozoic granitoids of the Patagonian batholith (Fig. 1). Fossil localities in this vast complex are extremely rare. Devonian trilobites are known from black slaty rocks at Buill (Biese, 1953, Fortey et al., 1992), the Late Triassic (Norian) bivalve Monotis in the Chonos Archipelago (Fang et al., 1998), and Early Permian fusulinids (Douglas and Nestell, 1976) and radiolaria (Ling et al., 1987) in the limestones of the Madre de Dios/Duque de York area (Cecioni, 1956). The relationship of the Buill rocks to the subduction complex has not been firmly established.
The age for the metamorphic rocks was based on stratigraphical relationships and Late Paleozoic Rb–Sr and K–Ar age determinations in both the schists and cross-cutting granites. Fossiliferous Late Triassic rocks unconformably overlie the subduction complex in central Chile, and Late Jurassic acid volcanic rocks of the Tobı́fera series overlie the metamorphic basement at Peninsula Staines in southern Chile (Allen, 1982). There is a similar volcanic unconformity on the Trinity Peninsula group metasandstones of the Antarctic Peninsula. However, a Mesozoic-Cenozoic metamorphic age for the Scotia metamorphic complex has been established by extensive geochronological work by Tanner et al., 1982, Hervé et al., 1990a, Hervé et al., 1991, Trouw et al., 1990, among others.
Glaucophane-bearing metabasites in the Diego Ramı́rez Archipelago (Wilson et al., 1989) yield Jurassic metamorphic (?) ages (Davidson et al., 1989). On the basis of Rb–Sr whole-rock systematics, Mesozoic reactivation of the metamorphic basement was suggested by Nelson et al. (1980) for Cordillera Darwin (Tierra del Fuego), by Davidson et al. (1987) for the Chonos metamorphic complex (CMC), and by Hervé et al. (1990b) for the western metamorphic series of central Chile, but the age of the sedimentary protolith in all cases was still considered Late Paleozoic. However, SHRIMP U–Pb dating of detrital zircon has established the depositional age of the CMC as Late Triassic (Hervé, 1998, Hervé and Fanning, 2001).
The purpose of this paper is to present a more systematic review of the chronology of the low grade metasedimentary complexes of southern Chile based on new SHRIMP U–Pb zircon analyses. These data constrain the depositional ages of the rocks and indicate the characteristics of their provenance, which can be interpreted in terms of the geodynamic evolution of this part of the Gondwana margin.
Section snippets
Geological background
The general context of the studied area is shown in Fig. 1. Two elongated outcrop zones of mainly low grade metasedimentary rocks occur to the east and west, respectively, of the Mesozoic-Cenozoic Patagonian batholith. Smaller patches of higher-grade metamorphic rocks are also included in the batholith.
The eastern Andes metamorphic complex (EAMC) crops out to the east of the batholith but reaches the coastal canals and islands south of 48°S. It comprises a mainly turbidite succession but also
Method
Metasedimentary samples of the various basement units were collected for U–Pb zircon dating by SHRIMP (sensitive high resolution ion microprobe) at The Australian National University (SHRIMP I and II). The procedures followed are outlined by Ireland, 1992, Williams, 1998. In general, zircon concentrates were poured onto double-sided tape with Duluth gabbro reference zircons and then cast into an epoxy mount. An area was selected at random, and the requisite grains were analyzed without regard
Samples
Latitude and longitude readings were obtained by global positioning systems, except for samples for which no decimal part is shown for the minutes figure. Place names not shown in Fig. 1 can be located using a good quality atlas or navigational charts.
Detrital zircon age spectra
The Tera-Wasserburg diagrams (Fig. 2, Fig. 3, Fig. 4) plot the total ratios, uncorrected for common Pb, and show that the data generally plot close to Concordia. Apparent discordance may be due to either a high common Pb content in the area analyzed, perhaps arising during crystallization (the data plotted in this diagram are conventionally uncorrected), or a postcrystallization disturbance (most likely, recent loss of radiogenic Pb, but loss or gain of U and common Pb are also feasible).
Discussion
Among the youngest ages in each pattern are some that may be affected by radiogenic Pb loss during the low grade metamorphism of the complexes (Late Permian in the EAMC, Late Triassic in the CMC and the Madre de Dios complex). Other than this issue, the differences in the zircon populations observed among the different metamorphic complexes can be interpreted in terms of different contributions from recognizable source regions.
The EAMC rocks have a much more significant contribution from
Conclusions
U–Pb dating indicates highly variable maximum depositional ages and source areas for the sedimentary protolith units of the metamorphic basement of southern Chile. The easternmost units of the EAMC and the Penı́nsula Staines have detrital zircons no younger than Ordovician, whereas the western units of this complex range from Carboniferous-Permian. A Devonian-Carboniferous sedimentation age would be consistent with the sparse fossil evidence. Late Jurassic metamorphism, probably related to
Acknowledgements
Field and laboratory expenses were supported by Fondecyt grants, including 1010412 to FH. RJP gratefully acknowledges the tenure of a Leverhulme Trust Emeritus Fellowship. Captains Victor, Conrado, and Conrado Jr. Alvarez, in their boats 21 de Mayo, Penguin, and Explorador, took us safely to the remote sampling areas. M.J. de Wit and M. Pimentel are thanked for critical reviews of the manuscript that resulted in major improvements. This paper is a contribution to IGCP Project 436 (Pacific
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