Elsevier

Gondwana Research

Volume 42, February 2017, Pages 104-125
Gondwana Research

Hot subduction in the middle Jurassic and partial melting of oceanic crust in Chilean Patagonia

https://doi.org/10.1016/j.gr.2016.10.007Get rights and content

Highlights

  • Partial melting of MORB-derived garnet amphibolites and generation of trondhjemite.

  • U-Pb zircon rims and garnet Sm-Nd dating indicate peak conditions at c.162 Ma.

  • The field record of a hot subduction zone (19°/km) during the Jurassic in S. Chile.

Abstract

Rare remnants of a Mesozoic subduction high pressure (HP) accretionary complex are exposed on Diego de Almagro Island in Chilean Patagonia. We herein focus on the Lazaro unit, a coherent slice of oceanic crust exposed on this island that has been first affected by high temperature (HT) metamorphism followed by a lower temperature deformation event (LT). Its Pressure-Temperature-time (P-T-t) evolution is reconstructed using field and petrographic observations, phase relations, thermobarometry and geochronology. Remnants of a primary amphibolite to HP granulite-facies event in mafic rocks comprising garnet (with ilmenite exsolutions), diopside, trondhjemitic melt, pargasite, plagioclase ± epidote are reported for the first time in neosomes, indicating peak P-T conditions of 1.1–1.3 GPa and c. 750 °C. This peak T paragenesis has been thoroughly overprinted by a phengite-chlorite-actinolite assemblage during isobaric cooling down to c. 450 °C. U-Pb dating of zircon metamorphic rims from a metasedimentary rock yielded a homogeneous age population of 162 ± 2 Ma for the HT event. Sm-Nd dating of two peritectic garnet-bearing samples yield ages of 163 ± 2 Ma and 163 ± 18 Ma for the HT event. Multi-mineral Rb-Sr dating of a metasedimentary rock overprinted by LT deformation suggests retrograde shearing between 120 and 80 Ma. Our results show that the HT event in the Lazaro unit took place at around 160–165 Ma, shortly before the onset of Patagonian Batholith emplacement. Partial melting of subducted oceanic crust reported in the Lazaro unit is related to the early stages of hot subduction along the Gondwana western margin. The Lazaro unit remained at c. 40 km depth along the subduction interface for > 80 Ma, recording the deformation and long-term cooling of the subduction channel environment until the upper Cretaceous.

Introduction

During the consumption of oceanic lithosphere, fluids and melts from the lower plate are transferred to the upper plate, leading to partial melting of the ultramafic wedge and the formation of volcanic arcs, ultimately contributing to the chemical differentiation of the Earth (Tatsumi and Eggins, 1995). Mass transfer processes across the subduction interface have been extensively studied via geochemical, petrological and geophysical investigations (Peacock et al., 1994, Oncken et al., 2003, Maruyama et al., 2009, Spandler and Pirard, 2013, Schmidt and Poli, 2014). Although subject to contamination during migration through the upper plate, arc magmas and volcanic rocks yield valuable information on chemical differentiation and volatile transport highlighting the contribution of various lithological compounds to the melting process (e.g. Plank and Langmuir, 1993, Walowski et al., 2015). Furthermore, high pressure rocks and exhumed former subduction interfaces have the potential to provide in situ insights into natural processes taking place within the mantle wedge region and beneath volcanic arcs (e.g. Hermann et al., 2006, John et al., 2012).

Only a few natural examples documenting partial melting processes and trondhjemite-tonalite genesis in oceanic subduction zones have been reported (e.g. Catalina Schists: Sorensen and Barton, 1987; Cuba mélanges: García-Casco et al., 2008, Lázaro and García-Casco, 2008; N. Iran: Rossetti et al., 2010). The apparent scarcity of such occurrences can be explained by the rarity of material exhumed from sub-arc depths (> 100 km) where temperatures are high enough (> 700 °C) to enable partial melting of the oceanic crust along normal subduction gradients (e.g. Syracuse et al., 2010). Known occurrences of melt-bearing, exhumed oceanic crust localities correspond to shallower rocks subducted along a hot prograde pressure-temperature (P-T) path (c. 20°/km). Such conditions may have been reached (i) during Archean times (e.g. Van Hunen and Moyen, 2012), (ii) shortly after subduction initiation (e.g. Stern, 2004) or (iii) during subduction of young oceanic lithosphere (e.g. Blanco-Quintero et al., 2011).

We herein report a new occurrence of high-pressure partial melting of subducted oceanic crust on the remote Diego de Almagro Island (Fig. 1; DAI, Chilean Patagonia) and discuss the meaning of this event in the light of new thermobarometric, geochemical and geochronological data. This island represents a unique vestige of a fossil accretionary system developed at the southwestern margin of Gondwana (Hervé and Fanning, 2003, Willner et al., 2004a), where different slices of oceanic crust were underplated between c. 120 Ma and c. 80 Ma and metamorphosed under blueschist to eclogite-facies conditions (Hyppolito et al., 2016). The new results presented here open a window for better understanding the origin of partially molten rocks in this paleo-subduction system and their tectonic significance in a regional context.

Section snippets

Geological setting

A very lengthy mountain chain, the Chilean Coastal Cordillera, is exposed almost continuously between latitudes 28° S and 55° S, and includes subduction complexes of Late Paleozoic and Mesozoic ages developed at the southwestern margin of the Gondwana continent via basal and frontal accretionary processes (Hervé, 1988, Glodny et al., 2005, Willner, 2005, Kato and Godoy, 2015). Only three occurrences of albite-epidote amphibolite-facies subduction channel rocks (with counter-clockwise P-T paths)

Rock types and field relationships of the Lazaro unit

Two field missions focusing on the southern coast of the Diego de Almagro Island were organized in 2007 and 2015. Sampling, petrological observations and structural measurements led to the characterization of the Lazaro unit, a coherent unit forming the eastern and south-eastern part of the island where abundant amphibolite and orthogneiss bodies have been reported (Hervé and Fanning, 2003). Our observations, in line with Hervé and Fanning (2003) and Willner et al. (2004a), show that the Lazaro

Analytical methods

Petrological study has been performed on a set of 20 representative samples located on Fig. 1c. Electron probe microanalyses were performed at the GFZ Potsdam with a JEOL-JXA 8230 probe under common analytical conditions (15 kV, 20 nA, wavelength-dispersive spectroscopy mode), using a 10 μm beam. Standards used for the calibration as follows: orthoclase (Al, Si, K), fluorite (F), rutile (Ti), Cr2O3 (Cr), wollastonite (Ca), tugtupite (Cl), albite (Na), MgO (Mg), Fe2O3 (Fe) and rhodonite (Mn). Table

Garnet amphibolite and garnet-bearing restites

All the 17 mafic samples from the Lazaro unit studied here were variably affected by retrogression and deformation. In the most re-equilibrated samples, the peak T event is nearly erased and the rocks have been transformed into greenschist facies mylonites (e.g. sample #29a; Electronic Appendix 2). Only samples where the HT event is best preserved exhibit rounded diopside crystals included within garnet (Figs. 3a, e and 4a), since matrix diopside is usually pseudomorphed by hornblende (Fig. 4

Single-equilibrium thermobarometry

In order to constrain the temperature reached by the Lazaro unit rocks during the HT metamorphic event, the garnet-clinopyroxene thermometers of Ravna (2000) and Ellis and Green (1979) have been used for samples #25 and #2-31b, in which peak metamorphic diopside has been preserved during retrogression. These temperatures have been calculated for a fixed pressure of 1.2 GPa. This pressure estimate was obtained on a garnet amphibolite sample (#27) following the calibration of Kohn and Spear (1990)

Rb-Sr geochronology

The Rb-Sr internal mineral isochron approach is particularly well suited for dating ductile deformation events in white-mica bearing metamorphic rocks. Deformation-induced recrystallization and re-equilibration of mineral phases (such as white mica, albite, apatite and titanite) leads to complete Sr-isotopic re-equilibration and reset of ages under moderate and high temperature (Inger and Cliff, 1994). Isotopic inheritance due to incomplete resetting of the pre-deformation isotopic system may

Pressure-Temperature-time history of Lazaro unit

Lazaro unit rocks reveal a two-fold tectono-metamorphic history visible in the structural record in the field and in the mineral zoning patterns. The prograde, burial history has not been preserved due to the high temperature of peak metamorphism and a long-term re-equilibration during cooling and exhumation (see also Willner et al., 2004b). Our P-T estimates show that peak metamorphic conditions reached c. 1.2 GPa and 750 °C (~ 40 km depth using an integrated rock density of 3) at the transition

Conclusions

Evidence for partial melting of subducted oceanic crust has been discovered in the Lazaro unit (Diego de Almagro Island). Thermobarometric results show that wet melting took place at around 1.2 GPa and c. 750 °C, leading to the formation of trondhjemitic melts and peritectic garnet. U-Pb dating of zircon rims and Sm-Nd geochronology on garnet-bearing HP granulite-facies assemblages reveal an age of c. 162 ± 2 Ma for this HT event. Combined with other ages showing that emplacement of the Patagonian

Acknowledgements

Jesus Muñoz is acknowledged for field work assistance and discussions. Silvio Ferrero and Patrick O′Brien are acknowledged for insightful discussions and Philippe Yamato for sharing numerical modeling results. This project has been funded by a Deutsche Forschungsgemeinschaft (DFG) project to S.A. (AN1113-1), and São Paulo Research Foundation (FAPESP) (#2004/10203-7, #2012/01191-1) and received support for analytical costs at CIC from the University of Granada. T.H. acknowledges the grant

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