Hot subduction in the middle Jurassic and partial melting of oceanic crust in Chilean Patagonia
Graphical abstract
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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2022, Chemical GeologyCitation Excerpt :The scarcity of amphibole in mantle rocks from modern subduction zones cannot account for the compositional variations in Hf/Sm and Zr/Sm observed in the magmas (Pearce et al., 1992). As such, slab melting of altered oceanic crust in the amphibolite facies has been proposed as a cause of these variations, which is consistent with the observations of amphibolite melting in subducted slabs with a warm P-T path (Angiboust et al., 2017; Prigent et al., 2018). Below, we examine the composition of slab fluid markers in magmatism spanning the history of the IBM margin.
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2021, TectonophysicsCitation Excerpt :In this context, the inception of the RVB occurred in the Middle Jurassic by rifting of the pre-Jurassic basement complexes, which were overlain by Jurassic pyroclastic, volcaniclastic and sedimentary successions (Bruhn et al., 1978; Forsythe and Allen, 1980; Dalziel, 1981; Stern and De Wit, 2003; Calderón et al., 2007; Hervé et al., 2008, 2010a). The continental basement in Patagonia comprises Paleozoic to early Mesozoic accretionary complexes of the proto-Pacific subduction zone (Nelson et al., 1980; Kohn et al., 1993; Hervé et al., 2003, 2008, 2010a; Hervé and Fanning, 2003; Willner et al., 2004; Hyppolito et al., 2016; Angiboust et al., 2017, 2018; Suárez et al., 2019). The Paleozoic-Lower Triassic Eastern Andes Metamorphic Complex extends from the Ultima Esperanza region (~ 51°S) to the Estrecho de Magallanes (Fig. 1 A), and consists of low-grade metapsammopelitic schists with minor intercalations of marbles and metabasic bodies (Forsythe and Allen, 1980; Allen, 1982; Hervé et al., 2003; Hervé et al., 2008; Betka et al., 2015).
Metamorphic imprint of ridge subduction on the Neo-Tethyan ophiolites from the Saka Unit (Central Pontides, northern Turkey)
2020, Journal of Asian Earth SciencesCitation Excerpt :The distribution of seismicity, the location of the underplating site(s), and the depth of melt genesis are extensively controlled by the thermal structure, which also strongly influences the metamorphic dehydration reactions (e.g. Abers et al., 2013; Oncken et al., 2003; Peacock and Wang, 1999). In addition, the thermal histories of subduction zones exhibit progressive cooling over time (Ma to tens of Ma), from the inception of the subduction to the mature stage (Angiboust et al., 2016; García-Casco et al., 2008; Grove and Bebout, 1995). However, throughout the subduction lifespan, transient warming episodes of the subduction environment may be triggered by the underthrusting of oceanic spreading ridges or of young oceanic lithosphere, and by modification of the features of the subduction, such as decreases in convergence velocity, changes to the subduction dip angle (e.g. Cloos, 1993; Iwamori, 2000; Dickinson, 1973; Stern, 2004), and hot-spot-related heating (Hässig et al., 2016a, 2016b).
The metamorphic rocks of the Nunatak Viedma in the Southern Patagonian Andes: Provenance sources and implications for the early Mesozoic Patagonia-Antarctic Peninsula connection
2019, Journal of South American Earth SciencesCitation Excerpt :A regionally widespread angular unconformity with the overlying Upper Jurassic volcano-sedimentary rocks along the Patagonian hinterland (Ghiglione et al., 2009; Giacosa et al., 2012a,b; Zerfass et al., 2017), gives a minimum age for deformation of the folded basement units. On the other hand, units of the western Metamorphic Complexes exhibit a geological history related to subduction dynamics and accretion of exotic terranes during the Triassic-Jurassic (Hervé et al., 2003, 2008; Hervé and Fanning, 2001; Willner et al., 2009; Angiboust et al., 2017). From correlation of Paleozoic-early Mesozoic magmatic and tecto-metamorphic events (Pankhurst et al., 2003; Hervé et al., 2006; Castillo et al., 2016; Heredia et al., 2016, 2018; Riley et al., 2016; González et al., 2018), it has been proposed that South America, Patagonia, and the Antarctic Peninsula were contiguous regions forming the SW margin of Gondwana.