Lithospheric Loss in the Andean Convergent Margin During the Triassic: Geochemical Evidence from Igneous Rocks of Northern Chile (24°30’

Home , Choiyoi Group, Llano de Chocolate Beds, San Rafael orogeny

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F u n 2 d 6 la serena octubre 2015 ada en 19 Lithospheric loss in the Andean convergent margin during the Triassic: geochemical evidence from igneous rocks of northern Chile (24°30’ – 30°00’S).

Verónica Oliveros*, Paulina Vásquez, Christian Creixell, Javiera González, Mauricio Espinoza, Friedrich Lucassen and Mihai. N. Ducea. Departamento Ciencias de la Tierra, Universidad de Concepción, Edmundo Larenas 129, Concepción, Chile Servicio Nacional de Geología y Minería, A. Santa María 0140, Providencia, Santiago, Chile. Isotope Geochemistry Laboratory, GEOMAR, Universität Bremen, Germany. Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA. Universitatea Bucuresti, Facultatea de Geologie Geoﬁzica, Strada N. Balcescu Nr 1, Bucuresti, Romania

*Contact email: [email protected]

Abstract. Several volcanic, volcano-sedimentary and continental basins in western Argentina and Chile during plutonic units cropping out in northern Chile between 24°30’ the Late Permian to Triassic period, supported the idea of and 30°S have been characterized petrographically and arrested subduction (Franzese and Spalleti, 2001; Kleiman geochemically. More than 50 new geochemical and isotopic and Japas, 2009). By the other hand, recent studies results from these units are presented and compared to proposed margin-parallel transpressional deformation of previous published data for Late Paleozoic to Late Jurassic the margin during the Triassic, associated to oblique igneous rocks. We propose that the continental margin subduction (Kato and Godoy, 2015). The lack of a underwent signiﬁcant lithospheric loss, in a continuous recognizable chain of igneous units that could represent the subduction setting, at some time during the Permian to the Permian to Triassic magmatic arc is also seen as an Triassic. Delamination of the lithospheric roots of the Paleozoic arc is a likely process to account for the evidence of this proposed tectonic setting. geochemical evolution of the magmas in the studied time frame, although stretching of the continental plate, without In this work we present new geochemical and isotopic data signiﬁcant crustal growth due to arc magmatism or for volcanic and plutonic rocks of more than 10 geological sedimentary accretion is also a plausible scenario. The units ranging in age from Carboniferous to Early Jurassic, westwards shift of the magmatic arc, located at the that crop out in the Coastal Cordillera, Precordillera or Principal Cordillera during the Carboniferous to the Early- Principal Cordillera of northern Chile between 24°30’ and Middle Triassic, to the present-day Coastal Cordillera could 30°S. These data are compared to a compilation of more have taken place at ca. 210 Ma. than 460 sets of geochemical results published from rocks of Carboniferous to Late Jurassic in age, cropping out Keywords: Triassic, subduction, magmatism, mainly in Chile but also in Argentina between 18° and geochemistry. 40°S. Our results allow us to propose a model of significant lithospheric loss in the proto-Andean margin during the Triassic. This process would have taken place 1 Introduction after a major orogenic event in the Permian, without the interruption of the subduction of oceanic lithosphere under Crustal growth at the present-day western portion of the the South-American plate, as revealed by the dominant South-American continental plate margin has been calk-alkaline affinities of the Palaeozoic and Mesozoic dominated by terrane accretion and subduction throughout igneous rocks that compose the present-day Andean the Phanerozoic eon. For the Andean region, whereas the forearc in northern Chile. existence of a subduction setting during the Late Palaeozoic and from the Early Jurassic until now has been relatively well proven, the nature of the margin evolution 2 Methods, Samples, Results from Late Permian to Late Triassic has been more difficult to determine. Several authors have proposed geodynamic changes in this time frame, particularly the interruption in Fifty-five samples of volcanic, subvolcanic and plutonic the subduction due to either terrane accretion (Mpodozis rocks from Carboniferous to Early Jurassic units were and Kay, 1992) or extensional forces driven by Pangea’s collected for petrographical and geochemical analysis. The breakup (Uliana et al. 1989). The existence of voluminous studied units in the Coastal Cordillera correspond to the acid explosive magmatism during the Permian (the Llano de Chocolate Beds, the Canto del Agua, Agua Chica Choiyoi group) after a regional scale orogenic event (San and Cifuncho Fms and related subvolcanic bodies; as well Rafael orogenic phase) along with the development of as Permian, Middle Triassic and Upper Triassic plutons. several hundred kilometers-long, NNW-SSE oriented, The studied units in the Precordillera and Principal 864 AT 1 GeoloGía ReGional y Geodinámica andina

Cordillera correspond to the Los Tilos and Guanaco Sonso samples, specifically three samples of the Norian-Rhaetian sequences, the La Totora, San Félix, Quebrada del Salitre, La Totora Fm., have transitional to alkaline affinities Sierra de Varas, Cerro Guanaco La Tabla and Algarrobal and/or plot in the whithin plate or oceanic island fields of formations and related subvolcanic bodies, and the Guanta, the discrimination diagrams (Fig. 1b), but the remaining Chollay, Colorado and Punta del Viento plutonic samples fall within, or very close to, the volcanic arc complexes and related dykes. Whole-rock major and trace basalts of continental arc granites fields(Fig. 1b,c). This element composition were obtained through XRF and ICP- pattern is confirmed when the analysed samples are MS for all samples, along with Sr-Nd-Pb isotopic ratio compared to the whole database. measured by TIMS. An interesting feature of the igneous rocks from Late The petrography of the volcanic and subvolcanic rocks Paleozoic to Triassic is their systematic shifts in specific indicates that they range from basalts to rhyolite, but the geochemical parameters that are linked to the extent of most abundant lithologies are dacites. Basalts and basaltic- lithospheric involvement in the magma sources. Thus, andesites occur mainly in the Upper Triassic units (La LaN/YbN ratios of the studied samples decrease from 4.0 at Totora, upper San Félix, Agua Chica, Sierra de Varas, ca. 300 Ma to 3.0 at ca. 250 Ma and to 1.5 at ca. 200 Ma, Guanaco Sonso and Quebrada del Salitre formations). although the younger group has a large dispersion between They have only porphyritic textures, with plagioclase, 1.0 and 3.0 (Fig. 1d). A similar pattern is observed for the clinopyroxene, hornblende and quartz as the main whole dataset including previous published data (Fig. 1d). phenocrysts. Groundmass is often largely altered and This shift suggests that crust over the magmatic arc consists of variable amounts of plagioclase, quartz, k- became thinner from the Carboniferous to the Early feldspar, undetermined mafics and Fe-Ti oxides. Plutonic Jurassic. However, other key ratios that represent proxies rocks range from quartz granodiorite and tonalite to to crustal thickness, such as Sr/Y (Chapman et al, in press) granite, with few gabbros occurring in the principal do not follow a decreasing pattern form the Carboniferous cordillera at ~30°S. to the Jurassic but rather the opposite.

The geochemical results indicate that the majority of the The loss of the crustal or lithospheric signal in the studied volcanic and plutonic rocks have calk-alkaline Paleozoic to Mesozoic Andean magmas is also evidenced affinities (Fig. 1a). The trend is observed either in the by their isotopic composition. Nd isotopes are a more major and highly mobile element contents or in the trace reliable proxy to the lithospheric component of arc and more refractory HFS (high field strength) elements. magmas than Sr or Pb since their ratios are little affected There is a consistent enrichment in LILE (large ion by low grade metamorphism or hydrothermal alteration, lithophile elements) over HFSE and marked Nb-Ta troughs which is very common in Andean rocks. The εNd in all the studied samples. REE (rare earth elements) parameter also decreases with time from Carboniferous to patterns vary from very steep to flat with a systematic Jurassic, a trend that is observed either for the set of decreasing in the LaN/YbN and LaN/SmN ratios from the studied samples or the whole database (Fig. 1e). This Carboniferous to the Lower Jurassic samples (Fig. 2d). would imply a tectonic evolution that involved loss of lithosphere and crustal thinning at some time by the end The isotopic composition of the rocks is variable, initial the Paleozoic and beginning of the Mesozoic era. 87Sr/86Sr ranges between 0.696 and 0.712 with over 95% of Delamination or foundering of the arc roots is a process the samples in between 0.701 and 0.709. Abnormal low that can account for this lithospheric loss.. This is a initial ratios are interpreted as extreme Rb loss due to plausible explanation taking into account that a major alteration. Initial 143Nd/144Nd ranges between 0.511848 and trigger for this process is a previous crustal thickening, as 0.512518 with over 95% of the samples in between has been proposed for the Carboniferous-Early Permian, 0.512470 and 0.512060 and initial εNd ranging between associated to the San Rafael Orogeny. Another likely +4.00 and -4.72. The isotopic composition of lead is less scenario is the stretching of the lithosphere. For both cases, variable, ranging between 18.67 and 18.33, 15.76 and regional Frontal cordillera geology (28°-29° S) suggest 15.53 and 38.91 and 36.55, for 206Pb/204Pb, 207Pb/204Pb and major tectonic changes took place during the Early 208Pb/204Pb respectively. Triassic, with evidences of extensional tectonics as voluminous emplacement of mafic dike swarms at 240-230 Ma, onset of extensional basins (San Félix basin) and large 3 Discussion exhumation of basement and Early-Middle Triassic plutons, covered by Late Triassic sequences, coeval with 3.1 Magmatic sources and their evolution from extensional or transtensional deformation. This large the Late Carboniferous to the Late Jurassic. amount of uplift and exhumation processes is more able to be produced by processes like buoyancy of the lithosphere The observed geochemical signature would indicate that triggered by delamination rather than crustal stretching the generation of magmas was controlled by fluid induced (Ducea, 2011). melting of a depleted asthenospheric mantle source. Few 865 SIM 1 GEOLOGÍA DEL TRIÁSICO ANDINO

3.2 Westward shifting of the magmatic arc Kleiman, L.E.; Japas M.S. 2009. The Choiyoi volcanic province at during the Late Triassic? 34°S–36°S (San Rafael, Mendoza, Argentina):Implications for the Late Palaeozoic evolution of the southwestern margin of Gondwana. Tectonophysics 473: 283–299. New and previous published ages for volcanic and Mpodozis C.; Kay S.K. 1992. Late Paleozoic to Triassic evolution of volcaniclastic rocks of the Agua Chica, Canto del Agua the Gondwana margin: Evidence from Chilean frontal and Cifuncho Fms as well as the Algodones Granite and cordilleran batholiths (28°S to 31°S). Geological Society of Carrizal Bajo plutonic complex, indicate that the earlier America Bulletin104: 999-1014. igneous activity in the Coastal Cordillera during the Pearce, J.A.; Harris, B.W.; Tindle, A.G. 1984. Trace element Mesozoic era started at ca. 210 Ma (Welkner et al. 2006, discrimination diagrams for the tectonic interpretation of Espinoza et al., 2015). By this age, it is likely that the granitic rocks. Journal of Petrology 25:956-983. magmatism in the present-day precordillera and Principal Cordillera had diminished after the built-up of the Chollay Rossel, P.; Oliveros, V.; Ducea, M.N.; Charrier, R.; Scaillet, S.; arc batholith during the Early-Middle Triassic (Salazar et Retamal, L;, Figueroa, O. 2013. The Early Andean subduction system as an analogue to island arcs: evidence from across-arc al., 2013) as evidence by the rather limited Upper Triassic geochemical variations in northern Chile. Lithos 179:211-230. volcanic sequences and the absence of significant Upper Triassic plutonic rocks in this area. Although few data are Salazar, E.; Coloma, F.; Creixell, C. 2013. Geología Del Área available, the volcanic samples belonging to the Upper El Tránsito-Lagunillas, Región de Atacama. Servicio Nacional de Triassic La Totora Formation show a distinct geochemical Geología y Minería, Carta Geológica de Chile, Serie Geología signature with transitional to alkaline affinities (Fig. 1b), Básica 149: 119 pp. Santiago. whereas geochemical features of subduction-related Uliana, M.A.; Biddle, K.T.; Cerdán, J. 1989. Mesozoic extension and magmatism are still present in these rocks. A similar the formation of Argentina sedimentary basins. Extensional pattern is observed for the Upper Jurassic volcanic back- tectonics and stratigraphy of the North Atlantic Margin, arc units in northern Chile (Rossel et al., 2013) suggesting American Association of Petroleum Geologists, Memoir 46, that a back-arc setting for this region could have been 599-613. developed as early as the Uppermost Triassic and Welkner, D.; Arévalo, C.; Godoy, E. 2006. Geología de la carta consequently the Andean arc shifted to its actual position Freirina – El Morado, Región de Atacama. Servicio Nacional de during this time frame. Geología y Minería, Carta geológica de Chile, Serie Geología Básica, Nº 100, 50 p., 1 mapa escala 1:100.000.

Acknowledgements Wood, D.A. 1980, The application of a Th-Hf-Ta diagram to problems of tectonomagmatic classification and to establishing This work has been funded through the Fondecyt Grant the nature of crustal contamination of basaltic lavas of the #1120715 (V. Oliveros, P. Vásquez, C. Creixell), the Plan British Tertiary volcanic province: Earth and Planetary Science Nacional of the Servicio Nacional de Geología y Minería Letters 50: 11–30. and two Conicyt doctoral fellowships (M. Espinoza, J. González).

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(26°–42° S). International Geology Review 57 (5-8): 606-628. 866 AT 1 GeoloGía ReGional y Geodinámica andina

Figure 1. a) AFM diagram (Irvine and Baragar, 1971), b)Th-Hf-Nb diagram (Wood, 1980),and c) Rb-Yb-Ta (Pearce et al., 1984) diagrams for the studied samples; d) LaN/SmN versus age and d) εNd(t) versus age for the studied units and the compiled database for Carboniferous to Upper Jurassic units in Chile between 18º-40ºS. Symbols correspond to the analysed samples and colour lines mark the field of all the results from the database, according to their assigned age (light blue: Jurassic, purple: Triassic, red: Permian, green: Carboniferous). Lw: Lower, Md: Middle, Up: Upper, Cis: Cisuralian, Guad: Guadalupian, Lop: Lopingian, (E,N)-MORB: (normal, enriched) mid-ocean ridge basalts, OIB: oceanic island basalts, AB: arc basalts, syn-COLG: syn-collisional granites, VAG: volcanic arc granites, WPG: within-plate granites, ORG: orogenic granites.

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