The Paleozoic Basement Beneath the Mesozoic Cover of the Andean Principal Cordillera in Central Chile (34°00'- 34°30'S): Tracks from Inherited Zircon Crystals
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O EOL GIC G A D D A E D C E I H C I L E O S F u n 2 d 6 la serena octubre 2015 ada en 19 The Paleozoic Basement beneath the Mesozoic Cover of the Andean Principal Cordillera in Central Chile (34°00'- 34°30'S): Tracks from Inherited Zircon Crystals Marcia Muñoz1*, Reynaldo Charrier2,3, Patricia Sigoña3 and Mark Fanning4 1Advanced Mining and Technology Center (AMTC), Universidad de Chile, Av. Tupper 2007, Edificio AMTC, Santiago, Chile 2Facultad de Ingeniería, Universidad Andres Bello, Campus República, Santiago, Chile 3Departamento de Geología, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile 4Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia * email: [email protected] Abstract. In this contribution we document and discuss the published for Late Paleozoic inherited zircon crystals that presence of Late Paleozoic crystalline basement beneath characterize the crystalline basement of that age in the the Mesozoic cover exposed in the High Andes of Central High Andes of Central Chile. We also compare and discuss Chile (34°00'-34°30'S). This has been tracked from the our results with data obtained from the Paleozoic common occurrence of inherited zircon crystals of that age crystalline basement that crops out at different locations, in in Cenozoic igneous rocks from the region. The latter have been analyzed for age and Hf-O isotopic composition by in- Chile and Argentina, along the Andean margin. situ single crystal analyses. Based on the similarities among age distributions, we suggest that the inferred To date, two batholiths of Late Paleozoic age have been basement corresponds to the southern prolongation of the recognized in the Andean margin of Chile and Argentina, Late Paleozoic-Triassic batholith exposed further north, in namely the Late Carboniferous Coastal Batholith south of the Chilean High Andes between 28-31°S, but the Triassic 33ºS, and the mostly Permian Frontal Cordillera Batholith components are missing from the record. We also indicate in the High Andes north of that latitude (Fig 1). Their that this whole belt along the High Andes represents an outcrops apparently have no prolongations north and south independent and partly overlapping magmatic event of 33ºS respectively, which represents a conundrum for the respect to that recorded in the Late Paleozoic components of the Coastal Batholith exposed from 33°S southwards. geology of this region and for understanding Andean Late The isotopic composition of the inherited zircons reveals a Paleozoic evolution. Some recent studies have provided more crustal-like signature than the latter units, but still new data and/or raised new hypotheses concerning these defines a trend towards less incorporation of upper crustal issues. material in the genesis of magmas with decreasing age. Regarding the virtual termination of the Late Paleozoic Key Words: Late Paleozoic, Crystalline Basement, Coastal Batholith northwards from 33°S, Godoy and Inherited Zircon Crystals, Chilean Andes Navarro (2014) have suggested that it should continue but remain hidden beneath the Late Paleozoic cover 1 Introduction (accretionary complex). This hypothesis, based on the metamorphic P-T° conditions recorded in this cover, Our studies on the genesis of Andean Cenozoic magmas in questions the actual magnitude and extent of subduction central Chile have revealed the incorporation of older erosion processes in the margin; the latter process having crustal components that they interacted with whilst been argued on the basis of the absence of the Late ascending through the upper continental lithosphere (e.g., Paleozoic Coastal Batholith northwards from 33°S. In Muñoz et al., 2013). In particular, the presence of support of these authors hypothesis, Sigoña et al. (this presumed Paleozoic igneous rocks is recorded by the volume) have reported the presence of granitic enclaves of common occurrence of inherited zircon crystals of that age Late Carboniferous age entrained within a dike that in igneous rocks of the region. These are distributed in the intrudes the Paleozoic Metamorphic Basement High Andes, near the Chilean-Argentinean border between (accretionary complex) in the coastal area at ~32°S. This 34°00'-34°30'S (Fig. 1), and are either emplaced in or suggests that the Coastal Batholith exposed south of 33ºS overly Mesozoic volcanic and sedimentary units. probably extends at depth to this region. As these Therefore, in this area the inherited material provides an observations suggest, the Late Paleozoic Coastal Batholith inferred direct sampling of the Paleozoic basement beneath is considered to constitute a continuous feature along the the Mesozoic cover. Chilean coast implying the existence of two parallel batholithic swaths north from at least 33ºS. In our studies we have routinely determined U-Pb ages and O isotopic compositions by SHRIMP and Hf isotopic For the region south of 33°S, where the Permian Frontal composition by LA-ICP-MS. We present in this Cordillera Batholith is apparently absent, Rossel et al. contribution both new unpublished data and previously (2014) obtained an age of 333 ± 4.4 Ma in a granitic 92 AT 1 GeoloGía ReGional y Geodinámica andina boulder contained within a immeridgian conglomerate 2 Sampled nits deposit of the Chilean High Andes. According to the authors, this finding suggests that plutonic bodies with a 2.1 eneral Characteristics and eologic Setting similar age as those exposed in the Frontal Cordillera Batholith might have been exposed and eroded in this From our sampling, a total of four igneous units show Late region during late urassic times. These in turn could Paleozoic inherited zircon crystals (Fig. 2) (i) three represent a prolongation of the mentioned batholith to the intrusive rocks, namely Paso Colina (~1.1 Ma), Cachapoal Andean region south of 33ºS. (~12.1 Ma) and eria (~ Ma), and (i) a volcanic flow from the Cerro Listado Pleistocene series (~0.3 Ma; locations indicated in Fig 1; ages from Muñoz et al. (2013) and our own unpublished data). These units, as with Andean Cenozoic magmatism in general, show compositional features of arc-related magmas, which has resulted from the continuous plate convergence throughout this period (e.g., ay et al., 200). In addition, they also show a variable involvement of crustal components incorporated while ascending through the overriding lithosphere (e.g., Hildreth Moorbath, 1) and/or in the asthenospheric source (e.g., Stern, 11). The Andean region where the sampled units are located exposes a nearly north-south trending belt of Mesozoic volcanic and sedimentary series distributed both sides of the Chilean-Argentinean border. Towards the west, in Chile, the outcrops are exclusively dominated by Cenozoic and Mesozoic rocks, except near the coastline. Here, the northern tip of the Coastal Batholith is exposed, which hosts a significant proportion of Late Paleozoic rocks (Fig 1). The geology towards the east, in Argentina, is mainly represented by the southern termination of the Frontal Cordillera, an uplifted block of Late Paleozoic to Triassic crystalline basement (Fig 1). 2.2 Inherited Zircons: Main Features, -Pb Ages and f- Isotopic Composition As the primary aim of our studies is the Cenozoic evolution, our data sets on inherited components is not as extensive. Nevertheless, the main characteristics of the few inherited components we have analyzed are persistently present amongst different samples and so, in the following, we treat these as a single group. Figure 1. Map showing the approximate location of the samples studied in this work and the distribution of Late Paleozoic crystalline basement rocks along Chile and Argentina. Sources from Segemar (1), Sernageomin (2002), Ramos et al. (2004), eckart et al. (2014) and Maksaev et al. (2014). If the above mentioned hypotheses are correct, then it would be possible to extend two parallel batholithic swaths north and south of 33ºS, a slightly older one along the coast and a younger one along the high Andean cordillera. Figure 2. (a) Cathodoluminescence image of analyzed zircons to This would in turn suggest an eastward shift of the illustrate the mode of occurrence of the inherited material. magmatic arc from late Carboniferous to Permian times. Indicated in green are the U-Pb pits and the corresponding ages. (b) Histogram that shows the age distribution for all the inherited material in the Cenozoic igneous rocks studied. 93 ST 1 TECTÓNICA Y DEFORMACIÓN CORTICAL ANDINA As revealed from the U-Pb age determinations, inherited the time span of 21-21 Ma recorded by the lower components in zircon populations are predominantly Choiyoi Group in western Argentina (Fig. 3; e.g., Rocha- present as crystal cores, rimmed by primary igneous zircon Campos et al., 2011). The Coastal Batholith, exposed at the (Fig. 2). Rarely, complete whole crystals correspond to this same latitude as units here studied (Fig. 2), has been shown component, with both core and rims recording the older to record the oldest ages amongst the Chilean crystalline ages (Fig. 2). Paleozoic basement (Fig. 3; eckart et al., 2014; Maksaev et al., 2014). However, it has to be noted that the age distribution is partly overlapping between ~320-20 Ma and this indicates a time span of at least ~30 Ma during which all of these magmatic events coexisted (Fig. 3). The Hf isotopic composition of the inherited zircons shows a crustal-like signature with most values of Hf(t) between -3 and - (Fig. 4). Regarding the Chilean crystalline Paleozoic basement located further north and at the coast, the inherited zircons show a comparatively more evolved signature and do not display any discernible pattern towards less evolved compositions with younging age (Fig. 4). In contrast, these units have O isotopic compositions that trend towards progressively less involvement of upper crustal material in the genesis of magmas with decreasing age (Fig. 4). This is seen in the inherited zircons where the 1O values vary from .2 to 4.