Devonian magmatism in the Coastal Ranges of the Chilean Lake district: The Zarao Metatrachyte

QUEZADA, P.; VILDOSO, F. ; MARAMBIO, C. ; PALAPE, C.; HERVE, F; CALDERÓN, M.; RAPELA, C.W.; PANKHURST, R.J; FANNING, C.M.

Congreso; XV Congreso Geológico Chileno; 2018

Introduction. The Zarao metatrachyte is a small and isolated Early-Middle Devonian felsic subvolcanic intrusion spatially associatedwith mafic schists. It was initially included in the ?Metabasitas de Estaquilla?, a sub-unit of the Permian to Triassic Bahia Mansa Metamorphic Complex (Duhart et al., 2001) although no contact relationships can be seen in the field. It is located on the easternslope of the Coastal Ranges of the Lake District (c. 41º30?S), around 50 km west of Los Muermos (Fig 1). Between 39º and 43ºS,Devonian magmatism is interpreted as forming two discontinuous belts of granitoids, one located near the western edge of the North Patagonian Massif and the other in the western slope of the main Andes ? the latter along the strike of the Liquiñe-OfquiFault Zone (Fig. 1). Both were generated in a supra-subduction zone environment (Quezada et al., 2015) and zircon O-Hf isotopedata indicate that continental crust contributions to the magmas are significant in granitoids of the NPM but absent in the plutonicrocks of the Chilean Andes (Hervé et al., 2016). The latter is suggested to have been generated in Chaitenia, an oceanic terrane.Pillowed metabasalts (Hervé et al., 2018, in press) and trilobite-bearing slates (Fortey et al., 1992) of Devonian age cropping out in the Andes of Lake District are the most probable host rocks of Chaitenia. In the Los Muermos area, metamorphic rocks found at >1600 m depth in ENAP drill cores were included by McDonough et al. (as cited by Duhart et al., 2001) in the Llanquihue Basement Complex (Fig. 1). Detrital zircon grains indicate a Middle Devonian (c. 385 Ma) maximum possible sedimentation age for the unit(Hervé et al., 2016), which probably represents an accretionary complex developed in the forearc of the NPM during the Devonian.The origin and relationship of the Zarao trachyte with Chaitenia and the Llanquihue Basement Complex is here investigated through whole-rock geochemistry coupled with in-situ zircon O-Hf isotope analyses. Materials and methods Whole-rock geochemistrywas performed on two samples of the Zarao metatrachyte (FO14108, CM2-04-08) and on two nearby mafic schists (CM2-06-08,CM2-08-08). In-situ zircon O-Hf analyses were carried out in one sample of the Zarao metatrachyte (FO14175). Major elements and trace elements were measured using ICP-OES and ICP-MS, respectively, at ACTLABS Laboratories (Canada). O- and Hf isotopeanalytical work was carried out at the Research School of Earth Sciences, The Australian National University, Canberra.Results 1. General aspects Vildoso (2017) included the studied rocks in the ?Metavolcanitas de Zarao?, an informal unit, on thebasis of petrographic observations and considering the older age of Zarao metatrachyte when compared with the Metabasitas deEstaquilla unit (Duhart et al., 2001). Two tectonothermal metamorphic events (D1 and D2) are recognized in the ?Metavolcanitasde Zarao? unit. D1 was generated under greenschist facies and its associated foliation (S1) has NE/SW orientation, dipping to SE.The mineral assemblage in the mafic schists is Bt + Chl + Ep + Act + Qz + Ttn + Ab. Secondary assemblages in the metatrachytes are Qz + Chl + Ttn + Bt + WM. D2 was coeval with fragile/ductile deformation and is well preserved in the metatrachytes with anassociated mineral assemblage consisting of Qz + Chl + Bt + Gt. The original texture of the trachyte is porphyritic, with K-feldspar crystals up to 1 cm. Plagioclase and quartz are also preserved. They are included in a fine-grained groundmass of feldspar, quartzand opaque minerals. In some thin samples, aligned microliths of feldspar are seen in thin section. The metamafic rocks of theZarao area are fine-grained amphibole and zoisite schists with nematoblastic to granonematoblastic texture oriented according to S1. Deformed plagioclase porphyroclasts are preserved. Zoisite, titanite and albite porphyroblasts are fractured and veins of Qz +Bt + Chl + opaques cut the foliation.