THE CUSHAMEN FORMATION AT SIERRA DE MAMIL CHOIQUE: DATING A BASEMENT SEPTA, SW MARGIN OF THE NORTH PATAGONIAN MASSIF

LOPEZ DE LUCHI, MONICA G; GRILLO VIDAL, CAROLINA; MARTINEZ DOPICO, CARMEN IRENE; MONTENEGRO, TERESITA; ROSSELLO, E.

Puerto Madryn

Congreso; XXI Congreso Geológico Argentino; 2022

The Paleozoic metamorphic basement in NW Patagonia is represented by the Colohuincul Complex(CC) along the eastern foothills of the Andean Cordillera and by the Cushamen Formation (CF) along thewestern border of the North Patagonian Massif. The latter forms a poorly exposed NNW trending belt from39º 45´S/ 70º 36´W to 42º 48´S/ 69º 30´W. The general features of the regional metamorphism in CF hadbeen reported for some areas (Cerredo and López de Luchi 1998, Giacosa et al. 2004, López de Luchi et al.2010, Marcos et al. 2020, Renda et al. 2021 and references therein). Ages of the sedimentary and igneousprotoliths and metamorphism are still under discussion. Detrital ages (see Table 1 for values and references)encompass groups of data that cover similar intervals in CC and CF but in most cases the youngest detritalzircon is ca. 440 Ma in CC whereas, although the youngest more prominent peak is also ca. 440 Ma in CF,smaller peak or spots indicate ages around 370 Ma. U-Pb SHRIMP dating of zircon rims from a medium grademetaclastic rock near the type locality for CF suggested a 335 Ma -earliest Permian metamorphism (Hervéet al. 2005) whereas for amphibolite grade paragneiss from south of El Maitén zircon metamorphic rimsindicate 330, 340 and 365 Ma (Pankhurst et al. 2006). On the other hand the metamorphism of the CF wasdated as early- middle Devonian (Lucassen et al. 2004, López de Luchi et al. 2010 and references therein).Marcos et al. (2020) bracketed what they considered as the CF fi rst metamorphic event between 330-300Ma. In this contribution we present the results of age dating by LAICPMS on zircon concentrates from a CF septa in the central part of the Sierra de Mamil Choique and we made a comparison of these detrital /metamorphic ages with the published data for CF. The Sierra de Mamil Choique (Fig. 1A) is composed of of three metaluminous to mildy peraluminousgranitoids units (Cerredo and López de Luchi 1998): biotite+- amphibole banded tonalite-granodiorite andbiotite+-muscovite monzogranite-granodiorite, which are intruded by biotite-muscovite monzogranites/leucomonzogranites CF septa appear as a discontinuous ENE- EW trending belt in the central part of theSierra. Individual outcrops are made up by a series of parallel 2-4 m wide septa of highly variable lengththat mostly dip with high angle to the south. Granite facies in clear cut contact with the septa are highlydeformed and in many cases interlayered and folded with axial planes parallel to the metamorphic fabric.The dated sample RC74a (41º50´3.84"S 70º13´36.24"W) a biotitic schist with local muscovite blast, wastaken from one of the northermost septa (Figs. 1A y 1B). Zircons were dated using an Agilent 8900 triplequadrupole ICP-MS (ICP-QQQ) at LateAndes (Salta, Argentina). 77 measurements generally include one spot per analyzed zircon. However, if the morphology allows, in some zircons 2 or more spots were madeto corroborate agreement in the ages calculated for the same grain. The spot diameter vary between20, 24 or 30 μm according to the size and homogeneity of the zircons. U-Pb ages were calculated basedon concordant zircons (90-110%). Peaks at 229.2, 285.8, 514.8, 606, 667.2, 1089.6, 1285.2, 1785.1 Maappear in the probability density diagram (Fig. 1C). The youngest peak is defi ned by three zircons withvery low Th/U and lacks geological meaning since the U-Pb SHRIMP age of the intruding granite is ca 277Ma (our own unpublished data). The following peak at 285.8 Ma (n=4) yields an age that within errors isalmost coincident with the age of for the banded tonalite of the Mamil Choique granitoids (Pankhurstet al. 2006). Zircons that contribute to this peak exhibit very low Th/U from 0 to 0.03 which could beconsidered as metamorphic. The remaining peaks are similar to those of the examples of CF (Table 1).Two features are remarkably, the gap between the peaks at 285.8 and 440-480 Ma and the intensity ofthe peaks at 1089.6 and 1285.2. If all the available detrital data for CF are considered the interval 1000-1300 Ma is always present but the peak is less intense than for the early Cambrian-Ediacaran ages that isopposite to the case of sample RC74a. The RC74a spectra with a more prominent peak at the Grenvillianinterval is similar to those exhibited by the zircons of the sample of Cuesta de Rahue and Estero El Loco(Herve et al. 2018), Arroyo Pescado schist (Uriz et al. 2019) and in one example of CC at san Martíndelos Andes (Serra Varela et al. 2016). It remains to understand the signifi cance of the Permian peak thatmay result either from zircons derived from thin layers of the intruding granitoids or from some contactrelated process as a fl uid assisted thermal overprint.