Metasedimentary and metaigneous protoliths pn Cushamen Formation. Rio Chico area. Rio Negro Province

CERREDO, M. E. ; LÓPEZ DE LUCHI, M. G

Neuquen

Congreso; XVIII Congreso Geológico Argentino; 2011

Asociación Geológica Argentina

The metamorphic series of CF is made up of metasedimentary and metaigneous rocks; magmatic additions accompanied the entire CF evolution since the pre-metamorphic stage. Near Río Chico bimodal volcanism emplaced in the CF sedimentary basin is represented by rare thin dikes and layers of dacitic and basaltic composition (López de Luchi et al. 2002). Variably foliated tonalitic to leucogranitic rocks (ranging from pegmatite through aplite textures) occurring as sheets, veins and dykes within the metamorphic series have been reported for the whole CF belt. The metamorphic series display a complex structural evolution characterized by several deformation phases. The two older deformations, D1 and D2, produced foliations and have been recognized in the whole CF belt and interpreted as part of the prograde metamorphic evolution, whereas the later D3 and D4 phases have only been reported for the northern areas (from Comallo to Río Chico) and interpreted to accompany the retrograde, uplift path of CF. Several episodes of synkynematic and intertectonic magmatic additions accompanied the D1-D3 evolution of CF. The metamorphic series of CF underwent a medium-pressure regional metamorphism; it is regionally dominated by upper greenschist to amphibolite facies mineral assemblages, except for the restricted exposures of Cañadón Chacay Huarruca, Río Chico area where lower greenschist facies have been reported There is a significant difference in the lithological composition of the metamorphic series between the lower greenschist facies (chlorite zone) and the upper greenschist to amphibolite facies (muscovite-biotite ± garnet zones) at Río Chico area. Although presently available analytical data are scarce as to define the nature of this shift, some preliminary diagnostic features are briefly outlined here. The amphibolite facies metamorphic series is exposed along the Cañadón La Angostura and discontinuously along the valley of Chico river; it is composed of an interlayering of muscovite-biotite±garnet schists and quartz-rich schists with scarce layers of tourmaline-bearing schists. Given the Mg-rich character of tourmaline, this lithological assemblage has been interpreted as indicative of exhalative volcanic processes contemporaneous with siliciclastic deposition. On geochemical classification diagrams, the muscovite-schists are mostly shales and the quartz-rich schists plot in the arkose field; major element compositional data suggest their provenance from felsic-acid igneous detritus as well as recycled mature polycyclic quartzose detritus. Tectonic discrimination diagrams based on major element contents point to an active continental margin as a likely setting for the deposition of the sedimentary protoliths. The dominant felsic provenance is considered to be associated with mature continental margins or pull-apart basins. The greenschist facies metamorphic series exposed along Cañadón Chacay Huarruca is composed of an interlayering of metapelites and basic metavulcanites; in addition rare dikes of acid metavulcanites are found sparsely distributed. Metabasalts are subalcaline, with Mg# 65-70, Cr 250-300 ppm; their flat REE pattern (La/YbCN ~ 1-1.8, La/SmCN ~ 0.6-1.3), without Eu trough indicate a general similitude with modern oceanic arc lavas, but Ba is low; rocks depict distinctive troughs for Ti, Ta and Nb on multielement plots. Several interelemental ratios (i.e. La/Yb vs Yb, Sr/Y vs Y, Nb vs Y, Rb vs Y+Nb) suggest an active margin setting for their emplacement. Overall, chemical data of metabasalts indicate an oceanic arc like signature. The acid dikes in turn are calalcaline, peraluminous, medium to high-K dacites with marked Eu anomaly (~ 0.7), moderately fractionated REE patterns (La/YbCN ~ 7, La/Sm CN ~ 3) and typical arc signatures (i.e.: LILE enrichment, Nb and Ti troughs in multielement plots). Data are scarce in order to obtain sound conclusions but a thin crust might be inferred. Therefore an island arc seems to be the more suitable scenario for the metabasic rocks contemporaneous with the deposition of the sedimentary protoliths of the low-greenschist facies of CF. Variably foliated metagranitoids occurring as veins and sheets within the amphibolite grade schists of CF at Río Chico are calc-alkaline, peraluminous (ASI clustered between 1-1.35) Bt-tonalites to Ms (± Bt ±Grt)-granites to leucogranites, with SiO2 ranging from 61% to 78%, with relatively low K/Rb (120?260), Rb/Sr (0.3?0.6). Trends present continuous negative slopes for TiO2, FeOtot, MnO, MgO, and CaO, smooth positive trend is defined for Al2O3, whereas K2O, Na2O and P2O5 show considerable scatter. Rb/Sr ratios are between 0.36-0.54 for metatonalites and 0.35-0.64 for metagranites which are higher than average upper crustal values (0.26, Rudnick and Gao, 2003); K/Rb range 160-260 somewhat below upper crustal average (280, Rudnick and Gao, 2003). Within the classification schema of Maniar and Piccoli (1989) the metagranitoids indicate either an active margin (island or continental arc) or collisional setting. Metagranitoids of Cañadón La Angostura yielded a tightly constrained (MSWD= 0.76) Rb-Sr WR isochron of 374.4±7.3 Ma, with associated 87Sr/86Sri=0.71129, suggesting crustal sources. The pre-metamorphic evolution of CF might be pointing to an early stage characterized by mature sediments (shales/arkoses associated with exhalative volcanic processes), followed by synsedimentary magmatism in a thin crust (likely an oceanic island arc). Basin inversion and metamorphism would have evolved either in a thickened crust or in a crust of normal thickness which has received advective heat contributions in order to produce crustal partial melts. Analytical work in progress will allow to better constrain the nature and geodynamic meaning of these changes in the sedimentary filling of Cushamen basin and its later evolution