The Vinquis granitic complex: an example of Carboniferous F-rich peraluminous A-type granite in the pre-Andean SW Gondwana margin, Sierras Pampeanas, Argentina.

MORALES CÁMERA, M.M.; DAHLQUIST, J.A.; BASEI, M.; CAMPOS, M.

Florianópolis

Simposio; 8th Hutton Symposium on Granites and Related Rocks; 2015

Universidade de São Paulo (USP) y Universidade Federal do Rio Grande do Sul (UFRGS)

A-type granites are rocks with distinct mineralogical and geochemical characteristics. However, the genesis of A-type granites has been controversial, leading to much discussion over the past 30years. In general, petrogenetic models for A-type granites commonly invoke igneous source rocks and petrogenesis of peraluminous A-type granites is especially poorly known. The Vinquis granitic complex (VGC), Sierras Pampeanas, Argentina, is formed by unusual F-rich peraluminous A-type monzogranites. A precise LA-MC-ICP-MS U-Pb zircon crystallization age of353 ± 9 indicates that the VGC was emplaced during Early Carboniferous time.The VGC displays high and restricted SiO2 contents between 71.6% and 74.8 wt.%. On both [FeOt/(FeOt+MgO)] vs. SiO2 and [(Na2O+K2O)−CaO] vs. SiO2 diagrams the samples plot in the ferroan and alkaline-calcic to calco-alkaline fields with Fe* = [(FeOt/(FeOt+MgO)] = 0.79 % and [(Na2O+K2O)−CaO]= 7.38% (average values, n = 20), thus showing an A-type granitoid signature.Relatively high concentrations of the some High Field Strength Elements (averages in ppm, Nb = 19, Th = 15, and Ga = 21) and moderate to flat REE patterns [(La/Yb)N = 11.42] showing significant negative Eu anomalies (Eu/Eu* = 0.41) are also typical features of A-type granites. However, the granites are peraluminous with aluminium saturation index ranging from 1.11 to 1.22 and distinctive high P2O5 content (av. 0.32 wt%). A high F content in the magma is inferred from biotite compositions with high F concentration (av. F = 1.17?1.44 wt%). The VGC has some geochemical differences from Carboniferous metaluminous A-type granites of the Sierras Pampeanas (Dahlquist et al. 2010). The latter have a strong ferroan signature (Fe* = 0.94), lower P2O5 content (0.08 wt%), higher High Field Strength Element concentrations, flat REE pattern ([(La/Yb)N = 6.47], and pronounced negative Eu-anomalies (0.20). The Rb/Ba and Rb/Srratios reported for the metaluminous A-type granites (Dahlquist et al. 2010) are higher than in the VGC. The particular A-type signature of the VGC can be attributed to a metasedimentary source as suggested by Anderson and Bender (1989) for the anorogenic peraluminous granites of the USA. Hf isotope data from magmatic zircon and granite whole-rock Sm-Nd (av. eHf353 = -6.32, eNd353 = -7.1), together with abundant inherited Ordovician and Cambrian zircon (n =10), strongly suggest a dominant metasedimentary source. The data indicate that VGC was emplaced during the Carboniferous magmatic event that included intraplate- and arc-type magmatism in SW Gondwana (Dahlquist, this volume). We conclude that the VGC was emplaced in an intraplate setting and that this magmatism included both metaluminous and peraluminous A-type granites.