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Resumen:
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The petrogenetic nature of A-type granites is a controversial problem. The Vinquis batholith in the Sierras Pampeanas of Argentina contains unusual F-rich and strongly peraluminous A-type monzogranites. A new LA-MC-ICP-MS U–Pb zircon crystallization age of 355±7Ma indicates emplacement in latest Devonian or earliest Carboniferous time, overlapping with extensive metaluminous A-type magmatism in the area. The monzogranites have a restricted range of SiO2 content (71.5–74.8 %), they are poor in Ca (0.54–1.4% CaO) and rich in FeOt, with relatively high FeOt/(FeOt+MgO) values ranging from 0.77 to 0.86 (average = 0.80) Both [FeOt/ (FeOt+MgO)] vs. SiO2 and [(Na2O+K2O)?CaO] vs. SiO2 plots indicate ferroan and alkali-calcic signatures typical of A-type granitoids. The samples have MgO/TiO2>1.2 and are moderately enriched in total alkalis (average 8.18%), with high K2O/Na2O values of 1.40–2.24. The granites are strongly peraluminous, with ASI (molar 12O3/[CaO+Na2O+K2O]) values of 1.2 to 1.3. The high P2O5 content (0.23–0.37%) is distinctive and close to values reported for other Paleozoic F-rich peraluminous A-type granites in the Sierras Pampeanas. They have moderate contents of high field strength elements (e.g., Zr, Nb, Th, Y, etc.) and moderately fractionated to flat REE patterns [(La/Yb)N in the range 4.8–19.6] showing significant negative Eu anomalies (Eu/Eu*=0.41). Biotite has a distinctive composition, with relatively high Fe2+/(Fe2++Mg) ratios (0.61–0.74) and high F (0.55–1.42wt.%) content. Together with the whole-rock chemistry this may be useful in identifying strongly peraluminous A-type granites. In addition, the Rb/Sr vs. Th+Zr+Ce diagram may be an appropriate discriminant between metaluminous and peralkaline A-type granites, strongly peraluminous A-type granites and strongly peraluminous orogenic granites. The geochemical evidence indicates that differentiation of the granitic rocks occurred by mineral fractionation from a F-rich peraluminous parental magma, dominant of plagioclase, K-feldspar, biotite, and accessory minerals such as zircon, monazite, xenotime, and oxides. The peraluminous composition and isotope data (?Hf,355 and ?Nd,355 ranging from ?9.5 to ?1.5 and ?6.1 to ?7.8, respectively), together with abundant inherited Ordovician and Cambrian zircon, strongly suggest a dominantly metasedimentary source. The whole-rock and biotite compositions indicate that the Vinquis batholith crystallized under mainly oxidizing conditions, whereas Early Carboniferous metaluminous to weakly peraluminous A-type granites of the Sierras Pampeanas crystallized under dominantly reduced conditions.
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