F-rich strongly peraluminous A-type magmatism in the pre-Andean foreland Sierras Pampeanas, Argentina: Geochemical, geochronological, isotopic constraints and petrogenesis

MORALES CÁMERA, M.M.; DAHLQUIST, J.A.; BASEI, M.; GALINDO, C.; CAMPOS, M.; FACETTI, NICOLAS

LITHOS

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2017 vol. 277 p. 210 - 227

0024-4937

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±7 Ma indicates emplacement in latest Devonian or earliest Carboniferous time, overlappingwith extensivemetaluminous A-typemagmatismin 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 N 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 Al2O3/[CaO + Na2O + K2O]) values of 1.2 to 1.3. The high P2O5 content (0.23?0.37%) is distinctive and close to valuesreported 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?1.42 wt.%) 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 betweenmetaluminous andperalkaline 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 fractionationfrom 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 (εHf,355 and εNd,355 ranging from−9.5 to−1.5 and−6.1 to−7.8, respectively), togetherwith abundant inherited Ordovician and Cambrian zircon, strongly suggest a dominantlymetasedimentary 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.