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

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Morales Cámera, Matías M. and Dahlquist, Juan Andrés and Stipp Basei, Miguel Angelo and Galindo Francisco, Mª del Carmen and Campos Neto, Mario da Costa and Facetti, Nicolás (2017) F-rich strongly peraluminous A-type magmatism in the pre-Andean foreland Sierras Pampeanas, Argentina:Geochemical, geochronological, isotopic constraints and petrogenesis. Lithos, 277 . pp. 210-227. ISSN ISSN: 0024-4937 ; ESSN: 1872-6143

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Official URL: https://www.sciencedirect.com/journal/lithos/vol/277




Abstract

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 (εHf,355 and εNd,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.


Item Type:Article
Uncontrolled Keywords:Strongly peraluminous Carboniferous A-type granites, Geochronological, Geochemistry, Isotopes, Pre-Andean margin, Oxidized A-type granites
Subjects:Sciences > Geology > Geochemistry
Sciences > Geology > Petrology
ID Code:51014
Deposited On:05 Feb 2019 07:36
Last Modified:05 Nov 2019 12:23

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