The Andaluca plutonic unit, Vinquis Intrusive Complex, Argentina: An assessment of mantle role in the genesis of Early Carboniferous weakly peraluminous A-type granites in the pre-Andean SW Gondwana margin

Abstract

During the Early Carboniferous, significant intracontinental magmatism was developed in the retro-arc region of the SW margin of Gondwana between ca. 27° and 31°S. This magmatism consisted of metaluminous to weakly peraluminous A-type granites, strongly peraluminous A-type granites (A- to S- hybrid granitoids) and, in a lesser extent, (sub-)volcanic felsic (rhyolites and trachytes) and mafic (alkaline basalts and lamprophyres) rocks. The Vinquis Intrusive Complex (VIC) registers these two compositional varieties of A-type granites: the Vinquis Unit (VU) represents the strongly peraluminous granites and the Andaluca Unit (AU) the weakly peraluminous granites. The adjacent Zapata Intrusive Complex (ZIC) also contains metaluminous to weakly peraluminous A-type granites. The AU is a semi-elliptical plutonic body of approximately 60 km 2 located in the southwestern part of the VIC, intruding the Vinquis Unit. The AU comprises three dominant rock types: i) quartz monzonite, ii) syenogranite and iii) alkali feldspar granite that can be classified as F-rich (calculated F content: 1950–5700 ppm), ferroan (FeO t /(FeO t + MgO) = 0.86–0.95) and weakly peraluminous (ASI = 1.03–1.10) A-type (Zr + Nb + Ce + Y = 364–570 ppm; Ga = 20–23 ppm) granitoids (SiO 2 wt% = 67.8–75.4% wt%). This unit was emplaced in early Carboniferous time (U Pb zircon crystallization ages of 346 ± 3 Ma and 342 ± 3 Ma). The AU has 87 Sr/ 86 Sr t , εNd t and εHf t values ranging from 0.7092 to 0.7140, −2.6 to −3.6 and − 16.5 to −3.3, respectively. These isotopic suggest that the parental magma of AU dominantly involved variable mixtures of mantle-derived components (magmas/fluids) and peraluminous continental crust. The extremely felsic granites of the AU (SiO 2 > ~72%) with a strongly fractionated composition (Rb > 336 and Sr < 70 ppm and Eu/Eu* < 0.22) supports extensive fractional crystallization. The high contents of fluorine and HFSE in AU could be explained as derived from an F- HFSE- and alkali-rich parental mafic magma plus contributions from F-bearing minerals assimilated from the country rock. Likewise, the magmatic differentiation processes from these reduced mafic magmas could explain the ferroan character of the AU granitoids. We envision three stages in the origin of these rocks: (1) fluids and magmas from the asthenospheric mantle metasomatize and melt the subcontinental lithospheric mantle (SCLM), the alkali-rich mafic melts produced are emplaced at the base of the crust, transfer heat to the crust and melt it, generating strongly peraluminous A-type magmas; (2) continued introduction of alkali-rich mafic magmas into the deep crust produce extensive assimilation of the preheated crust in equilibrium with mafic cumulates (generated by reaction and/or fractionation); (3) the hot magmas so produced migrate into the middle-upper crust to produce ferroan weakly peraluminous A-type granitoids. • The Andaluca plutonic unit of the Vinquis Intrusive Complex crystallized between 346 ± 3 Ma and 342 ± 3 Ma. • Andaluca Unit shows a F-rich weakly peraluminous ferroan A 2 -type affinity. • The high contents of fluorine and HFSE in Andaluca Unit can be explained from an F-rich parental alkaline mafic magma. • The Andaluca Unit granitoids can be generated through combined crustal assimilation and fractional crystallization of magmas derived from the metasomatized subcontinental lithospheric mantle.