Early Carboniferous mid-alkaline granite, Sierra de Famatina (NW Argentina): a crustal growth by addition of juvenile material

ALASINO, PABLO H.; DAHLQUIST, JUAN A.; PANKHURST, ROBERT J.; GALINDO, CARMEN; CASQUET, C¨¦SAR; RAPELA, CARLOS W.; LARROVERE, MARIANO; FANNING, C.M.

Heidelberg

Conferencia; 22° International Lateinamerika-Kolloquium; 2011

In the Carboniferous times a minor but widespread type of mid-alkaline, mainly granitic, magmatism occurred along more than 1000 km in the Sierras Pampeanas of NW Argentina. It is represented by small and scattered plutons and usually sub-circular forms. A newly discovered intrusion representative of the Carboniferous magmatism is report here, the Cerro La Gloria pluton (CLGP) on the western flank of Famatina. Following standard practice for young ages dated by SHRIMP, we calculated 238U/206Pb ages after correcting for common Pb on the basis of the measured 207Pb of the FAM177 sample of CLG pluton. The 12 ages range give a well-defined mean of 349 ± 3 Ma (MSWD = 1.1). The CLGP is circular with a radio of 5 km and an area of ca. 80 km2. Host rocks are Early Ordovician plutonic rocks. Contacts are sharp and irregular on the local scale. Its main plutonic facies is a pink to red porphyritic to inequigranular medium to coarse grained biotite syenogranite and an assemblage consist of microcline (43-49%), quartz (32-41%) and plagioclase (Ab88-95) (11-16%) (n=4) and accessories less than 2 modal % (biotite, ferro-edenite, magnetite, ilmenite, epidote, zircon, allanite and sphene). Sometimes cm-size mafic microgranular enclaves are found that show ovoid forms. They show similar mineralogy that its host although some have showy olivine crystals. The CLG samples are felsic or evolved in composition (SiO2 = 75-82 %). On the alkalis vs. silica classification diagram, the rocks plot dominantly at the silica-rich end on the limit field of alkaline granite and granite. They are located between the midalkaline and subalkaline lineages defined by sigma index equal to 2.5 (after Rittmann 1957). The granitic rocks are metaluminous to slightly peraluminous (ASI=0.96-1.05) although they have a relatively high agpaitic index (AI=0.90-0.98). They also are poor in CaO (0.3-0.8 %) and extremely rich in FeOt relative to MgO (FeOt/(FeOt+MgO=0.9-1.0). The CLG granitoids are enriched in TiO2 relative to MgO (where TiO2/MgO>1) and they are moderately enriched in total alkalis (8.0-8.7 %), with relatively high K2O content (K2O/Na2O=1.1-1.4). High Field Strength Elements of the CLG pluton such as Y, Nb, Ga, Ta and Th are relatively high and they are comparable to others Carboniferous A-type granites of the proto-Andean foreland in Argentina (e.g. Dahlquist et al. 2010 and reference therein). All studied plutonic rocks exceed the limit values of Ce+Nb+Zr+Y and 10000*Ga/Al (Whalen et al., 1987), they are plot in the A-type granites field on the discrimination diagram. Their REE patterns are flat or slightly enriched in LREE with values of LaN/YbN from 2.6 to 20.3 and significant negative Eu-anomalies (EuN/Eu*N = 0.01-0.33) suggesting plagioclase fractionation in the magma or retention in the source. Ranges of εNdt values are between -0.58 and 0.46. Nd model ages calculated according to DePaolo et al. (1991) are between 1.07 and 1.16 Ga, i.e., Proterozoic. In conclusion, the CLG pluton reveals a high participation of juvenile material of mantle character, probably by melting of underplating basaltic material in the lower crust with subsequent fractionation process. In comparison with those Carboniferous granitoids that outcrops to East (i.e., Sierras de Fiambalá, Velasco and Zapata, Dahlquist et al., 2010 and reference therein), the CLGP shows lower contribution from evolved crustal material suggesting a probable thinning of the crust in this region.