Early Carboniferous mid-alkaline granite, Famatina (NW Argentina): Crustal growth by addition of juvenile material

ALASINO, P.H.; DAHLQUIST, J.A.; PANKHURST, R.J; GALINDO, C.; CASQUET, C.; RAPELA, C.W.; LARROVERE, M. ; FANNING, C.M.

Heidelberg

Congreso; 22nd International Coloquium on Latin America Earth Sciences; 2011

University of Heidelberg

During the Carboniferous 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 scattered plutons with usually sub-circular forms. A newly discovered pluton representative of this magmatism is reported from Cerro La Gloria on the western flank of the Sierra de Famatina. Separated zircons from one sample were dated by U?Pb SHRIMP, 12 analyses yielding a weighted mean 238U/206Pb age of 349 ± 3 Ma (MSWD = 1.1). The pluton 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. The main plutonic facies is a pink to red, porphyritic to inequigranular, medium to coarse grained biotite syenogranite consisting of microcline (43-49%), quartz (32-41%) and plagioclase (Ab88-95) (11-16%) (n=4), with biotite, ferro-edenite, magnetite, ilmenite, epidote, zircon, allanite and sphene as accessory minerals (< 2%). The Cerro La Gloria samples are felsic (SiO2 = 75-82 %). On the alkalis vs. silica classification diagram, they plot dominantly at the silica-rich end of the boundary between the alkaline granite and granite fields. They are intermediate between the midalkaline and subalkaline lineages (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 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 are relatively abundant. All the studied plutonic rocks are plot in the A-type granite field in the discrimination diagram of Whalen et al. (1987). REE patterns are flat or slightly enriched in LREE (LaN/YbN 2.6-20.3) and have very significant negative Eu-anomalies (EuN/Eu*N = 0.01-0.33) suggesting plagioclase fractionation in the magma or retention in the source. Three determined åNdt values range between ‑0.58 and 0.46. In conclusion, the Carboniferous Cerro La Gloria magma was largely derived from juvenile material of mantle character, probably by melting of underplating basaltic material in the lower crust with subsequent fractionation. In comparison with Carboniferous granitoid outcrops to the east (e.g., Dahlquist et al., 2010), the Cerro La Gloria pluton shows a lesser contribution from evolved crustal material, suggesting a probable thinning of the crust in this region.