Química mineral de los granitos Sanagasta, Huaco y La Chinchilla, Sierra de Velasco, La Rioja, Argentina

GROSSE, P., ROSSI, J., SARDI, F. & TOSELLI, A.

Buenos Aires, Argentina

Congreso; VIII Congreso de Mineralogía y Metalogenia; 2006

Microprobe chemical analyses of feldspars, micas and apatites from the Sanagasta (GS), Huaco (GH) and La Chinchilla (SLC) granites of the Sierra de Velasco, La Rioja province, have been carried out. K-feldspar compositions are mostly Or88-93 in all three granites and no chemical difference is found to account for their pink color in the GS. This coloration could be due to higher oxygen fugacity conditions during the evolution of the GS, consistent with the presence of magnetite. Plagioclase is oligoclase in the GS (An18-24) and the GH (An10-23), and almost pure albite in the SLC (An1-2). Biotites from the GS and the GH are Fe-biotite and siderophyllite. They are characterized by a high Fe/(Fe+Mg) ratio (0.76-0.82), typical of felsic granites, and in discrimination diagrams they plot alternatively in the peraluminous and calc-alkaline fields. Biotites from the GS are slightly poorer in Al and Na and richer in Si than biotites from the GH. The mica of the SLC is zinnwaldite, rich in Li, and has a very high Fe/(Fe+Mg) ratio (0.94-0.97). Apatites from the GH have higher contents in Mn, Na, Fe and F, and lower contents in Si and LREE than those from the GS, similar to the differences shown by apatites from S- and I-type granites, respectively, although both have compositions comparable to those of felsic granites, either S- or I-type. The mineral chemistry of the GS and the GH suggest that both are evolved granites. The GH has mineral compositions comparable to those of felsic S-type granites while the GS shows characteristics intermediate between I- and S-type granites. The SLC is a highly fractionated granite similar to some A-type granites and probably originated from a water- and volatile-rich melt.