Highly fractionated Devonian granitoids of the Sierra de Luis, Argentina: sources for Qz-W veins

LÓPEZ DE LUCHI, M. G., ; SIEGESMUND ,S; STEENKEN, A.; WEMMER, K.

Brock University, St. Catharine, Ontario

Congreso; GAC-MAC 2004, Lake to Lake St. Catharine 2004; 2004

Geological Association of Canada-Mineralogicla Association of Canada

>Late stage highly evolved red leucogranites intruded the voluminous zonal elliptical Devonian batholiths of the Sierra de San Luis. They make up circular or elongated stocks and dykes emplaced in a N to NNE fracture and are spatially associated with W rich quartz veins. Monzogranites and alkali-feldspar granites are characterised by sub- and hyper-solvus textures, late magmatic interstitial biotite and interstitial or vein-fluorite. They are geochemically evolved (SiO2=74?78%), metaluminous to mildly peraluminous, poor in Ca, Fe,Mg, Sr, Ba, Zr, Sc, V and Pb, and strongly enriched in incompatible elements such as Li, Rb, Cs, Sn, Nb and Y. Rocks are rich in fluorine and phosphorus; (0·6?1·8 wt%F and 0·4?0·8 wt%P2O5). Contents of Sn and W are relatively high (10?50 ppm Sn, 20?80 ppm W). The REE contents are generally very low. The chondrite-normalized patterns are relatively flat (La/YbN 4-8) having prominent negative Eu anomalies. High degree of magmatic fractionation is demonstrated by low K/Rb and Zr/Hf ratios (90-120, 11?18, respectively), high U/Th ratio (4?6) and the higher concentrations of rare metals. Nb and Ta (50?70 ppm Nb and 30?50 ppm Ta) compared with the host monzogranites (25?50 and 10?25 ppm, respectively). N-NNE fracture system are developed inside the red granites and circular fractures in the host. Breccia textures, miarolitic cavities and hydrothermal alteration of the host granites are recognized. The veins consist mainly of quartz, muscovite, tourmaline, fluorite, apatite, epidote and beryl, with accessory scheelite, wolframite, titanite, pyrite, chalcopyrite, sphalerite, magnetite, hematite, rutile and ilmenite. Tourmaline-beryl Devonian undeformed pegmatoids are locally enriched in wolframite. Concentrations of F, Nb, Y and Rb in the least differentiated red granites similar to those in the surrounding high K calc-alkaline monzogranites suggests that higher concentrations in the red granite are mostly related to special fractionation processes. A F- rich is evidenced by primary fluorite in the granites. The HFSE may preferentially partitioned into this phase. Fluorine also facilitated the strong enrichment of W, Sn, Th, U in the residual melt, by protracting the crystal fractionation due to lowered solidus temperature and melt viscosity. Contemporaneous ages and emplacement style of the granites, undeformed pegmatoids and Qz veins suggest a genetic connection with the highly evolved red granites as the source for the mineralized Qz veins.