May 2020
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220 Reads
Revista de la Asociacion Geologica Argentina
Petrography and geochemistry of the El Retamo Basanite: new evidence of Triassic magmatism in Sierra de Valle Fértil, province of San Juan. The El Retamo Basanite, located in the northern sector of Sierra de Valle Fértil, is a new evidence of alkaline volcanism related to the Triassic for this region. It is interpreted as the remnant of a lava flow overlying the crystalline basement of the Valle Fértil Complex, topographically elevated by the Andean tectonics. It has an almost aphyric to microporphyritic texture with pheno- and microphenocrysts of olivine, clinopyroxene, minor plagioclase and occasional nepheline set in a trachytic groundmass of similar composition. The different mineral phases show disequilibrium textures related to rapid ascent and/or rapid crystal growth. The rock includes quartz and plagioclase xenocrysts, xenoliths of basement rocks and small mafic enclaves of undetermined origin. Chemically, the rock is classified as basanite or alkaline basalt with normative Ne, Ol and Di. The trace element, including REEs, patterns indicate absence of negative Nb and Ta anomalies similar to OIB-type basalts of an asthenospheric mantle origin. The negative slope between REEs in chondrite-normalized diagrams as well as calculated ratios between trace elements (for example: (La/Yb)N =13,7-16,5; (Tb/Yb)N >2; (Gd/Yb)N >2,5; Ba/Nb ≈12), suggest the presence of residual garnet at the source and little or no crustal contamination. This new outcrop is, so far, the least differentiated in the Triassic alkaline volcanic suite described in the Sierra de Valle Fértil.