The Intragranitic Rare-Element Pegmatites from the Punilla District, Pampean Pegmatite Province, Argentina: LA-ICP-MS U-Pb CGM Ages and Petrogenetic Affiliation

Abstract

Columbite-group minerals from four beryl-columbite-phosphate subtype intragranitic pegmatites from the Punilla Pegmatite District, emplaced in the Achala monzogranite batholith, have been geochemically and isotopically investigated. 206Pb/238U ages are between 329 and 389 Ma and grouped in two clusters: 344–354 Ma in the eastern part and ca. 378 Ma in the westernmost locality.The columbite-group minerals compositions are Nb and Fe rich, mainly columbite-(Fe) with some columbite-(Mn), generally showing Ta# , 0.31 and Mn# 0.55. The trace element normalized patterns show a depletion of Li, Sc, Sn, and Sb and a primary enrichment in Y, Yb, Ti, Zr, W, Pb, and U. The REEN contents show similar trends for most of the analyzed columbite-group minerals,with LREEN , MREEN , HREEN patterns of type 1 with a significant to accentuated negative Eu-anomaly. The major and trace element composition of the columbite-group minerals, added to the F-rich chemistry of biotite and magmatic muscovite of the Achala monzogranite and its metallogenesis, show a definitive Nb-Y-F (NYF) signature to the Punilla Pegmatite District pegmatites with exceptionally local occurrence of accessory Li minerals in one case. Their formation is considered to be the result of inward crystallization of residual melts emplaced in the hosting parental granite. The Achala monzogranite can be interpreted as an A2 type granite based on its Nb-Ta contents. The diachronism between the ages of hosting monzogranites, some of them considered high heat production granites, and the eastern pegmatites of Punilla Pegmatite District is attributed to the fact that the 206Pb/238U zircon dates of the former are ages of antecrysts that mainly crystallized in an underlying deep magma chamber.