The Chemical and B-Isotope Composition of Tourmaline from Intra-Granitic Pegmatites in the Las Chacras-Potrerillos Batholith, Argentina

Abstract

ABSTRACT The Devonian Las Chacras-Potrerillos batholith comprises six nested monzonitic to granitic intrusions with metaluminous to weakly peraluminous composition and a Sr-Nd isotopic signature indicating a dominantly juvenile mantle-derived source. The chemically most evolved units in the southern batholith contain a large number of intra-granitic, pod-shaped tourmaline-bearing pegmatites. This study uses in situ chemical and boron isotopic analyses of tourmaline from nine of these pegmatites to discuss their relationship to the respective host intrusions and the implications of their B-isotope composition for the source and evolution of the magmas. The tourmalines reveal a diversity in element composition (e.g., FeO, MgO, TiO2, CaO, MnO, F) which distinguishes individual pegmatites from one another. However, all have a narrow δ11B range of –13.7 to –10.5‰ (n = 100) which indicates a relatively uniform magmatic system and similar temperature conditions during tourmaline crystallization. The average δ11B value of –11.7‰ is typical for S-type granites and is within the range reported for peraluminous granites, pegmatites, and metamorphic units of the Ordovician basement into which the Las Chacras-Potrerillos batholith intruded. The B-isotope evidence argues for a crustal boron source like that of the Ordovician basement, in contrast to the metaluminous to weakly peraluminous composition and juvenile initial Sr and Nd isotope ratios of the Las Chacras-Potrerillos batholith magmas. We propose that the boron was not derived from the magma source region but was incorporated from dehydration melting of clastic metasedimentary rocks higher up in the crustal column.