Chemical and boron isotope composition of tourmaline from pegmatites and their host rocks, Sierra de San Luis, Argentina

Abstract

We report chemical and B-isotope analyses of tourmaline from Ordovician S-type granites, an aplite, LCT-type (lithiumcesium- tantalum) pegmatites, and metamorphic rocks of the Conlara Metamorphic Complex (CMC) in Sierra de San Luis, Argentina. For comparison, tourmaline from three LCT pegmatites in the adjacent Pringles Metamorphic Complex was also studied. Metamorphic tourmaline from the CMC has intermediate schorl-dravite compositions, with variable Fe# [100∗Fe/(Fe Mg)] from 32 to 79. The d11B values range from -14.8 to -8.9%, which are typical values for continental metasediments and granites, ruling out a marine origin for the tourmalinite protoliths. Tourmaline from the S-type granites and aplite is more homogeneous, with Fe# from 48 to 60. The d11B range (-14 to -9.8%) of granitic tourmaline is within that of the metamorphic tourmaline, supporting the idea of boron recycling in the CMC during partial melting to form the granites. Tourmaline from CMC-hosted pegmatites is compositionally diverse and we distinguished three groups based on Fe#: Group 1: 42 to 50, Group 2: 50 to 62, and Group 3: 62 to 93. Regardless of strong variations in Fe#, tourmaline from all pegmatites in the CMC has d11B values from -10.3 to -7.8%. These values overlap with the range of related granites but are about 2 permil higher, which we attribute to crystallization of 10B-enriched minerals (mica and tourmaline) in the evolved magma from which the pegmatites formed. Pegmatites from the Pringles Metamorphic Complex contain tourmaline with a similar overall range of Fe# (45 to 84) as in the CMC but lower d11B values (-13.2 to -11.2%).