Boron recycling in the continental crust of the central Andes from the Palaeozoic to Mesozoic, NW Argentina

Whole-rock chemical composition and B-11/B-10 isotope ratios in tourmaline was investigated to study the geochemical recycling of boron during the evolution of the Andean basement from the Palaeozoic to Mesozoic. In the basement (Cambrian to Ordovician high-grade paragneisses, migmatites and orthogneisses, the Eocambrian Puncoviscana Formation, and Paleozoic-Mesozoic granitoid igneous rocks) whole-rock B contents are generally below 100 ppm, but B contents of similar to1 wt% are found in cogenetic aplite and pegmatite dikes and in tourmaline-quartz rocks. In the metasedimentary rocks, no systematic variation in B content because of metamorphic grade and no correlation of B with other incompatible elements are apparent. Tourmalines from the high-grade metamorphic basement yield delta B-11 values ranging from -11.2 to -6.8 parts per thousand and isotope fractionation during migmatisation was small. Metamorphic tourmalines from the Puncoviscana Formation have delta B-11 values between -6.3 and -5.8 parts per thousand. The calculated (corrected for fractionation) delta B-11 values of -6 to -2 parts per thousand for the sedimentary protolith of the metamorphic basement indicate a continental B source with subordinate marine input. Tourmalines from Palaeozoic and Mesozoic granitoids display an identical range of delta B-11 values from -12 to -5.3 parts per thousand and indicate a similarly homogeneous B source throughout time. Tourmalines from pegmatites and tourmaline-quartz rocks record the average delta B-11 values of the parental granitic magma. We assume that B in the Palaeozoic and Mesozoic granitoids is derived from the local metamorphic basement supporting the hypothesis that recycling of the lower Palaeozoic crust is the dominant process in granitic magma formation from Palaeozoic to Mesozoic.