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 evo lution of the Andean basement from the Palaeozoic to Mesozoic. In the basem ent (Cambrian to Ordovician high-grade paragneisses, migmatites and orthogn eisses, the Eocambrian Puncoviscana Formation, and Paleozoic-Mesozoic grani toid 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 d ikes and in tourmaline-quartz rocks. In the metasedimentary rocks, no syste matic variation in B content because of metamorphic grade and no correlatio n 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 w as small. Metamorphic tourmalines from the Puncoviscana Formation have delt a B-11 values between -6.3 and -5.8 parts per thousand. The calculated (cor rected for fractionation) delta B-11 values of -6 to -2 parts per thousand for the sedimentary protolith of the metamorphic basement indicate a contin ental B source with subordinate marine input. Tourmalines from Palaeozoic a nd 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 sour ce throughout time. Tourmalines from pegmatites and tourmaline-quartz rocks record the average delta B-11 values of the parental granitic magma. We as sume that B in the Palaeozoic and Mesozoic granitoids is derived from the l ocal metamorphic basement supporting the hypothesis that recycling of the l ower Palaeozoic crust is the dominant process in granitic magma formation f rom Palaeozoic to Mesozoic.