Geochemical significance of lithium and boron isotopic heterogeneity evolving during the crystallization of granitic melts

Abstract

We present Li and B isotope data for muscovite, biotite, and feldspar + quartz separated from two-mica granite and biotite granite samples from the Huayang-Wulong granite suite (south Qinling, central China). Our data demonstrate systematic differences in the Li and B isotopic compositions among these minerals. Our results indicate that early-crystallizing minerals have lower δ7Li and δ11B values than the original melt and that residual melts and late magmatic fluids may acquire anomalously high δ7Li and δ11B values. Furthermore, our data imply that (1) late melts and magmatic fluids do not reflect the composition of their source melt, (2) minerals that crystallized over a large segment of magma evolution may be isotopically zoned, and (3) mineral-selective alteration by late magmatic fluids camouflages the source of the fluid, whose δ7Li and δ11B values reflect the isotopic compositions of the altered minerals rather than the composition of the remaining rock.