查看更多>>摘要：The relationship between fluid activity and anatexis in the Himalaya is ambiguous in regard to whether it is dominated by fluid-saturated melting triggered by Lesser Himalayan Sequence (LHS) fluids or dehydration melting of Greater Himalayan Sequence (GHS) metasedimentary rocks. Boron isotopes are a sensitive probe for tracing the source of fluids and their role during partial melting. However, a lack of constraints on LHS fluid activity has limited our understanding of anatectic processes in the Himalayan orogen. Here we report the results of in situ tourmaline and muscovite B isotope analyses of a suite of samples from the LHS and GHS, including chlorite schist from the LHS, quartz-tourmaline rock from the Main Central Thrust Zone, and leucogranites, leucosomes, mica schist, and garnet amphibolite from the GHS. Tourmaline B isotopic compositions fall in the range of delta B-11 = -18.3 parts per thousand to -8.2 parts per thousand, with tourmalines from leucogranites showing delta B-11 values of -17.2 parts per thousand to -8.2 parts per thousand, from leucosomes of -11.8 parts per thousand to -10.7 parts per thousand, from schists of -17.8 parts per thousand to -13.9 parts per thousand, from quartz-tourmaline rock of -18.3 parts per thousand to -12.8 parts per thousand, and from garnet amphibolite of -16.9 parts per thousand to -13.6 parts per thousand. Muscovites from leucogranite and mica schist yield relatively low B isotopes of -23.3 parts per thousand to -17.2 parts per thousand and -22.4 parts per thousand to -15.4 parts per thousand, respectively. Different source compositions, prograde devolatilization, tourmaline fractionation, fluid exsolution, and external fluid metasomatism are inferred to be the main mechanisms responsible for the varied B isotopes in this study. Our data suggest that the LHS is characterized by depleted B-11, with LHS-derived dehydration fluids calculated to have delta B-11 values of -15.2 parts per thousand to -12.0 parts per thousand at 600 degrees C, whereas metamorphic fluids in the GHS have delta B-11 values of -11.8 parts per thousand to -4.8 parts per thousand. Thus, we propose that dehydration of LHS hydrous minerals produced relatively light-B fluids. Consequently, leucogranites derived from hydration melting triggered by LHS fluids exhibit light B isotopic compositions compared with those derived from dehydration melting in the GHS. To investigate the source of heavy B in GHS leucogranites, we modeled B isotopic fractionation during muscovite pmgrade devolatilization, with results showing that substantial fractionation (0 11 13 > 7 parts per thousand) would occur during muscovite dehydration. Hence, B lost from micas could be a source of heavy B in the leucogranites. On the basis of these results, we propose a model whereby LHS hydrous metasediments underwent intense dehydration, with the resultant fluids migrating upward and triggering hydration melting on the edge of melt channel in the GHS. Hence, tourmalines that crystallized from the hydrated magmas inherited the low delta B-11 characteristic of the LHS fluids, whereas those derived from dehydration melting of micas in the interior of melt channel acquired higher delta B-11 values.
NSTL
Elsevier