HR: 0800h
AN: V31A-1408    [Abstracts]
TI: Chemical and Mineralogical Patterns Along a Late-Archean Crustal Profile in Southern India: Signatures of a High Temperature Brine?
AU: * Hansen, E C
EM: hansen@hope.edu
AF: Geology Department, Hope College, 35 E. 12th Street, Holland, MI 49423 United States
AU: Harlov, D E
EM: dharlov@gfz-potsdam.de
AF: GeoforschungsZentrum Potsdam, Telegrafenberg, Potsdam, D-14473 Germany
AB: Several features are present in a regional dehydration zone in Tamil Nadu, Southern India that have been attributed to the action of high temperature brines in smaller scale more localized dehydration zoneselsewhere (Harlov and Forster, 2002. J Pet. 43: 769-824). Many of these same features can also be explained by a model in which dehydration is the result of partial melting during high-grade metamorphism. An 8-km thick section of late Archean, mid to lower crust was studied along a 100 km traverse from Krishnagiri to Salem. Intermediate to felsic orthognesises make up the bulk of the terrane. In the northern amphibolite-facies zone these contain hornblende and biotite. First clinopyroxene and then orthopyroxene appear in the central transitional zone and become more abundant southwards as biotite and hornblende abundances decrease. Orthopyroxene is the dominant ferromagnesian silicate in the southern zone and garnet is widespread. Among the features of this traverse that can be explained by the migration of a high temperature brine upwards through the crust are: 1) the presence of K-feldspar grain boundary veins that increase in abundance with increasing metamorphic grade , 2) monazite grains within and on the rim of apatites in orthopyroxene-bearing gneisses, 3) increases in F concentrations in apatite and biotite and increases in the calculated HF-fugacity/H2O-fugacity ratios with increasing metamorphic grade, 4) depletion in whole rock Th and U, 5) depletion in both whole rock Rb and Rb in biotite southwards with increasing grade, 6) relatively high oxygen fugacities (2.5 log units above QFM) in orthopyroxene-bearing rocks, 7) increases in the abundance of hematite-rich hemo-ilmenites southwards with increasing grade, and 8) redistribution of REE from titanite in amphibolite-facies gneisses to apatite in granulite-facies gneisses. Monazite rims on apatite and K-feldspar grain boundary veins are also consistent with a model of metamorphism involving dehydration melting of intermediate igneous rocks. However, the presence of monazite inclusions in apatite and the detailed pattern of trace-element depletion are not as easily explained by this model. The formation of these features by brines acting on a regional scale would require a source of large amounts of these fluids in the lower crust or upper mantle.
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting