EFFECTS OF INTERACTION BETWEEN ULTRAMAFIC TECTONITE AND MAFIC MAGMA ON ND-PB-SR ISOTOPIC SYSTEMS IN THE NEOPROTEROZOIC CHAYA MASSIF, BAIKAL-MUYA OPHIOLITE BELT

Sm-Nd, Rb-Sr and U-Pb isotopic systems have been studied in minerals a nd whole rocks of harzburgites and mafic cumulates from the Chaya Mass if, Baikal-Muya ophiolite belt, eastern Siberia, in order to determine the relationship between mantle ultramafic and crustal mafic sections . Geological relations in the Chaya Massif indicate that the mafic mag mas were emplaced into, and interacted with older solid peridotite, Ha nd picked, acid-leached, primary rock-forming and accessory minerals ( olivine, orthopyroxene, clinopyroxene and plagioclase) from the two ha rzbugite samples show coherent behavior and yield Sm-147/Nd-144-Nd-143 /Nd-144 and U-238/Pb-204-Pb-206/Pb-204 mineral isochrons, correspondin g to ages of 640 +/- 58 Ma (95% confidence level) and 620 +/- 71 Ma, r espectively. These values are indistinguishable from the crystallizati on age of the Chaya mafic units of 627 +/- 25 Ma (a weighted average o f internal isochron Sm-Nd ages of four mafic cumulates). The Rb-Sr and Sm-Nd isotopic systems in the harzburgite whole-rock samples were dis turbed by hydrothermal alteration. These alteration-related isotopic s hifts mimic the trend of variations in primary isotopic compositions i n the mafic sequence, thus emphasizing that isotopic data for ultramaf ic rocks should be interpreted with great caution. On the basis of ini tial Sr and Nd values, ultramafic and mafic rocks of the Chaya Massif can be divided into two groups: (1) harzburgites and the lower mafic u nit gabbronorites with epsilon(Nd) = +6.6 to +7.1 and epsilon(Sr) = -1 1 to -16; and (2) websterite of the lower unit and gabbronorites of th e upper mafic unit: epsilon(Nd) = +4.6 to +6.1 and epsilon(Sr) = -8 to -9. Initial Pb isotopic ratios are identical in all rocks studied, wi th mean values of Pb-206/Pb-204 = 16.994 +/- 0.023 and Pb-207/Pb-204 = 15.363 +/- 0.015. The similarity of ages and initial isotopic ratios within the first group indicates that the isotopic systems in the pre- existing depleted peridotite were reset by extensive interaction with basaltic magma during formation of the mafic crustal sequence. The iso topic data agree with a hypothesized formation of the Chaya Massif in a suprasubduction-zone environment.