LEAD ISOTOPIC COMPOSITIONS OF THE WESTERN DHARWAR CRATON, SOUTHERN INDIA - EVIDENCE FOR DISTINCT MIDDLE ARCHEAN TERRANES IN A LATE ARCHEAN CRATON

Lead and Sr isotopic compositions of whole rocks and feldspar separates from several field-defined suites of rocks from the Western Dharwar craton of southern India elucidate a complex history of the area. The evolution of the craton earlier than almost-equal-to 3.0 Ga is difficult to determine because of thermal and metasomatic overprinting at almost-equal-to 3.0 Ga. Many whole rocks were completely reset then, but feldspar separates preserve some of the earlier history. Apparently, a series of terranes evolved from the mantle over the period > 3.4 to almost-equal-to 3.0 Ga to form the Western Dharwar craton. In each case, metamorphism and anatexis of older terranes accompanied extraction of juvenile material from the mantle. The oldest gneisses (almost-equal-to 3.4 Ga) had radiogenic Pb and Sr isotopic compositions when they formed and were derived from older crustal protoliths that were plausibly accreted to the crust at, or even before, 3.8 Ga. The old gneisses were greatly depleted in Rb, probably at almost-equal-to 3.4 Ga when the protoliths of younger magmatic rocks may have separated from the mantle. Younger gneisses apparently formed from these mafic-intermediate protoliths in the age range of almost-equal-to 3.2 to almost-equal-to 3.0 Ga. The almost-equal-to 3.0 Ga event in the central part of the craton was accompanied by intrusion of small diapiric trondhjemites, formed by melting of older crust, and penecontemporaneous metasomatism of country rocks by hydrous fluids carrying U and other lithophilic elements. The resultant deep crust was greatly depleted in heat-producing elements, and the craton has been inactive since almost-equal-to 3.0 Ga except for production of minor granites from depleted source regions at 2.5-2.6 Ga. The lack of activity at almost-equal-to 2.5 Ga contrasts strongly with extensive magmatism and metamorphism in surrounding areas that had not undergone depletion at almost-equal-to 3.0 Ga. Lower-crustal depletion also has caused present-day reduced heat flow to be very low.