ISOTOPE SYSTEMATICS OF GRANITES AND GNEISSES OF THE NANGA PARBAT MASSSIF, PAKISTAN HIMALAYA

Isotopic analyses of gneisses and anatectic granites from the Nanga Pa rbat-Haramosh Massif (Pakistan Himalaya), a zone of pronounced thermal activity and recent high-grade metamorphism in the Indian Plate, reve al information about the conditions of granite genesis and the nature of Sr exchange in high-grade gneisses and granites. Sr-87/Sr-86 ratios for both gneisses and granites are extremely high and heterogeneous ( 0.7721-1.0642), indicating that both granites and gneisses have an anc ient metasedimentary crustal source. Whole rock Rb-Sr data for gneisse s scatter around a reference isochron with an age of 1.8 Ga. Although the Nanga Parbat leucogranites have similar Sr-87/Sr-86 ratios to the surrounding gneisses, their Nd isotopic compositions ((epsilon Nd = -2 3 to -25) are higher than those of the gneisses and migmatites (epsilo n Nd = -26 to -29), indicating that the granites' source is not the pr esently exposed level of gneisses. This result is consistent with othe r studies that suggest these granites formed as a result of vapor-abse nt melting during the recent, rapid uplift of Nanga Parbat. Biotite an d feldspar mineral separate Rb-Sr data for six gneisses and one granit e have near-horizontal or negative slopes land ages) on a Rb-Sr isotop ic evolution diagram. This behavior is best explained by: (1) a recent (<5 Ma) local homogenization of Sr isotopes during the young metamorp hism; and (2) after peak metamorphism, Sr isotope exchange occurred be tween biotite and carbonate minerals after feldspar became closed to S r exchange. This exchange took place within the last 2 my and was medi ated by metamorphic or magmatic fluids which augmented Sr exchange wit h carbonate/calcsilicate lenses and/or deposited secondary calcite vei ns in the granites and gneisses. This proposed Sr exchange between sil icates and carbonates could have significant implications for the inte rpretation of the seawater Sr curve because it enables carbonate disso lution to contribute radiogenic Sr to the dissolved load in streams.