Isotope systematics of granites and gneisses of the Nanga Parbat Massif, Pakistan Himalaya

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Geological Sciences

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Isotopic analyses of gneisses and anatectic granites from the Nanga Parbat-Haramosh Massif (Pakistan Himalaya), a zone of pronounced thermal activity and recent high-grade metamorphism in the Indian Plate, reveal information about the conditions of granite genesis and the nature of Sr exchange in high-grade gneisses and granites. 87Sr/86Sr ratios for both gneisses and granites are extremely high and heterogeneous (0.7721-1.0642), indicating that both granites and gneisses have an ancient metasedimentary crustal source. Whole rock Rb-Sr data for gneisses scatter around a reference isochron with an age of 1.8 Ga. Although the Nanga Parbat leucogranites have similar 87Sr/86Sr ratios to the surrounding gneisses, their Nd isotopic compositions (εNd = -23 to -25) are higher than those of the gneisses and migmatites (εNd = -26 to -29), indicating that the granites' source is not the presently exposed level of gneisses. This result is consistent with other studies that suggest these granites formed as a result of vapor-absent melting during the recent, rapid uplift of Nanga Parbat.

Biotite and feldspar mineral separate Rb-Sr data for six gneisses and one granite have near-horizontal or negative slopes (and ages) on a Rb-Sr isotopic evolution diagram. This behavior is best explained by: (1) a recent ( <5 Ma) local homogenization of Sr isotopes during the young metamorphism; and (2) after peak metamorphism, Sr isotope exchange occurred between biotite and carbonate minerals after feldspar became closed to Sr exchange. This exchange took place within the last 2 my and was mediated by metamorphic or magmatic fluids which augmented Sr exchange with carbonate/calcsilicate lenses and/or deposited secondary calcite veins in the granites and gneisses. This proposed Sr exchange between silicates and carbonates could have significant implications for the interpretation of the seawater Sr curve because it enables carbonate dissolution to contribute radiogenic Sr to the dissolved load in streams.


This article was originally published in American Journal of Science. The full-text article from the publisher can be found here.

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American Journal of Science


© 1998 American Journal of Science