Petrogenetic and tectonic significance of young leucogranites from the northwestern Himalaya, Pakistan

Abstract

In the Himalaya of northwestern Pakistan, small discordant bodies of leucogranite intrude metamorphosed basement of the Indian crust. As the first step in assessing the tectonic significance of these granites, we have used the SHRIMP ion microprobe to obtain U‐Pb dates on zircon. Zircons of igneous appearance are scarce in the leucogranites, and with one exception are associated with xenocrystic components, which occur as discrete grains and as cores rimmed by young zircon. Zircons of igneous appearance are high in U [5000 to 40,000 ppm], low in Th/U [0.01 to 0.05], and frequently colored a distinct blue. Xenocrystic components give a range of Proterozoic ages consistent with ages previously reported for Indian crust. In sharp contrast, zircons of igneous appearance yield clusters of concordant Tertiary ages. Two leucogranite dikes from southern localities yield rather old emplacement ages of about 35 Ma (Swat) and about 50 Ma (Naran). Together with previously determined 40Ar/39Ar and fission track cooling ages, these ages document immediate and rapid metamorphism and denudation following Eocene collision. Three leucogranite dikes from northern localities within the Nanga Parbat‐Haramosh Massif are very young, having emplacement ages of about 2.3 Ma, 5 Ma, and 7 Ma. The formation and emplacement of these leucogranites during the rapid late Tertiary denudation of the Nanga Parbat region suggests to us that decompression melting may be a viable mechanism for leucogranite genesis.