Early Miocene anatexis identified in the western syntaxis, Pakistan Himalaya

Evidence for typical Himalayan Early to Middle Miocene anatexis has remaine d elusive in the Nanga Parbat massif in the western Himalaya of Pakistan; p revious work has identified only young plutonism (10-1 Ma). New U-(Th)-Pb d ata from the Southern Chichi granite, a leucogranite in southern Nanga Parb at, reveal that crustal melting occurred during the Early Miocene. This lar gely undeformed, fine-grained pluton intrudes the Indian metasedimentary co ver sequence adjacent to the Rupal shear, a major shear zone at Nanga Parba t. Tn-Pb ion microprobe analyses of monazites from the Chichi granite yield ages between 22 Ma and 16 Ma, with the majority of analyses lying at 19-18 Ma. U/Pb zircon analyses yield ages which fall along a chord with a lower intercept age of 19 Ma. The zircons also contain an similar to 1850 Ma inhe rited component. These data indicate that the Early Miocene anatexis that i s ubiquitous in central portions of the Himalayan orogen, unreported anywhe re in the NW Himalaya, also occurred in the western Himalayan syntaxis, and demonstrates that Nanga Parbat has a protracted melting history. A small g ranitic dike that cross-cuts the outer portion of the Rupal shear yields mo nazite ages between 22 Ma and 9 Ma, where the young ages correlate with hig h U concentrations. Ar-40-Ar-39 biotite ages from adjacent gneisses indicat e cooling by 10 Ma, requiring significant displacement on this portion of t he Rupal shear to be older than similar to 10 Ma and possibly as old as sim ilar to 20 Ma. (C) 1999 Elsevier Science B.V. All rights reserved.