Early Miocene anatexis identified in the western syntaxis, Pakistan Himalaya

摘要

Evidence for typical Himalayan Early to Middle Miocene anatexis has remained elusive in the Nanga Parbat massif in the western Himalaya of Pakistan; previous work has identified only young plutonism (10-1 Ma). New U-(Th)-Pb data from the Southern Chichi granite, a leucogranite in southern Nanga Parbat, reveal that crustal melting occurred during the Early Miocene. This largely undeformed, fine-grained pluton intrudes the Indian metasedimentary cover sequence adjacent to the Rupal shear, a major shear zone at Nanga Parbat. Th-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 anlayses yield ages which all along a chord with a lower intercept age of 19 Ma. The zircons also contain an approx 1850 Ma inherited component. These data indicate that the Early Miocene anatexis that is ubiquitous in central portions of the Himalayan orogen, unreported anywhere in the NW Himalaya, also occurred in the western Himalayan syntaxis, and demonstrates that Nanga Parbat has a protracted melting history. A small granitic dike that cross-cuts the outer portion of the Rupal shear yields monazite ages between 22 Ma and 9 Ma, where the young ages correlate with high U concentrations. ~(40)Ar-~(39)Ar biotite ages from adjacent gneisses indicate cooling by 10 Ma, requiring significant displacement on this portion of the Rupal shear to be older than approx 10 Ma and possibly as old as approx 20 Ma.