THE QIANGTANG METAMORPHIC BELT IN CENTRAL TIBET: EVIDENCE FOR LARGE-MAGNITUDE TRANSLATION AND UNDERPLATING OF MELANGE BENEATH CONTINENTAL CRUST DURING FLAT-SLAB SUBDUCTION

There is growing evidence that large volumes of sediment and overriding continental crust can be subducted at ocean-continent convergent margins. Less clear is how and to what extent this subducted material is underplated beneath and exhumed within continental crust. The >500-km-long and up to 100-km-wide E-W trending Qiangtang metamorphic belt in central Tibet provides a spectacular natural laboratory for addressing these questions. The belt consists of blueschist-bearing melange, with a matrix of mainly metasedimentary schists, and occurs in the footwall of early Mesozoic domal low-angle normal faults structurally beneath continental margin strata. This setting requires the melange to have underplated continental crust prior to normal faulting. The melange could have been underplated from a nearby suture within the Qiangtang terrane and exhumed in part within a subduction channel, on the basis of reports that the melange separates upper Paleozoic strata of contrasting affinity (Cathaysian in north, Gondwanan in south) and equilibrated at widely variable pressures (e.g., Li and Zheng, 1993, Inter. Geol. Rev.). However, preliminary results of an integrated structural, thermobarometric, and geochronologic study favor a competing model in which Qiangtang melange was sourced from the Jinsha suture ~200 km to the north during flat-slab subduction, and then exhumed by crustal-scale normal faulting (Kapp et al., 2000, Geology). Mafic epidote-bearing blueschists and amphibolites in the melange equilibrated at >10 kbar; the epidote amphibolites occur within an extensional mylonitic shear zone and are interpreted to have equilibrated during mylonitization. Equilibration temperatures range from ~660ºC to ~425ºC and are anomalously high for typical subduction zones. Detrital zircons from Carboniferous sandstones north of the melange, Carboniferous sandstones south of the melange, and metasandstones in the melange, yielded broadly similar LA-ICP-MS U-Pb age spectra (ages range from Archean to Devonian). This challenges the notion that Qiangtang melange separates upper Paleozoic strata of contrasting affinity and raises the possibility that melange metasandstones include Carboniferous continental margin strata that were tectonically eroded and incorporated into the melange during subduction.