Geochemistry and metamorphism of metabasites, and spatial variation of P-T paths across the Bhutan Himalaya: Implications for the exhumation of the Greater Himalayan Sequence.

摘要

This study was designed to investigate the provenance and metamorphic history of metabasites and metasediments in the GHS of western Bhutan, with the aim of improving our understanding of the tectonic evolution of this poorly known part of the Himalaya. In so doing, a secondary aim was to use the results of this study to test the applicability of the channel flow-extrusion models as viable hypotheses for the tectonic evolution of this segment of the orogen.; Metabasites found in the high- and 'mid'-structural levels of the GHS are within-plate basalts that were intruded into the distal parts of the North Indian margin during a period of Paleo-Mesoproterozoic magmatism, long before the onset of Himalayan orogenesis. Temperature estimates made on the metabasites and the encompassing metasediments reveals that peak-T conditions decrease with structural position from ∼ 900°C immediately below the STD down to ∼ 650-700°C proximal to the MCT zone. Associated pressures show a slight increase ranging from 9 kbar at the STD to 11 kbar at the MCT, consistent with observations across the orogen. U-Pb ages on zircon rims from the 'high' and 'mid'-structural levels suggest peak-T conditions in the GHS persisted to at least 21 Ma. Amphibole and biotite argon ages imply cooling through 500°C and 350°C by 15 Ma and 13 Ma respectively. The associated P-T-t paths vary with structural position and display marked similarities with paths predicted by thermomechanical model HT111 of Beaumont et al. (2004). The similarity between model results and the data suggests exhumation of the GHS as two hot-channels or domes that have been superimposed upon one another. The absence of a marked time gap in the geochronological data suggests extrusion and exhumation of the domes did not occur in pulses but was a continuous process. At the base of the GHS, the Jaishidanda Formation comprises protomylonities and corresponds with a zone of intense top-to-the-south shearing, probably correlating with the MCT zone. Isotopic data and the intercalation of rocks of varying metamorphic grade reveals that the zone of intense penetrative shear developed over a metamorphic discontinuity between GHS material that was subjected to high-T for protracted periods (homogeneous garnets) versus rocks that were not. Thus, the MCT zone in south-western Bhutan has developed in rocks entirely of GHS affinity implying that not everywhere in the Himalaya does the MCT zone correlate with a protolith boundary between the LHS and GHS.