Скачать или смотреть Field and petrographic constraints on the structural and metamorphic evolution of the.... I. Cawood

Field and petrographic constraints on the structural and metamorphic evolution of the Zanskar Himalaya, Suru Valley, NW India

Cawood, I., Searle, M., Weller, O., St-Onge M., Palin, R., Roberts, N. and Ahmad, T.
The metamorphic rocks of the Greater Himalayan Sequence (GHS) provide a key record of how continental crust accommodates deformation during continental collision. In this study, we focus on the Suru Valley of the western Himalaya, where the GHS is bound to the northeast by the Zanskar Shear zone (ZSZ), and to the southwest by the Main Central Thrust (MCT). This region provides a unique exposure of early widespread lower-temperature, higher-pressure (kyanite-grade) mineral assemblages. This contrasts to other areas of the GHS which equilibrated late in the orogenic cycle, under higher-temperature, lower-pressure (sillimanite-grade) conditions. The aim of the study is to integrate mineral growth and deformation fabric relationships at the macro- to micro-scale, with phase equilibria modelling and geochronology, to quantify the P-T-t-d evolution of the GHS, with the results of the structural analysis the focus of this presentation.

Field and petrographic observations reveal three sets of penetrative planar and/or linear structural elements (S1, S2, S3 and associated L1, L2, L3, and F2). These structural elements are folded by late open NE and SW plunging F4 folds. The dominant S2 fabric comprises a NW- to NE-dipping crenulation cleavage of S1 with locally developed shear structures documenting thrust-sense displacements. F2 are tight to isoclinal, m- to km-scale structures resulting in significant thickening of the GHS. The GHS in the study area displays a complete Barrovian metamorphic sequence from chlorite- to sillimanite-K-feldspar grade. The trace of mapped mineral isograds appear broadly concordant with S2 and are folded by the F4 structures. Peak-pressure and peak-temperature metamorphic minerals display the following relationships to the dominant penetrative fabric. Chlorite is aligned with S2. Biotite is aligned with S2, but at higher grades also overgrows S2. Garnet cores are characterized by an internal fabric comprising spiral or concordant inclusion trails with respect to S2, whereas inclusion-poor garnet rims overgrow S2. Staurolite overgrows S2 and preserves S2 concordant inclusion trails. Kyanite overgrows S2. Sillimanite shows both alignment and misalignment with respect to S2. Taken as a set, these observations suggest that the thermal peak of metamorphism outlasted deformation in the core of the GHS in the Zanskar Himalaya.

Penetrative, top-to-NE, normal shear sense fabrics associated with the ZSZ overprint the compressional fabrics of the GHS. North of Sankoo, the ZSZ is bound by undeformed and deformed Dras Volcanics in the hanging wall and footwall respectively, whereas east of Rangdum the shear zone juxtaposes low-grade metasedimentary rocks of the Tethyan Himalayan Series (THS) in the hanging wall and mixed semipelite, metacarbonate and pelite of the GHS in the footwall.

It is anticipated that forthcoming P-T-t data for peak-pressure and peak-temperature metamorphic assemblages from a broad range of exposed structural levels will help to constrain models for how the continental crust was thickened, metamorphosed, and exhumed in the Zanskar Himalaya, with potential along-strike implications for the evolution of the Himalayan metamorphic core.