Tectonic discontinuity, partial melting and exhumation in the Garhwal Himalaya (Northwest India): Constrains from spatial and temporal pressure-temperature conditions along the Bhagirathi valley

Abstract

The pattern and timing of tectonic discontinuity and partial melting in the Garhwal Himalaya, resulting from the India–Asia collision, are crucial factors for understanding the evolution of the Himalayan orogen, but these parameters remain poorly constrained. Middle- to high-grade metamorphic rocks from the Main Central Thrust (MCT) zone and the High Himalaya Crystalline Sequence (HHCS) along the Bhagirathi valley in northwest India have been investigated in this study. The primary mineral assemblage of the gneisses and migmatites in the HHCS is garnet + biotite + white mica + quartz + kyanite + rutile ± ilmenite ± sillimanite. The metamorphic grade at thermal peak rapidly increases upwards across the MCT from garnet schists (515–550 °C, 8–9 kbar) in the footwall to garnet-bearing migmatite (705–735 °C, 10–13 kbar) in the hanging wall. The P–T condition decreased upwards in kyanite gneiss of the lower HHCS (690 °C, 8.3 kbar), yielding a high field pressure gradient near the MCT. The prominent metamorphic discontinuity could have been caused by the southward extrusion of the HHCS along with the MCT. The base of the HHCS experienced high-grade metamorphism at 38–25 Ma, mainly at 31.9 ± 0.5 Ma. The new data, combined with previous studies, suggest that the prominent metamorphic discontinuity is characterized by the different periods of peak metamorphism and the overprint in the hanging wall (Eocene–Oligocene metamorphism and Early Miocene overprint) and footwall (Early Miocene metamorphism and Late Miocene to Pliocene overprint) of the MCT. The second metamorphic discontinuity occurred near the migmatite zone in the lower-middle HHCS, which experienced the anatexis (690–740 °C, 7–9 kbar) at 37–25 Ma, mainly at 33.9 ± 1.2 Ma. The Bhagirathi HHCS experienced a regional Barrovian-type Eohimalayan metamorphism associated with fluid-present melting of muscovite + plagioclase + quartz + H2O → melt ± kyanite at 34–32 Ma without a strong Miocene overprint. The period of partial melting is significantly older than the intrusion of ca. 22 Ma Early Miocene Gangotri leucogranites at the top of the HHCS. Protracted periods of crystallization (38–22 Ma) indicate a long duration of more than ca. 15 Myr of anatexis, from initiation of migmatization to pluton formation in the Bhagirathi HHCS. The protracted fluid-present melting and subsequent rapid exhumation in the Bhagirathi HHCS could be associated with heterogeneous extrusion forming the tectonic discontinuity in the lower-middle HHCS. The metamorphic grade decreased towards the upper HHCS (620 °C, 6.5 kbar) across the Jhala Normal Fault where occurs in the right-way-up sequence. On the other hand, in the central Himalaya, the thrusting occurred between upper HHCS and lower-middle HHCS in the inverted metamorphic sequence. The Early Miocene migmatites in the footwall of the significant tectonic discontinuity are dominant, and diachronous extrusion accompanied with the fluid-absent melting reactions is the common mechanism to exhume the HHCS, unlike the heterogeneous extrusion in the northwest Himalaya.