Timing of Himalayan ultrahigh‐pressure metamorphism: sinking rate and subduction angle of the Indian continental crust beneath Asia

Abstract

Coesite relics were discovered as inclusions in clinopyroxene in eclogite and as inclusions in zircon in felsic and pelitic gneisses from Higher Himalayan Crystalline rocks in the upper Kaghan Valley, northwest Himalaya. The metamorphic peak conditions of the coesite-bearing eclogites are estimated to be 27-32 kbar and 700-770°C, using garnet-pyroxene-phengite geobarometry and garnet-pyroxene geothermometry, respectively. Cathodoluminescence (CL) and backscattered electron (BSE) imaging distinguished three different domains in zircon: inner detrital core, widely spaced euhedral oscillatory zones, and thin, broadly zoned outermost rims. Each zircon domain contains a characteristic suite of micrometre-sized mineral inclusions which were identified by in situ laser Raman microspectroscopy. Core and mantle domains contain quartz, apatite, plagioclase, muscovite and rutile. In contrast, the rim domains contain coesite and minor muscovite. Quartz inclusions were identified in all coesite-bearing zircon grains, but not coexisting with coesite in the same growth domain (rim domain). 206 Pb ⁄ 238 U zircon ages reveal that the quartz-bearing mantle domains and the coesite-bearing rim were formed at c. 50 Ma and 46.2 ± 0.7 Ma, respectively. These facts demonstrate that the continental materials were buried to 100 km within 7-9 Myr after initiation of the India-Asia collision (palaeomagnetic data from the Indian oceanic floor supports an initial India-Asia contact at 55-53 Ma). Combination of the sinking rate of 1.1-1.4 cm year )1 with Indian plate velocity of 4.5 cm year )1 suggests that the Indian continent subducted to about 100 km depth at an average subduction angle of 14-19°.

Detrital zircon U-Pb-He double dating: A method of quantifying long- and short-term exhumation rates in collisional orogens

“…The range of the U/Pb ages indicates a protracted episode as long as 25 Myr for regional metamorphism of the host rocks at deep crust, i.e., a process from continental collision to postcollisional reworking. The upper U/Pb age limit yielded by the zircon rims in this study is consistent with the U/Pb ages of 44±3 Ma for the rim of zircon with a Permian core, a garnet-omphacite Sm-Nd isochron age of 49 ± 6 Ma, a phengite Rb-Sr isochron age of 43 ± 1 Ma for the Kaghan eclogite in Pakistan (Tonarini et al, 1993) as well as a U/Pb age of 50 Ma for overgrown quartz-bearing rims and a U/Pb age of 46 Ma for coesite-bearing rim (Kaneko et al, 2003;Parrish et al, 2006). The long episode of 40-50 Myr is common for high-pressure (HP) to ultrahigh-pressure (UHP) metamorphism in continent-continent collisional orogens such as the Dabei-Sulu in China and the Western Gneiss Region in Norway, whereas the short episode of 5-10 Myr is prominent in arc-continent collisional orogens such as the Himalayas and Alps (Zheng, 2012;Zheng et al, 2013).…”

Yin and yang of continental crust creation and destruction by plate tectonic processes

“…P-wave tomography identifies high-velocity zones beneath the Himalayan collision zone that can be traced to >1000 km deep in the mantle; these must be stilldescending slabs of subducted Tethys oceanic and Indian continental lithosphere (Van der Voo et al 1999). We also have direct evidence that at least some Indian continental crust is deeply subducted, because ultra-high pressure (UHP) metamorphic terranes -which contain metamorphic coesite, diamond, unusual Si-rich garnets, and/or K-bearing pyroxenes -are known from near Nanga Parbat and Tso Morai in the Himalayas (O'Brien et al 2001;Kaneko et al 2003;Leech et al 2005). UHP minerals demonstrate that these continental fragments descended to at least 90-140 km depth (Ernst 2006).…”

Determination of SiO2 Raman spectrum indicating the transformation from coesite to quartz in Gföhl migmatitic gneisses in the Moldanubian Zone, Czech Republic

“…Determination of SiO 2 Raman spectrum indicating the transformation inclusions were detected from the core, although all the SiO 2 inclusions in the rim were composed of quartz. This observation is in contrast to the normal observation in the other UHP terranes, including the Kaghan Valley in the Pakistan Himalaya (Kaneko et al, 2003). This issue is beyond the scope of this paper, and it will be discussed elsewhere.…”