Secular change of metamorphic features in the Himalayan orogen during the Cenozoic and its tectonic implications

The Himalayan orogen is an archetype of collisional orogens built by the continental collision between India and Asia in the Cenozoic, and it has been a natural laboratory for studying the tectonic evolution of continental collision zones. Various tectonic models have been proposed to interpret the formation and evolution of the Himalayan orogen. However, these models were primarily based on local observations of structural geology and petrology, and some of these models even contradict to each other. The widespread exposure of metamorphic rocks and their metamorphic features are crucial aspects for reconstructing the tectonic evolution of the collisional orogen, as it provides robust records of metamorphic pressure (P) and temperature (T) at different ages. In this study, we present a systematic review on the metamorphic features of crustal rocks from the Western through the Central to the Eastern Himalayas. In doing so, we outline the spatial and temporal changes of metamorphic features (including P-T paths and T/P ratios) at first and then discuss their tectonic implications.In general, the metamorphic evolution of the Himalayan orogen can be categorized into three periods: (1) precollisional period from the Early Jurassic to Paleocene, which is represented by low T/P Alpine type metamorphism forming high-pressure (HP) blueschists in the Western to Central Himalayas; (2) syn-collisional period from the Paleocene to Eocene, which is divided into two stages. The early stage from the Paleocene to early Eocene is characterized by continental deep subduction at low T/P ratios to form Alpine type HP to ultrahighpressure (UHP) eclogites in the Western Himalaya, whereas the late stage from the early to late Eocene is characterized by continental hard collision at medium T/P ratios to form Barrovian type medium-pressure (MP) amphibolite and HP granulite in the Central to Eastern Himalayas; (3) post-collisional period from the Oligocene through Miocene to Pliocene, which is also divided into two stages. The early stage during the Oligocene is represented by continuous metamorphism at medium T/P ratios. Tectonic reactivation for the transition from the moderate to high T/P metamorphism would occur locally in the early stage, but became extensive in the late stage when metamorphic T/P ratios were high enough to cause intensive anatexis in the Miocene, giving rise to Buchan type HT to UHT metamorphic rocks in the Central to Eastern Himalayas.The inspection of different metamorphic facies series throughout the entire Himalayan orogen indicates that the Indian continental crust was collisionally thickened at first and then underwent large-scale reworking through intracontinental extension in the post-collisional period. The deep subduction of oceanic and continental crust is characterized by Alpine type metamorphism at low T/P ratios, the continental hard collision is represented by Barrovian type metamorphism at medium T/P ratios, and the post-collisional reworking is indicated by Buchan type metamorphism at high T/P ratios. The difference in metamorphic T/P ratios is spatiotemporally distinguishable in the Himalayan orogen, documenting the tectonic transition from oceanic subduction through continental subduction and collision to post-collisional rifting along the convergent plate margin. Therefore, the secular change of metamorphic T/P ratios can be used to retrieve the change of geothermal gradients and dynamic regimes from syn-collisional compression to post-collisional extension in the lifetime of the convergent continental margin. As such, the orogenic wedge is a holistic model for the spatiotemporal occurrence of Himalayan sequences cross the collisional orogen, and the failed continental rifting is a viable mechanism for high T/P metamorphism and granitic magmatism in the intracontinental orogen.