Petrologic case for Eocene slab breakoff during the Indo-Asian collision

The Greater Himalayan Sequence is the metamorphic core of the Himalaya and has been a focus of considerable study, yet its petrologic evolution remains controversial. Pre-Oligocene metamorphism was nearly obliterated by Miocene metamorphism and melting, which many workers ascribe to shear heating, unusually high concentrations of radioactive elements accompanying burial, and/or several kilometers of Miocene exhumation due to extensional faulting. Sparse Oligocene to Eocene ages are often assigned to a tectonically unspecified event. An alternative slab-breakoff model (subduction of Greater Himalayan rocks to ∼100 km depth in the Eocene followed by buoyant extrusion due to decoupling of the oceanic lithosphere) has also been proposed based on rare Eocene eclogites, but without explanation of Miocene melting or metamorphic petrogenesis. We argue that slab breakoff readily explains the Eocene eclogites, Miocene partial melts, and late Eocene K-rich magmas in southeastern Tibet, and that metamorphic and plutonic ages help define the timing and rates of breakoff and extrusion. This model implies that (1) much of the Greater Himalayan Sequence was subducted to depths greater than commonly considered, (2) fluid-absent, decompression melting at 30 35 km depth was the consequence of as much as 100 km of extrusion, rather than radioactive heating or a smaller, crustal level extensional or erosional event, and (3) eruption of Eocene, K-rich Tibetan Plateau lavas has no implication for topography of the Tibetan Plateau.