UHP METAMORPHISM AND CONTINENTAL SUBDUCTION/COLLISION- THE HIMALAYAN EXAMPLE

More than 20 documented ultrahigh-pressure (UHP) terranes in the world demonstrate that not only is continental crust subducted to depths as great as 150 km, but also that some supracrustal rocks were subsequently exhumed to the earth's surface. UHP terranes are composed of mainly supracrustal rocks and minor blocks, lenses and boudins of mafic eclogite and garnet peridotite. Most quartzofeldspathic units are thoroughly back reacted; only eclogite and garnet peridotite retain scatter UHP phases such as coesite or diamond, indicative of P > 2.5 GPa.. These index minerals are restricted to micron-scale inclusions in chemically and mechanically resistant zircon, garnet, clinopyroxene and a few other strong container minerals, and are difficult to identify by conventional petrologic studies. The continental rocks were subjected to UHP metamorphism at T ranging from ~700 to 950 °C and P > 2.8 to 5.0 GPa, corresponding to depths of ~100 to 150 km. These UHP units were subsequently exhumed to crustal depths and subjected to intense hydration and amphibolite-facies overprint. Widespread Barrovian-type metamorphism in many collisional orogens may have masked an earlier, higher pressure metamorphic history. We suspect that coesite-bearing UHP rocks were once generated in the majority of exhumed collisional orogens. The recent finding of rare Himalayan coesite-bearing eclogite lenses enclosed by kyanite-sillimanite gneisses in both the Kaghan Valley, Pakistan and the Tso-Morari Complex of India is a typical example. We use the Himalayan model to illustrate UHP metamorphism and subduction of continental crustal rocks to mantle depths and the subsequent Barrovian-type overprint during exhumation. Himalayan UHP eclogites and adjacent gneisses were formed at mantle depths > 100 km at 46 to 52 Ma. These rocks were extruded to crustal depths and subjected to Barrovian-zone amphibolite- to granulite-facies metamorphism; recrystallization and associated magmatism occurred at 30 to 15 Ma. The Himalayan metamorphic belt was domally uplifted and the mountain-building process initiated since 11 Ma, when underthrusting of the Indian tectosphere beneath the Lesser Himalayas occurred.