Ultrahigh-pressure minerals and metamorphic terranes – The view from China

Abstract

Ultrahigh-pressure (UHP) metamorphism refers to mineralogical modifications of continental and oceanic crustal protoliths ± associated mafic-ultramafic rocks initially formed or emplaced in shallow levels of the lithosphere, but which subsequently have experienced P– T conditions within or above the coesite stability field (>∼2.7 GPa, ∼700 °C). Typical products include eclogite, garnet peridotite, and UHP varieties of metapelite, quartzite, marble, paragneiss, and orthogneiss. UHP metamorphic assemblages require relatively cold lithospheric subduction to mantle depths; some recrystallization even occurs under “forbidden” P– T conditions, characterized by a geotherm of <5 °C/km. In appropriate bulk compositions, UHP metamorphism produces coesite, microdiamond and other indicator phases such as majoritic garnet, TiO 2 with α-PbO 2 structure, supersilicic clinopyroxene, high- P clinoenstatite, K-cymrite and stishovite. Globally, at least 20 coesite-bearing eclogitic, eight diamond-bearing, and five majoritic garnet-bearing UHP regions have been documented thus far; they are mostly of Phanerozoic ages. The presence of majoritic garnet, and even apparent stishovite pseudomorph in supracrustal rocks suggests continental subduction to mantle depths exceeding 300 km; such UHP metamorphic terranes should be distinguished from deep-seated mantle xenoliths that contain UHP minerals. Cold subduction zones may be sites of major recycling of H 2O back into the mantle; high- P experiments on mafic-ultramafic bulk compositions reveal that many important hydrous and formally anhydrous phases are stable under such UHP conditions. The current explosion of research on continental UHP terranes reflects their significance for mantle dynamics and the tectonics of continental subduction, collision, exhumation, mantle–slab interactions, and geochemical recycling. A further characterization of UHP phases and positive identification of UHP minerals requires new experimental studies coupled with state-of-the-art analyses. For example, the very rare occurrence of microdiamond inclusions in zircons from Dabie–Sulu UHP rocks may reflect high f O 2 attending recrystallization inasmuch as epidote is rather common. Rutile needles within garnets from Sulu UHP eclogitic rocks may not be the result of exsolution, so in such cases the apparent UHP pressure may have been over estimated. Hadean igneous microdiamond inclusions in Jack Hills detrital zircons could have originated from mantle xenoliths whereas abundant detrital Phanerozoic diamonds containing inclusions of coesite and other eclogitic minerals from New South Wales might have been derived from unexposed UHP metamorphic terranes. Micro-mineral intergrowth and nano-size minerals may hold important key to deciphering the actual P– T paths of subduction and mantle return flow. Although most exhumed terranes have returned surfaceward relatively rapidly after short time of UHP condition, the long duration of storage at great depth and slow exhumation for the largest UHP terranes remain as major problems.