High‐pressure granulites: formation, recovery of peak conditions and implications for tectonics

Abstract

High‐pressure granulites are characterised by the key associations garnet‐clinopyroxene‐plagioclase‐quartz (in basic rocks) and kyanite‐K‐feldspar (metapelites and felsic rocks) and are typically orthopyroxene‐free in both basic and felsic bulk compositions. In regional metamorphic areas, two essential varieties exist: a high‐ to ultrahigh‐temperature group and a group representing overprinted eclogites. The high‐ to ultrahigh‐temperature type formerly contained high‐temperature ternary feldspar (now mesoperthite) coexisting with kyanite, is associated with garnet peridotites, and formed at conditions above 900 °C and 1.5 GPa. Clinopyroxene in subordinate basic rocks is Al‐rich and textural evidence points to a high‐pressure–high‐temperature melting history. The second variety contains symplectite‐like or poikilitic clinopyroxene‐plagioclase intergrowths indicating former plagioclase‐free, i.e. eclogite facies assemblages. This type of rock formed at conditions straddling the high‐pressure amphibolite/high‐pressure granulite field at around 700–850 °C, 1.0–1.4 GPa. Importantly, in the majority of high‐pressure granulites, orthopyroxene is secondary and is a product of reactions at pressures lower than the peak recorded pressure. In contrast to low‐ and medium‐pressure granulites, which form at conditions attainable in the mid to lower levels of normal continental crust, high‐pressure granulites (of nonxenolith origin) mostly represent rocks formed as a result of short‐lived tectonic events that led to crustal thickening or subduction of the crust into the mantle. Short times at high‐temperature conditions are reflected in the preservation of prograde zoning in garnet and pyroxene. High‐pressure granulites of both regional types, although rare, are known from both old and young metamorphic terranes (e.g. c. 45 Ma, Namche Barwa, E Himalaya; 400–340 Ma, European Variscides; 1.8 Ga Hengshan, China; 1.9 Ga, Snowbird, Saskatchewan and 2.5 Ga Jianping, China). This spread of ages supports proposals suggesting that thermal and tectonic processes in the lithosphere have not changed significantly since at least the end of the Archean.

Silurian high-pressure granulites from Central Qiangtang, Tibet: Constraints on early Paleozoic collision along the northeastern margin of Gondwana

“…Locally, these symplectites completely replace garnet grains and occur as pseudomorphs after garnets. These symplectic or coronitic textures have been observed in many granulite terranes and are generally regarded to indicate that the rock experienced near-isothermal decompression after peak metamorphism (Guo et al, 2002;Harley, 1989Harley, , 1992Thost et al, 1991;Kumar and Chacko, 1994;Carswell and O'Brien, 1993;O'Brien and Rötzler, 2003;Zhao et al, 2001). Rarely, garnet is also surrounded by orthopyroxene (opx S ) + plagioclase (pl S1 ) + amphibole (amp S1 ) (Fig.…”

“…ab = jd + qtz (Holland, 1980); grt-in and pl-out for quartz tholeiite metabasite composition (Green and Ringwood, 1967); alumino-silicate stability fields (Salje, 1986); fields of metamorphic fades and subfacies (Spear, 1993). crustal thickening (Brown, 1993(Brown, , 2001(Brown, , 2007England and Thompson, 1984;Harley, 1989Harley, , 1992Harley, , 2008O'Brien and Rötzler, 2003;Liu et al, 2009). The high peak P-T conditions implies that protoliths of HP basic granulites experienced burial to depths of ca.…”

“…A near-isothermal decompression path and symplectites or coronas widely distributed in HP basic granulites indicates that the rock experienced a rapid exhumation from the depth of 45-50 km to the middle crustal level of 20-25 km. This process likely occurred during collision along a convergent plate margin, followed by extensional exhumation as the collisional orogen collapsed (O'Brien and Rötzler, 2003;Brown, 2006). The M 3 assemblage probably formed when the HP basic granulites were being exhumed to the middle crustal level of ca.…”

“…Belomorian tectonic province; Mints et al, 2010) to Cenozoic (e.g. Himalayas; Liu and Zhong, 1997), are important indicators of plate convergence and collision following the subduction of oceanic or continental crust (O'Brien and Rötzler, 2003;Ernst and Liou, 2008). These rocks are frequently interpreted to have formed from the earth's deeper lithosphere and are subject to crustal thickening and thinning processes in continental collisional environments (Brown, 2006;Clarke et al, 2000;Harley, 1989;Möller et al, 2000;O'Brien and Rötzler, 2003).…”

Collision zone metamorphism in Vietnam and adjacent South-eastern Asia: Proposition for Trans Vietnam Orogenic Belt

“…6a), which should be a breakdown product from large single -phase mineral. Aug 68-80 ), which is equivalent with clinopyroxene from eclogite -facies and high -P granulite -facies (O'Brien and Rötzler, 2003). Hence Mg -rich garnet, sodic augite and quartz were equilibrated at high -P/ -T conditions, and subsequently these minerals broke down to form orthopyroxene -bearing assemblages during decompression stage as follows:…”