Coesite in eclogite from the North Qaidam Mountains and its implications

Abstract

Coesite provides direct evidence for ultrahigh pressure metamorphism. Although coesite has been found as inclusions in zircon in paragneiss of the north Qaidam Mountains, it has never been identified in eclogite. In this contribution, based on petrographic observations and in situ Raman microprobe spectroscopy, coesite was identified as inclusions in garnet of eclogite from the Aercituoshan, Dulan UHP metamorphic unit, north Qaidam Mountains. Coesite is partly replaced by quartz, showing a palisade texture. This is the first report on coesite in eclogite from the north Qaidam Mountains, and is also supported by garnet-omphacite-phengite geothermobarometry (2.7-3.25 GPa, 670-730℃). Coesite and its pseudomorphs have not been found in eclogites and associated rocks of other units of the north Qaidam Mountains. Further studies are required to confirm if all metamorphic units in the north Qaidam Mountains underwent the ultrahigh-pressure metamorphism.North Qaidam Mountains, Dulan, eclogite, coesite, Ultrahigh pressure metamorphismThe ultrahigh-pressure (UHP) metamorphism refers to the metamorphism above the coesite-quartz transition. Therefore, although a number of mineralogical indicators have been considered as evidence for UHP metamorphism, coesite is still one of the most direct indicators for UHP metamorphism.Over the past ten years, a new HP/UHP metamorphic terrane has been confirmed along the north Qaidam Mountains in western China [1][2][3][4][5][6][7][8][9][10][11][12][13] . It has been correlated to the south Altyn Tagh HP/UHP metamorphic terrane, and constrains a 400 km offset for the Altyn Tagh fault. Therefore, South Altyn-North Qaidam was considered as a single HP-UHP metamorphic belt truncated by a large strike-slip fault, similar to the case of the Dabie-Sulu HP-UHP metamorphic zone which was truncated by the sinistral Tanlu strike-slip fault in eastern China, and split into the Dabie and Sulu regions [14][15][16][17][18][19][20][21] . However, in contrast to Dabie-Sulu in eastern China, where UHP metamorphic evidence has been commonly recognized in different rock types, evidence of UHP metamorphism is relatively rare in the north Qaidam (NQD) and south Altyn Tagh (SAT) [11,22,23] . Up to now, coesite was only reported as inclusions in zircons of amphibolite-facies paragneiss in the Dulan area, eastern NQD. This high-pressure silica polymorph has never been positively identified in eclogite from the NQD and SAT. Although polycrystalline quartz inclusions in garnet and omphacite in the Jianggelesayi eclogite of SAT and Dulan eclogite of NQD were interpreted as pseudomorphs after coesite, the polycrystalline aggregates cannot be simply the result of a transformation from coesite because they contain small amounts of K-feldspar [24,25] .Recently, detailed field work and petrographic observations have led to the first discovery of coesite inclusions in garnet of eclogite in the northern slope of Aercituoshan, southern Dulan HP/UHP metamorphic unit, north Qaidam Mountains.

UHP metamorphic evolution and SHRIMP geochronology of a coesite-bearing meta-ophiolitic gabbro in the North Qaidam, NW China

“…The coesite inclusions reported here represent unequivocal evidence for UHP metamorphism for the North Qaidam eclogite and deep subduction of oceanic lithosphere (Zhang et al, 2008) and continental crustal materials (Song et al, 2005b;Zhang et al, 2008). Recently, well-preserved coesite inclusion has also been recognized in Aercitoushan, North Qaidam (Zhang et al, 2009), which further suggests some eclogites in Dulan have undergone UHP metamorphism definitely.…”

Petrology and geochronology of ultrahigh-pressure granitic gneiss from South Dulan, North Qaidam belt, NW China

“…Studies on different type eclogites in this region (Song et al 2003a;Zhang et al 2009dZhang et al , 2010 show that the peak-P pressure of coesite-bearing continental-type eclogites and the UHPM oceanic crust-mantle sequence is similar 27-34 kbar. Applying Grt-Cpx Fe 2+ -Mg exchange thermometer (Ravna 2000) on garnet mantle and both the enclosed omphacite inclusions and the omphacites in matrix, 682-748°C at 27-34 kbar was obtained for the peak-P stage of the enclosed eclogite (Figure 10d).…”

“…Therefore, detailed metamorphic evolution, geochronology, and geochemistry studies on different rock types in HP-UHP belts can provide vital information for deciphering the formation and evolution of an orogen. The North Qaidam is a Palaeozoic UHP belt in northwest China (Figure 1a, Yang et al 2001Yang et al , 2005Song et al 2003aSong et al , 2004Song et al , 2005aSong et al , 2005bChen et al 2005Chen et al , 2007Chen et al , 2008Chen et al , 2009aChen et al , 2009bChen et al , 2012Zhang et al 2009aZhang et al , 2009bZhang et al , 2009dZhang et al , 2010Liu et al 2012). UHP metamorphic rocks including garnet peridotite, eclogites, and pelitic gneisses occur as blocks or layers within para-and orthogneisses discontinuously from west to east in four places: (1) Yuka, (2) Lüliangshan, (3) Xitieshan, and (4) Dulan ( Figure 1b).…”

Petrology and P–T path of high-pressure granulite from the Dulan area, North Qaidam Mountains, northwestern China

“…Eclogite with peak metamorphic ages between 420 and 490 Ma is distributed over a distance of 350 km, at localities near Yuka, Xitieshan and Dulan. (Yang et al, 2002Song et al, 2003aSong et al, ,b, 2006Zhang et al, 2001Zhang et al, , 2005Zhang et al, , 2006Zhang et al, , 2009bMattinson et al, 2006aMattinson et al, , 2007Zhang et al, 2008aZhang et al, ,b, 2009a. Garnet peridotite from the Luliangshan area (Yang and Deng, 1994) is thought to have been exhumed from mantle depths in excess of 200 km (Song et al, 2004(Song et al, , 2005.…”