Ultra high temperature metamorphism of mafic granulites from South Altyn Orogen, West China: A result from the rapid exhumation of deeply subducted continental crust

Abstract

The South Altyn orogen in West China contains ultra high pressure (UHP) terranes formed by ultra-deep (>150-300 km) subduction of continental crust. Mafic granulites which together with ultramafic interlayers occur as blocks in massive felsic granulites in the Bashiwake UHP terrane, are mainly composed of garnet, clinopyroxene, plagioclase, amphibole, rutile/ilmenite, and quartz with or without kyanite and sapphirine. The kyanite/sapphirine-bearing granulites are interpreted to have experienced decompression-dominated evolution from eclogite facies conditions with peak pressures of 4-7 GPa to high pressure (HP)-ultra high temperature (UHT) granulite facies conditions and further to low pressure (LP)-UHT facies conditions based on petrographic observations, phase equilibria modelling, and thermobarometry. The HP-UHT granulite facies conditions are constrained to be 2.3-1.6 GPa/1,000-1,070°C based on the observed mineral assemblages of garnet+clinopyroxene+rutile+plagioclase+amphibole±quartz and measured mineral compositions including the core-rim increasing anorthite in plagioclase (X An = 0.52-0.58), core-rim decreasing jadeite in clinopyroxene (X Jd = 0.20-0.15), and TiO 2 in amphibole (Ti M2 /2 = 0.14-0.18). The LP-UHT granulite facies conditions are identified from the symplectites of sapphirine+plagio-clase+spinel, formed by the metastable reaction between garnet and kyanite at <0.6-0.7 GPa/940-1,030°C based on the calculated stability of the symplectite assemblages and sapphirine-spinel thermometer results. The common granulites without kyanite/sapphirine are identified to record a similar decompression evolution, including eclogite, HP-UHT granulite, and LP-UHT granulite facies conditions, and a subsequent isobaric cooling stage. The decompression under HP-UHT granulite facies is estimated to be from 2.3 to 1.3 GPa at~1,040°C on the basis of textural records, anorthite content in plagioclase (X An = 0.25-0.32), and grossular content in garnet (X Grs = 0.22-0.19). The further decompression to LP-UHT facies is defined to be >0.2-0.3 GPa based on the calculated stability for hematite-bearing ilmenite. The isobaric cooling evolution is inferred mainly from the amphibole (Ti M2 /2 = 0.14-0.08) growth due to the crystallization of residual melts, consistent with a temperature decrease from >1,000°C to~800°C at 0.4 GPa. Zircon U-Pb dating for the two types of mafic granulite yields similar protolith and metamorphic ages of c. 900 Ma and c. 500 Ma respectively. However, the metamorphic age is interpreted to represent the HP-UHT granulite stage for the kyanite/sapphirine-bearing granulites, but the isobaric cooling stage for the common granulites on the basis of phase equilibria modelling results. The two types of mafic granulite should share the same metamorphic evolution, but show contrasting features in petrography, details of metamorphic reactions in each stage, thermobarometric results, and also the meaning of zircon ages as a result of their different bulk-rock compositions. Moreover, the UHT metamorphism in U...

Paleoproterozoic UHT metamorphism with isobaric cooling (IBC) followed by decompression–heating in the Khondalite Belt (North China Craton): New evidence from two sapphirine formation processes

“…The microstructures of sapphirine‐bearing symplectites have generally been interpreted as reflecting near‐ITD in an order of 0.1–0.3 GPa or heating with or without decompression in other granulite terranes (Brandt et al, ; Cadéron et al, ; Harley, ; Harley et al, ; Jiao, Guo, Wang, & Peng, ; Kawasaki & Motoyoshi, ; Kriegsman & Schumacher, ; Waters, ). In addition, Spr + Pl intergrowths, associated with Spl + Pl and Crn + Pl intergrowths after Ky have been commonly interpreted as reflecting rapid extensive decompression (from 1.5–3.5 GPa to 1.1 GPa or below) with or without slight heating over a wide T range of 700–1,000ºC, such as in Ky eclogites (Baldwin, Powell, Williams, & Goncalves, ; Liati & Seidel, ; Tumiati et al, ), and in HP mafic/felsic granulites (Baldwin et al, ; Dong, Wei, & Zhang, ; Johansson & Möller, ; Petrakakis, Schuster‐Bourgin, Habler, & Abart, ; Rötzler & Romer, ; Zhang, Mattinson, Meng, & Wan, ). Our interpretation for the second type of Spr is consistent with the ITD path inference.…”

The metamorphic PT history of Precambrian Belomorian eclogites (Shirokaya Salma), Russia

“…Consequently, we conclude that the zonation of Grt II is the result of different diffusion rates of Ca, Mg and Fe; it formed at (or near‐to‐)peak eclogite facies conditions (e.g. Dong et al., 2019) but cannot be utilized for the estimation of PT conditions by pseudosection calculations. Meanwhile, considering possible ion exchange reactions, amphibole decomposition, and/or partial melting in the matrix, the increase of Grs at the garnet rim may alternatively implicate a temperature increase with pressure decrease (e.g.…”

P–T–t Path of Garnetites in South Altyn Tagh, West China: A Complete Record of the Ultradeep Subduction and Exhumation of Continental Crust

“…These ages are consistent with those from mafic granulites and host felsic rocks from Bashiwake area ( c . 900–940 Ma, Dong et al, and refs. therein).…”

“…Hence in this contribution, we focus on garnetites from the South Altyn Tagh, which occurs as interlayers with UHP garnet clinopyroxenites and garnet peridotites hosted by felsic granulites (Dong et al, ). It could provide us unique sights into the ultradeep subduction processes as the garnet grains have preserved notable prograde zonings and plenty of multiphase inclusions.…”