Spinel + quartz-bearing ultrahigh-temperature granulites from Xumayao,Inner Mongolia Suture Zone,North China Craton:Petrology,phase equilibria and counterclockwise p-T path

The Khondalite Belt within the Inner Mongolia Suture Zone （IMSZ） in the North China Craton is a classic example for Paleoproterozoic ultrahigh-temperature （UHT） metamorphism. Here we report new spinel-bearing metapelitic granulites from a new locality at Xumayao within the southern domain of the IMSZ. Petrological studies and thermodynamic modeling of the spinel -- quartz-bearing assemblage shows that these rocks experienced extreme metamorphism at UHT conditions. Spinel occurs in two textural settings：（1） high Xzn（Zn/（Mg ＋ FeH -- Zn） = 0.071--0.232） spinel with perthitic K-feld- spar, sillimanite and quartz in the rock matrix; and （2）low Xzn （0.045-0.070） spinel as inclusions within garnet porphyroblasts in association with quartz and sillimanite. Our phase equilibria modeling indicates two main stages during the metamorphic evolution of these rocks：（l） near-isobaric cooling from 975 ℃ to 875 ℃ around 8 kbar, represented by the formation of garnet porphyroblasts from spinel and quartz; and（2）cooling and decompression from 850 ℃, 8 kbar to below 750 ℃- 6.5 kbar, represented by the break-down of garnet. The spinel ＋ quartz assemblage is considered to have been stable at peak metamorphism, formed through the break-down of cordierite, indi- caring a near isothermal compression process. Our study confirms the regional extent of UHT metamor- phism within the IMSZ associated with the Paleoproterozoic subduction-collision process.

Sapphirine granulites from Panasapattu,Eastern Ghats belt,India:Ultrahigh-temperature metamorphism in a Proterozoic convergent plate margin

We report equilibrium sapphirine ＋ quartz assemblage in biotite-orthopyroxene-garnet granulites from a new locality in Panasapattu of Paderu region in the Eastern Ghats granulite belt, which provide new evidence for ultrahigh-temperature （UHT） metamorphism at 1030-1050 ℃ and 10 kbar in this region. The development of migmatitic texture, stabilization of the garnet-orthopyroxene- plagioclase-K-feldspar association, prograde biotite inclusions within garnet and sapphirine as well as sapphirine and cordierite inclusions within garnet in these granulites indicate that the observed peak assemblages probably formed during prograde dehydration melting of a Bt-Sill-Qtz assemblage, and constrain the prograde stage of the p--T path. The core domains of orthopyroxene porphyroblasts have up to w（Al203） 9.6%, which suggest that the temperatures reached up to 1150 ~C suggesting extreme crustal metamorphism. These conditions were also confirmed by the garnet-orthopyroxene thermobar- ometery, which yields a p--T range of 1012--960 ~C and 9.4 kbar. The p--T phase topologies computed using isochemical sections calculated in the model system Na20-CaO-K20--FeO-MgO-A1203-SiO2 --H2O （NCKFMASH） for metapelites, garnet-free sapphirine granulites and garnet-bearing sapphirine granulites match the melt-bearing assemblages observed in these rocks. Isochemical sections constructed in the NCKFMASH system for an average sub-aluminous metapelite bulk composition, and contoured for modal proportions of melt and garnet, as well as for the compositional isopleths of garnet, predict phase and reaction relations that are consistent with those observed in the rocks. Garnet and orthopyroxene contain Ti-rich phlogopite inclusions, suggesting formation by prograde melting reactions at the expense of phlogopite during ultrahigh-temperature conditions. These p--T results underestimate ＇peak＇ condi- tions, in part as a result of the modification of garnet compositions in the domains where some melt was retained. The post-peak evolution is constrained by a succession of melt-present reactions that occur at p 〈 10 kbar, inferred from micro-structural relations among various minerals. After high-temperature decompression from the metamorphic peak, the p--T path followed a near isobaric cooling stage to T 〈 900℃. The UHT rocks investigated in this study occur within a continental collision suture which witnessed prolonged subduction-accretion history prior to the final collision. We correlate the extreme metamorphism and the stabilization of UHT mineral assemblages to heat and volatile input from an upwelled asthenosphere during subduction--collision tectonics in a Proterozoic convergent plate margin.