Petrological evidence for isobaric cooling of ultrahightemperature pelitic granulites from the Khondalite Belt,North China Craton

The ultrahigh-temperature(UHT) pelitic granulites from the Khondalite Belt, North China Craton(NCC), contain ilmenite in the matrix, which has been partially replaced by rutile. Based on this observation and the growth of biotite by garnet-consuming reaction, the UHT rocks are inferred to have recorded three metamorphic stages: the peak metamorphic stage(M1) and two retrograde metamorphic stages(M2 and M3). The M1 stage is represented by the assemblage of perthite+sillimanite+ ilmenite in the matrix, and quartz inclusions bearing(in the cores) garnet porphyroblasts. The M2 stage is defined by rutile-replacing ilmenite and growth of garnet mantles and rims containing acicular sillimanite inclusions, with the garnet+ perthite+ sillimanite+rutile+ ilmenite+ quartz assemblage. The M3 stage is recorded by the growth of biotite in the matrix, with the garnet+ biotite+ perthite+ sillimanite+rutile+ilmenite+quartz assemblage. Based on phase equilibrium modeling, an isobaric cooling path is reconstructed, which is consistent with the idea that mantle-derived magma provided the heat for the UHT metamorphism in the Khondalite Belt, NCC.

Precipitation of rutile needles in garnet from sillimanite-bearing pelitic granulite from the Khondalite Belt, North China Craton

Pelitic granulite from the Khondalite Belt,North China Craton,is composed of garnet,sillimanite,perthite,biotite and quartz.Oriented rutile needles in garnet are reported for the first time from the granulite.The rutile needles have been identified by optical microscopy and confirmed by in situ Raman spectroscopy,where they show the characteristic bands at 446–448 and 610 cm-1.Because of their very strong shape preferred orientation in3–4 easily observable directions,these rutile needles are interpreted to have formed by precipitation during exhumation and cooling following peak metamorphism.The ternary feldspar geothermometer gives a peak metamorphic temperature of*980°C at 10 kbar.The presence of oriented rutile needles in garnet is suggested to be a valuable indicator of ultrahigh-temperature(UHT)metamorphism in the Khondalite Belt and therefore likely in other UHT terrains.