Metamorphic evolution and tectonic significance of ultrahigh‐temperature mafic granulites in the southern part of the Shiguai area, Daqingshan, China

Ultrahigh-temperature (UHT) mafic granulites in the southern part of the Shiguai area in Daqingshan, China, crop out within the Shanheyuan Gneiss of the Khondalite Belt (KB), a Paleoproterozoic tectonic belt in the Western Block of the North China Craton (NCC). The UHT mafic granulites occur within hypersthene-bearing dioritic-granitic gneiss as irregular lenses and deformed dike swarms. Four metamorphic generation assemblages (M-1-M-4) are recognized in the studied samples. M-1 is defined by the enclosed orthopyroxene and clinopyroxene within the "red eye" corona of the garnet, representing the prograde metamorphic stage. M-2 (UHT granulite-facies) is marked by the appearance of "red eye" corona of the garnet, with mineral assemblages of grt-cpx-opx-pl-ilm-q. M-3 is defined by spinel-rich symplectites between garnet and orthopyroxene, including grt-opx-pl-spl-mt-ilm. M-4 (amphibolite facies) is featured by the development of amphibole. Phase equilibrium modelling and some critical thermobarometers are combined to constrain the P-T evolution of the studied samples. The P-T conditions for the M-1-M-4 are respectively estimated at ~8 kbar/~771-852 degrees C, ~9.4-10.0 kbar/~930-965 degrees C, ~5.0-5.4 kbar/~865-910 degrees C, ~5.0-5.2 kbar/~675-682 degrees C, which define a clockwise P-T path, involving near-isothermal decompression following the peak metamorphism. The P-T path indicates that the rocks underwent a sequence of tectonothermal processes that started with crustal thickening (M-1-M-2), followed by rapid crustal uplift (M-3) and cooling (M-4). This process is inferred that the metamorphism of the UHT mafic granulites was associated with rapid exhumation and uplift that followed the continent-continent collision and amalgamation of the Yinshan and Ordos blocks. This amalgamation formed the Paleoproterozoic KB in the Western Block of the NCC.