Genesis of ultrahigh-temperature orogenic lower crust: Insights from the porphyritic garnet granitoids in the eastern Khondalite belt, North China craton

The widespread occurrence of ultrahigh-temperature (UHT) metamorphism in collisional orogenic belts has been extensively documented, but how such extreme thermal conditions are achieved in the partially molten lower crust remains enigmatic, particularly considering the impact of widespread garnet granitoids. Here, we investigated the UHT porphyritic garnet granitoids in the eastern Khondalite belt, North China craton, using petrography, mineralogy, geochemistry, geochronology, and electron backscatter diffraction (EBSD) fabric analyses to elucidate the genesis of UHT metamorphism in this region. The porphyritic garnet granitoids coexist with UHT pelitic granulites and are predominantly composed of K-feldspar, quartz, plagioclase, garnet, and biotite, with minor amounts of orthopyroxene. Geochemically, the rocks show a strongly peraluminous composition (alumina saturation index [ASI] = 1.10−1.28) and are characterized by relatively low silica (60.58−68.10 wt%) and high ferromagnesian (TFeO + MgO = 5.54−11.12 wt%) contents compared to typical granitic melts. Macroscopic and microscopic structures indicate that the porphyritic garnet granitoids underwent significant crystal accumulation and melt loss/extraction processes aligning with supersolidus deformation during the formation of cumulates. Ternary feldspar thermometry and Al-in-orthopyroxene thermometry indicate that the porphyritic garnet granitoids formed under UHT conditions. Combined with previous studies, our zircon U-Pb ages reveal that formation of the UHT porphyritic garnet granitoids and the regional geodynamic evolution were closely related to the following processes: (1) Initially, upwelling asthenospheric mantle facilitated fertile crust, generating abundant magma at ca. 1.95 Ga (first-stage anatexis) and triggering the accumulation of solids/crystals in a dominantly magmatic layer, along with upward melt migration, and (2) then, the cumulates sustained elevated temperatures due to ongoing upwelling of mantle material and accompanying mafic magmatism, ultimately reaching UHT conditions (second-stage anatexis) at ca. 1.92 Ga. Consequently, the coexisting pelitic granulites and porphyritic garnet granitoids likely experienced simultaneous UHT metamorphism and anatexis during the postorogenic extension stage. The present findings potentially offer a broader perspective on the mechanisms driving UHT metamorphism in partially molten orogenic lower crust.