Low Water Content of the Lower Continental Crust Induced by Decompression Melting: Evidence From the Weichang Granulite Xenoliths in the Northern North China Craton

Abstract The water content of granulite xenoliths from the Cenozoic Weichang basalts in the northern North China Craton (NCC) is investigated to constrain the nature and evolution of the lower crust. Granulites are categorized into felsic and mafic groups, characterized by widely developed glass‐bearing sieve textures in their constituent minerals. Spinel‐plagioclase‐clinopyroxene symplectites of garnet were observed in one mafic granulite. Compared to the Archean crystalline basement, the felsic granulites show similar mineral compositions, whereas the mafic granulites exhibit higher Mg# and Al 2 O 3 content in pyroxenes. Al 2 O 3 content of clinopyroxene in some felsic granulites decreases progressively from the core to the rim and extends into the sieve‐textured zones. Pyroxenes and plagioclases in the sieve‐textured zones exhibit higher Mg# and CaO than those in the clean zones, respectively. Water content in clinopyroxene varies at both inter‐ and intra‐grain scales, and shows a positive correlation with Al 2 O 3 . Overall, the water contents of clinopyroxene (89–286 wt. ppm) and orthopyroxene (38–192 wt. ppm) in the Weichang granulites are lower than those in the Precambrian granulites, which represent the ancient lower crust of the NCC. These observations indicate that the present‐day lower crust beneath Weichang is composed predominantly of reworked Precambrian felsic granulites and newly accreted Phanerozoic mafic granulites, which experienced decompression melting during the Cenozoic and have low water contents. Combined with previous studies, we propose that the lower crust beneath the northern NCC underwent a prolonged dehydration process during the Carboniferous‐Cretaceous, induced by intense decompression melting and the removal of voluminous granitic magmas. We infer that this water‐poor, high‐strength lower crust governed the cessation of felsic magmatism and influenced the chemical properties of Cenozoic basalts in the northern NCC.