Mesozoic adakitic rocks from the Xuzhou-Suzhou area, eastern China: Evidence for partial melting of delaminated lower continental crust

摘要

Adakitic rocks in the Xuzhou-Suzhou area, eastern China, consist of dioritic and monzodioritic porphyries and were dated at 131-132 Ma by the SHRIMP U-Pb zircon method. These rocks have high MgO content (1.47-5.73%), high Mg~# values (0.49-0.61), and high La/Yb and Sr/Y ratios. These features are similar to rocks derived from partial melting of a subducted oceanic slab. However, their high initial ~(87)Sr/~(86)Sr (0.7053-0.7075) and low ε_(Nd)(t) values (-4.43 to -13.14) are inconsistent with the origin from slab melting. These rocks often contain garnet residual crystals and eclogite, garnet clinopyroxenite, and garnet amphibolite xenoliths. Petrographical characteristics and estimated P-T conditions of these xenoliths indicate that they were once deeply subducted and subsequently underwent rapid exhumation in the early Mesozoic. Garnet residual crystals from the porphyries show similar chemical compositions to garnets from garnet clinopyroxenite and garnet amphibolite xenoliths. Ages of the inherited zircons of the xenoliths and their host rocks likely indicate that sources for the adakitic magma and protoliths of the eclogite and garnet clinopyroxenite xenoliths in the study area were from Precambrian basement of the North China Craton. The data also suggest that the lower continental crust in the eastern North China Craton was thickened during the early Mesozoic and delaminated in the early Cretaceous. The high-Mg adakitic magma resulted from partial melting of this delaminated lower continental crust and its subsequent interaction with the mantle during upward transport, leaving garnet as the residual phase.