Zircon SHRIMP U Pb ages and in-situ Hf isotopic analysis for the Mesozoic intrusions in South Taihang, North China craton: Evidence for hybridization between mantle-derived magmas and crustal components
The North China craton (NCC) was stabilized in Paleo-Proterozoic times, and the eastern part of it was remobilized in the Mesozoic, as is indicated by the presence of voluminous Mesozoic magmas in eastern NCC. In particular, the lithospheric mantle beneath eastern NCC was significantly thinned during the intense Mesozoic tectono-magmatic activity, which makes the NCC different from most other ancient cratons. The Mesozoic magmas may thus provide crucial evidence regarding the processes that transformed the NCC from craton to a tectonically active region. The South Taihang magmatic complex lies in the western part of the Mesozoic magmatic zone in the NCC. The complex falls into three main plutonic rocks: the Hongshan syenite, the Xishu gabbroic diorite-monzonite and the Wu'an monzonitic rocks. Zircon U-Pb dating reveals that the three plutonic rocks were emplaced contemporaneously at ̃ 132 Ma. In-situ Hf isotopic analyses for the dated zircons indicate that the Hongshan syenites show homogeneous ∊Hf( t) values (- 10 to - 13) which are slightly lower than those (- 9.0) for the enriched subcontinental lithospheric mantle (SCLM), as revealed by zircons from mafic rocks that originated from melting of SCLM. This suggests that the parental magma to the syenites was likely derived from melting of the enriched SCLM and contaminated slightly by old lower continental crustal material during ascent. The Xishu and Wu'an gabbroic diorites to monzonitic rocks, however, show significantly varied Hf isotopic compositions, with ∊Hf( t) = - 9.2 to - 16 and - 15.5 to - 24, respectively. The wide range of ∊Hf( t) values for a single sample suggests that the Xishu and Wu'an plutons formed through a process of hybridization between enriched mantle-derived mafic magma and crustal components in variable proportions. This is supported by the textural and compositional disequilibrium of plagioclase phenocrysts from the two plutons. The Wu'an monzonitic rocks show lower ∊Hf( t) values than the Xishu gabbroic diorite-monzonite, suggesting more incorporation of crustal components in the source. The sudden surge of magmatic activity in the South Taihang orogen took place as a result of lithospheric extension that caused partial fusion of the enriched portions of SCLM generating voluminous mafic magmas. Underplating of the mafic magmas in the lower crust consequently caused partial melting of the old basement rocks to produce granitic melts, which was followed by mixing between the mafic and granitic magmas.