Geochemical, mineralogical and chronological studies of mafic-intermediate dykes in the Jiaodong Peninsula: implications for Late Mesozoic mantle source metasomatism and lithospheric thinning of the eastern North China Craton

摘要

The Late Mesozoic tectonic evolution of the eastern North China Craton (NCC) remains controversial. In this study, a suite of Early Cretaceous mafic-intermediate dyke swarms found in the same section in the Jiaodong Peninsula, eastern NCC, were used to trace the nature of lithospheric mantle and crust-mantle interaction. Zircon U-Pb dating demonstrates that these dykes emplaced into the Jiaodong Peninsula during the same period (112-121 Ma). Diorite dykes have SiO2 contents of 52.7-54.1 wt.%, Na2O+K2O contents of 5.77-6.10 wt.%, a limited range of MgO (4.43-5.13 wt.%), Cr (64.8-85.5 ppm) and Ni (23.6-27.7 ppm), and a narrow range of (Sr-87/Sr-86)(i) isotopic ratios (0.709240-0.709246), epsilon(Nd)(t) values (-20.0 to -19.1) and epsilon(Hf)(t) values (ca. -23.9). Lamprophyre and dolerite dykes have low contents of SiO2 (50.5-55.4 wt.%) and Na2O+K2O (4.88-7.28 wt.%), high contents of Al2O3 (14.6-15.8 wt.%), MgO (5.83-8.60 wt.%), Cr (210-292 ppm) and Ni (114-149 ppm), and a narrow range (Sr-87/Sr-86)(i) isotopic ratios (0.709134-0.709788), epsilon(Nd)(t) values (-19.9 to -17.4) and epsilon(Hf)(t) values (-22.1 to -15.1). Both intermediate and mafic dykes exhibit strong large ion lithophile elements (LILE) enrichment and high field strength element (HFSE) depletion. These geochemical characteristics demonstrate that the magma was primarily derived from relatively low degrees of the partial melting of garnet-lherzolite mantle, which was modified by fluids from a subducting oceanic slab with marine sediments. Both the mafic dykes and intermediate dykes originated from the same magma source and formed in different stages of magma fractionation crystallization. The magma experienced the fractional crystallization of clinopyroxene and more ferromagnesian minerals in a hydrous condition, thus forming mafic dykes. Then, the residual calc-alkaline liquids upwelled into a low-H2O and low-pressure environment, in which more plagioclase crystallized from liquid, thus increasing the degree of differentiation and producing the Ca- and Al-rich diorite dyke. Combined with Late Mesozoic mantle-derived mafic-intermediate dykes and crust-derived volcanic rocks in the region, the eastern NCC has experienced considerable lithospheric thinning along with regional extensional tectonics, which was caused by the prolonged thermo-mechanical-chemical erosion on the lithosphere induced by the rollback of the subducted paleo-Pacific plate.