Chronology And Geochemistry Of Mesozoic High-Mg Diorites In Western Shandong: Constraints On Lithospheric Evolution Of The North China Craton

Zircon U-Pb chronology and trace element and Sr-Nd-Pb isotopic geochemistry are carried out for Mesozoic high-Mg diorites in western Shandong to constrain the time and mechanism of the North China Craton (NCC) destruction. Main results are as follows:1. Mesozoic high-Mg diorites from the western Shandong consist mainly of gabbro-diorite- monzodiorite and formed in the Early Cretaceous.2. High-Mg diorites, being most common, are characterized by enrichment in MgO and Na2O, light rare earth elements (LREEs) and large ion lithophile elements (LILEs), and depletion in heavy REE (HREEs) and high field strength elements (HFSEs). Their ⁸⁷Sr/⁸⁶Sr(t) ratios andεNd(t)values range from 0.7048 to 0.7108 and -3.95 to -16.60, respectively. Their Pb isotopic compositions can be divided into two groups, i.e., Group I: the initial ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios being 17.18-17.67, 15.44-15.55 and 37.54-38.47, respectively; and Group II: the initial ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios being 16.42-17.11, 15.25-15.38 and 36.56-37.10, respectively. 3. Mesozoic high-Mg intrusions are characterized by high initial ⁸⁷Sr/⁸⁶Sr ratios and lowεNd(t)values. Their Pb isotopic compositions display spatially a variation trend, i.e., ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios gradually decrease from the Tan-Lu fault zone to northwest Combined with the mantle peridotite xenoliths and Archean inherited zircons in the host rocks, it is suggested that the primary magma was derived from the lithospheric mantle or asthenosphere to be involved by lower crust. From southeast to northwest, the Yangtze craton (YC) basement gradually decreases and the NCC basement increases in the magmatic source.The mantle xenoliths from Mesozoic high-Mg diorites consist mainly of spinel harzburgite and chromite-bearing dunite. The mineral equilibration temperatures (less than 1000°C) and chemical data (average Mg # =91.4) indicate that they represent the residues after large degrees of polybaric melting. However, the enrichment in LREEs and LILEs and high initial ⁸⁷Sr/⁸⁶Sr ratios as well as lowεNd(t)values imply that they was later subjected to modification by the infiltration of an adakitic melt.4. The formation of Mesozoic high-Mg diorites requires a thickened continental crust, which is proved by the discovery of eclogite xenoliths in coeval adakitic rocks in the Xu-Huai area. The thickening of the eastern NCC resulted from the subduction of the YC beneath the NCC in Early Mesozoic. The silica-rich melt metasomatism in the mantle peridotite xenoliths, i.e., melt-peridotite interaction, provide direct evidence for high-Mg feature of the Mesozoic diorites in western Shandong.