Nature of the Early Cretaceous lithospheric mantle in western Shangdong: Constraints from geochronology, geochemistry and Sr-Nd-Pb-Hf isotopic data of Dakunlun gabbros and diabases

The LA-ICP-MS zircon U-Pb dating, Hf isotope, whole-rock geochemical and Sr-Nd-Pb data are reported for the Dakunlun gabbros and diabases in western Shandong to understand their nature of the Early Cretaceous lithospheric mantle. Few zircons from Dakunlun gabbros and the zircons from diabases all display striped absorption in cathodoluminescence images. It also indicates a magmatic origin that the high Th/U ratio (0. 11 similar to 3. 12). Moreover, most of zircons in the gabbros are captured and magmatic origin. The weighted mean Pb-206/U-238 ages of the magmatic zircons are 132 +/- 3Ma and 112 +/- 1Ma in the gabbros and diabases, respectively, implying an Early Cretaceous in age. The concordia ages of the captured zircons range from 2469Ma to 1752Ma, suggesting the existence of ancient crustal material. Geochemically, the Dakunlun gabbros and diabases are characterized by low SiO2 (49. 08% similar to 52. 61%), high MgO (4. 52%similar to 9. 67% ) , Na2O (2.58% similar to 3. 49%) and high Mg-# (45 similar to 65 ). Moreover, they are enriched in light rare earth elements and large ion lithophile elements (e.g. , Rb, Ba, Th) , and are depleted in heavy rare earth elements and high field strength elements (e. g. , Nb, Ta, Zr, and Hf). They have (La/Yb)(N) = 8. 98 similar to 10.2 and weak negative Eu anomaly (delta Eu = 0.83 similar to 0. 93). The (Sr-87/Sr-86), ratios vary between 0.7046 and 0.7060 and the epsilon(Nd) (t) vary between - 16. 71 and - 15. 54 in the gabbros. Whole rockinitial Pb-206/(204) Pb, Pb-207/Pb-204 and Pb-208/Pb-204 ratios of the gabbros range from 16. 596 to 16. 776, 15. 255 to 15. 290 and 36. 328 to 36. 648 , respectively. The epsilon(Hf)(t) values range from -12.0 to - 2. 5 in the magmatic zircons from diabases. The above data show that the Early Cretaceous Dakunlun gabbros and diabases are consistent with the period of lithospheric thinning beneath the eastern North China Craton. The primary magma was derived by partial melting of the enriched lithospheric mantle modified by the ancient lower crustal material. They were formed in an extensional tectonic environment related to the lithospheric thinning.