Petrogenesis of Fushan high-Mg# diorites from the southern Taihang Mts. in the central North China Craton: Resulting from interaction of peridotite-melt derived from partial melting of delaminated lower continental crust.

This paper reports chronological and geochemical data of the Fushan high-Mg-# diorites from the southern Taihang Mts. and discusses its petrogenesis and nature of the Mesozoic lithospheric mantle beneath the central North China Craton together with the mantle peridotite xenoliths entrained by them. The Fushan dioritic intrusion is composed mainly of olivine-bearing hornblende-diorite, hornblende-diorite and diorite. Based on their shape and structure, zircons from the hornblende-diorite hosting in peridotite xenoliths can be subdivided into two types, i.e., one being the crystallizing ones from the host magma with zonal homogeneous absorption and euhedral-subhedral shape, the other being inherited or captured ones with core-rim structure, round and euhedral-subhedral shapes. The former has high Th/U ratios (1.10 similar to 4.08), indicating a magmatic origin. Their Pb-206/U-238 ages range from 123Ma to 128Ma, yielding a mean Pb-206/U-238 age of 125 +/- 1 Ma (n = 12). This age represents the crystallization time of the Foshan high-Mg-# diorite. The latter's Th/U ratios are between 0.32 and 2.03, and they yield three age populations of 2503 +/- 11Ma, 2181 +/- 26Ma and 1778 +/- 36Ma. SiO2 and MgO contents of the Fushan dioritic rocks range from 56.69% to 59.21% and from 3.60% to 6.33%, respectively. Meanwhile, the dioritic rocks are characterized by high Mg-# (0.51 similar to 0.64), enrichment in Na (Na2O/K2O ratios more than 1), Cr (93.1 x 10(-6) similar to 420 x 10(-6)) and Ni (35.1 x 10(-6) similar to 137 x 10(-6)), and intensive enrichments in light rare earth elements (LREE) and large ion lithophile elements (LILEs) and depletion in high Field strength elements (HFSEs). Their initial Sr-87/Sr-86 and Pb-206/Pb-204 ratios as well as epsilon(Nd) (t) values range from 0.70581 to 0.70641, front 17.052 to 17.512, and from -8.30 to -16.56, respectively. Taken together, combined with the nature of the mantle peridotite xenoliths and the occurrence of a large amount of the Paleoproterozoic inherited or captured ziorcons in the host rocks, it is suggested that the primary magma for the Fushan high-Mg-# diorites should be derived from the partial melting of a delaminated lower continental crust and subsequently developed the melt-peridotite reaction.