Zheng Jianping

Geochemical Characteristics and Emplacement Ages of the Mengyin Kimberlites, Shandong Province, China

New major- and trace-element compositional data and isotopic analyses of the Mengyin (MY) kimberlites are provided in this paper. Samples were selected to be contamination free and to represent intrinsic features and characteristics of the source. In comparison with the kimberlites of South Africa, MY kimberlites have moderate SiO2 and TiO2; higher Th, U, and Nb; and lower V, Co, Ni, Rb, Sr, and Ba. The characteristics of MY kimberlites are similar to Group IA kimberlites and are distinct from Groups IB and II kimberlites from South Africa. The initial values of 87Sr/86Sr and 143Nd/144Nd are 0.70364 to 0.70421 and 0.51193 to 0.51203, respectively. The eNd values of 0.25 to +2.1 are similar to Group I kimberlites, and represent a depleted mantle source that was slightly more enriched than that of Group I South African kimberlites. An Rb-Sr phlogopite age (493 ± 10 Ma) presented in this work represents the emplacement age of the MY kimberlites.

Helium and argon isotopes of the Tertiary basic igneous rocks from Shandong Peninsula and implications for the magma origin

Helium (He) and Argon (Ar) isotopic compositions of the Tertiary basic igneous rocks were determined by the high temperature melting extraction method. The selected samples for the studies included alkaline basalts and diabases from the Jiyang basin, and the surrounding Shanwang and Qixia outcrops in the Shandong Peninsula, eastern China. The results show that the Paleogene basalts and diabases from the Jiyang basin yielded a wide range of 4He abundance of (73.70–804.16)×10−8 cm3 STP·g−1, with 3He/4He ratios of 0.374–2.959 Ra, which was lower than the MORB but evidently higher than the continental crust value. The Neogene alkaline basalts from the Jiyang basin, Shanwang and Qixia outcrops have variable 4He abundances ((42.34−286.72)×10−8 cm3 STP·g−1), and “continental crust-like” 3He/4He ratios (0.013−0.074 Ra). All of them contain atmospheric-like 40Ar/36Ar ratio (395.4–1312.7), reflecting the mantle sources with air components. Their low 3He/4He ratios are interpreted as the enrichment of the radiogenic 4He mainly inherited from the mantle. He and Ar systematics show the mixing of MORB-type, air and a 4He enriched member in the mantle source, suggesting that these igneous rocks originated from the depleted asthenospheric mantle mixed with an EMI component. Therefore, the present He and Ar isotopes do not support the viewpoints that the Cenozoic igneous rocks of Eastern North China were the products of mantle plume(s) activities.

Deep-derived enclaves (belonging to middle-lower crust metamorphic rocks) in the Liuhe-Xiangduo area, eastern Tibet: Evidence from petrogeochemistry *

Petrological and geochemical studies of deep-derived enclaves from the Liuhe-Xiangduo area, eastern Tibet, showed that the enclaves involve five types of rocks, i. e., garnet diopsidite, garnet amphibolite, garnet hornblendite, amphibolite and hornblendite, whose main mineral assemblages are Grt + Di + Hbl, Grt + P1 + Hbl + Di, Grt + Hbl + P1, P1 + Hbl, and Hb1 + Bt, respectively. The enclaves exhibit typical crystalloblastic texture, and growth zones are well developed in garnet (Grt) in the enclaves. In view of major element geochemistry, the deep-derived enclaves are characterized by high MgO and FeO *, ranging from 12. 00% to 12. 30% and 8. 15% to 10. 94%, respectively. The protolith restoration of metamorphic rocks revealed that the enclaves belong to ortho-metamorphic rocks. The REE abundances vary over a wide range, and ΣREE ranges from 53.39 to 129.04 μg/g. The REE patterns slightly incline toward the HREE side with weak LREE enrichment. The contents of Rb, Sr, and Ba range from 8. 34 to 101μg/g, 165 to 1485 μg/g, and 105 to 721 μg/g, respectively. The primitive mantle-normalized spider diagrams of trace elements show obvious negative Nb, Ta, Zr and Hf anomalies. Sr-Nd isotopic compositions of the enclaves indicated that the potential source of deep-derived enclaves is similar to the depletedmantle, and their (87Sr/86Sr)i ratios vary from 0.706314 to 0.707198, (147Nd/144Nd)i ratios from 0.512947 to 0.513046, and ɛNd(T) values from +7.0 to +9.0, respectively. The potential source of the enclaves is obviously different from the EM2-type mantle from which high-K igneous rocks stemmed (the host rocks), i. e., there is no direct genetic relationship between the enclaves and the host rocks. Deep-derived enclaves in the host rocks belong to mafic xenoliths, and those in the Liuhe-Xiangduo area, eastern Tibet, are some middle-lower crust ortho-metamorphic rocks which were accidentally captured at 20–50 km level by rapidly entrained high-temperature high-K magma, whose source is considered to be located at 50-km depth or so.