ABSTRACT We present the major and trace elements and Sr, Nd, and Pb isotopes in mid-ocean ridge basalts (MORB) from the East Pacific Rise (EPR) at 2.6–3.1°S. These samples are low-K tholeiites and show significant variation in their major element compositions (e.g. 4.60–8.18 wt% MgO, 8.34–12.12 wt% CaO, 9.78–14.25 wt% Fe2O3, and 0.06–0.34 K2O wt%). Trace element abundances of the 2.6–3.1°S MORB are variably depleted (e.g. (La/Sm), N = 0.51–0.78, Zr/Y = 2.35–3.42, Th/La = 0.035–0.056, and Ce/Yb = 2.38–3.96) but closely resemble the average N-MORB. In the compatible elements (Ni and Cr) against incompatible element Zr plots, the 2.6–3.1°S MORB show well-defined negative correlations, together with a liquid line of descent (LLD) modelling and petrographic observations, implying a significant role of olivine, plagioclase and clinopyroxene fractionation during magma evolution. When compared to global MORB and peridotites, the 2.6–3.1°S MORB and most of the other axial lavas from the South EPR show similar Zn/Fe, Zn/Mn, and Fe/Mn ratios, attesting to a peridotite-dominated mantle lithology. However, the relationships between incompatible trace element ratios, such as Zr/Rb and Nb/Sm, and the negative correlation between Zr/Nb and 87Sr/86Sr indicate a geochemically heterogeneous mantle source. The mantle beneath the South EPR likely consists of two components, with the enriched component residing as physically distinct domains (e.g. veins or dikes) in the depleted peridotite matrix. In the Sr–Nd–Pb isotope space, the South EPR MORB lie along the mixing lines between the depleted MORB mantle (DMM) and the ‘C’-like Pukapuka endmember. We infer that low-F melts derived from these enriched materials may cause localized mantle heterogeneity (veins or dikes) via an infiltration process. Subsequent melting of the refertilized mantle may impart an isotopically distinct characteristic to South EPR MORB. 相似文献
The Zhengguang deposit, a representative large gold deposit in the Duobaoshan ore field in NE China, is located in the northeast of the Central Asian Orogenic Belt (CAOB). Ore body emplacement was structurally controlled and occurs mainly at the contact zone between the strata of Duobaoshan Formation and an Ordovician diorite stock. The diorite rocks have a close genetic relationship with Au mineralization. Re–Os isotope dating of Au-bearing pyrite yields an isochron age of 506 ± 44 Ma (MSWD = 15). Based on present and previous dating results, it can be concluded that the Zhengguang deposit formed at ~480 Ma. The mineralization time of the Zhengguang deposit is nearly identical to those of the Duobaoshan and Tongshan deposits, indicating they are all derived from the same metallogenic system. The Duobaoshan-style porphyry Cu–Mo mineralization may exist at deeper levels at Zhengguang. The geochemical characteristics of the Zhengguang dioritic rocks presented in this paper are similar to those of bajaitic high-Mg andesite, and the magmas originated from a mantle wedge metasomatized by melts from a subducting oceanic slab at an active continental margin setting. The Ordovician magmatic–metallogenic events in the Duobaoshan ore field were caused by the westward subduction of an oceanic slab located between the Xing’an and Songliao blocks. It is worth pointing out that the Zhengguang deposit is the oldest known Phanerozoic Au deposit in NE China. Further studies of this deposit will improve understanding of the regularity of ore formation and aid mineralization forecast across the Duobaoshan region. 相似文献
In terms of observational analysis and numerical simulations,study is performed of thesynergistic effects of the tropical Indian Ocean and Pacific SSTA on the atmosphere in northern win-ter.It turns out that the SSTA features show in-phase variation for the Indian Ocean and the central/eastern Pacific at low latitudes and anti-phase change compared with that in the neighborhood of thewestern Pacific“warming pool”.When negative SSTA in the western Pacific and positive SSTA inthe central/eastern Pacific and the Indian Ocean are available,the 500 hPa height anomaly exhibits awave train of a positive-negative-positive anomaly following a great-circle path emanating from thesubtropical western Pacific via the NW portion to North America,with enhanced westerly jet overmidlatitude East Asia and the NW Pacific,and v.v.The comparison to the results from the numeri-cal modeling of the tropical western Pacific SSTA reveals the importance of the synergistic effects ofthe oceans on atmospheric circulations. 相似文献
Due to its structure,rock and mineral composition,fluid and other factors,the granite Buried Hill Reservoir is highly heterogeneous with a complex longitudinal structure and a reservoir space made up of a combination of dissolution pores and fractures.This paper is based on current understanding of tectonic evolution in the northern part of the South China Sea,in conjunction with the seismic phase characteristics.It is determined that the meshed fault system was formed by three stages of movement-tectonic compression orogeny during the Indochinese epoch,strike-slip compression-tension during the Yanshanian Period,early fracture extension activation during the Himalayan-which controlled the distribution of the Buried Hill Reservoir.Drilling revealed two types of buried hills,faulted anticline and fault horst,their longitudinal structure and the reservoir space type being significantly different.The mineral composition,reservoir space and diagenetic characteristics of the reservoir rocks and minerals were analyzed by lithogeochemistry,micro section and logging etc.,it thus being determined that the Mesozoic rocks of the Songnan Low Uplift in the Qiongdongnan Basin are mainly composed of syenogranite,granodiorite,monzogranite,which is the material basis for the development of the Buried Hill Reservoir.The content of felsic and other brittle minerals is more than 70%,making it easy for it to be transformed into fractures.At the same time,the weathering resistance of granodiorite and monzogranite is weaker than that of syenogranite,which is easily weathered and destroyed,forming a thick sand gravel weathering zone.With increasing depth of burial,weathering and dissolution gradually weaken,the deep acidic fluid improving the reservoir property of internal fractures and expanding the vertical distribution range of the reservoirs.The research results lay a foundation for the exploration of Buried Hill in the deep-water area of the Qiongdongnan Basin. 相似文献
Well che89, located in the Chepaizi area in the northwest margin of Junggar basin, acquires high production industrial oil flow, which is an important breakthrough in the exploration of the south foreland slope area of Junggar basin. The Chepaizi area is near two hydrocarbon generation depressions of Sikeshu and Shawan, which have sets of hydrocarbon source rock of Carboniferous to Jurassic as well as Upper Tertiary. Geological and geochemical parameters are proper for the accumulation of mixed source crude oil. Carbon isotope, group composition and biomarkers of crude oil in Upper Tertiary of well Che89 show that the features of crude oil in Upper Tertiary Shawan Formation are between that of Permian and Jurassic, some of them are similar to these two, and some are of difference, they should be the mixed source of Permian and Jurassic. Geochemical analysis and geological study show that sand extract of Lower Tertiary Wulunguhe Formation has the same source as the crude oil and sand extract of Upper Tertiary Shawan Formation, but they are not charged in the same period. Oil/gas of Wulunguhe Formation is charged before Upper Tertiary sedimentation, and suffered serious biodegradation and oxidation and rinsing, which provide a proof in another aspect that the crude oil of Upper Tertiary Shawan Formation of well Che89 is not from hydrocarbon source rock of Lower Tertiary.