Molar tooth (briefly called MT) carbonate is one of the Proterozoic carbonates with enigmatic sedimentary structure. According to the morphology of MT, it can be divided into two main genesis-morphology types, i.e., autochthonous and allochthonous, and each type can be further divided into a series of subtypes. The autochthonous MT can be divided into filamentous (MF1), ribbon, nodular and dotted type (MF4), in which the ribbon one can be subdivided into simple vertical (MF2) and complex ribbon (MF3), broken ribbon by storm in situ (MF5) and MT within nodular limestone (MF6). Allochthonous MT includes conglomerate clasts and horizontal detritus. Studying on the links between MT and the host rocks on five stratigraphic sections in the southeastern Jilin and eastern Liaoning indicates that the morphology of MT is closely related to host rocks, and seven genetic types by relationships between MT and the host rocks with facies interpretations are classified and discussed in this paper. The sedimentary environments of MT formation will be diverse if their morphologies are different.
292 chemical composition data and 82 isotopic composition data of gas samples collected from the Taibei Depression of the Turpan-Hami Basin, West China, were used in the study of their origin. Non-hydrocarbon gas is poor in most samples whereas abundant nitrogen in some samples is positively correlated with δ13C1. Although methane is the main constituent, higher molecular gaseous hydrocarbons, from ethane to pentane, are detected in most samples, in accordance with the distribution of oil reservoirs. The stable carbon isotope ratios of methane, eth-ane and propane are defined as d13C1: -45.5‰ to -33.5‰, d13C2: -30.2‰ to -10.5‰, and d13C3: 27.6‰ to -11.2‰, respectively. According to the distribution of carbon isotope ratios, 2 families of gas can be grouped, most showing normal distribution of carbon isotopes, and others having obvious heavier carbon isotopes and being of abnormal distribution. Based on the isotopic composition, the disagreement between the relationship of Δ(d13C1-d13C2) and d13C2 and that of Δ(d13C1-d13C2) and d13C2, and the calculated Ro, there are oil-associated gas, coal-derived gas and mixture of them. Other samples with obviously heavier isotopic compositions from the Yanmuxi oilfield of the Taibei Depression have been degraded by organisms. 相似文献
AbstractThe Gangdese batholith, Tibet, records the opening and closing of the Neo-Tethyan ocean and the resultant collision between the Indian and Eurasian plates. The Mesozoic magmatic rocks play a crucial role in understanding the formation and evolution of the Neo-Tethyan tectonic realm. This study focuses on Jurassic intrusive rocks in the Xietongmen area of the southern margin of the Lhasa terrane adjacent to the Yarlung–Tsangpo suture. Zircon U–Pb dating yielded Middle Jurassic dates for ca 170?Ma hornblende gabbro and ca 173?Ma granodiorite intrusions. All of the samples are medium- to high-K calc-alkaline, and the majority are metaluminous and enriched in the large ion lithophile elements and depleted in the high-field-strength elements. This indicates the magma was generated in a subduction-related tectonic setting. The intrusive rocks have high and positive εHf(t) values (hornblende gabbro: 13.3–18.7; granodiorite: 14.2–17.6) that yield Hf model ages younger than 312?Ma. These new data, combined with the results of previous research, suggest that the Jurassic igneous rocks were derived from a metasomatised region of an asthenospheric mantle wedge. Extremely depleted Sr–Nd–Pb–Hf isotope compositions are similar to the Yarlung ophiolite and igneous rocks within other intra-oceanic island arcs. Together with the existence of sandstone that is identified as the product of the oceanic island arc environment, we suggest formation in an intra-oceanic island arc.
The new zircon U–Pb dating has yielded Middle Jurassic ages for the ca 170?Ma hornblende gabbro and ca 173?Ma granodiorite phases of the Xietongmen intrusion.
Jurassic igneous rocks formed from a metasomatised asthenospheric mantle wedge by northward subduction of the Neo-Tethys oceanic crust beneath the southern margin of the Lhasa terrane.
Late Triassic–Jurassic igneous rocks, which are characterised by highly depleted isotopic compositions within the Southern Lhasa sub-terrane, record residual intra-oceanic island arcs in the eastern Tethyan belt.
