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Because few well-preserved hydrothermal chimneys have been found in terrestrial sedimentary rocks, research on paleo-thermal vents in geological history is relatively sparse. In this study, we present our original discovery of "hydrothermal chimneys" from the Chang 7 source rocks of the Triassic Yanchang Formation in the Ordos Basin, China, and provide the best evidence for deciphering hydrothermal activity preserved in the geological record (i.e., sedimentary rocks). Three possible chimney samples (i.e., samples 1551.6, 1551.6–2 and 1574.4) were collected for this study; they were interbedded with mudstones and oil shales, indicative of a deep-lake sedimentary environment. All three samples consist mainly of anhydrite, pyrite, and dolomite with the formation of mineral zoning across the walls of these structures, suggesting a sulfate-dominated stage and a carbonate-sulfide replacement stage. Moreover, their in situ geochemistry is characterized by high Eu, U, Th, Sr, Mn and U/Th ratios, which are typical indicators of hydrothermal vents. In addition, their S isotopes range from 7.89% to 10.88%, near the values of magma sulfur, implying a possible magmatic trigger for these hydrothermal vents. All this evidence shows that the Triassic sedimentary rocks of the Ordos Basin probably contain hydrothermal chimneys. Comparing ancient hydrothermal chimneys to modern hydrothermal chimneys, we should note the important implications of ancient chimneys; their formation mechanism may have been related to oil production, and they are possible indicators for future oil investigations. Further, they have great significance for studying the hydrothermal properties of primary dolomite. [ABSTRACT FROM AUTHOR]

The Sulu Orogen preserves the Neoproterozoic tectonic-magmatic events, corresponding to the breaking up of the Rodinia supercontinent. The ages and petrogenesis of meta-igneous rocks in the Liansandao area in the northern Sulu Orogen are not well-constrained. This study reports zircon U-Pb ages and Hf isotopes of these rocks from the Liansandao area. Three meta-igneous rock samples give similar weighted mean 206Pb/238U ages of 744±11, 767±12, and 762±15 Ma, respectively, indicating the Neoproterozoic crystallization ages. These rocks formed coevally with the Wulian and Yangkou intrusions that located along the Yantai-Qingdao-Wulian fault zone. The Neoproterozoic ages indicate that the meta-igneous rocks from the Liansandao area have affinity to the Yangtze Block. The three samples have εHf(t)Hf(t) values of -7.2–-10.5, -6.0–-17.5, and -6.8–-12.0, respectively. These negative εHf(t) values indicate a primarily crustal source. However, the widely various εHf(t) values that are higher than the continental crust, suggesting magma mixing between mantle-derived materials and the continental crust or source heterogeneity. Combined with the Hf model ages and geochemical characteristics, the monzodiorite (sample LSD-2) is most likely to be mantle-derived magma then interacted with ancient continental crust, and the granitic protolith (samples LSD-1 and LSD-3) in the Liansandao area might derive from the re-melting of a Paleoproterozoic continental crust at ~750 Ma, resulting from the upwelling and underplating of mantle-derived magma formed in an extensional setting due to the break- up of the Rodinia supercontinent. [ABSTRACT FROM AUTHOR]

The cycling of various isomorphs of authigenic silica minerals is a complex and long-term process. A special type of composite quartz (Qc) grains in tuffaceous shale of Permian Lucaogou Formation in the sediment-starved volcanically and hydrothermally active intracontinental lacustrine Santanghu rift basin (NW China) is studied in detail to demonstrate such processes. Samples from one well in the central basin were subject to petrographic, elemental chemical, and fluid inclusion analyses. About 200 Qc-bearing laminae are 0.1-2 mm and mainly 1 mm thick and intercalated within tuffaceous shale laminae. The Qc grains occur as framework grains and are dispersed in igneous feldspar-dominated matrix, suggesting episodic accumulation. The Qc grains are bedding-parallel, uniform in size (100 s µm), elongate, and radial in crystal pattern, suggesting a biogenic origin. Qc grains are composed of a core of anhedral microcrystalline quartz and an outer part of subhedral mega-quartz grains, whose edges are composed of small euhedral quartz crystals, indicating multiple episodic processes of recrystallization and overgrowth. Abundance of Al and Ti in quartz crystals and estimated temperature from fluid inclusions in Qc grains indicate that processes are related to hydrothermal fluids. Finally, the Qc grains are interpreted as original silica precipitation in microorganism (algae?) cysts, which were reworked by bottom currents and altered by hydrothermal fluids to recrystalize and overgrow during penecontemporaneous shallow burial. It is postulated that episodic volcanic and hydrothermal activities had changed lake water chemistry, temperature, and nutrient supply, resulting in variations in microorganic productivities and silica cycling. The transformation of authigenic silica from amorphous to well crystallized had occurred in a short time span during shallow burial. [ABSTRACT FROM AUTHOR]

Santanghu area in northeastern Xinjiang region of Northwest China is an important component of the Central Asian Orogenic Belt (CAOB), in which the dynamic mechanism of Permian magmatism is controversial. In Santanghu area is exposed a thick succession of the Middle Permian basalts, including a small amount of picritic basalts and andesites, known as the Tiaohu Formation. The picritic basalts contain cumulate olivine, and have whole-rock Mg up to 0.68-0.77; the basalts exhibit porphyritic or doleritic textures, and have relatively low Mg of 0.41-0.54, typical of evolved magmas. The mafic-ultramafic rocks of the Tiaohu Formation are slightly enriched in Light Rare Earth Elements (LREEs), and exhibit negative Nb and Ti anomalies. They also have high TiO content, and Nb/Y and Zr/Yb ratios greater than those of island arc volcanic rocks. Relatively low initial Sr isotopic ratios and high positive ɛ( t) and ɛ( t) values argue against contamination by ancient continental crust, and suggest formation of the Tiaohu Formation by partial melting of relatively refractory depleted lithospheric mantle that underwent metasomatism and extraction by fluid from the subducted slab. In addition, up to 38% olivine in picritic basalts indicates high-degree partial melting of lithospheric mantle, and the underlying Lucaogou Formation contains fragments of ultra-alkaline magmatic rocks that originated in the deep mantle. These observations imply wide-spread underplating in Santanghu area, which may have been associated with a mantle plume. [ABSTRACT FROM AUTHOR]

The Santanghu area is located on the northeastern margin of the Junggar Basin, northern Xinjiang, Northwest China. The Carboniferous volcanic rocks in this area are widely distributed in Kaokesaiergaishan, Santanghu, Daheishan and Naomaohu districts, which are located to the north of the Kalameili Fault. These rocks, sourced from a cognate magma, consist of basic, intermediate, and acidic lavas, and pyroclastic rock. The basic volcanic rocks are enriched with large-ion lithophile elements (LILE), but are relatively depleted in high field strength elements (HFSE), and have an obvious negative Nb-Ta-Ti anomaly. They were most probably derived from a depleted mantle source, and during their ascent, these magmas were not contaminated by the crustal material as they underwent magma crystallization differentiation. Based on the Carboniferous volcanic assemblage and geochemical data, it is apparent that the early Carboniferous volcanism occurred in a subduction-related tectonic setting. New LA-ICP-MS zircon U-Pb analyses constrain the age of the andesite within the volcanic rocks as the early Carboniferous (328.9-331.3 Ma). Combined with the regional geological record, comprehensive analysis of the isotope geochronological data indicates that the subduction of the Junggar Ocean predates the early Carboniferous, and that the Santanghu island arc magmatism was induced by the subduction of the Junggar Ocean in the Carboniferous. [ABSTRACT FROM AUTHOR]

The Permian Lucaogou Formation is an important hydrocarbon source rock in the Junggar, Turpan, and Santanghu basins in Xinjiang, NW China. For the first time, dolostones associated with mantle-originated exhalative hydrothermal fluid flows are discovered in Yuejingou section in the Santanghu Basin area. They include dolomicrite, doloarenite, and a small amount of dolorudite, and are finely (0.05-0.15 cm thick) interlaminated with lime micrite and dolomicritic analcime laminites. Alkali feldspar and analcime grains are common in doloarenite and are interpreted as having been derived from analcime phonolites and peralkaline magmatic rocks. These magmatic fragments were brought up from subsurface by hydrothermal fluid flow and had experienced exhalative brecciation, transport, and deposition on the lake floor. The matrix consists dominantly of dolomite and ankerite smaller than 0.01 mm. The dolostones can be subdivided into four types on the basis of mineral composition and content. The detrital analcime and alkaline feldspar grains and tuff lithics are interpreted as intraclasts, which were deposited in an under-filled starved lake basin. The δ values of dolostones are −5‰ to −21.1‰, and −11.9‰ on average; the Sr/Sr ratios of dolostones are 0.70457 to 0.706194, and 0.705005 on average. These values, in combination with evidence of multi-episodes of peralkaline extrusion, suggest a mantle origin of the hydrothermal fluids, which may have promoted primary dolomite formation. The fluid from the upper mantle caused serpentinization of ultramafic rocks that intruded into the lower crust to obtain Mg and Fe, and injected the ions into the lake water as the Mg and Fe sources for dolomite and ankerite. Hydrothermal fluids associated with peralkaline magmatic rocks also provided Ca, Mg, Fe, and CO. Explosive breccias formed and dolostones were convoluted near the vent of hydrothermal fluid exhalation, whereas laminated dolostones formed farther away from the vent. The dolostones are primary dolomite deposition in an intracontinental rift basin and associated with mantle-originated hydrothermal fluids. They provide an insight into the origin of dolomite formation in the geologic history and clues to understand the sedimentary environments and tectonic conditions in northern Xinjiang during the late Paleozoic. [ABSTRACT FROM AUTHOR]

The properties and tectonic significance of the fault bound zone on the northern margin of the Central Tianshan belt are key issues to understand the tectonic framework and evolutionary history of the Tianshan Orogenic Belt. Based on the geological and geochemical studies in the Tianshan orogenic belt, it is suggested that the ophiolitic slices found in the Bingdaban area represent the remaining oceanic crust of the Early Paleozoic ocean between the Hazakstan and Zhungaer blocks. Mainly composed of basalts, gabbros and diabases, the ophiolites were overthrust onto the boundary fault between the Northern Tianshan and Central Tianshan belts. The major element geochemistry is characterized by high TiO2 (1.50%–2.25%) and MgO (6.64%–9.35%), low K2O (0.06%–0.41%) and P2O5 (0.1%–0.2%), and Na2O>K2O as well. Low ΣREE and depletion in LREE indicate that the original magma was derived from a depleted mantle source. Compared with a primitive mantle, the geochemistry of the basalts from the Bingdaban area is featureded by depletion in Th, U, Nb, La, Ce and Pr, and unfractionated in HFS elements. The ratios of Zr/Nb, Nb/La, Hf/Ta, Th/Yb and Hf/Th are similar to those of the typical N-MORB. It can be interpreted that the basalts in the Bingdaban area were derived from a depleted mantle source, and formed in a matured mid-oceanic ridge setting during the matured evolutionary stage of the Northern Tianshan ocean. In comparison with the basalts, the diabases from the Bingdaban area show higher contents of Al2O3, ΣREE and HFS elements as well as unfractionated incompatible elements except Cs, Rb and Ba, and about 10 times the values of the primitive mantle. Thus, the diabases are thought to be derived from a primitive mantle and similar to the typical E-MORB. The diabases also have slight Nb depletion accompanying no apparent Th enrichment compared with N-MORB. From studies of the regional geology and all above evidence, it can be suggested that the diabases from the Bingdaban area were formed in the mid-oceanic ridge of the Northern Tianshan ocean during the initial spreading stage. [ABSTRACT FROM AUTHOR]

The Turpan-Hami and Santanghu basins are the late Paleozoic-Mesozoic-Cenozoic reworked intracontinental basins that superposed on the folded basement of the Paleozoic orogenic belt. 40Ar/39Ar geochronological study of the basalts developed in the basins reveals that the formation period is Permain (293–266 Ma). From geochemical comparison of the basalts, the Santanghu basalts exhibit a strong depletion in Nb and Ta, and a selective enrichment in HFSE, reflecting that the source region is influenced by the subducted components related to subduction of the ancient oceanic crust and characterized with “lagged arc volcanic rocks”. In contrast, the Turpan-Hami basalts show a slight depletion in Nb and Ta, high Th/Ta ratio, similar to the basalts formed in an intracontinental extensional zone or in an initial rift. Combined with the formation period of the ophiolite and ophiolite mélange zones and regional magmatic activities occurring on the post-orogenic extensional background in northern Xinjiang, it can be inferred from these geochemical characteristics that the tectonic background for forming the Turpan-Hami and Santanghu basins is closely related to the regional extension after the continent-continent collisional orogeny. The basalts of the two basins came from different sources on the post-orogenic extensional background of the similar basin-forming dynamics. Although the settings are all intracontinental rift, the source for the Santanghu basalts is obviously subjected to the metasomatism of the subducted components, implying the existence of the previous subduction. [ABSTRACT FROM AUTHOR]

The Songshugou dunite body, which occupies an area of about 20 km2, is the largest ultramafic massif in the eastern Qinling orogenic belt, Central China. The major component of this body is dunitic rocks including mylonitic dunite and coarse-grained dunite; they occupy about 95 vol% of the total body. Petrography, mineral composition, major and trace elements and primitive melt inclusions have been investigated in this paper; all revealed that this dunite body is the product of melt-rock interaction by porous percolation flow. In comparison with dunite sills or veins in harzburgite from the basal part of Oman ophiolite, this dunite body is characterized by lower contents of Al2O3, CaO and HREE but higher content of highly incompatible elements and Zr and Hf. The LREE enriched patterns and primitive mantle normalized spidergrams of trace elements are also different from the Oman dunite. Combining with melt inclusions observed in olivines, we conclude that this dunite body is the product of large- scale porous percolation flow of high-MgO melts within depleted mantle peridotites. The high-MgO magma, in essence, was most probably produced at the hot head of an upwelling mantle plume. The occurrence of the Songshugou dunite body is closely associated with the activity of mantle super-plume in Neo-Proterozoic era in the Yangtze Craton. [ABSTRACT FROM AUTHOR]

Zircons from two high-pressure granulite samples from the Yushugou ophiolitic terrane, southern Tianshan have been investigated by cathodoluminescence, LA- ICPMS and ion microp robe (SHRIMP) for their internal textures, trace elemental compositions and in situ dating. The weighted mean ages of these two samples are 392±7 and 390± 11 Ma, respectively, representing the granulite-facies metamorphic age of the ophiolitic terrane, and indicating that the southern Tianshan ocean initiated its northward subduction since the early Devonian. [ABSTRACT FROM AUTHOR]

Reports the zircon ion microprobe (SHRIMP) dating results for Songshugou garnet pyroxenite and Fushui gabbro. Suggestion that the Fushui gabbro-diorite complex body possesses characters of Alaska-type island-arc mafic-ultramafic plutonic complex which is an important component of the island-arc magmatic belt; Kinds of rocks that constructed the Early Paleozoic tectonic pattern of plate subduction and collision; Implications for the understanding of the tectonic evolution of the Qinling Orogenic Belt.

Presents a study that examined the geochemistry of basic dykes in the Wudang block of south Qinling, China and discussed their tectonic significance. Regional geology; Geochemistry of the basic dykes; Origin and tectonic implications.

Presents a study that focused on the strontium, neodymium, lead and oxygen isotopes of basic dyke swarms in the Wudang Block and basic volcanics of the Yaolinghe Group in China. General geology; Sample and analysis; Isotope geochemistry of the elements.

The chemistry of the basic volcanic rocks at the south margin of the Qinling orogenic belt (SMQOB) suggests that they were formed in an intraplate tectonic setting. The REE distribution patterns show these rocks are strongly enriched in LREE with high ΣREE, and their trace elements geochemistry is similar to that of continental flood basalt. All the above evidence suggests that the Caledonian basic volcanic rocks in the SMQOB were tholeiitic basalts formed in an intraplate spreading-initial rift tectonic setting. The characteristics of regional geology and geochemistry indicate that there was an intraplate spreading-rift tectonic setting between the South Qinling block and the Yangtze block in the Caledonian epoch. The dynamic spreading in this district began in the Early Caledonian and then the intraplate spreading-initial rifts were formed in the late Caledonian. As a result of spreading of the Tethys and geodynamic processes in deep mantle, the Mianlue-Huashan oceanic basin was formed between the Qinling block and the Yangtze block in Devonian, and the Qinling microplate was separated from the northern part of the Yangtze plate. [ABSTRACT FROM AUTHOR]

The Yushugou terrane of deepcrustal granulite facies in southern Tianshan consists of two parts, granulite and metaperidotite. The whole terrane is a metamorphism of (high-pressure) grunulite facies, and typical mineral associations are: Gt-Cpx-P1-Tit-Ilm (±Qz) (silica-saturated and oversaturated mafic rocks), Gt-Ky (pseudomorph)-P1-Ru-Ilm± Qz (metapelitic rocks) and Spi-Opx-Cpx-01 (meta-ultramafic rocks). The peak-stage P-T conditions are 795— 964°, 0.97—1.42 GPa, which are obtained with mineral chemistry, assemblage analyses and P-T estimation. The Sm-Nd isochron age of peak-stage metamorphic minerals is (315 ± 3.62) Ma. All of these indicate that the terrane is a deep-crustal body, which subducts to the depth of 40—50 km in the middle late-Paleozoic, undergoing metamorphism of (high-pressure) granulite facies, and exhumed again to the surface by tectonic uplifting. [ABSTRACT FROM AUTHOR]

On the basis of the synthetic studies of geology and geochemistry, an ophiolitic tectonic melange waa discovered in Sanligang-Sanyang area, the western part of Xiangfan-Guangji fault, the south margin of the Qinling Orogenic Belt. It is composed of different tectonic blocks with different lithological features and ages, mainly including the Huashan ophiolite blocks, Xiaofu Island-arc volcanic blocks, pelagic sediments, fore-arc volcanic-sedimentary system, and the massif of the basement and the covering strata of the Yangtze Block. These massifs were emplaced in the western part of Xiangfan-Guangji fault, the boundary between the Qinling Orogenic Belt and Yangtze Block, contacting each other by a shear zone or chaotic matrix. The characteristics of geochemistry indicate that the bash of the Huashan ophiolite are similar to mid-oceanic ridge basalts (MORB) formed in an initial oceanic baain, and the Xiaofu volcanic rocks are formed in a tectonic setting of island arc. The ophiolitic tectonic melange is the fragments of subduction wedge, which implies that there has been an oceanic basin between Qinling Block and Yangtze Block. [ABSTRACT FROM AUTHOR]

Focuses on the geological significance of the basic dyke swarms in the Wudang block in China. Geochemical characterization by continental tholeiites; Samarium-neodymium age; Presence of an ancient continental block in the Proterozoic.