High Ba-Sr granitic rocks are widespread in Phanerozoic orogenic systems, and their petrogenesis is important for revealing the evolutionary process of the Proto-Tethys Ocean in the North Qilian orogenic belt. This paper presents a combination of zircon U-Pb age, whole-rock major and trace element concentrations, and Sr-Nd-Hf isotopic data for Caowa high Ba-Sr dioritic intrusion from the eastern part of the North Qilian orogenic belt, aiming to decipher its petrogenesis and tectonic setting. LA-ICP-MS zircon U-Pb dating yield an emplacement age of 450 ± 2 Ma for the Caowa intrusion, indicating a magmatic activity of the Late Ordovician. The Caowa quartz diorites contain moderate contents of SiO2, MgO, Mg#, and resultant high concentrations of Na2O + K2O, Fe2O3T, and Al2O3, displaying calc-alkaline and metaluminous characteristics. The studied samples have relatively elevated Ba (up to 1165 ppm) and Sr (561 to 646 ppm) contents, with obvious enrichment in LILEs (e.g., Ba, Th, U) and depletions in HFSEs (e.g., Nb, Ta, Ti), resembling those of typical high Ba-Sr granitoids in subduction zones. Together with enriched Sr-Nd isotopic composition [(87Sr/86Sr)i = 0.7082–0.7086, εNd(t) = −5.1 to −4.9], and the wide ranges of zircon εHf(t) values (−13.2 to +8.5), it suggests that these high Ba-Sr quartz diorites were derived from a mixture magma source between the ancient crust materials and the enriched lithospheric mantle metasomatized by fluid released from subducted oceanic crust or sediment. Taking into account the ophiolites, high pressure metamorphic rocks, and arc magmatic rocks in the region, we infer that due to the influence of the northward subduction of the Qilian Proto-Tethys Ocean, the Laohushan oceanic crust of the North Qilian back-arc basin was subducted during the Late Ordovician and resulted in extensive metasomatism of lithospheric mantle by fluids derived from oceanic crust or sediments, and the Caowa high Ba-Sr quartz diorites were generated in the process of crust–mantle interaction during the Late Ordovician.
Abstract Autogenic processes are widely found in various sedimentary systems and they play an important role in the depositional evolution and corresponding sedimentary architecture. However, autogenic processes are often affected by changing allogenic factors and are difficult to be identified and analyzed from modern and ancient records. Through the flume tank experiment under constant boundary conditions, the depositional process, evolution principles, and the sedimentary architecture of a river-dominated delta was presented, and a corresponding sedimentary architecture model was constructed. The evolution of river-dominated delta controlled only by autogenic process is obviously periodic, and each autogenic cycle can be divided into an initial progradational stage, a middle retrogratational stage, and a late aggradational–progradational stage. In the initial progradational stage, one feeder channel incised into the delta plain, mouth bar(s) was formed in front of the channel mouth, and small-scale crevasse splays were formed on the delta plain. In the middle retrogradational stage, the feeder channel was blocked by the mouth bar(s) which grew out of water at the end of the initial stage, and a set of large-scale distributary splay complexes were formed on the delta plain. These distributary splay complexes were retrogradationally overlapped due to the continuous migration of the bifurcation point of the feeder channel. In the late aggradational–progradational stage, the feeder channel branched into several radial distributary channels, overlapped distributary channels were formed on the delta plain, and terminal lobe complexes were formed at the end of distributary channels. The three sedimentary layers formed in the three stages constituted an autogenic succession. The experimental delta consisted of six autogenic depositional successions. Dynamic allocation of accommodation space and the following adaptive sediments filling were the two main driving factors of the autogenic evolution of deltas.
The study of source rocks is a key component in the exploration and discovery of hydrocarbon plays in the offshore petroliferous basins of China. Geochemical analyses, drilling, seismic, and microfossil data were integrated to document the formation mechanism of lacustrine source rocks contained in the Wenchang Formation in the Zhu I Depression of the Pearl River Mouth Basin, China. Several factors that control the development of lacustrine source rocks were evaluated in this study, including tectonics, sedimentary conditions, palaeoclimate, the supply of organic matter, and redox conditions. The deposition and occurrence of source rocks is a combined function of these factors. Lacustrine source rocks are characterized by high organic matter contents (average TOC value = 1.33 wt.%) and are dominated by type I and type II 1–2 kerogen , which primarily originate from planktonic algae. The rifting of basement rocks, syn‐sedimentary faults, and palaeoclimate played significant roles in controlling the geochemical properties of the source rocks (e.g., controlling the abundance and types of organic matter). The Eocene lacustrine source rocks were deposited when the palaeoclimate was characterized by humid to semi‐humid, subtropical to tropical weather that did not have a stable temperature or humidity. Controlled by large‐scale faults, “Half‐graben patterns” and “Graben patterns” are the two main palaeotopographic patterns where the thermal stratification of deep lake water resulted in flourishing planktonic algae in its surface zones. Dysoxic to anoxic conditions prevailed during the deposition of the lacustrine source rocks. An underfilled basin situation caused the occurrence of abundant organic matter during the period of maximum surface flooding.
Based on detailed investigation of the modern sedimentation of the distributive fluvial system of Shule River and the data of unmanned aerial vehicle (UAV) aerial photography and satellite remote sensing, the sedimentary characteristics and differences of distributive fluvial system in arid areas are analyzed. By comparing the changes in slope, river morphology and sedimentary characteristics in different sections from the apex to the toe, the distributive fluvial system of Shule River can be divided into three facies belts: "proximal", "middle" and "distal". The proximal belt has the largest slope and strongest hydrodynamic condition, mainly appears as large-scale braided river deposits; the fluvial bars in this belt are mainly composed of gravels, the gravels have good roundness and certain directionality, and are medium-large boulders, with low sand content; the main microfacies in this belt are braided channel and flood plain. The middle belt with slope smaller than the proximal belt, is mainly composed of braided bifurcating river deposits. Due to branching and infiltration, this belt has weaker hydrodynamic conditions, so some of the distributive rivers dry up, appearing as ephemeral rivers. This belt has small lenticular sandbodies, fine to medium gravels, higher sand content, and mainly braided channel, flood plain and aeolian dune microfacies. The distal belt has the smallest slope and flat terrain, where the river begins to transform from braided river to meandering river, the sediment is mainly sand. Due to the influence of slope, this belt has weaker erosion toward source and stronger lateral erosion, and point bars developing around the edge of the active lobes. In this belt, the river is completely meandering, and the main microfacies are braided channel, meandering channel, flood plain, aeolian dune, lake and swamp.
The petrogenesis of high Ba-Sr granitoids provide a great significance to penetrate the Proto-Tethys evolution in the North Qilian orogenic belt. This paper presents a combination of zircon U-Pb age, whole-rock major and trace element concentrations, and Sr-Nd-Hf isotopic data for Caowa high Ba-Sr dioritic intrusion from the eastern part of the North Qilian orogenic belt, aiming to decipher its petrogenesis and tectonic setting. LA-ICP-MS zircon U-Pb dating yields an emplacement age of 447±3 Ma for the Caowa intrusion, indicating a magmatic activity of the late Ordovician. The Caowa quartz diorites contain moderate contents of SiO2, MgO, Mg# and resultant high concentrations of Na2O+K2O, Fe2O3T and Al2O3, displaying calc-alkaline and metaluminous characteristics. Their relatively elevated Ba (up to 1165 ppm) and Sr (561 to 646 ppm) contents, with obvious enrichment in LILEs (e.g. Ba、Th、U) and depletion in HFSEs (e.g. Nb、Ta、Ti) resemble those of typical high Ba-Sr granitoids in subduction zone. Together with enriched Sr-Nd isotopic compositons[(87Sr/86Sr)i=0.7082−0.7086, εNd(t)= -5.1 to -4.9], and relatively extensive εHf(t) values (-13.2 to +8.5) of zircons, it suggests that these high Ba-Sr quartz diorites were derived from a mixture magma source between the ancient crust materials and the enriched lithospheric mantle metasomatised by fluid was released from subducted oceanic crust or sediment. Taking into account the ophiolites, high pressure metamorphic rocks and arc magmatic rocks in the region, we infer that affected by the northward subduction of the Qilian Proto-Tethys ocean, the Laohushan oceanic crust of the North Qilian back-arc basin was subducted during the Late Ordovician and resulted in extensive metasomatism of lithospheric mantle by fluids derived from oceanic crust or sediments, and the Caowa high Ba-Sr quartz diorites generated in the process of crust-mantle interaction during the Late Ordovician.
The Cambrian Yuertus Formation and Ordovician Saergan and Yingan formation source rocks, which TOC contents of 0.38%–4.30%, are well developed in the Keping area of the Tarim Basin. Reservoir bitumen had been found in the Cambrian Wusongger Formation and Shayilike Formation. In this study, the geochemical characteristics of the bitumen and source rocks were analyzed through biomarkers for oil-source correlation. The results show that the characteristics of the bitumen and Yuertus Formation source rocks are similar. Comparatively, the Yuertus Formation source rocks and bitumen have lower Pr/Ph values and higher C 28 /C 29 regular steranes values. The maturity characteristics and depositional environment of the Cambrian source rocks in the Keping area and the platform basin areas are similar. Plots of Ph/ n- C 18 versus Pr/ n- C 17 , Ts/(Ts+Tm) versus 4-/1-MDBT (methyl dibenzothiophene), and DBT/P (dibenzothiophene/phenanthrene) versus Pr/Ph distinguish the bitumen and source rocks well. As an original plot, we found that the Fla/Py (fluoranthene/pyrene) versus MP/P (methyl-phenanthrene/phenanthrene) intersection plot can be used to identify the possible sources of polycyclic aromatic hydrocarbons (PAHs) to a certain extent and can distinguish between the Cambrian and Ordovician source rocks in this study. Comprehensive analysis revealed that the bitumen samples most likely originated from the Yuertus Formation source rocks. It was also found that the biomarker characteristics such as the shape type of the C 27 -C 28 -C 29 regular steranes, triarylosteranes, and triarylosteroids are not applicable to distinguishing the Cambrian and Ordovician source rocks in the Keping area. These research findings provide references for studying the Lower Paleozoic oil-source correlation in the platform in the Tarim Basin.
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