Genesis of the Weiquan Ag–Polymetallic Deposit in East Tianshan, China: Evidence from Zircon U–Pb Geochronology and C–H–O–S–Pb Isotope Systematics

The Weiquan Ag-polymetallic deposit is located on the southern margin of the Central Asian Orogenic Belt and in the western segment of the Aqishan-Yamansu arc belt in East Tianshan,northwestern China. Its orebodies, controlled by faults, occur in the lower Carboniferous volcanosedimentary rocks of the Yamansu Formation as irregular veins and lenses. Four stages of mineralization have been recognized on the basis of mineral assemblages, ore fabrics, and crosscutting relationships among the ore veins. Stage I is the skarn stage(garnet + pyroxene), Stage Ⅱ is the retrograde alteration stage(epidote + chlorite + magnetite ± hematite 士 actinolite ± quartz),Stage Ⅲ is the sulfide stage(Ag and Bi minerals + pyrite + chalcopyrite + galena + sphalerite + quartz ± calcite ± tetrahedrite),and Stage IV is the carbonate stage(quartz + calcite ± pyrite). Skarnization,silicification, carbonatization,epidotization,chloritization, sericitization, and actinolitization are the principal types of hydrothermal alteration. LAICP-MS U-Pb dating yielded ages of 326.5±4.5 and 298.5±1.5 Ma for zircons from the tuff and diorite porphyry, respectively. Given that the tuff is wall rock and that the orebodies are cut by a late diorite porphyry dike, the ages of the tuff and the diorite porphyry provide lower and upper time limits on the age of ore formation. The δ~(13)C values of the calcite samples range from-2.5‰ to 2.3‰, the δ~(18)O_(H2 O) and δD_(VSMOW) values of the sulfide stage(Stage Ⅲ) vary from 1.1‰ to 5.2‰ and-111.7‰ to-66.1‰, respectively,and the δ~(13)C, δ~(18)O_(H2 O) and δD_(V-SMOW) values of calcite in one Stage IV sample are 1.5‰,-0.3‰, and-115.6‰, respectively. Carbon, hydrogen, and oxygen isotopic compositions indicate that the ore-forming fluids evolved gradually from magmatic to meteoric sources. The δ~(34)S_(V-CDT) values of the sulfides have a large range from-6.9‰ to 1.4‰, with an average of-2.2‰, indicating a magmatic source, possibly with sedimentary contributions. The ~(206)Pb/~(204)Pb, ~(207)Pb/~(204)Pb, and ~(208)Pb/~(204)Pb ratios of the sulfides are 17.9848-18.2785,15.5188-15.6536, and 37.8125-38.4650, respectively, and one whole-rock sample at Weiquan yields~(206)Pb/~(204)Pb,~(207)Pb/~(204)Pb, and ~(208)Pb/~(204)Pb ratios of 18.2060, 15.5674, and 38.0511,respectively. Lead isotopic systems suggest that the ore-forming materials of the Weiquan deposit were derived from a mixed source involving mantle and crustal components. Based on geological features, zircon U-Pb dating, and C-H-OS-Pb isotopic data, it can be concluded that the Weiquan polymetallic deposit is a skarn type that formed in a tectonic setting spanning a period from subduction to post-collision. The ore materials were sourced from magmatic ore-forming fluids that mixed with components derived from host rocks during their ascent, and a gradual mixing with meteoric water took place in the later stages.     

Geochemical and Petrological Studies on the Early Carboniferous Sidingheishan Mafic–Ultramafic Iintrusion in the Southern Margin of the Central Asian Orogenic Belt, NW China

The Sidingheishan mafic-ultramafic intrusion is located in the eastern part of the Northern Tianshan Mountain, along the southern margin of the Central Asian Orogenic Belt in northern Xinjiang autonomous region of China. The Sidingheishan intrusion is mainly composed of wehrlite, olivine websterite, olivine gabbro, gabbro and hornblende gabbro. At least two pulses of magma were involved in the formation of the intrusion. The first pulse of magma produced an olivine-free unit and the second pulse produced an olivine-bearing unit. The magmas intruded the Devonian granites and granodiorites.An age of 351.4±5.8 Ma(Early Carboniferous) for the Sidingheishan intrusion has been determined by U-Pb SHRIMP analysis of zircon grains separated from the olivine gabbro unit. A U-Pb age of 359.2±6.4 Ma from the gabbro unit has been obtained by LA-ICP-MS. Olivine of the Sidingheishan intrusion reaches 82.52 mole% Fo and 1414 ppm Ni. On the basis of olivine-liquid equilibria, it has been calculated that the MgO and FeO included in the parental magma of a wehrlite sample were approximately10.43 wt% and 13.14 wt%, respectively. The Sidingheishan intrusive rocks are characterized by moderate enrichments in Th and Sm, slight enrichments in light REE, and depletions in Nb, Ta, Zr and Hf. The ε_(Nd)(t) values in the rock units vary from +6.70 to +9.64, and initial ~(87)Sr/~(86)Sr ratios range between 0.7035 and0.7042. Initial ~(206)Pb/~(204)Pb, ~(207)Pb/~(204)Pb and ~(208)Pb/~(204)Pb values fall in the ranges of 17.23-17.91,15.45-15.54 and 37.54-38.09 respectively. These characteristics are collectively similar to the Heishan intrusion and the Early Carboniferous subduction related volcanic rocks in the Santanghu Basin, North Tianshan and Beishan area. The low(La/Gd)_(PM) values between 0.26 and 1.77 indicate that the magma of the Sidingheishan intrusion was most likely derived from a depleted spinel-peridotite mantle.(Th/Nb)_(PM)ratios from 0.59 to 20.25 indicate contamination of the parental magma in the upper crust.Crystallization modeling methods suggest that the parental magma of the Sidingheishan intrusion was generated by flush melting of the asthenosphere and subsequently there was about 10 vol%contamination from a granitic melt. This was followed by about 5 vol% assimilation of upper crustal rocks. Thus, the high-Mg basaltic parental magma of Sidingheishan intrusion is interpreted to have formed from partial melting of the asthenosphere during the break-off of a subducted slab.     

Ordovician Proto–basin in South China and its Tectonic Implications: Evidence from the Detrital Zircon U–Pb Ages of the Ordovician in Central Hunan,China

Caledonian orogeny is another important tectonic event in South China Block after the breakup of the Rodinia supercontinent. With a view to constrain the tectonic evolution and proto–basin in South China, this paper reports the geochemical and zircon U-Pb dating data of the Ordovician strata in central Hunan, South China. Geochemical features and paleocurrent directions suggest that the lower Ordovician deposited in a passive continental margin basin with a provenance of quartzose components and...     

Subduction of a spreading ridge within the Bangong Co–Nujiang Tethys Ocean: Evidence from Early Cretaceous mafic dykes in the Duolong porphyry Cu–Au deposit,western Tibet

The intrusion of mafic dykes into a near-trench accretionary prism, and continental margin magmatism with characteristics that differ from those of adjacent arc magmatism, are direct manifestations of the subduction of a spreading ocean ridge and the formation of a slab window. In this paper, we investigated mafic dykes intruded into the accretionary prism that hosts the Duolong porphyry Cu–Au deposit (DCAD) of western Tibet. LA-ICP-MS analysis of U–Pb in zircon indicates that the dykes formed during the Early Cretaceous (126–127 Ma). The dykes are characterized by ε_Hf(t) values from + 2.44 to + 11.8. Twenty-nine mafic dyke samples were divided into three groups based on their locations and geochemical compositions: group I has Nb = 8.31–10.2 ppm, Nb/La = 0.71–1.20, and Nb/U = 21.4–37.9; group II has Nb = 40.5–52.6 ppm, Nb/La = 0.84–1.58, and Nb/U = 18.8–47.8; and group III has Nb = 65.7–105 ppm, Nb/La = 1.35–2.08, and Nb/U = 36.5–73.8. Group I is classified as Nb-enriched basalts (ENBs), whereas groups II and III are classified as high-Nb basalts (HNBs). Both the ENBs and HNBs were derived from an adakite-metasomatized mantle wedge that subsequently underwent crystallization of olivine and clinopyroxene. The compositional variations of the studied dykes resulted mainly from mantle source heterogeneity. The volume of the slab melts gradually increases from group I (ENBs) to group II (HNBs) and group III (HNBs), leading to gradually increasing incompatible element concentrations. Considering their geochemical characteristics and field relationships, as well as the unique characteristics of continental margin magmatism in the DCAD, we propose that the dykes emplaced in an extensional accretionary prism were derived from the northward subduction of a spreading ridge in the Bangong Co–Nujiang Tethys Ocean during the Early Cretaceous. The ridge subduction event was also responsible for the generation of coeval adakites, intermediate–felsic intrusions, the Maierze bimodal volcanic rocks as well as the metallogenesis of the DCAD.     

Surface Processes Driving Intracontinental Basin Subsidence in the Context of India–Eurasia Collision: Evidence from Flexural Subsidence Modeling of the Cenozoic Southern Tarim Basin along the West Kunlun Foreland, NW Tibetan Plateau

The India–Eurasia collision has produced a number of Cenozoic deep intracontinental basins, which bear important information for revealing the far-afield responses to the remote collision. Despite their significance, their subsiding mechanism remains the subject of debate, with end-member models attributing it to either orogenic or sedimentary load. In this study, we conduct flexural subsidence modeling with a two-dimensional finite elastic plate model on the Hotan–Mazatagh section along the southern Tarim Basin, which defines a key region in the foreland of the West Kunlun Orogen, along the NW margin of the Tibetan Plateau. The modeling results indicate that the orogenic load of West Kunlun triggers the southern Tarim Basin to subside by up to less than ~6 km, with its impact weakening towards the basin interiors until ~230 km north from the Karakax fault. The sedimentary load, consisting of Cenozoic strata, forces the basin to subside by ~2 to ~7 km. In combination with the retreat of the proto-Paratethys Sea and the paleogeographic reorganization of the Tarim Basin, we propose that surface processes, in particular a shift from an exorheic to an endorheic drainage system associated with the consequent thick sedimentary load, played a decisive role in forming deep intracontinental basins in the context of the India–Eurasia collision.     

Origin of Halite in the Lanping–Simao Basin, Southeastern Tibetan Plateau, China: Evidence from Sr and Cl Isotopes

The Lanping?Simao Basin is located on the southeastern Tibetan Plateau, China, and contains massive evaporites. The origin of evaporites in the basin has been hotly debated because of the strong transformation by tectonic movement. Forty halite samples from borehole MK-3 in the Mengyejing area of the basin were collected and analyzed using XRD, Cl-Sr isotopes and chemical compositions to trace the origin of the evaporites in the basin. The Br × 103/Cl ratios of the halite samples are between 0 and 0.55, most of which are synchronized with the law of seawater evaporation and at the stage of halite precipitation from seawater, indicating that the evaporites are mainly of marine origin. The 87Sr/86Sr ratios range from 0.707489 to 0.711279; after correction, the 87Sr/86Sr 145 Ma ratios range from 0.704721 to 0.707611, equivalent with the 87Sr/86Sr ratios of seawater at 145 Ma, indicating a marine origin. The decay of 87Rb in the evaporite during deposition, change of the depositional environment and the unsealed environment at a later period resulted in the present 87Sr/86Sr ratios of some samples being high. The δ37Cl value compositions range from ?0.38‰ to 0.83‰, which is consistent with the δ37Cl value composition of the world marine halite (?0.6‰ to 0.4‰), further confirming that seawater is the main origin. In addition, the high δ37Cl value of some samples at the boundary of the upper and lower evaporite layers might be related to the influence of δ37Cl-rich brine and the incomplete dissolution of the halite.     

2.3 Ga Magmatism and 1.94 Ga Metamorphism in the Xiatang Area,Southern Margin of the North China Craton——Evidence from Whole-rock Geochemistry and Zircon Geochronology and Hf Isotope

本文报道了华北克拉通南缘豫西鲁山下汤地区古元古代片麻状花岗岩和黑云角闪斜长片麻岩的全岩地球化学和锆石SHRIMP U-Pb年龄和Hf同位素组成.岩石呈包体形式存在于中元古代花岗岩中.片麻状花岗岩具深熔特征,岩浆锆石年龄为2.30 Ga;岩石高SiO2和K2O,低∑FeO、MgO和CaO,具稀土总量较高(∑REE=165.8×10-6)、轻重稀土分离较强[(La/Yb)n=37.8]及弱负铕异常(Eu/Eu(*)=0.76)的稀土模式;εNd(t)(t=2.30 Ga)=-0.75; tDM(Nd) =2.66 Ga.黑云角闪斜长片麻岩变质原岩为辉长闪长岩,捕获锆石年龄为2.25 Ga;岩石低SiO2 和MgO,高Al2O3和P2O5,具稀土总量高(∑REE=373.4×10-6)、轻重稀土分离不强[ (La/Yb)n=9.4]及较强负铕异常( Eu/Eu*=0.44)的稀土模式;εNd(t)(t=2.25 Ga)=-1.21;tDM(Nd) =2.75 Ga.片麻状花岗岩和黑云角闪斜长片麻岩都记录了1.94 Ga变质锆石年龄.片麻状花岗岩的岩浆锆石组成域的εHf(t)(t=2.30 Ga)=-6.71～0.38,tDMI(Hf) =2627 ～2910 Ma,tDM2(CC)(Hf)=2823 ～ 3255 Ma.黑云角闪斜长片麻岩的捕获锆石组成域的εHf(t)(t=2.25 Ga)=-19.58～ -1.73,tDM1 (Hf) =2664～3360 Ma,tDM2(CC)(Hf)=2968 ～4011 Ma.结合前人资料,得出如下结论:华北克拉通南缘豫陕晋结合部地区存在—规模较大的约2.3 Ga地质体分布区;华北克拉通南缘很可能存在规模巨大的＞2.7 Ga基底;中部造山带与孔兹岩带具有类似的古元古代晚期构造热事件演化历史.