Search

The petrogenesis and geodynamic setting of magmatic rocks in Eastern Tianshan, Xinjiang, are attracting increasing attention. This study explores these issues by providing SHRIMP zircon U–Pb dating, and element geochemical and Hf isotope data of magmatic rocks exposed in the Yandong region of Eastern Tianshan, Xinjiang. SHRIMP zircon U–Pb dating data indicate that the magmatic activities occurred around 335 Ma, and that mineralization of the Yandong porphyry Cu deposit occurred during the same period as that of the rock formation in the region or slightly later. All the magmatic rocks in Yandong are sub-alkaline or high-K, calc-alkaline with A/CNK values ranging from 0.91 to 2.70, enriched in K, Rb, and Ba and clearly depleted in Nb, Ta, P, Ti, and Th, and have geochemical characteristics typical of adakites (i.e, high Al2O3, high La/Yb ratios and Sr contents, low Y and HREE contents, and positive Eu anomalies).In situHf isotopic analyses of zircons that had previously been dated by SHRIMP yielded positive initial εHf(t) values ranging from +9.4 to +15.8. We suggest that the Palaeo-Tianshan oceanic crust subducted northward beneath the Dananhu-Tousuquan arc belt during the early Carboniferous, and the Yandong magmatic rocks were induced by partial melting of the subducted Palaeo-Tianshan oceanic slab, subsequently having been hybridized by peridotite in the mantle wedge. The slab melts play a crucial role particularly in the relatively closed magma system responsible for generating the Yandong porphyry Cu deposit. [ABSTRACT FROM AUTHOR]

Abstract: The Tulasu basin is an important epithermal Au mineralization district in western Tianshan in northwestern China. Geochemical and geochronological studies of the volcanic host rocks (the Dahalajunshan Formation) and associated intrusive rocks, particularly the enrichment of large-ion lithophile elements and depletion of high field strength elements, suggest that the Tulasu basin was located in a magmatic-arc setting from Late-Devonian to Early-Carboniferous, associated with the southward subduction of the North Tianshan Ocean beneath the Kazakhstan–Yili plate. Geologic and geochemical characteristics of the Au Cu–polymetallic deposits in the Tulasu basin suggest that the mineralization is related to the Late-Devonian to Early-Carboniferous arc magmatism, and is mainly of epithermal nature, including both the adularia–sericite type (Axi, Tawuerbieke and Tabei) and alunite–kaolinite or acid–sulfate type (Jingxi–Yelmend and Tieliekesayi). However, some evidence of porphyry-type mineralization has also been found, including the presence of mineralized porphyry enclaves in volcanic rocks hosting the Tawuerbieke Au prospect, and the development of porphyry-style Cu mineralization overprinted by vein-type mineralization at Kexiaxi. Considering the magmatic-arc setting that is favorable for both epithermal and porphyry types of mineralization, and the development of such mineralization systems in other parts of western Tianshan and the Altaid Belt (e.g., Uzbekistan, Kazakhstan and Kyrgyzstan), we suggest that major porphyry Cu Au deposits may have been developed in the Tulasu basin, probably beneath or adjacent to known epithermal Au mineralization. [Copyright &y& Elsevier]

The large to middle-sized magmatic Ni–Cu sulfide deposits are spatially situated in the eastern portion of Eastern Tianshan, whereas those in the western portion have been poorly understood due to their quantity and scale. The Haibaotan Ni–Cu sulfide occurrence is newly discovered in the western portion of Eastern Tianshan, which provides an opportunity for understanding how its magmatic conduit system was generated and evolved. The Haibaotan mafic–ultramafic intrusion is dominated by peridotite, pyroxenite, olivine gabbro, and gabbro, and sulfide orebodies occur mainly within gabbro. Gabbro from the Haibaotan intrusion is dated at 315.5 ± 1.9 Ma, which suggests that the Haibaotan intrusion is generated in a subduction evironment. The age is much earlier than the emplacement of other mineralized mafic–ultramafic intrusions (~ 280 Ma) in Eastern Tianshan. High zircon εHf(t) values (+ 7.09 to + 18.44) indicate that the parental magmas of the Haibaotan intrusion have been derived from a metasomatized mantle source. Additionally, mafic–ultramafic rocks in the Haibaotan intrusion exhibit enrichment of large ion lithophile elements (LILE) and light rare earth elements (LREE), together with a depletion of Zr, Ti, and Nb, which is indicative of a subduction-modified mantle source. Core–rim variations of olivine forsterite (Fo) and Ni contents imply that the Haibaotan intrusion resulted from successive magma pulses, and the variable PGE tenors reflect that the sulfide droplets scavenged metals from multiple magma pulses. The low Se/S ratios of sulfide mineralization suggest that the addition of crustal sulfur triggered sulfide segregation in the shallow magma conduit system. The evolution history of the Haibaotan magma conduit system meets the requirements for generating a magmatic Ni–Cu deposit. This study highlights that the subduction environments are favorable for the formation of magmatic sulfide deposits in Eastern Tianshan. [ABSTRACT FROM AUTHOR]

Significant variations of oxygen fugacity (f O 2) in silicate magmas are widely suggested to play an important role in formation of magmatic Ni–Cu deposits in convergent margin settings. Copper isotopes have potential to track redox change of magmatic systems, but the linkage of silicate magma oxygen fugacity with Cu isotope fractionation in magmatic Ni–Cu mineralization systems is still uncertain. A combined study of Cu isotopes and estimated oxygen fugacities can provide novel and direct insights into this issue. In this study, four magmatic Ni–Cu deposits in Eastern Tianshan (NW China) were investigated in order to reveal the f O 2 variations in silicate magmas as a function of redox-induced Cu isotope fractionation. The silicate magma f O 2 of the four deposits widely varies from ∼QFM –2.2 to + 0.6, from ∼QFM –0.7 to 0.0, from ∼QFM –1.4 to –0.2, and from ∼QFM –1.2 to –0.6 for the Tulaergen, Huangshandong, Huangshannan, and Hulu deposits, respectively. Models using olivine compositions, Cu/Pd ratios, and estimated silicate magma f O 2 suggest that the silicate magmas gradually became more oxidized as Ni–Cu mineralization proceeded. The closed-system R factor equation and the Rayleigh equation were used to model mineralization processes and related Cu isotope fractionation. Modelling using δ65Cu values and the estimated f O 2 indicates that the evolved silicate magmas with high f O 2 tend to incorporate heavier Cu isotopes. Our studies indicate that the f O 2 variation of silicate magmas is the key factor governing Cu isotope fractionation in magmatic Ni–Cu mineralization systems in convergent tectonic settings, which is potentially applicable for other magmatic Ni–Cu deposits worldwide. [ABSTRACT FROM AUTHOR]

Eastern Tianshan is an important nickel ore belt in China, in which large to middle-sized magmatic Ni–Cu sulfide deposits occur spatially in the eastern part. The Baixintan magmatic Ni–Cu sulfide deposit is an important discovery in the western part of Eastern Tianshan, which provides an ideal target to track the evolution of the magmatic mineralization system in this part. The Baixintan intrusion is composed of the northeastern mafic units and the southwestern mafic–ultramafic units. The olivine Fo contents of the Baixintan intrusion range from 81.12 to 84.46 mol.%. Numerical calculations yield a parental magma composition with 9.14 wt.% MgO. The Rayleigh simulation using olivine Fo and Ni contents shows that the parental magma of the Baixintan intrusion has experienced ~ 2% olivine crystallization, and then evolved magma became sulfide-saturated. Calculations by the mass-balance R factor equation using PGE data indicate that the formation of the mafic and mafic–ultramafic units of the Baixintan intrusion resulted from multiple influxes of magmas. The Baixintan intrusion is highly comparable with those mafic–ultramafic intrusions that host large magmatic Ni–Cu sulfide deposits in the eastern part of Eastern Tianshan. The mafic–ultramafic intrusions in the western part of Eastern Tianshan should be carefully evaluated for the potential of hosting large magmatic Ni–Cu deposits. [ABSTRACT FROM AUTHOR]

The Central Asian Orogenic Belt (CAOB) records a critical stage of Phanerozoic continental growth directly related to substantial juvenile additions during accretionary orogenesis. However, the processes and mechanisms of how juvenile continental growth remain controversial. The Central Tianshan Block (CTB) is a crucial part of the Tianshan orogenic belt. It was involved in the subduction—accretion of the western Paleo-Asian Ocean and the final amalgamation of the Tianshan Orogen in the southwestern CAOB. Multiphase late Paleozoic magmatism is recorded in the Katebasu district, forming one of the most critical granitoid stocks that host significant gold mineralization (i.e., Katebasu Au deposit, 89 t Au @ 2.7 g/t) in the CTB. Based on our new U–Pb ages, geochemical and Sr–Nd–Hf isotopic systematics, in combination with previously published datasets from the adjacent regions, we propose that the Katebasu granitoid stock records the regional tectonic switching from compression to extension of the CTB during the Devonian to Early Carboniferous. The compressional tectonic regime produced extensive juvenile granitoid crust. In contrast, subsequent extensional setting was probably related to slab rollback that may have triggered incremental involvement of juvenile mantle components in producing additional continental crust. Such a tectonic model probably plays an important role in the tectonic evolution and crustal growth of the CTB during the late Paleozoic. On a larger scale, such a long-term "rejuvenation" process probably plays a significant role in the generation of widespread juvenile magmatism and continental growth during the formation of the CAOB. [ABSTRACT FROM AUTHOR]

• Pb isotopic data of sulfide, metasedimentary and magmatic rocks at Chinese South Tianshan. • Paleoproterozoic metasedimentary rocks is likely to contribute 70% to 90% of the metal source. • Crustal fertilization and subsequently regional tectonic uplift and thermal conduction play key roles in formation of lode gold deposits. The Tianshan hosts numerous giant and world-class lode gold deposits, forming one of the world's largest gold provinces. However, the sources of gold remain equivocal. Herein, we present a comprehensive Pb isotopic investigation of the Precambrian basement, Neoproterozoic mafic dykes, and the Lower Paleozoic sequences of the South Tianshan in NW China. These data were compared with existing Pb isotope datasets from major lode gold deposits and coeval granitoid and mantle-related rocks, to trace the sources of gold and constrain the relative contributions from potential reservoirs. Gold-bearing sulfides from major lode gold deposits have relatively uniform Pb isotopic ratios, which are significantly different from regional coeval granitoid and mantle-related rocks. Conversely, the Pb isotopic characteristics from gold-bearing sulfides are comparable to those from the Paleoproterozoic metasedimentary rocks, showing well-defined linear relations. Binary mixing modeling shows that the Paleoproterozoic metasedimentary rocks might have contributed 70% to 90% for the source of Pb and, by inference other metals. Integrating geochemical and isotopic features from major lode gold deposits in the region, we propose that it invokes fertilization of ancient Au archives during metamorphic volatilization in the deep crust during subduction of the South Tianshan Ocean. Following the final closure of the South Tianshan Ocean, the regional tectonic uplift and thermal effect during widespread granitoids emplacement associated with the amalgamation of the Tarim with Kazakhstan-Yili terranes, mobilizing the deep roots of the fertilized ancient Au reservoirs, leading to the formation of the gold deposit hosted in fault systems of the South Tianshan. [ABSTRACT FROM AUTHOR]

Magmatism associated with plate subduction and collision plays an important role in crustal growth during the evolution of the Earth, but the detailed mechanisms regarding how magmatism contributes to the vertical growth of the crust remain unclear, in part due to controversies on the petrogenesis of intermediate-felsic rocks. In the Central Asian Orogenic Belt (CAOB), although it has been recognized that both pre-existing crustal component and juvenile mantle-derived melt played a role in the crustal evolution of the orogens, the mechanics of how they contributed to vertical crustal growth remain uncertain. This study presents new geochronological, major and trace element geochemical, and Nd–Hf isotopic data for the Late Paleozoic intermediate-felsic volcanic rocks of the northern Yili terrane, Western Tianshan Orogen, with the aims to constrain the petrogenesis of the andesitic magma and to offer new insights into the deep interaction processes between crustal- and mantle-derived magmas. According to zircon U–Pb dating, trace element and isotopic compositional data, these volcanic rocks can be divided into two groups. Group 1 rocks (365–375 Ma) have relatively low whole-rock ε Nd (t) values (−4.2 – −1.6) and display high contents of Th (11.8–20.4 ppm), U (2.7–5.1 ppm), Nb (14.1–36.7 ppm) and Ta (1.1–3.1 ppm) that suggest they were derived from the partial melting of pre-existing crust. Group 2 rocks (346–349 Ma) show REE and trace elemental patterns typical of subduction-related calc-alkaline volcanic rocks with relatively high whole-rock ε Nd (t) values (−1.89 – +2.4), and with variable zircon Hf isotopic values (ε Hf (t) = +1.9 – +13.5) that can be attributed a juvenile crust with variable input of mantle material. Hf–Nd isotopic decoupling of the group 2 rocks suggests a replenishment by more mafic magmas. Samples from the entire Yili terrane show an increasing trend in ε Nd (t) values with younger ages that suggest an increasing involvement of melted juvenile crustal rocks. Overall, these results suggest a crustal evolutionary scenario in which: 1) pre-existing crustal components were melted and ascended to the upper crust (group 1) during the southward subduction of the Junggar-North Tianshan ocean in the early stage; and, 2) a later stage in which newly input mantle material solidified to form new arc crust that was also melted by basaltic underplating and that was probably associated with the rollback of the Junggar oceanic slab. This two-stage crustal growth model seems to be an efficient process for vertical crustal growth during the formation of an accretionary orogen. • The DVF volcanic rocks are divided into two groups according to their ages and source rocks. • Underplating of mantle-derived magmas was associated with rollback of Junggar oceanic slab. • Two-stage crustal growth model works well for accretionary orogenic process. [ABSTRACT FROM AUTHOR]

Various analytical techniques have been developed to determine the solution composition of fluid inclusions, including destructive, non-destructive, single-inclusion, and bulk-inclusion methods. Cryogenic Raman spectroscopy, as a non-destructive and single-inclusion method, has emerged as a potentially powerful tool of quantitative analysis of fluid inclusion composition. A method of point analysis using cryogenic Raman spectroscopy has been previously proposed to quantitatively estimate the solute composition of H2O-NaCl-CaCl2 solutions, but there are uncertainties related to heterogeneity of frozen fluid inclusions and potential bias in the processing of Raman spectra. A new method of quantitative analysis of solute composition of H2O-NaCl-CaCl2 solutions using Raman mapping technology is proposed in this study, which can overcome the problems encountered in the point analysis. It is shown that the NaCl/(NaCl + CaCl2) molar ratio of the solution, X(NaCl, m), can be related to the area fraction of hydrohalite over hydrohalite plus antarcticite, Fhydrohalite, by the equation X(NaCl, m) = 1.1435 Fhydrohalite − 0.0884, where Fhydrohalite = hydrohalite area/(hydrohalite area + antarcticite area). This equation suggests that the molar fraction of a salt component may be estimated from the fraction of the Raman peak area of the relevant hydrate. This study has established a new way of estimating solute composition of fluid inclusions using cryogenic Raman mapping technique, which may be extended to other solutions. [ABSTRACT FROM AUTHOR]

The Hongshan deposit is one of the largest Cu-polymetallic deposits in the Zhongdian area, southwest China. Two types of Cu–Mo ores, mainly developed in the skarns, have been recognized in the Hongshan deposit, i.e., massive or layered skarn and vein-type, with the former being dominant. The highly andraditic composition of garnet (Adr100 to Adr64Gr32) and diopsidic composition of pyroxene (Di90Hd9 to Di1Hd99) indicate the layered skarn ores are of magmatic-hydrothermal origin that formed under oxidized conditions. Sm–Nd dating of garnet yield a well-constrained isochron age of 76.48 ± 7.29 Ma (MSWD = 1.2) for the layered skarn ores. This age was consistent with the Re–Os age for the pyrrhotite from the layered skarn ores, and thereby indicated that the layered skarn mineralization was formed in the Late Cretaceous, rather than in the Triassic as was previously thought. The coincidence of the geochronology from the layered skarn ores and vein-type mineralization further indicated that both ores were the result of a single genetic event, rather than multiple events. The recognition of the Late Cretaceous post-collisional porphyry–skarn Cu–Mo–W belt in the Zhongdian area exhibited a promising prospecting potential. [ABSTRACT FROM AUTHOR]

• ca. 983–905 Ma leucogranites and ca. 779 Ma mafic dikes were recognized in the northern Yili Block. • Leucogranites with an S-type affinity were derived by partial melting of pelitic and mafic rocks, whereas mafic dikes were formed from subduction-modified SCLM. • The Yili Block underwent early Neoproterozoic collisional orogenesis and switched to intraplate extension at ca. 779–776 Ma. • Contemporaneous tectonic evolution occurred in adjacent Kazakhstan and northern Tarim Craton. Understanding the tectonic evolution of the many micro-continents in the Central Asian Orogenic Belt is a complex and controversial issue. One such micro-continent, the Yili Block is characterized by Neoproterozoic magmatic rocks, which can provide important constraints on the tectono-magmatic history of the region. Herein, we present a systematic investigation of the petrology, whole-rock geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes on newly identified leucogranite intrusions and gabbroic dikes in the Wenquan district of the northern Yili Block. Two leucogranite samples yielded zircon U-Pb ages of 983 ± 9 Ma and 905 ± 9 Ma, respectively. Both of muscovite-bearing leucogranites have S-type affinities and show distinctive geochemical compositions similar to peraluminous granites. Furthermore, the leucogranites exhibit low Rb/Sr ratios (0.10–0.49) and elevated zircon εHf (t) values (+3.04 - +13.59), indicating that they were derived from partial melting of supracrustal pelitic and mafic rocks. In contrast, one gabbroic dike sample yielded a zircon U-Pb age of 779 ± 5 Ma. The gabbroic samples have distinctly high Th/Yb (1.10–1.68) and low Nb/La (0.37–0.46) ratios, with enrichment in large ion lithophile elements and depletion in Nb, Ta and Ti, implying that they were sourced from a metasomatic subcontinental lithospheric mantle. Integrated with other available data, we propose that the Yili Block underwent continuously collisional orogenesis during the early Neoproterozoic (ca. 983–780 Ma), with a subsequent tectonic shift from collisional to an intraplate extensional setting at ca. 779–776 Ma. A comparable magmatic evolution also occurred in adjacent micro-continents of the Kazakhstan region and northern Tarim Craton, indicating that these micro-continents may have shared a common Neoproterozoic geodynamic evolution. [ABSTRACT FROM AUTHOR]

Syn- to post-collisional magmatism in the Chinese western Tianshan Orogen is recorded in the Late Carboniferous to Early Permian magmatic rocks of the Awulale Mountains. However, the orogen's tectono-magmatic evolution remains controversial. In particular, two contrasting genetic models have been proposed, i.e., post-collisional extension vs. mantle plume. Here, we report zircon U-Pb ages, whole-rock geochemical data and Nd-Hf isotopic data from the mafic dikes and granitoids of the western Awulale Mountains. These data better constrain the rock's petrogenesis and geodynamic processes following the amalgamation of the Tianshan Orogen. In combination with previous data, we have identified two magmatic episodes in the Awulale Mountains. The Late Carboniferous (ca. 319–307 Ma) episode emplaced the mafic gabbro dikes and intermediate-acidic granitoids with I-type and adakitic affinity. The latest Carboniferous-Permian (ca. 302–286 Ma) episode emplaced bimodal intrusions containing mafic diabase dikes, acidic adakite and I- and A-type granites. Mafic dikes and granitoids of both episodes show depleted whole-rock Nd and zircon Hf isotopic compositions (εNd (t) : +2.21 ~ +7.54, εHf (t) : +6.48 ~ +15.97), indicating a juvenile magmatic source. However, the mafic dikes have variable geochemical features that clearly suggest their derivation from two distinct mantle sources during the two magmatic episodes. The earlier gabbro dikes have lower Th/Yb (0.08–0.12) and Zr/Yb ratios (15.32–25.45) that resemble a normal mid-ocean ridge basalt (N-MORB) from a depleted mantle source. In contrast, the later diabase dikes show enrichment in the light rare earth elements (LREE) with relatively higher Th/Yb (0.22–0.86) and Zr/Yb ratios (38.05–68.97) that are similar to an enriched mid-ocean ridge basalt (E-MORB) mantle source. Also, the Nb/Ta ratios from the earlier adakitic granitoids (21.26–23.28) differ markedly from the later acidic adakites (0.41–19.50), which implies the transition from a thicker to a thinner crust. Integrating our results with the previous geological records, leads us to propose a two-stage tectonic model for the Chinese western Tianshan Orogen's transition from syn - to post-collision. The Late Carboniferous stage of magmatism was triggered by the break-off of the North Tianshan Oceanic slab as the Yili Block and Junggar Terrane collided. The subsequent latest Carboniferous-Permian stage followed the switch to a post-collisional setting with delamination of the thickened lower crust and the intrusion of voluminous lithospheric mantle-induced bimodal magmatism. • Two episodes of contrasting magmatism have been identified in Awulale. • Early magmatism was triggered by slab break-off in a syn -collisional setting. • Late magmatism was linked to post-collisional crustal delamination. [ABSTRACT FROM AUTHOR]