Search

The closure of Paleo-Asian Ocean is considered to have occurred along the Solonker Suture in the southernmost segment of the Central Asian Orogenic Belt (CAOB), the largest Phanerozoic accretionary orogen on the globe. The suture branches to the east to form the northern Hegenshan–Heihe Suture and the southern Solonker–Changchun Suture. The Hegenshan–Heihe Suture is an ideal natural laboratory for studying the post-collisional geodynamic processes operating in a soft collision zone driven by divergent double-sided subduction. Here we report results from an integrated study of the petrology, geochronology, geochemistry, and Sr–Nd–Hf isotopic compositions of the Early Carboniferous–Early Permian magmatic suite in the Hailar Basin of the Xing’an–Erguna Block. The Early Carboniferous igneous rocks are represented by 356–349 Ma andesitic tuffs, exhibiting typical subduction-related features, such as enrichment in large-ion lithophile elements and depletion in high-field-strength elements. These features, together with the relatively depleted Sr–Nd–Hf isotopic compositions, constant Nb/Y values, but highly variable Rb/Y and Ba values indicate that these rocks were generated by partial melting of a depleted mantle wedge metasomatized by slab-derived fluids. The Late Carboniferous–Early Permian magmatic suite (317–295 Ma) is characterized by high Sr contents (313–1080 ppm) and low Y contents (5–13 ppm), and these can be subdivided into calc-alkaline adakitic rocks and high-K calc-alkaline adakitic rocks. The calc-alkaline adakitic rocks have higher values of Sr/Y, (Sm/Yb)source normalized, and Mg#, and lower values of Y, Ybsource normalized, and K2O/Na2O than the high-K calc-alkaline adakitic rocks, which suggests that the former was generated by partial melting of foundered lower continental crust and the latter by partial melting of normal lower continental crust. Based on our new data, in conjunction with those in previous studies, we conclude that the tectonic evolution of the Hegenshan–Heihe Suture involved Early Carboniferous double-sided subduction of the Nenjiang Ocean, latest Early Carboniferous soft collision between the Xing’an–Erguna and Songliao blocks, and Late Carboniferous–Early Permian post-collisional extension. We also propose a new geodynamic scenario in which removal of the lithospheric root might have occurred in a soft collision zone during the post-collision period via repeated and localized lithospheric dripping, which results from combined effects of hydration weakening of the lithosphere caused by pre-collision subduction and asthenospheric stirring triggered by slab break-off.

Late Mesozoic Nb-rich basaltic andesites and high-Mg adakitic volcanic rocks from the Hailar–Tamtsag Basin, northeast China, provide important insights into the recycling processes of crustal materials and their role in late Mesozoic lithospheric thinning. The Late Jurassic Nb-rich basaltic andesites (154 ± 4 Ma) are enriched in large-ion lithophile and light rare earth elements, slightly depleted in high-field-strength elements, and have high TiO2, P2O5, and Nb contents, and (Nb/Th)PM and Nb/U ratios, which together with the relatively depleted Sr–Nd–Hf isotopic compositions indicate a derivation from a mantle wedge metasomatized by hydrous melts from subducted oceanic crust. The Early Cretaceous high-Mg adakitic volcanic rocks (129–117 Ma) are characterized by low Y and heavy rare earth element contents, and high Sr contents and Sr/Y ratios, similar to those of rocks derived from partial melting of an eclogitic source. They also have high Rb/Sr, K2O/Na2O, and Mg# values, and high MgO, Cr, and Ni contents. These geochemical features suggest that the adakitic lavas were derived from partial melting of delaminated lower continental crust, followed by interaction of the resulting melts with mantle material during their ascent. Our data, along with available geological, paleomagnetic, and geophysical evidence, lead us to propose that recycling of Paleo-Pacific oceanic crustal materials into the upper mantle due to flat-slab subduction and rollback of the Paleo-Pacific Plate during the late Mesozoic likely provided the precondition for lithospheric thinning in northeast China, with consequent lithospheric delamination causing recycling of continental crustal materials and further lithospheric thinning.

Much controversy exists regarding the evolution and termination of accretionary orogens where typical continent-continent collision features are lacking. The Central Asian Orogenic Belt provides a key opportunity to explore the genetic relationships between the formation of high-Mg igneous associations and the evolution of accretionary orogens. In this study, two-stage Triassic magmatic events were identified in the southeastern segment of the Central Asian Orogenic Belt (including ca. 247 Ma andesites and ca. 222 Ma diorites), through precise secondary ion mass spectrometry (SIMS) and laser ablation–inductively coupled plasma– mass spectrometry (LA-ICP-MS) zircon dating. The Olenekian andesites show typical sanukitoid affinities with high TiO2, MgO, Ni, and Cr contents, and enrichment in large ion lithophile elements (LILEs; K, Ba, and Sr). They yield depleted Hf isotopic compositions and high δ18O values, which along with Hf-O isotope modeling results indicate that they originated from the partial melting of mantle wedge peridotites with input of 20% terrigenous sediments. The Norian diorites have moderate SiO2, relatively high MgO, Cr, and Ni contents, Mg# values of 63–65, high Sr, low Y and Yb contents, relevant high La/Yb ratios, and negligible Eu anomalies, which makes them comparable to high-Mg andesites with adakitic affinity. Their depleted isotopic compositions are similar to coeval A type granites that resulted from lithospheric delamination after the final collision. In addition, with high K2O/Na2O ratios and incompatible element ratios similar to those of the continental crust (e.g., Nb/U, Ce/Pb, and Nd/ Sm), they conform to an origin in which partial melting of delaminated mafic lower crust subsequently interacted with mantle materials. These two-stage abnormal Triassic high Mg rocks archive a hot slab window triggered by the break-off of the Paleo-Asian oceanic slab and lower crustal delamination related to collapse of the southeastern segment of the Central Asian Orogenic Belt. These findings provide beneficial information for the reconstruction of the East Asian blocks in Pangea. [ABSTRACT FROM AUTHOR]

High-silica granitoids signal maturity of continental crust and are also closely associated with rare metal mineralization. However, the possible factors controlling the upper crustal differentiation and rare metal mineralization have not been well-constrained. In this work, we report zircon U-Pb ages, trace elements, Hf-O isotopes, and wholerock elemental and Sr-Nd-Hf isotopic data on six high-silica granitic intrusions from the Southern Great Xing'an Range Metallogenic Belt (SGXRMB), NE China, with a view to elucidate their source, differentiation mechanism, and rare metal mineralization potential. Zircon U-Pb dating of the granites (including porphyritic granite and alkalifeldspar granite) yields Early Cretaceous ages of ca. 144-135 Ma. Petrographic and geochemical features including the high SiO2, DI, and TE1,3 values, and similar Sr-Nd-Hf-O isotopes suggest that the rocks are weakly peraluminous, highly evolved I-type granites sharing a common silicic magma reservoir. Integrated isotope modeling suggests a complex source region for the evolved I-type granites involving dominantly juvenile lower crustal components with subordinate older continental basement and possible contribution of recycled pelagic sediments. The highsilica granite in the Jingpeng-Lindong region and the quartz diorite-monzonite, granodiorite, and monzogranite in the Lindong-Zalute region show close spatial-temporal distribution, common source and consistent variations in their whole-rock zircon compositions, indicating melt extraction processes in a highly crystalline mush rejuvenated by the injection of high temperature magma and F-enriched volatile filter-pressing, with the former derived from initial interstitial melts leaving behind residual silicic cumulates represented by the latter. Detailed comparisons of the rare metal-bearing and barren highsilica granites within the SGXRMB show that simple anatectic or fractional crystallization processes cannot account for the rare metal granites. Fluid-melt interactions combined with a high degree of crystallization differentiation and changes in melt structures are proposed as the potential mechanisms for generating the rare metal mineralization in I-type granitic magmas. [ABSTRACT FROM AUTHOR]

We undertook zircon U-Pb dating and geochemical analyses of volcanic rocks from the Baiyingaolao Formation in the central Great Xing'an Range, northeastern China, with an aim to determine their age, petrogenesis and sources, which are important for understanding the Late Mesozoic tectonic evolution of the eastern section of the Central Asian Orogenic Belt. Lithologically, the Baiyingaolao Formation is composed mainly of rhyolites and rhyolitic tuffs, with minor trachy dacites. The zircons from three rhyolitic tuffs and two rhyolites are euhedral-subhedral in shape, display fine-scale oscillatory growth zoning and have high Th/U ratios (0.72-2.60), indicating a magmatic origin. The results of LA-ICP-MS zircon U-Pb dating indicate that the volcanic rocks from the Baiyingaolao Formation in the study area formed during the Early Cretaceous time with ages of 134-130 Ma. Petrological and geochemical characteristics of these volcanic rocks suggest that they are all highly fractionated I-type igneous rocks, and their parental magmas were likely derived from the partial melting of lower crustal materials with plagioclase, hornblende and apatite as the residual phases. In addition, the volcanics sampled in this paper, tectonically located in the Xing'an terrain, have high initial 176Hf/177Hf ratios (0.282770-0.283035) and positive εHf(t) values (2.42-11.96), combined with young Hf two-stage model ages of 1134-541 Ma, reflecting that the crustal growth of the Xing'an terrain occurred during Neoproterozoic and Phanerozoic times. These data, combined with previous studies on the contemporaneous magma-tectonic activities in NE China, suggest that the generation of the Early Cretaceous volcanic rocks in the central Great Xing'an Range was related to the lithospheric delamination caused by the subduction of the Paleo-Pacific plate. [ABSTRACT FROM AUTHOR]