Search

NE China is the easternmost part of the Central Asian Orogenic Belt (CAOB). The area is distinguished by widespread occurrence of Phanerozoic granitic rocks. In the companion paper (Part I), we established the Jurassic ages (184–137 Ma) for three granitic plutons: Xinhuatun, Lamashan and Yiershi. We also used geochemical data to argue that these rocks are highly fractionated I-type granites. In this paper, we present Sr–Nd–O isotope data of the three plutons and 32 additional samples to delineate the nature of their source, to determine the proportion of mantle to crustal components in the generation of the voluminous granitoids and to discuss crustal growth in the Phanerozoic.Despite their difference in emplacement age, Sr–Nd isotopic analyses reveal that these Jurassic granites have common isotopic characteristics. They all have low initial 87Sr/86Sr ratios (0.7045±0.0015), positive ϵNd(T) values (+1.3 to +2.8), and young Sm–Nd model ages (720–840 Ma). These characteristics are indicative of juvenile nature for these granites. Other Late Paleozoic to Mesozoic granites in this region also show the same features. Sr–Nd and oxygen isotopic data suggest that the magmatic evolution of the granites can be explained in terms of two-stage processes: (1) formation of parental magmas by melting of a relatively juvenile crust, which is probably a mixed lithology formed by pre-existing lower crust intruded or underplated by mantle-derived basaltic magma, and (2) extensive magmatic differentiation of the parental magmas in a slow cooling environment.The widespread distribution of juvenile granitoids in NE China indicates a massive transfer of mantle material to the crust in a post-orogenic tectonic setting. Several recent studies have documented that juvenile granitoids of Paleozoic to Mesozoic ages are ubiquitous in the Central Asian Orogenic Belt, hence suggesting a significant growth of the continental crust in the Phanerozoic. [Copyright &y& Elsevier]

The Paleozoic to early Mesozoic tectonic framework and evolution of Northeast China, especially the Jiamusi block and its related structural belts, are highly debated. In this paper, geochemical, geochronological and isotopic analyses were carried out on the blueschist in the Heilongjiang Complex to address these issues. The Heilongjiang Complex defines the suture belt between the Jiamusi block and the Songliao block in NE China, and the blueschist is a major composition for this complex, coexisting with mafic-ultramafic rocks, greenschist, quartzite and mica schist. The blueschist has a mineral association of sodic amphibole, epidote, chlorite, phengite, albite and quartz with accessory phases of apatite, titanite, zircon and ilmenite. Together with the lithological association, the major and trace element compositions present that the protoliths of the blueschist can be divided into the alkaline and tholeiitic basalts and have OIB affinities, formed in an ocean island setting, indicated by the (La/Yb) N values of 3.57 – 11.54, and the (La/Sm) N values of 0.69 – 3.64. The high and positive ε Nd (t) values of + 3.7 to + 9.0, and relative enrichment in Nb (vs. Th) and Ta (vs. U) show that both the alkaline and tholeiitic basalts may be derived from the asthenospheric mantle with insignificant crustal contamination. Magmatic zircons from the blueschist in Yilan area yield a 206 Pb/ 238 U age of 281 ± 3 Ma, interpreted as its protolithic age. The youngest ages of ~ 200 Ma of the detrital zircons in the associated mica schist from Mudanjiang area place constraints on the timing of metamorphism for the blueschist. These indicate that a big ocean existed between the Jiamusi and Songliao blocks at least since the early Permian, and the blueschist formed since the late Triassic to late Jurassic by the subduction of this ocean. Such an ocean during the Permian – Jurassic is difficult to be interpreted by the tectonic evolution of the Paleo-Asian Ocean. [ABSTRACT FROM AUTHOR]

Located in the east section of the Central Asian orogen in northeastern China, the Xilamulun district comprises several newly discovered molybdenum deposits, primarily of porphyry type and Mesozoic ages. This district is divided by the Xilamulun fault into the southern and the northern parts. In this paper, we present new zircon U–Pb dating, trace elements and Hf isotope, and/or whole rock Sr–Nd–Pb isotopic results for the host granitoids from three Mo deposits (Yangchang, Haisugou and Shabutai) in northern Xilamulun. Our aim is to constrain the age and petrogenesis of these intrusions and their implications for Mo mineralization. Zircon U–Pb LA-ICP-MS dating shows that the monzogranites from the Shabutai and Yangchang deposits formed at 138.4 ± 1.5 and 137.4 ± 2.1 Ma, respectively, which is identical to the molybdenite Re–Os ages and coeval well with the other Mo deposits in this region, thereby indicating an Early Cretaceous magmatism and Mo mineralization event. Zircon Ce/Nd ratios from the mineralized intrusions are significantly higher than the barren granites, implying that the mineralization-related magmas are characterized by higher oxygen fugacity. These mineralized intrusions share similar zircon in-situ Hf and whole rock Sr–Nd isotopic compositions, with slightly negative to positive ε Hf (t) ranging from − 0.8 to + 10.0, restricted ε Nd (t) values from − 3.7 to + 1.6 but a little variable ( 87 Sr/ 86 Sr) i ratios between 0.7021 and 0.7074, indicative of formation from primary magmas generated from a dominantly juvenile lower crust source derived from depleted mantle, despite diverse consequent processes (e.g., magma mixing, fractional crystallization and crustal contamination) during their evolution. The Pb isotopes (whole rock) also show a narrow range of initial compositions, with ( 206 Pb/ 204 Pb) i = 18.03–18.88, ( 207 Pb/ 204 Pb) i = 15.48–15.58 and ( 208 Pb/ 204 Pb) i = 37.72–38.28, in agreement with Sr–Nd–Hf isotopes reflecting the dominance of a mantle component. An integration of geochronological and isotopic data points to three stages of Mo mineralization events (i.e., Triassic, Late Jurassic, and Early Cretaceous) associated with magmas generated from three different source regions in the Xilamulun district, NE China. The variation in the origin of the magmas from which the porphyry Mo systems were generated suggests that the formation of Mo deposit lies not in the composition of magma sources. Other factors, including high magma oxygen fugacity, may have taken a fundamental role in Mo enrichment and subsequent mineralization. [ABSTRACT FROM AUTHOR]

The Mongol–Okhotsk suture belt played an important role in the tectonic evolution of northeast Asia during the Mesozoic. However, few studies have examined the influence of this tectonic belt on the geological evolution of northeast China. In this paper, we present zircon U–Pb geochronology, major and trace element geochemistry, and zircon Hf–O isotopic data for Early Jurassic volcanic rocks in the Erguna Massif of northeast China, with the aim of constraining the evolution of the Mongol–Okhotsk suture belt and its influence on the tectonic history of China during the Early Jurassic. Zircon U–Pb dating indicates that the trachybasalt and basaltic andesite in the study area were erupted between 193 ± 5 Ma and 181 ± 9 Ma (i.e., in the Early Jurassic). These Early Jurassic volcanic rocks belong to the high-K calc-alkaline series and are enriched in large ion lithophile elements and light rare earth elements, as well as being depleted in heavy rare earth elements and high field strength elements such as Nb and Ta. The rocks show a small negative Eu anomaly. The zircon εHf (182 Ma) values of the volcanic rocks range from − 1.9 to + 5.1, corresponding to T DM1 values of 640–901 Ma and T DM2 values of 901–1345 Ma. Zircons from two volcanic rocks yield δ 18 O values of 7.2‰ ± 1.5‰ (n = 19) and 6.6‰ ± 0.7‰ (n = 35). Geochemically, these Early Jurassic volcanic rocks are similar to those from active continental margin settings, and their primary magmas could have been derived from the partial melting of a lithospheric mantle wedge modified by fluid from a subducted slab. The discovery of Early Jurassic calc-alkaline volcanic rocks in the Erguna Massif, together with the coeval porphyry Cu–Mo deposits, indicates that an active continental margin existed in the Erguna area during the Early Jurassic. Taken together, we conclude that southward subduction of the Mongol–Okhotsk oceanic plate took place beneath the Erguna Massif during the Early Jurassic. [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]

Abstract: In this paper, we present new zircon U–Pb dating, Hf isotope, major and trace elements, and Sr–Nd isotope data for Early–Middle Triassic intrusive rocks in the Erguna Massif of NE China, in the eastern segment of the Central Asian Orogenic Belt (CAOB). Our aim is to elucidate the tectonic evolution of the Mongol–Okhotsk Ocean and its influence on NE China. Zircons from five representative intrusions in the Erguna Massif are euhedral–subhedral in shape, display oscillatory growth zoning in CL images, and have Th/U ratios of 0.17–1.54, and in combination these features indicate that the zircons are of igneous origin. Zircon U–Pb dating results demonstrate that the Early–Middle Triassic magmatism in the Erguna Massif took place between 241 and 247Ma. The Early–Middle Triassic rocks are composed mainly of a suite of diorites, quartz–diorites, granodiorites, monzogranites, and syenogranites. They have SiO2 =57.71–72.86wt.%, Mg#=19–52, Al2O3 =14.27–17.23wt.%, and Na2O/K2O=0.67–1.59. Chemically, they are metaluminous to peraluminous and belong to the high-K calc-alkaline series. Moreover, they are enriched in light rare earth elements and large ion lithophile elements, and depleted in heavy rare earth elements and high field strength elements (e.g., Nb, Ta, and Ti). The whole-rock εNd(t) and zircon εHf(t) values of these samples range from −4.68 to −3.42, and from −2.0 to +6.6, respectively, indicative of formation from primary magmas generated by partial melting of juvenile crustal material with a contribution from mantle-derived mafic magmas. These magmas subsequently underwent fractional crystallization, contamination, and magma mixing. Geochemically, these Early–Middle Triassic intrusive rocks have affinities with intrusive rocks from active continental margin settings. We conclude, therefore, that the Early–Middle Triassic magmatism in the Erguna Massif was generated within an active continental margin setting related to the southwards subduction of the Mongol–Okhotsk oceanic plate beneath the Erguna Massif. This conclusion is supported by the occurrence of Early Mesozoic porphyry and skarn deposits along the southeastern side of the Mongol–Okhotsk suture belt. [Copyright &y& Elsevier]

Northeastern (NE) China is the easternmost part of the Central Asian Orogenic Belt (CAOB), which is celebrated for its accretionary tectonics and the world''s most important juvenile crust production in the Phanerozoic era. Abundant granitoids occur in the Great Xing''an, Lesser Xing''an and Zhangguangcai Ranges in NE China. This paper presents partial results of a series of studies on the granitoids from this region, aiming to understand their role in the building of new continental crust in eastern Asia. Three composite granite plutons (Xinhuatun, Lamashan and Yiershi) were chosen for geochemical and isotopic study in order to determine their emplacement ages and petrogenesis. Petrographically, they range from granodiorite (minor), monzogranite, syenogranite to alkali-feldspar granite. Quartz and perthitic feldspar are principal phases, accompanied by minor amounts of plagioclase, biotite (<5%) and other accessory minerals. In addition, many contain abundant miarolitic cavities which suggest that they were emplaced at shallow levels with extensive fractional crystallization. Geochemically, the granites are silica-rich, peraluminous and have high contents of alkalis. They invariably show enrichment in light rare earth elements (LREE) and significant negative Eu anomalies. All the granitic rocks demonstrate the characteristic negative anomalies in Ba, Nb, Sr, P, Eu, and Ti, and a positive anomaly in Pb in the spidergram.The emplacement of the Xinhuatun pluton took place at 184±4 Ma as revealed by zircon SHRIMP U–Pb data. This is also supported by the slightly younger Rb–Sr whole-rock (WR) isochron age of 173±3 Ma. A whole-rock (WR) Rb–Sr isochron age of 154±3 Ma was obtained for the Lamashan pluton, which is interpreted as close to the time of emplacement. The Yiershi pluton was intruded at about 140 Ma as evidenced by a zircon U–Pb age of 137±2 Ma and WR Rb–Sr isochron age of 143±5 Ma. Biotite-WR Rb–Sr isochrons and 40Ar/39Ar ages of feldspars allow us to estimate the cooling rate of each pluton.Geochemical data suggest that the rocks are highly fractionated I-type granites. Fractionation of biotite and feldspars was the principal process of magmatic differentiation and responsible for major element variation. Rb, Sr and Ba concentrations were controlled by feldspar separation, whereas REE elements were fractionated by accessory minerals, such as apatite, allanite and monazite. [Copyright &y& Elsevier]