Wen-Chun Ge

Highly fractionated I-type granites in NE China (II): isotopic geochemistry and implications for crustal growth in the Phanerozoic

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 87 Sr/ 86 Sr ratios (0.7045F0.0015), positive eNd(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. D 2003 Elsevier Science B.V. All rights reserved.

Metamorphic P-T Path of the Southern Jilin Complex: Implications for Tectonic Evolution of the Eastern Block of the North China Craton

The Southern Jilin complex represents the northeasternmost exposure of the basement rocks in the Eastern block of the North China craton and consists of intensely foliated tonalitictrondhjemitic-granodioritic (TTG) gneisses, weakly foliated syn-tectonic potassium granites, posttectonic hypersthene-quartz diorite and clinopyroxene granodiorite and a minor amount of supracrustal rocks. The supracrustal rocks include ultramafic to felsic volcanic rocks, BIF, felsic paragneisses, pelitic gneisses, calc-silicates, and marbles metamorphosed from amphibolite to granulite facies. The petrological evidence from the mafic granulites and pelitic gneisses indicates three metamorphic mineral assemblages (M1 to M3). The early prograde assemblage (M1) is preserved as mineral inclusions within minerals of the peak assemblages, represented by hornblende + plagioclase + quartz ± biotite in the mafic granulites and biotite + plagioclase + quartz in the pelitic gneisses, with P-T conditions estimated at ~0.6 GPa and ~750°C. The peak assemblage is represented by orthopyroxene + clinopyroxene + garnet + plagioclase + quartz in the mafic granulites and garnet + sillimanite + plagioclase + quartz + biotite in pelitic gneisses, with P-T conditions estimated at 0.84-0.87 GPa and 800-850°C. The post-peak assemblage is characterized by garnet + quartz symplectic coronas in mafic granulites and kyanite replacing sillimanite in the pelitic gneisses, which probably occurred at ~0.85 GPa and 760°C. These mineral assemblages and their P-T estimates define an anticlockwise P-T path involving nearly isobaric cooling, reflecting an origin related to the intrusion and underplating of large amounts of mantle-derived magmas, which are considered to have been related to the interaction of upwelling mantle plumes with the lithosphere in the Eastern block of the North China craton in the Late Archean.

Petrogenesis of Early Cretaceous volcanic rocks of the Manketouebo Formation in the Wuchagou region, central Great Xing’an Range, NE China, and tectonic implications: geochronological, geochemical, and Hf isotopic evidence

ABSTRACT This study presents new whole-rock major and trace element geochemistry, zircon U–Pb ages, and Hf-isotope compositions for volcanic rocks from the Manketouebo Formation of the central Great Xing’an Range, NE China. These data provide precise ages and information on the petrogenesis and source of the magmas that formed this formation, furthering our understanding of the geodynamic setting of the large-scale late Mesozoic magmatism in the Great Xing’an Range and other areas in NE China. The Manketouebo Formation in the study area is dominated by rhyolites and rhyolitic tuffs with minor trachydacites. The LA-ICP-MS zircon U–Pb dating indicates that these volcanic rocks formed between 143 and 139 Ma. The volcanic rocks contain high silica (66.70–79.91 wt.%) and total alkali (5.93–9.72 wt.%) concentrations, and low concentrations of MgO (0.08–1.15 wt.%), total FeO (0.68–4.50 wt.%), and CaO (0.10–2.56 wt.%). They are enriched in large-ion lithophile elements (LILEs; e.g. Rb, Th, and U) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs; e.g. Nb, Ta, Ti, and P) and heavy rare earth elements (HREEs), indicating that they are similar to highly fractionated I-type igneous rocks. All of the magmatic zircons from the analysed samples have high initial 176Hf/177Hf ratios (0.282900–0.283093), positive εHf(t) values (7.48–14.19), and young Hf two-stage model ages (954–344 Ma) that suggest the primary magma that formed the volcanic rocks of the Manketouebo Formation was derived from the partial melting of Neoproterozoic to Phanerozoic juvenile crustal material, indicating in turn that significant crustal growth occurred at this time within the Xing’an Terrane. These data, combined with previous research into the spatial–temporal distribution of Mesozoic volcanic rocks in NE China, suggest that the Early Cretaceous magmatism in the Great Xing’an Range was influenced by both the subduction of the Palaeo-Pacific Plate and the closure of the Mongol–Okhotsk Ocean. This was a crucial period in the transformation from the Mongol–Okhotsk Ocean to the Palaeo-Pacific tectonic regimes. In summary, the early stages of Early Cretaceous magmatism in this area were related to the closure of the Mongol–Okhotsk Ocean, whereas the later stages of magmatism in this area and elsewhere in NE China were related to the subduction of the Palaeo-Pacific Plate.

Lower Cretaceous alkali feldspar granites in the central part of the Great Xing’an Range, northeastern China: chronology, geochemistry and tectonic implications

The Mingshui–Jilasitai–Suolun area, located in the central part of the Great Xing’an Range, is characterized by large volumes of alkali feldspar granites. However, the formation time and tectonic setting of these rocks remains controversial owing to a lack of precise geochronological and detailed geochemical data. In this paper, we report new SIMS U–Pb zircon ages and mineralogical, petrographical and geochemical data for Lower Cretaceous alkali feldspar granites from the Mingshui–Jilasitai–Suolun area. The SIMS zircon dating results indicate that these granites formed at 133.6–135.9 Ma. The mineralogical, petrographical and geochemical data show that these granitic rocks belong to highly fractionated I-type granites. Combined with the regional geology data, we propose that the formation of the Lower Cretaceous alkali feldspar granitic rocks was related to an extension induced by delamination of the lithosphere that arose from subduction of the Palaeo-Pacific plate.

Age assignment and geological significance of the “Budate Group” in the Hailar Basin

Age and nature of the Budate Group in the Hailar Basin are of great significance in studying the evolution of the Hailar Basin and the Xing’an-Mongolian Orogenic Belt (XMOB). Zircon U-Pb ages of eight volcanic rocks from the Budate Group and two basement granites in the Hailar Basin were reported in this study. The dating results indicated that the formation of these volcanic rocks was consistent with the emplacement of Late Paleozoic basement granite in age (356–290 Ma), i.e., Early Carboniferous to Early Permian rather than Early Mesozoic. Combined with regional geology, it was concluded that the Budate Group is the component of the basement of the Hailar Basin, and these volcanic rocks were formed at an active continental margin, which is related to the closure of the Hegen Mountains suture zone. The captured Neoproterozoic zircons (814–873 Ma) were probably derived from the Neoproterozoic basement materials in the Ergun Block, implying the close genetic relationships between the Ergun Block (including the Hailar Basin) and the Siberian Block.

Subduction history of the Paleo-Pacific slab beneath Eurasian continent: Mesozoic-Paleogene magmatic records in Northeast Asia

This paper presents a review on the rock associations, geochemistry, and spatial distribution of Mesozoic-Paleogene igneous rocks in Northeast Asia. The record of magmatism is used to evaluate the spatial-temporal extent and influence of multiple tectonic regimes during the Mesozoic, as well as the onset and history of Paleo-Pacific slab subduction beneath Eurasian continent. Mesozoic-Paleogene magmatism at the continental margin of Northeast Asia can be subdivided into nine stages that took place in the Early-Middle Triassic, Late Triassic, Early Jurassic, Middle Jurassic, Late Jurassic, early Early Cretaceous, late Early Cretaceous, Late Cretaceous, and Paleogene, respectively. The Triassic magmatism is mainly composed of adakitic rocks, bimodal rocks, alkaline igneous rocks, and A-type granites and rhyolites that formed in syn-collisional to post-collisional extensional settings related to the final closure of the Paleo-Asian Ocean. However, Triassic calc-alkaline igneous rocks in the Erguna-Xing’an massifs were associated with the southward subduction of the Mongol-Okhotsk oceanic slab. A passive continental margin setting existed in Northeast Asia during the Triassic. Early Jurassic calc-alkaline igneous rocks have a geochemical affinity to arc-like magmatism, whereas coeval intracontinental magmatism is composed of bimodal igneous rocks and A-type granites. Spatial variations in the potassium contents of Early Jurassic igneous rocks from the continental margin to intracontinental region, together with the presence of an Early Jurassic accretionary complex, reveal that the onset of the Paleo- Pacific slab subduction beneath Eurasian continent occurred in the Early Jurassic. Middle Jurassic to early Early Cretaceous magmatism did not take place at the continental margin of Northeast Asia. This observation, combined with the occurrence of low-altitude biological assemblages and the age population of detrital zircons in an Early Cretaceous accretionary complex, indicates that a strike-slip tectonic regime existed between the continental margin and Paleo-Pacific slab during the Middle Jurassic to early Early Cretaceous. The widespread occurrence of late Early Cretaceous calc-alkaline igneous rocks, I-type granites, and adakitic rocks suggests low-angle subduction of the Paleo-Pacific slab beneath Eurasian continent at this time. The eastward narrowing of the distribution of igneous rocks from the Late Cretaceous to Paleogene, and the change from an intracontinental to continental margin setting, suggest the eastward movement of Eurasian continent and rollback of the Paleo- Pacific slab at this time.

Formation of the Permian Taipinggou igneous rocks, north of Luobei (Northeast China): implications for the subduction of the Mudanjiang Ocean beneath the Bureya–Jiamusi Massif

ABSTRACT Controversy has long surrounded the tectonic framework and evolution of the Mudanjiang Ocean between the Bureya–Jiamusi–Khanka Massif and Songnen–Zhangguangcai Range Massif, which are located in the easternmost segment of the Central Asian Orogenic Belt. To address these issues, we present zircon U-Pb ages, geochemical data, and zircon Hf isotopic compositions of the Taipinggou amphibolite and metagabbro exposed along the boundary area of Bureya–Jiamusi Massif and Songnen–Zhangguangcai Range Massif. Magmatic zircons from the amphibolite and metagabbro yield 206Pb/238U ages of 267 ± 2 Ma and 264 ± 2 Ma, respectively, which are interpreted as protolith ages. The geochemical data of the amphibolite samples show transitional characteristics of calcalkaline to tholeiitic series, with high MgO concentrations (9.44–10.48 wt.%) and Mg-numbers (73–75). These samples are enriched in large ion lithophile elements (e.g. Rb, Ba, and K) and light rare earth elements and are depleted in high-field-strength elements (e.g. Nb, Ta, and Ti) and heavy rare earth elements, with εHf(t) values of −6.63 to −3.26. It is inferred that the parental magma originated from an enriched lithospheric mantle that had been metasomatized by fluids derived from subducted oceanic slab. During magma evolution, the magma that formed the amphibolite mainly experienced accumulation with a shallow-level evolutionary process involving fractional crystallization. The Taipinggou metagabbro samples are subalkaline series and also characterized by enrichment in large ion lithophile elements (e.g. Rb, Ba, and K) and light rare earth elements and by depletion in Nb–Ta–P–Ti, with εHf(t) values of −3.09 to +1.16. The Taipinggou metagabbro and amphibolite have similar geochemical and Hf isotopic compositions, indicating a common parental magma source but with different degrees of magmatic differentiation. Based on the new geochronological and geochemical data presented in this study, we propose that both the Taipinggou metagabbro and amphibolite formed in a Middle Permian continental arc setting, closely related to eastward subduction beneath the Bureya–Jiamusi Massif. Combined with previous studies and regional geological observations, we suggest that a double-side subduction model is favoured for the Late Palaeozoic–Early Mesozoic geodynamic processes along the boundary area of Bureya–Jiamusi–Khanka Massif and Songnen–Zhangguangcai Range Massif. GRAPHICAL ABSTRACT

Late Carboniferous to early Permian subduction-related intrusive rocks from the Huolongmen region in the Xing’an Block, NE China: new insight into evolution of the Nenjiang–Heihe suture

ABSTRACT The Hegenshan–Nenjiang–Heihe suture is a key record of the Palaeo-Asian Ocean and provides critical evidence regarding termination of the eastern Central Asian Orogenic Belt. Here we determine the age, petrogenesis, and tectonic setting of the late Palaeozoic intrusive rocks from the Huolongmen region, northeastern margin of the Xing’an Block, and propose a new Ordovician to Triassic evolution model for the Nenjiang–Heihe suture. The late Palaeozoic Huolongmen intrusive rocks consist of the 317 million year gabbro diorite, the 323–312 million year I-type monzogranite, and the 291 million year A2-type alkali feldspar granite, as indicated by the laser ablation inductively coupled plasma mass spectrometry zircon U–Pb ages and petrographic–geochemical signatures. Zircon Hf isotopic, geochemical, and regional geological data suggest that the 323–312 million year gabbro diorite and I-type monzogranite were both emplaced in a continental arc setting, with the former generated by partial melting of a depleted rutile-bearing lithospheric mantle wedge that was modified by slab-derived fluids, and the latter formed by partial melting of a juvenile lower crust that involved contamination of pre-existing arc materials. The 291 million year A2-type granite was emplaced in a subduction-related extension and generated from anataxis of intermediate to acid igneous rocks of the middle to upper crust at relatively low-pressure conditions. In our new model, the evolution of the Nenjiang–Heihe suture could be roughly divided into three stages, including (1) the episodic northwestward subduction of the Palaeo-Asian oceanic plate beneath the Xing’an Block changing gradually from a high angle to a low angle at 480–290 Ma; (2) the coexistence of the slab break-off and the northwestward subduction at 290–260 Ma; and (3) the soft collision between the Xing’an and Songliao blocks at 260–244 Ma.