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  • Neoproterozoic I-type granites in the Central Tianshan Block (NW China): geochronology, geochemistry, and tectonic implications

    分类: 地球科学 >> 地理学 提交时间: 2022-01-30 合作期刊: 《干旱区科学》

    摘要:

    The Central Tianshan Block is one of numerous microcontinental blocks within the Central Asian Orogenic Belt (CAOB) that overlies Precambrian basement rocks. Constraining the evolution of these ancient basement rocks is central to understanding the accretionary and collisional tectonics of the CAOB, and their place within the Rodinia supercontinent. However, to date, the timing and tectonic settings in which the basement rocks in the Central Tianshan Block formed are poorly constrained, with only sparse geochemical and geochronological data from granitic rocks within the northern segment of the block. Here, we present a systematic study combining U–Pb geochronology, whole-rock geochemistry, and the Sr–Nd isotopic compositions of newly-identified granitic gneisses from the Bingdaban area of Central Tianshan Block. The analyzed samples yield a weighted mean Neoproterozoic 206Pb/238U ages of 975–911 Ma. These weakly-peraluminous granitic rocks show a common geochemical I-type granite affinity. The granitic gneisses are calc-alkaline and enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), but they are depleted in high field strength elements (HFSEs); these characteristics are similar to those of typical subduction-related magmatism. All samples show initial (87Sr/86Sr)(t) ratios between 0.705136 and 0.706745. Values for ƐNd(t) in the granitic gneisses are in the range from –5.7 to –1.2, which correspond to Nd model ages of 2.0–1.7 Ga, indicating a role for Mesoproterozoic to Paleoproterozoic rocks in the generation of the granitic protoliths. The documented geochemical features indicate that the protoliths for the gneisses have a similar petrogenesis and magmatic source, which may reflect partial melting of thickened crust with the addition of small amounts of mantle-derived material. The Central Tianshan Block probably constitute part of an exterior orogen that developed along the margin of the Rodinian supercontinent during the Early Neoproterozoic and underwent a transition from subduction to syn-collision compression at 975–911 Ma.

  • Geochronology, geochemistry, and Sr–Nd isotopes of Early Carboniferous magmatism in southern West Junggar, northwestern China: Implications for Junggar oceanic plate subduction

    分类: 地球科学 >> 地理学 提交时间: 2021-12-06 合作期刊: 《干旱区科学》

    摘要:West Junggar is a key area for understanding intra-oceanic plate subduction and the final closure of the Junggar Ocean. Knowledge of the Carboniferous tectonic evolution of the Junggar Ocean region is required for understanding the tectonic framework and accretionary processes in West Junggar, Central Asian Orogenic Belt. A series of Early Carboniferous volcanic and intrusive rocks, namely, basaltic andesite, andesite, dacite, and diorite, occur in the Mayile area of southern West Junggar, northwestern China. Our new LA-ICPMS zircon U–Pb geochronological data reveal that diorite intruded at 334 (±1) Ma, and that basaltic andesite was erupted at 334 (±4) Ma. These intrusive and volcanic rocks are calc-alkaline, display moderate MgO (1.62%–4.18%) contents and Mg# values (40–59), and low Cr (14.5×10–6–47.2×10–6) and Ni (7.5×10–6–34.6×10–6) contents, and are characterized by enrichment in light rare-earth elements and large-ion lithophile elements and depletion in heavy rare-earth elements and high-field-strength elements, meaning that they belong to typical subduction-zone island-arc magma. The samples show low initial 87Sr/86Sr ratios (range of 0.703649–0.705008), positive εNd(t) values (range of 4.8–6.2 and mean of 5.4), and young TDM Nd model ages ranging from 1016 to 616 Ma, indicating a magmatic origin from depleted mantle involving partial melting of 10%–25% garnet and spinel lherzolite. Combining our results with those of previous studies, we suggest that these rocks were formed as a result of northwestward subduction of the Junggar oceanic plate, which caused partial melting of sub-arc mantle. We conclude that intra-oceanic arc magmatism was extensive in West Junggar during the Early Carboniferous.

  • Syn-subduction intra-continental shearing during Oligocene in Indochina, SE Asia

    分类: 地球科学 >> 地质学 提交时间: 2021-12-01

    摘要:Indochina developed continental-scale shear zones that record Cenozoic tectonic processes in SE Asia. Previous extrusion models that linked these shear zones with the northward indentation of the Indian continent, conflict with distributed conjugate strike-slip pairs and post-Oligocene rotation in Indochina. This paper presents evidence of coeval shearing along the Mogok-Shan Scarp fault in Myanmar, the western boundary of the Indochina block, previously proposed as a product of northward indentation of Indian Plate. The Kyanigan quarry along the northern Mogok-Shan Scarp fault exposes paragneiss, marble and quartzite schist. ‘σ’ structures, cored with garnet, in paragneiss record right-lateral shear sense, consistent with ‘σ’ and ’δ’ structures in L-tectonites in the Moulmein granitic mylonite to the south. U-Pb ages of metamorphic zircons of paragneiss and a biotite 39Ar-40Ar age of quartzite schist constrain deformation in the Kyanigan quarry to 33-21 Ma; a biotite 39Ar-40Ar age of mylonite at Moulmein is 26 Ma. These ages demonstrate Oligocene right-lateral shearing along the Mogok-Shan Scarp fault, coeval with other shear zones in Indochina. These Oligocene shear zones and strike-slip faults are conjugate structures recording left-lateral shear sense on NW-striking and right-lateral shear sense on N-S to NE-striking features. After restoration of ~80° clockwise rotation, the Oligocene conjugate strike-slip pairs in Indochina reflect approximate N-S-directed shortening, corresponding to northward subduction of the Indian ocean. This suggests that continental-scale intra-continental shearing may have been triggered by syn-subduction compression in SE Asia.

  • Sub-parallel ridge-trench interaction and an alternative model for the Silurian-Devonian archipelago in Western Junggar and North-Central Tianshan in NW China

    分类: 地球科学 >> 地质学 提交时间: 2021-06-22

    摘要:Plate boundary evolution and interpreted paleogeography commonly involve the interaction between oceanic ridges and trenches. Western Junggar and Central-North Tianshan, NW China, have previously been regarded as independent orogens, but this model was challenged by the discovery of their similar tropic-subtropic coral assemblages and of Precambrian and Early Paleozoic clasts transmitted from the Central Tianshan in Early to Middle Paleozoic sediments in SW Junggar, suggesting they had been in contact by then. The presence of Late Silurian-Early Devonian adakitic rocks in northern SW Junggar was explained by slab roll-back, which required an active subducting slab in SW Junggar. However, this model is inconsistent with the cessation of arc magmatism immediately after the adakitic magmatism indicating there was no subduction in SW Junggar at those times. By re-evaluating the relevance of four diagnostic features of modern Californian ridge subduction, we propose a new model to explain the Late Silurian-Early Devonian orogenic framework of NW China in terms of sub-parallel ridge-trench interaction. The transmitted clasts from Central Tianshan in Early Paleozoic sediments in SW Junggar, at the same time as the Late Ordovician and Middle Silurian coral assemblages, and the complementary accretionary complex in SW Junggar and magmatic arc in Central Tianshan, are all contributary evidence for an Early Paleozoic subduction system. This subduction zone was intruded by two 446-380 Ma double magmatic belts: 1. A ridge subduction-generated adakitic gabbro-basalt-andesite-diorite-granite-rhyolite suite that intruded a 504-446 Ma accretionary complex in SW Junggar, and 2. a magmatic arc in Central Tianshan. A buoyant subducting ridge rifted and separated these magmatic belts resulting in the opening of a new 414-325 Ma ocean, which is preserved in OIB- and MORB-type ophiolites in SW Junggar and North Tianshan. These geological and geochemical relationships resemble those generated by oceanic spreading in the modern Gulf of California, which led to separation of magmatic belts in Baja California and mainland Mexico. The new ocean split the Central Tianshan magmatic arc from the Early Paleozoic Mt. Xiemisitai-Barleik-Mayile accretionary complex in SW Junggar, which changed to a new passive continental margin that led to deposition of Devonian shallow marine-terrestrial sediments, and to cessation of magmatism at 379-349 Ma. These features, including the Middle Paleozoic orogenic architecture and the geochemical properties of specific magmatic rocks, indicate the development of a Late Silurian-Early Devonian sub-parallel-to-trench ridge subduction. This model not only interprets phenomena mentioned in the above earlier models, but also reconciles unresolved spatial relationships between Western Junggar and the Tianshan. Subsequent closure of the new ocean and subduction of its ridge in the Late Paleozoic gave rise to the archipelagic framework that we see today in NW China. An analysis of worldwide examples of sub-parallel ridge-trench interaction demonstrates that a ridge can undergo multiple episodes of subduction before a terminal ocean closure. In this context our new tectonic model for the western Junggar and Tianshan regions may provide an improved understanding of the structure and evolution of ridge-subduction events in ancient orogenic belts.