Plate movements, ductile deformation and geochronology of the Sanbagawa belt, SW Japan: tectonic significance of 89–88 Ma Lu–Hf eclogite ages

Prograde P–T paths and thermal modelling suggest metamorphism in the Sanbagawa belt represents unusually warm conditions for subduction-type metamorphic belts, and these likely reflect conditions of a convergent margin a few million years before the arrival of an active spreading ridge. Radiometric age data and kinematic indicators of ductile deformation suggest the Sanbagawa belt formed in a Cretaceous convergent margin associated with a plate movement vector that had a large sinistral oblique component with respect to the belt, the East Asian margin. Plate reconstructions for the Cretaceous to Tertiary for this region show that the only plausible plate compatible with such motion at this time is the Izanagi plate. These reconstructions also show that progressively younger sections of the Izanagi plate were subducted beneath eastern Asia, i.e. a spreading ridge approached, until 85–83 Ma when the Izanagi Plate ceased to exist as an independent plate. The major reorganization of plates and associated movements around this time is likely to be the age of major interaction between the ridge and convergent margin. The ridge-approach model for the Sanbagawa metamorphism, therefore, predicts that peak metamorphism is a few million years older than this age range. New Lu–Hf dating of eclogite in the Sanbagawa belt gives ages of 89–88 Ma, in excellent agreement with the prediction. Combining this estimate for the peak age of metamorphism with published P–T-t results implies vertical exhumation rates of greater than 2.5 cm yr⁻¹. This high rate of exhumation can explain the lack of a significant thermal overprint in the Sanbagawa belt during subduction of the ridge.     

Magmatic and geodynamic evolution of Urumieh–Dokhtar basic volcanism,Central Iran: major,trace element,isotopic, and geochronologic implications

Basic volcanic rocks from the West Nain area of the Urumieh–Dokhtar Magmatic Assemblage demonstrate significant subduction-related geochemical characteristics; these along with the new age data obtained for the volcanic rocks shed new light on the geodynamic evolution of the Iranian segment of Alpine–Himalayan orogeny. The late Oligocene (26.5 Ma) high-Nb basic volcanic rocks are likely to represent a transient rather enriched asthenospheric mantle underlying the otherwise dominantly Eocene–early Oligocene West Nain island arc. Lithospheric mantle geochemical signatures of the low-Zr volcanic rocks (20.6 Ma) and high-Th volcanic rocks (19.7 Ma) imply replacement of the underlying mantle. The substitution of asthenospheric mantle by a lithospheric mantle wedge might have been associated with – or perhaps caused by – an increase in the subduction rate. Culmination of the West Nain magmatism into slab melting that produced the early Miocene (18.7 Ma) adakitic rocks is compatible with subsequent ascent that triggered slab decompression melting.     

Formation of the world's largest molybdenum metallogenic belt: a plate-tectonic perspective on the Qinling molybdenum deposits

Qinling ore belt is the largest known molybdenum belt in the world with a total reserve of >5 Mt of Mo metal. Based on the geochemical behaviour of Mo, the structural settings of the Qinling orogenic belt, and geological events in eastern China, we propose that tectonic settings are of critical importance to the formation of these ore deposits. Molybdenum is very rare in the earth with an abundance of ～0.8 ppm in the continental crust. Both surface- and magmatic-hydrothermal enrichment processes are required for Mo mineralization. It can be easily oxidized to form water-soluble MoO₄ ^– in the surface environment, especially in the Phanaerozoic, and then precipitated under anoxic conditions. Therefore, closed or semi-closed water bodies with large catchment areas and high chemical erosion rates are the most favourable locations for Mo-enriched sediments. The Qinling orogenic belt was located in the tropics during crustal collisions, such that the chemical erosion was presumably intense, whereas the Erlangping back-arc basin was probably a closed or semi-closed water body as a result of plate convergence. More than 90% of the Mo reserves so far discovered in the Qinling molybdenum belt are associated with the Palaeozoic Erlangping back-arc basin. Compiled Re–Os isotopic ages for porphyry deposits (including several carbonate vein deposits) that have been dated show peaks during 220 million years (>0.32 Mt), 145 million years (>?3.5 Mt), and 115 million years (>?0.84 Mt), which correlate well with the three major episodes of granitoid magmatism since the Triassic. The ～220 million year episode of mineralization, represented by the Huanglongpu carbonate vein-type deposit and the Wenquan porphyry deposit, coincided with the formation of the South Qinling syn-orogenic granites as well as the Dabie ultrahigh-pressure metamorphic rocks, suggests a genetic relationship with the collision between South and North China Blocks. The ～145 Ma porphyry Mo deposits, representing the main mineralization, are attributed to reactivation by ridge subduction along the lower Yangtze River belt to the east of the Qinling orogen ～150–140 Ma. The ～115 Ma Mo deposits likely reflect slab rollback of the northwestwards subducting Pacific plate ～125–110 Ma.     

Geochronological and geochemical constraints on the origin of the 304 ± 5 Ma Karamay A-type granites from West Junggar,Northwest China: implications for understanding the Central Asian Orogenic Belt

U–Pb zircon geochronological, geochemical, and whole-rock Sr–Nd isotopic analyses are reported for a suite of Karamay A-type granites from the Central Asian Orogenic Belt (CAOB) in the western Junggar region of northern Xinjiang, Northwest China, with the aim of investigating the sources and petrogenesis of A-type granites. The Karamay pluton includes monzogranite and syenogranite. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating yielded a concordant weighted mean ²⁰⁶Pb/²³⁸U age of 304 ± 5 Ma (n = 11), defining a late Carboniferous magmatic event. Geochemically, the rock suite is characterized by high SiO₂, FeOt/MgO, total alkalies (K₂O + Na₂O), Zr, Nb, Y, Ta, Ga/Al, and rare earth elements (REEs) (except for Eu), and low contents of MgO, CaO, and P₂O₅, with negative Ba, Sr, P, Eu, and Ti anomalies. These features indicate an A-type affinity for the Karamay granitic intrusions. Isotopically, they display consistently depleted Sr–Nd isotopic compositions (initial ⁸⁷Sr/⁸⁶Sr = 0.7014–0.7022, ?_Nd(t) = +5.6–+7.0). Geochronological, geochemical, and isotopic data suggest that the Karamay A-type granites were derived from remelting juvenile lower crust, followed by fractional crystallization. The Karamay A-type granites as well as widespread late Carboniferous magmatism in the western Junggar region of the southwestern CAOB may have been related to ridge subduction and a resultant slab window. This further demonstrates the importance of the late Palaeozoic granitic magmatism in terms of vertical crustal growth in northern Xinjiang.     

Geochemistry of peridotites from the Yap Trench,Western Pacific: implications for subduction zone mantle evolution

ABSTRACT

This study examines the major and trace elements of peridotites from the Yap Trench in the western Pacific to investigate mantle evolution beneath a subduction zone. Major element results show that the peridotites are low in Al₂O₃ (0.31–0.65 wt.%) and CaO (0.04–0.07 wt.%) contents and high in Mg# (Mg/(Mg+Fe)) (0.91–0.92) and have spinels with Cr# (Cr/(Cr+Al)) higher than 0.6 (0.61–0.73). Trace element results show that the peridotites have extremely low heavy rare earth element (HREE) contents compared with abyssal peridotites but have U-shaped chondrite-normalized rare earth element (REE) patterns. The degree of mantle melting estimated based on the major elements, HREEs, and spinel Cr# range from 19% to 25%, indicating that the Yap Trench peridotites may be residues of melting associated with the presence of water in the mantle source. In addition to light rare earth element (LREE) enrichment, the peridotites are characterized by high contents of highly incompatible elements, positive U and Sr anomalies, negative Ti anomalies, and high Zr/Hf ratios. The correlations between these elements and both the degree of serpentinization and high field strength element (HFSE) contents suggest that fluid alteration alone cannot account for the enrichment of the peridotites and that at least the enrichment of LREEs was likely caused by melt–mantle interaction. Comparison between the peridotites and the depletion trend defined by the primitive mantle (PM) and the depleted mantle (DM) suggests that the Yap Trench mantle was modified by subduction-related melt characterized by high contents of incompatible elements, high Zr/Hf ratios, and low HFSE contents. Hydrous melting may have been enhanced by tectonic erosion of the subducting Caroline Plate with complex tectonic morphostructures at the earliest stages of subduction initiation.     

Boninitic and tholeiitic basaltic lavas and dikes from dispersed Jurassic East Othris ophiolitic units,Greece: petrogenesis and geodynamic implications

ABSTRACT

Pillow lavas, massive lava flows, and sub-volcanic dikes of tholeiitic basaltic composition are found to be members of the Vrinena, Aerino, Eretria, and Velestino dispersed Middle–Upper Jurassic ophiolitic units in East Othris. The Vrinena and Eretria ophiolitic units appear to have been emplaced onto the Pelagonian continental margin during the Upper Jurassic–Lower Cretaceous, whereas the Aerino and Velestino units seem to have been finally emplaced during post-Palaeocene times. Geochemically these are divided into two groups: Group I includes subduction-related boninites and low-Ti basalts from the Vrinena and Aerino units, and Group II high-Ti basalts show spreading-type characteristics occurring in the Eretria and Velestino units. Primary magma of the Group I volcanics appears to have been formed after high partial melting degrees (~18%) of a highly depleted harzburgitic mantle source, under relatively high temperatures (mantle potential temperature ~1372°C). Petrogenetic modelling also suggests that the primary magma of the Group II volcanics were formed after lower partial melting degrees (~7%) of a moderately depleted mantle source. The petrological and geochemical data from the East Othris dispersed and diversely emplaced ophiolitic units provide evidence of a common intra-oceanic supra-subduction zone (SSZ) origin within the Pindos oceanic strand of the Western Tethys. Specifically, Group I lavas and dikes from Vrinena seem to represent the extrusive part of an almost complete fore- to island-arc ophiolitic sequence. Dikes of Aerino most likely correspond to fore-arc magmatic material that intruded within exhumed serpentinized ultramafic rocks through a subduction channel that developed close to the slab and towards the fore-arc and the accretionary prism. The Group II volcanics either corresponded to a fore-arc magmatic expression, which extruded earlier than Group I volcanics and prior to the establishment of a mature subduction zone, or represent back-arc to island-arc magmatism that was contemporaneous to the fore-arc magmatic activity during rollback subduction.     

Partial melting of subducted Southern Qiangtang crust in northern Tibet: evidence from the geochemistry and geochronology of the Riwanchaka granodiorite porphyry in Central Qiangtang

ABSTRACT

This article presents new zircon U–Pb geochronology, Hf isotopic, and whole-rock major- and trace-element geochemical data that provide insights into the petrogenesis and tectonic history of the Riwanchaka granodiorite porphyries of Central Qiangtang, Tibet. Zircon U–Pb ages of 236–230 Ma indicate an early Late Triassic age of emplacement of the porphyries, and zircon Hf isotopic data yield εHf(t) values of – 7.0 to – 1.5 and ancient zircon Hf crustal model ages (T_DM^C) of 1524–1220 Ma. The granodiorite porphyries are characterized by low K₂O contents, high Mg# values, and relatively high Cr and Ni contents. They are classified as I-type calc-alkaline granite and are considered to have formed through the anatexis of ancient mafic crustal rocks with contributions from mantle-derived components. The geochemistry and isotopic compositions of all samples are similar to those of magmatic rocks that originated in the South Qiangtang crust. However, field observations indicate that the pluton intrudes the North Qiangtang crust, and we propose that the granodiorite porphyries were derived by partial melting of subducted continental crust of the South Qiangtang terrane. These new data have been integrated with data from previous studies to construct a new model of slab rollback during northward subduction of the Southern Qiangtang continental crust at ca. 245–226 Ma, thereby improving our understanding of magmatic processes involved in continental subduction in collision settings.     

深源地震机理的回顾:现状与问题

深源地震(deep earthquakes)对于研究地球内部结构及板块的运动、动力学机制等起着重要的作用。从和达清夫首次发现深源地震至今80多年的时间里深源地震得到了广泛关注,获得大批高质量的成果。但是,其成因机制至今仍然是未解之谜。目前,大家广泛接受以脱水脆裂为中源地震的成因机制,反裂隙断层为深源地震的成因机制。剪切失稳等机制也能对中源与深源地震的成因做出较好的解释,但仍需进一步的实验研究。中国东北珲春等地区位于环太平洋地震带上,也是中国唯一存在的深源地震带。对地表地质构造、矿物岩石物理学、深部探测和高温高压实验及数值模拟分析的综合研究是解决深源地震机制问题的有效途径。加强中国东北深源地震机理与西太平洋板块俯冲的研究,对进一步认识我国东部大地构造格局演化有着重要的深部背景意义。     

西藏哈海岗钨钼多金属矿床地质特征及辉钼矿铼-锇同位素定年

西藏墨竹工卡县哈海岗钨钼多金属矿床是近年来新发现的以钨钼为主的多金属矿床,也是念青唐古拉成矿带发现的第一个钨钼多金属矿床。文章简要地介绍了该矿床基本地质特征,利用Re-Os同位素定年方法对采自哈海岗矽卡岩型钨钼矿体中的2件辉钼矿进行了成矿年代的测定,获得的辉钼矿Re-Os同位素模式年龄分别为(140.1±2.8) Ma和(143.3±5.2) Ma,平均值为(141.7±4) Ma。测年数据表明,矿床成矿时代属于早白垩世,可能系班公湖—怒江洋向南俯冲碰撞阶段成矿作用的产物,早于其东侧亚贵拉、洞中拉石英斑岩的成岩年龄(126 Ma左右)。矿床成矿时代的厘定丰富了冈底斯—念青唐古拉成矿带矿床成矿系列,指示冈底斯成矿带存在早白垩世的成矿作用,该带至少存在四期独立的钼成矿事件;同时对于研究区域岩浆演化与成矿作用以及区域找矿预测也具有十分重要的意义。     

Subduction of the Bangong–Nujiang Ocean: constraints from granites in the Bangong Co area,Tibet

We report geochronologic, whole‐rock geochemical and Sr–Nd isotopic analyses of the granites that are exposed to both the north and the inside of the Bangong–Nujiang Suture (BNS) zone as well as the implications for the Mesozoic history of Tibet. To the north of the BNS, the Larelaxin pluton consists of I‐type quartz diorite and highly fractionated I‐type biotite granite. The Larelaxin pluton is enriched in large‐ion lithophile elements (LILE) but depleted in high‐field‐strength elements (HFSE); therefore, it exhibits the features of volcanic arc rocks. The initial Sr (0.7102 to 0.7215) and negative εNd (t) (−2.91 to −5.20) values imply a mixture of depleted mantle and continental crust. The mean ²⁰⁶Pb/²³⁸U zircon age is 168 Ma; we therefore propose that the Bangong–Nujiang Ocean (BNO) had already been subducted beneath the Qiangtang terrain by the middle Jurassic. Inside the BNS, the Rutog granites intruded into the Lagongtang and Duoren formations, which show a continental margin and a forearc basin sedimentary facies, respectively. The mean ²⁰⁶Pb/²³⁸U zircon age is 101 Ma. The Rutog granites are monzogranites with a high Na/K ratio (Na₂O/K₂O > 1) and a high LILE/HFSE ratio, and A/CNK < 1.1. The high Sr/Y ratio (22 to 56) implies that these granites are adakitic. The low initial Sr (0.7044 to 0.7055) and positive εNd (t) (+1.46 to +2.70) values indicate that the protolith of the Rutog granites originated mainly from a depleted source. We attribute the Rutog plutonism to the development of an oceanic arc during the continuing northward subduction of the BNO and propose that the Rutog adakitic granites were formed by melting of the subducted BNO crust with limited crustal contamination. Copyright © 2013 John Wiley & Sons, Ltd.