Petrochemistry of granulite xenoliths from the Cenozoic Qiangtang volcanic field,northern Tibetan Plateau: implications for lower crust composition and genesis of the volcanism

High-K calc-alkaline magmas from the Cenozoic Qiangtang volcanic field, northern Tibetan Plateau, contain lower crustal two-pyroxene and clinopyroxene granulite xenoliths. The petrology and geochemistry of six mafic and three felsic xenoliths from the Hol Xil area south of Ulan Ul Lake are discussed. Mafic granulites (Pl, Opx, Cpx, Ksp, and Bt) contain 48.76–58.61% SiO₂, 18.34–24.50% Al₂O₃, 3.16–5.41% Na₂O, 1.58–3.01% K₂O, low Mg# (30–67), LREE and LILE enrichment, high Rb/Sr (0.09–0.21), (La/Yb)_N (17.32–49.35), low Nb/Ta (9.76–14.92), and variable Eu anomalies (Eu?=?0.19?0.89). They also have more evolved Sr-Nd-Pb isotopic compositions in comparison with the host dacites ⁸⁷Sr/⁸⁶Sr (0.710812 vs. 0.713241), ?_Sr (+169.13 vs.?+203.88), ¹⁴³Nd/¹⁴⁴Nd (0.512113 vs. 0.512397), ?_Nd (?4.70 to??10.05), ²⁰⁶Pb/²⁰⁴Pb (18.7000 vs. 18.9565), ²⁰⁷Pb/²⁰⁴Pb (15.7135 vs. 15.7662), and ²⁰⁸Pb/²⁰⁴Pb (39.1090 vs. 39.4733). Felsic granulites (Qtz, Pl, Ksp, Bt, and Cpx) show enrichment of LREE and LILE and have evolved Sr-Nd-Pb isotopic compositions with (La/Yb)_N (2.04–10.82), ⁸⁷Sr/⁸⁶Sr (0.712041–0.729088), ?_Sr (+180.71–+430.59), ¹⁴³Nd/¹⁴⁴Nd (0.512230–0.512388); ?_Nd (?4.74 to??7.96), ²⁰⁶Pb/²⁰⁴Pb (18.9250–19.1717), ²⁰⁷Pb/²⁰⁴Pb (15.7662–15.7720), and ²⁰⁸Pb/²⁰⁴Pb (39.2109–39.6467). These geochemical data suggest that the protolith of the mafic granulites could have been a hybrid mafic magma (e.g. enriched mantle type II) or metasomatized restite derived from the partial melting of metamafic-intermediate rocks rather than basaltic cumulates, whereas the felsic granulite protolith was a quartzofeldspathic S-type granitic rock. We argue that the lower crust of the northern Tibetan Plateau is hot and heterogeneous rather than wholly gabbroic. Interaction between the mantle-derived magma and the metasedimentary/granitic lower crust of the Tibetan Plateau may have played an important role in the generation of shoshonitic and high-K calc-alkaline andesite-dacite rocks.     

Detrital zircon ages in Palaeozoic and Mesozoic basement assemblages of the Peninsular Ranges batholith,Baja California,Mexico: constraints for depositional ages and provenance

The origin and continuity of Phanerozoic lithostratigraphic terranes in southern and Baja California remain an unsolved issue in Cordilleran tectonics. We present data from eight detrital zircon samples collected across the southern extent of the Peninsular Ranges that help constrain the provenance of detritus and the depositional ages of these basement units. Detrital zircon signatures from units in the eastern Peninsular Ranges correlate with Palaeozoic passive margin assemblages in the southwestern North American Cordillera. Units in the central belt, which consists of Triassic–Jurassic metasedimentary turbidite assemblages that probably deformed in an accretionary prism setting, and Cretaceous metasedimentary and metavolcanic units that represent the remnants of a continental margin arc, were derived from both proximal and more distal sources. The westernmost units, which are locally structurally interleaved with the Triassic through Cretaceous units of the central belt, are Cretaceous deposits that represent a series of collapsed basin complexes located within and flanking the Cretaceous Alisitos volcanic island arc. Cretaceous intra-arc units show little influx of cratonal material until approximately 110 Ma, whereas coeval sediments on the northern and eastern flanks of the Alisitos arc contain abundant cratonal detritus. Intra-arc strata younger than approximately 110 Ma contain large amounts of Proterozoic and older detrital zircons. These data suggest that basins associated with the Alisitos arc were either too distant or somehow shielded from North American detritus before 110 Ma. In the case of the former, increased influx of continental detritus after 110 Ma would support a tectonic model in which the arc was separated from North America by an ocean basin and, as the arc approached the continent, associated depositional centres were close enough to receive input from continental sources.     

Origin of the Dachang gold deposit,NW China: constraints from H,O, S,and Pb isotope data

The Dachang gold deposit is located in the Late Triassic Songpan-Ganzi Fold Belt, NE Tibetan Plateau. Gold ore is concentrated as veins along secondary faults and fracture zones in the Bayan Har Group metaturbidites. No exposed felsic plutons are present in the vicinity of the deposit. The auriferous veins contain <15% sulphide minerals, mainly arsenopyrite, pyrite, and stibnite. Gold is commonly enclosed within arsenopyrite and pyrite. Typical alteration around the ore bodies includes silicification, sericitization, and weak carbonatization.

Gold-bearing quartz samples have δ¹⁸O values of 16.9–21.2‰ (V-SMOW) from which δ¹⁸O_H2O values of 6.2–9.6‰ can be calculated from the fluid inclusion temperatures (or 10.0 to 12.7‰ if we used the average arsenopyrite geothermometer temperature of 301°C). The δD values of fluid inclusions in quartz range from –90‰ to –72‰. δ³⁴S values of gold-bearing sulphides mainly range from –5.9‰ to –2.8‰ (V-CDT). Pyrite and arsenopyrite in ores have ²⁰⁶Pb/²⁰⁴Pb ratios of 18.2888 to 18.4702, ²⁰⁷Pb/²⁰⁴Pb ratios of 15.5763 to 15.6712, and ²⁰⁸Pb/²⁰⁴Pb ratios of 38.2298 to 38.8212. These isotopic compositions indicate that the ore-forming fluids were of metamorphic origin, and the S and Pb may have been derived from the host metaturbidites of the Bayan Har Group. The Dachang Au deposit has geological and geochemical features similar to orogenic gold deposits. We propose that the ores formed when the Songpan-Ganzi Fold Belt was intensely deformed by Late Triassic folding and thrusting. Large-scale thrusting resulted in regional allochthons of different scales, followed by secondary faults or fracture zones that controlled the ore bodies.     

Indosinian Orogenesis in the Lhasa Terrane,Tibet: New Muscovite 40Ar‐39Ar Geochronology and Evolutionary Process

Muscovite 40Ar-39Ar dating of muscovite-quartz schist, eclogite and retrograde eclogite indicates an Indosinian orogenesis occurred at 220–240 Ma in the Lhasa terrane, which is caused by the closure of Paleo-Tethyan ocean basin and the following collision of the northern Lhasa terrane and southern Gondwana land. This Indosinian orogenesis is further confirmed by the regional sedimentary characteristics, magmatic activity and ophiolite mélange. This evidence suggests that the Indosinian orogenic belt in the Lhasa terrane is widely distributed from the Coqen county in the west, and then extends eastward through the Ningzhong and Sumdo area, finally turning around the eastern Himalayan syntaxis into the Bomi county. Based on the evolutionary process, the geological development of Lhasa terrane from early Paleozoic to early Mesozoic can be divided into seven stages. All of the seven stages make up a whole Wilson circle and reveal a perfect evolutionary process of the Paleo-Tethys ocean between the northern Lhasa terrane and southern Gondwana land. The Indosinian orogenisis is a significant event for the evolution of the Lhasa terrane as well as the Tibetan Plateau.     

Kurosegawa Terrane in Southwest Japan: Disrupted Remnants of a Gondwana-Derived Terrane

The Kurosegawa Terrane is an anomalous, disrupted, Paleozoic and Mesozoic lithotectonic assemblage characterized by fragments of continent and continental margins. It is located in Southwest Japan where it lies between two Mesozoic subduction complex terranes. The Kurosegawa Terrane is an exotic and far-travelled geologic entity with respect to its present position. Limestones of the Kurosegawa Terrane formed along a continental margin yield fusulinacean fossils Cancellina, Colania and Lepidolina. Accordingly, the Kurosegawa Terrane was once situated within the Colania-Lepidolina territory in the East Tethys-Panthalassa region at a palaeo-equatorial latitude, possibly close to the eastern margin of the South China and/or Indochina-East Malaya continental blocks. These blocks had rifted from Gondwana by late Devonian. They drifted northwards, passing through the Colania-Lepidolina territory in mid-Permian time, and amalgamated with the proto-Asian continent during the late Triassic. Subsequently, during the Cretaceous, parts of the allochthonous continental blocks and their associated tectonic collage were transpressed, dispersed, and displaced from the southeastern periphery of Asia towards the north. As a result, the Kurosegawa Terrane is formed as a disrupted allochthonous terrane, characterized by a serpentinite melange zone, lying between the adjoining Mesozoic subduction complex terranes.     

西藏仁错地区前寒武纪基底对北拉萨地块起源的约束

北拉萨地块是青藏高原前寒武纪岩石出露最为广泛的地区之一,该地块的新元古代演化历史及其在超大陆演化中所扮演的角色一直存有争议.围绕这一科学问题,本文收集整理了北拉萨地块仁错地区前寒武纪念青唐古拉岩群的同位素年代学、地球化学和同位素地球化学资料,从中发现了 3期新元古代岩浆-沉积-变质记录,包括:①925～886 Ma,岩...     

Age and geochemistry of the Early Mesozoic granitoid massifs of the southern Bureya terrane of the Russian Far East

A massif of porhyritic microcline biotite granites located in the southern part of the Bureya (Turan) terrane has an age of 185 ± 1 Ma. The granites are characterized by (K₂O + Na₂O) > 8%, a K₂O predominance over Na₂O, and a moderately differentiated REE distribution pattern ((La/Yb)_n = 14.7–28.5). The obtained age indicates that at least one stage of the Early Mesozoic granitoid magmatism in the Bureya terrane occurred in the Early Jurassic. The formation of early Mesozoic granitoids was presumably related to collision between North Asian and Sino-Korean cratons, and the intervening Amur superterrane, although a subduction origin also cannot be completely ruled out.     

中国大陆科学钻探主孔榴辉岩Sm-Nd年龄及其地质意义

南苏鲁中国大陆科学钻探主孔CCSD-MH及地表的榴辉岩Sm—Nd同位素年龄测试结果显示，每件样品中全岩（Wr)与石榴石(Grt)和绿辉石(Omp)点均在一条直线上。除样品B210R186以外，其余5件榴辉岩均给出十分相近的Sm—Nd等时线年龄，位于202．6-219Ma之间，这与榴辉岩围岩——片麻岩锆石边缘记录的近等温减压退变质年龄(200-220Ma)十分接近，表明测试的榴辉岩中石榴石的Sm—Nd同位素体系有可能在近等温减压退变质时重置，并在向角闪岩相退变质过渡时封闭。因此，笔者测定的202．6-219Ma的Sm-Nd全岩-矿物等时线年龄，应代表了南苏鲁榴辉岩在构造折返过程中近等温减压阶段的退变质年龄，而不能代表苏鲁地体的超高压变质年龄。     

大别-苏鲁地区晚中生代镁铁质岩地球化学对造山带岩石圈地幔性质的约束

来自大别-苏鲁地区晚中生代镁铁质岩石表现出类似于岛弧火山岩的富集大离子亲石元素(LILE)、轻稀土元素(LREE)和相对亏损高场强元素(HFSE)的微量元素地球化学特征,和高度富集的放射成因 Sr(I_(Sr)=0.7065～0.7090)和低放射成因 Nd(ε_(Nd)(t)=-19～-10)的同位素组成;同时它们显示出一定程度的 Nb/Ta 和 Zr/Hf 内部分馏特征,反映其地幔源区曾受到了相对富金红石和 CO_2的熔体交代作用。我们倾向认为深俯冲陆壳在俯冲或折返过程中发生部分熔融作用形成的熔体与地幔反应是形成大别-苏鲁地区造山带富集岩石圈地幔的重要机制。