大别-苏鲁造山带橄榄岩记录的碰撞造山过程中地幔楔的地壳交代作用

地壳交代作用是洋壳俯冲带板片-地幔界面的普遍现象,由于地幔楔样品的缺乏,其识别存在困难。而碰撞造山带广泛出露的地幔楔来源的造山带橄榄岩则是理想的研究对象。本文对大别-苏鲁造山带橄榄岩的已有研究成果进行了系统总结。这些成果表明这些橄榄岩在大洋俯冲向大陆碰撞转换的不同阶段经历了多期地壳来源流体的交代作用。地壳交代作用不仅改变了地幔楔橄榄岩的地球化学成分,而且导致了交代矿物的生长以及超镁铁质交代体的形成。这些交代体或作为同碰撞和碰撞后镁铁质火成岩的地幔源区,或将地壳组分传输到深部地幔,或释放流体交代大陆俯冲隧道中的俯冲陆壳。本文对地壳交代作用研究中存在的重要问题和解决思路提出了建议。     

Provenance and origins of a Late Paleozoic accretionary complex within the Khangai–Khentei belt in the Central Asian Orogenic Belt,central Mongolia

We have investigated the petrography, geochemistry, and detrital zircon U–Pb LA-ICPMS dating of sandstone from the Gorkhi Formation of the Khangai–Khentei belt in the Ulaanbaatar area, central Mongolia. These data are used to constrain the provenance and source rock composition of the accretionary complex, which is linked to subduction of the Paleo-Asian Ocean within the Central Asian Orogenic Belt during the Middle Devonian to Early Carboniferous. Field and microscopic observations of the modal composition of sandstone and constituent mineral chemistry indicate that the sandstone of the Gorkhi Formation is feldspathic arenite, enriched in saussuritized plagioclase. Geochemical data show that most of the sandstone and shale were derived from a continental margin to continental island arc setting, with plutonic rocks being the source rocks. Detrital zircon ²⁰⁶Pb/²³⁸U ages of two sandstones yields age peaks of 322 ± 3 and 346 ± 3 Ma. The zircon ²⁰⁶Pb/²³⁸U age of a quartz–pumpellyite vein that cuts sandstone has a weighted mean age of 339 ± 3 Ma. Based on these zircon ages, we infer that the depositional age of sandstone within the Gorkhi Formation ranges from 320 to 340 Ma (i.e., Early Carboniferous). The provenance and depositional age of the Gorkhi Formation suggest that the evolution of the accretionary complex was influenced by the intrusion and erosion of plutonic rocks during the Early Carboniferous. We also suggest that spatial and temporal changes in the provenance of the accretionary complex in the Khangai–Khentei belt, which developed aound the southern continental margin of the Siberian Craton in relation to island arc activity, were influenced by northward subduction of the Paleo-Asian Ocean plate.     

东昆仑卡尔却卡地区中志留世A型花岗岩岩石成因及构造环境

东昆仑造山带西部卡尔却卡地区出露大量似斑状二长花岗岩,对其展开岩相特征、锆石U-Pb年龄和岩石地球化学研究,结果显示,似斑状二长花岗岩年龄为425±2.3 Ma,表明其形成于中志留世。岩石具有高SiO2（69.0%~71.2%）和K2O（4.3%~5.54 %）含量,富集大离子亲石元素（Rb、K、Th和U）,亏损高场强元素（Nb、Ta和Ti）,强烈的负Eu异常（Eu/Eu*=0.40~0.51）特征。岩石较低的Cr、Ni含量（Cr=13.81×10-6~32.9×10-6,Ni=5.48×106~12.05×10-6）、低的Ce/Pb（2.01~4.19）和Nb/U（4.1~7.57）值,表明该岩体来自上地壳的部分熔融。岩体高10000Ga/Al值（3.22~3.42）、Zr+Nb+Ce+Y元素含量（459×10-6~656×10-6）和锆石饱和温度（827~881℃）,表明其为A型花岗岩。构造图解和高的Y/Nb值（2.93~3.15）指示,该岩体为A2型花岗岩,形成于碰撞后伸展背景。结合区域岩浆岩的时空分布,表明东昆仑造山带从中志留世开始进入碰撞后伸展阶段。     

Zircon U–Pb age and Hf isotope evidence for contrasting origin of bimodal protoliths for ultrahigh-pressure metamorphic rocks from the Chinese Continental Scientific Drilling project

A combined study of zircon morphology, U–Pb ages and Hf isotopes as well as whole‐rock major and trace elements was carried out for ultrahigh‐pressure (UHP) eclogite and felsic gneiss from the main hole (MH) of the Chinese Continental Scientific Drilling (CCSD) project in the Sulu orogen. The results show contrasting Hf isotope compositions for bimodal UHP metaigneous rocks, pointing to contrasting origins for their protoliths (thus dual‐bimodal compositions). The samples of interest were from two continuous core segments from CCSD MH at depths of 734.21–737.16 m (I) and 929.67–932.86 m (II) respectively. Zircon U–Pb dating for four samples from the two core segments yields two groups of ages at 784 ± 17 and 222 ± 3 Ma, respectively, corresponding to protolith formation during supercontinental rifting and metamorphic growth during continental collision. Although the Triassic UHP metamorphism significantly reset the zircon U–Pb system of UHP rocks, the Hf isotope compositions of igneous zircon can be used to trace their protolith origin. Contrasting types of initial Hf isotope ratios are, respectively, correlated with segments I and II, regardless of their lithochemistry. The first type shows positive ?_Hf(t) values of 7.8 ± 3.1 to 6.0 ± 3.0, with young Hf model age of 1.03 and 1.11 Ga. The second type exhibits negative ?_Hf(t) values of ?6.9 ± 1.6 to ?9.1 ± 1.1, with old Hf model ages of 2.11 and 2.25 Ga. It appears that the UHP rocks from the two segments have protoliths of contrasting origin. Consistent results are also obtained from their trace element compositions suggesting that mid‐Neoproterozoic protoliths of bimodal UHP metaigneous rocks formed during supercontinental rifting at the northern margin of the South China Block. Thus, the first type of bimodal magmatism formed by rapid reworking of juvenile crust, whereas the second type of bimodal magmatism was principally generated by rift anatexis of Paleoproterozoic crust. Melting of orogenic lithosphere has potential to bring about bimodal magmatism with contrasting origins. Because arc–continent collision zones are the best place to accumulate both juvenile and ancient crusts, the contrasting types of bimodal magmatism are proposed to occur in an arc–continent collision orogen during the supercontinental rifting, in response to the attempted breakup of the supercontinent Rodinia at c. 780 Ma.     

Chloritoid–glaucophane schist in the north Qilian orogen, NW China: phase equilibria and P–T path from garnet zonation

Chloritoid–glaucophane‐bearing rocks are widespread in the high‐pressure belt of the north Qilian orogen, NW China. They are interbedded and cofacial with felsic schists originated from greywackes, mafic garnet blueschists and low‐T eclogites. Two representative chloritoid–glaucophane‐bearing assemblages are chloritoid + glaucophane + garnet + talc + quartz (sample Q5‐49) and chloritoid + glaucophane + garnet + phengite + epidote + quartz (sample Q5‐12). Garnet in sample Q5‐49 is coarse‐, medium‐ and fine‐grained and shows two types of zonation patterns. In pattern I, X_grs is constant as X_py rises, and in pattern II X_grs decreases as X_py rises. Phase equilibrium modelling in the NC(K)MnFMASH system with Thermocalc 3.25 indicates that pattern I can be formed during progressive metamorphism in lawsonite‐stable assemblages, while pattern II zonation can be formed with further heating after lawsonite has been consumed. Garnet growth in Q5‐49 is consistent with a continuous progressive metamorphic process from ～14.5 kbar at 470 °C to ～22.5 kbar at 560 °C. Garnet in sample Q5‐12 develops with pattern I zonation, which is consistent with a progressive metamorphic process from ～21 kbar at 540 °C to ～23.5 kbar at 580 °C with lawsonite present in the whole garnet growth. The latter sample shows the highest P–T conditions of the reported chloritoid–glaucophane‐bearing assemblages. Phase equilibrium calculation in the NCKFMASH system with a recent mixing model of amphibole indicates that chloritoid + glaucophane paragenesis does not have a low‐pressure limit of 18–19 kbar as previously suggested, but has a much larger pressure range from 7–8 to 27–30 kbar, with the low‐pressure part being within the stability field of albite.     

Granular experiments of thrust wedges: Insights relevant to methane hydrate exploration at the Nankai accretionary prism

The accumulation mechanism of methane hydrates has been a central issue in previous hydrate research regarding the Nankai accretionary prism, southwest of Japan. Expulsion of formation fluids is significant during the prism accretion process, and the migration of these methane-bearing fluids exerts a strong control on the accumulation of hydrates. Two types of fluid pathways, inter-granular porosity and faults, need to be evaluated to understand hydrate accumulation. Fluid migration along faults can be partly modeled by examining faulting activity. Our study modeled the accretion process by using two granular methods that approximated the geologic body as an assemblage of particles: (1) analog experiments using granular materials, and (2) a numerical simulation based on the distinct element method. The analog experiments closely reproduced the prism geometry observed in seismic profiles across the Nankai accretionary prism. Digital image correlation analysis indicated that the frontal thrust is generally active but older structures are also frequently reactivated. The numerical simulations produced prism geometries similar to those of the analog experiments. The velocity distributions of the particles showed evidence of episodic faulting and reactivation, but the internal stress field exhibited little change in the deeper part of the prism during deformation. The frequent and substantial changes in fault activity displayed by the models indicate episodic fluid flow along fault surfaces. Active frontal thrusting suggests that formation fluids generally migrate from deep within the prism to the deformation front, but may move along reactivated older faults. Inter-granular permeability also fluctuates, as it is controlled by temporal and spatial variations in the internal stress field. However, fluid flow is likely to be relatively stable in the deeper segment of the prism.     

青岛连三岛地区“荆山岩群”高压变质岩的变质时代和变质演化特征

青岛连三岛地区原划为古元古界荆山岩群中出露各类片岩、片麻岩。本文在前人研究的基础上,对连三岛地区出露的含榴黑云母斜长片麻岩、含榴云母片岩和含榴黑云母钾长片麻岩,进行详细的岩相学和矿物化学研究,确定其变质温压条件及其P-T演化轨迹,并采用LA-ICP-MS锆石U-Pb定年,获得了3个样品的原岩时代和变质时代,为全面深入认识其变质属性提供了进一步的重要依据。根据岩相学和矿物化学成分可以识别出两期矿物组合：第一期（峰期变质阶段）为Grt1+Kfs+Aln+Ph+Qtz;第二期（退变质阶段）为Grt2+Pl+Ep+Bt+Qtz;依据多硅白云母Si压力计、锆石Ti温度计以及GB-GBPQ矿物温压计,确定其峰期变质和退变质的温压条件分别为T=600~817℃、P=2.4~2.6GPa和T=431~456℃、P=0.48~0.82GPa。结合白云母部分熔融现象,上述两个变质阶段构成了一个折返早期升温降压,后穿过多硅白云母熔融反应线,最后降温降压的顺时针型演化的P-T轨迹。CL图像显示3个样品的锆石均具有典型的岩浆核-变质边结构;结合LA-ICP-MS锆石原位微区U-Pb定年和微量元素分析,3个样品分别获得了769~756Ma、~223Ma和213~216Ma三组年龄,分别与苏鲁造山带其他高压-超高压变质岩的新元古代原岩时代（700~800Ma）、峰期变质时代（240~225Ma）和角闪岩相退变质时代（215~205Ma）一致。对样品含榴黑云母斜长片麻岩（17LSD-1）和含榴云母片岩（18LSD-2）进行锆石Hf同位素分析,获得样品17LSD-1的εHf（t）=-23.2~2.8、tDM2C（Hf）=1712~2845Ma,表明其原岩主要来源于古元古代陆壳重熔;样品18LSD-2的εHf（t）=-13.9~8.6、tDM2C（Hf）=1113~2358Ma,表明样品形成时壳源物质成分占主导地位,同时部分幔源或新生地壳物质的加入,导致少部分εHf（t）偏正值。Hf同位素结果表明连三岛变质岩原岩的形成与扬子板块新太古代-早古元古代的陆壳重熔有关。对比前人的相关数据,无论是原岩时代、变质年龄还是变质演化特征,本文研究的连三岛地区片岩/片麻岩与苏鲁造山带的部分变质岩均具有相似的原岩属性和变质属性,因此推断其应归属为苏鲁超高压变质带的一部分,是三叠纪扬子板块向华北板块俯冲碰撞引发的高压-超高压变质事件的产物,不应再作为岩石地层单元划归为"荆山岩群"。     

Types,geological features and geodynamic significances of gold-copper deposits in the Kanggurtag metallogenic belt,eastern Tianshan,NW China

Occupying the middle of the central Asia Paleozoic accretionary and collisional orogenic belt, the eastern Tianshan area has a great economic potential due to Au-Cu mineralization during syn- and post- orogenic events. In the Kanggurtag Au-Cu metallogenic belt, three major types of gold deposits have been recognized: ductile-shear-zone-hosted gold deposits, magmatic hydrothermal gold deposits, and epithermal gold deposits. Four kinds of copper deposits have also been identified recently: the porphyry-type, the skarn-type, the magmatic type, and volcanic/sedimentary-type. Tectonically, the development of these late Paleozoic gold and copper deposits was closely associated with the subduction and collision of the ancient Tianshan ocean that intervened between the Tarim craton and the Siberian block. In the early to mid-Carboniferous, N-dipping subduction beneath the Dananhu arc generated magmatic intrusions, leading to formation of the porphyry Cu deposits. The magmatic front migrated southward to form the Yamansu arc upon the Kanggurtag accretionary wedge. In the latest Carboniferous to early Permian, during the closure of the ancient Tianshan ocean, large mafic-ultramafic complexes were emplaced, resulting in several magmatic copper-nickel deposits. Gold deposits of the shear-zone-type are controlled by the Kanggurtag ductile shear zone, which is related to collisional orogenesis. The epithermal gold deposits are associated with extensional tectonics and post-tectonic volcanic activity. The tectonic settings, geological features, and temporal and spatial distributions of these different types of gold and copper deposits reflect, to a great extent, the accretionary and collisional tectonics that occurred between the northern margin of the Tarim block and the southern margin of the Siberian block.     

西昆仑-塔西南坳陷晚古生代以来的沉积构造演化

自柯克亚深层油气勘探取得突破以来,塔西南坳陷一直受到各类地质学家的广泛关注,有关该盆地的形成和演化历史及其油气资源评价近年来更是成为人们的研究热点.本文在总结前人资料的基础上,探讨塔西南和西昆仑地区自晚古生代以来所经历的构造及沉积格架的演变过程,对塔西南坳陷性质及其演化阶段划分所存在的争议进行了归纳,分析了塔西南-西昆仑这一盆山体系形成和演化中的构造变形和沉积记录.总体来说,根据现有沉积和构造变形资料,中生代之前西昆仑和塔西南坳陷分别处于同一构造背景下的不同沉积单元;二者之间盆山体系的形成主要自晚侏罗世-早白垩世,中-上新世是造盆造山作用机制发生重大转折的时期,或者说早更新世末的构造运动基本上奠定了西昆仑.塔里木盆地南缘现今的盆-山构造格架.     

浅议苏鲁造山带基底大地构造属性--以荣成-威海地区为例

苏鲁造山带的基底在大地构造上是归属华北板块还是扬子板块,抑或二者兼而有之还是二者都不是?存在着不同意见.依据造山带变质表壳岩的区域性对比和岩浆演化序列,特别是通过Nd同位素示踪所提供的信息,文章从事件演化的角度综合研究分析认为,苏鲁造山带基底的大地构造属性,既不应归于华北板块,也不宜归于扬子板块,而应作为一个独立的大地构造单元划出为宜.