东天山碰撞造山与金铜成矿系统分析

根据碰撞带不同单元按构造一岩石地层划分原则,分出有序和无序两套地层岩石组合,分属两个不同的构造一火山活动带,碰撞造山与韧性剪切带强时空耦合．碰撞带两侧岛弧火山岩和带内碰撞花岗岩特征和成岩成矿时代、地球化学省等表明其较特殊的碰撞造山和陆内造山成盆多阶段演化特点。与碰撞造山有关的金铜矿分七种成因类型。现划10个成矿区带分属两个古陆边缘成矿系统,金矿成矿可分为五个阶段。金铜矿成带分布与碰撞造山演化有关,空间上北部铜矿带,南部金矿带,为今后进一步找矿指明了方向,提出了具体靶区。     

The transition from subduction to continental collision: crustal structure in the North Canterbury region, New Zealand

The North Canterbury region marks the transition from Pacific plate subduction to continental collision in the South Island of New Zealand. Details of the seismicity, structure and tectonics of this region have been revealed by an 11-week microearthquake survey using 24 portable digital seismographs. Arrival time data from a well-recorded subset of microearthquakes have been combined with those from three explosions at the corners of the microearthquake network in a simultaneous inversion for both hypocentres and velocity structure. The velocity structure is consistent with the crust in North Canterbury being an extension of the converging Chatham Rise. The crust is about 27 km thick, and consists of an 11 km thick seismic upper crust and 7 km thick seismic lower crust, with the middle part of the crust being relatively aseismic. Seismic velocities are consistent with the upper and middle crust being composed of greywacke and schist respectively, while several lines of evidence suggest that the lower crust is the lower part of the old oceanic crust on which the overlying rocks were originally deposited.
The distribution of relocated earthquakes deeper than 15 km indicates that the seismic lower crust changes dip markedly near 43S. To the south-west it is subhorizontal, while to the north-east it dips north-west at about 10. Fault-plane solutions for these earthquakes also change near 43S. For events to the south, P -axes trend approximately normal to the plate boundary (reflecting continental collision), while for events to the north, T -axes are aligned down the dip of the subducted plate (reflecting slab pull). While lithospheric subduction is continuous across the transition, it is not clear whether the lower crust near 43S is flexed or torn.     

也谈大兴安岭南段前进场岩体成因类型和形成环境

前进场岩体在岩石地球化学方面以过铝、富集ＬＲＥＥ、具中等负Ｅｕ异常为特征，∑ＲＥＥ介于１０７．８１－１４３．２４μｇ／ｇ之间，结合其野外地质特征和区域构造分析，本文认为该岩体为板块碰撞期间于晚石炭世由沉积物质部分熔融而形成的Ｓ型花岗岩。     

New age constraints on the cooling and unroofing history of the Trans-Himalayan Ladakh Batholith (Kargil area), N. W. India

Thermotectonic history of the Trans-Himalayan Ladakh Batholith in the Kargil area, N. W. India, is inferred from new age data obtained here in conjunction with previously published ages. Fission-track (FT) ages on apatite fall around 20±2 Ma recording cooling through temperatures of ∼100°C and indicating an unroofing of 4 km of the Ladakh Range since the Early Miocene. Coexisting apatite and zircon FT ages from two samples in Kargil show the rocks to have cooled at an average rate of 5–6°C/Ma in the past 40 Ma. Zircon FT ages together with mica K−Ar cooling ages from the Ladakh Batholith cluster around 40–50 Ma, probably indicating an Eocene phase of uplift and erosion that affected the bulk of the batholith after the continental collision of India with the Ladakh arc at 55 Ma. Components of the granitoids in Upper Eocene-Lower Oligocene sediments of the Indus Molasse in Ladakh supports this idea. Three hornblende K−Ar ages of 90 Ma, 55 Ma, and 35 Ma are also reported; these distinctly different ages probably reflect cooling through 500–550°C of three phases of I-type plutonism in Ladakh also evidenced by other available radiometric data: 102 Ma (mid-Cretaceous), 60 Ma (Palaeocene), and 40 Ma (Late Eocene); the last phase being localised sheet injections. The geodynamic implications of the age data for the India-Asia collision are discussed.     

A Study on the Forming Conditions of Basalts in Seamounts of the South China Sea

Volcanic rocks in seamounts of the South China Sea consist mainly of alkali basalt, tholeiitic basalt, trachyandesitic pumice, dacite, etc. Inclusions in the minerals of the volcanic rocks are mainly amorphous melt inclusions, which reflects that the volcanic rocks are characterized by submarine eruption and rapid cooling on the seafloor. Furthermore, fluid-melt inclusions have been discovered for the first time in alkali basalts and mantle-derived xenoliths. indicating a process of differentiation between magma and fluid in the course of mantle partial melting. Alkali basalts and inclusions may have been formed in this nonhomogeneous system. Rock-forming temperatures of four seamounts were estimated as follows: the Zhongnan seamount alkali basalt 1155 ∼ 1185 °C; the Xianbei seamount alkali basalt 960 ∼ 1200 °C; tholeiitic basalt 1040 ∼ 1230 °C; the Daimao seamount tholeiitic basalt 1245 ∼ 1280 °C; and the Jianfeng seamount trachyandestic pumice 880 ∼ 1140 °C. Equilibrium pressures of alkali basalts in the Zhongnan and Xianbei seamounts are 13.57 and 8.8 × 10⁸ Pa, respectively. Pyroxene equilibrium temperatures of mantle xenoliths from the Xianbei seamount were estimated at 1073 ∼ 1121 °C, and pressures at (15.58 ∼ 22.47)×10⁸Pa, suggesting a deep-source (e.g. the asthenosphere) for the alkali basalts. This project was financially supported by the National Natural Science Foundation of China and Guangzhou Marine Geology Survey.     

Ridge collision and in situ greenstones in accretionary complexes: An example from the Late Cretaceous Ryukyu Islands and southwest Japan margin

Abstract Mélange units containing greenstones are common throughout the Cretaceous-Miocene Shimanto Supergroup in the Ryukyu Is and southwest Japan. Most greenstones in the accretionary complex originated in oceanic spreading ridges and seamounts, and they formed far from the convergent margin. Some mélange-like units in the supergroup, however, contain greenstones that were extruded upon and intruded into unconsolidated fine-grained terrigenous clastic sediments. It is inferred that eruption of the in situ greenstones resulted from igneous activity in the trench area. Geochemical signatures indicate that the greenstone protoliths were similar to mafic lavas generated at spreading ridges. Fossil ages of the strata containing in situ greenstones become younger over a distance of 1300 km eastward from Amami-Oshima (Cenomanian-Turonian) in the Ryukyu Is to central Japan (Late Maestrichtian-earliest Paleocene), implying that a site of igneous activity in the trench area migrated eastward along the Ryukyu Is and southwest Japan margin. Plate reconstructions of the northwest Pacific Ocean suggest the presence of the Kula-Pacific ridge near Late Cretaceous to early Paleogene Japan. In this context, it is suggested that the greenstones formed in response to Kula-Pacific ridge-forearc collision.
Ancient ridge-forearc collisions are best recognized by the presence of mid-ocean ridge basalt (MORB) extruded on sediments inferred to have accumulated in the trench area. Diachronous occurrences of the strata associated with these MORB in an orogenic belt are useful for documenting the ridge collision through time.     

Silurian Sedimentation in the South Qilian Belt: Arc-Continent Collision-related Deposition in the NE Tibet Plateau?

The South Qilian belt mainly comprises an early Paleozoic arc-ophiolite complex, accretionary prism, microcontinental block, and foreland basin. These elements represent accretion-collision during Cambrian to Silurian time in response to closure of the Proto-Tethyan Ocean in the NE of the present-day Tibet Plateau. Closure of the Proto-Tethyan Ocean between the Central Qilian block and the Oulongbuluke block and the associated collision took place from NE to SW in a zipper-like style. Sediment would have been dispersed longitudinally SW-ward with a progressive facies migration from marginal alluvial sediments toward slope deep-water and deep-sea turbidites. This migration path indicates an ocean basin that shrank toward the SW. The Balonggongga'er Formation in the western South Qilian belt represents the fill of a latest Ordovician-Silurian remnant ocean basin that separated the Oulongbuluke block from the Central Qilian block, and records Silurian closure of the Proto-Tethyan Ocean and subduction beneath the Central Qilian block. However, alluvial deposits in the Lajishan area were accumulated in a retro-foreland basin, indicating that continent-continent collision in the eastern South Qilian belt occurred at c. 450–440 Ma. These results demonstrate that the Proto-Tethyan Ocean closed diachronously during early Paleozoic time.     

郯庐断裂带起源与大陆斜向汇聚

郯庐断裂带起源于华北克拉通与扬子板块的汇聚过程中已被多数学者所认同。该断裂带的起源机制，涉及到这两个大陆板块的汇聚方式。可是，对于这一重要问题却长期存在着不同的认识。本文依据郯庐断裂带内部及其两侧前陆上构造与年代学研究成果，综合分析该断裂带的起源机制。郯庐断裂带内部残留的起源期构造为左行走滑韧性剪切带，所获得的白云母40Ar/39Ar同位素年龄为239~217 Ma。华北克拉通边缘的徐淮弧形逆冲-推覆构造，及九江地区扬子板块上的弧形褶皱带，分别为苏鲁造山带、大别造山带点碰撞的产物。郯庐断裂带西侧的华北克拉通边缘，在汇聚过程中呈现为刚性陆块的特征，没有出现大规模的牵引弯曲现象。而东侧的扬子板块前陆构造，在汇聚过程中却明显出现了大规模的牵引弯曲现象。断裂带东侧的张八岭隆起北段，出露了扬子板块上中地壳韧性拆离带，其中所获得的白云母40Ar/39Ar同位素年龄为245~218 Ma，其滑动方向受控于郯庐断裂带起源期的左行走滑运动。这一系列构造与年代学信息，表明郯庐断裂带起源于华北克拉通与扬子板块的汇聚过程中，代表了这两个大陆的斜向汇聚边界。该断裂带起源期的活动时限，与大别、苏鲁造山带内大陆深俯冲时间相吻合。在大别造山带北部的华北克拉通，原始应存在着向南的突出体(嵌入体)，从而导致嵌入式碰撞与嵌入体边界的大陆斜向汇聚(起源期郯庐断裂带)，符合嵌入式碰撞导致板片撕裂模式。     

Thermobaric structure of a traverse across western Dabieshan: implications for collision tectonics between the Sino-Korean and Yangtze cratons

As the best preserved high‐ and ultrahigh‐pressure (HP and UHP) metamorphic terrane in the Qinling‐Dabieshan‐Sulu orogen, western Dabieshan is divided into six lithotectonic units along a traverse across the orogen, i.e. from north to south, the Nanwan, Balifan, Huwan, Xinxian, Hong'an and Mulanshan units. In this terrane five eclogite‐bearing zones (I–V) are developed. The garnet and clinopyroxene in eclogites from these zones exhibit chemical zoning, suggesting that the rims record general peak temperature and pressure. Thermobarometric study indicates that the peak P–T conditions of eclogite are 550–570°C and 21 kbar for Zone I, 470–500°C and 14–17 kbar for Zone II, 620–670°C and 26–29 kbar for Zone III, 530–560°C and 20–22 kbar for Zone IV, and 490–510°C and 19–20 kbar for Zone V. The symmetrical thermobaric pattern, in conjunction with structural and geochronological data, demonstrates that the Huwan and Hong'an units belong to the same HP slice overlying the UHP slice. This pattern, together with the Mulanshan LT/HP blueschist–greenschist belt in the south, roughly constitutes a ‘normal’ metamorphic zonation. However, clear metamorphic gaps occur between different slices. It is inferred that the LT/HP, HP and UHP slices were broken up from the downgoing slab during subduction and reached different depths along different geothermal gradients. The successive subduction of underlying slices leads to a nearly concomitant uplift of overlying slices, whereas exhumation of the deepest UHP slice was effected by underthrusting of the lower crust of the Yangtze craton.