Changes of Late Mesozoic Tectonic Regimes around the Ordos Basin(North China)and their Geodynamic Implications

A synthesis is given in this paper on late Mesozoic deformation pattern in the zones around the Ordos Basin based on lithostratigraphic and structural analyses. A relative chronology of the late Mesozoic tectonic stress evolution was established from the field analyses of fault kinematics and constrained by stratigraphic contact relationships. The results show alternation of tectonic compressional and extensional regimes. The Ordos Basin and its surroundings were in weak N-S to NNE-SSW extension during the Early to Middle Jurassic, which reactivated E-W-trending basement fractures. The tectonic regime changed to a multi-directional compressional one during the Late Jurassic, which resulted in crustal shortening deformation along the marginal zones of the Ordos Basin. Then it changed to an extensional one during the Early Cretaceous, which rifted the western, northwestern and southeastern margins of the Ordos Basin. A NW-SE compression occurred during the Late Cretaceous and caused the termination of sedimentation and uplift of the Ordos Basin. This phased evolution of the late Mesozoic tectonic stress regimes and associated deformation pattern around the Ordos Basin best records the changes in regional geodynamic settings in East Asia, from the Early to Middle Jurassic post-orogenic extension following the Triassic collision between the North and South China Blocks, to the Late Jurassic multi-directional compressions produced by synchronous convergence of the three plates （the Siberian Plate to the north, Paleo-Pacific Plate to the east and Lhasa Block to the west） towards the East Asian continent. Early Cretaceous extension might be the response to collapse and lithospheric thinning of the North China Craton.

Kinematic History and Changes in the Tectonic Stress Regime during the Cenozoic along the Qinling and Southern Tanlu Fault Zones

Since the mid-late Eocene, North China has been subjected to extensional stress, resulting in the formation and development of basins. The dynamic origin of this crustal extension has long been an issue of debate. This paper presents the results of kinematic analyses of faults obtained from two seperated areas in North China. In the Weihe graben situated on the southernmost margin of the Ordos block, analyses of fault kinematics were coupled with an analysis of the basin's subsidence history. Three successive extensional tectonic phases accompaning the basin's formation and development have been distinguished. The Palaeogene extension was oriented in a WNW-ESE direction; the Neogene extension in a NE-SW direction and the Pliocene-Quaternary extension in a NW-SE direction. Such changes have also been recorded by fault kinematics along the southern Tanlu fault zone. This has been demonstrated by three successive sets of fault striations indicating normal dip slip resulting from NW-SE extension, then left-lateral slip with a normal component resulting from NE-SW extension, and right-lateral slip with a minor normal component, respectively. The kinematic history of faults and their chronological evolution indicate changes in continental dynamics acting in North China over Cenozoic time.

Petrogenesis of the Early Cretaceous Laojunshan monzogranite at the southern margin of the North China Craton: Constrains on the transition of the tectonic regime

1 Introduction Voluminous Mesozoic magmatic rocks containing abundant Au-Mo polymetallic mineralization resources are developed in the Xiaoqinling-Xiong’ershan district of the southern margin of the North China Craton(NCC).

Evolution of the Late Cenozoic Tectonic Stress Regime in the Tianshui Basin,Northeast Tibetan Plateau

The Tianshui Basin,located inside the western Qinling orogenic belt and northeastern margin of the Tibetan Plateau （Fig.1）,is a NE-trending Late Cenozoic basin,which documents the neotectonic response of the northeastward growth of Tibetan Plateau.

Molecular Biomarker Characteristics of the Linxi Formation Source Rocks in the Middle-Western Region of Inner Mongolia:New evidence for late-stage tectonic evolution of the Paleo-Asian Ocean

Objective Two important geological issues have long been controversial in the Xing-Meng area of North China. The first concerns the final closure of Paleo-Asian Ocean in Xing-Meng area, and the other concerns the folding and lifting of the Xing-Meng Trough. The focus of thses issues is the Late Permian sedimentary environment, which is generally considered to be either an exclusively continental environment or from the closed inland sea environment in the Early to Middle stage to continental lacustrine environment in the late stage. In recent years, we have successively discovered abundant typical marine fossils （e.g., bryozoans and calcareous algae） in the Upper Permian thick limestone layer from Linxi County and Ar Horqin Banner in eastern region of Inner Mongolia and Jiutain County in Jilin Province. These significant findings have attracted the attention from fellow academics.

Petrology, Geochronology and Geochemistry of the Xar Moron River Ophiolite: Implications for the Tectonic Evolution of the Paleo-Asian Ocean

As the largest accretionary orogen, the crustal tectonic framework and evolution of the Central Asian Orogenic Belt(CAOB) have always been one of the hot topics among geologists(Seng?r et al., 1993, 1996;Jahn et al., 2000 a;Badarch et al., 2002;Windley et al., 2007;Li et al., 2009). The formation of the main part of the crust in the CAOB involved continuous lateral accretion of island arcs and accretionary complexes along the margins of the Siberian, Sino-Korean and Tarim paleocontinents and the final collision between these continental margins because of the subduction of the Paleo-Asian Ocean plate since Mesoproterozoic. The ophiolites, which represent the fragments of ancient oceanic lithosphere, are the direct evidence for the study of the evolution of orogenic belts. Based on field geological survey, the mantle peridotite(serpentinite), gabbro, basalt and radiolarian bedded chert, which were deemed as the "ophiolite trinity", were identified as isolated blocks in the matrix of pelitic siltstone and silty mudstone in the Kedanshan, Xingshuwa and Jiujingzi areas along the Xar Monron River in southeast Inner Mongolia of China. Besides, there were plenty of other exotic blocks, such as limestone and sandstone, in the matrix. Both of the matrix and blocks underwent strong foliated deformation. All of these rocks above constitute a tectonic mélange. Zircon U-Pb dating for the gabbro blocks in the Xingshuwa and Jiujingzi ophiolites reveals that they were formed in early Permian(275-280 Ma). The ages of the gabbros, together with the middle Permian radiolaria fossils in the chert reported by Wang and Fan(1997), indicate that the oceanic basin was not closed in early-middle Permian. The geochemical compositions of the basaltic blocks distributed in different locations in the Xingshuwa tectonic mélange display different genetic types of normal mid-ocean ridge basalt(N-MORB), enriched mid-ocean ridge basalts(E-MORB), oceanic island basalt(OIB), island arc basalt and continental marginal arc basalt, which indicates what they represented is a complex oceanic basin. Combining with the studies on regional magmatism, strata and structure data, it is suggested that the Xar Moron River Ophiolite belt represented the final suture zone of the Paleo-Asian Ocean in the southeast Inner Mongolia, and the ocean did not close before late Permian.

End Late Paleozoic tectonic stress field in the southern edge of Junggar Basin

This paper presents the end Late Paleozoic tectonic stress field in the southern edge of Junggar Basin by interpreting stress-response structures （dykes, folds, faults with slickenside and conjugate joints）. The direction of the maximum principal stress axes is interpreted to be NW-SE （about 325°）, and the accommodated motion among plates is assigned as the driving force of this tectonic stress field. The average value of the stress index Rt is about 2.09, which indicates a variation from strike-slip to compressive tectonic stress regime in the study area during the end Late Paleozoic period. The reconstruction of the tectonic field in the southern edge of Junggar Basin provides insights into the tectonic deformation processes around the southern Junggar Basin and contributes to the further understanding of basin evolution and tectonic settings during the culmination of the Paleo- zoic.

Zircon U-Pb-Hf isotopic and geochemical characteristics of the Xierzi biotite monzogranite pluton,Linxi,Inner Mongolia and its tectonic implications

The opening, subduction and final closure of the Paleo-Asian Ocean led to the formation of the Central Asian Orogenic Belt. Controversy has long surrounded the timing of final closure of the Paleo-Asian Ocean. Here we present zircon U-Pb ages and petrological, geochemical and in situ Hf isotope data for the Xierzi biotite monzogranite pluton, Linxi, SE Inner Mongolia. U-Pb dating of zircon by LA-ICP-MS yields a middle Permian emplacement age(268.7 ± 2.3 Ma) for the Xierzi pluton that is dominated by biotite monzogranites with high SiO_2(71.2-72.8 wt.%),alkali(Na_2 O + K_2 O =8.05-8.44 wt.%), Al_2 O_3(14.4-15.2 wt.%) and Fe_2 O_3~T relative to low MgO contents, yielding Fe_2 O_3~T/MgO ratios of 2.87-3.44, and plotting within the high-K calc-alkaline field on a SiO_2 vs. K_2 O diagram. The aluminum saturation indexes(A/CNK) of the biotite monzogranites range from 1.06 to 1.19, corresponding to weakly to strongly peraluminous. They are enriched in rare earth elements(REE), high field strength elements(HFSEs; Zr,Hf). and large ion lithophile elements(LILEs; Rb, U, Th). The LREEs are enriched relative to the HREEs,with a distinct negative Eu anomaly in a chondrite-normalized REE diagram. Geochemically, the Xierzi biotite monzogranite is classified as an aluminous A-type granite, with all samples plotting within the A2-type granite field on a Y/Nb vs. Rb/Nb diagram. Zircon ε_(Hf)(t) values and two-stage modal ages of the zircons within the pluton range from +4.80 to +13.65 and from 983 to 418 Ma, respectively, indicating that the primary magma was generated through partial melting of felsic rocks from juvenile crust.Consequently, these results demonstrate that the Xierzi pluton formed under the post-orogenic extensional setting after arc-continent collision in the middle Permian.

Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt in NE China

The eastern Central Asian Orogenic Belt(CAOB)in NE China is a key area for investigating continental growth.However,the complexity of its Paleozoic geological history has meant that the tectonic development of this belt is not fully understood.NE China is composed of the Erguna and Jiamusi blocks in the northern and eastern parts and the Xing’an and Songliao-Xilinhot accretionary terranes in the central and southern parts.The Erguna and Jiamusi blocks have Precambrian basements with Siberia and Gondwana affinities,respectively.In contrast,the Xing’an and Songliao-Xilinhot accretionary terranes were formed via subduction and collision processes.These blocks and terranes were separated by the Xinlin-Xiguitu,Heilongjiang,Nenjiang,and Solonker oceans from north to south,and these oceans closed during the Cambrian(ca.500 Ma),Late Silurian(ca.420 Ma),early Late Carboniferous(ca.320 Ma),and Late Permian to Middle Triassic(260-240 Ma),respectively,forming the Xinlin-Xiguitu,Mudanjiang-Yilan,Hegenshan-Heihe,Solonker-Linxi,and Changchun-Yanji suture zones.Two oceanic tectonic cycles took place in the eastern Paleo-Asian Ocean(PAO),namely,the Early Paleozoic cycle involving the Xinlin-Xiguitu and Heilongjiang oceans and the late Paleozoic cycle involving the Nenjiang-Solonker oceans.The Paleozoic tectonic pattern of the eastern CAOB generally shows structural features that trend east-west.The timing of accretion and collision events of the eastern CAOB during the Paleozoic youngs progressively from north to south.The branch ocean basins of the eastern PAO closed from west to east in a scissor-like manner.A bi-directional subduction regime dominated during the narrowing and closure process of the eastern PAO,which led to“soft collision”of tectonic units on each side,forming huge accretionary orogenic belts in central Asia.

Geochemistry and Tectonic History of Seamount Remnants in the Xingshuwa Subduction Accretionary Complex of the Xar Moron Area,Eastern Margin of the Central Asian Orogenic Belt

This study focuses on the geology,geochemistry,zircon U-Pb geochronology and tectonic settings of the three types of seamount basalts from the Xingshuwa subduction accretionary complex in the Xar Moron area,eastern margin of the Central Asian Orogenic Belt(CAOB).The seamount remnants are composed of carbonate’cap’sediments,large volumes of pillow and massive basalts,carbonate breccia slope facies and radiolarian cherts.Group 1 basalts are characterized by high contents of P2 O5 and TiO2 with alkaline affinity and LREE enrichment,indicating that they are derived from intraplate magma.Group 2 basalts display N-MORB LREE depletion patterns,indicating that they were formed at a mid-ocean ridge.Group 3 basalts have shown distinct Nb depletion and high Th/Yb ratios,indicating that they were generated in an island arc tectonic setting.The zircon U-Pb age of Group 1 basalt sample XWT18-131 is 576.4±9.4 Ma,suggesting that the oceanic island seamount was the product of intraplate magmatism related to a mantle plume or’hot spot’in the late Neoproterozoic.The zircon U-Pb age of Group 2 basalt sample XWT18-132 is 483±22 Ma,indicating that the Paleo-Asian Ocean(PAO)was continuously expanding in the Early Ordovician.The zircon U-Pb age of Group 3 basalt sample XWT18-101 is 240.5±8.2 Ma,suggesting that this area underwent the evolutionary path of ocean-continent transition,developing towards continentalization during the Middle Triassic.Thus,we believe that there was both mantle plume-related intraplate magmatism and intraoceanic subduction during the evolution of the PAO,the CAOB possibly being an evolutionary model of an intraoceanic subduction and mantle plume magmatism complex.

Geotectonic significance of the Neoproterozoic ophiolitic metagabbros of Muiswirab area,South Eastern Desert,Egypt:constraints from their mineralogical and geochemical characteristics

Petrological and geochemical studies of Neo-proterozoic metagabbros were carried out in the Muiswirab area,South Eastern Desert of Egypt.The Muiswirab area comprises of ophiolitic metagabbroic rocks(MOM),which are tectonically thrusted over a thick pile of metavolcanic rocks and intruded by syn-and post-tectonic granitoid rocks.The whole-rock geochemical variations coupled with chemical compositions of mineral constituents are used to attain the genesis and tectonic evolution of the studied metagabbros.The geothermobarometric investiga-tion of the analyzed amphiboles from(MOM)revealed that these metagabbros underwent regional metamorphism under lower to upper greenschist facies(biotite zone)conditions(at a temperature of 450 to 500℃and pressure of 1-3 kbar).Geochemically,the metagabbros(MOM)show tholeiitic affinity and cxhibiting both arc-and MORB-like characters as evidenced by their clinopyrox-ene compositions and the Ti/V ratios(11.84-31.65),which considered as prominent features of forearc tectonic regime.The geochemical features suggest a probable fractionation of olivine±clinopyroxene±plagioclase as well as insignificant crustal contamination.The parental magma of the investigated MOM rocks seems to be developed in a sub-arc mantle wedge setting due to the enrichments of LILE(e.g..Rb,Ba,Sr,Pb)over HFSE(e.g,Ti,Nb,Y,Zr,Hf,Ta).The studied MOM rocks have lower values of Nb/U relative to MORB and OIB indicating that their geochemical variation produced due to the enrichment of a lithosphere mantle by OIB like components.The ratios of Zn/Fet,La/Sm,Sm/Yb,Th/Yb and Nb/Yb indicate that the MOM rocks represent a fragment of oceanic crust originated at a supra-subduction zone environment and their parental magma developed by 5-30%partial melting of a spinel lherzolite mantle rather than pyroxenite in an island arc setting and conformable with most of the Egyptian ophiolitic metagabbros.

华北克拉通新太古代晚期岩浆作用:对构造体制和克拉通化的启示

在对华北克拉通太古宙基底作简要介绍的基础上,本文总结了华北克拉通新太古代晚期(主要为2.55~2.5 Ga期间)岩浆岩的锆石年龄分布模式、地球化学和Nd-Hf-O同位素组成特征。华北克拉通新太古代晚期变质基底的主要特征如下:(1)新太古代晚期岩石在华北克拉通广泛分布,但许多地区都存在中太古代晚期—新太古代早期地质记录。(2)岩浆锆石年龄主要变化于2.55~2.5 Ga,年龄峰值约为2.52 Ga。(3)与新太古代早期以前(>2.6 Ga)的英云闪长岩-奥长花岗岩-花岗闪长岩(TTG)相比,新太古代晚期英云闪长岩和花岗闪长岩比例明显增大。富钾花岗岩、闪长-辉长岩(包括闪长岩、辉长岩及两者之间过渡岩石)、赞岐岩(Sanukite,主体为富镁闪长岩)分布范围和规模也明显增大。大规模富钾花岗岩主要分布于华北克拉通东部,构成新太古代晚期双岩浆岩带的富钾花岗岩带。(4)在新太古代晚期变质基底分布区,几乎都有变质表壳岩存在。它们以较小规模存在于TTG和富钾花岗岩中。岩石类型包括变玄武质岩石、变质安山质-英安质火山岩/火山碎屑岩和变质碎屑沉积岩。一些地区存在变质超基性岩。(5)总体上,新太古代晚期地质事件存在如下演化规律:首先是表壳岩形成,然后是TTG侵入,最后是变质变形和富钾花岗岩形成。2.6~2.55 Ga为华北克拉通岩浆构造的“寂静期”。(6)新太古代晚期TTG岩石的Sr/Y和La/Yb比值存在很大变化,中-高压TTG岩石大量形成表明新太古代晚期陆壳规模、厚度发生了明显增大。至少部分富钾花岗岩在形成过程中有沉积物参与。(7)不同类型TTG岩石具有类似的全岩Nd同位素和岩浆锆石Hf同位素组成,Nd-Hf同位素亏损地幔模式年龄主要分布在3.0~2.5 Ga,与中太古代晚期—新太古代早期岩石的模式年龄相近或稍偏年轻。富钾花岗岩Nd-Hf同位素组成特征受物源区早期形成演化历史制约。岩浆锆石O同位素组成与全球太古宙岩浆锆石类似,但显示更大的变化范围。结合其他研究,上述证据表明:(1)中太古代晚期—新太古代早期是华北克拉通陆壳增生最重要时期,这与全球其他许多克拉通类似,不同之处在于华北克拉通遭受了新太古代晚期构造岩浆热事件强烈改造;(2)在华北克拉通,类似于现代板块运动的构造体制在新太古代晚期开始启动;(3)规模最大的BIF(条带状铁建造)沿华北克拉通东部古老陆块西缘的双岩浆岩带分布,鞍本和冀东之间、冀东和鲁西之间是BIF找矿重要靶区;(4)华北克拉通在新太古代晚期完成初始克拉通化。

南北朝·大地变迁的动力--漫步地震五千年(8)

地下应力场是地壳形变、岩体破坏、地震发生的直接力源,岩体的稳定性问题十分重要。通过国际合作已经完成了全球GPS速度场和地应力图的更新,有了动力源的板块就不再是一堆僵化构造体,它们运动的活力和内部的应力状况被第一次揭示出来。本文介绍了地应力原地测量的方法,应力张量结构的划分原则,全球和中国应力图的基本特征。对于地震的震源机制解,作者绘制了多种三维透视图,便于读者以看图识字的方式掌握震源机制解的地质含义。最后,以两个实例对比了实测应力图和震源机制解的异同,分析了当地动力学的背景。

Permian tectonic evolution and continental accretion in the eastern Central Asian Orogenic Belt:A perspective from the intrusive rocks

The tectonic evolution and history of continental accretion of the eastern Central Asian Orogenic Belt(CAOB)are not yet fully understood.In this study,we investigate Permian intrusive rocks from the Jiamusi Block of the eastern CAOB to constrain the tectonic evolution and continental accretion of this region during the late-stage evolution of the Paleo-Asian Ocean.Our new data show that Early Permian gabbro-diorites were derived from the partial melting of depleted mantle metasomatized by oceanic-slab-released fluids.Middle Permian adakitic granites have low Na2O and MgO and high K2O contents,indicating a thickened-lower-crust source.Late Permian S-type granites were derived from the partial melting of continental crust.A compilation of the available geochronological data for Permian intrusive rocks(including adakitic and A-,S-,and I-type granites and mafic rocks)from the eastern CAOB reveals that the A-type granites formed mainly during the Early–Middle Permian,S-type and adakitic granites mostly during the Middle–Late Permian,and I-type granites and mantle-derived mafic rocks throughout the Permian.The A-type granites,which are proposed to have been sourced from thinned continental crust,indicate an extensional setting in the eastern CAOB during the Early Permian.The Middle–Late Permian adakitic granites imply a thickened continental crust,which indicates a compressional setting.Therefore,the eastern CAOB underwent a transition from extension to compression during the Middle Permian,which was probably triggered by the late-stage subduction of Paleo-Asian oceanic crust.Considering the petrogenesis of the intrusive rocks and inferred regional tectonic evolution of the eastern CAOB,we propose that vertical underplating of mantle-and oceanic-slabderived magmas contributed the materials for continental crust accretion.

Time range of Mesozoic tectonic regime inversion in eastern North China Block

An important tectonic inversion took place in eastern North China Block(NCB) during Mesozoic, which caused a great lithosphere thinning, reconstruction of basin-range series, powerful interaction between mantle and crust, a vast granitic intrusion and volcanism, and large-scale metallogenic explosion. The time range of the Mesozoic tectonic regime inversion in the eastern North China Block is one of the key issues to understand mechanism of tectonic regime inversion. Our updated results for recognizing the time range are mainly obtained from the following aspects: structural analyses along northern and southern margins of the NCB and within the NCB for revealing tectonic inversion from compression to extension and structural striking from ~EW to NNE; geothermic analyses of the eastern sedimental basins for a great change of thermal history and regime; basin analysis for basin inversion from compression to extension and basin migration from ~EW to NNE; petrological and geochemical studies of volcanic rocks and lowermost crust xenoliths for recognizing peak period of mantle upwelling and intense interaction between mantle and crust, and main metallogenic epoch. All the studies of the above give the same time range from~150-140 Ma to ~110-100Ma, peaking at ~120 Ma.

Emplacement and deformation of Shigujian syntectonic granite in central part of the Dabie orogen:Implications for tectonic regime transformation

The Shigujian pluton is a gneissic quartz monzonite located in Tiantangzhai area in central part of the Dabie orogen.Anisotropy of magnetic susceptibility(AMS) data show that most magnetic foliations dip steeply to southeast.About 85% of sampling points dip from 40° to 90°.Magnetic foliations are generally parallel to the foliations measured in the field.The pluton has NWW-SEE trending lineations in the southeast and NE-SW trending lineations in central part and north,but the lineations plunge to SW in central part and to NE in the north.All plunges are moderate.The anisotropy degree(P) is between 1.065 and 1.532 and the shape parameter(T) is between 0.005 and 0.694.A Flinn diagram of the magnetic fabrics shows that the value of K is less than 1.The analysis of AMS suggests that the pluton was emplaced and deformed under a SE-NW compressional stress regime.The analysis of quartz C-axis fabrics indicates that the pluton was deformed under compressional stress and deformation temperatures range from 400 to 500℃.Microstructures indicate that the pluton is deformed in near solidus conditions and the pluton is a synkinematic intrusion.The emplacement of the Shigujian granite is inferred to have taken place syntectonically.The zircon U-Pb dating of the granite suggests that the pluton was intruded at 141±2.3 Ma.By synthesizing all data,it seems that the Shigujian pluton was emplaced in a compressional environment and the transformation time of the Dabie orogen from compression to extension took place after 141 Ma.The structural evolution of the Dabie orogen was controlled by the Pacific tectonic domain when the Shigujian pluton was emplaced,whereas the adjacent Tiantangzhai complex massif is the result of an extensional environment.

Geochronology, Geochemistry and Sr-Nd-Hf Isotopes of Early–Middle Triassic Adakitic Plutons in Central-eastern Jilin Province, NE China: Constraints on the Non-synchronous Closure of Paleo-Asian Ocean

The Changchun–Yanji suture zone(CYSZ) in NE China is considered as the suture between the North China Craton(NCC) and Central Asian Orogenic Belt(CAOB). The geochronology, geochemistry and Sr-NdHf isotopes of Early–Middle Triassic adakitic plutions from the CYSZ, are presented in this paper to discuss their petrogenesis and tectonic setting, as well as to constrain the timing and style of the Paleo-Asian Ocean's final closure. In Early Triassic, the Dayushan pluton(ca. 250 Ma) from western CYSZ has negative ε_(Nd)(t) values, bidirectional provenances(NCC and CAOB) of ε_(Hf)(t), which are formed in a collision tectonic setting. In contrast, in eastern CYSZ, the early Triassic samples in Liangshan(ca. 242 Ma) were high Mg~# values, positive ε_(Nd)(t), single provenances(CAOB) of ε_(Hf)(t) resulting from a subduction setting. In the Middle Triassic, the Atype granites in western CYSZ are found in previous studies representing a post-collisional extensional environment, whereas syn-collisional Lianyanfeng granites(ca. 237 Ma) in eastern CYSZ with low ISr and large scale ε_(Nd)(t) and ε_(Hf)(t) values from bidirectional provenances(NCC and CAOB), represent a collisional setting. The Paleo-Asian Ocean's occurred in a scissor-like fashion along the CYSZ during the Triassic period.

Radical change in tectonic regime and paleo-environment at the end of Neoarchean evidenced by a unique metal co-existing deposit

Banded iron formation and Cu-Zn sulfide deposits within volcanic-argillaceous sequences(as volcanogenic massive sulphide deposits(VMS)-like type) occur together in the Qingyuan greenstone belt of the North China Craton,recording the first appearance of oxidized ores and sulfide ores co-existing in the early Earth.The unique metal co-existing deposits should meet two requirements:tectonic setting and sedimentary environment.As regards to tectonic setting,plate-like tectonics might have started since the end of the Neoarchean because continents had grown large enough and there occurred volcanic arcs and backarc basins similar to modern ones in a way.Partial melting of subducted continental crust is conductive to providing ore-forming elements.As for sedimentary environment,late Neoarchean seawater was rich in Fe^(2+) and anoxic.Instantaneous oxidation of the seawater resulted possibly from frequent submarine volcanic eruptions and facilitated precipitation of the banded iron formation.At this point,it is also favorable for the enrichment of Cu and Zn ions in seawater.The VMS-like deposits tended to form when the seawater was reduced again.Studies of isotopic elements like sulfur,oxygen,iron and silicon support the above geological processes.It is shown that the geologic conditions only existed in the late Neoarchean and Paleoproterozoic for a short period of time.The banded iron formations disappeared around 1.85 Ga,and the associated sulfide metal deposits also became dominant sedimentary exhalative deposits in the meso-Neoproterozoic Boring Billion,as a result of increasing oxidation of the oceans and the increasing maturity of the continental crust.This study is significant not only for decoding metallogenic genesis but also helping understand rapid change in Precambrian tectonic regimes and Earth’s environments.

Middle-Miocene transformation of tectonic regime in the Himalayan orogen

Understanding the multiple tectonic transformations during the Himalayan orogeny is significant in evaluating the evolution of Himalayan orogen.In the Gyirong area in south Tibet,deformed leucogranitic veins in the biotite-plagioclase gneisses of Greater Himalayan crystalline complex(GHC) constitute south-vergent asymmetric folds.The reconstruction of the veins shows that they experienced two generations of deformation under different tectonic regimes:an earlier top-to-north extension and a later top-to-south thrusting,implying a tectonic transformation from N-S extension to N-S shortening.Zircons LA-ICP-MS U-Pb dating of the leucogranite shows that it was emplaced during 21.03-18.7 Ma.The data suggest that the tectonic transformation occurred after 18.7 Ma.The chronological data of South Tibet detachment system(STDS) and North-South trending rift(NSTR) from Gyirong and other areas indicate that the Himalayan orogeny was in a period of tectonic transformation from N-S extension to N-S shortening during 19-13 Ma.The transformation of tectonic regime was probably controlled by the India-Asia convergence rate.An increase in the convergence rate resulted in N-S shortening of the orogen,thrusting and folding,with coeval formation of the NSTR in Tibet.A decrease in the convergence rate led to N-S extension and reactivation of the STDS.

喜马拉雅造山带始新世和中新世的地壳深熔条件:来自花岗岩的地球化学证据

与花岗岩岩浆作用相关的地壳深熔条件与汇聚板块边缘的构造体制和热结构密切相关。对喜马拉雅新生代花岗岩来说,不同时代的地壳源区深熔条件被两组花岗岩记录下来。第一组形成于43~44 Ma的始新世;第二组形成于17~18 Ma的中新世,具有典型高喜马拉雅淡色花岗岩的地球化学组成。始新世花岗岩经历了分离结晶作用,形成了两个亚组,一组具有高镁铁度、高CaO含量和高Na2O/K2O和Sr/Y值,另一组则恰恰相反。Sr-Nd同位素组成表明,始新世花岗岩的源区物质由角闪岩和变泥质岩组成。相平衡模拟结果显示,始新世花岗岩的原始熔体由角闪岩和变泥质岩构成的混合源区在(850±50)℃和(0.85±0.05)GPa条件下部分熔融产生。而中新世花岗岩的源区熔融条件与典型高喜马拉雅淡色花岗岩的一致,为750~770℃和0.6~0.8 GPa,源岩为变泥质岩。综合包括变质岩石学在内的多学科研究结果,碰撞造山带从同碰撞阶段向碰撞晚期和碰撞后阶段的演化过程中发生了热结构的变化。同碰撞阶段的超高压变质岩记录了低的地温梯度(<10℃/km),碰撞晚期的始新世花岗岩形成于高的地温梯度(约30℃/km),碰撞后阶段的中新世花岗岩也形成于高的地温梯度(约25~35℃/km)。地温梯度的改变可以通过碰撞造山带构造体制的变化来解释,即由同碰撞阶段的挤压体制转变为始新世碰撞晚期和中新世碰撞后的伸展体制。