Intra-continental Orogeny:Insights from Magnetotelluric Data into the Mesozoic Uplift History of the Eastern Jiangnan Orogen in South China

Despite extensive efforts to understand the tectonic evolution of the Jiangnan Orogen in South China,the orogenic process and its mechanism remain a matter of dispute.Previous geodynamic studies have mostly focused on collisional orogeny,which is commonly invoked to explain the Jiangnan Orogen.However,it is difficult for such hypotheses to reconcile all the geological and geophysical data,especially the absence of ultrahigh-pressure metamorphic rocks.Based on the magnetotelluric data,we present a group of resistivity models produced through the combination of two-dimensional and three-dimensional inversions,revealing the geo-electrical structures of Jiangnan and a typical collisional orogen.In our models,the resistive crust is separated into three parts by a prominent conductive layer with opposite dipping directions on both sides.A special thrust-nappe system,which is different from that developed in a typical collisional process,is revealed in the Jiangnan Orogen.This structure suggests a process different from the simple collisional orogeny.To interpret our observations,an'intra-continental orogeny'is proposed to address the development of the Jiangnan Orogen in the Mesozoic.Furthermore,this'reworked'process may contain at least two stages caused by the decoupling of the lithosphere,which is revealed by an extra conductive layer beneath Jiangnan.

The late Early-Paleozoic granitic magmatism in Northwestern Fujian, China: constraints on intraplate orogeny in the South China block

The Early Paleozoic tectono-thermal event was a significant orogenic activity during the Phanerozoic era,which had a profound impact on the early crust of the South China Block(SCB) and established the foundation for later tectonic activity.The Wuyi-Yunkai orogenic belt in Southeastern China was extensively exposed to Early Paleozoic magmatism,the genetic mechanism of which remains controversial.To shed light on this issue,detailed petrological,geochemical,and zircon U-Pb-Hf isotopic studies were carried out on two granitoids,namely the Yuntongshan pluton and the Gaoqiao pluton,identified in the central Wuyishan.Zircon U-Pb chronology of the Yuntongshan and Gaoqiao bodies yielded ages of437±4 Ma(MSWD=2.2) and 404±2 Ma(MSWD=12),respectively,indicating that they were emplaced during the Early Silurian and Early Devonian periods.These granitoids are primarily composed of biotite-granite and biotite-monzonitic-granites,with high concentrations of S_(i)O_(2)(73.59-75.91 wt%),K_(2)O+Na_(2)O(8.31-8.73wt%),and low contents of MgO,CaO,Cr,Ni.They are classified as high-K calc-alkaline and weakly metaluminous-strongly peraluminous S-type granites.These granitoids are enriched in light rare earth elements(LREEs) and large ion lithophile elements(LILEs) and depleted in heavy rare earth elements(HREEs) and high field strength elements(HFSEs) with arc affinity.The εHf(t) values of-3.3 to-15.4 with two-stage Hf model ages ranging from 2829 to 1644 Ma,combined with the presence of Neoproterozoic inherited zircons,suggest that the primary magma of these granitoids was derived from the partial melting of Neoproterozoic crust with a Paleoproterozoic crustal model age.These findings,combined with the spatio-temporal distribution of regional magmatism,reveal that the late Early-Paleozoic granitoids formed in the intraplate orogenic background originating from the subduction of the proto-Tethys Ocean and proto-Pacific Ocean around the margin of the east Gondwana supercontinent.

Zircon U-Pb age of Fengxian acid pyroclastic rocks and its enlightenment to the existence of Pan-African orogeny in the West Qinling Orogenic Belt, China

1.Objective The formation of East Gondwana and its combination with West Gondwana is a result of a series of complex orogenic events,known as Pan-African orogeny.The Northern Qinling Mountain is a microcontinent that formed on the ancient oceanic crust in the northern of Yangtze craton before Neoproterozoic.It is likely that the North Qinling,even the Yangtze craton and its surrounding oceanic crust belong to the ancient Gondwana tectonic domain and be a part of East Gondwana supercontinent(Chen Q,2007).

青藏高原陆陆碰撞-挤出活动构造体系控震作用:以1990年以来强震活动为例

青藏高原是地中海-喜马拉雅地震带上强震活动最频繁的区域之一,深入认识该区的活动构造体系控震效应对于区域强震危险性分析具有重要科学意义。从陆陆碰撞-挤出活动构造体系角度,对青藏高原自1990年以来的MW≥6.0强震活动及控震构造机制进行分析发现,青藏高原陆陆碰撞-挤出构造体系对区域强震活动起到显著控制作用,区域强震事件尤其是MW≥6.5地震主要出现在构造体系的主要边界断裂带上,并显示出相对有规律的时空迁移过程,而且青藏高原东部的多层次挤出-旋转活动构造体系构成了1990年以来强震过程的主要控震构造,其次是喜马拉雅主前缘逆冲断裂带。因此,青藏高原挤出构造体系应是未来强震活动趋势分析最值得关注的区域,尤其是当前最为活跃的巴颜喀拉次级挤出构造单元。对比分析土耳其安纳托利亚板块及周边的强震活动发现,该区具有类似的陆陆碰撞-挤出构造体系及控震效应,表明该构造体系是陆内造山中的一种典型的控震构造。进一步综合分析认为,活动构造体系控震效应的主要表现:一是构造体系中主要断块的边界断裂带通常是强震活动的主要场所;二是构造体系中不同构造带的强震活动常具有联动效应或相互触发关系,其中的复杂或特殊构造部位则是易出现双震或震群活动的场所;三是当构造体系中某个构造单元或构造带处于活跃阶段时,便会出现强震丛集现象。另外,充分认识构造体系中主要活动断层间的协调变形关系,活动断层带上的强震活动的分段破裂行为,以及活动断层上强震原地复发通常存在“周期长、准周期性和丛集性”的特点等,有助于在根据活动构造体系分析区域未来强震活动趋势时更为准确地判定活动断层带的未来强震危险性。

基墨里造山与松潘-甘孜锂矿链成因的探讨

晚三叠世—早侏罗世,基墨里大陆和泛华夏陆块与欧亚大陆的碰撞,形成了4000 km长的基墨里造山带。位于三大陆之间、现今青藏高原北部的松潘-甘孜地体的造山属性的确定,对于探索松潘-甘孜锂矿链的成因有重要意义。松潘-甘孜地体西部和北部的早侏罗世陆相火山岩、底砾岩和煤系地层(~201 Ma)不整合在晚三叠世褶皱地层及花岗岩体之上,为晚三叠世—早侏罗世的基墨里造山时限提供了直接证据。松潘-甘孜地体中的甲基卡、可尔因、扎乌龙和白龙山-大红柳滩等稀有金属矿集区具有共同特征:赋存在由核部高分异S型花岗岩和幔部中晚三叠世浊积岩组成的片麻岩穹隆构造中、经历了巴罗-巴肯变质作用、含锂伟晶岩脉侵位在花岗岩体上部的伸展空间。通过对松潘-甘孜地体区域地质调查和对矿集区的变质、变形、岩浆和成矿作用及同位素年代学研究,提出该地体经历晚三叠世—早中侏罗世的基墨里造山过程:(1) 230~220 Ma:地壳缩短和加厚阶段,以盖层大规模强烈褶皱、逆冲带,盖层与基底之间向南的滑脱变形为特征,伴随深熔和巴罗式变质作用;(2) 220~190 Ma:地壳减压折返阶段,大量花岗岩侵位在中晚三叠世浊积岩中、形成片麻岩穹隆并伴随巴肯式变质作用。由于花岗质岩浆的高度分异及岩浆不混溶作用,导致侵位在片麻岩穹隆顶部伸展空间的伟晶岩稀有金属富集成矿。此外,伟晶岩型锂矿科学钻探(JSD)揭示了甲基卡矿集区多层次穹状花岗岩席控制含锂伟晶岩脉的成矿机制,推测大规模中下地壳基底深熔驱动岩浆上升,岩浆体沿上地壳中的构造面推叠形成岩席。

大陆下地壳

大陆下地壳是连接岩石圈地幔和上地壳的“纽带”,是地壳与地幔交换最活跃的部位。上地幔与下地壳的部分熔融及下地壳一些岩石的拆沉还可直接导致壳-幔物质的交换、循环与重组。换言之,下地壳是壳-幔作用的一个极其重要的场所,底垫、拆沉、深熔、高级变质和其他作用都在下地壳中发生和实现。然而,下地壳是以往研究地球深部和浅部关系时被“跳”过去的部位,没有得到足够的重视。克拉通化定义为大陆原来混沌的原地壳分异并形成稳定的上地壳和下地壳,并由此构建了稳定的壳-幔结构,这种空前的稳定关系从形成起一直维持到现在,是大陆演化、洋-陆与壳-幔相互作用的基础。在板块边界的造山过程中,如洋-陆的俯冲碰撞特别是陆-陆碰撞,可以形成不同大陆地块的陆壳叠置、加厚、垮塌、拆沉、底垫和重新稳定,在造山带根部形成新的下地壳,即造山带型下地壳。本文重点讨论了克拉通型下地壳演化过程,强调了其动力学意义及其在大陆动力学研究中的重要地位,建议在深地研究和学科布局中给与充分重视。

锂的圈层循环与资源富集过程:从高原盐湖到造山带伟晶岩

盐湖卤水型锂矿与富锂伟晶岩是目前最为主要的可利用锂资源,前者是经地表风化/高温水岩淋滤形成的水体通过蒸发作用浓缩而成的卤水,而后者被认为是陆表风化沉积物熔融产物经高演化形成的富锂硬岩。大陆地壳是此类表生物质循环最根本的物源,因而探究富锂地壳的形成及其表生风化剥蚀对于理解盐湖与伟晶岩锂成矿至关重要。本文将从元素循环视角切入,结合内生高温-外生低温过程中锂的元素地球化学行为,梳理锂在俯冲带各圈层的循环过程,探讨富锂地壳的形成机制及其对高原盐湖与造山带伟晶岩锂资源发育的影响。研究发现:(1)俯冲板片对弧岩浆锂的物质贡献有限;(2)厚地壳环境下的弧岩浆具有更高的锂含量;(3)造山带富锂弧岩石的循环应是盐湖与伟晶岩锂成矿的重要控制因素。

内蒙古达茂旗北部包尔汉图群时代及中早-中三叠世变质锆石年龄的启示

【研究目的】包尔汉图群主要分布于达茂旗北部地区,是研究古亚洲洋早古生代构造演化的关键部位。【研究方法】本文对达茂旗一带包尔汉图中基性火山岩进行岩石学、地球化学、同位素地球化学和年代学研究,以探讨其形成时代和成岩。【研究结果】本次工作在包尔汉图群片麻状斜长角闪岩LA-ICP-MS法锆石U-Pb成岩年龄为495±2.0 Ma,同时在角闪绿帘岩和斜长角闪岩中获得了变质锆石年龄分别为242.3±1.1 Ma和249±3 Ma。【结论】片麻状斜长角闪岩地球化学特征表现为富集Rb、Th等大离子亲石元素,亏损Nb、Ta、P、Ti等高场强元素,表明包尔汉图群在寒武纪(Є3)已有火山喷发,其形成与古亚洲洋由北向南的俯冲作用有关。角闪绿帘岩和斜长角闪岩具MORB型地球化学特征,其中的捕获锆石既有华北板块古老的再循环陆壳物质,又有古生代的新生基性下地壳物质,可以限制古亚洲洋闭合时代应早于249 Ma。

阿尔金北缘塔东南坳陷一带深部卤水钾盐矿地质特征与成矿规律浅析

阿尔金北缘分布一系列钾盐矿床,是我国重要的钾盐矿生产基地。以阿尔金北缘、塔里木东南坳陷区南部边缘钾盐矿床为研究对象,开展地质、物探及钻探等方面研究。研究表明,该钾盐矿床成因类型为陆成盐湖矿床,以钾为主伴生有钠、镁盐、溴,形成液体矿和固体矿,液体矿由潜卤水和承压卤水矿体组成,固体矿由盐壳石盐矿组成。深部卤水钾盐矿床物质主要来自南部阿尔金山区和塔里木盆地古盐再溶解补给。阿尔金山前若羌坳陷中丰富的盐类组份来源、半封闭构造环境和多风干旱气候条件是形成卤水钾盐矿床的主要条件。阿尔金北缘一带盆地沉降中心可能存在深藏含钾卤水矿床,该区域具良好的找矿前景。

华夏块体及邻区新元古代-早古生代沉积记录与华南洋演化:问题、讨论与建议

华南作为罗迪尼亚超大陆和冈瓦纳大陆的组成部分,在新元古代至早古生代经历了大陆汇聚和裂解、雪球地球事件、早期生命演化等一系列重大地质事件,但组成华南大陆的两大块体之一的华夏块体构造属性及其与扬子陆块拼贴过程等最基本的认识仍然存在争议,其核心是华南洋演化问题。本文在扬子东南缘至华夏地区新元古代-早古生代火山-沉积序列对比基础上,总结了扬子型和华夏型沉积建造组合类型在华南地区的时空演化过程,进而对华夏块体构造属性和新元古代至早古生代的不同演化模式进行了讨论,重点对比探讨了陆内造山模式和增生造山模式的认识。为准确约束华南洋的性质与演化过程,本文建议在地质调查工作中加强中南地区新元古代-早古生代沉积序列、沉积时代、古地理恢复及构造背景等,以及信宜-增城带、郴州-临武-钦防带两侧地层对比和钦防海槽的深入研究。

Characteristics of Gold Mineralization at the Baguiomo Gold Panning Site, Koudougou Region, Burkina Faso, West Africa

The Birimian Baguiomo formations are located in the northern part of the Boromo greenstone belt. In this belt, the volcanic rocks (tholeiitic basalt, calcalkaline basalt, andesite) hosting the gold mineralization are located in the Kwademen-Baguiomo shear zone. This mineralization, located only a few kilometers from the Kwademen gold deposit, is uncharacterized and, together with the latter, could constitute a gold potential capable of being economically exploitable. It is in this sense that this work is carried out with a view to characterizing the gold mineralization of the Baguiomo gold panning site. To carry out this work, we have made direct field measurements, combined with microstructures, and combined all this with data from geochemical rock analysis of the basalts that are the main host formations. Geochemical data show that tholeitic basalts formed from a mantle plume that was emplaced in an oceanic plateau context. Calc-alkaline basalts and andesites are comparable to Paleoproterozoic tholeitic basalts (PTH3), which are slightly enriched in light rare earths. Fertility tests show that these basalts concentrate between 3 and 6 ppb of gold at the time of accretion, which is sufficient for remobilization of this primary gold during the Eburnian orogeny to yield a deposit of around 4 - 5 Moz. Gold mineralization is associated with pyrite crystals when the latter are disseminated in the rock mass, whereas it is associated with hematite in quartz veins concordant with S1 shear deformation. It is mainly the pyrite crystals in the pressure shadows that contain the gold grains, whose development would be synchronous with micro-shear zone reactivation during the first phase of D1B deformation. The second phase of D2B deformation, which is a crenulation or fracture schistosity, does not significantly affect the shear deformation that controls mineralization.

Pan-African metamorphism and magmatism in the Prydz Belt,East Antarctica:a geochronological perspective

The Prydz Belt in East Antarctica underwent extensive reworking during the late Neoprotertozoic–early Paleozoic Pan-African orogeny,which is characterized by granulite facies,clockwise P–T paths,and high dT/dP values.This study compiles the existing age and composition data of zircon and monazite from metamorphic rocks and links their key characteristics to the metamorphic evolution of the Prydz Belt.The frequency of zircon U–Pb ages starts to increase noticeably from~555 Ma,peaking between 530 Ma and 520 Ma,followed by a dramatic decline after 520 Ma.High Th/U values(>0.1)of zircon are observed from~545 Ma,displaying a noticeable increasing trend in Th/U values before a rapid decline from~520 Ma.The frequency of monazite ages progressively increases from~540 Ma,reaching its peak at 515 Ma,and then rapidly decreases after 490 Ma.Combined with the crystallization behaviors of zircon and monazite,this study suggests that the systematic changes in Th/U values of zircon after 545 Ma indicate a transition in the thermal regime of the Prydz Belt towards the cooling stage.Abundant growth of zircon and monazite corresponds to the post-peak cooling process,while the crystallization peak of monazite lags behind that of zircon by~5–15 Ma,which indicates a relatively low cooling rate.Though the granitic intrusions accompanied the entire metamorphic evolution,the majority of them are younger than 520 Ma.The results suggest that the Pan-African event likely peaked at~555–545 Ma and gradually cooled to near-solidus conditions at~520–510 Ma,with a relatively slow average cooling process.

基于背景噪声反演华南块体岩石圈三维S波速度结构

利用背景噪声方法,对布设在华南块体及其周边区域数个流动地震台阵中共704个地震台站的连续数据进行分析,反演得到华南岩石圈的三维S波速度结构。结果表明,地表附近S波速度结构与华南块体的地表构造高度相关,低速异常区集中在四川盆地和江汉盆地等沉积层较厚的区域。江南造山带的中上地壳呈现明显的高速异常,该高速异常即为华夏块体与扬子块体的分界区域。华夏块体中地壳低速异常区与该区域广泛分布的中生代花岗岩对应较好。华南上地幔岩石圈的东部与西部存在显著的速度差异,西部呈现显著的高速异常,反映西部四川盆地底部稳固的克拉通结构;上地幔的低速异常揭示华南东部岩石圈因太平洋板块俯冲的影响而明显减薄(<70km)。推测华夏块体以琼州海峡为中心的上地幔显著低速区可能对应海南地幔柱的热物质上涌。

Regional Tectonic Transformation in East Kunlun Orogenic Belt in Early Paleozoic: Constraints from the Geochronology and Geochemistry of Helegangnaren Alkali-feldspar Granite

The Helegangnaren feldspar granite exposed in the eastern part of East Kunlun, is characterized by high concentrations of SiO2 and alkaline, low abundances of Fe, Mg and Ca, metaluminous-weak peraluminous. Trace elements analysis shows that the granite is depleted extremely in Ba, Sr and Eu, and rich in some large-ion lithophile elements and high field strength elements. Besides, the granite has high Ga contents, the values of 104（Ga/AI） vary from 2.50 to 2.77, which is mainly greater than the lower limit of A-type granites （2.6）, and is higher than the I- and S- type granites＇ average （2.1 and 2.28, respectively）. Rare earth element （REE） is characterized by relatively high fractionations of light REE （LREE） and heavy REE （HREE） （LREE/HREE=9.3-13.60, （La/Yb）N=10.92-18.02）, pronounced negative Eu anomalies （JEn=0.08-0.13）, and exhibits right- dipping gull pattern. Major elements, rare elements and trace elements features show the granite is ascribed to A-type granite and A2 subtype in tectonic genetic type. They are plotted into post-collision or within-plate area in a variety of tectonic discriminations. Geological and geochemical data comprehensively suggest that the granite is formed in a post-collision extensive tectonic setting. Laser ablation inductively coupled plasma mass spectrometry （LA-ICP-MS） zircon U-Pb dating yields a weighted mean age of 425 Ma, belonging to Middle Silurian, which is similar to the age of the post- collision geological events in the region. The differences of magmatic rocks in formation age, rocks assemblage and rocks series systematically indicate that the regional tectonic stress regime in the East Kunlun orogenic belt experienced a major transformation from compress to extension in Middle Silurianin, and the Helegangnaren feldspar granite intruded in the early stage of tectonic transformation.

福建德化蒲洋、浦城仙阳地区早古生代埃达克质岩成因探讨及其地质意义

华南加里东造山作用的类型仍存在争议,焦点集中在是否存在加里东期的大洋俯冲造山作用。根据福建德化蒲洋花岗闪长岩和浦城黄墩、仙阳等地的二长花岗岩年代学,岩石地球化学特征研究认为,岩石具埃达克质岩特征,成岩年龄为450~455 Ma,侵位时间与早古生代华夏地块西南部的云开造山运动的俯冲时限(460~440 Ma)吻合,结合区域资料,认为是大洋俯冲板片部分熔融成因,为该时期华南板块存在洋板块俯冲造山作用提供了重要证据,且指示初始俯冲时限不晚于455 Ma。

^(40)Ar/^(39)Ar Thermochronological Constraints on the Timing of Collisional Orogeny in the Mian-Lite Collision Belt,Southern Qinling Mountains

Silurian, Devonian and Carboniferous geological bodies in the Mianxian-Lueyang (Mian-Lue) collisional belt (MLB) and its neighbouring areas, southern Qinling Mountains, China, show similar characteristics of having undergone deformation of two stages. The earlier one, which is inferred to be related to collisional orogeny between the Yangtze and Sino-Korean palaeocontinents based on previous geological data, is responsible for large-scale, north-verging recumbent folds and overthrusts, and associated with low greenschist fades metamorphism. 40Ar/39Ar dating of three muscovite samples taken from different localities yields plateau ages of 226.9±0.9 and 219.5±1.4 Ma and an apparent age of 194.5±3.0 Ma. Thus, the late Triassic collision between the Yangtze and Sino-Korean palaeocontinents has been constrained.

The Yanshanian Orogeny and Two Kinds of Yanshanides in Eastern-Central China

The Tan-Lu Fault was once a transform fault in the Paleotethys, west of which was the Qinling-Dabie Ocean separating the Yangtze Craton from the North China Craton, and east of which was the Su-Lu Ocean separating the Su-Wan Block from the Jiao-Liao Craton. The Qinling-Dabie Ocean closed in the Indosinian orogeny, which created the China-Southeast Asia Subcontinent, with the Tan-Lu Fault becoming a marginal shear zone along the newly-formed amalgamated subcontinent. The Su-Lu Ocean subducted partly in the Indosinian.orogeny, but not closed. In the Jurassic and Early Cretaceous, the Su-Wan Block drifted northwards with subduction of the Su-Lu Ocean and moved westwards to converge the subcontinent by sinistral sheafing of the ENE-striking fractures. The Su-Lu Ocean finally closed and the Su-Wan Block collided with the Jiao-Liao Craton in the Early Cretaceous, which constituted a part of the magnificent interplate Yanshanides. The interplate orogeny rejuvenated the fossil sutures and deep fractures, as well as the Indosinian orogen, and the intraplate （intracontinental） Yanshanian orogeny occurred in the subcontinent. The East Asia Yanshanides, consisting of the interplate orogens in the outer side and the intraplate orogens in the inner side, collapsed quickly in the latest Early Cretaceous and Late Cretaceous. The eastern China area entered a tensile period from the Eogene, and the tectonic differentiation between the central and eastern China areas since the Jurassic was further strengthened.

Intracontinental Collisional Orogeny During Late Permian-Middle Triassic in South China: Sedimentary Records of the Shiwandashan Basin

Sedimentary response to an orogenic process is important for determining whether South China had compressional or extensional orogeny during the period from the Late Permian to the Middle Triassic besides the tectonic and magmatologic evidence. An intracontinental collision event took place between the Yangtze and Cathaysia blocks in the Late Permian. Beginning at the Late Triassic, the tectonic movement was completely changed in nature and entered a post-collisional extensional orogenic and basin-making process. This paper presents sedimentological evidence from the Late Permian to the Middle Triassic in the Shiwandashan basin at the southwestern end of the junction zone between the Yangtze and Cathaysia blocks.

Zircon geochronology reveals polyphase magmatism and crustal anatexis in the Buchan Block,NE Scotland:Implications for the Grampian Orogeny

The type locality for high-temperature,low-pressure regional metamorphism,the Buchan Block in NE Scotland,exhibits profound differences to the rest of the Grampian Terrane.These differences have led some to regard the Buchan Block as an exotic crustal fragment comprising Precambrian basement gneisses and cover rocks thrust into their current position during Grampian orogenesis.Although rocks of the Buchan Block are now generally correlated with Dalradian strata elsewhere,the origin of the gneisses and the cause of the high heat flow and associated magmatism is debated.We report SIMS U-Pb and LA-ICPMS Hf isotopic data in zircon from high-grade rocks from the northeast(Inzie Head Gneiss)and northwest(Portsoy)corners of the Buchan Block.Around Inzie Head,upper amphibolite to granulite facies metasedimentary gneisses coexist with diorite sheets that were emplaced contemporaneously with partial melting of their host rocks,at least locally.U-Pb geochronology indicates a crystallisation age for the diorite of 486±9 Ma.Highly-deformed diorites within the Portsoy Gabbro have a crystallisation age of 493±8 Ma.Ages of ca.490 Ma for magmatism and high-grade metamorphism,which are broadly contemporaneous with ophiolite obduction and the onset of orogenesis,are significantly older than the established peak of Grampian metamorphism(ca.470 Ma).We propose a new model for the Grampian Orogeny involving punctuated tectonothermal activity due to tectonic switching during accretionary orogenesis.Rollback of a NW-dipping subduction zone at ca.490 Ma produced a back-arc environment(the Buchan Block)with associated arc magmatism and high dT/dP metamorphism.Arrival of an outboard arc resulted in shortening(the initial phase of the Grampian Orogeny)at ca.488 Ma.Rollback of a NW-dipping subduction zone to the SE of the ca.488 Ma suture began at 473 Ma and led to lithospheric-scale extension,decompression melting and advective heating of the middle crust,producing the widespread ca.470 Ma Grampian(classic Barrovian and Buchan)regional metamorphism.Resumed hinge advance and the final phase of shortening cut off the heat supply at ca.465 Ma,marking the end of the Grampian Orogeny.

Isotope Geochronologic and Geochemical Constraints on the Magmatic Associations of the Collisional Orogenic Zone in the West Kunlun Orogen, China

The Jiajiwaxi pluton in the southern portion of the West Kunlun Range can be divided into two collision-related intrusive rock series, i.e., a gabbro-quartz diorite-granodiorite series that formed at 224±2.0 Ma and a monzonitic granite-syenogranite series that formed at 222±2.0 Ma. The systematic analysis of zircon U-Pb geochronology and bulk geochemistry is used to discuss the magmatic origin （material source and thermal source）, tectonic setting, genesis and geotectonic implications of these rocks. The results of this analysis indicate that the parent magma of the first series, representing a transition from I-type to S-type granites, formed from thermally triggered partial melting of deep crustal components in an early island-arc-type igneous complex, similar to an I-type granite, during the continental collision orogenic stage. The parent magma of the second series, corresponding to an S-type granite, formed from the partial melting of forearc accretionary wedge sediments in a subduction zone in the late Palaeozoic-Triassic. During continued collision, the second series magma was emplaced into the first series pluton along a central fault zone in the original island arc region, forming an immiscible puncture-type complex. The deep tectonothermal events associated with the continent-continent collision during the orogenic cycle are constrained by the compositions and origins of the two series. The new information provided by this paper will aid in future research into the dynamic mechanisms affecting magmatic evolution in the West Kunlun orogenic belt.