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1.
续海金  宋衍茹  叶凯 《岩石学报》2013,29(5):1594-1606
威海-荣成地区混合岩化花岗质片麻岩为研究超高压地体折返过程中的部分熔融提供了理想的窗口。本文通过对荣成地区鲍村花岗质片麻岩中浅色条带野外观察、岩相学以及锆石的CL图像、U-Pb定年、微量元素和Lu-Hf同位素的系统研究表明,鲍村花岗质片麻岩中的浅色条带为部分熔融的产物,其主要矿物组成为石英+斜长石+钾长石+黑云母。锆石CL图像显示清楚的核-边结构:继承岩浆核和新生岩浆边。继承核的206Pb/238U协和年龄为620±8～784±7Ma(Mean=701±33Ma);具有典型的岩浆锆石的微量元素特征,如重稀土(HREE)和Y富集,强烈的正Ce异常和负Eu异常,以及极低的(Gd/Lu)N和Hf/Y比值;εHf(t=700Ma)值为-13.0～-8.0(Mean=-10.9±0.8),相应的tDM2为2120～2429Ma(Mean=2303±51Ma)。新生边的206Pb/238U协和年龄为211±6～244±5Ma(226±10Ma);虽然微量元素的整体含量低于继承核,但也具有岩浆锆石的微量元素特征,如稀土元素配分模式显示HREE明显上翘的特征,正Ce异常和负Eu异常,以及极低的(Gd/Lu)N比值;εHf(t=225Ma)值为-17.3～-14.2(Mean=-15.2±0.6),相应的tDM2为2152～2347Ma(Mean=2214±33Ma)。上述特征表明,部分熔融的原岩是由古元古代(2.2～2.3Ga)的地壳物质在新元古代(701±33Ma)重熔而成,部分熔融可能发生在超高压地体折返早期的热折返阶段(榴辉岩相-高压麻粒岩相条件下),时代应为226±10Ma。威海-荣成混合岩化片麻岩内广泛发育的富含钾长石和石英的酸性岩脉(约220～210Ma),可能为折返晚期的角闪岩相冷凝阶段,部分熔融形成的熔体经历了结晶分异作用的产物。  相似文献   

2.
本文对丹凤地区秦岭岩群含柯石英超高压变质地体长英质片麻岩中的混合岩化长英质浅色体和含石榴子石暗色包体的花岗质脉体进行了详细的矿物学、地球化学和锆石U-Pb年代学以及Lu-Hf同位素研究。其中,长英质浅色体显示了近原位熔融的高硅、富钾的过铝质花岗岩地球化学特征;锆石的CL图像呈灰黑色,均匀无结构或云雾状内部结构,Th/U比值0. 008,并含有钾长石、斜长石、石英和磷灰石等包裹体,显示深熔锆石的特征;花岗质脉体暗色包体中的石榴子石显示核-边成分环带,其中核部成分与秦岭岩群长英质片麻岩中石榴子石成分一致,边部Sps含量升高,显示熔体改造或退变质扩散特征,寄主花岗质脉体显示重稀土强烈亏损的与石榴子石平衡的熔体特征,指示它们是秦岭岩群含石榴子石长英质片麻岩部分熔融的产物。锆石LA-ICP-MS定年得到长英质浅色体和花岗质脉体的结晶年龄分别为445±4Ma和420±1Ma,明显晚于本区的超高压变质时代,而与折返过程中麻粒岩相和角闪岩相退变质叠加的时代基本一致。结合区域地质和前人的研究成果,提出秦岭岩群在深俯冲板块的折返过程中,分别在445Ma和420Ma发生了两期部分熔融作用。  相似文献   

3.
柴北缘大陆深俯冲板片折返过程中的深熔作用研究   总被引：3，自引：2，他引：1  
柴北缘锡铁山地区长英质（花岗）片麻岩普遍经历了不同程度的部分熔融作用,常见新生的花岗质浅色体呈层状、脉状或网络状分布于长英质片麻岩中,并显示出混合岩化的特征。岩相学观察结果显示长英质片麻岩保留了关键的深熔作用显微结构证据:（1）石榴石内部发育有钾长石、石英和斜长石组成的矿物包裹体;（2）长石颗粒边界出现由石英+钾长石±斜长石±白云母组成的楔形矿物集合体;（3）云母颗粒边界发育尖锐的、不规则的微斜长石,而且云母边界溶蚀明显,形成锯齿状不规则的边界;（4）石英、斜长石或钾长石颗粒边界发育圆珠状（stringofbeads）结构,而且颗粒边界或三联点中尖锐状微斜长石与周围矿物的形成较小的二面角。阴极发光图像和锆石U-Pb定年结果表明花岗质浅色体中的锆石具有明显的核、幔、边三层结构,而且具有明显不同的年龄结果。发光较强的继承性锆石岩浆核部的206Pb/238U年龄约为~910Ma,而且具有高的Th/U比值;弱发光的变质锆石幔的206Pb/238U年龄结果约为~450Ma。新生的锆石增生边中等程度发光,并发育震荡环带和较低的Th/U比值,与世界典型地区混合岩中深熔锆石的特征十分相似,其206Pb/238U年龄结果为432±3Ma。野外关系、显微结构特征和年代学的研究结果显示柴北缘锡铁山地区花岗质浅色体可能是其寄主岩石长英质片麻岩在折返到高压麻粒岩相条件下深熔作用的产物,而且白云母的脱水熔融是引发岩石发生深熔作用的主要机制。柴北缘地区已有的资料综合研究表明,大陆深俯冲板片在俯冲/碰撞和折返过程中可能经历了多重深熔作用。  相似文献   

4.
柴北缘鱼卡河榴辉岩围岩的变质时代及其地质意义   总被引：17，自引：1，他引：16  
陈丹玲  孙勇  刘良 《地学前缘》2007,14(1):108-116
利用阴极发光和LA-ICP-MS原位分析方法,对柴北缘鱼卡河超高压榴辉岩的直接围岩——石榴石白云母角闪钠长片麻岩和含蓝晶石的石榴石云母片岩进行了详细的锆石微量元素和U-Th-Pb同位素分析。结果表明,石榴石白云母角闪钠长片麻岩中的锆石主体呈浑圆状形态,内部结构以扇状和冷杉树状为主,稀土总量和重稀土含量均较低,重稀土富集程度低,具明显正Eu异常,Th/U比值均小于0.1,指示其形成在与石榴石平衡共生而不含斜长石的变质条件下。而含蓝晶石的石榴石云母片岩中的锆石显示明显的核-边结构,核部显示碎屑锆石特征,边部为与石榴石共生的变质新生锆石。LA-ICP-MS原位定年获得两种围岩的峰期变质年龄分别为(431±3)Ma和(432±19)Ma。这两组年龄在误差范围内一致,而且与已获得的紧邻的两类超高压榴辉岩的变质年龄((436±3)Ma和(431±4)Ma)以及带内指示大陆深俯冲作用时代的都兰含柯石英副片麻岩锆石的柯石英微区年龄(424~432Ma)完全一致。参考这两类岩石与超高压榴辉岩的野外产状关系、含蓝晶石的石榴石云母片岩的原岩特征以及榴辉岩中超过变质年龄达300Ma的原岩残核的存在,共同表明,柴北缘鱼卡河超高压变质地体是典型的大陆深俯冲碰撞作用的产物,本区大陆深俯冲发生的时代为430~435Ma的早古生代。  相似文献   

5.
马旭东  陈丹玲 《地质通报》2006,25(1):99-103
柴达木盆地北缘大柴旦地区超高压变质岩的围岩--长英质片麻岩的主要矿物组合为石榴子石 蓝晶石 多硅白云母 条纹长石 石英 斜长石,形成压力在800MPa以上.在阴极发光图像内部结构研究的基础上,利用LA-ICP-MS对该长英质片麻岩中锆石的变质微区进行了详细的原位微区U-Pb定年,得到206Pb.238U加权平均年龄为427.2 Ma±4.5 Ma,对应的Th/U为0.01～0.06.该年龄与本区已有的榴辉岩的围岩超高压泥质片麻岩、石榴子石橄榄岩的变质年龄在误差范围内一致.  相似文献   

6.
越来越多的证据表明,超高压地体在俯冲和折返过程中可以经历部分熔融[1-2]。部分熔融会显著影响深俯冲板块的物理化学性质,在超高压地体的演化过程中扮演着极其重要的角色[3-4]。长英质片麻岩是超高压地体中最常见且最易熔的一种岩石类型,目前已报道的部分熔融现象普遍同该类岩石紧密相关。不同于长英质片麻岩,基性榴辉岩通常保留了峰期超高压矿物组合且相对难熔,同时其经历部分熔融的现象鲜有报道。超高压榴辉岩中石英±钾长石±斜长石±其他矿物多相固体包裹体的研究表明,大别—苏鲁超高压榴辉岩在折返早期可能经历过低程度部分熔融[5-6],然而关于该类多相固体包裹体的成因目前仍存在争议。本研究对大别山甘家岭M型超高压榴辉岩进行了详细的岩石学和矿物学研究并发现了部分熔融的多种证据。与此同时,对榴辉岩中的多相固体包裹体进行了详细的显微构造和成分分析,相关研究结果为其形成机制提供了新的认识。甘家岭超高压榴辉岩峰期矿物组合由石榴子石、绿辉石、多硅白云母、柯石英、菱镁矿、白云石、金红石、磷灰石和锆石等组成,局部经折返熔融形成以石榴子石和石英为主要组成的残余体。相较于榴辉岩,残余体的Si O2、P2O5和REE含量升高,其他主量元素和LILE含量降低。榴辉岩和残余体在显微尺度上保留了部分熔融的多种结构证据:斜长石和绿帘石薄膜/条带、斜长石+石英熔体囊假象、斜长石+钾长石交生体、绿帘石的震荡环带等。榴辉岩和残余体中的石榴子石含有相似的多相固体包裹体,不过该现象在残余体中更为典型。石榴子石中的多相固体包裹体主要分为两类:第一类由石英±钾长石±斜长石±其他矿物组成,整体形态非常接近柯石英;第二类核部由金红石组成,其周围被斜长石+钾长石±石英包裹,包裹体的整体形态非常接近核部的金红石。两类包裹体均发育楔入状和颈缩构造,包裹体同石榴子石之间的接触边界明显受控于寄主矿物晶面,这些特征暗示两类包裹体的形成均同熔流体活动紧密相关。第一类多相固体包裹体富集LILE(Cs、Rb、Ba、K、Pb、Li、Be和Sr)但其LREE和HFSE(Nb、Ta和Ti)含量变化较大,后两类元素的含量分别同包裹体中的绿帘石和含钛矿物多少紧密相关。考虑到两类包裹体的整体形态分别接近早期的柯石英和金红石包裹体并且其化学组成多变,本研究认为这些包裹体是由外界的熔流体渗入到石榴子石中并同早期的矿物包裹体相互反应形成的。石榴子石中发育两期裂隙,早期裂隙已经愈合并在电子背散射图像上表现为脉络状亮纹,晚期裂隙切割早期愈合裂隙并被后期蚀变的绿泥石充填,本研究将前者解释为早期熔流体的渗入通道。此种成因机制下,利用上述多相固体包裹体恢复早期的熔流体成分将会非常困难。  相似文献   

7.
深俯冲陆壳物质部分熔融产生的熔体,实验岩石学方面已有广泛报道,而天然初始熔体的组分却难以厘定。对此,本文从苏鲁超高压地体荣成混合岩中识别出了深俯冲花岗质陆壳部分熔融产生的天然初始熔体组成。野外露头显示,混合岩中主要矿物组成为钾长石+斜长石+石英的浅色熔体呈不连续的条带状与残余体互层产出,指示了原位或近源区的部分熔融特征。混合岩浅色体锆石CL图像呈明显的核-边结构,继承核部为扬子板块来源的岩浆锆石,形成时代为721±24Ma;新生边部CL图像具震荡环带结构,微量元素上REE呈明显左倾,具有Eu的负异常及Ce的正异常,低的Hf/Y和Th/U比值,具深熔锆石特征,指示形成于花岗质陆壳物质的部分熔融。边部U-Pb谐和年龄为225.9±2Ma,略晚于苏鲁超高压地体超高压峰期变质年龄,表明初始熔融发生在超高压地体折返早期。浅色熔体的全岩地球化学特征表明,主量元素上具有高SiO_2、K_2O及Na_2O含量,低的Fe_2O_3~T、MgO及CaO含量,A/CNK=1.02~1.04,呈弱过铝质亚碱性花岗岩的特征,这与实验岩石学中富硅陆壳物质部分熔融产生的熔体组分极为相近;微量元素上富集大离子亲石元素(如Rb、Ba、Pb等),亏损Nb、Ta、Ti等高场强元素,REE呈较为平坦的配分模式,具弱的Eu负异常并亏损Sr。本文通过上述对天然样品研究,厘定了深俯冲花岗质陆壳部分熔融及其初始熔体的组成,为理解大陆俯冲带壳幔相互作用提供了关键依据。  相似文献   

8.
南苏鲁超高压变质地体中罗迪尼亚超大陆裂解事件的记录   总被引：14，自引：11，他引：14  
通过苏鲁超高压变质地体南部不同类型超高压变质岩石的原岩重塑．揭示超高压变质岩的原岩形成于由大陆玄武质岩石、辉长岩、表壳岩和花岗岩组成的被动陆缘拉伸构造环境。中国大陆科学钻探主孔中不同类型超高压变质岩石的锆石SHRIMP U-Pb定年表明。花岗质片麻岩原岩年龄为780～680Ma；榴辉岩、石榴角闪岩的原岩年龄为765～730Ma，副片麻岩中包含了730Ma、680Ma、621Ma和较年轻的继承性碎屑锆石和结晶锆石年龄。结合前人的研究成果表明，苏鲁超高压变质地体南部正片麻岩类和榴辉岩的原岩所代表的花岗岩浆和基性岩浆活动为罗迪尼亚超大陆形成后的新元古代裂解事件的产物．而副片麻岩的原岩为新元古代．古生代时期形成的扬子被动陆缘的沉积-火山表壳盖层，它们与结晶基底一起在240～220Ma期间经历了超高压变质作用。  相似文献   

9.
榴辉岩与围岩的变质程度和变质年龄研究是超高压竞质地体形成机制研究的关键.已有研究表明,柴北缘野马滩榴辉岩和围岩副片麻岩共同经历了超高压变质作用,榴辉岩的变质年龄为458±7Ma,但有关副片麻岩的变质时代存在争议,从而制约了本区超高压变质地体形成机制的讨论.本文选择新发现的野马滩超高压榴辉岩中的副片麻岩夹层进行详细的年代学和地球化学研究.锆石的阴极发光图象显示所选锆石具有清晰的核.边结构,为典型的变质碎屑锆石特征.LA-ICP-MS定年方法获得锆石核部原岩形成年龄为大于1000Ma,边部变质事件年龄为458±6Ma.该年龄与紧邻的超高压榴辉岩的变质年龄一致,指示二者形成于同一地质事件.岩石地球化学研究表明,片麻岩的原岩为形成于陆缘环境的含泥硅质岩.综合榴辉岩与片麻岩原岩形成构造背景、变质程度及产状关系,提出野马滩榴辉岩与围岩属"原地"关系,可能形成于陆壳的深俯冲作用.  相似文献   

10.
柴达木盆地北缘大柴旦地区超高压变质岩的围岩——长英质片麻岩的主要矿物组合为石榴子石+蓝晶石+多硅白云母+条纹长石+石英+斜长石,形成压力在800MPa以上。在阴极发光图像内部结构研究的基础上,利用LA-ICP-MS对该长英质片麻岩中锆石的变质微区进行了详细的原位微区U-Pb定年,得到206Pb/238U加权平均年龄为427.2Ma±4.5Ma,对应的Th/U为0.01～0.06。该年龄与本区已有的榴辉岩的围岩超高压泥质片麻岩、石榴子石橄榄岩的变质年龄在误差范围内一致。  相似文献   

11.
Coesite‐bearing eclogites from >100 km2 in the southern Dulan area, North Qaidam Mountains (NQM) of western China, contain zircon that records protolith crystallization and ultra high pressure (UHP) metamorphism. Sensitive High‐Resolution Ion Microprobe (Mass Spectrometer) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry U–Pb analyses from cathodoluminescence (CL)‐dark zircon cores in a coesite‐bearing eclogite yield an upper intercept age of 838 ± 50 Ma, and oscillatory zoned cores in a kyanite‐bearing eclogite gave a weighted mean 206Pb/238U age of 832 ± 20 Ma. These zircon cores yield steep heavy rare earth element (HREE) slopes and negative Eu anomalies that suggest a magmatic origin. Thus, c. 835 Ma is interpreted as the eclogite protolith age. Unzoned CL‐grey or ‐bright zircon and zircon rims from four samples yield weighted mean ages of 430 ± 4, 438 ± 2, 446 ± 10 and 446 ± 3 Ma, flat HREE patterns without Eu anomalies, and contain inclusions of garnet, omphacite, rutile, phengite and rare coesite. These ages are interpreted to record 16 ± 5 Myr of UHP metamorphism. These new UHP ages overlap the age range of both eclogite and paragneiss from the northern Dulan area, suggesting that all UHP rock types in the Dulan area belong to the same tectonic unit. Our results are consistent with slow continental subduction, but do not match oceanic subduction and diapiric exhumation UHP model predictions. These new data suggest that, similar to eclogites in other HP/UHP units of the NQM and South Altyn Tagh, protoliths of the eclogites in the Dulan area formed in a continental setting during the Neoproterozoic, and then subducted to mantle depth together with continental materials during the Early Palaeozoic.  相似文献   

12.
Composite granite–quartz veins occur in retrogressed ultrahigh pressure (UHP) eclogite enclosed in gneiss at General's Hill in the central Sulu belt, eastern China. The granite in the veins has a high‐pressure (HP) mineral assemblage of dominantly quartz+phengite+allanite/epidote+garnet that yields pressures of 2.5–2.1 GPa (Si‐in‐phengite barometry) and temperatures of 850–780°C (Ti‐in‐zircon thermometry) at 2.5 GPa (~20°C lower at 2.1 GPa). Zircon overgrowths on inherited cores and new grains of zircon from both components of the composite veins crystallized at c. 221 Ma. This age overlaps the timing of HP retrograde recrystallization dated at 225–215 Ma from multiple localities in the Sulu belt, consistent with the HP conditions retrieved from the granite. The εHf(t) values of new zircon from both components of the composite veins and the Sr–Nd isotope compositions of the granite consistently lie between values for gneiss and eclogite, whereas δ18O values of new zircon are similar in the veins and the crustal rocks. These data are consistent with zircon growth from a blended fluid generated internally within the gneiss and the eclogite, without any ingress of fluid from an external source. However, at the peak metamorphic pressure, which could have reached 7 GPa, the rocks were likely fluid absent. During initial exhumation under UHP conditions, exsolution of H2O from nominally anhydrous minerals generated a grain boundary supercritical fluid in both gneiss and eclogite. As exhumation progressed, the volume of fluid increased allowing it to migrate by diffusing porous flow from grain boundaries into channels and drain from the dominant gneiss through the subordinate eclogite. This produced a blended fluid intermediate in its isotope composition between the two end‐members, as recorded by the composite veins. During exhumation from UHP (coesite) eclogite to HP (quartz) eclogite facies conditions, the supercritical fluid evolved by dissolution of the silicate mineral matrix, becoming increasingly solute‐rich, more ‘granitic’ and more viscous until it became trapped. As crystallization began by diffusive loss of H2O to the host eclogite concomitant with ongoing exhumation of the crust, the trapped supercritical fluid intersected the solvus for the granite–H2O system, allowing phase separation and formation of the composite granite–quartz veins. Subsequently, during the transition from HP eclogite to amphibolite facies conditions, minor phengite breakdown melting is recorded in both the granite and the gneiss by K‐feldspar+plagioclase+biotite aggregates located around phengite and by K‐feldspar veinlets along grain boundaries. Phase equilibria modelling of the granite indicates that this late‐stage melting records P–T conditions towards the end of the exhumation, with the subsolidus assemblage yielding 0.7–1.1 GPa at <670°C. Thus, the composite granite–quartz veins represent a rare example of a natural system recording how the fluid phase evolved during exhumation of continental crust. The successive availability of different fluid phases attending retrograde metamorphism from UHP eclogite to amphibolite facies conditions will affect the transport of trace elements through the continental crust and the role of these fluids as metasomatic agents interacting with the mantle wedge in the subduction channel.  相似文献   

13.
Fluid availability during high‐grade metamorphism is a critical factor in dictating petrological, geochemical and isotopic reequilibration between metamorphic minerals, with fluid‐absent metamorphism commonly resulting in neither zircon growth/recrystallization for U‐Pb dating nor Sm‐Nd isotopic resetting for isochron dating. While peak ultra‐high pressure (UHP) metamorphism is characterized by fluid immobility, high‐pressure (HP) eclogite‐facies recrystallization during exhumation is expected to take place in the presence of fluid. A multichronological study of UHP eclogite from the Sulu orogen of China indicates zircon growth at 216 ± 3 Ma as well as mineral Sm‐Nd and Rb‐Sr reequilibration at 216 ± 5 Ma, which are uniformly younger than UHP metamorphic ages of 231 ± 4 to 227 ± 2 Ma as dated by the SHRIMP U‐Pb method for coesite‐bearing domains of zircon. O isotope reequilibration was achieved between the Sm‐Nd and Rb‐Sr isochron minerals, but Hf isotopes were not homogenized between different grains of zircon. The HP eclogite‐facies recrystallization is also evident from petrography. Thus this process occurred during exhumation with fluid availability from decompression dehydration of hydrous minerals and the exsolution of hydroxyl from nominally anhydrous minerals. This provides significant amounts of internally derived fluid for extensive retrogression within the UHP metamorphosed slabs. Based on available experimental diffusion data, the consistent reequilibration of U‐Pb, Sm‐Nd, Rb‐Sr and O isotope systems in the eclogite minerals demonstrates that time‐scale for the HP eclogite‐facies recrystallization is c. 1.9–9.3 Myr or less. This provides a maximum estimate for duration of the fluid‐facilitated process in the HP eclogite‐facies regime during the exhumation of deeply subducted continental crust.  相似文献   

14.
The amalgamation of South (SCB) and North China Blocks (NCB) along the Qinling‐Dabie orogenic belt involved several stages of high pressure (HP)‐ultra high pressure (UHP) metamorphism. The new discovery of UHP metamorphic rocks in the North Qinling (NQ) terrane can provide valuable information on this process. However, no precise age for the UHP metamorphism in the NQ terrane has been documented yet, and thus hinders deciphering of the evolution of the whole Qinling‐Dabie‐Sulu orogenic belt. This article reports an integrated study of U–Pb age, trace element, mineral inclusion and Hf isotope composition of zircon from an eclogite, a quartz vein and a schist in the NQ terrane. The zircon cores in the eclogite are characterized by oscillatory zoning or weak zoning, high Th/U and 176Lu/177Hf ratios, pronounced Eu anomalies and steep heavy rare earth element (HREE) patterns. The zircon cores yield an age of 796 ± 13 Ma, which is taken as the protolith formation age of the eclogite, and implies that the NQ terrane may belong to the SCB before it collided with the NCB. The ?Hf(t) values vary from ?11.3 to 3.2 and corresponding two‐stage Hf model ages are 2402 to 1495 Ma, suggesting the protolith was derived from an enriched mantle. In contrast, the metamorphic zircon rims show no zoning or weak zoning, very low Th/U and 176Lu/177Hf ratios, insignificant Eu anomalies and flat HREE patterns. They contain inclusions of garnet, omphacite and phengite, suggesting that the metamorphic zircon formed under eclogite facies metamorphic conditions, and their weighted mean 206Pb/238U age of 485.9 ± 3.8 Ma was interpreted to date the timing of the eclogite facies metamorphism. Zircon in the quartz vein is characterized by perfect euhedral habit, some oscillatory zoning, low Th/U ratios and variable HREE contents. It yields a weighted mean U–Pb age of 480.5 ± 2.5 Ma, which registers the age of fluid activity during exhumation. Zircon in the schist is mostly detrital and U–Pb age peaks at c. 1950 to 1850, 1800 to 1600, 1560 to 1460 and 1400 to 1260 Ma with an oldest grain of 2517 Ma, also suggesting that the NQ terrane may have an affinity to the SCB. Accordingly, the amalgamation between the SCB and the NCB is a multistage process that spans c. 300 Myr, which includes: the formation of the Erlangping intra‐oceanic arc zone onto the NCB before c. 490 Ma, the c. 485 Ma crustal subduction and UHP metamorphism of the NQ terrane, the c. 430 Ma arc‐continent collision and granulite facies metamorphism, the 420 to 400 Ma extension and rifting in relation to the opening of the Palaeo‐Tethyan ocean, the c. 310 Ma HP eclogite facies metamorphism of oceanic crust and associated continental basement, and the final 250 to 220 Ma continental subduction and HP–UHP metamorphism.  相似文献   

15.
大别山北部榴辉岩及英云闪长质片麻岩的锆石U-Pb年龄分析表明：北部榴辉岩相峰期变质时代为226～230Ma左右;北部塔儿河一带英云闪长质片麻岩经历过印支期变质事件;大别山北部与南部超高压岩石中一致的（226～230Ma）高压或超高压变质年龄表明,北部镁铁-超镁铁质岩带中部分岩石也曾作为扬子俯冲陆壳的一部分,在印支期发生过高压或超高压变质作用;本区锆石发生过两期变质增生事件,一是印支期高压或超高压变质,另一期是燕山期热变质事件;榴辉岩及英云闪长质片麻岩的原岩形成时代为晚元古代;锆石U-Pb年龄可用多期变质增生模型来解释。  相似文献   

16.
In order to decipher element mobility in ultrahigh-pressure (UHP) eclogite-facies metamorphic rocks during subduction and exhumation of continental crust, major-trace elements and Sr-Nd isotopes were systematically investigated for two continuous core segments of about 3 m length from the Chinese Continental Scientific Drilling (CCSD) project in the Sulu orogen. The segments are composed of lithological transitions between UHP eclogite and granitic gneiss. The eclogite exhibits a large variation in major and some trace elements such as LILE (e.g., Rb, Ba and K) and LREE, but a relatively limited range in HFSE and HREE. This suggests high mobility of LILE and LREE but immobility of HFSE and HREE during continental collision-zone metamorphism. Some eclogites have andesitic compositions with high SiO2, alkalis, LREE, and LILE but low CaO, MgO and FeO contents. These features likely result from chemical exchange with gneisses, possibly due to the metasomatism of felsic melt produced by partial melting of the associated gneisses during the exhumation. On the other hand, some eclogites appear to have geochemical affinity to refractory rocks formed by melt extraction as evidenced by strong LREE and LILE depletion and the absence of hydrous minerals. These results provide evidence of melt-induced element mobility in the UHP metamorphic rocks. In particular, large variations in the abundance of such elements as SiO2, LREE and LILE occur at the contact between eclogite and granitic gneiss, indicating their mobility between different slab components. Petrographic observations also show the presence of felsic veins on small scales in the UHP metamorphic rocks, demonstrating the occurrence of hydrous melt in local open-systems during the continental collision. As a whole, nevertheless, the protolith nature dictates the geochemical differences in both eclogite and granitic gneiss between the two core segments because mass transport during the subduction-zone metamorphism is principally dictated by the lithological differences at contact. The eclogite and granitic gneiss from the first core segment have high εNd(t) values, whereas those from the second core segment show relatively low εNd(t) values in concordance with majority of UHP metaigneous rocks outcropped along the Dabie-Sulu orogenic belt. Thus contrasting origins of bimodal igneous rocks were involved in the continental collision, demonstrating that the subducted continental crust is the magmatic product of active rifting margin during supercontinental breakup in the middle Neoproterozoic.  相似文献   

17.
从都兰北带榴辉岩的片麻岩围岩的锆石中发现了柯石英包裹体和石墨包裹体，说明该超高压带的峰期变质作用已达柯石英稳定区间（＞2.8GPa）但小于金刚石的稳定区间（＜3.5GPa），从而确定柴达木盆地北缘存在早古生代超高压变质作用和陆－陆碰撞作用，为中国中部存在一条横贯东西的早古生代（大致500－400Ma）高压超高压变质带的推断提供了新的关键性证据。  相似文献   

18.
Hydrothermally altered rocks are products of fluid–rock interactions, and typically preserve numerous quartz veins that formed as chemical precipitates from fluids that fill up cracks. Thus, quartz veins are the record of the fluid system that involved fracture flow in the direction of changing temperature or pressure. In order to decipher the fluid activity in the Sulu ultrahigh-pressure (UHP) terrane in eastern China, quartz veins together with an adjacent eclogite lens and the host gneiss were studied. In one location a deformed quartz vein is located at the boundary between the host gneiss and the eclogite lens. The amphibolite-facies overprinting of the eclogite lens decreases from the rim to the core of the lens, with fresh eclogite preserved in the core. The foliated biotite gneiss contains felsic veins and residual phengites. Zircon rims from the gneiss are characterized by melt-related signatures with steep HREE patterns, high Hf contents and negative Eu anomalies, and a pool of weighted average 206Pb/238U analyses reveal an age of 219 ± 3 Ma (2σ), which is younger than the UHP metamorphic age (236 ± 2 Ma, 2σ) recorded by zircons from the eclogite lens. This suggests that the gneiss in the Sulu UHP terrane could have suffered from partial melting due to phengite dehydration during the “hot” exhumation stage.The formation age of the quartz vein (219 ± 2 Ma, 2σ) defined by zircon rims agrees well with the partial melting time (219 ± 3 Ma, 2σ) of the host gneiss. The initial 176Hf/177Hf ratios of zircon rims from the quartz vein are obviously lower than zircons from the eclogite lens, but overlap with the coeval zircon domains from the nearby granite dikes produced by partial melting of orthogneiss. These observations suggest that the quartz vein and corresponding fluid flow could be associated with partial melting of the host gneiss. On the other hand, amphibole-bearing and HREE-rich zircon rims from the amphibolite pool an amphibolite-facies metamorphic age of 217 ± 5 Ma (2σ), overlap with the formation age of the quartz vein. This implies that retrogression of the eclogite lens could have been caused by melting-induced fluid flow. Based on the above observations, we speculate that partial melting of the gneiss in the continental subduction-related UHP belt could have induced a significant fluid flow during the exhumation stage, and thus contributed significantly to the extensive retrogression of eclogites in the Sulu UHP terrane.  相似文献   

19.
Coesite- and microdiamond- bearing ultra-high pressure (UHP) eclogites in the North Qinling terrane have been widely retrogressed to amphibolites. Previous geochronological studies on these UHP rocks mainly focused on the timing of peak eclogite facies metamorphism. The Kanfenggou UHP metamorphic domain is one of the best-preserved coesite-bearing eclogite occurrences in the North Qinling terrane. In this study, mafic amphibolites and host schists from this domain were collected for 40Ar/39Ar dating to constrain their retrograde evolution. Two generations of amphibole are recognized based on their mineral parageneses and 40Ar/39Ar ages. A first generation of amphibole from garnet amphibolites yielded irregularly-shaped age spectra with anomalously old apparent ages. Isochron ages of 484–473 Ma and initial 40Ar/36Ar ratios of 3695–774 are obtained from this generation of amphibole, indicating incorporation of excess argon. Second generation amphibole occurs in epidote amphibolites yielded flat age spectra with plateau ages of 464–462 Ma without evidence for excess argon. These ages suggest that the amphibolite-facies metamorphism has taken place as early as 484 Ma and lasted until 462 Ma for the North Qinling UHP metamorphic rocks. Phengite from the country-rock schists yielded 40Ar/39Ar plateau ages of 426–396 Ma, with higher phengite Si contents associated with the older the plateau ages. Based on our new 40Ar/39Ar ages and previous zircon UPb geochronological data, we construct a new detailed pressure-temperature-time (P-T-t) path illustrating the retrograde metamorphism and exhumation rate of the North Qinling eclogites and host schists. The P-T-t path suggests that these UHP metamorphic rocks experienced initial medium-to-high exhumation rates (ca. 8.7 mm/yr) during the Early Ordovician (489–484 Ma), which was mainly derived from buoyancy forces. Subsequently, the exhumation rate decreased gradually from ~0.8 to 0.3 mm/yr from 484 to 426 Ma, which was probably governed by extension and/or erosion.  相似文献   

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