苏鲁地体超高压和退变质时代的厘定:来自片麻岩锆石微区SHRIMP U-Pb定年的证据

锆石微区矿物包体的激光拉曼和阴极发光测试以及相应的SHRIMP U-Pb定年结果表明,苏鲁地体片麻岩锆石微区记录了十分复杂的年代学信息。其中副片麻岩锆石核部记录了345～743 Ma的继承性锆石年龄,标志着原岩碎屑锆石来源的复杂性;含柯石英的锆石微区记录了220～234 Ma的超高压变质年龄;而含石英包体的边部则记录了202～219 Ma的退变质年龄。正片麻岩继承性锆石核部所记录的年龄为574～680 Ma,表明原岩锆石曾经历了部分Pb丢失,原岩的形成年龄应大于680 Ma;含柯石英锆石微区所记录的超高压变质年龄为224～242 Ma;而锆石边部所记录的退变质年龄为209～219 Ma。两类片麻岩锆石微区所隐藏的超高压变质和退变质年龄信息十分相近,平均值分别为229±4Ma和211±4Ma,标志着苏鲁地体超高压变质时代应为印支期,相应的构造抬升速率约5.6 km/Ma。该项成果不仅确定了苏鲁地体超高压变质和退变质时代,而且对于深入探讨苏鲁地体快速折返过程中的动力学机制有着重要的科学意义。     

苏鲁超高压变质带中非超高压花岗质片麻岩的准确识别:来自锆石微区矿物包体及SHRIMP U-Pb定年的证据

通过隐藏在锆石微区矿物包体激光拉曼的系统鉴定和阴极发光图像特征的详细研究,配合相应的锆石微区SHRIMP U-Pb定年测试,发现苏鲁地体超高压变质带中确实存在非超高压变质的花岗质片麻岩。该类岩石中的锆石晶体自核部到边部所保存的矿物包体以不含超高压矿物为特征,相应的阴极发光图像具有典型岩浆结晶锆石的核部和幔部,以及变质的再生边的特点。其中岩浆结晶锆石微区记录的~(238)U-~(206)Pb年龄为404～748Ma,表明原岩中部分锆石可能经历了Pb丢失,也不排除后期热事件因素的影响,原岩的形成年龄应大于748 Ma;而锆石的再生边所记录的~(238)U-~(206)Pb。年龄为204～214 Ma,与研究区经历超高压变质的副片麻岩和花岗质片麻岩锆石微区所记录的苏鲁地体快速折返过程中角闪岩相退变质年龄(~(238)U-~(206)Pb年龄的平均值为211±4 Ma,刘福来等,2003a)十分相似。上述特征表明,苏鲁地体超高压变质带中的部分花岗质片麻岩在超高压变质事件之前就已经形成,但并未“参与”深俯冲—超高压的变质演化过程,而是在苏鲁地体快速折返的角闪岩相退变质过程中与超高压岩片“拼贴”在一起。该项成果不仅为正确识别非超高压变质岩石提供了一个新的研究方法,而且对进一步深入探讨苏鲁地体超高压和非超高压岩片的“拼贴”机制有着重     

苏鲁-大别超高压岩石中锆石SHRIMP U-Pb定年研究——综述和最新进展

如何建立苏鲁-大别超高压岩石深俯冲-超高压-快速折返过程连续而完整的P-T-t轨迹及精细的年代谱系,是目前地学界研究的热点。而变质锆石是否记录深俯冲石英榴辉岩相进变质阶段的年代学信息和超高压峰期变质时代的准确归属,是目前苏鲁-大别超高压变质带需要深入研究的核心问题。本文在对前人同位素年代学方面所取得的成果进行系统总结的基础上。采用锆石中矿物包体激光拉曼和电子探针测试、锆石阴极发光图像成因分析以及SHRIMP U-Pb定年等综合研究手段,确定苏鲁-大别地体榴辉岩及其强退变质围岩在深俯冲-构造折返过程中主要经历了四个阶段的变质演化:深俯冲石英榴辉岩相进变质(Ⅰ)、超高压峰期变质(Ⅱ)、构造折返初期石英榴辉岩相退变质(Ⅲ)和构造折返晚期角闪岩相退变质(Ⅳ)。研究发现,扬子板块(中)新元古代巨量的陆壳物质在早三叠纪(246～244Ma)俯冲到华北板块之下约65km的深处。发生了石英榴辉岩相进变质,相应的变质温压条件为T=542～693℃,P=1.7～2.02GPa。这些高压石英榴辉岩相岩石在中-新三叠纪继续向下俯冲,在235～225Ma期间,俯冲的深度至少达到了170km的地幔深处,并发生了峰期柯石英榴辉岩相超高压变质,相应的变质温压条件为T=722～866℃,P>5.5GPa。苏鲁-大别超高压地体自石英榴辉岩相进变质阶段到超高压峰期变质阶段的俯冲速率为7.0km/Myr。这些超高压岩石在219～216Ma期间,发生了第一次构造抬升至75km的深处,并经历了石英榴辉岩相退变质作用的改造,退变质温压条件为T=730～780℃,P=1.7～2.6GPa。这些退变质岩石在212～205Ma期间,又经历了第二次抬升至25km中-下地壳深处,并叠加了角闪岩相退变质作用,该阶段变质温压条件为T=610～710℃,P=0.7～1.2GPa。苏鲁-大别超高压地体两次构造抬升的速率大致相同,为5.6km/Myr。该项成果不仅确定了苏鲁-大别榴辉岩及其强退变质岩石深俯冲过程石英榴辉岩相进变质-超高压峰期变质、构造折返过程石英榴辉岩相-角闪岩相退变质连续而完整的变质演化P-T-t轨迹及精细的年代谱系,而且对于重新建立苏鲁-大别巨量陆壳物质快速超深俯冲-快速折返的动力学模式有着重要的科学意义。     

关于含柯石英锆石SHRIMP测年获得大别-苏鲁超高压变质带超高压变质作用年龄的讨论

对大别一苏鲁超高压变质(UHPM)带部分含柯石英包体的锆石及部分进行SHRIMP U—Pb测年的锆石进行划分和讨论后得出为大别-苏鲁UHPM带超高压变质作用(UHPM)年龄而进行含柯石英锆石SHRIMP U—Pb测年．宜从有结晶环带且在环带域或环带核部有柯石英包体的锆石的核部或环带域进行SHRIMP U—Pb测年．不宜从锆石边部获得UHPM年龄。着重讨论杨经绥等(2002)的两颗含柯石英锆石的结构，柯石英包体所在部位。测年数据的意义，得出苏鲁UHPM带UHPM年龄是578Ma(或略大于578Ma)。且这一认识与苏鲁UHPM带内区域地质情况相吻和。锆石中(多在边部)给出的印支期年龄值反映印支期地质作用(流体改造、重结晶等)改造事件的时代。     

大别山岳西县石关混合岩锆石SHRIMP定年及其地质意义

安徽岳西县石关混合岩属于原北大别变质杂岩的一部分。锆石SHRIMPU-Pb定年结果:第一次变质时间为232Ma,相当于晚三叠世早期,第二次变质时间为207Ma,相当于晚三叠世晚期。复合锆石的核部(继承锆石)为发育韵律环带的岩浆碎屑锆石,其年龄值分别为560Ma、444Ma、394Ma、378Ma和331Ma,暗示其原岩非岩浆岩,而应为沉积岩;研究结果还表明,北大别石关混合岩经历了两次变质作用:①超高压变质作用,变质时间为232Ma(n=5加权平均年龄);②退变质作用,时间为207Ma。与苏鲁地区超高压变质和退变质时间一致。     

青藏高原拉萨地块松多榴辉岩的锆石SHRIMP U-Pb年龄及锆石中的包裹体

拉萨地块榴辉岩样品中的锆石SHRIMP U-Pb年龄值为(242.4±15.2)～(291.9±12.8)Ma,平均261.7Ma±5.3Ma。所有锆石均含有大量的包裹体,主要分布在锆石核部。最常见的矿物包裹体是石榴子石,其次为石英、磷灰石、金红石和绿辉石,可见角闪石、榍石、多硅白云母和钠长石。包裹体具3种组合:榴辉岩相(Grt Omp Rt Phe)、角闪岩相(Amp Spn Ab)和不确定相(Qtz Ap)。锆石中的矿物包裹体与岩石中对应矿物的成分相同。包裹体集中在锆石核部和榴辉岩相矿物的大量出现表明锆石生长发生于变质峰期或峰期之后不久。锆石的Th/U比值均很低,具变质成因锆石的典型特征。区域地质资料对比表明,榴辉岩的原岩可能形成于石炭纪—二叠纪早期,是古特提斯洋盆裂解的产物。     

大别-苏鲁超高压榴辉岩中石榴子石和绿辉石的微量元素研究

对9件大别-苏鲁超高压榴辉岩主要矿物石榴子石和绿辉石进行的激光熔样-等离子质谱(LA-ICP-MS)微量元素研究结果表明,所研究的榴辉岩可由石榴子石稀土元素特征分为两类(Group 1和Group 2)Group 1的石榴子石具有一般石榴子石的贫轻稀土、富重稀土的平滑型配分模式,Group 2的石榴子石则表现出独特的高正Eu异常和平坦型重稀土元素的配分模式.可以认为Group 2的原岩属于堆晶辉长岩,而Group 1更可能是正常的玄武岩.矿物颗粒和两种主矿物的主量元素和微量元素分析表明,所研究的榴辉岩达到了矿物规模上的微量元素分配平衡.     

大别山超高压变质带中“脉状”榴辉岩的锆石U-Pb年龄

榴辉岩广泛分布于大别山超高压变质带中,出露规模大小不等,成因类型复杂,围岩类型多样,不仅产于基性、超基性岩中,还产于片麻岩、大理岩、石英硬玉岩、变质花岗岩中。对大别山超高压变质带出露的两处"脉状"榴辉岩进行锆石U Pb年龄测定,分别获得上交点年龄(858±25)Ma和(845±150)Ma,下交点年龄(249±20)Ma和(237±13)Ma,表明其原岩形成时代为新元古代,印支期发生过高压—超高压变质作用,与大别山超高压变质带中其他类型的榴辉岩年龄基本一致。     

苏鲁地体古元古代花岗质片麻岩锆石的U-Pb定年、REE和Lu-Hf同位素特征

苏鲁超高压变质带北部的威海和桃行地区出露大量含黑云母和绿帘石的花岗质片麻岩。激光拉曼测试和阴极发光图像分析结果表明,含黑云母和绿帘石花岗质片麻岩(SLU1和SLU2)的锆石中保存大量矿物包裹体,主要为石英(Qtz)+钾长石(Kfs)+钠长石(Ab)±磷灰石(Ap),与基质中的矿物组合完全一致。阴极发光图像显示典型岩浆结晶环带的特点,稀土元素配分模式显示轻稀土元素明显亏损,而重稀土元素明显富集的特点,具有强烈的负Eu异常(Eu/Eu?鄢=0.03～0.13)。锆石U-Pb SHRIMP和LA-ICP-MS定年结果表明,该类花岗质片麻岩的原岩形成时代为古元古代 (1825±6~1838±3Ma),并经历了三叠纪超高压变质作用(215± 33~227±35Ma) 的改造。Lu-Hf同位素研究结果证明,该类花岗质片麻岩的原岩是中—新太古代古老地壳再活化和部分熔融的产物。上述特征表明,苏鲁超高压变质带中不仅存在与Rodinia超大陆裂解有关的花岗岩,而且也存在古元古代由古老地壳部分熔融而成的花岗岩。     

南阿尔金巴什瓦克榴辉岩的变质演化:对南阿尔金超高压变质作用的启示

高压/超高压变质带中的榴辉岩对于揭示造山带的热演化历史及造山过程具有重要的研究意义.本文通过对南阿尔金巴什瓦克地区榴辉岩的详细岩相学研究,认为该榴辉岩经历了榴辉岩相的峰期变质阶段(M1,946～1026℃和2.9～3.2 GPa)、峰后退变质阶段(M2,硬玉+石英=钠长石反应线附近)以及晚期麻粒岩相-角闪岩相的变质阶段(M3,～0.8 GPa,789～841℃).此外,锆石U-Pb年代学结果表明该榴辉岩的变质时代为492.8±2.8 Ma (MSWD=0.77),结合锆石中的石榴子石、绿辉石、金红石和石英包裹体及前人所获得的年代学数据,认为该变质时代记录了早古生代榴辉岩相的变质事件,进而恢复了南阿尔金榴辉岩所记录的顺时针P-T-t轨迹.本研究为南阿尔金巴什瓦克地区超高压变质作用提供了有效的矿物学与岩石学证据,同时为探讨南阿尔金动力学演化过程提供了重要的约束条件.     

SHRIMP U-Pb zircon dating from Sulu-Dabie dolomitic marble, eastern China: constraints on prograde, ultrahigh-pressure and retrograde metamorphic ages

Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircon from Sulu‐Dabie dolomitic marbles is characterized by distinctive domains of inherited (detrital), prograde, ultrahigh‐pressure (UHP) and retrograde metamorphic growths. The inherited zircon domains are dark‐luminescent in CL images and contain mineral inclusions of Qtz + Cal + Ap. The prograde metamorphic domains are white‐luminescent in CL images and preserve a quartz eclogite facies assemblage of Qtz + Dol + Grt + Omp + Phe + Ap, formed at 542–693 °C and 1.8–2.1 GPa. In contrast, the UHP metamorphic domains are grey‐luminescent in CL images, retain the UHP assemblage of Coe + Grt + Omp + Arg + Mgs + Ap, and record UHP conditions of 739–866 °C and >5.5 GPa. The outermost retrograde rims have dark‐luminescent CL images, and contain low‐P minerals such as calcite, related to the regional amphibolite facies retrogression. Laser ablation ICP‐MS trace‐element data show striking difference between the inherited cores of mostly magmatic origin and zircon domains grown in response to prograde, UHP and retrograde metamorphism. SHRIMP U‐Pb dating on these zoned zircon identified four discrete ²⁰⁶Pb/²³⁸U age groups: 1823–503 Ma is recorded in the inherited (detrital) zircon derived from various Proterozoic protoliths, the prograde domains record the quartz eclogite facies metamorphism at 254–239 Ma, the UHP growth domains occurred at 238–230 Ma, and the late amphibolite facies retrogressive overprint in the outermost rims was restricted to 218–206 Ma. Thus, Proterozoic continental materials of the Yangtze craton were subducted to 55–60 km depth during the Early Triassic and recrystallized at quartz eclogite facies conditions. Then these metamorphic rocks were further subducted to depths of 165–175 km in the Middle Triassic and experienced UHP metamorphism, and finally these UHP metamorphic rocks were exhumed to mid‐crustal levels (about 30 km) in the Late Triassic and overprinted by regional amphibolite facies metamorphism. The subduction and exhumation rates deduced from the SHRIMP data and metamorphic P–T conditions are 9–10 km Myr^?1 and 6.4 km Myr^?1, respectively, and these rapid subduction–exhumation rates may explain the obtained P–T–t path. Such a fast exhumation suggests that Sulu‐Dabie UHP rocks that returned towards crustal depths were driven by buoyant forces, caused as a consequence of slab breakoff at mantle depth.     

U—Pb Dating of Marble Assoicated With Eclogite from the Dabie Mountains,East China

Marble is associated with ultrahigh pressure eclogite in the Dabie Mountains, East China. U-Pb isotope data for the marble define a²³⁸U-²⁰⁶Pb isochron age of 435 ± 45 Ma with an initial²⁰⁶Pb/²⁰⁴Pb value of 18.075 ± 0.006. The age of ∼435 Ma is interpreted to represent the time of limestone diagenesis rather than the time of metamorphic resetting accompanied by the formation of the ultrahigh pressure eclogites at Triassic. The paleontologic and stratigraphic studies also favor a local excursion in the carbon isotopic composition of the latest Ordovician water within a sedimentary basin between the Sino-Korean and Yangtze cratons. Because the latest Ordovician limestone was involved in the eclogite formation, the continental collision between the Yangtze and Sino-Korean plates would postdate Ordovician. This project was financially supported by the National Natural Science Foundation of China and the Chinese Academy of Sciences.     

Petrochemistry, oxygen isotopes and U-Pb SHRIMP geochronology of mafic–ultramafic bodies from the Sulu UHP terrane, China

Two Rongcheng eclogite‐bearing peridotite bodies (Chijiadian and Macaokuang) occur as lenses within the country rock gneiss of the northern Sulu terrane. The Chijiadian ultramafic body consists of garnet lherzolite, whereas the Macaokuang body is mainly meta‐dunite. Both ultramafics are characterized by high MgO contents, low fertile element concentrations and total REE contents, which suggests that they were derived from depleted, residual mantle. High FeO contents, an LREE‐enriched pattern and trace‐element contents indicate that the bulk‐rock compositions of these ultramafic rocks were modified by metasomatism. Oxygen‐isotope compositions of analysed garnet, olivine, clinopyroxene and orthopyroxene from these two ultramafic bodies are between +5.2‰ and +6.2‰ (δ¹⁸O), in the range of typical mantle values (+5.1 to +6.6‰). The eclogite enclosed within the Chijiadian lherzolite shows an LREE‐enriched pattern and was formed by melts derived from variable degrees (0.005–0.05) of partial melting of peridotite. It has higher δ¹⁸O values (+7.6‰ for garnet and +7.7‰ for omphacite) than those of lherzolite. Small O‐isotope fractionations (ΔCpx‐Ol: 0.4‰, ΔCpx‐Grt: 0.1‰, ΔGrt‐Ol: 0.3–0.4‰) in both eclogite and ultramafic rocks suggest isotopic equilibrium at high temperature. The P–T estimates suggest that these rocks experienced subduction‐zone ultrahigh‐pressure (UHP) metamorphism at ～700–800 °C, 5 GPa, with a low geothermal gradient. Zircon from the Macaokuang eclogite contains inclusions of garnet and diopside. The 225 ± 2 Ma U/Pb age obtained from these zircon may date either the prograde conditions just before peak metamorphism or the UHP metamorphic event, and therefore constrains the timing of subduction‐related UHP metamorphism for the Rongcheng mafic–ultramafic bodies.     

大别超高压变质地体的岩石组合及其演化--以皖西南碧溪岭和双河地区为例

碧溪岭和双河地区是大别超高压地体内2个典型的露头区,通过野外露头观察、岩相学和岩石化学研究发现,岩石组合主要为榴辉岩、斜长角闪岩、片麻岩和面理化花岗岩。榴辉岩具有壳源的特点,是地壳岩石在地幔深处发生超高压变质作用形成的;斜长角闪岩为板块折返期间榴辉岩的退变质产物;片麻岩为斜长角闪岩在折返过程中在合适的环境下深熔递进演变的产物;而面理化花岗岩是片麻岩部分熔融的结果。榴辉岩与片麻岩和面理化花岗岩之间为经过改造的原地关系。     

Tracing the protolith, UHP metamorphism, and exhumation ages of orthogneiss from the SW Sulu terrane (eastern China): SHRIMP U–Pb dating of mineral inclusion-bearing zircons

Orthogneisses are the major country rocks hosting eclogites in the Sulu UHP terrane, eastern China. All of the analyzed orthogneiss cores from the main drilling hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) have similar major and trace element compositions and a granite protolith. These rocks have relatively high LREE/HREE ratios, strong negative Eu anomalies (Eu/Eu*=0.20–0.39), and negative Ba anomalies (Ba/Ba*=0.25–0.64). Coesite and coesite-bearing UHP mineral assemblages are common inclusions in zircons separated from orthogneiss, paragneiss, amphibolite, and (retrograded) eclogite of the CCSD-MH. This suggests that the eclogite, together with its country rocks, experienced in situ ultrahigh-pressure (UHP) metamorphism. Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircons from the orthogneisses are zoned and that they have distinct mineral inclusions in the different zones. Most zircons retain early magmatic cores with abundant low-pressure mineral inclusions, which are mantled with metamorphic zircon-containing inclusions of coesite and other UHP minerals. The outermost rims on these grains contain low-pressure mineral inclusions, such as quartz and albite. SHRIMP U–Pb dating of the zoned zircons gives three discrete and meaningful groups of ages: Proterozoic ages for the protolith, 227±2 Ma for the coesite-bearing mantles, and 209±3 Ma for the amphibolite facies retrograde rims. The widespread occurrence of UHP mineral inclusions in zircons from the Sulu metamorphic belt dated at about 227 Ma suggests that voluminous continental crust experienced late Triassic subduction to depths of at least 120 km and perhaps more than 200 km. Eighteen million years later, the terrane was rapidly exhumed to midcrustal levels, and the UHP rocks were overprinted by amphibolite facies metamorphism. The exhumation rate deduced from the zircon age data and previously obtained metamorphic P–T data is estimated to be 5.6–11.0 km/Ma. Such rapid exhumation of the Sulu UHP terrane may be due to the buoyancy forces produced by subduction of low-density continental material into the deep mantle.     

松潘-甘孜地体内花岗岩锆石SHRIMP U-Pb定年及其构造意义

松潘-甘孜地体总体上是一个三角状褶皱带,其北侧、东南侧及东侧分别与东昆仑-西秦岭构造带、金沙江构造带及龙门山构造带相邻。地体内几乎全部被三叠系浊积岩所覆盖,其中侵位了很多花岗岩体。显然,这些花岗岩的岩浆特征、来源与侵位时代,对研究松潘-甘孜地体基底性质、构造演化等问题具有重要意义。本文报道利用SHRIMP定年技术对一些花岗岩岩体锆石所进行的精确测年数据。测试结果表明:(1)松潘-甘孜地体内的花岗岩体主要形成于晚三叠世,但岩浆活动可延续到早侏罗世晚期;(2)早期花岗岩浆活动与三叠纪系褶皱变形大致同期,指示这一时期的花岗岩浆的形成可能与三叠系下部大型拆离滑脱构造相关;(3)花岗岩结晶锆石普遍具有浑圆状或不规则状较老的继承锆石核,这些核部继承锆石可能代表花岗岩浆在上升过程中从不同地层内捕获的碎屑锆石,或者是下地壳岩石深熔残留锆石。根据SHRING U-Pb定年,这些继承锆石的年龄分别为二叠纪、加里东期、晚元古代和早元古代。元古代碎屑锆石的存在也可能指示松潘-甘孜地体具前寒武系基底。     

北苏鲁威海地区超基性岩的原岩形成时代和超高压变质时代

超基性岩是苏鲁超高压变质地体中一类特殊且十分重要的岩石类型,它们通常呈规模不一的块状、条带状或不规则透镜状 (体) 赋存于区域大面积出露的花岗质片麻岩中。锆石中矿物包体激光拉曼测试、阴极发光图像分析和不同性质锆石LA-ICP-MS U-Pb定年等综合研究结果表明,北苏鲁威海地区含橄榄石辉石岩 (样品W1和W2) 中锆石的成因十分复杂,可进一步划分3种不同类型锆石。其中第一类锆石呈自形-半自形晶,阴极发光图像显示清晰的岩浆结晶环带,矿物包体主要为Ol+Cpx+Ap, 记录的²⁰⁷Pb/²⁰⁶Pb年龄为1835~1845Ma,应代表含橄榄石辉石岩的原岩形成时代;第二类为变质重结晶锆石,呈半自形-他形晶,阴极发光图像显示模糊的岩浆结晶环带,矿物包体与第一类完全一致,记录的²⁰⁶Pb/²³⁸U年龄变化范围大,为250~784Ma之间,表明部分继承性岩浆结晶锆石明显受到后期岩浆-变质热事件的影响而发生不完全重结晶和Pb丢失,进而使其记录的年龄相对偏新;第三类锆石呈他形晶,为典型的变质锆石,阴极发光图像十分均匀,矿物包体相对少见,主要为Grt+Cpx,记录的²⁰⁶Pb/²³⁸U年龄为230~234Ma, 且与苏鲁地体榴辉岩及其围岩中含柯石英锆石微区记录的超高压变质年龄 (225~235Ma) 十分一致,应代表含橄榄石辉石岩的峰期超高压变质时代。超基性岩中超高压变质锆石的准确识别表明苏鲁地体在峰期超高压变质阶段的确存在流体,流体的存在对超高压变质锆石的形成起着至关重要的作用。该项研究不仅准确厘定北苏鲁威海地区超基性岩的原岩形成时代和超高压变质时代,而且对于深入探讨苏鲁-大别超高压地体流体行为、演化规律及其水-岩相互作用机理具有重要的科学意义。