中国大陆科学钻探主孔0～4500米变质岩石锆石中保存的超高压矿物包体

中国大陆科学钻探主孔0-4500米的岩心主要由榴辉岩、斜长角闪岩、副片麻岩、正片麻岩以及少量的超基性岩所组成。岩相学研究结果表明,榴辉岩的围岩普遍经历了强烈角闪岩相退变质作用的改造,峰期超高压变质的矿物组合已完全被后期退变质过程中角闪岩相矿物组合所替代。采用激光拉曼技术,配备电子探针和阴极发光测试,发现主孔224件岩心中有121件(包括榴辉岩、斜长角闪岩、副片麻岩和正片麻岩)样品的锆石中普遍隐藏以柯石英为代表的超高压矿物包体,且不同岩石类型锆石中所保存的超高压矿物包体组合存在明显差异。(含多硅白云母)金红石石英榴辉岩锆石中保存的典型超高压包体矿物组合为柯石英 石榴石、柯石英 石榴石 绿辉石 金红石和柯石英 多硅白云母 磷灰石。黑云绿帘斜长角闪岩锆石中保存的超高压矿物组合为柯石英 石榴石 绿辉石、柯石英 石榴石 多硅白云母和柯石英 绿辉石 金红石,与榴辉岩所保存的超高压矿物组合十分相似,表明该类斜长角闪岩是由超高压榴辉岩在构造折返过程中退变质而成。在副片麻岩类岩石,如石榴绿帘黑云二长片麻岩锆石中,代表性的超高压包体矿物组合为柯石英 多硅白云母和柯石英 石榴石等;而在石榴黑云角闪钠长片麻岩锆石中,则保存柯石英 硬玉 石榴石 磷灰石、柯石英 硬玉 多硅白云母 磷灰石和柯石英 石榴石 磷灰石等超高压矿物包体。在正片麻岩锆石中,标志性的超高压矿物包体为柯石英、柯石英 多硅白云母、柯石英 蓝晶石 磷灰石和柯石英 蓝晶石 榍石等。此外,在南苏鲁东海至临沭一带的地表露头以及一系列卫星孔岩心的锆石中,也普遍发现以柯石英为代表的标志性超高压矿物包体,表明在南苏鲁地区由榴辉岩及其围岩的原岩所组成的巨量陆壳物质(方圆>5000km2,厚度超过4.5km)曾整体发生深俯冲,并经历了超高压变质作用。该项研究对于重塑苏鲁-大别超高压变质带俯冲-折返的动力学模式有着重要的科学意义。     

隐藏在苏鲁地体斜长角闪岩锆石中超高压变质作用的信息

运用激光拉曼和阴极发光技术，配备电子探针测试，发现苏鲁地体地表露头和中国大陆科学钻探工程预先导孔CCSD-PP1和CCSD-PP2斜长角闪岩锆石中均保存以柯石英为代表的典型超高压矿物组合：柯石英 石榴石 绿辉石 金红石；柯石英 石榴石 绿辉石；柯石英 石榴石 绿辉石 多硅白云母 金红石 磷灰石；柯石英 绿辉石 金红石；柯石英 菱镁矿。该类矿物包体组合与苏鲁地体超高压榴辉岩的峰期矿物组合十分相似，表明斜长角闪岩可能是由超高压变质的榴辉岩在构造抬升过程中退变质而成。     

华中榴辉岩及其成因的若干问题

榴辉岩是华中高压变质带的特征岩类之一。根据其共生的矿物组合可划分出四类榴辉岩:角闪(蓝闪)榴辉岩,蓝晶石榴辉岩、柯石英榴辉岩和石英榴辉岩。其中蓝闪榴辉岩主要分布于团麻断裂以西地区;柯石英榴辉岩均出露于团麻断裂以东地区。根据矿物对地质温压计和变质矿物组合估测,西部地段榴辉岩的形成条件为p=1.1～1.5GPa,t=540～600℃,中东部地段榴辉岩为p≥2.5GPa,t=700～800℃。 根据岩石组合、矿物组合和构造环境分析,绝大多数(如果不是全部的话)榴辉岩与围岩经历了相同的高压—超高压变质作用,是同一构造运动旋回中不同构造部位的变质产物。蓝闪榴辉岩和柯石英榴辉岩的地质环境和退变质作用特征具有重要的大地构造意义。     

北秦岭官坡地区高压—超高压榴辉岩岩相学及变质作用研究

北秦岭官坡地区的榴辉岩及含柯石英榴辉岩产在帮岭岩群的北侧，主要由绿辉石和石榴石组成，部分石榴石和绿辉石中含柯石英包体。此外还含有退变质的多硅白云母、角闪石、黝帘石和纳长石等矿物，根据变质矿物之间的替代关系及共生组合规律，榴辉岩退变质作用可划分为四个阶段，各阶段代表性矿物组合依次为：柯石英＋绿辉石＋石榴石；石英＋绿辉石＋石榴石；多硅白云母＋绿辉石＋石榴石＋石英；角闪石＋斜长石＋白云母＋黑云母。这四个     

大别山超高压榴辉岩带榴辉岩的特征和变质作用

大别山超高压柯石英榴辉岩带中多处产出含柯石英榴辉岩,岩石中广泛分布柯石英假象。它们产在片麻岩和大理岩中,并有不同的特征矿物组合。与鄂北等地高压型榴辉岩比较,超高压(柯石英)榴辉岩的绿辉石富硬玉组分,而石榴石为一般的富铁铝榴石贫镁铝榴石的石榴石。钙质角闪石和钠钙质冻蓝闪石是超高压(柯石英)榴辉岩中主要的次生角闪石类型。榴辉岩的原岩是陆内拉张过程中形成的广义拉斑玄武岩,它们在中朝一扬子大陆板块汇聚造山的俯冲大地构造背景中,发生渐进变质。变质时地热增温率极低。造山运动后期,榴辉岩随构造运动抬升又经历两阶段迭加变质。     

河南大别山区榴辉岩及蓝晶石石英岩中柯石英的发现

团麻断裂以西的河南大别山区榴辉岩及其围岩蓝晶石石英岩中首次发现柯石英。含柯石英榴辉岩的矿物共生组合为石榴石+绿辉石+蓝晶石+多硅白云母+石英(柯石英)+金红石,其形成温压条件为T=610～740℃,P≥2.8 GPa。含柯石英榴辉岩及其围岩同时经历了超高压变质作用,可能由扬子板块向华北板块深俯冲作用形成,俯冲深度可迭90km以上。河南大别山区含柯石英榴辉岩的发现扩大了中国中东部含柯石英榴辉岩带的分布区。     

滇西双江县勐库地区(退变)榴辉岩的岩石学、矿物学特征

滇西双江县勐库镇以北的那卡河、控角等地出露的(退变)榴辉岩以构造岩片的形式产于奥陶纪湾河蛇绿岩混杂岩带中。根据退变质程度差异依次划分为退变榴辉岩、榴闪岩、含石榴斜长角闪岩,主要矿物成分为角闪石、石榴子石、单斜辉石、斜长石,次要矿物成分为金红石、多硅白云母、硬玉、绿帘石、黑云母、磷灰石、石英等,特征的高压-超高压变质矿物绿辉石仅残余在少量的石榴子石、角闪石中,以包裹体形式产出。岩石学和矿物学研究显示,该榴辉岩峰期变质矿物组合为:绿辉石+石榴子石+金红石+石英+多硅白云母。石榴子石中的部分石英包裹体周围发育放射状的胀裂纹,暗示其可能由柯石英转化而成。由此推断,该榴辉岩可能经历了超高压变质作用。     

鲁东造山带榴辉岩变质作用特征及其动力学机制

岩相学研究表明,广泛发育鲁东造山逞中的榴辉岩可以划分为四种类型：第一类为含柯石英及其假象榴辉岩;么二类为含蓝晶石 、黝帘石及多硅白云母等榴辉岩;第三类为石榴石－绿辉石－石英组合榴辉岩;第四类为（角闪石）石榴石－辉及有关岩石（非典型“榴辉岩”）。这四类榴辉岩峰期前进变质矿物共生组合及Ｐ－Ｔ条件估算结果均显示它们应属于高温与超高压变质作用产物,其变质作用ＰＴｔ轨迹表现为顺时针演化趋势,反映出板块俯冲碰     

含柯石英超高压榴辉岩的早期变形--以苏北东海碱场榴辉岩体为例

以超高压矿物组合的各种后成合晶及冠状体等卸载不平衡结构为参考标志，将含柯石英的超高压榴辉岩的交形序列分成两个部分。后成合晶及冠状体发育之前的变形为早期变形，是在大陆深俯冲和碰撞条件下发育的超高压变质变形组构。后成合晶及冠状体发育之后的变形为晚期变形，是在超高压岩石折返剥露过程中，主要是在角闪岩相甚至绿片岩相条件下发育的。构造上江苏省北部东海县碱场合柯石英榴辉岩体，分为块状榴辉岩和面理化榴辉岩两种类型，分别代表超高压变质岩早期变形的两个构造阶段或世代(D1、D2)。详细描述了它们的矿物组合、中小尺度及显微尺度下的组构特征，讨论了两者的几何关系和区域构造意义，强调指出，只有含柯石英榴辉岩的早期变形组构，才能记录和反映斜向大陆深俯冲及碰撞的动力学过程。     

柴达木盆地北缘鱼卡河含柯石英榴辉岩的确定及其意义

在柴达木盆地北缘鱼卡河边达肯大坂杂岩之花岗质片麻岩中发现典型的含柯石英榴辉岩。榴辉岩主要由石榴石、绿辉石和少量（多硅）白云母、柯石英和石英、角闪石、金红石等矿物组成。石榴石中铁铝、镁铝和钙铝榴石分子含量分别为５１％～５９％、２６％～３１％和１３％～１９％;绿辉石中硬玉分子含量为４５％～４８％;岩石中残留有ｂ０值极大的高压矿物多硅白云母（ｂ０＝９０７５×１０－１ｎｍ）;角闪石亦为高压类型的冻蓝闪石;最为重要的是确定了柯石英的存在。榴辉岩原始特征保存完好,仅遭受轻微的退变质作用和叠加变质作用。组成矿物可分为３个世代：（１）峰期矿物组合：石榴石＋绿辉石＋（多硅）白云母＋（柯）石英＋金红石;（２）退变交生组合：冻蓝闪石＋蠕虫状石英;（３）后期叠加变质矿物白云母。榴辉岩相变质作用发生在压力大于２８ＧＰａ的超高压至大约７３０℃、１７０ＧＰａ的高压环境,表明柴达木盆地北缘是一条重要的古板块汇聚边界。     

Multi-stage reaction history in different eclogite types from the Pakistan Himalaya and implications for exhumation processes

Metabasites were sampled from rock series of the subducted margin of the Indian Plate, the so-called Higher Himalayan Crystalline, in the Upper Kaghan Valley, Pakistan. These vary from corona dolerites, cropping out around Saif-ul-Muluk in the south, to coesite–eclogite close to the suture zone against rocks of the Kohistan arc in the north. Bulk rock major- and trace- element chemistry reveals essentially a single protolith as the source for five different eclogite types, which differ in fabric, modal mineralogy as well as in mineral chemistry. The study of newly-collected samples reveals coesite (confirmed by in situ Raman spectroscopy) in both garnet and omphacite. All eclogites show growth of amphiboles during exhumation. Within some coesite-bearing eclogites the presence of glaucophane cores to barroisite is noted whereas in most samples porphyroblastic sodic–calcic amphiboles are rimmed by more aluminous calcic amphibole (pargasite, tschermakite, and edenite). Eclogite facies rutile is replaced by ilmenite which itself is commonly surrounded by titanite. In addition, some eclogite bodies show leucocratic segregations containing phengite, quartz, zoisite and/or kyanite. The important implication is that the complex exhumation path shows stages of initial cooling during decompression (formation of glaucophane) followed by reheating: a very similar situation to that reported for the coesite-bearing eclogite series of the Tso Morari massif, India, 450 km to the south-east.     

Paragonite eclogites from Dabie Shan,China: re-equilibration during exhumation?

Abstract Paragonite in textural equilibrium with garnet, omphacite and kyanite is found in two eclogites in the ultrahigh-pressure metamorphic terrane in Dabie Shan, China. Equilibrium reactions between paragonite, omphacite and kyanite indicate a pressure of about 19 kbar at c . 700° C. However, one of the paragonite eclogites also contains clear quartz pseudomorphs after coesite as inclusions in garnet, suggesting minimum pressures of 27 kbar at the same temperature. The disparate pressure estimates from the same rock suggest that the matrix minerals in the ultrahigh-pressure eclogites have recrystallized at lower pressures and do not represent the peak ultrahigh-pressure assemblages. This hypothesis is tested by calibrating a garnet + zoisite/clinozoisite + kyanite + quartz/coesite geobarometer and applying it to the appropriate eclogite facies rocks from ultrahigh- and high-pressure terranes. These four minerals coexist from 10 to 60 kbar and in this wide pressure range the grossular content of garnet reflects the equilibrium pressure on the basis of the reaction zoisite/clinozoisite = grossular + kyanite + quartz/coesite + H₂O. The results of the geobarometer agree well with independent pressure estimates from eclogites from other orogenic belts. For the paragonite eclogites in Dabie Shan the geobarometer indicates pressures in the quartz stability field, confirming that the former coesite-bearing paragonite-eclogite has re-equilibrated at lower pressures. On the other hand, garnets from other coesite-bearing but paragonite-free kyanite-zoisite eclogites show a very wide variation in grossular content, corresponding to a pressure variation from coesite into the quartz field. This wide variation, partly due to a rimward decrease in grossular component in garnet, is caused by partial equilibration of the mineral assemblage during the exhumation.     

超高压变质作用中水的相变增压

据静水压力梯度计算, 榴辉岩形成深度至少要70km, 含柯石英榴辉岩的形成深度至少要12 0km, 而同时含有柯石英和金刚石的榴辉岩形成深度至少要145km, 超高压变质岩要从如此深的部位折返到地表是不可想象的.大量资料表明水参与了超高压变质作用过程.通过矿物的脱水行为、水的相变和地温曲线等研究, 指出由于存在水的相变增压等多种增压因素, 大大提高了超高压变质作用过程中的地压梯度, 据静水压力计算出的超高压变质岩形成深度只是最大深度, 其形成时的实际深度要比该最大深度小得多.      

大别山五庙超高压变质岩的变形构造

含柯石英榴辉岩、高压片麻岩的五庙超高压变质岩与外围低压片麻岩以逆冲断层呈构造侵位关系,而在其内部榴辉岩与高压片麻岩为非构造接触,并且发育有榴辉岩相静态重结晶构造以及在纯剪应力状态下的前角闪岩相变形构造Ｄ１和在顶端指向ＮＮＷ运动的单剪应力状态下的角闪岩相变形构造Ｄ２。     

Petrological and Geochemical Characteristics of Ultrahigh-Pressure Metamorphic Rocks from the Dabie-Sulu Terrane,East-Central China

The Dabie-Sulu ultrahigh-pressure (UHP) terrane is the largest in the world. Mafic-ultramafic rocks occur as ubiquitous minor components in it, and preserve UHP index minerals such as coesite and microdiamond. Eclogites and garnet peridotites together with their country rocks including quartzofeldspathic, pelitic, psammatic, and carbonate rocks were subjected to coeval UHP meta-morphism during the Triassic collision between the Sino-Korean and Yangtze cratons. This review summarizes petrological and geochemical characteristics of eclogites, garnet peridotites, and UHP metasediments from published data and ongoing research in the Dabie-Sulu belt of east-central China.

Parageneses of minerals from coesite-bearing eclogites encased in gneiss, garnet peridotite, and marble delineate clockwise P-T paths characterized by nearly isothermal decompression. Many eclogites have been overprinted by Barrovian-zone amphibolite- to granulite-facies assemblages, whereas others preserve primary igneous minerals and textures in the cores of coesite-bearing eclogitic blocks. In addition, eclogites contain UHP hydrous and carbonate phases including talc, epidote, zoisite, magnesite, and dolomite; these together with Ti-clinohumite, phlogopite, amphibole, chlorite, and possible talc in garnet peridotites and OH-topaz in kyanite quartzites document the role of hydrous mineral transport to the deep mantle in fluid-deficient UHP metamorphic regions. Both crustal- and mantle-derived garnet peridotites from the Dabie-Sulu region were recrystallized within the diamond stability field, with the Earth's lowest recorded geothermal gradient. <5°C/km, suggesting that UHP metamorphic recrystallization took place in a previously unrecognized, forbidden P-T region.

Geochemical and isotopic data indicate that Dabie-Sulu mafic-ultramafic rocks have diverse origins; their compositions in some cases have been complicated by metamorphic recrystallization, crustal contamination, and fluid metasomatism. Nevertheless, REE geochemical and Nd isotope data clearly indicate that they have “continental” affinities and cannot represent a subducted Tethyan oceanic slab. Garnet peridotites and their enclosing eclogites display variable isotopic compositions; mantle-derived fragments preserve a mantle signature, whereas crust-hosted mafic-ultramafics display distinct crustal contamination and metasomatism. Among the many outstanding projects remaining to be investigated, geochemical and isotopic constraints of mantle-derived garnet peridotites and eclogites should provide an additional window to our understanding of mantle heterogeneity, metasomatism, slab/mantle interactions, and lithospheric evolution of the Sino-Korean craton.     

大别山北部榴辉岩的Sm-Nd年龄测定及其对麻粒相退变质时间的制约

报道了大别山北部三个榴辉岩样品的矿物 Sm- Nd等时线年龄，它们分别为 (210± 6) Ma或 (214± 6) Ma、 (208± 38) Ma和 (208± 4) Ma。氧同位素研究表明，这些样品中的石榴子石与绿辉石之间处于氧同位素平衡状态，因此，该 Sm- Nd等时线定年结果可靠。本区榴辉岩的高压麻粒岩相退变质阶段的冷却年龄为 210 Ma左右；榴辉岩的钕同位素初始比ε Nd(t)(两个样品一个为－ 10左右，另一个为－ 2)基本上表现为陆壳岩石特征，可能类似于南部超高压带中的榴辉岩，为印支期扬子陆壳俯冲变质成因。它们的全岩δ 18O值较低，为＋ 2.4‰～＋ 3.6‰，可能指示其原岩同大别山南部超高压带中榴辉岩一样，在板块俯冲之前，经受过高温地表水热液蚀变。年代学结果表明，大别山北部榴辉岩在 230～ 210 Ma期间经历的是一等温或升温过程，这与大别山南部含柯石英榴辉岩在这一时期的快速冷却过程形成强烈对比，这对理解俯冲陆壳中不同构造岩片折返过程的差异有重要意义。     

Petrology and geochronology of ultrahigh-pressure granitic gneiss from South Dulan,North Qaidam belt,NW China

abstract

An integrated study including petrography, mineral chemistry, metamorphic P–T path modelling, and zircon U–Pb dating was conducted on a granitic gneiss and enclosed eclogite from South Dulan, North Qaidam UHP (ultrahigh-pressure) belt. The result shows that the granitic gneiss underwent a clockwise P–T path with a peak-P stage at 655–745°C, 30–34 kbar, and a subsequent peak-T stage at 815–870°C, 14–18 kbar, which is similar to the P–T estimates reported for coesite-bearing continental-type eclogites in this region. The enclosed eclogite resembles an olivine–pyroxene-rich cumulate in Qaidam block. It has a similar prograde P–T path with the country gneiss and experienced a peak-P stage of 682–748°C at 27–34 kbar. Zircon U–Pb dating yields an eclogite-facies metamorphic age of 447 ± 2 Ma for the granitic gneiss and 445 ± 6 Ma for the enclosed eclogite. These ages agree with metamorphic ages obtained from paragneisses (427–439 Ma), coesite-bearing continental-type eclogites (430–451 Ma), and UHPM (ultrahigh-pressure metamorphic) oceanic crust–mantle sequence (440–445 Ma) from South Dulan, as well as UHP eclogites, garnet peridotite, and gneisses from other units (460–420 Ma) within this belt reported by others. Similar metamorphic ages as well as P–T evolution documented in gneisses and intercalated eclogites imply that both rocks experienced a coeval UHP event. Summarizing all the published geochronology data, we argue that the North Qaidam UHP belt was mainly formed by continental deep subduction at ~460 to ~420 Ma. The UHPM oceanic crust-mantle sequence in South Dulan may represent oceanic lithosphere in the transition zone between oceanic and continental crust, which was dragged upward by the exhumed continental rocks after break-off of the dense oceanic crust.     

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.