蛇绿岩套中超基性岩体的岩石组合:蛇纹岩、异剥钙榴岩和蛇绿碳酸岩——以西阿尔卑斯Zermatt-Saas蛇绿岩为例

蛇纹岩、异剥钙榴岩和蛇绿碳酸岩是蛇绿岩套中超基性单元特有的3类岩石组合,该套岩石组合的形成过程复杂,经历了从地幔岩浆结晶分异、洋脊变质作用改造和俯冲-仰冲构造过程,记录了从地幔岩浆侵位到造山带形成、演化的全程信息。蛇纹岩由方辉橄榄岩、二辉橄榄岩和纯橄岩通过水化和氧化过程而形成;异剥钙榴岩由含水石榴石、符山石、绿帘石族矿物、透辉石和绿泥石等含水和含钙的硅酸盐矿物组成,是由基性岩经历钙交代和水化作用而形成;蛇绿碳酸岩则由高度破碎变形的蛇纹岩角砾和碳酸岩基质(方解石、白云石或菱镁矿)共同组成,碳酸钙主要来自海水参与蛇纹岩化过程产生的富钙热液。阿尔卑斯西部的Zermatt-Saas蛇绿岩体中这3种岩石的组合研究表明:蛇纹岩化过程发生在大洋变质时期,超基性岩体在海水的作用下形成蛇纹岩。蛇纹岩化过程中释放出主要来自斜方辉石和单斜辉石的钙,与水共同作用交代超基性岩体中的基性岩脉,从而形成异剥钙榴岩。蛇绿碳酸岩形成于俯冲变质之前或俯冲变质的早期。这3类岩石一经形成,都经历了其后的叠加变质作用,进而表明Zermatt-Saas蛇绿岩经历了大洋变质、与俯冲、折返和抬升有关的高压变质和区域变质、绿片岩相变质和晚期热液变质作用的pT轨迹演化,代表着西阿尔卑斯从洋脊变质作用到俯?     

缅甸硬玉岩区的硬玉化绿辉石岩

绿辉石岩是缅甸硬玉岩区一种新的岩石类型,普遍遭受多期次硬玉化.未发生硬玉化的绿辉石含较高TFeO和CaO,但较低的MgO,按化学成分分类部分已属霓石-普通辉石系列的中间相.这种绿辉石很可能是硬玉质的流体/熔体交代地幔辉石岩类的产物,与主期大规模硬玉岩的结晶、钠质-钠钙质角闪石边和钠铬辉石的形成同期或稍后,是该区第Ⅰ期的硬玉化.沿绿辉石的解理或裂隙交代并伴有充填的硬玉是在相对拉张的构造背景下形成的,是该区第Ⅱ期的硬玉化.切割早期绿辉石和硬玉的充填硬玉细脉可能是最晚一期的硬玉化.硬玉化绿辉石岩的结构与显微构造特征指示了在缅甸硬玉岩区,至少存在不少于3期的硬玉化的交代充填作用.文中讨论了硬玉化物质的可能来源,认为其可能与俯冲板片上的沉积物有密切关系.     

缅甸硬玉岩地区的热液型钠长石岩

产于俯冲带内的低温高压带的由单矿物构成的硬玉岩通常伴有钠长石岩,目前对于硬玉岩研究的关注度较高,而对于钠长石岩则相对较低,很少有相关论文报导.产于缅甸翡翠矿区的钠长石岩,经常与硬玉岩相伴而生,是良好的研究样品.钠长石岩的主要矿物成分是低温钠长石,其次含有硬玉、绿辉石、透辉石等辉石类矿物和钠透闪石、蓝透闪石、镁钠闪石等闪石类矿物,此外还有钠沸石等.钠长石沿着解理和裂隙交代硬玉,说明钠长石形成晚于硬玉岩.钠长岩中的主要组成矿物钠长石的形成温度小于300℃,且其形成压力小于0.5kb,推测是在硬玉岩抬升程中通过交代与沉淀作用形成.其内的透辉石有两种类型,一类可能是被交代的硬玉中的透辉石组分会渐进增加,最终形成透辉石.另一类是被绿辉石包裹的透辉石残留,其很有可能是早期来自地幔楔或者俯冲带岩石中的矿物残留,即异剥钙榴岩或辉石岩类,可以视作硬玉化绿辉石岩和硬玉化异剥钙榴岩的矿物学证据.热液型钠长石岩的存在进一步说明缅甸翡翠矿区钠化热液存在现象的普遍性与穿越性.     

金沙江蛇绿岩带蛇纹岩中异剥钙榴岩的成因及意义

金沙江蛇绿岩带绒角蛇绿岩由超镁铁岩与异剥辉长岩岩脉组成,在蛇绿岩就位过程中,异剥辉长岩岩脉碎裂为不规则的岩块包裹于超镁铁岩中,形成超镁铁岩中的构造包体。经过交代蚀变作用,超镁铁岩形成蛇纹岩,异剥辉长岩形成异剥钙榴岩。异剥钙榴岩的形成是超镁铁岩蛇纹石化和异剥辉长岩中斜长石钙铝榴石化循环作用的结果,并与对蛇绿岩起过作用的构造运动有关系,是变形作用伴随交代作用的产物。绒角蛇绿岩中的异剥钙榴岩的发现,说明金沙江蛇绿岩带不存在榴辉岩,其变质相属葡萄石一绿泥石低温低压相．也说明金沙江蛇绿岩带是洋壳残片“冷侵入”的产物。     

霍山地区混合岩与混合岩化作用研究

通过对1:25万临汾市幅区调混合岩与混合岩化作用的研究,认为区内混合岩化主要发育于不同构造阶段形成的片麻岩中。具代表性的岩石,按构造形态分类有:条痕—条带状混合岩化变质岩,囊团—脉状混合岩化变质岩,眼球状混合岩化变质岩;按混合岩化程度差别分类有:混合岩化变质岩,混合岩;按混合岩化的成因分类有:注入混合岩,变质交代混合岩,变质分异混合岩,深熔混合岩。     

俄罗斯极地乌拉尔Сыум-Кеу超基性岩体中的硬玉岩

俄罗斯极地乌拉尔Сыум-Кеу超基性岩体中的硬玉岩呈脉状在以叶蛇纹石为主的蛇纹岩中产出,硬玉岩由硬玉和绿辉石组成,根据硬玉岩的颜色、结构和构造可划分出三个世代,可能对应存在三期硬玉化过程。第一世代硬玉为主体,灰白色,粗粒结构,致密块状,硬玉分子(Jd)含量54%～88%;第二世代硬玉发育在灰白色硬玉中,呈浅绿色,细粒-隐晶结构,细脉状-囊状,硬玉分子(Jd)含量74%～86%;第三世代硬玉呈绿色-深绿色,半透明-透明,中-细粒结构,瘤状。第二、三世代硬玉达到珠宝首饰级,具有较高的商业价值。根据硬玉岩的产状和晶体具有韵律生长环带及流体包裹体发育等特征,认为硬玉岩是在高压低温环境中由富含Na、Al、Si的流体直接结晶形成的。     

缅甸硬玉岩中的流体包裹体

缅甸硬玉岩是世界上最大和最重要的玉石矿床之一,位于印度板块和欧亚板块之间的新特提斯洋缝合带中。研究表明,缅甸硬玉岩是新特提斯洋壳俯冲过程中橄榄岩经高压变质、交代作用形成的。对不同变质程度缅甸硬玉岩样品中的流体包裹体的研究表明,缅甸硬玉岩中含有4种类型的流体包裹体:1不含或含少量甲烷的低盐度水溶液包裹体(Ⅰ型),呈孤立状或小群(簇状)产于硬玉晶体核部,或沿着硬玉晶体的生长环带分布,具有原生生长结构;2含石盐子晶的H2O+Na Cl±CH4三相包裹体(Ⅱ型);3纯甲烷(CH4)包裹体(Ⅲ型),可以细分为高密度(Ⅲa)和低密度(Ⅲb)两种;4气相或空包裹体(Ⅳ型)。研究表明,缅甸硬玉岩及其相关岩石在形成和演化过程中发生了多期次流体交代事件。硬玉形成过程中,交代橄榄岩的流体相可能来自海水。首次在缅甸硬玉岩中识别出高盐度的含水包裹体和高密度的含CH4包裹体。高盐度的含水包裹体可能与硬玉岩重结晶过程相关,而高密度的CH4流体可能为俯冲板片的上地幔楔中超基性岩蛇纹石化过程的副产物。计算的流体包裹体等容线表明,硬玉岩演化过程中这些流体包裹体发生了不同程度的再平衡。     

俄罗斯极地乌拉尔Сыум-Key超基性岩体中的硬玉岩

俄罗斯极地乌拉尔Сыум-Key超基性岩体中的硬玉岩呈脉状在以叶蛇纹石为主的蛇纹岩中产出,硬玉岩由硬玉和绿辉石组成,根据硬玉岩的颜色、结构和构造可划分出三个世代,可能对应存在三期硬玉化过程.第一世代硬玉为主体,灰白色,粗粒结构,致密块状,硬玉分子(Jd)含量54%～88%;第二世代硬玉发育在灰白色硬玉中,呈浅绿色,细粒-隐晶结构,细脉状.囊状,硬玉分子(Jd)含量74%～86%;第三世代硬玉呈绿色-深绿色,半透明-透明,中-细粒结构,瘤状.第二、三世代硬玉达到珠宝首饰级,具有较高的商业价值.根据硬玉岩的产状和晶体具有韵律生长环带及流体包裹体发育等特征,认为硬玉岩是在高压低温环境中由富含Na、Al、Si的流体直接结晶形成的.     

滇西点苍山杂岩中混合岩的地质年代学分析及其区域构造内涵

位于红河-哀牢山断裂带北延的点苍山杂岩由中部深变质杂岩、西侧中生界浅变质-未变质的火山-沉积岩系和东部叠加退变质岩带组成.中部深变质杂岩包括类片麻岩-长英质岩类、云母片岩、大理岩类和角闪质岩石.岩石普遍遭受角闪岩相变质作用和混合岩化作用的改造,形成条带状、条纹状、条痕状和角砾状等具有不同构造特点的混合岩.混合岩中有两种不同的脉体物质(花岗质岩脉),它们含有具不同特点的锆石颗粒,即由混合岩化作用改造形成的低Th/U比值锆石颗粒和由岩浆结晶作用形成的高Th/U比值锆石颗粒.对于锆石颗粒采用SHRIMP U-Pb测年,揭示出二者具有相近的年龄值,分别为842.5±9.9Ma与833±9Ma.它们的存在揭示出从混合岩化作用向岩浆作用的转变.混合岩的变形作用、变质作用特点分析,尤其是对于花岗质脉体的锆石年代学研究揭示出,点苍山深变质杂岩与扬子地块基底具有显著的相似性.结合扬子地块新元古代整体构造-岩浆活动性规律,揭示出由板块俯冲与消减引起,早于新元古代Rodinia裂解时期地幔柱上涌事件.     

危地马拉硬玉岩与硬玉化绿辉石岩的岩石学特征及其地质意义

位于北美-加勒比板块俯冲带内的危地马拉硬玉岩区产有硬玉岩、榴辉岩、钠长岩等岩石类型,但绿辉石岩还没有详细报导过。研究样品为该区的硬玉岩和绿辉石岩,前者主要由Jd端元含量较高的硬玉组成,具有粒、柱状镶嵌结构。硬玉晶体具有显著的成分振荡环带,其核部及其背散射电子图像的浅色区Jd端元含量为94.81%～95.48%;暗色区Jd含量大于97.92%。后者绿辉石岩具有交代结构,主要由绿辉石和硬玉组成。其中绿辉石的CaO(9.01%～10.80%)和MgO(6.09%～7.94%)含量较高,FeO(2.84%～4.89%)含量较低;硬玉的CaO、MgO和FeO含量变化范围较大,分别为0.59%～4.30%,0.26%～3.05%,0.76%～2.87%。硬玉岩中流体包裹体的存在,以及绿辉石岩中显著的硬玉交代绿辉石现象说明其成因与缅甸硬玉一样,是硬玉质流体通过结晶-交代作用形成。硬玉岩中硬玉振荡的环带结构反映形成时温度-压力-组分体系存在振荡变化,平直连续的环带边界反映结晶时P-T条件位于硬玉稳定的低温高压区域。该区绿辉石岩的硬玉化作用清楚,能够观察到三期硬玉岩化作用现象,说明绿辉石岩可能是在硬玉岩的形成过程中,硬玉质流体与原岩辉石岩作用的结果,这进一步阐明了俯冲带内硬玉岩区硬玉质流体的广泛透入性与结晶交代作用的多样性。     

Element residence and transport during subduction-zone metasomatism: evidence from a jadeitite-serpentinite contact,Guatemala

Jadeitite is a rare constituent of serpentinite-matrix mélange bodies from certain subduction complexes. Most jadeitite crystallizes from Na-, Al-, and Si-bearing fluids that are apparently derived from multiple subduction-zone sources. Even though jadeitite is near-end-member NaAlSi₂O₆ in major element composition and is volumetrically minor in subduction complexes, its trace elements and stable isotopes appear to record fluid compositions not directly seen in other subduction zone metasomatic systems.

Prior to our work, how jadeitite-forming fluids interact with serpentinite host rocks and serpentinizing fluids were largely unknown, because serpentinite-to-jadeitite contacts are generally not exposed. In the Sierra de las Minas, Guatemala, we have studied a 3 m-wide pit transecting the contact between a mined-out jadeitite body and its host serpentinite. An apparent transition zone between the former jadeitite and nearby serpentinite exposed in the mine pit contains four texturally distinct rock types of differing outcrop colours, composed of albitites and meta-ultramafic rocks. (The jadeitite body is now represented only by a large spoil pile.) Seven samples from the contact zone, jadeitite from the spoil pile, a serpentinite outcrop approximately 1 m outside the pit, and a jadeitite nodule within the contact zone albitite were analysed for major, minor, and trace elements.

Abundances of Al₂O₃, Na₂O, MgO, FeO, Cr, Ni, and Sc track the contact between sheared albitite and foliated meta-ultramafic rocks. These elements change from values typical of Guatemalan jadeitites in the jadeitite block and albitites in the contact zone to values for Guatemalan meta-ultramafic rocks and serpentinites across the contact zone. In addition, the abundances of SiO₂, CaO, Fe₂O₃, K₂O, Rb, Cs, and Y show important features. Of greatest interest, perhaps, approximately 15 cm from the contact with meta-ultramafic rock, Zr, U, Hf, Pb, Ba, Sr, Y, and Cs in albitite are greatly enriched compared to elsewhere in the contact zone. Element enrichments spatially coincide with the appearance, increase in modal abundance, and/or increase in grain sizes of zircon, rare earth element (REE) rich epidote, titantite, and celsian within albitite. All of these ‘trace-element-rich’ accessory minerals show poikiloblastic inclusions of albite, which suggests that they grew concomitantly in the metasomatic zone.

Graphical and computational methods of evaluating mass changes of metasomatites relative to likely protoliths show that, near the contact, fewer minor and trace elements in albitite show 1:1 coordination with presumed protoliths. Most metasomatitites are enriched in large-ion lithophile elements (LILE) and heat-producing elements (HPE) relative to likely protoliths. Albitite near the contact with meta-ultramafic rocks also shows ultramafic components. Except for a Ca-rich actinolite schist zone, the meta-ultramafic rocks are depleted in LILE and HPE relative to serpentinite; host serpentinite is itself under-abundant in these elements relative to average upper mantle or chondrite.

In summary, the metasomatic zone shows more evidence for the introduction of components to albitite and actinolitic meta-ultramafic rock than it does for exchange of protolith components between jadeitite and serpentinite. The fluid that presumably formed the metasomatites was sufficiently rich in LILE and high-field-strength elements (HFSE) to both saturate and grow minerals in which Zr, Ba, and Ti are essential structural constituents and/or HFSE, LILE, and HPE minor to moderate substituents. These geochemically diverse element groups were fixed in albitite via the crystallization and growth of new accessory minerals within these rocks during albititization. The amount of LILE and HPE-depleted meta-ultramafic rock appears to be too small to call upon a local source for the LILE and HPE-enrichment seen in albitites. Therefore, LILE and HPE must be of exotic origin, carried and deposited by fluids within the albitites at the jadeitite-serpentinite contact. This contact clearly testifies to an alteration style that involved crystallization of ‘trace-element’-rich minerals during fluid flow; this process appears to be essential to mass transfer within subduction zones.     

Jadeitites, albitites and related rocks from the Motagua Fault Zone, Guatemala

Jadeitites from Guatemala are found as weathered blocks in tectonized serpentinite in a 15-km zone north of the Motagua Fault Zone. Rock types found with jadeitite include albitites, albite-mica rocks, omphacite/taramitic amphibole-bearing metabasites, chlorite-actinolite schists, talc-carbonate rocks and antigorite schists. In addition to the predominant jadeitic (Jd₉₃_₁₀₀) pyroxene, common phases in jadeitite include micas (paragonite and/or phengite ± rarer phlogopite), omphacite, albite, titanite /Pm zircon, apatite and graphite. Conditions of jadeitite formation are 100-400d? C, 5-11 kbar with 0.0 > log₁₀a_sio2≥= 0.7. Fluid inclusions, coarse textures, vein structures, and rhythmic zoning of pyroxene indicate an aqueuos fluid was involved. Jadeitites are either (1) metasomatic modifications of former felsic-to-pelitic inclusions that have undergone silica depletion plus efficient soda exchange and enrichment, or (2) solution precipitations derived from such a source. The close spatial relationship of faults and shear zones, serpentinites, and jadeitites suggests jadeitites form in a relatively high-P/T setting with substantial flow of sodic fluid in a tectonized zone. Most Guatemalan jadeitites are extensively altered to analcime, albite, taramitic amphibole, (clino)zoisite ± nepheline and preiswerkite. This alteration reflects depressurization /Pm heating to below the jadeite + fluid = analcime reaction at high a_Na. With progressive alteration, analcime and nepheline are replaced by albite; the increase in silica content may result from fluid flowing up a tectonized zone reaching saturation with an albite assemblage. Albitite phases, albite, actinolite, zoisite, /Pm chlorite, phengite, K-feldspar and quartz, record conditions of c. 3-8 kbar at T < 400d? C, indicating a clockwise P-T trajectory of the blocks. Barium aluminosilicates—banalsite, celsian, cymrite and hyalophane—are common minor late-stage phases in jadeitites and albite-rich rocks. Barian phengite is common in albite-mica rocks.     

缅甸硬玉岩中后成合晶冠状体的含水性研究

基于多期次流体活动在硬玉岩及后成合晶冠状体的交互作用过程中发挥了至关重要的作用,采用电子探针、显微红外光谱等测试方法,从微尺度角度重点对缅甸角闪石质硬玉岩中角闪石+铬硬玉+硬玉后成合晶冠状体的成分和结构羟基赋存状态进行了研究.结果显示,参与后成合晶冠状体形成的流体组分较为复杂且形成过程是多阶段的;后成合晶冠状体的共生矿...     

哈萨克斯坦伊特穆伦硬玉岩的矿物学特征

硬玉岩是一种较为少见的高压低温变质岩,不仅具有重要的科研价值,也是一种名贵的玉石材料。本文对产于哈萨克斯坦卡拉干达州伊特穆伦矿区的硬玉岩进行了岩相学观察和矿物化学成分分析,并与缅甸、俄罗斯和危地马拉的硬玉岩进行对比。结果表明,哈萨克斯坦硬玉岩主要组成矿物为硬玉、绿辉石以及少量方沸石和钠沸石,具有粒柱状变晶结构,硬玉矿物的平均化学成分为w(SiO₂)=58.38%,w(Al₂O₃)=21.88%,w(Na₂O)=12.69%,w(CaO)=3.40%,w(MgO)=2.58%,w(FeO)=0.29%。不同产地样品中绿辉石和硬玉的SiO₂含量相差不大。哈萨克斯坦样品中绿辉石Na₂O和FeO的含量略低于缅甸、俄罗斯和危地马拉,MgO和CaO稍高于其它3个产地,硬玉MgO和CaO略高于其它3个产地。     

Relicts of an intra-oceanic arc in the Sapi-Shergol mélange zone (Ladakh, NW Himalaya, India): implications for the closure of the Neo-Tethys Ocean

In the Ladakh–Zanskar area, relicts of both ophiolites and paleo-accretionary prism have been preserved in the Sapi-Shergol mélange zone. The paleo-accretionary prism, related to the northward subduction of the northern Neo-Tethys beneath the Ladakh Asian margin, mainly consists of tectonic intercalations of sedimentary and blueschist facies rocks. Whole rock chemical composition data provide new constraints on the origin of both the ophiolitic and the blueschist facies rocks. The ophiolitic rocks are interpreted as relicts of the south Ladakh intra-oceanic arc that were incorporated in the accretionary prism during imbrication of the arc. The blueschist facies rocks were previously interpreted as oceanic island basalts (OIB), but our new data suggest that the protolith of some of the blueschists is a calc-alkaline igneous rock that formed in an arc environment. These blueschists most likely originated from the south Ladakh intra-oceanic arc. This arc was accreted to the southern margin of Asia during the Late Cretaceous and the buried portion was metamorphosed under blueschist facies conditions. Following oceanic subduction, the external part of the arc was obducted to form the south Ladakh ophiolites or was incorporated into the Sapi-Shergol mélange zone. The incorporation of the south Ladakh arc into the accretionary prism implies that the complete closure of the Neo-Tethys likely occurred by Eocene time.     

Multiple origins of zircons in jadeitite

Jadeitites form from hydrothermal fluids during high pressure metamorphism in subduction environments; however, the origin of zircons in jadeitite is uncertain. We report ion microprobe analyses of δ¹⁸O and Ti in zircons, and bulk δ¹⁸O data for the jadeitite whole-rock from four terranes: Osayama serpentinite mélange, Japan; Syros mélange, Greece; the Motagua Fault zone, Guatemala; and the Franciscan Complex, California. In the Osayama jadeitite, two texturally contrasting groups of zircons are identified by cathodoluminescence and are distinct in δ¹⁸O: featureless or weakly zoned zircons with δ¹⁸O = 3.8 ± 0.6‰ (2 SD, VSMOW), and zircons with oscillatory or patchy zoning with higher δ¹⁸O = 5.0 ± 0.4‰. Zircons in phengite jadeitite from Guatemala and a jadeitite block from Syros have similar δ¹⁸O values to the latter from Osayama: Guatemala zircons are 4.8 ± 0.7‰, and the Syros zircons are 5.2 ± 0.5‰ in jadeitite and 5.2 ± 0.4‰ in associated omphacitite, glaucophanite and chlorite-actinolite rinds. The δ¹⁸O values for most zircons above fall within the range measured by ion microprobe in igneous zircons from oxide gabbros and plagiogranites in modern ocean crust (5.3 ± 0.8‰) and measured in bulk by laser fluorination of zircons in equilibrium with primitive magma compositions or the mantle (5.3 ± 0.6‰). Titanium concentrations in these zircons vary between 1 and 19 ppm, within the range for igneous zircons worldwide. Values of δ¹⁸O (whole-rock) ≅ δ¹⁸O (jadeite) and vary from 6.3 to 10.1‰ in jadeitites in all four areas.     

Pseudotachylytes from Corsica: fossil earthquakes from a subduction complex

Subduction zones are the most seismically active tectonic environment on Earth. Here we report fault‐rock evidence of palaeoearthquakes from a subduction complex. Pseudotachylyte veins formed by shear heating on faults at seismic strain rates contain dendrites, spherulites and acicular minerals characteristic of blueschist facies conditions. The veins have been found in both mantle peridotite and gabbro. Our discovery from Corsica opens a new avenue of research towards understanding mechanisms related to subduction zone earthquakes and the conditions at which these are released.     

俯冲带深部流体:来自超高压变质锆石晶内结构和包裹物的证据

利用阴极发光和激光拉曼研究了大别山俯冲带超高压基性和长英质变质岩中的锆石。超高压变质增生锆石环带或新生锆石颗粒从不同角度反映了俯冲带地幔深度流体作用信息。其一,在超高压变质锆石生长域中存在微米级流体包裹体。由于其十分细小,组成往往难以测定。但是在变基性岩中检测到富CO2的负晶形包裹体。其二,在超高压锆石生长域中较普遍地发现富含挥发份的多硅白云母和磷灰石包裹体,其与典型的超高压变质矿物———柯石英稳定共生。副矿物研究表明,即使是在新鲜的弱退变质的榴辉岩中,富含挥发份的矿物相———磷灰石仍是含量较高的矿物,说明在俯冲带的深部(>100km),流体仍可以稳定地保存在高挥发份的矿物晶体格架中。其三,超高压变质增生锆石环带的边界呈蚕食状或港湾状,指示流体参与了锆石的变质生长过程,即通过流体与锆石之间的相互作用导致原有锆石的溶解和新生锆石的沉淀结晶。考虑到如此高的锆溶解度,认为在地幔俯冲深度,流体的成分复杂,并不是简单的富水体系,其化学组成与深熔融体相似,且赋存状态应为含水融体相。     

南苏鲁造山带的超高压变质岩及岩石化学研究

在南苏鲁造山带核部，古老的表壳岩和花岗质侵人岩经历了三叠纪的超高压变质作用，在超高压变质岩石抬升过程中经历了强烈的角闪岩相退变质作用改造。据岩相学和岩石化学研究，可以区分出六大类典型超高压变质岩：榴辉岩、石榴石橄榄岩、石英硬玉岩、石榴石多硅白云母片岩、硬玉石英岩和石榴石绿辉石文石岩。这些岩石的角闪岩相退变质产物分别是斜长角闪岩、蛇纹岩、长英质片麻岩、长石石英云母片岩、石英岩和大理岩。地球化学研究揭示，榴辉岩的原岩很可能是形成在大陆内部构造环境的拉斑玄武岩，而石榴石橄榄岩可能是起源于亏损的残余地幔。石英硬玉岩原岩包括正变质的花岗岩和奥长花岗岩、副变质的酸性火山碎屑岩和长石石英砂岩。大面积分布的古老花岗岩很可能是形成在大陆或大陆边缘环境。长石石英云母片岩、石英岩和大理岩的原岩为沉积岩，与副变质的长英质片麻岩和基性火山岩—起构成了古老的表壳岩组合。双峰式的酸性和基性火山岩组合的存在也证明部分表壳岩是形成在大陆环境。因此，可以推测南苏鲁造山带核部的超高压变质岩原岩为形成在大陆板内环境的沉积岩—酸性和基性火山岩—花岗岩和奥长花岗岩建造。