Ionprobe U-Pb zircon ages from the high-pressure/low-temperature mélange of Syros, Greece: age diversity and the importance of pre-Eocene subduction

The Cycladic blueschist belt in the central Aegean Sea has experienced high‐pressure (HP) metamorphism during collisional processes between the Apulian microplate and Eurasia. The general geological and tectonometamorphic framework is well documented, but one aspect which is yet not sufficiently explored is the importance of HP mélanges which occur within volcano‐sedimentary successions. Unresolved issues concern the range in magmatic and metamorphic ages recorded by mélange blocks and the significance of eventual pre‐Eocene HP metamorphism. These aspects are here addressed in a U‐Pb zircon study focusing on the block–matrix association exposed on the island of Syros. Two gneisses from a tectonic slab of this mélange, consisting of an interlayered felsic gneiss‐glaucophanite sequence, yielded zircon ²⁰⁶Pb/²³⁸U ages of 240.1 ± 4.1 and 245.3 ± 4.9 Ma, respectively, similar to Triassic ages determined on zircon in meta‐volcanic rocks from structurally coherent sequences elsewhere in the Cyclades. This strongly suggests that parts of these successions have been incorporated in the mélanges and provides the first geochronological evidence that the provenance of mélange blocks/slabs is neither restricted to a single source nor confined to fragments of oceanic lithosphere. Zircon from a jadeitite and associated alteration zones (omphacitite, glaucophanite and chlorite‐actinolite rock) all yielded identical ²⁰⁶Pb/²³⁸U ages of c. 80 Ma. Similar Cretaceous U‐Pb zircon ages previously reported for mélange blocks have been interpreted by different authors to reflect magmatic or metamorphic ages. The present study adds a further argument in favour of the view that zircon formed newly in some rock types at c. 80 Ma, due to hydrothermal or metasomatic processes in a subduction zone environment, and supports the interpretation that the Cycladic blueschist belt records both Cretaceous and Eocene HP episodes and not only a single Tertiary HP event.     

藏北新生代两套钾玄质火山岩系列地球化学特征

岩石学和元素地球化学特征研究表明，藏北新生代自南向北沿可可西里岩带和喀喇昆仑—玉门岩带出露有两套钾玄质系列火山岩。它们富集大离子亲石元素(LILE)和轻稀土(LREE)及明显亏损Nb—Ta—Ti，同时具有板内和岛弧(陆弧)的双重特征。源区来源于可能与俯冲带流体有关的相似交代富集地幔，成岩过程主要经历了低度(＜10％)辉石分离结晶作用，同时，源区伴有地壳物质的混染作用。     

中国东部苏鲁榴辉岩超基性岩褶皱带的演化及其与大别(中国)和柯切塔夫(哈萨克斯坦)杂岩的对比(英文)

苏鲁褶皱带形成于元古宙 (2 2 33～ 185 5Ma)典型优地槽构造环境 ,主要由石榴橄榄岩、石榴辉石岩、榴辉岩等侵入岩 (柯石英深度相地幔岩浆房中形成 )和它们的火山沉积建造围岩一起经褶皱、变质而形成。变质作用经历了先蓝片岩相 (前花岗岩 )后片麻岩混合岩相过程。由于变质作用的不规律性 ,苏鲁褶皱带可分为 2个构造带 :(1)东部构造带 (蓝片岩 )和 (2 )西部构造带 (片麻岩混合岩 )。根据A·都城秋穗所识别的变质带系统 ,可将其作为一个双变质带。东部构造带以出现许多块状、条带状榴辉岩辉石岩橄榄岩组合的残余岩块为特征 ,其中还残留着高压的矿物 (石榴石、绿辉石、柯石英 ) ,而且有被混合岩和各种交代岩替代的显著标志。在中生代 ,苏鲁元古褶皱带受造山作用的影响活化 ,导致许多花岗岩体的侵入 ,使交代岩广泛发育。     

Petrological feature of spinel lherzolite xenolith from Oki-Dogo Island: An implication for variety of the upper mantle peridotite beneath southwestern Japan

Abstract   Spinel lherzolite is a minor component of the deep-seated xenolith suite in the Oki-Dogo alkaline basalts, whereas other types of ultramafic (e.g. pyroxenite and dunite) and mafic (e.g. granulite and gabbro) xenoliths are abundant. All spinel lherzolite xenoliths have spinel with a low Cr number (Cr#; < 0.26). They are anhydrous and are free of modal metasomatism. Their mineral assemblages and microtextures, combined with the high NiO content in olivine, suggest that they are of residual origin. But the Mg numbers of silicate minerals are lower (e.g. down to Fo₈₆) in some spinel lherzolites than in typical upper mantle residual peridotites. The clinopyroxene in the spinel lherzolite shows U-shaped chondrite-normalized rare-earth element (REE) patterns. The abundance of Fe-rich ultramafic and mafic cumulate xenoliths in Oki-Dogo alkali basalts suggests that the later formation of those Fe-rich cumulates from alkaline magma was the cause of Fe- and light REE (LREE)-enrichment in residual peridotite. The similar REE patterns are observed in spinel peridotite xenoliths from Kurose and also in those from the South-west Japan arc, which are non-metasomatized in terms of major-element chemistry (e.g. Fo > 89), and are rarely associated with Fe-rich cumulus mafic and ultramafic xenoliths. This indicates that the LREE-enrichment in mantle rocks has been more prominent and prevalent than Fe and other major-element enrichment during the metasomatism.     

地壳流体与地幔流体间的关系

通过铀、金矿床及地幔岩捕虏体研究，初步得出以下认识：（１）在热液作用中岩浆分异热液是不存在的；（２）碱交代作用是整个地幔交代作用和地壳交代作用的基本运作机制，现已构成有几十条具体规律的理论系统；碱交代岩是地幔流体转变为热液的化石记录；（３）拆离断层构造体系是地幔流体上升到地壳后的活动通道；（４）磷（Ｐ）是地幔流体的特征性示踪成分；（５）大洋缺氧事件、生物种属灭绝、厄尔尼诺等有共同的地幔流体致因；（６）天然气－油－盐－金属成矿是统一的热液成矿作用。软流体是今后区域成矿预测的重要新依据。软流体的顶部发育有次级、再次级的局部上凸部，这是矿带、矿田群的重要成矿控制因素和找矿新标志。     

地幔交代作用的微区微量元素证据——云南双沟蛇绿岩的质子探针研究

用质子探针技术对双沟地幔交代熔融物的微区微量元素进行了测定。研究表明,地幔橄榄岩中的残留矿物橄榄石和斜方辉石仅含Ni和Zn,而Ga、Ge、Sr、Rb、Zr、Y、As、Pb等微量元素主要集中在地幔部分熔融所产生的矿物(尤其是尖晶石和绿泥石)中。微区范围内微量元素的分布表明,尖晶石二辉橄榄岩地幔不均一性的尺度可达微米级,通常解释为尖晶石二辉橄榄岩在开放系统下地幔交代作用的结果。     

The Texture,Mineral Assemblages and Origin of Metamorphic Peridotite from Kuda Ophiolite,Xinjiang

There are two grades of network-type texture in metamorphic peridotite from the Kuda ophiolite,Xinjiang,China,The first graduation consists of coarse-grained olivine with plastic deformation fabrics,orthopyroxene and minor clinopyroxene and Cr-spinel.The second graduation consists of small minerals filling among the mineral grains of the first graduation.Four kinds of assemblages containing tremolite have been discovered in the second graduation,which were formed during mantle partial melting and mantle metasomatism of a highly depleted peridotite.     

华北克拉通-扬子克拉通与秦岭造山带古地幔组成及状态的对比

经研究，华北克拉通古地幔以含石榴石的二辉橄榄岩、方辉橄榄岩及纯橄榄岩为主．地幔交代作用强烈，岩石富集不相容元素，明显高于南非及西澳克拉通者．扬子克拉通古地幔主元素亏损程度差，交代作用较弱，岩石为石榴石／尖晶石二辉橄榄岩．秦岭造山带壳、幔交界带的岩石类型复杂，包括蛇纹石化尖晶石二辉橄榄岩（蛇纹岩）、石榴石辉石岩、榴闪岩、变辉长岩及变玄武岩等．蛇纹岩遭受强的塑性流变，剪切变形结构发育．与克拉通相比，地幔交代作用很弱．３个地幔域的古地温状态有差异，秦岭造山带最高，扬子克拉通居中，华北克拉通最低．上述结果表明，两克拉通的古地幔特征相差显著，暗示经历了不同的演化过程．交代作用可能发生在１７００Ｍａ以前和１１００～５００Ｍａ，后者强于前者．造山带地幔的剪切变形明显强于克拉通地幔     

Retrograded textures and associated mass transfer: evidence for aqueous fluid action during exhumation of the Qinglongshan eclogite, Southern Sulu ultrahigh pressure metamorphic terrane, eastern China

The Qinglongshan eclogites in the Southern Sulu ultrahigh pressure metamorphic (UHPM) terrane show very different retrograded textures from their counterparts in the Northern Sulu terrane, implying a different thermal history. Scanning electron and optical microscope observations indicate that the peak assemblage of the Qinglongshan eclogite is anhydrous, composed of Grt + OmpI + Rt + (Ky + coesite). These primary minerals were replaced by second and third stage minerals, resulting in symplectite pseudomorphs or coronas. The following relationships are inferred: OmpI → OmpII + Ab + Fe‐oxide symplectite (type I) and Rt → Rt + Ilm intergrowth; and, Ky → Pg, OmpII (+Pl) → Amp (+Pl) symplectite (type II), and Grt → Prg (+Fe‐oxide). Mineral chemistry and mass‐balance demonstrate that the pseudomorphed textures were developed by metasomatism involving dissolution and precipitation intensified by fluids along grain boundaries. The formation of symplectite type I produced Fe, Mg and Na but consumed Ca and Si. The Mg and Fe diffused to garnet where exchange of (Mg, Fe) with Ca of the garnet resulted in compositional zonation with decreased Ca towards the edge of garnet grains where Ca was consumed during symplectite formation. The replacement of kyanite by paragonite consumed the extra Na. In the later stage, fluid infiltration partially transformed symplectite type I to type II, and narrow rims of pargasite resorbed garnet from their boundaries. Mass balance suggests that the transformation and resorption would have been coupled during fluid infiltration. In the latest stage, epidote and quartz were precipitated at very late stage as a result of fluid activity along microfractures. Tentative P–T conditions based on mineral reactions and thermocalc software suggest that the retrograded eclogite did not record the granulite facies retrograde evolution characteristic of eclogites from the Northern Sulu terrane. The difference in retrograde evolution between the Southern and Northern Sulu eclogites suggests a different exhumation history.     

Roles for fluid and/or melt advection in forming high-P mafic migmatites, Fiordland, New Zealand

A series of striking migmatitic structures occur in rectilinear networks through western Fiordland, New Zealand, involving, for the most part, narrow anorthositic dykes that cut hornblende‐bearing orthogneiss. Adjacent to the dykes, host rocks show patchy, spatially restricted recrystallization and dehydration on a decimetre‐scale to garnet granulite. Although there is general agreement that the migration of silicate melt has formed at least parts of the structures, there is disagreement on the role of silicate melt in dehydrating the host rock. A variety of causal processes have been inferred, including metasomatism due to the ingress of a carbonic, mantle‐derived fluid; hornblende‐breakdown leading to water release and limited partial melting of host rocks; and dehydration induced by volatile scavenging by a migrating silicate melt. Variability in dyke assemblage, together with the correlation between dehydration structures and host rock silica content, are inconsistent with macroscopic metasomatism, and best match open system behaviour involving volatile scavenging by a migrating trondhjemitic liquid.