Semial ophiolite veins of United Arab Emirates―A fluid inclusions study

Calcite and quartz veins in the gabbroic and dunite rocks of the Semial ophiolite (UAE) were selected for fluid inclusion analysis. The inclusions contain both aqueous low-salinity and hydrocarbon-dominated fluids. Mi-crothermometry data indicate that the aqueous fluids contain 0.22 to 1.45 equivalent wt% NaCl and occasionally contain traces of hydrocarbons. Homogenization to liquid occurred between 91 and 152℃. Modeling based on these fluid inclusion observations indicates that the trapping conditions of t...     

中国东南部中生代基性岩脉时空分布与形成机理

中国东南部发育的基性岩脉见于中生代各个时期,其年龄从内陆至沿海逐渐变新,与区内花岗岩年龄有相同的变化趋势,但两者活动高峰期不重合。基性岩脉主要发育于花岗岩体内部的拉裂带,鲜见穿过花岗岩上方或周围的沉积变质盖层。中生代各时期的基性岩脉与相应时期的中酸性和基性火山岩浆活动具有清晰的分带现象。进一步研究表明,基性岩脉与同期基性火山岩应是同源不同相的产物,而中酸性喷发的岩浆与基性喷发(侵入)的岩浆,则可能分别来自壳内熔融层和壳下(地幔)熔融层或壳内岩浆层的上部和下部。中酸性与基性岩浆活动的"错峰"现象和基性岩脉带的"迁移"现象,均与本区中生代陆壳系统的内能变化有关。     

Zircon U-Pb Age,Geochemical, and Sr-Nd-Pb-Hf Isotopic Constraints on the Time Frame and Origin of Early Cretaceous Mafic Dykes in the Wuling Mountain Gravity Lineament,South China

In view of the importance of mafic dyke swarms and their contribution to current scientific problems relating to South China, herein, we present the findings of studies on twenty–five representative mafic dykes cropping out in Hunan Province and Guangxi Zhuang Autonomous Region, within the southern Wuling Mountain gravity lineament, China. These results include new zircon LA-ICP-MS U-Pb age, whole rock geochemical, Sr-Nd-Pb isotopic, and zircon Hf isotopic data for these dykes. The dykes formed between 131.5 ± 1.2 and 121.6 ± 1.1 Ma, and have typical doleritic textures. They fall into the alkaline and shoshonitic series, are enriched in light rare earth elements(LREE), some large ion lithophile elements(LILE; e.g., Rb, Ba, and Sr), Th, U, and Pb, and are depleted in Nb, Ta, Hf, and Ti. Moreover, the dolerites have high initial87 Sr/86 Sr ratios(0.7055–0.7057), negative εNd(t) and zircon εHf(t) values(-14.8 to-11.9,-30.4 to-14.9), and relatively constant initial Pb isotopic ratios(that are EM1-like, 16.77–16.94, 15.43–15.47, and 36.84–36.92 for 206 Pb/204 Pb,207 Pb/204 Pb, and 208 Pb/204 Pb, respectively). These results indicate that the dykes were likely derived from magma generated through low-degree partial melting(1.0%–10%) of an EM1-like garnet–lherzolite mantle source. The parental magmas fractionated olivine, clinopyroxene, plagioclase, and Ti-bearing phases with negligible crustal contamination, during ascent and dyke emplacement. Several possible models have been proposed to explain the origin of Mesozoic magmatism along the Wuling Mountain gravity lineament. Herein we propose a reasonable model for the origin of these mafic dykes,involving the collision between the paleo-Pacific Plate and South China, which led to subsequent lithospheric extension and asthenosphere upwelling, resulting in partial melting the underlying mantle lithosphere in the Early Cretaceous, to form the parental magmas to the WMGL mafic dykes, as studied.     

辽东青城子矿集区双顶沟岩体年代学及地球化学研究

青城子矿集区印支期岩浆作用形成岩基状双顶沟岩体及岩脉状石英二长斑岩等,双顶沟岩体根据其岩相学特征可划分为主体相和中心相两个岩相带,主体相内含有大量暗色微粒包体,并常见矿物不平衡结构,显示岩浆混合的特征。在岩石地球化学方面,包体与寄主岩石的主要氧化物之间具有良好的线性关系,寄主岩石和包体的稀土元素配分曲线和微量元素蛛网图形态相似,指示寄主岩石与包体在岩石形成过程中发生过成分交换及均一化,也显示岩浆混合特征。双顶沟岩体主体相具有高Ba-Sr花岗岩特征,可能为加厚下地壳部分熔融形成的熔体与富集地幔岩浆混合作用的产物,中心相则为主体相经过长石、角闪石、黑云母等的分离结晶作用而形成。石英二长斑岩脉可能为双顶沟岩体演化的浅成相,两者具有相同的源区和成因,矿集区内铅锌、金银矿床的形成可能与岩浆混合作用演化形成的石英二长斑岩相关。     

西藏尼木渐新世花岗岩中的岩浆混合作用:对岩石成因及陆壳增生的启示

尼木渐新世黑云母二长花岗岩中含有丰富的形态各异的暗色镁铁质微粒包体。本文报道了寄主岩和镁铁质包体的年代学、元素地球化学及Sr- Nd- Hf同位素组成,据此阐明了岩石的成因,并探讨了岩浆混合作用在成岩中的意义及其对陆壳增生的启示。锆石LA- ICP- MS U- Pb定年结果表明,寄主岩和镁铁质包体的成岩年龄在误差范围内基本一致,均为约30Ma,说明它们同时形成。元素地球化学组成上,寄主黑云母二长花岗岩为高钾钙碱性、准铝质,富集Rb、Th、U等大离子亲石元素,亏损Nb、Ta、Ti、P等高场强元素,且具有高Sr、低Y和Yb含量,Sr/Y比值高,缺乏明显铕负异常,表现出埃达克质岩石的特征。镁铁质包体贫硅,富铁、镁,具有与寄主岩相似的稀土与微量元素分布模式。二者的全岩Sr、Nd同位素和锆石Hf同位素组成也相近\[寄主岩:(87Sr/86Sr)i=0.7057~0.7064,εNd(t)=-1.45~0.35,εHf(t)=1.21~7.34;镁铁质包体:(87Sr/86Sr)i=0.7058~0.7064,εNd(t)=-2.23~-1.57,εHf(t)=2.40~7.04\]。综合分析表明,尼木渐新世黑云母二长花岗岩应主要起源于碰撞加厚的新生镁铁质下地壳的部分熔融,镁铁质包体应为幔源玄武质岩浆与其诱发加厚地壳熔融形成的埃达克质岩浆经不均匀混合作用的产物。结合对冈底斯带岩浆岩已有资料的全面分析,表明幔源玄武质岩浆的底侵及其诱发的岩浆混合作用既是冈底斯岩基形成的主要方式,也是导致青藏高原陆壳增生的重要途径。     

An example of post-collisional appinitic magmatism with an arc-like signature: the Wadi Nasb mafic intrusion,north Arabian–Nubian Shield,south Sinai,Egypt

We present new data for the Neoproterozoic mafic intrusion exposed in Wadi Nasb, south Sinai, Egypt (northernmost Arabian–Nubian Shield; ANS). The Nasb mafic intrusion (NMI) intrudes metasediments, Rutig volcanics, and diorite/granodiorite, and is intruded in turn by younger monzogranite and quartz-monzonite. Available geochronological data for the country rocks of the NMI provide a tight constraint on its age, between 619 and 610 Ma, during the hiatus between the lower and upper Rutig volcanics. The NMI is neither deformed nor metamorphosed, indicating post-collisional emplacement, and uralitization by late-magmatic and sub-solidus alteration is restricted to the margins of the intrusion. A quantitative fractionation model indicates a fractionating assemblage of 61% primary amphibole, 10% clinopyroxene, 28% plagioclase, 1% biotite, 0.4% apatite, and 0.15% Fe-Ti oxide. Contrary to the recent studies, we find that the nearby diorite of Gebel Sheikh El-Arab is not co-genetic with the appinitic gabbro of the NMI. Although there are volcanic xenoliths in the NMI, we find no chemical evidence requiring contamination by continental crust. A subduction-related signature in a post-orogenic intrusion requires the inheritance of geochemical tendencies from a previous subduction phase. Given that the fine-grained gabbro of the NMI is consistent with a near-primary mantle melt, we attribute this inheritance to persistence and later melting of the slab-modified mantle domains, as opposed to partial melting and assimilation of the juvenile continental crust. The fine-grained gabbro composition indicates derivation at temperature and pressure conditions similar to the sources of mid-ocean ridge basalts: mantle potential temperature near 1350°C and extent of melting about 7%. Such temperatures, neither so high as to require a plume nor so low as to be consistent with small degrees of melting of a volatile-rich source, are most consistent with a lithospheric delamination scenario, allowing the upwelling of fertile, subduction-modified asthenosphere to depths ≤50 km.     

西秦岭天水地区晚三叠世柴家庄二长花岗岩及其暗色包体地球化学——岩石成因及岩浆混合作用

晚三叠世花岗岩类在秦岭-大别造山带西端广泛分布,其成因机制及地球动力学背景的研究对于反演华北、扬子两大板块沿秦岭-大别造山带在三叠纪时期的拼合历史具有重要意义.本文选择西秦岭天水地区柴家庄晚三叠世二长花岗岩及其中的暗色包体进行精细的岩石学和地球化学研究.暗色包体中普遍发育针状磷灰石及斜长石捕掳晶,暗示岩浆混合作用;暗色包体具有较低的SiO₂（60.27%~60.38%）、高的Mg#（54~55）和Nb/Ta比值（14.8~16.6）,表明其来源于富集岩石圈地幔的部分熔融作用;寄主二长花岗岩表现出典型埃达克岩的地球化学特征,其富集Sr、Ba,亏损Y和HREE,岩石的Sr/Y比值介于88~98之间,Y/Yb比值介于13~15之间,暗示源区有石榴石残余.结合前人的研究结果,提出柴家庄二长花岗岩可能为增厚的造山带下地壳在碰撞后伸展环境下发生部分熔融作用的产物,可能与晚三叠世时期秦岭造山带的板片断离作用有关     

Juxtaposition of India and Antarctica During the Precambrian: Inferences from Geochemistry of Mafic Dykes

There are several geological, geochemical and geophysical evidences, which corroborate reconstruction of Gondwanaland and juxtaposition of India and Antarctica. Petrology of the Precambrian mafic dykes of East Antarctica and Central-East India also support juxtaposition of India and Antarctica. Mafic dykes of different generations are emplaced in the Archaean granite gneisses of these regions. These dykes appear to be an important tool to support juxtaposition of India and Antarctica. Geological and petrological data of the Central-East India Precambrian mafic dykes suggest four episodes of mafic magmatism in the region - three tholeiitic and one noritic (?). Similarly, East Antarctica also comprises four dyke suites, emplaced during three distinct periods. These suites are 2.4 Ga meta-tholeiites, 2.4 Ga high-Mg tholeiites, 1.8 Ga dolerites and 1.2–1.4 Ga dolerites. Geochemical compositions of these mafic dykes are compared and they show good relationships with each other. Similarities in petrological and geochemical characteristics of Precambrian mafic dykes of East Antarctica and Central-East India strongly support juxtaposition of these two continents.     

安徽铜陵地区幔源镁铁质团块研究及其地质意义

幔源岩石包体研究,是认识上地幔岩石圈物质组成、幔源岩浆演化及壳幔动力学过程的重要手段。铜陵地区小铜官山石英二长闪长岩中发育有微粒闪长质包体,并且这些微粒闪长质包体中不均匀地分布着镁铁质团块,三者的形成过程可视为铜陵地区岩浆演化的缩影,为了解本区深部岩浆作用过程提供了有力的证据。在前人研究的基础上,笔者借助电子探针、扫描电镜、电镜能谱和二次飞行时间离子探针(Tof-SIMS)对产于铜陵地区微粒闪长质包体中的镁铁质团块进行了详细的研究,首次获得了一套精确的矿物化学资料和元素分布图,总结了镁铁质团块的特征,并讨论了本区的深部岩浆作用过程。矿物学研究表明,镁铁质团块中的角闪石和辉石均已发生了不同程度的透闪石化和阳起石化蚀变,蚀变过程中,从镁钙闪石到镁角闪石,再到透闪石,随着Si的增加,角闪石呈现出Mg的富集和Ti、Al贫化的特点。团块中的富Cr磁铁矿、Ti磁铁矿和少量的铝直闪石指示了其具有深源性。Tof-SIMS元素分布图显示,透闪石主要由Al、Si、Ca、Sc、V、Cr、Mn、Cu和Sr元素组成,透辉石主要由Si、Mg、Ca、Cu和Rb组成。在铜陵地区,上地幔部分熔融形成一套玄武岩浆,受岩浆底侵作用影响,玄武岩浆上侵,进入下地壳深位岩浆房,与下地壳硅镁层发生同化混染作用,形成一套轻度演化的中基性(辉长质)玄武岩浆,镁铁质团块就是这类中基性玄武岩浆直接结晶形成的。后受构造作用影响,这类中基性玄武岩浆上侵到中地壳岩浆房(12~16 km),与中地壳的变质岩系发生同化混染和结晶分异作用形成一套中性闪长质岩浆,微粒闪长质包体就是这套闪长质岩浆发生结晶分异作用而形成的。镁铁质团块和微粒闪长质包体清楚地解释了铜陵地区深部岩浆作用过程,并有力地证明了铜陵地区中地壳的闪长质岩浆来源于下地壳的壳幔混源岩浆。     

Petrology and geochemistry of the ultramafic–mafic Mawpyut complex,Meghalaya: a Sylhet trap differentiation centre in northeastern India

The present article describes, for the first time, petrological and geochemical details of the Mawpyut differentiated complex which is related to the Sylhet trap located at Jaintia Hills district, Meghalaya, northeastern India. The Mawpyut complex occurs as an arcuate body that intrudes into the surrounding Shillong Group rocks. The complex in general contains ‘ultramafic’ and ‘mafic’ rocks, as well as minor syenitic veins that postdate the main units. The lithotypes correspond to cumulate and noncumulate units. The cumulate unit is represented by olivine clinopyroxenite, clinopyroxenite, plagioclase‐bearing ultramafic, olivine gabbronorite, mela‐gabbronorite, melagabbro, orthopyroxene gabbro, and gabbro, all with a pronounced cumulus texture. The noncumulate unit is marked by gabbro, monzonite, monzodiorite, and quartzsyenite. The use of several major and trace element variation diagrams suggests that magmatic differentiation led to the formation of cumulate and noncumulate units. In chondrite‐normalized REE diagrams the cumulate rocks show flat LREE and MREE patterns and a moderate positive Eu anomaly (in plagioclase‐bearing ultramafics) due to plagioclase cumulation. The rocks of the noncumulate unit show a strongly fractionated REE pattern and no Eu anomaly. The noncumulate mafic rocks are geochemically comparable to high‐phosphorous/high‐titanium basalts (HPT) indicative of low pressure fractional crystallization. In a primitive mantle‐normalized multielement diagram some of the cumulate rocks show pronounced negative anomalies for K and P, indicating anorogenic mafic magmatism in a within‐plate setting. The rocks of the noncumulate unit show a slight negative anomaly for Yb and a Nb–Ta trough, indicating a subduction‐related signature that perhaps is inherited from subducted sedimentary rocks incorporated during crustal contamination of the derived magma (left after crystal cumulation) with country rocks. Various trace element ratios for the cumulate mafic rocks indicate parent EMI/EMII/HIMU sources with a very limited crustal signature. The noncumulate mafic rocks (corresponding to the derived evolved magma) indicate EMI/EMII/HIMU sources with a pronounced crustal contamination. The Sr–Nd isotopic compositions of the Mawpyut samples typically plot in the continental flood basalt field, with an affinity to the EMII source. The isotopic compositions of the noncumulate rocks also clearly indicate crustal contamination. We suggest that partial melting (involving garnet in the residue) of the enriched mantle source EMI/EMII/HIMU could have derived the parental melt; this melt, in turn, underwent assimilation and fractional crystallization to produce the variety of cumulate‐noncumulate lithologies of the Mawpyut complex. Copyright © 2013 John Wiley & Sons, Ltd.