南黄海千里岩岛榴辉岩的地球化学特征及成因分析     

李敏  韩宗珠  许红  赵广涛  赵利  张贺  刘金庆 《中国海洋大学学报(自然科学版)》2014,44(11):59-66

南黄海千里岩榴辉岩对研究华北板块和扬子板块碰撞缝合线有重要意义。千里岩榴辉岩变形作用较弱,经历了多期退变质作用。千里岩榴辉岩原岩为拉斑系列的玄武岩,与大别苏鲁造山带榴辉岩相似。千里岩榴辉岩微量与稀土元素配分模式均与大别苏鲁造山带榴辉岩有较大差异。千里岩榴辉岩大离子亲石元素相对于N-MORB富集,高场强元素特征与N-MORB基本一致,无Nb、Ta、Zr、Ti负异常;稀土元素配分形式为LREE亏损的左倾型。千里岩榴辉岩主微量元素判别图与微量元素特征值均指示其形成的构造环境为弧后盆地。榴辉岩的207Pb/204Pb比值不同于华北板块而接近扬子板块,继承性锆石(747±19)Ma的SHRIMP U-Pb年龄说明其原岩经历了仅在扬子板块有记录的晋宁期热事件。该区所在的千里岩隆起区代表了扬子板块的基底。  相似文献   

潮致余流和潮混合对长江口外东北海域低盐水团的作用   总被引：1，自引：0，他引：1  

张文静  朱首贤  李训强  阮鲲  管卫兵  彭剑 《海洋学报》2014,36(3):9-18

长江冲淡水对黄海、东海水文环境有重要影响,它主要以羽状形态向外海扩展,在某些年份的特殊时间段也存在孤立的低盐水团现象。在低盐水团的动力机制研究中,风、径流量、台湾暖流、天文潮和斜压不稳定的作用已得到讨论。天文潮对冲淡水及低盐水团的影响主要包括潮致余流和潮混合,潮致余流作用仍缺乏讨论。本文对1983年8月低盐水团的动力机制进行数值模拟分析,重点讨论了潮致余流和潮混合的影响。结果表明:潮致Lagrange余流促使一部分冲淡水从口门向北输运,在32°N附近呈舌状转向东,有利于在口外东北海域形成低盐水团;小潮转大潮的垂向混合作用加强,浅水区表层盐度升高的速度快于较深水区,也有利于在口外东北海域形成低盐水团。  相似文献   

西藏松多榴辉岩的矿物原位微量元素地球化学研究     

李天福  杨经绥  李兆丽  徐向珍  任玉峰  陈松永 《岩石矿物学杂志》2012,31(6):858-874

对松多榴辉岩中单矿物进行的LA-ICP-MS原位微区微量元素分析研究结果表明,石榴石主要富集中、重稀土元素和Y,同时具有高丰度的Sc、V、Cr和Co等元素;绿辉石中的微量元素以中稀土元素、Sr、Sc、V、Cr、Co、Ni和Ti为主,含有一定量的Zr、Hf等。石榴石、绿辉石、角闪石和绿帘石中均显示轻稀土元素亏损的特点,表明在退变质过程中没有发生明显的富轻稀土元素的外来流体交代作用,因而其微量元素矿物地球化学的某些特点不同于苏鲁地区的榴辉岩。石榴石变斑晶中某些元素(如Ti、Zr)的分带性暗示了榴辉岩在紧随峰期变质之后的折返过程中发生了降压增温过程。榴辉岩主要变质矿物中微量元素的分配显然受到矿物主量元素的分配所控制,如MgO在石榴石和绿辉石之间的分配对Ni、Co、Ti分配的控制以及CaO的分配对Sr、Y、REE分配的控制等。退变质过程中矿物的形成或分解以及物理化学条件的改变都可以引起矿物间微量元素的重新分配。由绿辉石退变质而形成的角闪石,较之原先的绿辉石,其微量元素配分曲线总体特征会发生变化,但元素总体丰度相近,某些元素特点相似,又反映了绿辉石和角闪石之间的成生联系。金红石是Ti、Nb、Ta、Zr、Hf的主要赋存矿物,而与之共生的绿帘石所表现出来的高场强元素的亏损特征表明了金红石的存在所带来的影响。  相似文献   

The secondary origin of diamonds: multi-modal radiation tomography of diamondiferous mantle eclogites     

《International Geology Review》2012,54(9):1172-1180

Three-dimensional neutron and X-ray tomography reveals the textural and spatial relationship of diamonds and associated minerals in situ, in a unique suite of 17 diamondiferous eclogites. We emphasize the reporting of X-ray imaging on mantle xenoliths, which in combination with neutron imaging enables the clear identification of diamonds and interstitial metasomatic secondary minerals. In particular, neutrons are highly sensitive to hydrogen (H), allowing for the identification of OH- and H₂O-bearing metasomatic minerals. The identification of metasomatic minerals allows for the delineation of distinct metasomatic pathways through the eclogite xenoliths. Diamonds are readily identified as the darkest greyscales due to their low attenuation, and are typically surrounded by secondary minerals, never in contact with primary minerals, and always confined within metasomatic pathways. The ubiquitous occurrence of diamonds in association with pathways suggests a potential genetic link. Both octahedral and dodecahedral diamonds are observed within individual xenoliths, suggesting multiple heterogeneous growth and dissolution processes at small scales. The distinct age dichotomy between eclogite xenoliths and metasomatic mineral assemblages implies that the observed textural relationship of diamonds and late-stage metasomatic pathways for this suite of 17 eclogites casts doubt on the theory that eclogitic diamonds formed billions of years ago. Diamonds are interpreted to have formed from multiple growth episodes, with the last of these episodes represented by the metasomatic assemblages observed in this study. This further indicates that eclogitic diamond inclusions may span large time scales from ancient ages (>2 Ga) all the way to the last growth event, perhaps even close to the time of kimberlite emplacement (~360 Ma), which has significant implications for age-dating of diamonds and the study of diamonds as a whole.  相似文献   

P–T–time (phengite Ar closure) history of spatially close-outcropping HP and UHP oceanic eclogites (southwestern Tianshan): implication for a potential deep juxtaposing process during exhumation?     

《International Geology Review》2012,54(10):1270-1293

ABSTRACT

The Chinese southwestern Tianshan HP–UHP/LT metamorphic complex possesses well-preserved mafic layers, tectonic slices/blocks, boudins/lens of different sizes, and lithology embedded within dominant metavolcanoclastics. A recent study on the ultra-high pressure (UHP) eclogite revealed a short timescale of exhumation (≤10 Ma, ~315 ± 5 Ma). However, controversies still exist on some key questions: (1) the reasonable interpretation of spatially close-outcropped high pressure (HP) and UHP slices with respect to regional geodynamics, and (2) if the previous regional scatter Ar–Ar ages proved the existence of internally coherent sub-belts or troubled by dating on samples with notable ⁴⁰Ar retention. This study focusses on detailed P–T–time (phengite Ar closure) recovery of samples from a HP eclogite lens and its host rock, the UHP thick-layered eclogite. Based on data from bulk–rock, microprobe analysis, and muscovite Ar–Ar chronological dating, we link phengite growth to potential garnet growth stages via thermodynamic modelling. Facilitated by the P–T–Ar retention% graph, we collect all the regional muscovite Ar–Ar data together with results in this study for evaluating the significance of regional muscovite Ar–Ar ages and set back to geodynamics. According to modelling results, the HP lens eclogite reached peak metamorphism at ~550°C, 2.50 GPa with an Ar–Ar muscovite plateau age of 316.9 ± 1.0 Ma that could date the mass phengite growth event during prograde metamorphism. In contrast, the UHP layered eclogite experienced UHP peak burial at ~510°C, 2.95 GPa, and then to HP peak metamorphism at ~560°C, 2.60 GPa with ~311.6 ± 0.7 Ma plateau age that may constrain the cooling age during early exhumation. Noteworthy, both of them share a quite similar early exhumation path despite bearing contrasting prograde metamorphic experiences. With considering updated regional exhumation pattern, this might imply the existence of a potential deep juxtaposing (capture) process between HP slices and exhumating UHP complex, at about 45–60 km depth along subduction plate interface.  相似文献   

Review of melting experiments on carbonated eclogite and peridotite: insights into mantle metasomatism     

《International Geology Review》2012,54(12):1443-1455

Experimental studies on the partial melting of eclogite and peridotite provide important clues on mantle metasomatism. Here, we review results from some of the recent experiments and show that melting of carbonated eclogite and peridotite can produce carbonatitic to carbonated silicate melt, in which carbonates melt preferentially before Ti oxides and silicates. Low-degree melting results in carbonatitic melt coexisting with Ti oxides and silicates. This process also leads to the fractionation between some high-field strength elements (Nb, Ta, Zr, Hf, and HREE) and highly incompatible elements (U and Th) in the melt. When Ti oxides are nearly exhausted in eclogite, extremely high TiO₂ contents (e.g. 19 wt.%) are present in the melt with marked concentration of Nb and Ta. These results help to explain the features of carbonatitic metasomatism and the Nb–Ta spike in oceanic island basalts as identified in experimental studies. These studies also explain the reducing conditions that stabilize diamond in the deep mantle (>150 km) as well as the occurrence of diamond at different depths reported in various studies. Melting in such a reduced mantle can happen through redox reaction between diamond, pyroxene, and olivine, in which the initial liquid is a carbonated silicate melt. However, the theoretical oxygen fugacity (fO₂) in the asthenosphere is much lower than that predicted by the reaction and requires elevated fO₂, which can be caused by the addition of relatively oxidized materials from the lower mantle, deep asthenospheric material, and various recycled components. A combination of these processes generates locally oxidized domains in the deep mantle.  相似文献   

东昆仑西段夏日哈木-苏海图早古生代榴辉岩的发现及意义   总被引：16，自引：9，他引：7  

祁生胜  宋述光  史连昌  才航加  胡继春 《岩石学报》2014,30(11):3345-3356

1:25万区域地质调查在东昆仑西段昆中断裂以北的北昆仑变质基底中发现了呈构造透镜分布的榴辉岩及其退变质的榴闪岩,沿小灶火-苏海图-夏日哈木-拉宁灶火断续延伸长达20km.榴辉岩经历了强烈的退变质作用,大部分已退变为榴闪岩,由峰期变质矿物组合石榴石+绿辉石+金红石+钛铁矿和后期退变矿物组合普通角闪石(绿色)+斜长石组成,石榴石边缘发育似针状普通角闪石及微粒状斜长石组成的后合成晶结构,形成宽0.15~1cm的"白眼圈"结构.石榴石成分为Alm_52-53And_4.2Spess_0.7-1.2Gros_21.7-22Pyrope_19-20,绿辉石中硬玉分子量30.14mol%~37.7mol%.估算榴辉岩相变质温压条件P≈20kbar、T≈660~700℃,榴闪岩相退变温压条件P≈7kbar、T≈550℃,岩石经历了明显的降温降压过程.榴辉岩中锆石阴极发光图像显示大部分具有扇形分带或"杉树叶"结构,17颗锆石Th/U极低,在0.007~0.09之间,其(LA-ICP-MS)U-Pb年龄411.1±1.9Ma代表了榴辉岩相峰期变质年龄,结合区域构造岩浆事件,确定本次高压变质的峰期时代为晚志留-早泥盆世,其形成可能与大规模的陆-陆碰撞事件有关,这一发现为研究东昆仑造山带的构造演化具有重要意义.  相似文献   

新疆西南天山哈布腾苏河高压-超高压变质带中榴辉岩和蓝片岩的岩石学及变质演化     

施建荣  刘福来  刘平华  孟恩  刘超辉  杨红  王舫  蔡佳 《岩石矿物学杂志》2014,33(1):29-50

对西南天山哈布腾苏河一带出露的典型榴辉岩和蓝片岩进行了详细的岩相学、矿物化学和温压条件综合研究。榴辉岩可分为蓝闪石榴辉岩、钠云母榴辉岩、绿帘石榴辉岩和蓝闪石榴角闪岩（退变榴辉岩）4类,蓝片岩可分为含蓝闪石石榴白云母钠长片岩、石榴白云母蓝闪片岩和石榴白云母蓝闪石英片岩3类。新鲜榴辉岩主要矿物组合为石榴石+绿辉石+钠云母+绿帘石,退变榴辉岩则为石榴石+蓝闪石+角闪石;蓝片岩主要矿物组合为石榴石+蓝闪石+多硅白云母+钠云母+钠长石+石英。榴辉岩和蓝片岩中石榴石变斑晶均保存进变质生长环带,从核部到边部X_Mn和X_Fe降低,X_Mg和X_Ca升高,指示了升温进变质的演化过程。根据榴辉岩矿物共生组合、石榴石内部包体组合分布特征及传统地质温压计估算结果,确定榴辉岩经历了4阶段的变质演化：早期硬柱石蓝片岩相进变质阶段、峰期榴辉岩相变质阶段（t=543～579℃,p=1.5～1.6 GPa）、峰后绿帘蓝片岩相退变质阶段（t=～450℃,p<1.0 GPa）和晚期蓝闪绿片岩相退变质阶段（t<400℃,p<0.5 GPa）。利用p-T视剖面图计算的榴辉岩、蓝片岩峰期变质温压条件与传统地质温压计估算结果十分相近,其中榴辉岩的峰期变质条件t=520～550℃,p=1.7～1.9 GPa;蓝片岩峰期变质条件t=520～620℃,p=1.7～2.3 GPa。本文估算的榴辉岩峰期变质压力条件与前人根据柯石英的发现而认为研究区部分榴辉岩及其围岩曾经历超高压变质作用的认识明显相悖,原因可能如下：① 后期退变质作用引起研究区榴辉岩全岩成分、矿物化学成分的调整,在采用Grt-Cpx-Phe温压计和以全岩成分为基础的相平衡模拟方法估算峰期温压条件时受到影响,从而使估算峰期压力条件普遍偏低;② 西南天山的榴辉岩可能并非全都经历了超高压变质作用,高压、超高压榴辉岩可能分别代表了不同变基性岩块在不同俯冲深度变质的产物。  相似文献   

Multistage metamorphism and deformation in high‐pressure metabasites of the northern Adula Nappe Complex (Central Alps,Switzerland)     

Fabio Zulbati 《Geological Journal》2011,46(1):82-103

Metabasites from the northern Adula Nappe Complex (ANC) display a complex microstructural evolution recording episodes of deformation and metamorphic re‐equilibration that were obliterated in the surrounding phengite‐bearing schists. Pre‐D1 and D1 deformation episodes are preserved as mineral inclusions within garnet cores of some amphibole‐bearing eclogites and record high‐temperature greenschist‐/amphibolite‐facies conditions. D2 produced an eclogite‐facies foliation which developed at 580 ± 70°C and 19 ± 3 kbar. D3 was a composite deformation episode which can be divided into three sub‐episodes D3m, D3a and D3b which occurred as the metamorphism evolved from post‐eclogitic high‐pressure and low‐temperature conditions through to amphibolite‐facies conditions at 590 ± 30°C and 11.7 ± 1.3 kbar. The D3 deformation episode was responsible for the development of the S3 regional‐scale foliation in the surrounding schists, whilst D4 caused the development of an S4 greenschist foliation. The composite nature of the D3 episode indicates that rocks of the northern ANC experienced a protracted post‐eclogitic structural reworking and that the current structure of this part of the Alps is a late‐Alpine feature. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Triassic eclogite from northern Vietnam: inferences and geological significance   总被引：2，自引：0，他引：2  

N. NAKANO  Y. OSANAI  K. SAJEEV  Y. HAYASAKA  T. MIYAMOTO  N. T. MINH  M. OWADA  B. WINDLEY 《Journal of Metamorphic Geology》2010,28(1):59-76

T he first finding of low‐temperature eclogites from the Indochina region is reported. The eclogites occur along the Song Ma Suture zone in northern Vietnam, which is widely regarded as the boundary between the South China and Indochina cratons. The major lithology of the area is pelitic schist that contains garnet and phengite with or without biotite, chloritoid, staurolite and kyanite, and which encloses blocks and lenses of eclogite and amphibolite. The eclogites commonly consist of garnet, omphacite, phengite, rutile, quartz and/or epidote with secondary barroisite. Omphacite is commonly surrounded by a symplectite of Na‐poor omphacite and Na‐rich plagioclase. In highly retrograded domains, diopside + tremolite + plagioclase symplectites replace the primary phases. Estimated peak‐pressure metamorphic conditions based on isochemical phase diagrams for the eclogites are 2.1–2.2 GPa and 600–620 °C, even though thermobarometric results yield higher pressure and temperature conditions (2.6–2.8 GPa and 620–680 °C). The eclogites underwent a clockwise P–T trajectory with a post‐peak‐pressure increase of temperature to a maximum of >750 °C at 1.7 GPa and a subsequent cooling during decompression to 650 °C and 1.3 GPa, which was followed by additional cooling before close‐to‐isothermal decompression to ∼530 °C at 0.5 GPa. The surrounding pelitic schist (garnet–chloritoid–phengite) records similar metamorphic conditions (580–600 °C at 1.9–2.3 GPa) and a monazite chemical age of 243 ± 4 Ma. A few monazite inclusions within garnet and the cores of some zoned monazite in garnet–phengite schist record an older thermal event (424 ± 15 Ma). The present results indicate that the Indochina craton was deeply (>70 km) subducted beneath the South China craton in the Triassic. The Silurian cores of monazite grains may relate to an older non‐collisional event in the Indochina craton.  相似文献