Re–Os systematics of the Raobazhai peridotite massifs from the Dabie orogenic zone, eastern China

The Raobazhai ultramafic massif of the ultrahigh pressure Sulu–Dabie orogenic belt, central China, is thought to be a segment of subcontinental lithospheric mantle that was subducted and exhumed during the Triassic collision of the North China and Yangtze cratons. We performed a Re–Os isotopic study of peridotites from the massif, associated with major and trace element analysis and textural examination. Os (1.02 to 6.28 ppb) and Re (0.004 to 0.376 ppb) concentrations are typical of orogenic lherzolite values, and ¹⁸⁷Os/¹⁸⁸Os ratios (0.1157 to 0.1283) are all similar to or lower than the proposed primitive upper mantle value. ¹⁸⁷Os/¹⁸⁸Os is roughly correlated with ¹⁸⁷Re/¹⁸⁸Os, and strongly correlated with Al₂O₃. These correlations can be explained by radiogenic ingrowth of ¹⁸⁷Os since an ancient partial melting event. T_MA model ages (1.7 to 2.0 Ga) of refractory peridotites from the lower massif are consistent with the model age (1.8 Ga) obtained from the ¹⁸⁷Os/¹⁸⁸Os vs. Al₂O₃ correlation at ~1% Al₂O₃. This age cannot distinguish the cratonic provenance of the Raobazhai massif, since similar Re–Os model ages have been obtained from both the North China and the Yangtze cratons. The poor quality of the ¹⁸⁷Os/¹⁸⁸Os vs. ¹⁸⁷Re/¹⁸⁸Os correlation indicates that the Re/Os ratios were disturbed, perhaps during Triassic subduction. The mainly lherzolitic samples of the upper massif, which were most strongly affected by this process, have porphyroclastic textures with fine-grained olivine, pyroxene and amphibole neoblasts, suggesting Re mobility during recrystallization in the presence of fluids.Previous studies of ultramafic xenoliths from arc volcanics demonstrate that slab-derived melts or fluids can both scavenge mantle Os and add substantial amounts of radiogenic Os to the suprasubduction mantle. In Raobazhai, both trace element patterns and the abundance of hydrous phases provide evidence for extensive interaction with fluids during subduction and/or exhumation. Nevertheless, the strong correlation between ¹⁸⁷Os/¹⁸⁸Os and Al₂O₃, and the high Os concentrations of these rocks indicate that Os isotopic ratios, and probably even Os concentrations, were essentially unaffected by this process. Assuming that the arguments favoring a suprasubduction setting for the Raobazhai massif are valid, these data provide evidence that Os systematics are sometimes surprisingly robust, even above subduction zones.     

西大别造山带红安高压榴辉岩的变质演化：岩相学与Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-O(Fe 2O3)体系中相平衡关系

西大别造山带红安高压榴辉岩主要矿物为石榴石、绿辉石、冻蓝闪石、石英和绿帘石,有时可见蓝闪石、多硅白云母和钠云母。石榴石具有生长环带且边缘成分变化大,可分为代表峰期的Ⅰ型边（X_Mg高、Grs低）和受退变质改造的Ⅱ型边（X_Mg低、Grs高）。石榴石内蓝闪石包体发育冻蓝闪石退变边,说明包体不能完全反映进变质条件。基质绿辉石比包体绿辉石Jd含量低,在一个晶体内成分有明显变化和沿解理缝发育冻蓝闪石,显示峰后绿辉石有成分变化和退变质改造。基质中冻蓝闪石晶体较大,核部见有蓝闪石残留,说明二者有成因联系。冻蓝闪石和绿辉石都发育后成合晶结构,石榴石有韭闪石的反应冠状体。在THERMOCALC程序计算的P-T视剖面图中,石榴石Ⅰ型边反映的峰期P-T条件为2.4~2.6GPa、570~585℃,和基质中多硅白云母Si含量等值线限定范围一致,对应硬柱石蓝闪石榴辉岩组合。石榴石Ⅱ型边P-T范围为1.9~2.4GPa、530~570℃,低于峰期条件。在可能的峰后降压过程中,岩石先后主要经历了硬柱石脱水生成绿帘石和蓝闪石、绿辉石退变为冻蓝闪石的反应阶段。绿辉石、冻蓝闪石发育的后成合晶说明晚期退变过程缺乏流体,石榴石的韭闪石冠状体也可能在该阶段产生,都受局部成分域控制。红安高压榴辉岩中各矿物与成分代表不同变质阶段,称其为冻蓝闪石榴辉岩只是对现有主要组成矿物的描述,不是基于共生关系的严格岩石学命名。     

大别造山带研子岗碱性岩体成因及其构造意义：锆石U-Pb年龄和地球化学制约

出露于大别造山带西南部的研子岗碱性杂岩体侵位于元古代和新太古代随县群中,本文采用锆石LA-ICPMS U-Pb定年方法,获得岩体的主体岩性角闪正长岩的岩浆锆石年龄为133± 1Ma,代表岩体的结晶年龄,这一年龄也是大别造山带中已知形成时代最早的早白垩世碱性岩体的结晶年龄（单颗粒锆石U-Pb法）。研子岗碱性杂岩体具有碱性岩典型的富碱（K₂O＋Na₂O＝8.38%~11.26%）、低硅（SiO₂＝63.41%~66.51%）的特点。主要矿物为微斜条纹长石,暗色矿物主要为镁角闪石。地球化学特征表明,全部岩石均具有高Ba-Sr花岗岩类高Ba（1230× 10^-6~4865× 10^-6）、高Sr（583× 10^-6~2088× 10^-6）和无负Eu异常（Eu/Eu^*＝0.97~1.12）等特征,并具有A型花岗岩的部分地球化学特征。岩石的Y/Nb比（0.46~1.09）较低,具洋岛玄武岩的部分地球化学特征,(⁸⁷Sr/⁸⁶Sr) _i初始比值（0.70513~0.70543）较低,表明岩体的物源主要来源于幔源。Nd二阶段模式年龄(t_2DM=1859~1942Ma)和锆石Hf二阶段模式年龄(t_DM2=2130~2330Ma）较老,ε_Nd(133)值(-12.5~-11.4）和ε_Hf(133)值(-18.2~-15.4）较低。综合分析表明,岩体主要为古老的富集岩石圈地幔物质低程度部分熔融和随后地壳物质轻度AFC（混染和分离结晶作用）过程的产物,源岩熔融的热量主要来源于软流圈物质的上涌底侵作用提供。研子岗岩体形成于碰撞后构造环境,岩体的形成预示着大别造山带早白垩世造山过程的即将结束,板内时期的即将来临。     

北大别超高压榴辉岩的快速折返与缓慢冷却过程

岩石学研究表明,北大别超高压榴辉岩经过了超高压和高压榴辉岩相变质作用以及麻粒岩相叠加和角闪岩相退变质作用.其中,高压麻粒岩相和角闪岩相变质阶段形成的后成合晶以及石榴子石和单斜辉石等矿物中成分分带的存在,证明该区榴辉岩经历了一个快速折返过程;而不同变质阶段的温度、压力和形成时代,却反映该区榴辉岩在峰期超高压变质作用之后又经历了一个缓慢冷却过程.超高压岩石折返期间的缓慢冷却过程也许正是北大别长期难以发现柯石英和有关超高压证据的重要原因.因此,本文为大别山不同超高压岩片的差异折返模型的建立提供了新的证据.     

Deformation and metamorphism in the Fenes Nappe (southern Apuseni Mountains,Romania)Déformation et métamorphisme dans la nappe de Fenes (monts Apuseni méridionaux,Roumanie)

The Fenes Nappe belongs to the stack of tectonic units cropping out in the southern Apuseni Mts (Romania). It is characterised by a structural history consisting of two folding phases that developed during the time spanning from Early Aptian to Late Maastrichtian. The D1 phase produced west-northwest-verging, isoclinal to very tight folds, associated to a slaty cleavage. The main metamorphic imprint of the Fenes Nappe is linked to this deformation phase; illite and chlorite ‘crystallinity’ values indicate metamorphic conditions of the late diagenesis, close to the diagenetic zone/anchizone boundary. The subsequent D2 phase produced north-northwest-verging, parallel folds, not associated with synkinematic recrystallisation. These phases are interpreted as developed during a structural path, which includes burial at a depth of 8–10 km, followed by exhumation at shallower structural levels. To cite this article: A. Ellero et al., C. R. Geoscience 334 (2002) 347–354.     

TECTONICS OF THE DABIE OROGENIC BELT, CENTRAL CHINA

Tectonically the Dabie orogenic belt consists mainly of the Dabieshan Yanshanian uplifted zone and the Beihuaiyang Variscan-Indosinian folding zone. In the north boundary adjoining the North China Block, there are an Early Palaeozoic ophiolitic mixtite belt and the Hefei Mesozoic-Cenozoic faulted basin which overlaps on the suture belt. In the south of Dabie orogen, there is a secondary tectonic unit called Foreland thrust-faulted structural zone which was mainly formed by the intracontinental subductions during Mesozoic era. The study shows that the Dabie Block is a part of mid-late Proterozoic palaeo-island arc at the north margin of Yangtze Block. During Caledonian period, as a submerged uplift at the northen continental margin of Yangtze Block, the Dabie Block collided with the early Palaeozoic palaeo-island arc at the south margin of North China Block, resulting in the convergence of the North and South China Blocks and the disappearance of oceanic crust. Since then,large-scale intracontinental subductions were followed. Dabie Orogenic Belt is the product of overlapping of Yangtze Block, Dabie Block and North China Block under the mechanism of intracontinental subduction. Indosinian period is the period of chief deformation and high pressure dynamic metamorphism for Dabie Block, and Yanshan period is the main orogenic period in which the remelting of crust caused by basement shearing resulted in large scale thermometamorphism. The present tectonic framework of the orogen was finally formed by the rapid uplifting of the Dabieshan mountains and gliding southwards, which result in the developing of thrust belt on south side and the extensional tectonic movement on north side.     

U—Pb Dating of Marble Assoicated With Eclogite from the Dabie Mountains,East China

Marble is associated with ultrahigh pressure eclogite in the Dabie Mountains, East China. U-Pb isotope data for the marble define a²³⁸U-²⁰⁶Pb isochron age of 435 ± 45 Ma with an initial²⁰⁶Pb/²⁰⁴Pb value of 18.075 ± 0.006. The age of ∼435 Ma is interpreted to represent the time of limestone diagenesis rather than the time of metamorphic resetting accompanied by the formation of the ultrahigh pressure eclogites at Triassic. The paleontologic and stratigraphic studies also favor a local excursion in the carbon isotopic composition of the latest Ordovician water within a sedimentary basin between the Sino-Korean and Yangtze cratons. Because the latest Ordovician limestone was involved in the eclogite formation, the continental collision between the Yangtze and Sino-Korean plates would postdate Ordovician. This project was financially supported by the National Natural Science Foundation of China and the Chinese Academy of Sciences.     

剪切带中流体与金矿成矿作用的关系综述

流体广泛存在于剪切带中，在剪切带中汇集，常以通道式运移；流体的渗透和流动引起剪切带的成分变异和体积亏损；剪切中的流体往往携带大量成矿物质而成为矿流体，金矿的形成与流体关系密切；     

豫南─鄂北大别山北部高压角闪石榴辉岩的研究

一个高压角闪石榴辉岩带出现在豫南─鄂北大别山高压超高压变质单元的最北部。榴辉岩的矿物组合为石榴石＋绿辉石＋角闪石＋绿帘石＋多硅白云母＋石英＋金红石。采用Ｐｏｗｅｌｌ等（１９９４）的Ｔｈｅｒｍｏｃａｌｃ估计的温压条件：压力为１．８－２．４ＧＰａ，温度为４９０－５９２℃。这个带的榴辉岩在矿物组合，矿物成分和温压条件上明显不同于该高压变质单元中其它带的榴辉岩。     

大别造山带北部变质花岗岩岩石谱系单位划分及特征

1：5万区调工作将原大别杂岩解体划分为变质表壳岩、变质超镁铁质岩、变质花岗岩三部分。变质花岗岩广泛分布于大别遣山带中。通过区内变质花岗岩的研究发现，成分上构成了闪长质(石英闪长质)一石英二长闪长质、石英二长质的演化趋势；结构上由细粒结构一中细粒结构一中细粒粗斑状结构一中细粒巨斑状结构的规律性变化。参照岩石谱系单位划分原则对其进行划分、归并。在空间上相伴生，具有同源岩浆演化特征，建立了岳西片麻岩套，并对其岩石学、岩石化学、地球化学特征作了阐述，确定其属岛弧环境的钙碱性岩石，为正确认识大别遣山带的早期构造演化提供了重要的地质依据。