Experimental study on dehydration melting of natural biotite-plagioclase gneiss from High Himalayas and implications for Himalayan crust anatexis

Here we present the results of dehydration melting, melt morphology and fluid migration based on the dehydration melting experiments on natural bio-tite-plagioclase gneiss performed at the pressure of 1.0-1.4 GPa, and at the temperature of 770-1028℃. Experimental results demonstrate that: (i) most of melt tends to be distributed along mineral boundaries forming 'melt film' even the amount of melt is less than 5 vol%; melt connectivity is controlled not only by melt topology but also by melt fraction; (ii) dehydration melting involves a series of subprocesses including subsoiidus dehydration reaction, fluid migration, vapor-present melting and vapor-absent melting; (iii) experiments produce peraluminous granitic melt whose composition is similar to that of High Himalayan leucogranites (HHLG) and the residual phase assemblage is Pl+Qz+ Gat+Bio+Opx±Cpx+IIm/Rut±Kfs and can be comparable with granulites observed in Himalayas. The experiments provide the evidence that biotite-plagioclase gneiss is one of

Dynamic response mechanism and precursor characteristics of gneiss rockburst under different initial burial depths

To investigate the influence mechanism of geostress on rockburst characteristics,three groups of gneiss rockburst experiments were conducted under different initial geostress conditions.A high-speed photography system and acoustic emission(AE)monitoring system were used to monitor the entire rockburst process in real time.The experimental results show that when the initial burial depth increases from 928 m to 1320 m,the proportion of large fracture scale in rockburst increases by 154.54%,and the AE energy increases by 565.63%,reflecting that the degree and severity of rockburst increase with the increase of burial depth.And then,two mechanisms are proposed to explain this effect,including(i)the increase of initial geostress improves the energy storage capacity of gneiss,and then,the excess energy which can be converted into kinetic energy of debris ejection increases,consequently,a more pronounced violent ejection phenomenon is observed at rockburst;(ii)the increase of initial geostress causes more sufficient plate cracks of gneiss after unloading ofσh,which provides a basis for more severe ejection of rockburst.What’s more,a precursor with clear physical meaning for rockburst is proposed under the framework of dynamic response process of crack evolution.Finally,potential value in long term rockburst warning of the precursor obtained in this study is shown via the comparison of conventional precursor.

Geochronological and geochemical constraints on the granitic gneiss in the Huozhou Complex: implications for the tectonic evolution of the Trans-North China Orogen

The Trans-North China Orogen is a major Neoarchean Paleoproterozoic collisional orogenic belt above the North China Craton, formed due to prolonged and complex processes. Even though many NeoarcheanPaleoproterozoic magmatic and metamorphic activities have been reported, due to the Huozhou Complex’s small outcropping range, little attention has been paid to the origin of various igneous rocks of the Huozhou Complex in the center of the Trans-North China Orogen. The Huozhou Complex, located south of the Luè liang, Wutai, and Hengshan complexes, is an important window into the Early Precambrian structure and evolution of the North China Craton. Its magma and metamorphism are crucial to understanding the development of the structural evolution of the Trans-North China Orogen. The Huozhou metamorphic complex area exposes a range of Precambrian metamorphic rocks, among which the most extensively dispersed is felsic biotite plagioclase gneiss. In this study comprehensive geological field survey, micropetrology,chronology, geochemistry, and Hf isotope analysis were carried out for the Qinggangping and Anziping gneiss in the north of the Huozhou Complex. The results show that the magmatic zircon age of the Qinggangping gneiss is2196 ± 14 Ma, and its protolith is I-type granite, formed by partial melting of igneous rocks in the absence of weathering. Its source is mainly the juvenile crust from depleted mantle dating 2431–2719 Ma, with a small amount of mantle-derived material. The Anziping gneiss has a metamorphic zircon age of 1931 ± 13 Ma with an S-type granite protolith belonging to peraluminous granite.The Anziping gneiss is formed by recycling pre-existing crustal components at 2613–2848 Ma. A minor quantity of mantle-derived magma is also introduced to the crust simultaneously. The samples of Qinggangping gneiss and Anziping gneiss show the characteristics of obvious negative Nb, Ti, and P elements in the spider diagram of primitive mantle standardization. This implies that the rocks have the characteristics of magmatic rocks in an island arc or subduction environment, which could have formed in the tectonic environment of the continental margin arc.

苏鲁造山带新沂地区新元古代花岗片麻岩成因及对Rodinia超大陆裂解的响应

新沂地区花岗片麻岩位于苏鲁造山带的西缘。本文通过新沂地区花岗片麻岩岩相学、岩石地球化学、锆石U-Pb同位素年代学等方面的研究,探讨其成因与构造环境,以揭示Rodinia超大陆裂解事件在该地区的反响。研究认为:研究区花岗片麻岩属准铝质-弱过铝质A型花岗岩,具有高SiO_(2)、富碱、贫CaO、低Al_(2)O_(3)质量分数的特征,以及右倾海鸥型稀土元素配分模式,富集Rb、Zr、Hf等元素,严重亏损Sr、Eu、Nb、Ta等元素,形成年龄为746.0~742.5 Ma。新沂地区花岗片麻岩是来自下地壳物质为主、少量幔源物质的部分熔融,在岩浆演化过程中经历了以钾长石和斜长石为主的分离结晶,而后经过超高压变质作用最终形成。研究区花岗片麻岩形成于新元古代后碰撞伸展环境,是Rodinia超大陆裂解事件在苏鲁造山带新沂地区的最初响应。

柴北缘欧龙布鲁克地块中元古代晚期麻粒岩相变质作用——来自石榴夕线堇青石片麻岩的岩石学、相平衡模拟和U-Pb年代学的制约

在欧龙布鲁克地块乌兰北部地区察汗河岩群中识别出麻粒岩相石榴夕线堇青石片麻岩,其矿物组合为石榴子石、夕线石、堇青石、黑云母、斜长石、钛铁矿和少量钾长石等。岩相学观察显示,M1阶段矿物组合有斜长石±钾长石+石榴子石+夕线石+石英,M2阶段矿物组合有斜长石±钾长石+石榴子石+夕线石+石英+钛铁矿+黑云母,M3阶段矿物组合有堇青石+黑云母+钛铁矿+石英+石榴子石+斜长石±钾长石。相平衡模拟计算结果显示,该岩石的峰期温压条件为p=0.92~1.08 GPa,峰期温度t>790℃,峰期之后经历升温降压的p-T演化轨迹。锆石和独居石LA-ICP-MS U-Pb年代学研究获得的变质年龄分别为1133±14 Ma和1125±37 Ma,1133~1125 Ma应代表了该期麻粒岩相变质作用的时代。结合区域地质资料和已有的研究成果,我们认为乌兰北部察汗河岩群的石榴夕线堇青石片麻岩可能形成于大洋俯冲作用下的弧或弧后构造环境,乌兰北部的岩浆-变质杂岩带经历了从中元古代晚期-新元古代早期俯冲增生到碰撞造山的演化过程,是全球Rodinia超大陆汇聚过程的响应。

川西扎乌龙地区卡吉亚二云母花岗岩、稀有金属伟晶岩的地球化学、年代学特征及二者的成矿关系

锂-铍等稀有金属是中国乃至世界上重要的战略性关键金属矿产,美国、澳大利亚和日本等国早已将其列为禁止出口矿产名录,其战略地位非同一般。中国锂-铍矿产资源丰富,成矿背景优越,但地表第四纪覆盖严重,且分布不均。近年甜水海-松潘-甘孜造山带花岗伟晶岩型锂矿资源逐步被勘探发现,引起国内外学者广泛关注。川西石渠县扎乌龙锂矿作为该造山带中部重要的花岗伟晶岩型锂矿点,厘清该矿床锂资源聚集主控因素及其稀有金属富集规律将为甜水海-松潘-甘孜造山带下一步战略找矿行动提供指导和重要决策依据。本文对扎乌龙地区锂辉石钠长石伟晶岩脉、白云母钠长伟晶岩脉、电气石钠长石伟晶岩脉、卡吉亚二云母花岗岩体以及岩体边部的含电气石细晶岩、十字石黑云母片岩等进行系统采样分析,利用LA-ICP-MS独居石U-Th-Pb同位素定年技术,确定扎乌龙卡吉亚二云母花岗岩形成于晚三叠世213~211 Ma,含电气石细晶岩约形成于210 Ma,白云母钠长石伟晶岩约形成于210~200 Ma,富锂伟晶岩可能在205 Ma集中产出。结合前人年代学资料,综合分析认为扎乌龙矿区由二云母花岗岩、电气石细晶岩到白云母钠长石伟晶岩和锂辉石钠长石伟晶岩形成时代逐步年轻。210 Ma以后,岩浆演化逐步进入岩浆-热液阶段,在约10 Ma时间内发生了持续、多期次的岩浆侵位活动。此外,结合花岗岩向伟晶岩逐步降低的REE、K/Rb、Nb/Ta和Zr/Hf等微量特征值,以及高演化伟晶岩独居石矿物Nd、Eu负异常现象,表明花岗质岩浆的结晶分异作用在扎乌龙伟晶岩稀有金属富集过程中发挥着重要作用。

中下地壳切向分层流变的结果:喜马拉雅东段雅拉香波片麻岩穹隆

大陆中、下地壳切向(近水平)分层固态流动变形是地壳物质流动的重要形式之一,也是片麻岩穹隆的重要形成机制。雅拉香波穹隆位于特提斯喜马拉雅构造带的最东段,出露不同变质级别和时代的岩石地层,发育强烈的韧性剪切变形以及多期岩浆事件,是研究造山过程中构造变形和岩浆历史的天然实验室。本文以该穹隆为研究对象,进行了详细的野外构造解析和显微观察等工作,总结出以下三个特点:(1)雅拉香波穹隆内不同构造层次的岩石经历了相同的构造体制和不同变形条件改造:从浅部到深部,变形温度逐渐递增,由390℃到600℃;差应力逐渐减小,从24.58MPa减少至8.72MPa;应变速率逐渐加快,从1.27×10^(-13)~1.28×10^(-13)/s增加到5.19×10^(-11)~5.21×10^(-11)/s。以上体现了地壳活动带强烈的分层流变特点。(2)结合前人研究划分了穹隆变形的三个期次(D_(1)、D_(2)和D_(3)),其中D_(1)表现为上盘向南的剪切方向,D_(2)则表现为上盘向北的剪切方向。进一步,将主要变形期次D_(2)进一步划分为两个阶段,早期主要是以单剪为主导的剪切作用类型,而晚期则是以纯剪为主导的剪切作用类型。(3)根据D_(2)面理和线理的产状分布特点,可以得出,深部岩石线理的倾伏角近水平,而浅层次岩石的线理倾伏角近竖直。基于以上研究表明,雅拉香波穹隆各部分岩石均遭受了不同程度的剪切改造,不同构造层次的岩石具有几何学上的一致性以及运动学上的解耦,体现了穹隆发育过程中运动方向上的转变。结合穹隆各部位线理的倾伏角的变化规律,本文认为雅拉香波穹隆记录了中下地壳分层流动的过程,穹隆的形成主要受中下地壳近水平切向流动控制,辅以垂向流动的改造。

含氟流体助熔华南基底片麻岩的实验研究:对稀土成矿花岗岩成因的制约

为了明确含氟流体对花岗岩源区岩浆过程中稀土元素地球化学行为的影响,本文采用华南代表性的高稀土片麻岩和低稀土片麻岩作为初始物质,与~4%的流体(纯水、1.5 mol/L HF、1.0 mol/L NaF)在0.8 GPa、1000℃的条件下开展了流体助熔的部分熔融实验。实验结果表明,所有片麻岩的熔融程度达到40%以上,残余矿物组合主要为斜长石+石英+斜方辉石±单斜辉石+钛铁矿+磷灰石±黑云母。部分熔融产生的熔体的成分主要为花岗闪长质‒花岗质,总体具有高氟(0.11%~0.27%)、高水(0.38%~1.86%)和A2型花岗岩的特征;其中,高稀土片麻岩部分熔融形成的熔体为钾玄质和准铝质,而低稀土片麻岩熔融形成的熔体偏高钾钙碱性和强过铝质。在微量元素蛛网图上,两者均表现为富集K、Rb、Th、Ce、Sm、Y、Yb等元素,亏损Ba、Nb、Ta、Zr、Hf、Ti等元素;稀土元素配分曲线则表现为右倾、负Eu异常的特征。相对于原岩,含氟流体助熔会强化熔体中Si、K、Na、Rb、Sr、Ba、Th、U、LREE和F的富集,并增加熔体的La_(N)/Yb_(N)值。同时,实验也证实片麻岩部分熔融形成的熔体的稀土元素含量主要受原岩控制,高稀土片麻岩更易于熔出稀土元素初始富集的熔体;相对于原岩,各类流体助熔均会造成稀土元素在熔体中不同程度的富集,其中,含HF流体助熔最高可富集稀土46%~49%(近1.5倍富集)。我们因此认为在华南LREE型和HREE型成矿花岗岩形成过程中,含F流体发挥重要的作用。此外,本文也提出了一种新的A2型花岗岩形成机制,即含F流体助熔地壳基底岩石,这很好地解释了华南晚中生代大规模分布的与稀土、稀有金属及氟成矿相关的陆内A2型花岗岩的形成。

辽西兴城娘娘顶地区花岗质片麻岩形成时代与变形样式

辽西兴城地区出露的大面积花岗质岩石遭受了强烈的韧性变形改造,其独特的变形样式为研究华北克拉通东北缘中生代以来的构造演化提供了独特的视角。兴城娘娘顶地区出露的花岗质片麻岩塑性变形特征显著,矿物拉伸线理均以低角度向NNE倾伏,整体表现出上盘向SSW的逆冲型韧性剪切变形。最新的花岗质片麻岩中锆石U-Pb年代学结果显示,其原岩形成时代为(169.5±1.6)Ma,为中侏罗世。典型变形岩石的石英EBSD(电子背散射衍射)分析、流变学参数估算,指示变形岩石古差异应力值为13.8~17.7 MPa,应变速率为1.16×10^(-16)~2.20×10^(-14)s^(-1),变形温度介于350~450℃之间。综合研究认为该期变形为发育于中浅部地壳层次绿片岩相的缓慢变形,与晚侏罗世末期—早白垩世早期蒙古—鄂霍茨克洋俯冲闭合的远程效应和古太平洋板块俯冲作用的叠加影响有关。

内蒙古中部乌拉山-大青山地区富铝片麻岩年代学特征

内蒙古中部乌拉山-大青山出露的乌拉山岩群是一套变质程度达高角闪岩相-麻粒岩相的早前寒武纪高级变质岩系。为划分乌拉山-大青山地区富铝片麻岩的原岩形成时代与变质时代,通过对侵入乌拉山岩群黑云石榴斜长片麻岩的变质深成体—石英闪长质片麻岩和紫苏花岗质片麻岩变质深成体进行年代学研究。结果表明:紫苏花岗质片麻岩(TW7)的加权平均年龄为(2 462±13)Ma,花岗闪长质片麻岩(D1227)加权平均年龄为(2 512±24)Ma,由于后期构造事件对岩石的改造,岩体的实际年龄应更大一些。由此限定乌拉山富铝片麻岩原岩的形成年代早于2.5 Ga。石英闪长质片麻岩(TWB6)具有两期变质年龄,分别为2.2 Ga和1.9 Ga,指示乌拉山-大青山地区遭受过古元古代早期和晚期构造热事件叠加改造。上述研究结果表明,乌拉山-大青山地区富铝片麻岩最晚形成时代限定在2.5 Ga。

江苏东海双湖矿区建筑用片麻岩矿床地质特征及矿石加工技术性能

江苏省东海县双湖矿区建筑用片麻岩为宋山二长片麻岩的一部分,矿区范围即矿体范围。矿石自然类型为二长片麻岩,水饱和抗压强度为70.0~165.5MPa;组合分析测试结果显示优于建筑用石料Ⅰ类要求,矿石的吸水率满足建筑用石料要求,矿石化学成分中有害物质含量符合建筑用石料Ⅱ类指标要求。矿石加工技术性能结果显示,建筑用片麻岩矿经加工后的产品均可满足相应的利用要求。

北苏鲁威海地区共生伟晶岩脉-钙硅酸盐岩的形成机制和地质意义

威海位于苏鲁造山带最北端,出露片麻岩和榴辉岩等多类超高压岩石,是研究大陆俯冲带变质、深熔和熔体效应的理想靶区。基于详细野外观察,本文对该区的一套钙硅酸盐岩、伟晶岩脉和寄主片麻岩进行了岩相学、地球化学和锆石年代学等研究,旨在深入探究超高压变质岩的多期熔融及与之伴随的地质过程。阴极发光显示,三类岩石中的锆石整体具有核-边结构,继承核年龄为859±20Ma~272±6Ma,与苏鲁变质岩内大多残余岩浆锆石的年龄一致。新生锆石共记录了约237±4Ma、224±7Ma~218±2Ma和177±5Ma~166±3Ma等三组年龄。结合不同微区的稀土元素特征,本文认为,237±4Ma记录了峰期变质时代,而224±7Ma~218±2Ma和177±5Ma~166±3Ma年龄的锆石呈现典型的深熔锆石特征,分别代表俯冲陆壳折返至高压榴辉岩相和加厚地壳初始伸展阶段的部分熔融年代。伟晶岩与片麻岩中继承锆石的Hf同位素组成不同,表明形成伟晶岩的熔体并非寄主岩石直接熔融而来。此外,钙硅酸盐岩及其内部的单斜辉石具有极低的Cr、Ni等相容元素含量和负Eu异常特征,且均显示大离子亲石元素富集、高场强元素亏损的特点。并且,钙硅酸盐岩与苏鲁超高压片麻岩的Sr-Nd同位素组成接近,其内部继承/新生锆石与伟晶岩中相应锆石微区的Hf同位素组成一致。综合分析上述数据并结合前人成果,本文推测,钙硅酸盐岩为深俯冲陆壳折返阶段长英质熔体与片麻岩内部的大理岩团块发生熔岩反应所致。综上,本研究进一步约束了俯冲陆壳在折返及造山垮塌阶段两期熔融的时间及熔体来源等信息,并为大陆俯冲带深部的熔体效应和物质循环提供了参考。

大姚县秀水河铜金矿成矿地质条件及找矿前景探讨

大姚县秀水河铜金矿矿体主要赋存于花岗质片麻岩、糜棱岩化白云英片岩中,受构造破碎带控制,岩石劈理、节理裂隙发育,较破碎,具硅化、碳酸盐化等特征,矿化主要为铜矿化及褐铁矿化,见孔雀石、褐铁矿呈细脉状、薄膜状、浸染状沿破碎带分布,局部可见少量黄铜矿、蓝铜矿、斑铜矿呈散点状分布。通过区内土壤地球化学测量工作,发现区内元素组合异常与已知矿化带吻合较好,异常浓集中心与已知矿点相对应,且异常强度高,指示该区具有良好的成矿地质条件及较大的找矿前景。

辽西新太古代片麻岩金矿中金与伴生元素的多重分形模型剖析

新太古代片麻岩在辽西地区分布面积广阔,其中发育的金矿矿产资源丰富。为了研究辽西地区新太古代片麻岩金矿中共伴生元素对于Au元素成矿的重要性,本文以辽西地区新太古代片麻岩中典型金矿床及矿化点矿体的组合分析数据为样本,运用多重分形理论进行Ag、Cu、Pb、Zn、S元素质量分数与金元素质量分数的相关性分析,进而建立了基于多重分形理论的金与共伴生元素多重分形模型。根据多重分形的拟合分析结果显示,Au、Cu、Pb、Zn和S元素的拟合曲线分布状态均呈明显的下凸型,其均匀程度介于正态分布和指数分布之间,广义分维值均大于1.3,说明它们之间存在明显的相关性。而银元素的拟合曲线分布状态明显不同于其它元素,说明银元素的富集不受或受到其它元素的影响很小。结合元素亲和性特征,以硫元素为核心,分析了Au、Ag、Cu、Pb、Zn的亲硫性,并将新太古代片麻岩金矿中金的演化过程划分成四个阶段,分别为①黄铁矿富集阶段,②自然金伴生阶段,③交代充填阶段,④变质氧化阶段。根据多重分形模型的拟合分析结果,并结合前人的理论研究成果表明:新太古代片麻岩金矿中共伴生元素组合对于金元素成矿极为重要,它为后期找矿的方向及隐伏矿床的发现提供了重要的指示。

High- and Ultrahigh-pressure Metamorphism and Retrogressive Textures of Gneiss in the Donghai Area——Evidence from gneisses in drillhole ZK2304

In the gneisses from the drillhole ZK2304 of the Donghai area, there have been preserved high- and ultrahigh-pressure metamorphic mineral assemblages, a series of complicated retrogressive textures and relevant metamorphic reactions. In addition to garnet, jadeititic-clinopyroxene and rutile, other peak stage (M2) minerals in some gneisses include phengite, aragonite and coesite or quartz pseudomorphs after coesite. The typical peak-stage mineral assemblages in gneisses are characterized by garnet + jadeitic-clinopyroxene + rutile + coesite, garnet + jadeitic-clinopyroxene + phengite + rutile ± coesite and garnet + jadeitic-clinopyroxene + aragonite + rutile ± coesite. The grossular content (Gro) in garnet is high and may reach 50. 1 mol%. The SiO2 content of phengite ranges from 54.37% to 54.84% with 3.54-3.57 p.f.u. Quartz pseudomorphs after coesite occur as inclusions in garnet.The gneisses of the Donghai area have been subjected to multistage recrystallization and exhibit a closewise P-T evolutional path characterized by the near-isothermal decompression. The inclusion assemblage (Hb+Ep+Bi+Pl+Qz) within garnet and other minerals has recorded a pre-peak stage (Mi) epidote amphibole fades metamorphic event. High- and ultrahigh-pressure peak metamorphism (M2) took place at T=750-860℃ and P>2.7 GPa. The symplectitic assemblages after garnet, jadeitic-clinopyroxene and rutile imply a near-isothermal decompression metamorphism (M3, M4) during the rapid exhumation. Several lines of evidence of petrography and metamorphic reactions indicate that both gneisses and eclogites have experienced ultrahigh-pressure metamorphism in the Donghai area. This research may be of great significance for an in-depth study of the metamorphism and tectonic evolution in the Su-Lu ultrahigh-pressure metamorphic belt.

Geochemical Characteristics and Genesis of the Granitic Gneisses from the Southeastern Dabie Mountains

The granitic gneisses from the ultrahigh-pressure (UHP) metamorphic terrain of the southeastern Dabie Mountains encompass two types: monzonitic granitic gneiss and alkali-feldspar granitic gneiss, which are characterized by rich alkalis, poor CaO, high FeO/MgO, particularly high Ba, Rb, Th, Ta, REE (except Eu), Ga, Nb and Zn, and low Sr, Eu, Cr, Co and Ni. The gneisses, particularly the alkali-feldspar granitic gneiss, have typical chemical characteristics of A-type granites. They resulted from partial melting of crustal materials existing in the rift zone along the northern margin of the South China block during the Neoproterozoic. These gneisses might not have undergone UHP metamorphism during the late Triassic, but were involved into UHP rocks by the tectonic mixing process and kept the exhumation message of the UHP rocks from the middle and upper crust.

SHRIMP Dating and Recrystallization of Metamorphic Zircons from a Granitic Gneiss in the Sulu UHP Terrane

An unusual zircon SHRIMP dating result of a granitic gneiss from the Qinglongshan eclogite-gneiss roadcut section is presented in this paper. The very peculiar and complicated internal structures, as well as the very low Th/U ratios (0.01-0.08) of the zircons indicate that they were formed by metamorphic recrystallization. Strongly in contrast with previously published zircon U-Pb ages of the Dabie-Sulu UHP metamorphic rocks where protolith ages of 600-800 Ma are commonly recorded, only metamorphic age of 218±5 Ma, defined by 18 analytical spots either in rim or in core of zircons, are recorded in this granitic gneiss. This age represents the time of the complete metamorphic recrystallization overprint on primary magmatic zircons. The recrystallization was derived by the UHP metamorphism, and was strengthened by the early stage of retrograde metamorphic fluid activity.

Origin and Geological Significance of TTG Gneisses from the Maevatanana Greenstone Belt in North-Central Madagascar,and A Comparison with India

The Maevatanana greenstone belt in north-central Madagascar contains widespread exposures of tonalite-trondhjemite-granodiorite （TTG） gneisses, and is important for its concentrations of various metal deposits （e.g., chromium, niekle, iron, gold）. In this paper we report on the petrography, and major and trace element compositions of the TTG gneisses within the Berere Complex of the Maevatanana area, as well as LA-ICP-MS U-Pb ages and Lu-Hf isotopic compositions of zircons from the gneisses. The gneisses consist mainly of granitoid gneiss and biotite （± hornblende） plagiogneiss, and analysis of thin sections provides evidence of crushing, recrystallization, and metasomatism related to dynamic metamorphism. Samples have large variations in their major and trace element contents, with SiO2 = 55.87-68.06 wt%, Al2O3 = 13.9-17.8 wt%, and Na2O/K2O= 0.97-2.13. Geochemically, the granitoid gneisses and biotite plagiogneisses fall on a low-Al trondhjemite to granodiorite trend, while the biotite-hornblende plagiogneisses represent a high-Al tonalite TTG assemblage. Zircon U-Pb dating shows that the Berere Complex TTG gneisses formed at 2.5-2.4 Ga. Most εHf（t） values of zircons from the biotite （q- hornblende） plagiogneisses are positive, while most εHf（t） values from the granitoid gneisses are negative, suggesting a degree of crustal contamination. Two-stage Hf model ages suggest that the age of the protolith of the TTG gneisses was ca. 3.4-2.6 Ga, representing a period of paleocontinent formation in the Mesoarchean. Geothermometries indicate the temperature of metamorphism of the TTG gneisses was 522-612℃. Based on these data, the protolith of the TTG gneisses is inferred to have formed during the development of a Mesoarchean paleocontinent that is now widely exposed as a TTG gneiss belt （mostly lower amphibolite facies） in the Maevatanana area, and which records a geological evolution related to the subduction of an ancient oceanic crust and the collision of microcontinents during the formation of the Rodinia supercontinent. The lithological similarity of Precambrian basement, the close ages of metamorphism within greenstone belts and the comparable distribution of metamorphic grade all show a pronounced Precambrian geology similarity between Madagascar and India, which can provide significative clues in understanding the possible Precambrian Supercontinent tectonics, and also important constraints on the correlation of the two continental fragments.

Geochemistry of Gneisses from Dabie Complex and Tongbai Complex in Qinling-Tongbai-Dabie Orogenic Belt:Implications for Location of Yangtze-Sino-Korean Suture

The Dabie complex (DC) and the Tongbai complex (TBC) are separately distributed in the middle and eastern parts of the Qinling-Tongbai-Dabie orogenic belt. In this study, the Dabie complex can be divided into two units: one is the complex with no high pressure and ultrahigh pressure metamorphic rocks (DC1), and the other is the complex containing coesite-bearing eclogite lenses or boudins (DC2). Gneisses are predominant in the TBC, DC1 and DC2. Major and trace element data of gneisses in the TBC, DC1 and DC2 show them to be the orthogneisses. The gneisses in the DC1 have higher incompatible element contents and higher ratios of w(K 2O)/w(Na 2O) and w(La) n/w(Yb) n than those in the DC2. However, no obvious differences arise in other element contents and the ratios of w(La)/ w(Nb), w(Nb)/w(Th), w(Nb)/w(Hf), w(Ba)/w(La), w(Sm)/w(Nd) and w(Th)/w(U) between the gneisses in the DC2 and those in the DC1. These observations suggest that the protoliths of the gneisses in the DC2 have affinities to those in the DC1. The difference between the DC1 and DC2 gneisses in incompatible element contents could reflect the difference in their partial melting extent. The TBC gneisses are geochemically similar to the DC1 gneisses, suggesting that the TBC and DC1 gneisses are the same lithologic unit in the Qinling-Tongbai-Dabie orogenic belt and that they have experienced similar formations and evolution histories. In the Qinling-Tongbai area, the TBC is part of the northern blocks of the Yangtze craton. Given the similarity of geochemical characteristics, the rock assemblage and the ages between the TBC and DC1 gneisses, we can infer that the Dabie complex also belongs to the northern blocks of the Yangtze craton. In terms of the distribution of eclogites and metamorphic facies, we propose that the collisional suture in the Dabie area is distributed along the Xiaotian-Mozitan fault, at the contact with the Shang-Dan-Tongbai fault to the west.

Petrological Implication of the Albite Rims in the Felsic Gneisses of the Fuping Complex

The albite rim is present in most felsic gneisses of the Fuping Complex. The presence of the rim indicates the coexistence of plagioclase and K-feldspar in the rock. The rim is formed immediately after the myrmekite, and both textures were derived from the alteration of K-feldspar. The difference is that that there is no quartz present in the rim, and the rim is nearly albite and the anorthite content of the rim plagioclase is substantially lower than that of the myrmekite plagioclase. Formed at 400- 500~C the albite rim was derived from the K-feldspar composition adjustment in the late or post- magmatism stage. As the temperature decreased, the equilibrium between K-feldspar and plagioclase could be maintained, and reactions between the minerals occurred. The leucocratic veins in the complex show distinguished magma or migmatitic characteristics. The rim might form in the late magma or deuteric stage. The formation of the rim implies obvious granitic magmaor melt-injection activity. Typical metamorphic rocks cannot produce the rims. Anatexis after medium-high grade metamorphism might be subordinate. If present, the anatexis is water-present, but the rim texture cannot be taken as the symbol of anatexis.