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1.
峨眉山玄武岩的辉石研究   总被引：1，自引：0，他引：1       下载免费PDF全文
本文研究的峨眉山玄武岩的辉石均为单斜辉石。东川碱性火山岩中以透辉石为主,显示碱性岩系辉石的特征,但相对贫 LREE、Ti 等不相容元素,结晶时的 fO2较高;攀枝花岩带主要是透辉石质普通辉石;其他剖面和地区多为普通辉石,且具拉斑质岩系富钙辉石的成分和演化特点。攀枝花带和西岩区辉石在化学成分上比较原始,而东岩区和中岩区辉石的成分演化程度相对较高。含 Ti 的“其他”阳离子对在各岩区（带）的辉石中均限重要,暗示它们的形成与火山弧环境无关。  相似文献   

2.
大通沟南山地区基性-超基性岩体位于柴达木地块西北缘,侵位于新元古代地层。岩体多呈北东向展布,脉状、透镜状产出,岩石类型主要为橄榄辉石岩、辉石角闪石岩、辉石橄榄岩、橄榄二辉岩等,铜镍矿化主要赋存于橄榄辉石岩、辉石角闪石岩中。岩石表现出低硅、中镁、高钛、高钙等特征,富集轻稀土元素和大离子亲石元素(Th、U、Rb),相对亏损高场强元素(Nb、Ta、Ti)。岩石学及岩石地球化学研究表明岩浆在演化过程中发生了一定程度的结晶分异和同化混染,显示出良好的铜镍矿成矿条件。岩石MgO含量、m/f值等指标显示岩体具有良好的铜镍矿含矿性。通过对岩体形成环境、岩石地球化学、物化探特征综合研究,表明大通沟南山地区基性-超基性岩具有良好的铜镍硫化物矿床形成条件和找矿前景。  相似文献   

3.
北大巴山碱质基性—超基性潜火山杂岩的辉石矿物研究   总被引：2，自引：0，他引：2  
北大巴山碱质基性-超基性潜火山杂岩中有7种岩石含有新鲜的单斜辉石,它们分属于透辉石、普通辉石及霓辉石。单斜辉石可划分为5种成因类型:(1)斑晶、微斑晶及微晶;(2)深源捕虏晶;(3)半同源捕虏晶;(4)异源捕虏晶;(5)地幔型捕虏晶,本文涉及前4类辉石。单斜辉石的研究表明:(1)7种含辉石的岩石为同源岩浆结晶分异作用的产物,源岩浆为碱性-过碱性幔源岩浆。(2)初始岩浆形成于77km以下深度,上升过程中岩浆曾在3个不同深度储积。(3)岩浆上升至近地表处,在冷凝固结前曾有多次潜火山隐爆活动发生。(4)霓辉石捕虏晶的出现代表区内存在另一种碱基性岩浆活动。(5)辉石成分指示碱质基性-超基性次火山杂岩形成于陆边板内裂陷带。  相似文献   

4.
陕西凤县九子沟地区的奥陶纪草滩沟群中出露一套由透辉石岩组成的超镁铁质岩体,依据岩性和区域对比,被认为是形成于古生代而且可能是大洋岩石圈残片,但缺乏详细研究。野外地质考察发现,该透辉石岩在露头上与一套黑云母透辉石岩和(含透辉石)正长岩关系密切,而且以黑云母透辉石岩为主体。本文在详细岩石学研究的基础上,对其中的透辉石岩、黑云母透辉石岩以及(含透辉石)正长岩进行了透辉石矿物成分、锆石U-Pb年代学和Hf同位素研究。结果表明,三种岩石中的透辉石皆为岩浆型透辉石,而且透辉石成分在透辉石岩和(含透辉石)正长岩中完全相同,而在黑云母透辉石岩中出现向普通辉石转变的趋势。LA-ICP-MS锆石定年结果,得到透辉石岩、黑云母透辉石岩和(含透辉石)正长岩的形成年龄分别为235.2±1.6Ma、232.8±4.3Ma和230.7±2.1Ma,表明九子淘超镁铁岩-正长岩形成于中生代。透辉石岩和黑云母透辉石岩锆石具有相似的Hf同位素组成,初始~(176)Hf/~(177)Hf分别变化于0.282405~0.282589和0.282397~0.282607,ε_(Hf)(t)值介于-8.26~-1.37和-8.41~-0.84之间,显示富集岩石圈地幔特征;(含透辉石)正长岩锆石~(176)Hf/~(177)Hf介于0.282577~0.282825之间,ε_(Hf)(t)值为-2.06~+5.83,是富集岩石圈地幔低程度部分熔融的产物,与超镁铁岩来自不同的岩浆源区。综合区域地质已有研究,九子沟中生代超镁铁岩-正长岩体是在华北、扬子两大板块碰撞过程中构造体制由挤压向伸展转换阶段,由两个富集岩石圈地幔不同程度部分熔融的、独立演化岩浆先后侵位形成。  相似文献   

5.
东天山黄山岩带因发育众多镁铁质-超镁铁质岩体和岩浆铜镍硫化物矿床而备受地质学者关注。四顶黑山岩体位于该岩带最东端,具有层状岩体特征;岩石类型有单辉橄榄岩、橄榄辉石岩、橄榄苏长辉长岩、橄榄辉长岩、辉石角闪岩、辉长岩、角闪辉长岩、闪长岩,主要造岩矿物为贵橄榄石、古铜辉石、透辉石、普通角闪石和斜长石,岩体分异较好,蚀变较发育。通过矿物显微结构特征观察以及计算得出:四顶黑山岩体中橄榄石最先结晶,其开始结晶温度大约在1419℃左右;古铜辉石和透辉石在橄榄石之后开始结晶,结晶温度分别在1100℃左右和900～1100℃之间,两矿物相在岩石中可以共存。岩体中橄榄岩相、苏长岩相发育,以及贵橄榄石+古铜辉石的矿物组合特征,表明岩体具有形成铜镍(铂)矿床的有利条件。  相似文献   

6.
冀北小张家口基性-超基性杂岩体主要由辉石岩和次要的纯橄岩组成,并含有方辉橄榄岩透镜状捕掳体.岩石学和元素地球化学数据表明,辉石岩和纯橄岩可能是堆晶成因,而方辉橄榄岩可能是来自地幔源区的地幔岩捕掳体(抽取玄武质熔体之后的难熔残余).辉石岩和纯橄岩都表现出相当富集的Nd-Sr同位素特征,εND (t)=-2～10,ISr=0.7045～0.7081,表明它们的母岩浆来自富集的岩石圈地幔的部分熔融.化学成分和同位素成分一致表明,母岩浆在以橄榄石和辉石为主的分离结晶过程中,遭受下地壳不同程度的混染.辉石岩受显著混染,不能代表岩石圈地幔的同位素成分;而纯橄岩是最初的分离结晶体,仅受轻微下地壳混染,可近似反应岩石圈地幔源区的同位素成分.小张家口岩体的形成可能与古亚洲洋的缝合(240～250Ma)有关.  相似文献   

7.
玉古萨依基性超基性杂岩体位于东昆仑造山带祁漫塔格山西段玉古萨依地区。岩体侵入于祁漫塔格群变质砂岩中,由北西至南东方向岩性由超基性向基性过渡,可见明显的层状分异现象,普遍蚀变较强,辉石岩为主要的堆晶相。杂岩体岩石整体属于钙碱性系列,都属于富铁质或铁质基性超基性岩,富集轻稀土元素和大离子亲石元素,亏损重稀土元素和高场强元素,稀土元素配分曲线为右倾型。微量元素特征显示不同岩石类型均为同源岩浆演化的产物。岩体中角闪辉长岩的锆石U-Pb年龄为406±2.6Ma,形成于早泥盆世早期。岩石地球化学特征显示岩体形成于板内伸展环境下,表明在该时期区域内已经为碰撞后的伸展阶段。本次研究发现该杂岩体应为祁漫塔格早古生代蛇绿构造混杂岩带中非蛇绿岩成因的镁铁-超镁铁质层状侵入体。  相似文献   

8.
对位于秦岭-大别造山带的沙坪沟钼矿矿区出露的橄榄角闪石岩开展了系统的岩石学、岩相学及电子探针原位分析,厘定了其成岩物理化学条件,并探讨了岩石成因及形成的构造环境。橄榄角闪石岩主要造岩矿物为橄榄石、角闪石、金云母和辉石;副矿物为磷灰石、铬铁尖晶石、镁铁尖晶石、磁铁矿、磁黄铁矿、镍黄铁矿等。电子探针原位分析结果显示,橄榄石属于贵橄榄石系列,角闪石主要是韭闪石,辉石为透辉石,云母为金云母。造岩矿物结晶顺序为橄榄石-辉石-角闪石,结晶温度介于1 111. 64～1 223. 30℃之间,形成压力为350～740 MPa,形成于较高的氧逸度环境(角闪石结晶时的氧逸度范围为NNO+0. 59～NNO+2. 27)。角闪石和金云母的矿物学特征显示它们均是与幔源岩浆作用相关的原生矿物,表明该套基性-超基性岩与幔源岩浆作用有关。综合分析认为,在大别造山带中生代地壳减薄的伸展构造背景下,该橄榄角闪石岩母岩浆源于地幔,是经过地幔7. 92%部分熔融的玄武质岩浆经结晶分异作用形成的幔源岩浆底侵下地壳的产物。  相似文献   

9.
Shar.  RS 马瑞 《世界地质》1991,10(1):27-28
Sand Mata麻粒岩岩组形成印度半岛西北部Rajasthan地区太古宙片麻岩杂岩体。其主要岩石为泥质麻粒岩,由石榴石、矽线石、蓝晶石、黑云母、堇青石、紫苏辉石、石英和长石组成。在各种规模范围内可见由石榴石、石英、长石等组成的成分条带。该岩组另一个岩石类型为基性麻粒岩,其矿物组合为石榴石、透辉石、紫苏辉石、斜长石和少量石英。这些麻粒岩被苏长岩脉侵入,而岩脉的年龄还没有确定。它们主要特征是有变余辉绿结构,同时有时含有橄榄石。麻粒岩岩组为卵形,并被片麻岩-片岩-花岗岩杂岩体围绕,该杂岩体被称为条带状片麻岩杂岩体(Banded Gneissic Complex)。它们的接触界线为韧性剪切带,该剪切带具有陡的下倾线理。基性麻粒岩在紫苏辉石和斜长石表面具有石榴石—透辉石和石英的反应边。反应边矿物的随机方位表明结晶作用是在麻粒岩相变质的静态环境中进行的。石榴石—透辉石—石英组合在冷却过程中还没有形成,这可由其P-T估算以及苏长岩脉中石榴石的发  相似文献   

10.
岛弧苦橄岩非常稀少,但在阿尔泰山南缘却分布有世界上罕见的中泥盆世厚大苦橄岩层。本文对该地区苦橄岩及其共生的玄武岩中单斜辉石的矿物化学成分进行了电子探针分析,结果表明,苦橄岩中的单斜辉石全部为透辉石,玄武岩中的单斜辉石种属有普通辉石和透辉石两类,但玄武岩中的单斜辉石成分呈连续演化关系。根据单斜辉石-熔体平衡原理计算了苦橄岩中单斜辉石斑晶形成的温度和压力,其变化范围主要集中在1350～1387℃和16.6～21.3kPa,相应的深度为55～70km,大致代表了苦橄岩岩浆房的深度。与此不同的是,玄武岩中单斜辉石斑晶结晶的深度较苦橄岩浅,为37～41km,暗示玄武岩浆是在相对低压条件下分离结晶后形成的演化岩浆。基于单斜辉石的成分和演化,确定了该地区火山岩属亚碱性系列,其构造背景为岛弧环境。  相似文献   

11.
Petrologic, trace element and Sr-Nd isotopic studies of mantle xenoliths in Quaternary basalts from Huinan, NE China provide constraints on the origin of coarse-grained harzburgites and the nature of lithosphere-asthenosphere interaction during lithospheric thinning. The Huinan harzburgites have a secondary recrystallized texture and their composition deviates from the partial melting trend of residual peridotites. The convex-upward REE pattern and a positive Cr-Yb correlation in clinopyroxene imply an interaction with basaltic melts at a high melt/rock ratio. The Huinan harzburgites are therefore not simple residues of partial melting, but likely resulted from melt-rock interaction during which the percolating melts preferentially dissolved pyroxenes by precipitation of olivine, transforming lherzolite to harzburgite. The melt percolation-reaction enhanced grain boundary diffusion kinetics, and gave rise to the characteristic texture of these mantle rocks. These “reactive” harzburgites were eventually metasomatized by compositionally distinct small volume volatile-rich melts, which may be derived from the main harzburgite-forming event as a result of melt-consuming reaction. Most likely the formation of the Huinan harzburgites was coeval with thermo-tectonic erosion of the continental lithosphere by upwelling asthenospheric melts. Thermometric considerations suggest a relatively long time interval between lithospheric thinning and eruption of the host basalts, consistent with the contention that lithospheric thinning in eastern China may have peaked in the late Cretaceous.  相似文献   

12.
 We have investigated new samples from the Gees mantle xenolith suite (West Eifel), for which metasomatism by carbonatite melt has been suggested. The major metasomatic change is transformation of harzburgites into phlogopite-rich wehrlites. Silicate glasses are associated with all stages of transformation, and can be resolved into two major groups: a strongly undersaturated alkaline basanite similar to the host magma which infiltrated the xenoliths during ascent, and Si-Al-enriched, variably alkaline glass present exclusively within the xenoliths. Si-Al-rich glasses (up to 72 wt% SiO2 when associated with orthopyroxene (Opx) are usually interpreted in mantle xenoliths as products of decompressional breakdown of hydrous phases like amphibole. In the Gees suite, however, amphibole is not present, nor can the glass be related to phlogopite breakdown. The Si-Al-rich glass is compositionally similar to glasses occurring in many other xenolith suites including those related to carbonatite metasomatism. Petrographically the silicate glass is intimately associated with the metasomatic reactions in Gees, mainly conversion of harzburgite orthopyroxene to olivine + clinopyroxene. Both phases crystallize as microlites from the glass. The chemical composition of the Si-Al-enriched glass shows that it cannot be derived from decompressional melting of the Gees xenoliths, but must have been present prior to their entrainment in the host magma. Simple mass-balance calculations, based on modal analyses, yield a possible composition of the melt prior to ascent of the xenoliths, during which glass + microlite patches were modified by dissolution of olivine, orthopyroxene and spinel. This parental melt is a calc-alkaline andesite (55–60 wt% SiO2), characterized by high Al2O3 (ca. 18 wt%). The obtained composition is very similar to high-alumina, calc-alkaline melts that should form by AFC-type reactions between basalt and harzburgite wall rock according to the model of Kelemen (1990). Thus, we suggest that the Si-Al-enriched glasses of Gees, and possibly of other suites as well, are remnants of upper mantle hybrid melts, and that the Gees suite was metasomatized by silicate and not carbonatite melts. High-Mg, high-Ca composition of metasomatic olivine and clinopyroxene in mantle xenoliths have been explained by carbonatite metasomatism. As these features are also present in the Gees suite, we have calculated the equilibrium Ca contents of olivine and clinopyroxene using the QUI1F thermodynamical model, to show that they are a simple function of silica activity. High-Ca compositions are attained at low a SiO2 and can thus be produced during metasomatism by any melt that is Opx-undersaturated, irrespective of whether it is a carbonatite or a silicate melt. Such low a SiO2 is recorded by the microlites in the Gees Si-Al-rich glasses. Our results imply that xenolith suites cannot confidently be related to carbonatite metasomatism if the significance of silicate glasses, when present, is not investigated. Received: 2 March 1995 / Accepted: 12 June 1995  相似文献   

13.
A suite of mafic pyroxenite xenoliths and clinopyroxene megacrysts was brought to the surface by Cenozoic nephelinites of the Jbel Saghro Volcanic Field (Anti-Atlas, Morocco). The large population of samples was subdivided into five groups: (i) clinopyroxenites sensu stricto; (ii) olivine clinopyroxenites; (iii) mica-bearing clinopyroxenites; (iv) kaersutite-bearing clinopyroxenites; (v) clinopyroxene megacrysts. These xenoliths display a cumulate texture (adcumulate, heteradcumulate with poikilitic clinopyroxene including olivine). The clinopyroxenes have the composition of augite and show an appreciable variation of MgO (7.02–14.80 wt.%), TiO2 (0.58–5.76 wt.%) and Al2O3 (2.81–12.38 wt.%) contents in grains. The clinopyroxenes are characterized by convex upward chondrite-normalized REE patterns, they display very similar trace element compositions with low contents of incompatible elements such as Rb (0−0.9 ppm), Ba (0.1–8.3 ppm), Th (0.1−0.3 ppm), U (0.01−0.04 ppm) and Nb (1.3–3.2 ppm). REE contents of the calculated melts in equilibrium with the clinopyroxene megacrysts and clinopyroxene from pyroxenite xenoliths are similar to those of the nephelinites exposed in Jbel Saghro. Crystallization temperatures of pyroxenite xenoliths and clinopyroxene megacrysts range from 950 °C to 1150 °C. Clinopyroxene barometry yielded pressure of crystallization ranging from 0.4 to 0.8 GPa for pyroxenite xenoliths and 0.3 to 0.7 GPa for clinopyroxene megacrysts. This pressure range is in agreement with pyroxenite xenoliths and clinopyroxene megacrysts being crystallized from their parental melts at the lower and upper crust.  相似文献   

14.
徐则民 《地学前缘》2009,16(3):364-373
发育于斜坡非饱和带的结晶岩与碎屑岩腐岩多夹于残坡积土层和风化岩带之间,保留有裂隙及层理等构造,但质地松软。腐岩结构体与母岩色差显著,保留原岩结构和构造特征,矿物成分以次生粘土矿物、次生石英及褐铁矿最为常见。腐岩次生微孔隙发育,具有低密度、高孔隙度、高渗透率、高容水度及高持水度特征。腐岩发育是通过低渗透岩石结构体风化前锋的渐进性扩展实现的,非饱和带高湿度空气形成的凝结水和降水入渗形成的非饱和渗流是该过程的核心动力。气态水分子能够扩散到低渗透岩石结构体表面内侧一定宽度范围内的孔隙系统之中凝结为液态水并溶蚀造岩矿物。雨季入渗降水在结构体表面形成的薄膜水流通过分子扩散机制接受其内侧结构体粒间溶液中的溶出组分并将其携入饱水带,同时恢复粒间溶液的侵蚀动力。当上述过程循环到一定程度时,结构面内侧便形成一定厚度的腐岩壳。腐岩壳在雨季直接吸收、贮存流经其表面的液态渗入水,在核心石外侧形成富水结构层并在旱季向包气带或其内侧的风化前锋提供水汽。随着时间推移,腐岩壳越来越厚,核心石最终消失;当某一范围内的岩石结构体全部演变为腐岩结构体时,该范围内的岩体也就完成了向腐岩体的转变。  相似文献   

15.
ABSTRACT

The dismembered ophiolites in Wadi Arais area of the south Eastern Desert of Egypt are one of a series of Neoproterozoic ophiolites found within the Arabian–Nubian Shield (ANS). We present new major, trace, and rare earth element analyses and mineral composition data from samples of the Wadi Arais ophiolitic rocks with the goal of constraining their geotectonic setting. The suite includes serpentinized ultramafics (mantle section) and greenschist facies metagabbros (crustal section). The major and trace element characteristics of the metagabbro unit show a tholeiitic to calc-alkaline affinity. The serpentinized ultramafics display a bastite, or less commonly mesh, texture of serpentine minerals reflecting harzburgite and dunite protoliths, and unaltered relics of olivine, orthopyroxene, clinopyroxene, and chrome spinel can be found. Bulk-rock chemistry confirms harzburgite as the main protolith. The high Mg# (91.93–93.15) and low Al2O3/SiO2 ratios (0.01–0.02) of the serpentinized peridotite, together with the high Cr# (>0.6) of their Cr-spinels and the high NiO contents (0.39–0.49 wt.%) of their olivines, are consistent with residual mantle rocks that experienced high degrees of partial melt extraction. The high Cr# and low TiO2 contents (0.02–0.34 wt.%) of the Cr-spinels are most consistent with modern highly refractory fore-arc peridtotites and suggest that these rocks probably developed in a supra-subduction zone environment.  相似文献   

16.
新疆天山地区榆树沟-铜花山蛇绿岩特征和构造背景   总被引：5，自引：3，他引：2  
榆树沟-铜花山蛇绿岩出露于南天山北缘,属哈萨克斯坦-伊犁板块和塔里木板块之间的缝合带。蛇绿岩已被构造肢解,主要由超镁铁岩、堆晶岩、熔岩类组成。超镁铁岩以方辉橄榄岩为主,显示典型的亏损地幔岩特征,橄榄石为富镁型,Fo为90;斜方辉石为顽火辉石,En为90,单斜辉石含量少;副矿物铬尖晶石属低铬型,Cr#值(＝Cr/(Cr+Al)×100)16~28,Mg#值(=Mg/(Mg+Fe)×100)63~75,反映了深海橄榄岩特征。归一化后榆树沟超镁铁岩MgO含量38.84%~44.53%,Al2O3为1.51%~3.63%,CaO为0.42%~5.77%,成分近于大洋二辉橄榄岩;铜花山的超镁铁岩叠加碳酸岩化,LREE强烈富集,可能经历了俯冲洋壳流体改造。熔岩类在榆树沟和铜花山均有较大规模产出,其中榆树沟玄武岩为主,铜花山安山岩和英安岩较多。熔岩多已遭受绿片岩相海底热液蚀变。榆树沟玄武岩的REE含量总体比铜花山的低,稀土配分模式均为轻稀土富集型;玄武岩的微量元素特征表明其源区可能遭受过流体作用影响。铜花山三个高镁火山岩化学成份具有SiO2(32%~36%)含量很低、MgO(20.12%~28.50%)含量高、K2O+Na2O(0.06%~0.46%)含量小于2%的特征,综合分析可归为苦橄岩类。堆晶岩通常以构造小岩块产在超镁铁岩块中或基性熔岩中,以辉长岩为主,有少量橄榄辉石岩等,岩石蚀变、构造变形强烈。堆晶岩的成分接近玄武岩,存在LREE富集和略亏损平坦两种类型。岩石地球化学特征表明榆树沟和铜花山的蛇绿岩形成于MOR构造环境,但具有受俯冲带流体改造特征,并伴生有岛弧火山岩。  相似文献   

17.
The North Puruliya Shear zone (NPSZ) is characterized by occurrence of mafic-ultramafic rocks aligned parallel to the shear zone, intruding the high grade Proterozoic rocks of Chhotanagpur Gneissic Complex. The ultramafic rocks occur as small lenses, pockets, veins, thin dykes and are intimately associated with mafic (gabbro, norite) rocks. Pyroxenites (viz. olivine websterite, websterite, plagioclase websterite) and hornblendite are the two important members of the ultramafic rocks containing clinopyroxene, orthopyroxene, olivine, plagioclase, amphibole, phlogopite and ilmenite. The mafic-ultramafic rocks show evidence of shearing and retrogressive metamorphism. Linear correlation of chemical attributes suggests fractionation-controlled magmatic differentiation. Enrichment of LILE and LREE in the mafic-ultramafic suite suggests an enriched mantle source and pronounced negative Eu-anomalies in all the rock types except hornblendite suggest fractionation of plagioclase under low fO2 condition. Progressive iron enrichment trend in rocks of the mafic-ultramafic suite also indicate magmatic differentiation under low fO2 condition. Early fractionation and accumulation of clinopyroxene and plagioclase from a basaltic magma may have given rise to the ultramafic rocks of the area. Little change in the Nb/Zr and Ce/Zr ratios of ultramafic and mafic rocks (except alkali norite) strongly support low crustal contamination. A few samples of norite and gabbro-norites appeared to be variably contaminated by a crustal component or affected by late granitic intrusion resulting in enrichment of alkali in the former.  相似文献   

18.
同正常的残余方辉橄榄岩相比,辉南方辉橄榄岩具有异常高的HREE组成和特殊的二次重结晶结构,因此,它们不是上地幔经大程度部分熔融后的残余,而是熔体－岩石反应的结果。倒U型REE分配模式暗示这些样品经历了与玄武质熔体相互作用的历史,并达到了平衡,大量熔（流）体的存在有利于地幔岩石矿物颗粒的增长,从而形成特征的二次重结晶结构。这种“反应”型方辉橄榄岩的形成可能与上涌软流图对岩石圈地幔的热－化学浸蚀有关。“反应型”方辉橄榄岩形成之后,又受到了类似于碳酸岩或富挥发份小体积熔体的交代,因此辉南地区上地幔经历了多期地幔交代作用。  相似文献   

19.
 Garnet–hornblende–plagioclase gneisses rich in incompatible elements occur in the crystalline basement of the Austro-Alpine Silvretta nappe and are associated with clinopyroxene norites and harzburgite cumulates. It is proposed here that the gneisses were formerly oceanic plagiogranites. An εNd( 530 ) value of +5.6 for the gneisses as well as initial 87Sr/86Sr values of 0.7036–0.7037 for the gabbroic rocks and 0.7026–0.7027 for the ultramafic rocks suggest a mantle source for this rock association. The geochemical characteristics of the garnet–hornblende–plagioclase gneisses indicate that their precursors were derived by fractional crystallization from a basaltic parent magma, by the same process which produced the adjacent clinopyroxene norites and ultramafic cumulates as well. The combined U–Pb upper intercept ages of zircons from two gneiss samples yield an igneous crystallization age of 532±30 Ma, similar to previously dated (mostly calc-alkaline) orthogneisses in the same area. High-quality transparent zircons showed the least degree of discordance, but contain extremely low U and Pb levels. The rock suite, including this plagiogranite, was emplaced within oceanic crust which formed in the latest Precambrian–early Palaeozoic off the northern continental margin of Gondwana. Received: 26 April 1996 / Accepted: 2 August 1996  相似文献   

20.
西天山吐拉苏盆地安山岩中发现岩石包体及地质找矿意义   总被引：4，自引：0，他引：4  
新疆西天山斑岩型铜矿找矿前景良好,但面临攻坚;吐拉苏盆地是西天山浅成低温热液型金矿集区,晚古生代吐拉苏盆地是北天山洋向南向伊犁板块之下俯冲形成的“西南太平洋型”岛弧环境,在金矿深边部寻找斑岩型铜金矿值得关注。笔者最近在岛弧安山岩中发现硫化物矿化二长斑岩、细晶闪长岩等岩石包体。岩石包体发现于塔乌尔别克金矿区,多呈浑圆体或次棱角状,随机分布于安山岩中,与围岩界线截然,常见安山岩围绕岩石包体形成冷凝边。二长斑岩包体显示似斑状结构,斑晶由斜长石和钾长石组成;细晶闪长岩主要由斜长石和角闪石组成,细晶结构,块状构造;寄主安山岩为斑状结构,斑晶为斜长石和少量角闪石及单斜辉石,基质由短柱状斜长石及少量单斜辉石组成,玻基交织结构。二长斑岩包体、细晶闪长岩包体和寄主安山岩三者具有极为相似的微量元素和稀土元素组成,均富集大离子亲石元素、相对亏损Nb、Ta等高场强元素。这些岩石学和地球化学特点反映岩石包体是岛弧火山喷发过程中从下覆地质体捕掳成因,指示安山岩层之下存在同源斑岩体并可能已发生硫化物矿化。综合考虑吐拉苏盆地铜矿化点、酸性热液蚀变、高硫型和低硫型浅成低温热液金矿的组合关系,认为盆地内发育斑岩-浅成低温热液金铜成矿系统,塔乌尔别克是斑岩型铜金矿找矿重要靶区。  相似文献   

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