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1.
Biotite + plagioclase + quartz (BPQ) is a common assemblagein gneisses, metasediments and metamorphosed granitic to granodioriticintrusions. Melting experiments on an assemblage consistingof 24 vol. % quartz, 25 vol. % biotite (XMg = 0·38–0·40),42 vol. % plagioclase (An26–29), 9 vol. % alkali feldsparand minor apatite, titanite and epidote were conducted at 10,15 and 20 kbar between 800 and 900°C under fluid-absentconditions and with small amounts (2 and 4 wt %) of water addedto the system. At 10 kbar when 4 wt % of water was added tothe system the biotite melting reaction occurred below 800°Cand produced garnet + amphibole + melt. At 15 kbar the meltingreaction produced garnet + amphibole + melt with 2 wt % addedwater. At 20 kbar the amphibole occurred only at high temperature(900°C) and with 4 wt % added water. In this last case themelting reaction produced amphibole + clinopyroxene ±garnet + melt. Under fluid-absent conditions the melting reactionproduced garnet + plagioclase II + melt and left behind a plagioclaseI ± quartz residuum, with an increase in the modal amountof garnet with increasing pressure. The results show that itis not possible to generate hornblende in such compositionswithout the addition of at least 2–4 wt % H2O. This reflectsthe fact that conditions of low aH2O may prevent hornblendefrom being produced with peraluminous granitic liquids fromthe melting of biotite gneiss. Thus growth of hornblende inanatectic BPQ gneisses is an indication of addition of externalH2O-rich fluids during the partial melting event. KEY WORDS: biotite; dehydration; gneisses; hornblende; melt  相似文献   

2.
STUDY ON THE ISOTOPIC CHRONOLOGY AND THE TECTONIC SIGNIFICANCE OF DUGUER GRANITIC GNEISS IN CENTRAL QIANGTANG, TIBET  相似文献   

3.
孟恩  刘福来  刘建辉  施建荣 《岩石学报》2012,28(9):2793-2806
本文对辽东南长海地区花岗质片麻岩进行了系统的岩石学和地球化学研究,以便对其原岩性质及形成的构造环境给予制约。研究结果表明,研究区内花岗质片麻岩类可划分为富钠和富钾两类花岗质岩石,前者包括黑云二长花岗质片麻岩和花岗闪长质片麻岩,矿物组成主要包括斜长石、石英、黑云母及少量的钾长石等,后者则主要包括花岗质、二长花岗质和糜棱岩化花岗质片麻岩,其矿物组成以钾长石、斜长石、石英和次要的白云母和黑云母为主。地球化学分析结果显示,富钠花岗质片麻岩具有富硅、富钠、高铝、富集LREEs和LILEs、强烈亏损HREEs和HFSEs(Nb、Ta、P、Ti)、轻重稀土强烈分馏、并显示弱负Eu异常和Ba的相对亏损等特征;而富钾花岗质片麻岩则显示高硅、富钾、过铝质的地球化学属性、富集LREEs和LILEs、亏损HREEs和HFSEs,与富钠花岗质岩石相比,轻重稀土分馏程度相对较弱、HFSEs以及Ba相对于Rb和Th的亏损程度更强,并显示中等负Eu异常和强烈的Sr负异常等特征。上述特征表明,辽东南长海地区富钠花岗质片麻岩应起源于中酸性陆壳物质的部分熔融,并可能有玄武质物质的加入,原岩应为具有活动大陆边缘属性的花岗闪长岩,其形成应与狼林地块(或胶辽地块)东南及南部先存洋盆向陆块之下的俯冲作用相联系;而富钾花岗质片麻岩应形成于中酸性陆壳物质的部分熔融,原岩为具有碰撞型花岗岩属性的碱性花岗岩,其形成应与洋壳消减闭合、陆陆碰撞拼贴相联系。  相似文献   

4.
Omphacite and garnet coronas around amphibole occur in amphibolites in the Hong'an area, western Dabie Mountains, China. These amphibolites consist of an epidote–amphibolite facies assemblage of amphibole, garnet, albite, clinozoisite, paragonite, ilmenite and quartz, which is incompletely overprinted by an eclogite facies assemblage of garnet, omphacite and rutile. Coronas around amphibole can be divided into three types: an omphacite corona; a garnet–omphacite–rutile corona; and, a garnet–omphacite corona with less rutile. Chemographic analysis for local reaction domains in combination with petrographical observations show that reactions Amp + Ab + Pg = Omp +Czo + Qtz + H2O, and Amp + Ab = Omp ± Czo + Qtz + H2O may lead to the development of omphacite coronas. The garnet–omphacite–rutile corona was formed from the reaction Amp + Ab + Czo + Ilm ± Qtz = Omp + Grt + Rt + H2O. In garnet–omphacite coronas, the garnet corona grew during an early stage of epidote amphibolite facies metamorphism, whereas omphacite probably formed by the reactions forming the omphacite corona during the eclogite facies stage. It is estimated that these reactions occurred at 0.8–1.4 GPa and 480–610 °C using the garnet–clinopyroxene thermometer and omphacite barometer in the presence of albite.  相似文献   

5.
北大别东部大山坑片麻岩主要由钾长石、斜长石和石英组成,少量角闪石、黑云母和褐帘石,成分为二长花岗质。岩石以富碱（Na2O＋K2O）尤其是 K2O、贫Al2O3为特征,地球化学性质上表现为富集 K、Rb、Th、La、Ce等大离子亲石元素而亏损Nb、Ta、U等高场强元素及Sr元素,高的Ga含量、强的负Eu异常（δEu＝0．37）和相对较强的轻重稀土分馏程度（（La／Yb）N＝16．75）。岩石地球化学的总体特征与南大别水吼地区的 A型花岗片麻岩类似,意味着其原岩与南大别的 A型花岗片麻岩一样,可能也是在拉张状态下形成的一套偏碱性的花岗岩。该二长花岗片麻岩的锆石 U-Pb年龄为229．2±5．5 Ma,也与南大别超高压变质的年龄相似,指示北大别正片麻岩印支期可能也经历过超高压变质作用。南、北大别造山带可能具有相同的形成与演化历史,现南、北大别变质带之间的差异可能更多的是后造山历史不同,尤其是燕山期花岗岩侵入对两个带影响的不同造成的。  相似文献   

6.
The Ancient Gneiss Complex (AGC) of Swaziland, an Archean gray gneiss complex, lies southeast and south of the Barberton greenstone belt and includes the most structurally complex and highly metamorphosed portions of the eastern Kaapvaal craton. The AGC is not precisely dated but apparently is older than 3.4 Ga.The AGC consists of three major units: (a) a bimodal suite of closely interlayered siliceous, low-K gneisses and metabasalt; (b) homogeneous tonalite gneiss; and (c) interlayered siliceous microcline gneiss, metabasalt, and minor metasedimentary rocks — termed the metamorphite suite. A geologically younger gabbro-diorite-tonalite-trondhjemite suite, the Granodiorite Suite, is spatially associated with the AGC and intrusive into it.The bimodal suite consists largely of two types of low-K siliceous gneiss: one has SiO2 < 75%, Al2O3 > 14%, low Rb/Sr ratios, and depleted heavy rare earth elements (REE's); the other has SiO2 > 75%, Al2O3 < 13%, high Rb/Sr ratios, and relatively abundant REE's except for negative Eu anomalies. The interlayered metabasalt ranges from komatiitic to tholeiitic compositions. Lenses of quartz monzonitic gneiss of K2O/Na2O close to 1 form a minor part of the bimodal suite. Tonalitic to trondhjemitic migmatite locally is abundant and has major-element abundances similar to those of non-migmatitic varieties.The siliceous gneisses of the metamorphic suite show low Al2O, K2O/Na2O ratios of about 1, high Rb/Sr ratios, moderate REE abundances and negative Eu anomalies.K/Rb ratios of siliceous gneisses of the bimodal suite are very low (～130); of the tonalitic gneiss, low (～225); of the siliceous gneiss of the metamorphite suite, moderate (～300); and of the Granodiorite Suite, high (～400).Rocks of the AGC differ geochemically in several ways from the siliceous volcanic and hypabyssal rocks of the Upper Onverwacht Group and from the diapirs of tonalite and trondhjemite that intrude the Swaziland Group.  相似文献   

7.
Sushina nepheline syenite gneisses of Early Proterozoic North Singhbhum Mobile Belt (NSMB), eastern India suffered regional metamorphism under greenschist-amphibolite transitional facies condition. The Agpaitic Sushina nepheline syenite gneisses consist of albite, K-feldspar, nepheline (close to Morozewicz-Buerger composition), aegirine, biotite, epidote, piemontite, sodalite, cancrinite, natrolite and local alkali amphibole. Accessory phases include zircon, hematite, magnetite, rare pyrochlore and occasional eudialyte and manganoan calcic zirconosilicates. Mineral chemistry of albite, K-feldspar, nepheline, aegirine, alkali amphibole, natrolite and zirconium silicate minerals are described. The detailed textural features together with chemical data of some minerals indicate metamorphic overprint of these rocks. A new reaction is given for the genesis of metamorphic epidote. Metamorphic piemontite suggests greenschist facies metamorphism under high fO2 (Hematite-Magnetite buffer). Up to 15.34 mol% of jadeite component in aegirine suggests that the metamorphic grade of the nepheline syenite gneiss reached at least to greenschist-amphibolite transitional facies or higher. Nepheline geothermometry suggests temperature of metamorphism <500 °C, which is consistent with greenschist facies metamorphism of surrounding chlorite-biotite-garnet phyllite country rock.  相似文献   

8.
The transition zone between Archean low- and high-grade rocks in southern India represents eroded crustal levels representative of 15–20 km. It is comprised chiefly of tonalitic gneisses with some varieties showing incipient charnockitization and of minor amounts of granitic gneiss and charnockite, both of which appear to have developed from the tonalitic gneisses.Tonalitic gneisses and charnockites are similar in major and trace elements composition while granitic gneisses are relatively enriched in Rb, K, Th, Ba and light rare earth element (REE) and depleted in Cr and Sc. All three rock types exhibit enriched light REE patterns with variable positive Eu anomalies. Total REE content decreases with increasing Eu/Eu and SiO2 and with decreasing Fe2O3T and MgO in the tonalitic gneisses and charnockites.An internally consistent model for the production of the tonalitic gneisses involves partial melting of an enriched mafic source with variable ratios of hornblende to clinopyroxene. This source, in turn, is derived from an ultramafic mantle relatively enriched in incompatible elements. Granitic gneisses form from tonalitic gneisses by alkali metasomatism from chloride-bearing fluids with high H2O/CO2 ratios purged from the lower crust by CO2, and charnockites are produced from tonalitic gneisses (and granitic gneisses) by ischochemical CO2 metamorphism following the alkali metasomatism.  相似文献   

9.
Thorian brannerite mineralisation at the Crockers Well prospect occurs in sodic granitic rocks and associated sodic felsic gneisses. Field, mineralogical and chemical data support the derivation of the granitic rocks from the gneisses by anatexis during high grade metamorphism. The sodic granitic rocks are largely peraluminous, contain high Na2O, low K2O, CaO, Rb, Ba, Sr and ferromagnesian elements, and variable but commonly high U, Th, Nb, Ce, Y and F values. Many geochemical parameters are inherited from the compositionally similar sodic felsic gneisses, which are interpreted to be metamorphosed analcime-rich volcano-sedimentary rocks of original intermediate-felsic (alkaline) affinites. Significant U-Th mineralisation is restricted to fractures and local breccia bodies which contain a mineral assemblage rich in quartz, F-bearing phlogopite and minor fluorapatite, sodic plagioclase, niobian rutile, thorian brannerite, monazite, muscovite, chlorite, tourmaline and fluorite. Certain genetic analogies are proposed with porphyry Cu and stockwork Mo deposits, with mineral deposition having occurred in mechanically-induced fractures and breccia bodies developed during sub-solidus cooling of the sodic granitoids.  相似文献   

10.
南阿尔金地区榴辉岩特征及意义   总被引：3，自引：0，他引：3  
野外地质调查工作表明,阿尔金地区西起且末河到茫崖长达300km范围内的"阿尔金山岩群"中多处出露榴辉岩透镜体。榴辉岩在产状和岩石学特征上可以分为两种:第1种是中细粒结构,块状构造,退化变质较弱,围岩多与片麻状花岗岩或花岗闪长岩关系密切;第2种是细粒结构,片麻状构造,具有透入性片麻理,有明显退化变质现象,围岩多与大理岩、黑云变粒岩、石英片岩等关系密切。在分布上,榴辉岩限于古元古代"阿尔金山岩群"中,含有榴辉岩的阿尔金构造杂岩带是柴达木盆地北缘碰撞杂岩带的西延部分。  相似文献   

11.
刘福来  S.MARUYAMA 《地质论评》2001,47(2):164-168
中国大陆科学钻探工程预先导孔CCSD－PP1中存在两类不同性质的花岗质片麻岩，它们的岩相学性质，地球化学特征，锆石中的包体矿物组合和分布规律以及阴极发光图像均存在明显的判别，其中第一类花岗质片麻岩中的锆石呈完好的自形晶，不含任何超高压矿物包体，自晶体中心至边缘均具有典型的岩浆结晶生长环带，表明该类花岗质片麻岩未经历超高压变质作用，第二类花岗质征麻岩中的锆石呈半自形晶一它形晶，晶体核部保存典型的岩浆结晶生长环带，且无超高压矿物包体，具有继承性锆石的特点，而在晶体的过渡带及其与边缘的交界部位，普遍存在以柯石英粉 包体矿物，表明第二类花岗质片麻岩曾经历了超高压变质作用，且锆石在此变质过程进一步结晶生长，形成新的生长环带，该项成果对于深入探讨苏鲁超高压变质带的俯冲－折返机制及其与岩浆作用的相互关系以及对中国大陆科学钻探工程的选址和实施有着重要的科学意义。  相似文献   

12.
Jadeitites, albitites and related rocks from the Motagua Fault Zone, Guatemala   总被引：12，自引：0，他引：12  
Jadeitites from Guatemala are found as weathered blocks in tectonized serpentinite in a 15-km zone north of the Motagua Fault Zone. Rock types found with jadeitite include albitites, albite-mica rocks, omphacite/taramitic amphibole-bearing metabasites, chlorite-actinolite schists, talc-carbonate rocks and antigorite schists. In addition to the predominant jadeitic (Jd93_100) pyroxene, common phases in jadeitite include micas (paragonite and/or phengite ± rarer phlogopite), omphacite, albite, titanite /Pm zircon, apatite and graphite. Conditions of jadeitite formation are 100-400d? C, 5-11 kbar with 0.0 > log10asio2≥= 0.7. Fluid inclusions, coarse textures, vein structures, and rhythmic zoning of pyroxene indicate an aqueuos fluid was involved. Jadeitites are either (1) metasomatic modifications of former felsic-to-pelitic inclusions that have undergone silica depletion plus efficient soda exchange and enrichment, or (2) solution precipitations derived from such a source. The close spatial relationship of faults and shear zones, serpentinites, and jadeitites suggests jadeitites form in a relatively high-P/T setting with substantial flow of sodic fluid in a tectonized zone. Most Guatemalan jadeitites are extensively altered to analcime, albite, taramitic amphibole, (clino)zoisite ± nepheline and preiswerkite. This alteration reflects depressurization /Pm heating to below the jadeite + fluid = analcime reaction at high aNa. With progressive alteration, analcime and nepheline are replaced by albite; the increase in silica content may result from fluid flowing up a tectonized zone reaching saturation with an albite assemblage. Albitite phases, albite, actinolite, zoisite, /Pm chlorite, phengite, K-feldspar and quartz, record conditions of c. 3-8 kbar at T < 400d? C, indicating a clockwise P-T trajectory of the blocks. Barium aluminosilicates—banalsite, celsian, cymrite and hyalophane—are common minor late-stage phases in jadeitites and albite-rich rocks. Barian phengite is common in albite-mica rocks.  相似文献   

13.
河北武安坦岭多斑斜长斑岩中基质矿物特征及其研究意义   总被引：2，自引：0，他引：2  
河北武安坦岭斜长斑岩具有多斑斑状结构,基质为显微晶质结构。岩相学观察表明,斜长石斑晶有一个宽广的核部和一个宽度可变的条纹长石反应边,个别核部包含有角闪石、黑云母等矿物。基质矿物主要由蓝透闪石、条纹长石(An0Ab8.4Or91.5～An0.1Ab57.3Or42.6)、石英、钾长石(An0.3Ab5.9Or93.7～An0.3Ab4.7Or95.2)、钠长石(An0.2Ab98.3Or1.5～An0.1Ab99.2Or0.7)、磁铁矿、赤铁矿、钛铁矿、磷灰石、榍石和锆石等11种矿物组成。角闪石温压计计算结果得出,基质角闪石核部的结晶压力高于边部,核部为34.05 MPa,对应的结晶温度为660.35℃,结晶深度为1.29km;边部的结晶压力为24.32MPa,结晶温度为598.32℃,结晶深度为0.92km;而斜长石斑晶中的角闪石形成时压力为159.51～178.19MPa,温度为817.68～819.79℃,对应的形成深度为6.03～6.73km。基质角闪石在Al2O3-TiO2图上落在壳源区,而斑晶中的角闪石和黑云母都落在壳幔混合区。斜长石、条纹长石、磁铁矿和磷灰石的微量和稀土元素测试数据显示,其都具有相对富集LILE、亏损HFSE的特点,暗示了基质矿物的形成有流体参与。ICT三维扫描结果显示,斜长斑岩基质中的孔隙体积含量约为3.428%,铁质体积含量为4.371%,且铁氧化物和孔隙具弱连通性。通过讨论分析,笔者得出:(1)坦岭斜长斑岩中斜长石斑晶具有明显的交代结构,且晶体本身没有明显熔蚀现象,这些特征表明大量的斜长石斑晶快速上升,即"冻结岩浆房"的活化机制与流体密切相关;(2)斜长斑岩中基质矿物有十一种,且矿物类型复杂,不符合平衡系统矿物相律,应属于流体晶矿物组合;(3)坦岭斜长斑岩的基质"岩浆"可能是一种富Fe、K、P、Si、Na等元素的熔体-流体流;(4)多斑斜长斑岩的形成经历了(1)深度6～7km的深部岩浆房形成斜长石堆晶→(2)富Fe、K、P、Si、Na等元素的熔体-流体流加入深部岩浆房,冻结岩浆房活化→(3)由于流体超压,含大量斜长石斑晶的熔体-流体在地壳浅部(0.9～1.2km)呈小岩株状或岩脉状就位。多斑斜长斑岩为深部找矿提供了有力的线索。  相似文献   

14.
The oxygen and hydrogen isotope compositions of minerals and whole rock were determined for two types of gneiss (biotite gneiss and granitic gneiss) associated with ultrahigh pressure (UHP) eclogites in the Shuanghe district of the eastern Dabie Mountains. There are significant differences in δ18O between the two gneisses: the UHP biotite gneiss varying from −4.3‰ to 10.6‰ similar to the associated eclogites, whereas the non-UHP granitic gneiss ranges only from −3.8‰ to 1.2‰. The δD values are similar in the two gneisses with −37 to −64‰ for epidote/zoisite, −92 to −83‰ for amphibole, and −63 to −109‰ for biotite/phengite. Hydrogen isotope disequilibrium among the coexisting hydroxyl-bearing minerals is ascribed to retrograde exchange subsequent to amphibolite-facies metamorphism. Oxygen isotopic equilibrium has been preserved among various minerals in both gneisses regardless of the large variation in rock δ18O. Oxygen isotopic geothermometers yield different but regular temperatures corresponding to the closure temperatures of oxygen diffusion in the minerals. The metamorphic temperatures of both eclogite facies and amphibolite facies have been recovered in mineral pairs from the biotite gneiss. The isotopic temperatures for the granitic gneiss are mostly in accordance with amphibolite-facies metamorphism. However, high temperatures of 550 to 650 °C are obtained from those minerals resistant to retrograde oxygen isotope exchange, implying that the granitic gneiss may have experienced higher temperature metamorphism than expected from petrologic thermometers. The 18O-depletion of both gneisses is interpreted to result from meteoric-hydrothermal exchange before/during plate subduction. Therefore, the measured δ18O values of the gneisses reflect the oxygen isotope compositions of their protoliths prior to the UHP metamorphism. It is inferred that the UHP unit is in foreign contact with the non-UHP unit like a tectonic melange, but both of them experienced the two common stages of geodynamic evolution: (1) 18O-depletion prior to the UHP metamorphism, (2) uplifting since the amphibolite-facies metamorphism. Received: 5 May 1998 / Accepted: 27 August 1998  相似文献   

15.
华北克拉通阜平杂岩的深熔和混合岩化作用   总被引：3，自引：2，他引：1  
华北克拉通的阜平杂岩长英质岩石中常产出显著的浅色体、岩脉和花岗岩侵入体,并形成广泛的混合岩化作用。通过矿物自形晶的形成、黑云母向角闪石的转换和大量钠长石净边的出现以及其它与熔体活动有关结构的分析,浅色脉体和混合岩化作用的发生与外来熔体的注入有关。在长英质片麻岩中可出现明显的熔体注入,在一些不易片理化的岩石如石英岩中亦可形成浸染状熔体渗入。熔体汇集可形成浅色体、岩脉,直至花岗岩侵入体。而深熔作用本身形成熔体的作用在本区几乎可以忽略不计。在遭受渗透式混合岩化作用的过程中,岩石成分发生了改变,形成开放系统。随着渗透熔体的结晶,可形成一些岩浆锆石,在副片麻岩中则很容易被当作碎屑锆石。  相似文献   

16.
J. Parnell 《Lithos》1996,37(4):281-292
Moinian (Proterozoic) gneisses in Easter Ross-shire, Scotland, exhibit pronounced alteration where they have been subjected to the flow of hydrocarbon-bearing fluids from an adjacent Devonian sedimentary basin. Comparison of the petrography of altered and unaltered gneiss indicates that element redistribution accompanied the alteration process. The alteration involved creation of secondary porosity by dissolution of primary minerals in the gneiss, chloritization of biotite and precipitation of authigenic orthoclase. Cross-cutting veinlets include bitumen, authigenic quartz, orthoclase, albite, monazite and titanium oxides, attesting to redistribution of all the major components of the gneiss. Evidence for bitumen being coeval with albite/quartz, and an association of high porosity with bitumen indicate that hydrocarbon-bearing fluids were present during the alteration process. Bitumen also penetrated beyond the visible alteration zone and accreted around radioelement-bearing accessory minerals.

Organic geochemical data indicate that there is a local source for the bitumen in the Devonian sequence. Bitumen veining also occurs within the Devonian sequence. Fluid inclusion data from quartz associated with bitumen in the gneiss and the Devonian indicate minimum trapping temperatures of 120–140 and 100–120 °C respectively. The alteration was probably a result of a fluid flow episode relating to fault movement during basin subsidence.  相似文献   


17.
In the Tuoshi oilfield,located in the Cenozoic Jianghan Basin of southeastern China ,there have been found hydrocarbon reservoirs hosted in lacustrine sandstones of the Eogene Xingouzui Formation.The main diagenetic features identified in these sandstones include the dissolution of detrital K-feldspar and albite grains,the precipitation of quartz as overgrowths and /or cements ,and the precipitation and /or transformation of clay minerals.These diagenetic features were interpreted to have occurred in early,intermediate and late stages,based on the burial depth.The kinetics of fluid-mineral reactions and the concentrations of aqueous species au each stage of diagenesis were simulated numerically for these lacustrine sandstones,using a quasi-sta-tionary state approximation that incorporates simultaneous chemical reactions in a time-space continuum.During the early diagenetic stage,pore fluid was weakly acidic,which resulted in dissolution of K-feldspar and albite and,therefore,led to the release of K^ ,Na^ ,Al^3 and SiO2(aq) into the diagenetic fluid.The increased K^ ,Na^ ,Al^3 and SiO2(aq) concentrations in the diagenetic fluid caused the precipitation of quartz,kaolinite and illite.At the beginning of the intermediate diagenetic stage the concentration of H^ was built up due to the decomposition of organic matter,which was responsible for further dissolution of K-feldspar and albite and pre-cipitation of quartz,kaolinite,and illite.During the late diagenetic stage,the pore fluid was weakly alkaline,K-feldspar became stable and was precipitated with quartz and clay minerals.When the burial depth was greater than 3000 m,the pore fluids became supersaturated with respect to allbite,but undersaturated with respect to quartz,resulting in the precipitation of albite and the dissolution of quartz.The diagenetic reactions forecasted in the numerical modeling closely matched the diagenet-ic features identified by petrographic examination, and therefore,can help us to gain a better understanding of the diagenetic processes and associated porosity evolution in sandstone reservoirs.  相似文献   

18.
A syenite gneiss associated with biotite pyroxenite and biotite-muscovite gneiss forms an elongated mass covering about 150 km2 in the basement complex around Shaki in Western Nigeria. It lies conformably in the biotite-museovite gneiss to which it is similar in texture. The biotite pyroxenite occurs as patches of varying sizes widely distributed throughout the syenite but is not found in any of the surrounding rocks.18 chemical analyses and 62 modal analyses show that the syenite is composed essentially of microcline (in places slightly perthitic), albite, quartz, diopsidic augite and hornblende and is chemically characterised by the unusual combination of very high K2O with high MgO, FeO and CaO contents. The biotite pyroxenite and the syenite contain high amounts of the trace elements characteristic of both magnesian ultra-basic rocks and granitic rocks.The syenite and the biotite pyroxenite are believed to have originated through two metasomatic alteration processes; one characterised by CaO+FeO+MgO and the other by K2O. The two processes are believed to have been simultaneous and related. The process may be analogous to that obtaining during fenitisation.  相似文献   

19.
The petrology and mineralogy of lawsonite zone metabasites have been studied northeast of town of Tav?anli, NW Turkey. In the field the metabasites are characteristically green and lack foliation; the essential mineral assemblage being sodic pyroxene+ lawsonite+chlorite+quartz±sodic amphibole. Sodic pyroxene of aegirine-jadeite composition occurs as pseudomorphs after magmatic augite. Lawsonite and chlorite are the other two dominant minerals. Sodic amphibole forms progressively from a reaction between sodic pyroxene, chlorite and quartz, and an isograd representing the first abundant occurrence of sodic amphibole in basic rocks has been mapped. The widespread occurrence of sodic pyroxene pseudomorphs in other blueschist terrains indicates that the inferred sodic amphibole producing reaction is of general significance for blueschist metabasites.The conversion of greenstones with the assemblage albite+chlorite+actinolite directly into glaucophane-lawsonite blueschists without any intervening lawsonite zone illustrates the influence of the initial mineral assemblage on the reaction path.  相似文献   

20.
Late Proterozoic rocks of Tanol Formation in the Lesser Himalayas of Neelum Valley area are largely green schist to amphibolite facies rocks intruded by early Cambrian Jura granite gneiss and Jura granite representing Pan-African orogeny event in the area. These rocks are further intruded by pegmatites of acidic composition, aplites, and dolerite dykes. Based on field observations, texture, and petrographic character, three different categories of granite gneiss (i.e., highly porphyritic, coarse-grained two micas granite gneiss, medium-grained two micas granite gneiss, and leucocratic tourmaline-bearing muscovite granite gneiss), and granites (i.e., highly porphyritic coarse-grained two micas granite, medium-grained two micas granite, and leucocratic tourmaline-bearing coarse-grained muscovite granite) were classified. Thin section studies show that granite gneiss and granite are formed due to fractional crystallization, as revealed by zoning in plagioclase. The Al saturation index indicates that granite gneiss and granite are strongly peraluminous and S-type. Geochemical analysis shows that all granite gneisses are magnesian except one which is ferroan whereas all granites are ferroan except one which is magnesian. The CaO/Na2O ratio (>0.3) indicates that granitic melt of Jura granite gneiss and granite is pelite-psammite derived peraluminous granitic melt formed due to partial melting of Tanol Formation. The rare earth element (REE) patterns of the Jura granite and Jura granite gneiss indicate that granitic magma of Jura granite and Jura granite gneiss is formed due to partial melting of rocks that are similar in composition to that of upper continental crust.  相似文献   

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