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1.
Jadvyga Rimšaite 《Contributions to Mineralogy and Petrology》1964,10(2):152-183
Micas from magmatic and metamorphic rocks differ from one another in chemical composition and in trace element content. The
chemical composition of micas is discussed in relation to their occurrence, paragenesis and sequence of crystallization. On
the basis of previous studies of the relationship between the physical properties and the chemical composition of 34 chemically
analysed micas, reliable physical methods have been established which permit identification of different mica varieties in
the same rock. Structural formulae and trace element content of micas from basic and granitic rocks, as well as from skarns,
schists, ortho- and paragneisses are discussed. The relationship between the components of the tetrahedral and octahedral
layers and of the interlayer are illustrated as ratios.
Poorly differentiated, hybrid and metasomatic rocks often contain more than one variety of mica. Some prophyritic basalts
and lamprophyres contain an early phlogopite which is paragenetically related to pyroxene phenocrysts and late biotite which
occurs in the groundmass and in the fractures as a result of the crystallization of residual magma. The biotitemuscovite assemblage
was observed in granodiorites, quartz-monzonites, schists and gneisses. In the albite-K-feldspar granites, muscovite predominates
and the biotite is usually altered.
The chemical composition of micas from metamorphic rocks depends on the grade of metamorphism and on the nature of associated
minerals. The biotite from paragneisses contains considerable quantities of octahedral alumina. Pre-metamorphic micas show
variable deficiencies of the (OH, F) group. The micas are useful minerals in determining the degree of differentiation and
subsequent alteration of igneous rocks.
The present study was carried out on the basis of 34 recent complete chemical analyses andca 100 X-ray fluorescence analyses.
Dedicated to Professor Dr.Carl W. Correns on the occasion of his 70th birthday. 相似文献
2.
矿物微量元素组成用于火成岩构造背景判别 总被引：1，自引：0，他引：1
赵振华 《大地构造与成矿学》2016,(5):986-995
火成岩中的矿物,特别是稳定副矿物的微量元素组成可用于火成岩构造–岩浆背景的判别。产于不同构造背景火成岩中单颗粒锆石原位微量元素含量的准确测定,积累了大量资料。本文介绍了用锆石微量元素含量的二维核密度分布投影,构建了U/Yb-Nb/Yb、Sc/Yb-Nb/Yb和U/Yb-Sc/Yb构造背景判别图解,可以区分洋中脊、地幔柱影响的洋岛及俯冲有关的弧(大陆弧)等不同构造背景形成的火成岩。单斜辉石、尖晶石、黑云母和角闪石的微量元素或主量元素组成同样可用于构造背景识别。 相似文献
3.
W. R. Ryall 《Journal of Geochemical Exploration》1979,11(2):175-194
In the Pb-Zn-Ag lodes at Broken Hill, significant Hg remains located in sphalerite and in tetrahedrite inclusions in galena, despite prograde granulite facies regional metamorphism that has resulted in recrystallization of the ore. The stratigraphically uppermost No. 3 lens has the highest Hg content, with sphalerite and galena concentrates containing up to 149,000 ppb and 198,000 ppb Hg, respectively, in Ag-Sb-rich parts of the lode.The No. 2 and 1 lenses and A lode have generally low Hg contents, but narrow tetrahedrite-rich sections contain up to 83,000 ppb Hg in sphalerite concentrates and 82,000 ppb in galena concentrates. The stratigraphically lowermost B lode has a moderately high Hg content with sphalerite concentrates containing from 4400 to 41,000 ppb Hg and galena concentrates from 30 to 21,000 ppb Hg.Programmed heating determinations showed Hg in sphalerite concentrates to be contained principally in the sphalerite structure, with subordinate amounts in tetrahedrite inclusions. Mercury in galena concentrates is almost wholly contained in tetrahedrite inclusions, with lesser amounts in dyscrasite and pyrargyrite.Piercement bodies show increased Pb, Ag, Sb, As and Hg contents due to preferential migration of galena and tetrahedrite. The Hg content of sphalerite in these bodies is approximately doubled relative to the source lode, and the Hg content of galena concentrates is increased sevenfold, due to an increase of tetrahedrite inclusions.Coarse-grained sulfide-gangue pods and pegmatites, which reflect partial melting and remobilization of parts of the lode, have a mean Hg content less than the same volume of host lode. Within these bodies Hg is strongly partitioned into sphalerite, which contains from 14,000 to 33,000 ppb Hg, whereas galena contains only from 10 to 570 ppb Hg, due to the absence of Hg-bearing tetrahedrite inclusions.Within the lode environment, trace amounts of Hg in veins formed during retrograde metamorphism are located in sphalerite which contains up to 29,000 ppb Hg.The presence of significant amounts of Hg in Ag-Sb-As minerals in a siderite-rich vein within the main lode and in the Consols lode (located 700 m from the main lode) indicates that Hg remains associated with these elements over a wide range of conditions.In the wall rocks, there is no Hg halo which is exclusive of disseminated sulfide minerals. Under high-grade regional metamorphism, trace amounts of Hg in the Broken Hill lode are retained within pre-metamorphism host minerals. However, mobilization of Hg does take place in veins associated with retrograde metamorphism after localized breakdown of tetrahedrite. 相似文献
4.
Uranium in spinel peridotite inclusions in basalts from Sardinia 总被引：2，自引：0，他引：2
The uranium distribution in spinel peridotite inclusions and their host basalt from Sardinia, Italy, was determined by fission-track mapping. Whole-rock U concentrations range from 14 to 55 ppb. Although the partitioning of U among major silicate phases of the inclusions — olivine, orthopyroxene and clinopyroxene — remains roughly constant, the U content in the minerals is highly variable, e.g. ranging from 27 ppb to 177 ppb in clinopyroxene. The U variation in the minerals shows no apparent correlation with their major element chemistry. Liquid which equilibrated with the assemblages of inclusions with high U content, had U concentrations higher than those found in basaltic rocks. It is suggested that the inclusions were contaminated with a phase strongly enriched in U and subsequently recrystallized. The available data show that spinel peridotite inclusions of basaltic rocks frequently have complex multistage evolution and thus cannot provide a representative picture of the upper mantle radioactivity. 相似文献
5.
本文从酸碱电子理论出发,提出了一种新的岩浆岩碱度参数——相对碱度及其计算方法。岩石或矿物的相对碱度是其中的氧元素丢失电子能力的综合度量。规定硅作为其他阳离子对氧原子屏蔽作用的比较基准,用各金属氧化物与二氧化硅的反应焓变值作为它们相对于二氧化硅对矿物或岩石相对碱度的贡献。综合各氧化物的碱度贡献使得到岩石或矿物的相对碱度值。计算表明,主要造岩标准矿物和各种岩浆岩的相对碱度值顺序与传统用法基本一致。 相似文献
6.
In low- and middle-grade pelitic metamorphic rocks, the extentsof Tschermak substitution in muscovite, chlorite and biotitechange regularly with bulk-rock composition and external conditions.This paper gives a theoretical analysis of the changes. From equipotential lines for Al2O3 plotted on AFM diagrams,we have derived a series of Thompson-type muscovite compositiondiagrams, which show how the celadonite content of muscovitevaries with the associated ferromagnesian minerals or with thebulk-rock composition under constant external conditions. Thedistribution coefficient of the exchange reaction for Tschermaksubstitution between muscovite and chlorite varies greatly notonly with temperature but also with the extent of this substitutionin the two minerals because of their strong deviation from ideality.Muscovites with a high celadonite content (phengites) occurin low-temperature rocks in any of the high-, medium- and low-pressuretypes of metamorphism, probably because the exchange equilibriumfor Tschermak substitution between muscovite and chlorite isnot sensitive to pressure. When both Mg-Fe and Tschermak substitutionoccur in muscovite and some other silicates, a metapelite containingthree AFM phases together with muscovite and quartz has at leasttwo independent net-transfer reactions that take place sideby side with changing external conditions. The mass balancerequirement in the rock is imposed on a linear combination ofthe two reaction equations, leading to a constraint on the stoichiometricequations among phase components and the progressive compositionalchanges of muscovite, chlorite and biotite. From such a viewpoint,we examine reactions and progressive mineralogical changes inmetapelites, beginning with K-feldspar-bearing low-grade metapeliticrocks, in which biotite appears by reaction of K-feldspar withchlorite at a temperature lower than that of the biotite isograddefined for K-feldspar-free pelitic rocks. When both Mg-Fe and Tschermak substitution occur in muscoviteand some other silicates, a metapelite containing three AFMphases together with muscovite and quartz has at least two independentnet-transfer reactions that take place side by side with changingexternal conditions. The mass balance requirement in the rockis imposed on a linear combination of the two reaction equations,leading to a constraint on the stoichiometric equations amongphase components and the progressive compositional changes ofmuscovite, chlorite and biotite. From such a viewpoint, we examinereactions and progressive mineralogical changes in metapelites,beginning with K-feldspar-bearing low-grade metapelitic rocks,in which biotite appears by reaction of K-feldspar with chloriteat a temperature lower than that of the biotite isoerad definedfor K-feldsoar-free oelitic rocks. The equations for the reactions that produce biotite or biotite? garnet in K-feldspar-free metapelites have been derived. Combinedwith the composition relations of coexisting muscovite, chlorite,biotite and garnet, they lead to the inference that progressof these reactions with rising temperature causes a decreaseof the celadonite content of muscovite, the antigorite contentof chlorite and the phlogopite content of biotite in the metapelitesof the chlorite, biotite and lower garnet zones, and that thistrend may not continue into the staurolite zone. This inferencehas been verified by examination of the analytical data of mineralsin four progressive metamorphic terranes covering the high-,medium- and low-pressure types. The composition of chlorite is buffered by the garnet-producingreaction so that chlorites in garnet-bearing metamorphic rocksdevelop (Mg ? Fe)/Al2 ratios close to that of almandine at elevatedtemperatures. The maximum possible celadonite content in muscoviteunder given external conditions is realized in K-feldspar-bearingrocks, and decreases with rising temperature. 相似文献
7.
Jung-Woo Park Ian H. Campbell Ryan B. Ickert Charlotte M. Allen 《Contributions to Mineralogy and Petrology》2013,165(2):217-236
The behavior of the platinum group elements (PGE) and Re in felsic magmas is poorly understood due to scarcity of data. We report the concentrations of Ni, Cu, Re, and PGE in the compositionally diverse Boggy Plain zoned pluton (BPZP), which shows a variation of rock type from gabbro through granodiorite and granite to aplite with a SiO2 range from 52 to 74 wt %. In addition, major silicate and oxide minerals were analyzed for Ni, Cu, and Re, and a systematic sulfide study was carried out to investigate the role of silicate, oxide, and sulfide minerals on chalcophile element geochemistry of the BPZP. Mass balance calculation shows that the whole rock Cu budget hosted by silicate and oxide minerals is <13 wt % and that Cu is dominantly located in sulfide phases, whereas most of the whole rock Ni budget (>70 wt %) is held in major silicate and oxide minerals. Rhenium is dominantly hosted by magnetite and ilmenite. Ovoid-shaped sulfide blebs occur at the boundary between pyroxene phenocrysts and neighboring interstitial phases or within interstitial minerals in the gabbro and the granodiorite. The blebs are composed of pyrrhotite, pyrite, chalcopyrite, and S-bearing Fe-oxide, which contain total trace metals (Co, Ni, Cu, Ag, Pb) up to ~16 wt %. The mineral assemblage, occurrence, shape, and composition of the sulfide blebs are a typical of magmatic sulfide. PGE concentrations in the BPZP vary by more than two orders of magnitude from gabbro (2.7–7.8 ppb Pd, 0.025–0.116 ppb Ir) to aplite (0.05 ppb Pd, 0.001 ppb Ir). Nickel, Cu, Re, and PGE concentrations are positively correlated with MgO in all the rock types although there is a clear discontinuity between the granodiorite and the granite in the trends for Ni, Rh, and Ir when plotted against MgO. Cu/Pd values gradually increase from 6,100 to 52,600 as the MgO content decreases. The sulfide petrology and chalcophile element geochemistry of the BPZP show that sulfide saturation occurred in the late gabbroic stage of magma differentiation. Segregation and distribution of these sulfide blebs controlled Cu and PGE variations within the BPZP rocks although the magma of each rock type may have experienced a different magma evolution history in terms of crustal assimilation and crystal fractionation. The sulfide melt locked in the cumulate rocks must have sequestered a significant portion of the chalcophile elements, which restricted the availability of these metals to magmatic-hydrothermal ore fluids. Therefore, we suggest that the roof rocks that overlay the BPZP were not prospective for magmatic-hydrothermal Cu, Au, or Cu–Au deposits. 相似文献
8.
9.
The importance of oxygen isotope geochemistry in studies of terrestrial and extra-terrestrial silicate rocks was recognized nearly sixtyeight years ago soon after the discovery of O18 and O17. As early as 1934, the significance of oxygen isotope variations in rocks and minerals was stressed by Russian geochemists who also pioneered the discipline of silicate oxygen isotope geochemistry. It is now known that processes involving isotopic interaction between rock and water, magmatic differentiation, and metamorphic recrystallization fractionate oxygen isotopes in the lithosphere. δO18 (the conventional notation for reporting O18/O16 ratios in rocks and minerals) is highest in sedimentary rocks (17 to 35 ‰) and lowest in igneous rocks (4 to 12‰). Metamorphic rocks have intermediate values. δO18 in mafic minerals (1 to 8‰) is lower than in felsic minerals (8 to 16‰). In igneous and metamorphic rocks, quartz is most enriched in O18 (10 to 16‰) and magnetite the least (1 to 2‰). An important application of O18/O16 techniques is in geothermometry, where these are capable of elucidating several petrological processes. 相似文献
10.
The Tengchong volcanic field comprises numerous Quaternary volcanoes in SW China. The volcanic rocks are grouped into Units 1–4 from the oldest to youngest. Units 1, 3 and 4 are composed of trachybasalt, basaltic trachyandesite and trachyandesite, respectively, and Unit 2 consists of hornblende-bearing dacite. This rock assemblage resembles those of arc volcanic sequences related to oceanic slab subduction. Rocks of Units 1 and 3 contain olivine phenocrysts with Fo contents ranging from 65 to 85 mole%, indicating early fractionation of olivine and chromite prior to the eruption of magma. All the rocks from Units 1, 3 and 4 have very low PGE concentrations, with <0.05 ppb Ru and Rh, <0.2 ppb Pt and Pd, and Ir that is commonly close to, or slightly higher than detection limits (0.001 ppb). The small variations of Pt/Pd ratios (0.4–2.2) are explained by fractionation of silicate and oxide minerals. The 5-fold variations in Cu/Pd ratios (200,000–1,000,000) for the lavas at Tengchong, which do not vary systematically with fractionation, likely reflect retention of variable amounts of residual sulfide in the mantle source. In addition, all the rocks from Units 1, 3 and 4 have primitive mantle-normalized chalcophile element patterns depleted in PGE relative to Cu. Together with very low Cu/Zr ratios (0.06–0.24), these rocks are considered to have undergone variable degrees of sulfide-saturated differentiation in shallow crustal staging magma chambers. Large amounts of olivine and chromite crystallization probably triggered sulfide saturation of magma at depth for Units 1 and 3, whereas crustal contamination was responsible for sulfide saturation during ascent of magma for Unit 4. 相似文献
11.
Studies of Mesozoic granites associated with rare earth element (REE)‐rich weathered crust deposits in southernmost Jiangxi Province indicate that they have high‐K to shoshonite compositions and belong to ilmenite‐series I‐type granites. Of the studied rocks at 59–292 ppm of bulk REE content, the highest are seen in the biotite granites of Dingnan (358, 429 ppm) and mafic biotite granite of the Wuliting Granite (344 ppm) near the Dajishan tungsten mine, both areas where weathered‐crust REE deposits occur. REE‐bearing accessory minerals in these granites are mainly zircon, apatite and allanite, and REE‐fluorocarbonates are common. REE enrichment occurs in the rims of apatite crystals, and in fluorocarbonates that occur along grain boundaries of and cracks in major silicate minerals, and in fluorocarbonates that replaced altered biotite. It is therefore thought that a major part of the REE content of these granites was concentrated during deuteric activity, rather than during magmatic crystallization. The crack‐filling REE‐fluorocarbonates could subsequently have been easily leached out and deposited in weathered crust developed during a long period of exposure. 相似文献
12.
Lynton S. Land 《Aquatic Geochemistry》1995,1(2):137-145
Most sedimentary basins contain saline pore water. Saline formation waters can form during burial diagenesis as the result of normal processes of water/rock interaction involving incongruent halite dissolution, bittern salt destruction, and albitization of detrital plagioclase. The kinds of Na-Ca-Cl saline formation waters typical of sedimentary basins can also result from modification of surficial brines formed by the precipitation of NaCl from evaporated seawater. As the porosity of rocks is reduced during burial, discharge of saline formation waters contributes to crustal chloride cycling, and helps explain riverine chloride loads. During burial, the dissolution of metastable detrital minerals derived from crustal rocks in corrosive, saline water transfers incompatible elements such as Li and B from the igneous crust to the sedimentary crust. Similarly, albitization transfers Ca from the crustal silicate (igneous) mineral reservoir to the crustal carbonate and aqueous (sedimentary) reservoirs. Metamorphism and then melting of albite-enriched rocks accounts for the elevated sodium contents of igneous rocks relative to sedimentary rocks. In this way average sediments have become enriched through time in Ca, Cl, Br, S, Li, and B, and depleted in Na relative to average igneous crust. 相似文献
13.
Metagranodiorite samples from the Brossasco‐Isasca Unit, Dora‐Maira Massif, western Alps, show pseudomorphous and coronitic textures where igneous minerals were partially replaced by ultra‐high pressure (UHP) metamorphic assemblages. The original magmatic paragenesis consisted of quartz, plagioclase, K‐feldspar, biotite and minor phases. During UHP metamorphism, the plagioclase (site P) was replaced by zoisite, jadeite, quartz, K‐feldspar and kyanite, and coronitic reactions developed between biotite and adjacent minerals. At the original igneous biotite–quartz contact (site A), a single corona of poorly zoned garnet is developed, whereas at the biotite–K‐feldspar (site B) and biotite–plagioclase (site C) contacts, composite coronas are formed. Integration of results from petrographic observations, calculations of mineral stoichiometry and thermodynamic calculations of mineral stability has allowed the determination of the metamorphic reactions involved and the estimation of the metamorphic conditions, which reached as high as 24 kbar and 650 °C. Accurate microanalysis by energy‐dispersive spectroscopy (EDS) and statistical analysis of the data allowed us to identify, for the first time in a natural Na‐pyroxene of metagranitoid rocks, the end‐member Ca‐Eskola. 相似文献
14.
《Journal of African Earth Sciences》2007,47(3):153-173
The lithology of Malawi is characterized by Precambrian metamorphic and igneous rocks which form part of the polyphase East African Orogen. Rift-related sedimentation and igneous activity during the late Paleozoic (Karoo System) and the late Mesozoic (Chilwa Province) have produced a great variety of rocks that underwent strong chemical weathering and erosion when the entire region received its final shape by peneplanation and fluvial incision during the Cenozoic under (sub)tropical climatic conditions. Aluminum is a dominating element in minerals (e.g., corundum, kyanite, beryl, gibbsite) in this region. Some minerals were concentrated in deposits bound to Al-enriched host rocks (zircon, pyrochlore, eudialyte, uranium minerals) with high A/CNK ratios, whereas others, e.g., asbestos, chalcedony, monazite, kaolinite, ilmenite and garnet have host rocks of a low A/CNK ratio. Aluminum was used to categorize these various mineral deposits. The abundance of aluminum and accumulations of Nb, Zr, Ti, REE, Sr and Ba point for some mineralisations to similar subcrustal carbonatite-forming systems that were operative during periods of the Precambrian and the Mesozoic in Malawi. Aluminum variation does not only reflect differentiation in the various igneous rock series but it is also visible in the sedimentary realm during transport and weathering. In context with other elements such as Ti and P, Al provides an opportunity to reveal chemical relationships between rocks and mineral deposits. Spinel and Al-enriched silicate minerals can be used as pathfinder minerals in the stream sediments to guide the exploration geologist to non-metallic deposits. 相似文献
15.
The paper is based on a study of minerl assemblages in metamorphiciron formations in northere Quebec. Thirty-four mineral analyseshave been performed, using photometric methods of silicate analysis. Three facies of iron formation are represented, namely, quartz-speculariteiron formation, quartz-magnetite iron formation, and quartz-silicateiron formation. The silicate iron formation is made up mainlyof quartz, ferrohypersthene, ferroaugite, cummingtonite, andcalcite. Arfvedsonite and aegirine-hedenbergite occur in magnetite-richrocks. The analysed coexisting minerals are from the silicate ironformation and from overlying mafic rocks of igneous origin.The analysed assemblages include ferrohypersthene-ferro-augite-cummingtonite,ferrohypersthen-ferroaugite, cummingtonite-ferroaugite, diopside-hornblende,anthophyllite-hornblende, hypersthene-diopside-hornblende-garnet,and olivine-hypersthene-chlorite. The analyses show that (1) the minerals are nearly in equilibriumas shown by parallel tie lines, (2) cummingtonite generallyhas a higher MgO/FeO ratio than coexisting ferrohypersthene,(3) the phase higher in calcium has the higher MgO/FeO ratio,(4) the solubility gap between ferrohypersthene and ferroaugiteis 43 mole percent. The stability relationships of pyroxenes and amphiboles arediscussed in view of the new data. It is shown that FeO-CaO-SiO2systems often behave differently from MgO-CaO-SiO2 systems.Hedenbergite is probably not stable in the presence of excessCO2 in the area. The low variance ferrohypersthene-ferroaugite-cummingtonite-calcite-quartzassemblage indicates that the rocks were closed to H2O and CO2.The behaviour of the volatile components is discussed in viewof this fact. The source of the silicate minerals is assumed to be hydrousiron silicates and carbonates. A model of the sequence of chemicalreactions taking place in carbonate-silicate iron formationsduring metamorphism is suggested.
1Present address: Bolidens Gruvaktiebolag, Boliden, Sweden. 相似文献
16.
粤北大宝山矿区加里东期火山岩的厘定及其地质意义 总被引：6，自引：2，他引：4
粤北大宝山矿区一带出露一套与层状Fe-Cu-Pb-Zn矿化紧密共生的层状火成岩。层状火成岩产状与上覆页岩及大理岩和下伏碳质页岩基本一致。本文通过层状火成岩显微特征及锆石年龄探讨火成岩形成环境及时代。层状火成岩为斑状结构,块状构造,局部发育条带状构造及流动构造。层状火成岩的斑晶主要由火山喷发形成的棱角明显石英、斜长石、钾长石、少量角闪石、黑云母及黄铁矿等晶屑及少量岩屑组成,基质为火山熔浆凝结形成的全晶质矿物,主要由粒度极细的石英、钾长石、绢云母、白云母及少量黑云母等组成,石英和钾长石构成典型的霏细结构。层状火成岩含大量火山碎屑及具流动构造等现象表明其为火山碎屑熔岩类,主要为英安质凝灰熔岩、流纹质凝灰熔岩及角砾熔岩。大宝山矿区中部流纹质凝灰熔岩和东部丘坝一带英安质凝灰熔岩的锆石LA-ICP-MS U-Pb年龄分别为436.4±4.1Ma,MSWD=0.94和434.1±4.4Ma,MSWD=1.9,是加里东期形成的。据大宝山矿区熔岩锆石U-Pb年龄,结合层状熔岩和灰岩及碳质页岩紧密共生及区域深大断裂构造活动特征,我们认为大宝山与层状矿化关系密切的层状火成岩为加里东期海相火山熔岩,粤北一带加里东期海相火山活动可能和形成于加里东期的吴川-四会深大断裂活动有关。 相似文献
17.
18.
Gold abundance in granitic rocks of different geological periods in southern China has been estimated. A review of the quantitative data available indicates that unaltered granitic rocks have a rather restricted range in gold content, rarely exceeding 4 ppb and generally ranging from 1.4 to 3.3 ppb. The mean gold content tends to decrease from basic to acidic granitoids. This tendency suggests that gold isnot concentrated in the residual silicate meltduring the formation of granitic rocks. It is necessary to establish the background values of gold for various rock types although it seems that gold abundance data for the granitic rocks of southern China can necessarily provide any geochemical clues or guides to areas favorable or unfavorable for gold mineralization. 相似文献
19.
TANG Pan CHEN Yuchuan TANG Juxing WANG Ying ZHENG Wenbao LENG Qiufeng LIN Bin WU Chunneng 《《地质学报》英文版》2019,93(6):1947-1966
Biotite is an important hydrated ferromagnesian silicate mineral in igneous rocks and porphyry deposits. The determination of chemical compositions of biotite plays an important role in both igneous petrology and ore forming processes. This paper summarizes research results of magmatic and hydrothermal biotites exemplified by the Lakange porphyry Cu–Mo deposit and the Qulong porphyry Cu deposit in the Gangdese porphyry–skarn metallogenic belt, Tibet. Biotite mineral chemistry can provide critical insights into classification, geothermometer, geothermobarometry, oxygen fugacity, petrogenesis and tectonic setting, evaluating magmatic-hydrothermal process by halogen and halogen fugacity ratios, and distinguishing between barren and mineralized rocks. Biotite provides the latest mineralogical evidence on metallogenic prognosis and prospecting evaluation for porphyry Cu polymetallic deposits or magmatic hydrothermal deposits. 相似文献
20.
Microlites (minute spherulitic, dendritic, skeletal, acicular and poikilitic crystals) diagnostic of crystallization in quenched melt or glass in fault rocks have been used to infer fossil earthquakes. High‐P microlites and crystallites are described here in a variably eclogitized gabbro, the wallrock to the coesite‐bearing eclogite breccia at Yangkou in the Chinese Su‐Lu high‐P metamorphic belt. The studied hand specimens are free of discernible shear deformation, although microfractures are not uncommon under the microscope. In the least eclogitized gabbro, the metagabbro, stellate growths of high‐P minerals on the relict igneous minerals are common. Dendritic garnet crystals (<1?5 μm) grew around rutile and/or phengite replacing ilmenite and biotite, respectively. Skeletal garnet also rims broken flakes of igneous biotite and mechanically twinned augite. Radial intergrowths of omphacite and quartz developed around relict igneous orthopyroxene and are rimmed by skeletal or poikilitic garnet where a Ti‐bearing mineral relict is present. Acicular epidote, kyanite and phengite crystallites are randomly distributed in a matrix of Na‐rich plagioclase, forming the pseudomorphs after igneous plagioclase. In the more eclogitized gabbro, the coronitic eclogite located closer to the eclogite breccia, all the igneous minerals broke down into high‐P assemblages. Thick coronas of poikilitic garnet grew between the pseudomorphs after igneous plagioclase and ferromagnesian minerals. The igneous plagioclase is replaced by omphacite crystallites, with minor amounts of phengite and kyanite. Thermodynamic modelling of the plagioclase pseudomorphs shows an increase in P–T in the wallrock from the metagabbro to the coronitic eclogite, and the P–T variation is unrelated to H2O content. The fluid‐poor pressure overstepping scenario is unsupported both by phase diagram modelling and by whole‐rock chemical data, which show that the various types of eclogitized gabbro are all fairly dry. A large pressure difference of >2 GPa between the metagabbro and the coesite‐bearing eclogites ~20 m apart cannot be explained by the subduction hypothesis because this would require a depth difference of >60 km. The microlites and crystallites are evidence for dynamic crystallization due to rapid cooling because constitutional supercooling was unlikely for the plagioclase pseudomorphs. The lack of annealing of the broken biotite and augite overgrown by strain free skeletal garnet is consistent with a transient high‐P–T event at a low ambient temperature (<300 °C), probably in the crust. Therefore, the eclogitization of the wallrock to the eclogite breccia was also coseismic, as proposed earlier for the eclogite facies fault rocks. The outcrop‐scale P–T variation and the transient nature of the high‐P–T event are inconsistent with the other existing tectonic models for high‐P metamorphism. The fact that the less refractory but denser biotite is largely preserved while the more refractory but less dense plagioclase broke down completely into high‐P microlite assemblages in the metagabbro indicates a significant rise in pressure rather than temperature. Given that the metamorphic temperatures are far below the melting temperatures of most of the gabbroic minerals under fluid‐absent conditions, stress‐induced amorphization appears to be the more likely mechanism of the coseismic high‐P metamorphism. 相似文献