共查询到20条相似文献，搜索用时 31 毫秒
1.
高温变泥质岩石中石榴石-黑云母地质温度计的应用——以胶北荆山群富铝岩石为例 总被引：6，自引：0，他引：6
在对胶北荆山群麻粒岩相富铝岩石中石榴石、黑云母的成分环带进行深入研究基础上,选取不同粒径、与不同矿物相邻的石榴石、黑云母各微区点成分,利用石榴石-黑云母温度计分别进行了温度估算。确定在黑云母含量较高的岩石(V_(Grt)/V_(Bt)≤1)中,利用大颗粒石榴石(d≥1500μm)晶体核部(或靠近长英质矿物一侧的晶体幔部)成分与基质中远离石榴石等镁铁矿物处于长英质矿物之间的黑云母核部成分配合。通过石榴石-黑云母温度计可以获得相当可信的变质峰期温度。但是对于黑云母含量极低的岩石(V_(Grt)/V_(Bt)≥6),由于黑云母的成分普遍遭到了强烈改造。使得温度估算结果异常偏低,因此不适合采用石榴石-黑云母温度计估算峰期温度。同一岩石中,采用不同的相邻石榴石-黑云母矿物对晶体边缘成分获得的温度值差异较大,反映它们在峰期后发生Fe-Mg交换反应并达到封闭温度平衡状态的程度不同,因此利用石榴石-黑云母温度计难以获得准确的封闭温度。通过热力学计算,建立了一个新的石榴石-黑云母温度计公式。确定胶北荆山群所经历的变质峰期温度为720～770℃,峰期后最低相对封闭温度为480～500℃。 相似文献
2.
胶北荆山群富铝岩系石榴石扩散环带特征及其成因指示意义 总被引：9，自引：2，他引：9
通过详细的微区成分测定，发现胶北荆山群富铝岩系中石榴石普遍发育扩散环带，但扩散环带的发育程度及样式很不均匀，明显受与其相邻矿物的控制。与黑云母接触时，石榴石晶体边部的镁含量最低，环带最为发育，与堇青石接触时次之，与长英质矿物接触时则环带发育较弱或不发育。这种特征的石榴石扩散环带样式与传统认识有很大差异，反映降温过程中石榴石与黑云母等镁铁矿物之间的Fe-Mg交换作用主要是通过彼此接触的界面来实现，粒间流体对组分的传输作用有限。但是当岩石中黑云母大量存在而石榴石含量又较低时，由于体系水活度增高，粒间流体也会传输一定的Fe、Mg组分，导致与长英质矿物相邻的石榴石晶体边部发育微弱的扩散环带。通过分析，确定粒径大于1500斗m的石榴石晶体核部可以保存变质峰期的平衡成分，基质中远离石榴石等镁铁矿物处于长英质矿物之间的大颗粒黑云母颗粒核部也基本可以保存变质峰期的平衡成分。 相似文献
3.
浙西南遂昌-大柘地区八都岩群在印支期变质事件影响下发生变质变形,通过详细野外调查和岩相学研究,可将其划分为3期变质变形序列：S1变形期,NW向片麻理记录的残留紧闭褶皱,共生矿物组合为石榴子石变斑晶及其内部定向分布的包裹体矿物,石榴子石+黑云母+石英（泥质）和石榴子石+角闪石+斜长石+石英（长英质）;S2变形期,区域性宽缓褶皱及NE向缓倾透入性片麻理,共生矿物组合为石榴子石变斑晶及定向分布的基质矿物,矽线石+石榴子石+黑云母+石英+斜长石±钾长石（泥质）和石榴子石+钾长石+斜长石+黑云母+石英（长英质）;S3变形期,NE向陡倾透入性片麻理及韧脆性断裂大部分被花岗斑岩脉填充,共生矿物组合为石榴子石变斑晶及其周围退变矿物,石榴子石+矽线石+堇青石+斜长石+黑云母+石英±钾长石（泥质）和角闪石+斜长石+黑云母+钛铁矿（长英质）。结合前人研究成果,八都岩群印支期变质事件峰期变质程度达到麻粒岩相,显示顺时针近等温降压（ITD）型的p-T演化轨迹,S1-S3变质变形反映出从俯冲碰撞到快速折返冷却的演化过程,伴随S3同期侵位的花岗斑岩锆石U-Pb定年结果,将该演化过程完成时间约束在229.7 Ma,可能是浙西南地区对印支期古特提斯洋域内印支-华南-华北板块之间俯冲-碰撞过程的响应。 相似文献
4.
Liu Fulai Shen Qihan Geng Yuansheng Institute of Geology Chinese Academy of Geological Sciences Beijing Xu Xuechun Ma Rui Open Laboratory Changchun College of Geology Changchun Jilin 《《地质学报》英文版》1997,71(4):407-422
The high-temperature and high-pressure experiment on natural block rock indicates that dehydration-melting of hydrous biotite (Bi) and partial melting of felsic minerals in garnet-biotite-plagioclase gneiss are mainly controlled by temperature, while mineral phase transformation is not only controlled by temperature-pressure conditions but also genetically associated with hydrous mineral dehydration-melting and partial melting of felsic minerals. According to the characteristics of biotite dehydration-melting and garnet transformation reaction, three stages may be distinguished: (1) when the experimental temperature is 700℃, biotite transforms to ilmenite (Ilm) + magnetite (Mt) + H2O and garnet to magnetite (Mt); (2) when the temperature is 730-760℃, biotite is dehydrated and melted and transformed into K2O-rich melt + Ilm + Mt, and garnet, into hypersthene (Hy) + cordierite (Crd); (3) when the temperature is up to or higher than 790℃, biotite is dehydrated and melted and transformed into melt + Hy + 相似文献
5.
T. F. Shcherbakova 《Geochemistry International》2017,55(9):833-839
Zircon hosted in granite, which crystallized from local pools of anatectic melt among migmatites, in the Rikolatvi structure, Belomorian Mobile Belt, contains minute inclusions of various minerals, biotite and garnet among others. The compositions of the biotite and garnet in the microinclusions differ from those of the same minerals in the granite containing the zircon. The crystallization temperature of the anatectic melt was estimated by the biotite–garnet geothermometer and the composition of the biotite and garnet inclusions at ~800°C. 相似文献
6.
We present a compositional and textural analysis of shock-induced microtextures in garnet porphyroblasts in migmatitic garnet–cordierite–biotite paragneisses from the centre of the Vredefort impact structure, South Africa. Detailed imaging and major element analysis of deformation features in, and adjacent to, the garnet porphyroblasts record a complex, heterogeneous distribution of shock effects at the microscale. As the most competent silicate mineral in the assemblage, with the highest Hugoniot Elastic Limit and a wide pressure–temperature stability field, the porphyroblastic garnet preserves a more diverse shock deformation response compared to minerals such as quartz and feldspar, which underwent more comprehensive shock metamorphism and subsequent annealing. The garnet porphyroblasts display pre-impact fractures that are overprinted by later intra-granular Hertzian and distinctive planar fractures associated with the impact event. Shock-induced strain localization occurred along internal slip planes and defects, including pre-existing fractures and inclusion boundaries in the garnet. Symplectitic (kelyphitic) coronas commonly enclose the garnet porphyroblasts, and inhabit intra-granular fractures. The kelyphite assemblage in fractures with open communication beyond garnet grain boundaries is characterized by orthopyroxene—cordierite—sapphirine. Conversely, the kelyphite assemblage in closed-off intra-granular fractures is highly variable, comprising spatially restricted combinations of a secondary garnet phase with a majoritic component, Al-rich orthopyroxene, sapphirine and cordierite. The impedance contrast between garnet porphyroblasts and their inclusions further facilitated the formation of shock-induced features (Al-rich orthopyroxene coronas). Together, the textural and mineralogical data suggest that these features provide a record of oscillatory shock perturbations initiated under confining pressure beneath the transient crater floor. This occurred as the shocked rock volume underwent post-shock expansion, forming the core of the central uplift, and was followed by variable textural re-equilibration. This study thus provides a microtextural and mineralogical perspective of the shock regime within confined crust immediately prior to and during central uplift formation. 相似文献
7.
Porphyroblastic biotite and garnet in the Barrovian metapelites of the Imjingang belt, Korea, were investigated to unravel the sequence and mechanism of mineral growth. Poikiloblastic biotite contains straight inclusion trails (Si ) discontinuous to the major foliation, and develops clear zones at the grain margin. These microstructures suggest an initial growth of biotite between two contractional deformations (Dn−1 and Dn ) followed by an overgrowth during Dn . Although garnet poikiloblasts contain variable Si patterns, their major growth is likely to have occurred during Dn on the basis of compositional relationships among variable garnet types. Early poikiloblasts of both minerals were formed by chemical replacement of the matrix that consisted mainly of chlorite, muscovite and quartz. Subsequent growth of biotite was governed by a crack-filling mechanism, and was accompanied by the production of extensional cracks inside or around biotite, providing fluid pathways. The overgrowth of garnet was favoured at the biotite–garnet interface, and the consequence was a partial replacement of inclusion-poor garnet after biotite subsequent to Dn . In addition, clear zones and pressure shadows as well as the matrix around biotite porphyroblasts were replaced by garnet, suggesting an inheritance of various pre-existing microstructures in the Si pattern of garnet. Further attention is thus required for any attempt to delineate the microstructural interaction between deformation and metamorphism, particularly in a sample containing early-grown porphyroblasts. Microstructural evidence for the two-stage growth of biotite and garnet is present up to the kyanite zone, indicating that this growth mechanism is prevalent during progressive metamorphism of Barrovian metapelites. 相似文献
8.
Paul C. Hess 《Contributions to Mineralogy and Petrology》1971,30(3):177-195
Partial electron microprobe analyses of garnet, biotite and cordierite in sillimanite-K feldspar gneisses of the Brimfield Formation in south-central Massachusetts indicate that the compositions of these minerals are not constant in a thin section. The FeO/MgO mol ratio of biotite is sensitive to the nature of other FeO-MgO minerals occurring in close proximity. The most iron-rich biotites are those that do not contact either cordierite or garnet. The most iron-poor biotites occur as inclusions in garnet. Biotites in direct contact with either cordierite or garnet have intermediate FeO/MgO ratios. The bulk of a given grain of garnet or cordierite is homogeneous in composition. Chemical zoning is absent. All grains of garnet and cordierite in a thin section are constant in composition. However, where garnet and cordierite abut biotite, the FeO/MgO ratio of the garnet rim is increased and that of cordierite is decreased. The FeO/MgO ratios of garnet, cordierite and biotite bare a regular relation to each other indicating a possible equilibrium state. However the distribution coefficient defined by the compositions of minerals in direct contact are greater than those defined by the compositions of the interiors of garnet and cordierite matched with the compositions of biotites removed from these phases. This pattern is believed to be the result of two thermal events. The first event produced the mineral assemblages and widespread equilibrium was obtained. A subsequent retrograde event left the mineralogy intact but caused cation exchange reactions at immediate contacts between garnet, cordierite and biotite. The physical conditions of the first event are estimated at P=5–6 kb, T=700–750° C. The retrograde event occurred at lower temperatures and very low activities of H2O since no muscovite is developed at microcline-sillimanite contacts. 相似文献
9.
The products of shock metamorphism in the Jänisjärvi astrobleme in Karelia, Russia, are compared with the results of experiments in which spherical converging shock waves affected a spherical rock sample. The sample was loaded by a broad spectrum of shock pressures, which increased from ～20 GPa at the periphery of the rock sphere to > 200 GPa at its center. Experiments with rocks metamorphosed under the effect of spherical converging shock waves imitate collisions of cosmic bodies with the Earth’s surface, when transformations in rocks and minerals are induced by a single impact event. The shock-thermal decomposition of mafic minerals occurs in the same succession in nature and the experiments, with some differences between natural and experimentally produced shock-thermal aggregates likely accounted for by the smaller sizes of the experimental impact rock sample and, correspondingly, its more rapid quenching. Our shock experiments were the first to synthesize ringwoodite that was rich in Al2O3 and should be referred to as aluminous ringwoodite. The mineral was produced not via the martensite transition of olivine but by means of biotite replacement coupled with the migration of elements. The transformations of minerals by shock waves (amorphization and shock-thermal decomposition) were determined to be controlled mainly by the crystal structures of these minerals. The experimental products provide evidence of the migration of chemical elements within the crystal structure. The structural setting of ions in a mineral determines the onset of element migrations and the intensity of this process. 相似文献
10.
《International Geology Review》2012,54(9):1028-1036
Experimentally, at 66 kbar, biotite and lepidomelane are converted into associations of several minerals, at 1300–15000 and 900–1450°C, respectively, with the accompanying increase in the density; garnet is the densest product of both minerals; all of their original potassium passes into the low-density minerals during the conversion. The biotite and lepidomelane associations with garnet and other minerals are converted into biotite and lepidomelane above 1500° and 1450°C, respectively. Thus the range of instability of the micas, with its high pressures and relatively low temperatures, appears to be conducive to migration of potassium into upper horizons of the earth, e.g. by the gravitational differentation or by a fractionation in melting of the mantle substance. – V. P. Sokoloff. 相似文献
11.
沂水杂岩中变泥砂质岩石的岩石化学特征及年代 总被引：4，自引：3，他引：1
含夕线石十字石榴二云斜长片麻岩是沂水杂岩中首次发现的一种少见的变泥砂质岩石,包裹于沂水生心官庄岩浆杂岩体中,为残留的变质表壳岩透镜体,它经历了两期变质作用的改造.早期高角闪岩相变质与区域麻粒岩相变质有关,峰期矿物共生组合主要为:石榴子石(中心域)+黑云母±白云母+斜长石+石英,M1峰期变质温压条件为:T=660±10℃,P=5.7±0.3kb;晚期角闪岩相变质矿物共生组合为:十字石+石榴子石(边部域)+黑云母±白云母+斜长石±夕线石+石英,以形成大量自形-半自形十字石和具有明显的成分环带的石榴子石为特征,晚期石榴子石的形成由核部→边部经历了一降温降压过程,石榴子石核部:T=650±10℃,P=7.7±0.5Kb,石榴子石边部:T=578±10℃,P=4.7±0.1kb;晚期变质作用早期(石榴子石成核)阶段与埋深导致的部分熔融有关,晚期石榴子石生长阶段与岩浆热事件有关.锆石SHRIMP U-Pb定年结果表明:碎屑锆石不一致线上交点年龄为2695±32Ma,代表变泥砂质岩石源区岩浆岩的结晶年龄,变泥砂质岩石的早期变质变质作用年龄小于此值;晚期变质作用年龄为2537±5Ma. 相似文献
12.
Hartmut Schneider 《Contributions to Mineralogy and Petrology》1974,43(3):233-243
Thermal transformations in biotites depending on increasing postshock temperatures were studied.A decrease of the unit cell volume, caused by oxidation of octahedral Fe2+ to Fe3+ at temperature rangings of about 400–500° C was observed.In the same temperature range (500° C) the biotite lattice becomes thermodynamically unstable. Intersheet and octahedral layers are decomposed, whereas individual SiO4-tetrahedra are more resistant to thermal vibrations. The decomposition of the biotite leads to the formation of new minerals and amorphous phases. By X-ray analysesmagnetite, hercynite, pyroxene, feldspar, and quartz were identified.Thermal transformations of biotite induced by shock waves are characterized by states of distinct disequilibrium. 相似文献
13.
Geology of Ore Deposits - The evolution of rock-forming minerals (orthopyroxene, garnet, biotite, and amphibole) is studied in successive generations of granites and leucosomes. It is shown that... 相似文献
14.
Takeshi Ikeda 《Contributions to Mineralogy and Petrology》1998,132(4):321-335
The garnet-cordierite zone, the highest-grade zone of the Ryoke metamorphic rocks in the Yanai district, SW Japan, is defined
by the coexistence of garnet and cordierite in pelitic rocks. Three assemblages in this zone are studied in detail, i.e. spinel
+ cordierite + biotite, garnet + cordierite + biotite and garnet + biotite, all of which contain quartz, K-feldspar and plagioclase.
The Mg/(Fe + Mg) in the coexisting minerals decreases in the following order: cordierite, biotite, garnet and spinel. Two
facts described below are inconsistent with the paragenetic relation in the K2OFeOMgOAl2O3SiO2H2O (KFMASH) system in terms of an isophysical variation. First, garnet and biotite in the last assemblage have Mg/(Fe + Mg)
higher than those in the second. Second, the first two assemblages are described by the reaction,
while they occur in a single outcrop. The addition of MnO, ZnO and TiO2 to the system can resolve the inconsistencies as follows. The assemblage garnet + biotite can consist of garnet and biotite
higher in Mg/(Fe + Mg) than those in garnet + cordierite + biotite as long as they are enriched in spessartine and depleted
in Al, respectively. The assemblage garnet + cordierite + biotite becomes stable relative to spinel + cordierite + biotite
with increasing spessartine content or decreasing gahnite content and the Ti content of biotite. The constituent minerals
of the assemblages, spinel + cordierite + biotite and garnet + cordierite + biotite, preserve several reaction microstructures
indicative of prograde reactions,
and
together with retrograde reactions,
and
This suggests that the pressure-temperature path of the rocks includes an isobaric heating and an isobaric or decompressional
cooling. The high-grade areas consisting of the K-feldspar-cordierite zone, sillimanite-K-feldspar zone and garnet-cordierite
zone have prograde paths involving isobaric heating and show a southwards increase in pressure with a thermal maximum in the
middle. These high-grade zones are closely associated with the gneissose granitic rocks, suggesting that the Ryoke metamorphism,
one of the typical low-pressure type, is caused by the heat supply from the syn-tectonic granitic rocks that emplaced at the
middle level of the crust.
Received: 22 August 1997 / Accepted: 11 May 1998 相似文献
15.
中国大陆科学钻探主孔0～4500米变质岩石锆石中保存的超高压矿物包体 总被引：14，自引：9，他引：14
中国大陆科学钻探主孔0-4500米的岩心主要由榴辉岩、斜长角闪岩、副片麻岩、正片麻岩以及少量的超基性岩所组成。岩相学研究结果表明,榴辉岩的围岩普遍经历了强烈角闪岩相退变质作用的改造,峰期超高压变质的矿物组合已完全被后期退变质过程中角闪岩相矿物组合所替代。采用激光拉曼技术,配备电子探针和阴极发光测试,发现主孔224件岩心中有121件(包括榴辉岩、斜长角闪岩、副片麻岩和正片麻岩)样品的锆石中普遍隐藏以柯石英为代表的超高压矿物包体,且不同岩石类型锆石中所保存的超高压矿物包体组合存在明显差异。(含多硅白云母)金红石石英榴辉岩锆石中保存的典型超高压包体矿物组合为柯石英 石榴石、柯石英 石榴石 绿辉石 金红石和柯石英 多硅白云母 磷灰石。黑云绿帘斜长角闪岩锆石中保存的超高压矿物组合为柯石英 石榴石 绿辉石、柯石英 石榴石 多硅白云母和柯石英 绿辉石 金红石,与榴辉岩所保存的超高压矿物组合十分相似,表明该类斜长角闪岩是由超高压榴辉岩在构造折返过程中退变质而成。在副片麻岩类岩石,如石榴绿帘黑云二长片麻岩锆石中,代表性的超高压包体矿物组合为柯石英 多硅白云母和柯石英 石榴石等;而在石榴黑云角闪钠长片麻岩锆石中,则保存柯石英 硬玉 石榴石 磷灰石、柯石英 硬玉 多硅白云母 磷灰石和柯石英 石榴石 磷灰石等超高压矿物包体。在正片麻岩锆石中,标志性的超高压矿物包体为柯石英、柯石英 多硅白云母、柯石英 蓝晶石 磷灰石和柯石英 蓝晶石 榍石等。此外,在南苏鲁东海至临沭一带的地表露头以及一系列卫星孔岩心的锆石中,也普遍发现以柯石英为代表的标志性超高压矿物包体,表明在南苏鲁地区由榴辉岩及其围岩的原岩所组成的巨量陆壳物质(方圆>5000km2,厚度超过4.5km)曾整体发生深俯冲,并经历了超高压变质作用。该项研究对于重塑苏鲁-大别超高压变质带俯冲-折返的动力学模式有着重要的科学意义。 相似文献
16.
Rubbo Marco Borghi Alessandro Compagnoni Roberto 《Contributions to Mineralogy and Petrology》1999,137(4):289-303
Samples of the metagranodiorite from M. Mucrone (Sesia zone, Western Alps) show pseudomorphic and coronitic textures where
the igneous minerals were partially replaced by high-pressure metamorphic assemblages. The original magmatic paragenesis consisted
of quartz, plagioclase, K-feldspar, biotite and minor phases. During the eclogitic event the original plagioclase was fully
replaced by zoisite, jadeite and quartz ± K-feldspar pseudomorphic symplectites and the biotite was in part replaced by phengitic
mica. Moreover, a composite corona often developed around the biotite. This corona consists of a layer of phengite I and garnet
and, where the igneous biotite and feldspars were in contact, of an outer layer of phengite II intergrown with quartz. Biotite,
phengite and K-feldspar are homogeneous while garnet shows a strong composition zoning recording the kinetics of the metamorphic
reactions. A numerical simulation of the observed garnet zoning is performed assuming that intercrystalline diffusion and
plagioclase resorption were the slowest rate-determining processes during the prograde P-T path. The metamorphic paragenesis constrains the P-T path chosen in the simulation. The comparison between measured and calculated garnet zoning permits evaluation of the relative
weights of interface kinetics, grain-boundary and lattice diffusion.
Received: 26 November 1997 / Accepted: 6 August 1999 相似文献
17.
This paper presents monomineral and multiphase inclusions in garnet from eclogites and clinopyroxenites, which form layers and boudins in garnet peridotites from two areas in the Moldanubian zone of the Bohemian Massif. The garnet peridotites occur in felsic granulites and reached UHP conditions prior to their granulite facies overprint. In addition to complex compositional zoning, garnets from hosting eclogites and clinopyroxenites preserve inclusions of hydrous phases and alkali silicate minerals including: amphiboles, chlorites, micas and feldspars. Amphibole, biotite and apatite inclusions in garnet have a high concentration of halogens; CO2 and sulfur are involved in carbonates and sulfide inclusions, respectively. The inclusion patterns and compositional zoning in garnet in combination with textural relations among minerals, suggest that the ultramafic and mafic bodies are derived from lithospheric mantle above the subduction zone and were transformed into garnet pyroxenites and eclogites in the subduction zone. Based on compositional, mineral and textural relations, all of these rocks along with the surrounding crustal material were overprinted by granulite facies metamorphism during their exhumation. 相似文献
18.
Hassan Abd El-Razek Aly Shahin 《Arabian Journal of Geosciences》2013,6(7):2245-2259
Optical microscopy, X-ray diffraction (XRD), and back-scattered electron imaging (BSE) have been used to determine the mineralogical composition of the uraniferous iron and manganese oxides and the associated U-minerals hosted in biotite granite that occurred north east Gabal El Sela area south Eastern Desert, Egypt. These mineralizations were found as veinlets fractures filling associated with strongly kaolinitic alteration of the coarse-grained biotite granite. XRD determined that the geothite mineral form the main constituent of uraniferous iron oxide in addition to tapiolite, and kaolinite minerals, where as uraniferous manganese oxide composed of pyrolusite, ramsdellite, and cryptomelane. BSE confirmed that the associated uranium minerals represented by uranothorite, kazolite, and zentime in addition to columbite-bearing minerals. Uranothorite and columbite-bearing minerals are the most abundant minerals in this mineralization. Petrographically, biotite granite is composed mainly of quartz, in addition to K-feldspars, biotite and muscovite with minor zircon, garnet, apatite, uranium-rich thorite and iron oxide. Petrochemical studies and tectonic discrimination diagrams for this granite reveal that they are classified as granite to alkali feldspar granite, originated from calc-alkaline magma having peraluminous nature and developed in within-plate tectonic environment. Field radiometric measurements revealed the localization of two high radiometric anomalies associated with iron and manganese oxides, within this anomaly uranium content range from 65 to 85 ppm. Alpha Track-etch Detectors of radon gas registrations revealed high track density reach up to 15,448.7 Bq/m3. 相似文献
19.
In this contribution we have empirically calibrated the garnet–biotite–muscovite–aluminosilicate–quartz (GBMAQ) barometer using low- to medium–high-pressure, mid-grade metapelites. Application of the barometer suggests that the GBMAQ and GASP barometers show quite similar pressure estimates. Furthermore, metapelites within thermal contact aureole or very limited geographic area show no meaningful pressure diversity determined by the GBMAQ and GASP barometers which is the geological reality. The random error of the GBMAQ barometer is expected to be around ± 0.8 kbar, and this barometer shows no systematic bias with respect to either pressure, or temperature, or AlVI in muscovite, or Fe in biotite, or Fe in garnet. The GBMAQ barometer is thermodynamically consistent with the garnet–biotite geothermometer because they share the same activity models of both garnet and biotite. This barometer is especially useful for assemblages with Ca-poor garnet or Ca-poor plagioclase or plagioclase-absent metapelites. Application of this barometer beyond the calibration ranges, i.e., P–T range and chemical ranges of the minerals, is not encouraged. 相似文献
20.
Elaborating on the theme of a previous paper (1965) and in responseto Buddington‘s criticism (1966) the present discussionis primarily concerned with the high-grade meta-morphic reactionsand regional-metamorphic zoning in the Adirondacks. The hypersthene isograd, delineated by the first appearanceof orthopyroxene as a product of Pw-T-controlted reactions involvinghornblende and biotite in quartz-buartz-bearing rocks. separatesthe almandite-amphibolite-facies terrane of the lowlands fromthe granulite-facies region of the highlands. In the almandite-amphibolite facies of the lowlands at leastone isograd, the orthoclase-garnet isograd, (Ruddington's lstgarnet isograd), can be recognized which is delineated by thePw-T-controlled appearance of these minerals at the expenseof muscovite and part of the biotite and quartz. The granulite-facies terrane of the highlands is characterizedby the predominance of assemblages of the hornblende-granulitefacies in which orthopyroxene cocxists with hydrous mineralssuch as biotite and hornblende in apparent equilibrium. In theAdlrondacks there is no sizeable, regional development of thepyroxene-granulite facies where hydrous minerals are virtuallyabsent. Within the hornblende-granulite-facies terrane two isogradscan be recognized. The garent-clinopyroxene isograd (Buddington's3rd garnet isograd) delineates the regional development of garnet,clinopyroxene, and quartz by P1-T-controlled reaction betweenorthopyroxene and plagioclase. The isograd defines in the easternAdirondack a region of the hornblende-clinopyroxene-almanditesubfacies of the honblende-granulite facies. In the westernAdirondacks, near the hypersthene isograd, a yet uncharted cordieriteisograd may define the areal extent of the biotite-cordierite-almanditesubfacies of the hornblende-granulite facies. The central portionof the Adirondacks, where cordierite is absent and garnet andclinopyroxene do not coexist except in silica-deficient rocks,is the region of the hornblende-orthopyroxene-plagioclase subfaciesof the hornblende-granulite facies. 相似文献