草河群区域变质带及石榴石的研究

辽东草河群由北向南,可以分出比较完整的中压型巴罗式变质带、变质带的分界线或等变质度线和面基本上与地槽褶皱的构造方向一致,显示出带型区域变质作用和复合变质作用的特点。区域变质相带的矿物共生组合、斜长石号码、岩石类型、矿物的物理化学性质都具独自的特征。变质相带的变泥质岩石中的石榴石均属铁铝榴石,随变质度的增高,石榴石成分中的Al2O3、MgO、FeO Fe2O3含量增加,CaO、MnO含量降低。     

青海都兰榴辉岩的发现及对中国中央造山带内高压—超高压变质带？…

在青海都兰北东４０ｋｍ野马滩一带元古宙变质杂岩中,新发现走向近东西的榴辉岩;可分为南北两带,北带宽约５ｋｍ,南带宽３～５ｋｍ。北带榴辉岩相矿物组合为石榴石－绿辉石－多硅白云母－金红石,它们的退变质矿物主要为角闪石及少量高钠的斜长石类。而南带峰期变质矿物为石榴石－绿辉石－蓝晶石－金红石－黝帘石（？）,退变质矿物中则出现大量高钠质斜长石。带另一个明显的区别是北带铁铝榴石端元组分含量较南带的略高,而镁铝     

赣中变质岩带变质带的重新划分及变质作用P-T条件的确定

根据随变质作用增强而出现的特征变质矿物及矿物组合,对赣中变质岩带重新划分出五个变质带,由低到高依次为绢云母一绿泥石带、黑云母带、铁铝榴石带、十字石带、夕线石带。在对各变质带变质矿物进行详细研究的基础上,采用石榴石-黑云母(Gt-Bi)、斜长石-角闪石(Pl-Hb)、石榴石-十字石(Gt-St)、石榴石-角闪石(Gt-Hb)、石榴石-斜长石-夕线石-石英(Gt-Pl-Sil-Q)、石榴石-斜长石-黑云母-白云母(Gt-Pl-Bi-Mu)等多种地质温压计确定了各变质带的形成温度、压力,绢云母-绿泥石带的形成温度为250-350℃,压力为0.25-0.35GPa;黑云母带的形成温度为350-450℃±,压力为0.25-0.40GPa±;铁铝榴石带的形成温度为450-550℃±,压力为0.40～0.60GPa±;十字石带的形成温度为550-600℃±,压力为0.50-0.60GPa±;夕线石带的形成温度为600-650℃±,压力为0.55-0.65GPa±,赣中变质岩带峰期变质作用已达到角闪岩相。     

高压-超高压变质岩石中不同成因的石榴石

石榴石是高压-超高压变质岩石中最重要的变质矿物之一,是研究俯冲带深部变质和熔融过程的理想研究对象.通过对俯冲带内不同条件下形成的石榴石进行详细研究,确定了岩浆成因、变质成因和转熔成因石榴石.岩浆石榴石是岩浆熔体在冷却过程中结晶形成,成分主要为锰铝榴石-铁铝榴石,通常含有石英、长石、磷灰石等晶体包裹体.变质石榴石是在亚固相条件下通过变质反应形成,包裹体为参与变质反应的矿物组合;进变质生长的石榴石通常显示核部到边部锰铝榴石降低的特征.转熔石榴石是在超固相条件下通过转熔反应形成,通常含有晶体包裹体,其中既有从转熔熔体结晶的矿物包裹体,也有转熔反应残留的矿物包裹体.对超高压变质岩石中转熔石榴石的识别,可以为深俯冲陆壳岩石的部分熔融提供重要的岩石学证据,是大陆俯冲带部分熔融研究的重要进展之一.      

吉林中部呼兰群泥质岩递进变质带特征及其地质意义

下古生界呼兰群变质岩系位於吉林褶皱带南端,其中泥质岩变质片岩段具有明显的递进变质作用,并可划分为:铁铝石榴石带、十字石带、蓝晶石带、硅线石带四个矿物带,呈中压中温巴洛式变质带特征。本文对该变质带矿物组合、原岩建造、变质作用演化、成因机理及其地质意义进行了探讨。     

相山铀矿田变质基底片岩的变质作用研讨

在相山铀矿田的早-中元古代变质基底中，出露一套云母片岩。按照片岩中的变质特征矿物，可划分为黑云母带、石榴石带和十字石带，与著名的巴罗带中的三个变质带相对比具有相同的矿物组合。该套片岩主要由富铝矿物十字石、铁铝榴石和大量黑云母组成，因而其原岩以粘土岩为主。由于片岩中含有大量黑云母和石榴石，这为利用石榴石、黑云母探讨变质作用的温度压力条件提供了方便，利用矿物温度压力计得到的温度为500～600℃，压力约0.5Gpa。在晚元古代以前该地区至少发生过两次热动力变质作用。     

新疆西天山高压变质带的变质矿物与变质作用演化

新疆西天山高压变质带主要由石榴石，角闪石，绿辉石，多硅白云母，钠云母，绿帘石，绿泥石，钠长石，石英，榍石和金红石等组成，石榴石主要含铁铝榴石组份，角闪石有蓝闪石，亚铁蓝闪石，青铝闪石，冻蓝闪石等类型，变质矿物组合显示高压变质带经历了由硬柱石蓝片岩相，榴辉岩相，绿帘蓝片岩相至绿片岩相的变质作用演化进程。     

中国大陆科学钻探主孔(CCSD-MH)石榴石橄榄岩:一个经历了深俯冲作用的古生代超镁铁质侵入体

中国大陆科学钻探主孔中的超镁铁岩主要产在603.20～683.53m深度之间。超镁铁岩的上部直接围岩为高Ti-Fe型榴辉岩;内部夹有薄层状含柯石英高镁榴辉岩和厚层状多硅白云母榴辉岩;下部直接围岩为石英榴辉岩和普通榴辉岩。超镁铁岩的主体岩性为石榴石单辉橄榄岩,主要由橄榄石(60%～70%),石榴石(10%～25%),单斜辉石(5%～15%),斜方辉石(1%～5%)和少量金云母和钛铁矿或钛斜硅镁石组成。橄榄石Fo 79～89,其中一些以包裹体形式出现的高MgO橄榄石可能形成较早,主体橄榄石属变质重结晶阶段形成;石榴石以低CaO、高MgO和Cr_2O_3含量高(达3%)为特征,保留较好的进变质成分环带;单斜辉石Na_2O含量达到4%～5%,分为绿辉石和普通辉石类,属变质成因,结合矿物对的温压估算,岩石已经经历超高压变质作用。岩石成分研究表明,石榴石单辉橄榄岩与其顶、底板榴辉岩及其中的榴辉岩夹层有一较大的成分间断,其中MgO含量约相差10%左右,认为超镁铁岩与榴辉岩的原岩不属同一岩浆演化产物。锆石SHRIMP定年表明石榴石单辉橄榄岩原岩时代为古生代346～461Ma,超高压变质作用时代为早中生代220～240Ma。认为CCSD主孔石榴石橄榄岩为古生代的超镁铁质侵入体,在印支期的中国南北板块俯冲折返过程中经历了超高压变质作用。     

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

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。     

宜昌老林沟石榴石矿床地质特征

老林沟石榴石矿产于黄陵背斜北部类孔兹岩系中一套富铝铁质正常沉积的泥质粉砂岩、泥质岩中,在区域热动力变质作用下,铁铝榴石与夕线石共生,属于巴罗变质系列中夕线石带产物,根据矿物共生组合、指相特征矿物得出形成该矿产的区域变质的温压条件为:t为7 00℃,P为7 30MPa,变质相属低角闪岩相至高角闪岩相。     

信阳-舒城断裂带西段石榴云母片岩中石榴石的矿物学特征及其构造意义

桐柏山地区信阳-舒城断裂带西段石榴云母片岩中石榴石变斑晶保留了较多岩石形成过程中的变形-变质信息,它真实地反映了中国南北两大板块缝合带的形成条件和演化历史.石榴石探针成分分析结果表明其属于铁铝榴石,反映出经受中级区域变质作用的特征.在Nadi石榴石成分与变质带的关系图上主要投影在略偏石榴石带的蓝晶石带内,显示岩石遭受了...     

Pseudomorphs and associated microstructures of western Maine,USA

In metapelitic rocks of western Maine, a pluton-related M3 metamorphic gradient ranging in grade from garnet to upper sillimanite zone was superposed on a fairly uniform M2 regional metamorphic terrain characterized by the assemblage andalusite+staurolite+biotite+/−garnet. As a result, M2 assemblages re-equilibrated to the P, T, and aH₂O conditions of M3, and both prograde and retrograde pseudomorphism of M2 porphyroblasts occurred. The type of pseudomorph and degree of development is directly related to the rock's position within the M3 metamorphic gradient, a function of its proximity to the Mooselookmeguntic pluton. Several ‘hinge’ zones occur in which the M3 minerals that pseudomorphed a particular M2 phase change. For example, M2 garnet was replaced by M3 chlorite or biotite, depending on its position within the M3 gradient. Similarly, in a transition zone between M3 upper staurolite and lower sillimanite zones, M2 staurolite was stable and shows M3 growth rims. Downgrade from this transition zone, staurolite was pseudomorphed by chlorite and muscovite, whereas upgrade, the pseudomorphs contain muscovite and some biotite. M3 pseudomorphs commonly retain crystal shapes of the original M2 porphyroblasts, reflecting relatively low regional deviatoric stress during and after M3. Although evidence for textural disequilibrium is common, chemical equilibrium was closely approached during M3. This study demonstrates for M3 that: (1) the pseudomorphic replacement was a constant volume process, and (2) fabrics produced by tectonic events can be erased by subsequent deformation and/or sufficiently intense subsequent recrystallization.     

Almandine in Thermal Aureoles

The concept that almandine-rich garnet is an anomalous phasein thermal metamorphic assemblages is based partly upon therarity of almandine in hornfelses, and partly upon evidenceof breakdown of ‘regionally’ formed garnets whentheir host-rocks have been thermally metamorphosed. Where amphibolitefacies regional gneisses have been re-metamorphosed within thepyroxene-hornfels facies conditions of the late Caledonian Lochnagaraureole, garnet of composition Alm₈₀Py₁₁p⁶ (Gr+And)₄ has reactedwith biotite, sillimanite, and muscovite to give cordierite-richpseudomorphs; where armoured from reaction by immersion in quartzor feldspar, no signs of dissolution are seen. In some totallyreconstituted hornfelses, almandine garnet of composition AIm₈₀P₁₃Sp₄(Gr+And)3 appears to have coexisted stably with cordierite andorthoclase. It is concluded that the ‘unstable’garnet was breaking down not because its P/T stability fieldhad been exceeded, but because the alman-dine-bearing rock-compositionfield in the thermal (pyroxene-hornfels) facies was more restrictedthan that in the regional (amphibolite) facies. Chemical datafrom the hornfelses suggest that this restriction was mainlydue to the stability of cordierite—the plane spinel-cordierite-quartzrestricts garnet to those rocks with effective mol. (FeO+MgO-t-MnO)A12O3>1, while within that range cordierite-biotite tie-linesrestrict garnet to rocks of high (FeO+MnO)/MgO ratio (Fig. 4b). Recorded instances of garnet ‘instability’ in thermalaureoles show similar features to the Lochnagar aureole—garnetbreaks down by reaction; unequivocal instances of isochemicalbreakdown are rare. This, combined with the widespread occurrenceof almandine in volcanic and plutonic igneous rocks, suggeststhat almandine is a physically stable phase not only in thehornblende-hornfels facies, but also in the pyroxene-hornfelsfacies and possibly in portion of the sanidinite facies as well.The rarity of almandine in thermal aureoles is the result ofits very narrow rock composition field under the P/T conditionsof such environments. Comparison of thermal and granulite facies garnet-cordieriteassemblages suggests that P and T modify the almandine-bearingrock composition field mainly by modifying the limiting Mg/Feratio of garnet and the limiting Fe/Mg ratio of cordierite (Fig.10). The wide rock-composition field of almandine in the amphibolitefacies may be contingent upon the inhibition of cordierite byhydrous minerals under relatively high partial pressures ofwater.     

Shigar valley gemstones,their chemical composition and origin,Skardu, Gilgit-Baltistan,Pakistan

A variety of gemstones is being mined in the Shigar valley, Skardu, Pakistan. These include beryl (goshenite and aquamarine), tourmaline (schorl), garnet (almandine–spessartine), apatite, topaz, fluorite, zoisite, clinozoisite, and axinite, mostly occurring in complex or zoned pegmatites and metamorphic rocks. These have been analyzed using electron probe micro-analyzer and X-ray diffractometer. The mineral chemistry of each gemstone is similar to its respective typical gemstone variety with homogenous chemical composition. Field and chemical characteristics suggest that beryl, tourmaline, garnet, apatite, topaz, and fluorite are occurring in zoned pegmatites which are largely formed by magmatic hydrothermal fluids in the cavities and vugs within the intermediate zone. However, zoisite, clinozoisite, and axinite may have a metamorphic and/or metasomatic origin.     

Rare earth diffusion kinetics in garnet: Experimental studies and applications

We determined the diffusion coefficient of Sm in almandine garnet as function of temperature at 1 bar and fO₂ corresponding to that of wüstite-iron buffer, and to a limited extent, that of a few other selected rare earth elements in almandine and pyrope garnets. Both garnets were demonstrated to have metastably survived the diffusion annealing at conditions beyond their stability fields. The experimental diffusion profiles were analyzed by secondary ion mass spectrometry, and in addition, by Rutherford back scattering spectroscopy for two samples. Transmission electron microscopic study of an almandine crystal that was diffusion-annealed did not reveal any near-surface fast diffusion path. Using reasonable approximations and theoretical analysis of vacancy diffusion, the experimental data were used to develop an expression of rare earth element (REE) diffusion coefficient in garnet as a function of temperature, pressure, fO₂, ionic radius, and matrix composition. Calculation of the closure temperature for the Sm-Nd decay system in almandine garnet in a metamorphic terrain shows very good agreement with that constrained independently. Modeling of the REE evolution in melt and residual garnet suggests that for dry melting condition, the REE pattern in the melt should commonly conform closely to that expected for equilibrium melting. However, for much lower solidus temperatures that would prevail in the presence of a H₂O-CO₂ fluid, the concentration of light REE in the melt could be significantly lower than that under equilibrium melting condition. A reported core and rim differences in the REE content of a garnet crystal in a mantle xenolith in kimberlite have been reproduced by assuming that the REE zoning was a consequence of entrapment in a magma derived from an external source for ∼32,000 yr before the eruption.     

Mineralogy of New Caledonian metamorphic rocks

A Cretaceous to low-Tertiary sequence of interbedded pelites, cherts, basic and acidic volcanics and calcareous lenses has been metamorphosed by an Oligocene event. A complete intergradational metamorphic sequence is exposed in the Ouégoa destrict. The following metamorphic zones have been recognised: — (1) lowest-grade rocks consisting of quartz-sericite phyllites and pumpellyite metabasalts (2) lawsonite zone, characterized by the association of lawsonite and albite (3) epidote zone, characterised by epidote-omphacite-sodic hornblendealmandine bearing metabasalts and epidote-albite-almandine-glaucophane bearing metasediments; calcareous metasediments may also carry omphacite. The epidote and lawsonite zones are separated by a narrow belt of transitional rocks. Garnets occur in metasediments throughout the lawsonite zone as rare tiny crystals (<0.03 mm diam.). Garnets first appear in metabasalts in lawsonite-epidote transitional rocks. Garnets are widespread and abundant in epidote-zone metasediments and metabasalts. 45 garnets from rocks representative of all lithologies and metamorphic grades have been analysed with an electron-probe microanalyser. The garnets were consistently zoned. Garnets in lawsonite and low-grade epidote zones show a “bell-type” zoning with cores enriched in Mn relative to Fe and rims enriched in Fe, Mg and frequently Ca. Garnets from high-grade epidote-zone metapelites and metabasalts show, in addition, a shallow oscillatory zoning with complimentary variations in Mn and Fe equivalent to 5 mole- % spessartine and almandine. The Fe-for-Mn substitutional zoning, believed to be caused by a diffusion/saturation effect similar to that of the Rayleigh fractional model (Hollister, 1966), appears to have had superimposed on it the effects of parent-rock chemistry and metamorphic grade which control in a complex manner the composition of the cores and the rims of garnets. Garnets from different rock types and metamorphic grade are compositionally distinct. Garnets from lawsonite-zone rocks, irrespective of parent-rock chemistry appear to be spessartine. Garnets from epidote-zone metaigneous rocks and most metasediments are almandine. Garnets from epidote-zone metasediments with bulk-rock compositions which are manganiferous, or have high oxidation ratios, or both, may be spessartine-rich. Garnets from metabasalts are consistently more pyropic in both core and rim compositions than garnets from pelitic metasediments; the pyrope content of cores and rims of garnets from equivalent rock types and mineral assemblages increases with increasing metamorphic grade. Cores of garnets from epidote-zone pelites are richer in grossular than garnets from lower-grade pelites. The reaction which brings almandine garnet into Ouégoa district blueschist assemblages simultaneously with the replacement of lawsonite by epidote involves components of chlorites and sodic amphiboles and can be represented by the following simplified equation: ferroglaucophane+Fe-rich chlorite+lawsonite → glaucophane+Mg-rich chlorite+epidote+almandine.     

湖北省宜昌市彭家河石榴石矿床地质特征及成因分析

运用矿物学、岩石学的方法对宜昌黄陵地区彭家河石榴石矿床中含矿层的矿物组合、化学成分、变质相及原岩特征进行分析与研究,结果表明彭家河石榴石矿床的含矿层位为水月寺群的周家河组,石榴石主要赋存于含(石墨)矽线石榴黑云斜长片麻岩及石榴黑云斜长片麻岩中,前者石榴石含量较高10%～50%,后者含量稍低12%～36%,X衍射及电子探针分析矿床中石榴石以铁铝榴石为主,其次是镁铝榴石,少量钙铝榴石.运用变质反应平衡温压方程计算出成矿温压条件为:P=600 MPa～800 MPa,T=650℃～750℃;变质相为高角闪岩相,高角闪岩相的富铁铝质沉积变质岩系--孔兹岩系是寻找此类石榴石矿床的重要找矿标志.     

Microstructural features of albite porphyroblasts as indicators of sequential Barrovian metamorphic mineral growth in the Caledonides of the SW Scottish Highlands

Inclusion – porphyroblast and porphyroblast – porphyroblast relationships show that abundant albite in mica schists in the Caledonides of the SW Scottish Highlands are part of the Barrovian metamorphic assemblage. Growth early in the D2 deformational phase of porphyroblast cores followed the growth of Mn‐rich garnet but preceded the growth of porphyroblasts of the index mineral almandine. Two sets of inclusion trails in the albite correspond to the regionally expressed S1 and S2. Straight trails of muscovite, chlorite, quartz, epidote and the earliest growth of biotite make up S1. Crenulated trails express deformation of S1 early in D2 with muscovite, chlorite, biotite, quartz, epidote and the Mn‐rich garnet associated with the development of S2 crenulation cleavage. The geometries of these trails uniquely record early stages of D2 deformational history. An _0?3 growth is related to the temporal coincidence of the formation of S1–S2 crenulation cleavage hinges as favourable sites for nucleation and the release of large amounts of water from prograde reactions during tectonothermal reconstitution of first cycle immature sediments with a volcanic component. The main characteristics of the regionally expressed D2 schistosity were developed during the major grain coarsening that followed both albite and almandine porphyroblast growth. Essentially inclusion‐free An _4?19 rims grew on the inclusion‐containing cores in the almandine zone in the later stages of schistosity growth and unoriented porphyroblasts of muscovite, biotite and chlorite indicate that mineral growth extended from the later stages of D2 to post‐D2. Previous interpretations of the albite porphyroblast growth having been during D4 to post‐D4 contemporaneous with retrogression are inconsistent with the microstructural evidence.     

Submicron polyphase inclusions in garnet from the Tananao Metamorphic Complex, Taiwan: a key to unravelling otherwise unrecognized metamorphic events

Kyanite and staurolite occur in the Tananao Metamorphic Complex as submicron inclusions in almandine‐rich garnet from a metamorphosed palaeosol weathering horizon, near Hoping, eastern Taiwan. Quartz, rutile/brookite and zircon are also found as associated submicron inclusions in garnet. Employing the reaction ilmenite+kyanite+quartz=almandine+rutile, and the breakdown of staurolite and quartz as thermobarometers, these submicron‐scale minerals formed at >8.3–8.8 kbar and < 660–690 °C. This P–T estimate is different from that (i.e. 5–7 kbar and 530–550 °C) derived from matrix minerals, which include almandine‐rich garnet, muscovite, chlorite, chloritoid, plagioclase, quartz and ilmenite. These results suggest that submicron inclusions in garnet‐like materials may record portions of the otherwise undocumented prograde path or provide information about previous metamorphic events and thus yield new insights into orogenic belts.     

Garnet-cordierite-biotite equilibria in the Steinach aureole,Bavaria

Three garnet-biotite pairs and eleven garnet-cordierite-biotite triplets from the Steinach aureole (Oberpfalz, North-East Bavaria) were analyzed using an electron probe microanalyzer.The regional metamorphic muscovite-biotite schists contain garnets strongly zoned with Mn-Ca-rich centers and Fe-rich edges, the average composition being almandine 67 — spessartine 4 — pyrope 4 — grossular (+andradite) 25.The first contact garnet that is formed in mica schists of the outermost part of the aureole is small, virtually unzoned, and has an average composition of almandine 52 — spessartine 37 — pyrope 8 — grossular (+andradite) 3. With increasing metamorphic grade, there is a consistent trend to form garnets richer in Fe ending up with a composition almandine 84.5 — spessartine 5.5 — pyrope 7.5 — grossular (+andradite) 2.5. This trend is accompanied by a general increase in grain size and modal amount of garnet. Associated cordierites and biotites also become richer in Fe with increasing grade. While the garnets in the highest grade sillimanite hornfelses are poorly zoned, the transitional andalusite-sillimanite hornfelses contain garnets with distinct but variable zonation profiles.These facts can possibly be explained by the time-temperature relationships in the thermal aureole. In a phase diagram such as the Al-Fe-Mg-Mn tetrahedron, the limiting mineral compositions of a four-phase volume or a three-phase triangle are fixed by T and P (the latter remaining effectively constant within a thermal aureole). Thus, in garnet-cordierite-biotite assemblages, garnet zonation should be controlled by temperature variation rather than by a non-equilibrium depletion process. Taking into account the experimental data of Dahl (1968), a zoned garnet from a transitional andalusite-sillimanite hornfels would reflect a temperature increase of about 40° C during its growth. A hypothetical P-X diagram is proposed to show semi-quantitatively the compositional variation of garnet-cordierite pairs with varying pressures (T constant).