赣中周潭群变质岩中石榴石的演化史及其地质意义

周潭群变质岩系中石榴石、十字石和黑云母微区化学成分变化明显,石榴石变斑晶具典型的生长环带,由晶体中心向两侧边缘XMg,XFe值递增,XCa,XMn值递减,反映增温过程,晶体最边缘的化学成分反映变质峰期的温度条件。变质岩系经历了角闪岩相变质作用,划分为3个变质阶段。各阶段石榴石的分布、形态、矿物组合、化学成分及其成因条件存在一定差异。通过成因矿物学的系统研究,确定本区变质岩系石榴石的结晶生长演化史及变晶变形序列。     

阜平群的退变质作用：石榴石的生长和分解

本文通过阜平群下部基性麻粒岩中石榴石变斑晶的矿物化学、成分环带,包含变晶结构和冠状体结构的研究,结合石榴石生长前后温压条件估算,确定石榴石生长于区域麻粒岩高峰变质作用之后的近等压降温的热背景中,并由于后期阜平群的快速抬升而发生降压分解,最后利用石榴石的生长成分环带和降压分解现象揭示阜平群在麻粒岩相变质作用之后的退变质PTt轨迹。     

集安岩群石榴石成分特征及其动力学意义

集安岩群石榴石以富铁为特征，均属铁铝榴石。形成于高角闪岩相的石榴石成分比较稳定，＜ＦｅＯ＞平均含量３２．２０％，ＭｇＯ５．１７％，Ｘ＿（Ｍｇ）平均值０．２３；结晶于低角闪岩相的石榴石（ＦｅＯ＋ＭｇＯ）含量３３．５０％，（ＭｎＯ＋ＣａＯ）６．９１％。石榴石微区成分不均匀，晶体内具有一些微区成分环带，这种环带不是各进变质作用阶段ｐ－Ｔ条件平衡或近平衡的产物。石榴石边缘存在着明显的扩散环带，表现为晶体边缘相对晶体核部，Ｃａ含量和＜ＦｃＯ＞含量增高，ＭｇＯ含量降低，Ｘ＿（Ｍｇ）值变小，反映本区变质峰期后经历的是一个近等压冷却的地质动力学过程。     

日本南部柳井（Yanai）地区岭家变质带进变质过程中石榴石的成分分带模型

在日本南部柳井（Ｙａｎａｉ）地区，泥质和硅质岩石中的石榴石晶体有正环带、反环带及均一体。正环带晶体中Ｍｎ的含量从中心到边部呈现连续下降，而反环带晶体Ｍｎ的含量在边部呈现富集。正环带石榴石生长呈现明显的机械加粗、成核和生长或碰接和增生。在泥质岩石中，从正环带到均一体和反环带伴随变质程度增加而系统变化，可认为是变质温度的增加而使内部扩散增强的原因。在Ｃｏｒｄ一Ｉ带，估算温度为５６０±３０℃。泥质岩石中的石榴石粒度＞２．２ｍｍ，而且保留环带。而硅质岩石中的石榴石则为相似大小的均一体。这说明石榴石的整个生长带被均一化的程度受两种岩石类型和颗粒大小因素影响。在硅质岩石中少量的黑云母（Ｂｉ）可以解释在这种岩石类型中细颗粒石榴石的形成。因此，很容易发生均一化作用。在硅质岩中粗粒的均一体石榴石形成于进变质作用过程中细粒晶体的碰接。这些集合体很容易经过内部和晶界扩散而均一化。     

花岗岩拓扑学的反思

介绍了石榴石成分环带 (剖面 )的现代测试技术 ,强调了二维成分填图在当代成分环带研究中的重要性。根据石榴石成分环带 (剖面 )的变化特征 ,将其划分为匀变连续型成分环带和突变间断型成分环带。匀变连续型环带形成受平衡生长控制 ;突变间断型成因复杂 ,其中的台阶型可与多期变质、变质条件大幅度改变、变质反应性质改变或深熔作用干扰等过程有关 ;脉冲型又可分生长型环带(剖面 )和扩散型环带 (剖面 ) ,后者是变质顶峰后石榴石与包体矿物反应导致石榴石某种成分的晶内扩散有关。东昆仑小庙岩群石榴石的岩相学、主量微量元素环带和稀土元素配分环带特征表明其是两期变质作用的产物。     

集安岩群石榴石成分特征及其动力学意义

集安岩群石榴石以富铁为特征，均属铁铝榴石。形成于高角闪岩相的石榴石成分比较稳定，＜ＦｅＯ＞平均含量３２．２０％，ＭｇＯ５．１７％，ＸＭｇ平均植０．２３；结晶于低角闪岩相的石榴石（ＦｅＯ＋ＭｇＯ）含量３３．５０％，（ＭｎＯ＋ＣａＯ）６．９１％。石榴石微区成分不均匀，晶体内具有一些微区成分环带，这种环带不是各进变质作用阶段ｐ－Ｔ条件平衡或近平衡的产物。石榴石边缘存在着明显的扩散环带，表现为晶体边缘相对     

中国大陆科学钻探工程主孔榴辉岩的岩石学研究

中国大陆科学钻探工程5000m主孔位于苏鲁超高压变质带南部。该钻孔0～2000m主要由榴辉岩、片麻岩、石榴石橄榄岩和少量片岩和石英岩组成。累积厚度达1000多米的榴辉岩具有不同的矿物组成、不同的矿物含量和不同的全岩化学成分,可划分成富Si的石英榴辉岩、富Ti的金红石榴辉岩、富Al的多硅白云母和蓝晶石榴辉岩、富Mg的双矿物榴辉岩和具有正常玄武岩成分的普通榴辉岩。榴辉岩的原岩包括基性层状侵入岩和变质表壳岩。榴辉岩全岩成分对石榴石和绿辉石中某些化学组分的含量有明显控制,而且直接影响到变质条件估算的准确性。扩散成分环带的广泛发育表明超高压矿物在早期退变质过程中发生了成分再平衡。这一事实以及具有成分生长环带石榴石变斑晶的存在,为榴辉岩形成在更高温度(>940℃)和更高压力(>4.5GPa)条件下提供了有力的证据。     

陕西汉阴北部梅子垭组变质作用研究

梅子垭组是汉阴北部地区出露面积最大的地层,岩性主要为白云母石英片岩、含碳白云母石英片岩、含石榴子石黑云母变斑晶白云母石英片岩等,石榴子石和黑云母变斑晶在该岩层中普遍发育。该岩组构造变形复杂,经历了多期构造活动和变质变形过程。笔者在野外调查的基础上,通过分析研究区石榴子石、黑云母变斑晶的关系,采用石榴子石-黑云母地质温度计,对该区广泛出露的含石榴子石黑云母变斑晶白云母石英片岩进行研究,得到如下结论:研究区存在3期黑云母和石榴子石,石榴子石变斑晶具有明显的生长环带,而黑云母变斑晶没有环带;研究区变质作用的变质温度范围为介于511~572℃,主要为530~560℃;变质压力范围为0.16~0.84GPa,变质相属高绿片岩相。     

剪切带中变斑晶的生长及包裹体痕迹的演化

韧性剪切带中，由于变形分解作用的存在，岩石发生递进变形过程中，产于共轴或非共轴递进缩短带内的变斑晶不发生旋转，而变斑晶内的包裹体痕迹是递进变形过程中遗留在变斑晶内的变形变质痕迹。利用未旋转斑晶中的包裹体痕迹可以确定早期面理的取向，寻找构造演化的时间标志，确定变形变质的关系及其演化史。对北祁连托勒牧场大型走滑韧性剪切带中石榴石、黑云母等变斑晶及包裹体痕迹的研究，揭示了变斑晶的生长和包、裹体痕迹与褶劈理的演化有着重要联系以及剪切变形过程中变形变质演化史、应变速率的变化。递进变形相应地发生递增变质，但两者存在着一定的差异性。     

变形作用与变斑晶生长

理想状态下 ,晶体的生长是各向均匀的。但是 ,近年来变形作用与变斑晶生长关系研究表明 :变形变质过程中变斑晶并不是从中心各向同性地均匀生长 ,其生长与当时所处的应力环境、物质供给和流体相的参与有关。其中流体相的参与是变斑晶生长所需物质迁移的载体 ,为变斑晶生长的物质供给提供先决条件。变斑晶的生长不是连续的而是呈阶段性生长 ,变斑晶的生长相是与变形相相互消长的。变形变质过程中 ,变形与变斑晶生长阶段的关系可总结如下 :(1)变斑晶在未变形域或弱变形域成核后 ,晶体生长所需的物质供应充足时 ,晶体的生长是围绕晶核均匀生长…     

δ18O and yttrium zoning in garnet: time markers for fluid flow?

The relative timing of two discrete pulses of metamorphic fluid flow is constrained based on chemical zoning in several garnet crystals from Kvaløya, Troms, northern Norway. The garnet crystals measured 1–2 cm in diameter and were contained within c. 1.6 Ga, staurolite grade metasediments. Major element zoning indicates that garnet grew under normal prograde conditions in the garnet and/or staurolite zones. Timing constraints are based on comparisons between major and trace element chemical zoning, oxygen isotope (δ¹⁸O) zoning and deformational (inclusion trail) zoning in one of the garnet. We interpret at least two pulses of metamorphic fluid flow. The first pulse occurred during the syn‐tectonic growth interval. The δ¹⁸O zoning was reversed relative to ‘normal’ prograde zoning and the δ¹⁸O maximum was located within the syn‐tectonic growth zone, displaced 3–4 mm from the garnet core. The fluid might have been sourced in neighbouring calcareous pelites and may also have caused formation of an Y ring. The second (and subsequent) pulse(s) occurred during/after the post‐tectonic growth interval. δ¹⁸O was locally increased at the garnet rim, particularly where the rim was sheared. The incomplete rim was also enriched in calcium. Transport of oxygen and calcium by metamorphic fluids is well documented. Transport of Y is both problematic and poorly understood, but might have been facilitated by complexing with F and/or CO₂.     

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。     

Chemical and textural equilibration of garnet during amphibolite facies metamorphism: The influence of coupled dissolution–reprecipitation

Metamorphic equilibration requires chemical communication between minerals and may be inhibited through sluggish volume diffusion and or slow rates of dissolution in a fluid phase. Relatively slow diffusion and the perceived robust nature of chemical growth zoning may preclude garnet porphyroblasts from readily participating in low‐temperature amphibolite facies metamorphic reactions. Garnet is widely assumed to be a reactant in staurolite‐isograd reactions, and the evidence for this has been assessed in the Late Proterozoic Dalradian pelitic schists of the Scottish Highlands. The 3D imaging of garnet porphyroblasts in staurolite‐bearing schists reveals a good crystal shape and little evidence of marginal dissolution; however, there is also lack of evidence for the involvement of either chlorite or chloritoid in the reaction. Staurolite forms directly adjacent to the garnet, and its nucleation is strongly associated with deformation of the muscovite‐rich fabrics around the porphyroblasts. “Cloudy” fluid inclusion‐rich garnet forms in both marginal and internal parts of the garnet porphyroblast and is linked both to the production of staurolite and to the introduction of abundant quartz inclusions within the garnet. Such cloudy garnet typically has a Mg‐rich, Mn‐poor composition and is interpreted to have formed during a coupled dissolution–reprecipitation process, triggered by a local influx of fluid. All garnet in the muscovite‐bearing schists present in this area is potentially reactive, irrespective of the garnet composition, but very few of the schists contain staurolite. The staurolite‐producing reaction appears to be substantially overstepped during the relatively high‐pressure Barrovian regional metamorphism reflecting the limited permeability of the schists in peak metamorphic conditions. Fluid influx and hence reaction progress appear to be strongly controlled by subtle differences in deformation history. The remaining garnet fails to achieve chemical equilibrium during the reaction creating distinctive patchy compositional zoning. Such zoning in metamorphic garnet created during coupled dissolution–reprecipitation reactions may be difficult to recognize in higher grade pelites due to subsequent diffusive re‐equilibration. Fundamental assumptions about metamorphic processes are questioned by the lack of chemical equilibrium during this reaction and the restricted permeability of the regional metamorphic pelitic schists. In addition, the partial loss of prograde chemical and textural information from the garnet porphyroblasts cautions against their routine use as a reliable monitor of metamorphic history. However, the partial re‐equilibration of the porphyroblasts during coupled dissolution–reprecipitation opens possibilities of mapping reaction progress in garnet as a means of assessing fluid access during peak metamorphic conditions.     

松潘—甘孜造山带中丹巴地区变质岩演化及其地质意义

丹巴地区位于松潘—甘孜造山带中部。区域动热变质作用主要发生于印支晚期—喜马拉雅早期,与深层滑脱-逆冲作用有关。根据特征矿物组合．丹巴地区变质岩可划分出六个变质带：绢云母—绿泥石带、黑云母带、石榴子石带、十字石带、蓝晶石带和夕线石带。石榴子石生长环带代表每段时间矿物晶体边部的平衡,可用于推测石榴子石生长时的P－t轨迹。利用石榴子石—黑云母温度计和石榴子石—斜长石—Al2SiO5-石英压力计得到石榴子石的参考温度、压力,根据环带定量计算得到的变质作用p-t轨迹为顺时外形式,与其所处的构造背景为大陆碰撞造山带是一致的。     

The effect of metamorphic fluid flow on the nucleation and growth of garnets from Troms, North Norway

A spatial association is observed between the size distribution of garnet porphyroblasts and the size distribution of quartz veins in greenschist facies metapelites from Troms, North Norway. The size distribution of quartz veins reflects the flow regime of metamorphic fluids. The hypothesis that the flow regime of metamorphic fluids is also responsible for the size distribution of garnet crystals was tested by ascribing empirical acceleration parameters to the nucleation and growth rates of garnet crystals.
In regions where fluid flow was interpreted as pervasive', acceleration parameters for nucleation were high, whereas in regions where fluid flow was interpreted as channelled', acceleration parameters for growth were high. Accelerated crystal growth is further implied from the chemical zoning and crystal morphologies of garnets collected near discrete veins.
This spatial association may imply that fluid flow can be instrumental in controlling garnet crystallization. Fluid flow could affect garnet crystallization kinetics by facilitating thermal advection and/or mass transfer. In the study area, rhodochrosite (MnCO₃) veins provide evidence for mass transfer of Mn by fluid flow. An influx of Mn would expand the stability field of garnet to lower temperatures. The resulting thermal overstep could accelerate nucleation and/or growth of garnets.
The corollary of this study is that size distributions and chemical zoning of garnets, or other porphyroblast phases, can be used to study metamorphic fluid flow.     

内蒙古卓资孔兹岩系中石榴石成分环带及其在温压估算中的应用

内蒙卓资出一了大面积太古宙矽线石榴黑云二长片麻岩，其中石榴石的Ｍｇ、〈Ｆｅ〉、Ｍｇ／（Ｍｇ＋Ｆｅ）等呈明显的环带特点。研究表明，石榴石既存在生长环带，也具有扩散环带。其成分环带与石榴石生长过程和变质条件有关。不同成分环带的热力学计算，可以确定变质作用过程中不同阶段的温压条件。     

Prograde reactions and garnet zoning reversals in staurolite schist, British Columbia: significance for thermobarometric interpretations

Abstract An outcrop of staurolite-bearing pelitic schist from the Solitude Range in the south-western Rocky Mountains, British Columbia, was examined in order to determine the nature of prograde garnet- and staurolite-producing reactions using information from garnet zoning and inclusion mineralogy. Although not present as a matrix phase, chloritoid is present as inclusions in garnet and is interpreted to have participated in the simultaneous growth of garnet and staurolite by a reaction such as chloritoid + quartz = garnet + staurolite + H₂O.
A garnet zoning trend reversal, which is most pronounced with respect to almandine and grossular components, is present in the outer core of garnets. The location of the zoning reversal corresponds to the outer limit of chloritoid inclusions in garnet. As there is no evidence for polymetamorphism, the zoning reversal is interpreted to indicate continued garnet growth by prograde reaction(s) during a single metamorphic event after the exhaustion of chloritoid as a matrix phase.
Metamorphic conditions recorded by mineral rim compositions are 550–600° C at 6–7 kbar. Because there is no evidence for partial resorption of garnet during production of staurolite, we interpret these results to represent peak conditions.     

Modeling of zoning patterns in garnet: Thermodynamic and kinetic aspects

Results from the modeling of compositional zoning patterns in garnet porphyroblasts from the medium-grade metapelitic schist of northern Ladoga area are considered. The P-T pseudosections in the model KMnFMASH system were calculated for this purpose using THERMOCALC software (Powell et al., 1998). Particular emphasis is placed upon the effect of garnet growth kinetics on the model zoning profiles for Mn (Gulbin, 2013). They fit the observed profiles if intergranular diffusion-controlled growth is assumed for porphyroblasts. Additionally, a model of metamorphic fractional crystallization is used to characterize the oscillations in both the garnet core and rim. Starting from the assumption that a reservoir, where garnet grows, consists of chlorite, and that this mineral is intensely replaced with biotite and staurolite at the onset of crystallization, a partial release of Mn from the chlorite structure and the concentration of this component in intergranular space is inferred. In terms of the model under consideration, the coefficient of the Mn partition between garnet and reservoir temporarily increases at the early stage of garnet growth, giving rise to the enrichment of the intermediate zone of porphyroblasts in Mn. In addition to the modeling of garnet growth zoning, its subsequent diffusion modification is estimated on the basis of intracrystalline diffusion profile simulation. The reverse zoned, Mn-rich and Mg-poor garnet rims are related to retrograde growth of garnet at the late stage of porphyroblast formation. The data obtained are used to constrain metamorphic evolution and the P-T-t path of staurolite-bearing rocks in the northern domain of the studied area.     

Prograde pressure–temperature paths in the pelitic schists of the Sambagawa metamorphic belt, SW Japan

Prograde P–T paths recorded by the chemistry of minerals of subduction‐related metamorphic rocks allow inference of tectonic processes at convergent margins. This paper elucidates the changing P–T conditions during garnet growth in pelitic schists of the Sambagawa metamorphic belt, which is a subduction related metamorphic belt in the south‐western part of Japan. Three types of chemical zoning patterns were observed in garnet: Ca‐rich normal zoning, Ca‐poor normal zoning and intrasectoral zoning. Petrological studies indicate that normally‐zoned garnet grains grew keeping surface chemical equilibrium with the matrix, in the stable mineral assemblage of garnet + muscovite + chlorite + plagioclase + paragonite + epidote + quartz ± biotite. Pressure and temperature histories were inversely calculated from the normally‐zoned garnet in this assemblage, applying the differential thermodynamic method (Gibbs' method) with the latest available thermodynamic data set for minerals. The deduced P–T paths indicate slight increase of temperature with increasing pressure throughout garnet growth, having an average dP/dT of 0.4–0.5 GPa/100 °C. Garnet started growing at around 470 °C and 0.6 GPa to achieve the thermal and baric peak condition near the rim (520 °C, 0.9 GPa). The high‐temperature condition at relatively low pressure (for subduction related metamorphism) suggests that heating occurred before or simultaneously with subduction.     

Garnet polycrystals and the significance of clustered crystallization

Polycrystalline garnets are common in metamorphic rocks and may form as a result of close spacing of nuclei (if clustering is early) or impingement of larger grains (if clustering occurs later in the growth history). The timing of clustering relative to garnet growth is relevant to understanding the formation and evolution of porphyroblasts and evaluating the significance (if any) of clustering. Electron backscattered diffraction (EBSD) analysis of garnet-bearing metamorphic rocks reveals the presence of polycrystalline garnet in nine localities examined in this study: the northern Appalachians (Vermont, Maine, New York, USA); North American Cordillera (North Cascades Range, Washington; Snake Range, Nevada, USA); western Rocky Mountains (British Columbia, Canada); southern Menderes Massif (Turkey); Santander Massif (Colombia); and the Sanandaj–Sirjan zone (Hamadan, Iran). In some samples, polycrystals comprise ~20–30% of garnets analyzed, and chemical and textural evidence suggests that early coalescence of garnet polycrystals is common. Some early-coalescing polycrystals exhibit growth zoning that is concentric about the geometric center of the polycrystal. In thin section, these garnets may be undetectable as polycrystals based on morphology or zoning. In some polycrystals, zoning is unrelated to the location of internal grain boundaries; in others, Fe–Mn–Mg zoning has a different pattern than that of Ca; zoning patterns may vary on the scale of a single thin section. In addition, some polycrystals are characterized by high-angle misorientation boundaries that may be in special (non-random) orientations, an observation that indicates that these polycrystals are not random clusters of grains. The presence of internal grain boundaries may affect diffusion pathways and length scales, and may facilitate communication of porphyroblast interiors with matrix phases, thereby influencing reaction history of the rock and the composition/zoning of garnet.