喜马拉雅造山带片麻岩中石榴子石的多期生长

石榴子石是高级变质作用的重要矿物之一,能够保留成分环带和不同变质阶段的矿物或流体包裹体,为示踪寄主岩石经历的变质演化历史提供重要信息。本文对采自雅拉香波片麻岩穹窿内高级变质岩中石榴子石进行了详细的包裹体成分、主量元素环带和微量元素特征的研究,揭示出石榴子石黑云母片麻岩至少记录了五期岩浆或变质热事件。第Ⅰ期石榴子石为来源于源区的岩浆型石榴子石。第Ⅱ期、第Ⅲ期和第Ⅳ期石榴子石为变质型石榴子石,但不同期次变质作用的温压条件和生长介质、矿物组合不同。第Ⅴ期石榴子石为转熔型石榴子石,是黑云母脱水熔融形成,记录了喜马拉雅造山过程早期加厚地壳条件下的深熔作用。喜马拉雅造山带变质岩中石榴子石具有复杂的成因机制和演化历史,在应用石榴子石进行变质作用研究时,需要仔细甄别,否则会得到错误的结论。     

东昆仑东段金水口石榴堇青石花岗岩成因——石榴子石证据

为了查明东昆仑东段金水口地区的石榴堇青石花岗岩的成因和地质意义,对其中的石榴子石进行了岩相学和矿物化学研究。根据石榴子石的产状和成分,在该花岗岩中初略识别出岩浆型和转熔型两种成因的石榴子石。前者较少,与长石、石英共生,细小,自形,不含矿物包裹体,富MnO贫MgO,具MnO成分环带;后者较多,与黑云母共生,晶粒较大,它形,常含矿物包裹体,贫MnO富MgO,不具MnO成分环带。两种石榴子共存表明花岗岩为原地深熔成因或混合岩化成因,表明该地区的白沙河岩组片麻岩在早古生代曾经历过减压升温过程。     

华北地台北缘集宁地区古元古代片麻状石榴花岗岩的深熔成因及地质意义

本文对华北克拉通北缘集宁地区空间上密切共生的片麻状石榴花岗岩和孔兹岩系富铝片麻岩的岩相学、地球化学及年代学特征进行了对比研究。SHRIMP锆石U-Pb定年方面,在富铝片麻岩中获得了1910±10Ma和1839±13Ma变质锆石年龄,在片麻状石榴花岗岩中获得了1919±17Ma的变质重结晶锆石年龄。在石榴花岗岩的石榴石包裹体中识别出与富铝片麻岩相对应的进变质阶段(M1)和峰期阶段(M2)的矿物组合,由此确认富铝片麻岩的变质作用和导致石榴花岗岩形成的深熔作用是同一构造热事件的产物。通过对二者变质作用演化及特征变质矿物的对比,认为深熔作用主要发生在峰期后等温降压阶段(M3),石榴花岗岩中的石榴石为深熔作用过程中的残留矿物相或转熔矿物相,而石榴花岗岩则是混合有大量残留矿物相的熔体结晶的产物。对片麻状石榴花岗岩和富铝片麻岩的地球化学组成特征进行了对比分析,片麻状石榴花岗岩既有一定的继承性,又有十分明显的变异性。变异性表现为:1)石榴花岗岩主量和微量元素含量分布极不均匀,微量元素含量普遍低于源岩(Cs、Rb、Th、U、Nb、Ta、Zr、Hf等);2)大离子亲石元素Cs和生热元素U、Th亏损明显,Sr相对富集;3)高场强元素Nb、Ta、P、Ti的明显亏损;4)铕异常变化大,存在铕富集型、铕平坦型和铕亏损型共存的稀土配分曲线的岩石,这是深熔成因石榴花岗岩最突出的表现,也可能是原地-半原地深熔花岗岩的主要地球化学标志。综合区域上的地质资料,认为深熔作用与碰撞后伸展构造背景下基性岩浆底侵事件有关。     

内蒙古大青山地区石榴花岗岩的地质特征和岩相学特征

内蒙古大青山地区石榴花岗岩出露于哈德门沟-平方沟-虎本汉沟一带,沿麻粒岩系与孔兹岩系之间的构造不整合面呈不规则带状分布,从其野外地质特征和岩相学特征可确定石榴花岗岩与围岩为渐变过渡关系.石榴花岗岩中的变质表壳岩以及部分包体不仅在岩性上可与外围的同类岩石对比,而且也显示了明显的深熔作用改造的痕迹.现有证据表明石榴花岗岩总体上是由孔兹岩系的石榴黑云片麻岩原地熔融形成的,为深熔作用的产物.石榴花岗岩的研究为本区的深熔作用演化提供了依据.     

青藏高原东南缘点苍山-哀牢山杂岩带中片麻岩的变质演化及深熔作用特征

片麻岩是点苍山-哀牢山变质杂岩带最常见的岩石类型,主要由夕线石榴黑云二长片麻岩、石榴黑云斜长片麻岩以及含十字石蓝晶夕线石榴片麻岩所组成,其原岩的化学成分与华北克拉通典型孔兹岩系十分相似.岩相学、成因矿物学和变质反应性质研究以及温压条件估算结果表明,研究区片麻岩类岩石变质峰期的温压条件为T=700～770℃,P=0.5～0.8GPa,已达到或接近麻粒岩相变质,晚期退变质阶段的温压条件为T=600～650℃,P=0.35～0.45GPa.片麻岩变质演化的P-T轨迹具有顺时针型式.在麻粒岩相变质阶段,片麻岩类岩石普遍发生深熔作用,主要表现为含水矿物黑云母的脱水熔融和长英质矿物的部分熔融.该项研究对于进一步揭示青藏高原东南缘点苍山-哀牢山造山带的变质演化深熔作用机理及动力学过程具有重要的科学意义.     

通化集安群石榴子石和黑云母微区化学成分特征及其对变质作用p—T—t轨

集巡群片麻岩中石榴子石和黑云母接触带两侧边缘广泛发育Ｆｅ－Ｍｇ交换反应所形成的扩散环带,但晶体内部各自保持变质峰期的成分,中温带石榴子石可保留生长环带。     

东疆哈尔里克变质带中石榴子石的特征

石榴子石为东疆哈尔里克变质带中的常见矿物。从外形上可分为两种 类型,一种是晶形发育较好的石榴子石,为断裂区域变质作用形成;另一种是破碎状、裂纹 发育的石榴子石,为早期热变质作用形成,呈残留状出现。二者在空间上共存。对于晶形较 好的石榴子石的化学成分研究表明,石榴子石发育生长环带,在晶体粒径与核部MnO的含量 之间存在着相关关系。其峰期MnO的含量反映了递增变质带变质温度变化的趋势。     

通化集安群石榴子石和黑云母微区化学成分特征及其对变质作用p-T-t轨迹的指示意义

集安群片麻岩中石榴子石和黑云母接触带两侧边缘广泛发育Fe－Mg交换反应所形成的扩散环带,但晶体内部各自保持变质峰期的成分,中温带石榴子石可保留生长环带。经微区化学成分详细研究后,正确运用Grt－Bt温度计获得进变质阶段,变质高峰期和修期后Fe－Mg交换反应停止时的温度条件。按石榴子石和斜长石的微区钙含量变化确定了峰期和退变质阶段压力。据此建立起逆时针形式的变质作用p－T－t轨迹,反映该区早元古代陆壳内裂陷带的闭合过程。     

柴达木盆地北缘大柴旦和锡铁山榴辉岩中石榴子石环带对比及地质意义

大柴旦榴辉岩和锡铁山榴辉岩同为柴达木盆地北缘高压—超高压变质带的组成部分,但它们的岩相学特征、形成的PT条件及榴辉岩中石榴子石中保存的成分环带均具有明显的差异。大柴旦榴辉岩峰期变质的PT条件为T=620～730℃,P=2.3～2.8GPa,岩相学研究表明其经历了近等温降压或降温降压的后榴辉岩相历史,保存有进变质的矿物组合和石榴子石的生长环带及增生结构。相反,锡铁山榴辉岩的榴辉岩相的温度较高,岩相学研究显示其具有明显的后榴辉岩相的高温麻粒岩相的叠加,PT轨迹显示压力峰期早于温度峰期,具有略升温降压的后榴辉岩相变质历史,榴辉岩中石榴子石的成分环带表现为扩散环带,没有生长环带保留。大柴旦榴辉岩和锡铁山榴辉岩的不同的石榴子石环带特征反映了两者在形成过程中,特别是折返过程中不同的构造热历史。     

原地-半原地深熔花岗岩特征:以华北克拉通北缘包头地区石榴花岗岩为例

在华北克拉通北缘大青山地区,广泛的深熔作用导致新太古代晚期石榴花岗岩发育。石榴花岗岩空间上与新太古代晚期大青山表壳岩(主要为石榴黑云母片麻岩)共生,渐变过渡。宏观上岩性具有不均一性,在包头哈德门沟一条实测地质剖面上可以观察到石榴混合闪长岩、石榴混合石英闪长岩和石榴混合花岗闪长岩等不同岩石类型。岩相学研究表明,石榴花岗岩与源岩矿物组合相近,但含量却大不相同。石榴花岗岩和源岩中的石榴石特征十分相似。石榴石周围存在放射状黑云母,是典型的逆反应结构。研究表明深熔作用可能发生在具有顺时针P-T轨迹的变质作用演化峰期及峰期后近等温降压阶段,主要发生钾长石熔融和黑云母熔融。地球化学特征研究表明石榴花岗岩与石榴黑云母片麻岩具有亲缘关系,均表现出高Al2O3、CaO含量,K2O/Na2O比值存在较大变化,富集轻稀土和大离子亲石元素,亏损Nb、Ta、P和Ti。但是,石榴花岗岩存在组成变化,Eu富集型和亏损型稀土模式为原地-半原地深熔花岗岩的标志性特征。SHRIMP锆石年代学研究表明深熔作用发生在2.43Ga之前,与区内新太古代晚期水平顺层滑脱变形有关,揭示华北克拉通北缘新太古代晚期构造热事件。     

石榴子石Lu-Hf年代学研究进展

高Lu/Hf比的石榴子石可以用于构筑高精度的Lu-Hf等时线,因而石榴子石Lu-Hf法被广泛地应用于各类岩石的定年研究中。特别是在造山带研究中,石榴子石Lu-Hf法揭示了许多其他定年体系所不能揭示的信息。但是由于石榴子石生长历史复杂,影响石榴子石 Lu-Hf 定年结果的因素多样,合理解释石榴子石Lu-Hf年龄指示的地质意义成为石榴子石Lu-Hf年代学研究的难点。这些因素包括石榴子石的生长模式、石榴子石的成核过程、封闭温度、包裹体、石榴子石生长历史、矿物间元素平衡、几何效应以及样品的预处理方法等。在进行石榴子石 Lu-Hf 定年研究时需要对这些因素进行综合的判定,才能对年龄做出合理的解释。本文初步探讨了这些因素对石榴子石 Lu-Hf 定年结果的影响,并以苏鲁-大别造山带为例介绍了石榴子石Lu-Hf年代学在碰撞造山带研究中的进展,揭示了石榴子石Lu-Hf体系在造山带及超高压变质研究中的巨大潜力。     

A TEM study of the oriented orthopyroxene and forsterite inclusions in garnet from Otrøy garnet peridotite,WGR, Norway: new insights on crystallographic characteristics and growth energetics of exsolved pyroxene in relict majoritic garnet

Regularly oriented orthopyroxene (opx) and forsterite (fo) inclusions occur as opx + rutile (rt) or fo + rt inclusion domains in garnet (grt) from Otrøy peridotite. Electron diffraction characterization shows that forsterite inclusions do not have any specific crystallographic orientation relationships (COR) with the garnet host. In contrast, orthopyroxene inclusions have two sets of COR, that is, COR‐I: <111>_grt//<001>_opx and {110}_grt～//～{100}_opx (～13° off) and COR‐II: <111>_grt//<011>_opx and {110}_grt～//～{100}_opx (～14° off), in four garnet grains analysed. Both variants of orthopyroxene have a blade‐like habit with one pair of broad crystal faces parallel/sub‐parallel to {110}_grt plane and the long axis of the crystal, <001>_opx for COR‐I and <011>_opx for COR‐II, along <111>_grt direction. Whereas the lack of specific COR between forsterite and garnet, along with the presence of abundant infiltrating trails/veinlets decorated by fo + rt at garnet edges, provide compelling evidence for the formation of forsterite inclusions in garnet through the sequential cleaving–infiltrating–precipitating–healing process at low temperatures, the origin of the epitaxial orthopyroxene inclusions in garnet is not so obvious. In this connection, the reported COR, the crystal habit and the crystal growth energetics of the exsolved orthopyroxene in relict majoritic garnet were reviewed/clarified. The exsolved orthopyroxene in a relict majoritic garnet follows COR‐III: {112}_grt//{100}_opx and <111>_grt//<001>_opx. Based on the detailed trace analysis on published SEM images, these exsolved orthopyroxene inclusions are shown to have the crystal habit with one pair of broad crystal faces parallel to {112}_grt//{100}_opx and the long crystal axis along <111>_grt//<001>_opx. Such a crystal habit can be rationalized by the differences in oxygen sub‐lattices of both structures and represents the energetically favoured crystal shape of orthopyroxene inclusions in garnet formed by solid‐state exsolution mechanism. Considering the very different COR, crystal habit, as well as crystal growth direction, the orthopyroxene inclusions in garnet of the present sample most likely had been formed by mechanism(s) other than solid‐state exsolution, regardless of their regularly oriented appearance in garnet and the COR specification between orthopyroxene and garnet. In fact, the crystallographic characteristics of orthopyroxene and the similar chemical compositions of garnet at opx + rt inclusion domains, fo + rt inclusion domains/trails and garnet rim suggest that the orthopyroxene inclusions in the garnet are most likely formed by similar cleaving‐infiltration process as forsterite inclusions, though probably at an earlier stage of metamorphism. This work demonstrates that the oriented inclusions in host minerals, with or without specific COR, can arise from mechanism(s) other than solid‐state exsolution. Caution is thus needed in the interpretation of such COR, so that an erroneous identification of exhumation from UHP depths would not be made.     

Super-silicic garnet microstructures from an orogenic garnet peridotite, evidence for an ultra-deep (>6 GPa) origin

We report the field, petrographic and mineral chemical characteristics of relict super‐silicic (=majoritic) garnet microstructures from the Otrøy peridotites in the Western Gneiss Region, Norway. The evidence for the former existence of super‐silicic garnet consists of two‐pyroxene exsolution microstructures from garnet. Estimates of the initial composition of the super‐silicic garnet imply pressures of 6–6.5 GPa, indicating that the Otrøy garnet peridotites were derived from depths >185 km. The garnet peridotites consist of inter‐banded variable compositions with c. 50% garnet peridotite and 50% garnet‐free peridotite. Two distinct garnet types were identified: (a) normal matrix garnet, grain‐size ≤4 mm, and (b) large isolated single garnet crystals and/or (polycrystalline) garnet nodules up to 10 cm in size. Large garnet nodules occur only within limited bands within the garnet peridotites. The relicts of super‐silicic garnet were exclusively found in some (not all) of the larger garnet nodules. Petrographic observations revealed that the microstructure of nodular garnet consists of the following four characteristic elements. (1) Individual garnet nodules are polycrystalline, with grain sizes of 2–8 mm. Garnet grain boundaries are straight with well‐defined triple junctions. (2) Some garnet triple junctions and garnet grain boundaries are decorated by interstitial orthopyroxene. (3) Cores of larger polycrystalline garnet contain two‐pyroxene exsolution microstructures. (4) Precipitation‐free rims (2 mm thick) surround garnet cores containing the exsolved pyroxene microstructure. Pyroxene exsolution from super‐silicic garnet was subsequently followed by brittle–ductile deformation of garnet. Both exsolved pyroxene needles and laths become undulous or truncated by fractures. Simultaneous garnet plasticity is indicated by the occurrence of high densities of naturally decorated dislocations. Transmission electron microscopy observations indicate that decoration is due to Ti‐oxide precipitation. Estimates of the P–T conditions for mineral chemical equilibration were obtained from geothermobarometry. The mineral compositions equilibrated at mantle conditions around 805±40 °C and 3.2±0.2 GPa. These P–T estimates correspond to cold continental lithosphere conditions at depths of around 105 km. From a combination of both depth estimates it can be concluded that the microstructural memory of the rock extends backwards to twice as great a depth range as obtained by thermobarometric methods. Available geochronological and geochemical data of Norwegian garnet peridotites suggest a multi‐stage, multi‐orogenic exhumation history.     

含榴花岗片麻岩 Lu-Hf 年代学初探

对西大别四道河含石榴子石花岗片麻岩进行了锆石 U-Pb 和石榴子石 Lu-Hf 年代学测试,锆石 U-Pb谐和年龄为(223±1) Ma,石榴子石-全岩 Lu-Hf 等时线年龄为(212.2±0.7) Ma.石榴子石具有极高的母子体同位素比值(176Lu/177Hf =~300).结合锆石和石榴子石的微量元素特征,该锆石 U-Pb 年龄代表的可能是超高压/高压变质时间,石榴子石 Lu-Hf 年龄代表的是石榴子石重结晶时间,可能指示了后期退变质作用流体活动     

Melt inclusions in eclogite garnet: experimental study of natural processes

Experiment with poikilitic garnet at 3 GPa and 800 °C showed dehydration melting of its mineral inclusions, which is accompanied by the growth of (sub)euhedral garnet crystals inside the inclusion and/or xenomorphic garnets replacing the host mineral. The newly formed and host garnets differ drastically in composition. The inclusion surface is complicated by specific wedge-like protrusions or thin branches composed of melt or its crystallization products. The above features have been discovered in polymineral inclusions in garnet from low-temperature (650 °C) eclogite from the Yukon-Tanana terrane, Canada. The inclusions are interpreted as the crystallization products of in situ formed melt.     

Oriented inclusions of pyroxene,amphibole and rutile in garnet from the Lüliangshan garnet peridotite massif,North Qaidam UHPM belt,NW China: an electron backscatter diffraction study

Oriented inclusions of clinopyroxene, orthopyroxene, sodic amphibole and rutile have been identified in garnet from the Lüliangshan garnet peridotite massif in the North Qaidam ultrahigh‐pressure metamorphic (UHPM) belt, northern Tibetan Plateau, NW China. Electron backscatter diffraction (EBSD) analyses demonstrate that nearly half of the measured intracrystalline clinopyroxene (8 out of 17) have topotactic crystallographic relationships with host garnet, that is, (100)_Cpx//{112}_Grt, (010)_Cpx//{110}_Grt and [001]_Cpx//<111>_Grt. One‐fifth of the oriented sodic amphibole (23 out of 110) inclusions of have topotactic crystallographic relationships with host garnet, that is, (010)_Amp//{112}_Grt, (100)_Amp//{110}_Grt and [001]_Amp//<111>_Grt. Over a third of rutile (36 out of 99) inclusions also show a close crystallographic orientation relationship with host garnet in that one <103>_Rt and one <110>_Rt parallel to two <111>_Grt while the axes of [001]_Rt exhibit small girdles centred the axes of <111>_Grt. But, no ‘well‐fit’ crystallographic relationship was observed between orthopyroxene inclusions and host garnet. Considering a very long and complex history for the Lüliangshan garnet peridotite, we suggest that the low fit rates for these oriented minerals may result from several possible assumptions including different generations or multi‐stage formation mechanisms, heterogeneous nucleation and growth under non‐equilibrium conditions, and partial changes of initial crystallographic orientations of some inclusions. However, the residual quantitative ‘well‐fit’ crystallographic information is sufficient to indicate that the nucleation and growth of many pyroxene, amphibole and rutile are controlled by the lattice of the host garnet. The revealed close topotactic relationships accompanied by clear shape orientations provide quantitative microstructural evidence demonstrating a most likely exsolution/precipitate origin for at least some of the oriented phases of pyroxene, sodic amphibole and rutile from former majoritic garnet and support an ultra‐deep (>180 km depth) origin of the Lüliangshan garnet massif.     

When epitaxy controls garnet growth

Within a mica schist from the coesite-bearing Brossasco-Isasca Unit (Western Alps), microstructural analysis shows that Alpine garnet grains are aligned with the crenulated foliation. Garnet crystallographic orientation was analysed with electron backscatter diffraction (EBSD): the obtained crystallographic dispersion patterns and distribution patterns of misorientation axes suggest a strong parallelism of {110} garnet planes with a 56°W-dipping foliation. The data are interpreted as evidence for an epitaxial growth of garnet upon (001) biotite planes, sometime during and/or after dispersion of the biotite/garnet crystals from their initially foliation-parallel orientation by rotation about the Alpine crenulation axis. This interpretation is based on the comparison of the measured EBSD data with: (i) theoretical dispersion trajectories of garnet crystallographic data, (ii) numerically modelled pole figures, and (iii) numerically modelled misorientation axis distribution patterns. Our data suggest that epitaxial growth of garnet upon biotite is allowed by distortion of the pseudohexagonal basal oxygen ring structure on (001) biotite surfaces, and that distortion is driven by introduction of missing ions. Our data further suggest that the spatial distribution of precursor phases influences the distribution patterns of garnet within mica schists.     

On the interpretation of peak metamorphic temperatures in light of garnet diffusion during cooling

Abstract Finite difference models of Fe-Mg diffusion in garnet undergoing cooling from metamorphic peak conditions are used to infer the significance of temperatures calculated using garnet-biotite Fe-Mg exchange thermometry. For rocks cooled from high grades where the garnet was initially homogeneous, the calculated temperature (T_calc) using garnet core and matrix biotite depends on the size of the garnet, the ratio of garnet to biotite in the rock (V_garnet/V_biotite) and the cooling rate. For garnets with radii of 1 mm and V_garnet/V_biotite<1, T_calc is 633, 700 and 777°C for cooling rates of 1, 10 and 100°C/Ma. For V_garnet/V_biotite= 1 and 4 and a cooling rate of 10° C/Ma, T_calc is approximately 660 and 610° C, respectively. Smaller and larger garnets have lower and higher T_calc, respectively. These results suggest that peak metamorphic temperatures may be reliably attained from rocks crystallized at conditions below T_calc of the garnet core, provided that V_garnet/V_biotite is sufficiently small (<0.1) and that the composition of the biotite at the metamorphic peak has not been altered during cooling. Numerical experiments on amphibolite facies garnets with nominal peak temperatures of 550–600° C generate a ‘well’in Fe/(Fe + Mg) near the rim during cooling. Maximum calculated temperatures for the assemblage garnet + chlorite + biotite + muscovite + plagioclase + quartz using the Fe/(Fe + Mg) at the bottom of the ‘well’with matrix biotite range from 23–43° C to 5–12° C below the peak metamorphic temperature for cooling rates of 1 and 100° C/Ma, respectively. Maximum calculated temperatures for the assemblage garnet + staurolite + biotite + muscovite + plagioclase + quartz are approximately 70° C below the peak metamorphic temperature and are not strongly dependent on cooling rate. The results of this study indicate that it may be very difficult to calculate peak metamorphic temperatures using garnet-biotite Fe-Mg exchange thermometry on amphibolite facies rocks (T_max > 550° C) because the rim composition of the garnet, which is required to calculate the peak temperature, is that most easily destroyed by diffusion.     

西藏甲玛超大型铜矿石榴子石特征及成因意义

石石榴子石是西藏甲玛超大型铜多金属矿床的主要矽卡岩矿物,分布广、颜色变化大,石榴子石矽卡岩中矿化好,是开展甲玛矽卡岩成因研究的一个重要切入点。通过对甲玛矿区大量岩矿心的地质编录,归纳石榴子石的颜色、晶形、矿物组合、矿化等地质现象,总结石榴子石在甲玛矿床的空间分布、矿化特征等,结合显微镜下鉴定,分析石榴子石的矿物学特征,并通过电子探针分析其化学成分。甲玛石榴子石集中于矽卡岩中,少量分布于角岩、大理岩和斑岩中。石榴子石为钙铁-钙铝榴石系列,以钙铁榴石为主,发育颜色与成分环带,光性异常致部分非均质。深色环带的SiO2、Al2O3含量较浅色环带高,而TFeO含量相反。石榴子石形成于矽卡岩早期,为晚期Cu、Mo、Pb、Zn、Au、Ag等多金属硫化物沉淀富集提供了有利空间。甲玛石榴子石的矿物学特征、化学成分组成、REE特征、流体包裹体、H-O同位素特征等显示其属于接触交代成因,认为甲玛是典型的岩浆热液接触交代矽卡岩型铜多金属矿床。     

Metamorphism of metapelites: calculations of equilibrium assemblages and numerical simulations of the crystallization of garnet

Abstract This work uses a simplified model of equilibrium to predict the assemblage sequence and compositional zoning in garnet that should result from prograde metamorphism of common bulk compositions of pelitic rocks. An internally-consistent set of model thermodynamic data are derived for natural mineral compositions from natural assemblages. Equilibrium assemblages can be calculated for pelitic compositions with excess quartz and either muscovite or K-feldspar at any pressure and water pressure. The compositions and abundances of phases in equilibrium assemblages can be calculated where the elements Mg, Fe and Mn are exchanged among phases. The prograde metamorphic assemblage sequences and the effects of pressure on assemblages, predicted by the simulation method presented here, are similar enough to natural observations to suggest that the simulations can be used to analyse natural equilibrium and growth processes. The calculated phase diagrams at moderate and high crustal pressures explain the mineral assemblage sequence produced by prograde metamorphism in common pelitic compositions. Garnet appears by continuous reaction of biotite and chlorite as the garnet-biotite-chlorite divariant field migrates toward higher Mg/Fe ratios over the bulk composition. Staurolite appears in common bulk compositions when garnet and chlorite become incompatible. An aluminum silicate phase can appear when staurolite and chlorite react. Staurolite breaks down at an extremum point to produce garnet. Continuous reaction of biotite and sillimanite causes growth of abundant garnet. The reaction sequence involving garnet, staurolite and aluminum silicates is probably different at low pressure, but the main reason that staurolite and garnet are rare is the restricted compositional range over which their assemblages exist. Andalusite appears by the divariant reaction of chlorite and cordierite appears at low temperature in low pressure assemblages for common bulk compositions by the extremumpoint breakdown reaction of chlorite. Compositional zoning of garnet and the systematic variation of biotite composition in metamorphic sequences indicate that garnet is probably fractionated during growth. Fractionation of garnet causes garnet-consuming, univariant reactions to become multivariant. The metastable persistence of garnet should reduce the abundance and stability range of staurolite. Fractionation of even small quantities of garnet should deplete the equilibrating bulk composition of Mn, but have little effect otherwise. The simulations show that the prograde assemblage sequence in pelitic rocks can be complex in detail, with some assemblages lasting over temperature intervals of only a few degrees. The major prograde reactions that release water are the breakdown of chlorite to form garnet at low grade and the breakdown of muscovite at high grade. The volume of water released by formation of garnet at high grade is also important. These reactions have the capacity to buffer water pressure. The density of anhydrous pelitic rock increases markedly when chlorite breaks down and by the continuous reaction forming garnet at high grade. The heat content is controlled principally by heat capacity and continuous reactions. Discontinuous reactions have little thermal buffering capacity. Simulations of garnet fractionation show that commonly-observed garnet zoning profiles can be formed by garnet growth in the assemblage garnet-biotite-chlorite in common bulk compositions. A reversal of Fe-zoning in garnet can occur when garnet resumes growth above staurolite grade in the assemblage garnetbiotite-sillimanite. Discontinuities in zoning profiles can be caused only by disequilibrium. The disequilibrium can be due to either metastable persistence during a hiatus in growth or to growth by irreversible reaction. Because the appearance of garnet is controlled by a continuous rather than a discontinuous reaction, the appearance of garnet is very sensitive to bulk composition. The early development of garnet is also sensitive to the pressure and water pressure of metamorphism. As a consequence the first garnet isograd is of limited thermometric value. Metastable persistence of kyanite and manite at high grades could reduce the abundance of garnet and allow biotite to persist. Metastable persistence would also limit the of cordierite formation.