鄂尔多斯盆地正宁地区三叠系延长组长9油层组成岩作用

综合利用岩心、铸体薄片、扫描电镜等多种资料,对鄂尔多斯盆地正宁地区三叠系延长组长9油层组储集层的岩石类型、成岩作用类型、成岩相分布特征及主控因素进行了系统研究。延长组长9油层组储集层的岩石类型为细—中粒岩屑质长石砂岩和长石质岩屑砂岩,成岩作用类型主要为压实作用,同时还有绿泥石、方解石、铁方解石及硅质胶结作用,方解石交代作用和少量的溶蚀作用,储集层目前处于中成岩A期。根据层理频率和塑性颗粒含量将岩石划分为高频层理富片状塑性颗粒粉—细砂岩岩石相和低频层理富刚性矿物细—中砂岩岩石相,前者分布于水下分流河道边部,发育压实致密成岩相;后者形成于水下分流河道主河道和河口坝。根据砂体结构可划分为厚层单砂体和厚层多砂体。厚层单砂体为单期形成的河道或河口坝砂体,上部以方解石充填胶结成岩相为特征,下部以绿泥石衬边胶结成岩相为特征;厚层多砂体为多期砂体冲刷叠置形成,发育方解石绿泥石充填胶结成岩相。塑性颗粒含量对压实作用强度影响最大,叠加碳酸盐胶结致使储集层质量变差,而早期绿泥石环边胶结抑制压实作用,是有利储集层发育的重要条件。综合评价表明,延长组长9油层组有利储集层发育于绿泥石衬边胶结相,其次为绿泥石和方解石充填胶结相,最差储集层为方解石充填胶结相和压实致密相。     

鄂尔多斯盆地正宁地区三叠系延长组长9油层组成岩作用*

综合利用岩心、铸体薄片、扫描电镜等多种资料,对鄂尔多斯盆地正宁地区三叠系延长组长9油层组储集层的岩石类型、成岩作用类型、成岩相分布特征及主控因素进行了系统研究。延长组长9油层组储集层的岩石类型为细—中粒岩屑质长石砂岩和长石质岩屑砂岩,成岩作用类型主要为压实作用,同时还有绿泥石、方解石、铁方解石及硅质胶结作用,方解石交代作用和少量的溶蚀作用,储集层目前处于中成岩A期。根据层理频率和塑性颗粒含量将岩石划分为高频层理富片状塑性颗粒粉—细砂岩岩石相和低频层理富刚性矿物细—中砂岩岩石相,前者分布于水下分流河道边部,发育压实致密成岩相;后者形成于水下分流河道主河道和河口坝。根据砂体结构可划分为厚层单砂体和厚层多砂体。厚层单砂体为单期形成的河道或河口坝砂体,上部以方解石充填胶结成岩相为特征,下部以绿泥石衬边胶结成岩相为特征;厚层多砂体为多期砂体冲刷叠置形成,发育方解石绿泥石充填胶结成岩相。塑性颗粒含量对压实作用强度影响最大,叠加碳酸盐胶结致使储集层质量变差,而早期绿泥石环边胶结抑制压实作用,是有利储集层发育的重要条件。综合评价表明,延长组长9油层组有利储集层发育于绿泥石衬边胶结相,其次为绿泥石和方解石充填胶结相,最差储集层为方解石充填胶结相和压实致密相。     

帕米尔中部含刚玉变质杂岩

刚玉矿化位于穆兹科尔(Muzkol)变质杂岩体内.该杂岩体近纬向展布在帕米尔中部阿尔卑斯褶皱带的东部,属蓝晶石—夕线石型阿尔卑斯带变质杂岩.地质填图发现有4个变质岩区:低绿片岩相(白云母—绿泥石)区,高绿片岩相(黑云母—绿泥石)区,绿帘石—角闪岩相区,角闪岩相区,而刚玉矿化集中在绿帘石—角闪岩及周围角闪岩相变质体内的交代蚀变区.交代蚀变区内的刚玉矿化通常与层面或 大型同倾斜褶皱轴面平行.根据母岩成分可将含刚玉交代岩分为3类,分别位于方解石大理岩内、白云石大理岩内及片岩内.在大理岩内,刚玉成粉红色,与白云母、黑云母、方柱石、钙长石、正长石、金红石、电气石、磷灰石、黄铁矿及石墨共生,以Al含量变化大和Fe含量低为特点.在片岩内,刚玉成蓝色,与黑云母、绿泥石、电气石及磷灰石共生各种类型的含刚玉岩石均以K,Na,Mg的含量较高而Fe含量较低为特点.根据P—T—XCO2图估算含刚玉交代岩的稳定范围在610℃相似文献   

在泥质岩体系KFMASH(K_2O—FeO—MgO—Al_2O_3—SiO_2—H_2O)中计算出的矿物平衡

我们利用Holland和Powell(1990)的内部一致的热动力学数集,在0～18kb,460～720℃温压范围内计算出了KFMASH(K_2O—FeO—MgO—Al_2O3—SiO_2—H_2O)体系和KFASH、KMASH亚体系中的单变平衡线和不变平衡点,这些体系中含有十字石、硬绿泥石、绿泥石、黑云母、堇青石、石榴石、红柱石、矽线石和蓝晶石组合及过剩的白云母、石英和水。我们不仅获得了这些平衡的温压位置,而且还确定了涉及到MgFe—1和MgSiAl—2置换的矿物成分。除了在我们的KFMASH体系格子中由于没有Fe~(+3)而无硬绿泥石+黑云母或红柱石+黑云母组合的稳定范围以外,Harte和Hudson(1979)所确立的岩石一致格子是特别出色的。但考虑到格子中反应位置的不确定性以及黑云母和硬绿泥石可能富集了更多的那部分Fe,这些组合在格子中即是稳定的。我们检验了一个内部一致的数集是否象我们合理预测的与那些天然泥质岩石一样复杂的相关系,尽管结果与岩石学观察是有惊人的一致性,但仍需进一步的实验工作,特别是含堇青石和十字石的反应,在进一步改进以前,可以得出这种推算出的格     

鄂尔多斯盆地上三叠统延长组重矿物的成岩作用

砂岩中重矿物在埋藏成岩环境中会发生溶蚀甚至形成新的重矿物胶结物。利用重矿物特征进行物源分析是鄂尔多斯盆地上三叠统延长组物源分析的最重要方法之一,但石榴子石、榍石和绿帘石这3种重矿物在成岩作用期间发生了明显的后生变化。研究表明,延长卡中石榴子石发生溶蚀形成粒内溶孔和刻面石榴子石,部分被方解石、绿泥石或硅质交代;见自生榍石晶体及榍石次生加大,且榍石的生长具有多期次性;见自生绿帘石晶体及绿帘石次生加大,部分绿帘石被绿泥石或硅质交代;相对稳定性强的石榴子石部分被溶蚀,而相对稳定性弱的碎屑状榍石、绿帘石并未见到溶蚀现象,这与以往研究中对于重矿物稳定性的认识是不一致的,希望能引起未来研究者的重视。     

矿物成因族的演化及其地质找矿信息

基于对胶东、晋北寒武纪变质岩系、超基性岩、基性岩、花岗岩类、火山岩类、硅铁建造铁矿、金矿蚀变带内 1 1个不同地质产状的 34个角闪石、 2 51个绿泥石、 60个黑云母单矿物的提纯 ,矿物的共生组合、物理性质、化学成分、X光衍射分析、热分析、红外吸收光谱分析、双矿物地质温度计、矿物压力计的测定 ,提出了角闪石、绿泥石、黑云母矿物成因族的地壳物质演化、成岩成矿作用、变质岩原岩恢复、地层对比、变质相、蚀变带的演化规律和地质找矿信息。     

澜沧江杂岩带小黑江-上允地区蓝片岩的成因及变质演化

通过对澜沧江杂岩带小黑江-上允地区蓝片岩的岩相学、地球化学、成因矿物学以及相平衡模拟的综合研究,阐述蓝片岩的原岩以及变质演化过程。地球化学分析结果显示,蓝片岩具有一致的稀土元素配分模式,具弱Eu正或负异常,稀土元素和微量元素特征与OIB相似,其原岩可能为OIB型玄武岩。详细矿物学研究表明,本区蓝片岩记录了俯冲峰期蓝片岩相变质和峰期后绿片岩相变质两个变质阶段,其矿物组合分别为蓝闪石+钠长石+多硅白云母+绿泥石+绿帘石和蓝闪石+钠长石±阳起石+绿泥石+绿帘石。通过Na_2O-Ca O-Fe O-MgO-Al_2O_3-SiO_2-H_2O-O体系相平衡计算,得到两个阶段的压力范围分别约为0.95 GPa和0.40 GPa。     

鄂尔多斯盆地中南部三叠系延长组8油层组成岩作用及其对储层物性的控制

鄂尔多斯盆地中南部延长组8油层组主要成岩作用包括压实作用、石英次生加大、自生绿泥石膜生长、次生高岭石化、连晶方解石交代、长石溶蚀。根据铸体薄片,碳氧同位素分析,确定了各种主要成岩产物的空间分布和成因,分析了成岩产物分布与现今总面孔率的关系,从而确定8油层组的物性主要受石英次生加大、连晶方解石、长石溶孔、剩余原生孔隙分布的控制。石英次生加大和连晶方解石发育的地方,储层物性差;具自生绿泥石膜的剩余原生孔隙和长石溶孔发育的地方,储层物性好。     

内蒙古中部大黄花夕卡岩型铜多金属矿床特征

大黄花铜多金属矿位于赤峰-朝阳金多金属成矿带南部,矿化发生在燕山期正长花岗岩与石炭系酒局子组大理岩接触部位,局部构成铜（铁）矿体.夕卡岩矿物主要为石榴石、透辉石、透闪石、绿帘石、绿泥石、角闪石及阳起石,金属矿物有黄铜矿、辉铜矿、黝铜矿、斑铜矿、铜蓝及氧化次生矿物蓝铜矿、孔雀石、褐铁矿.围岩蚀变强烈,呈现清晰的蚀变分带现象,自接触带向外可分为较强的透闪石-阳起石化带,较强的透闪石-透辉石化带,弱蚀变的大理岩化带.金属元素组合特征明显,为Cu、Pb、Zn、Au、Ag组合.该铜多金属矿的形成与正长花岗岩的侵入作用有关,成矿时代为中侏罗世晚期,与朝阳-赤峰地区金多金属矿成岩成矿时代一致.     

内蒙古中部大黄花夕卡岩型铜多金属矿床特征

大黄花铜多金属矿位于赤峰-朝阳金多金属成矿带南部，矿化发生在燕山期正长花岗岩与石炭系酒局子组大理岩接触部位，局部构成铜（铁）矿体.夕卡岩矿物主要为石榴石、透辉石、透闪石、绿帘石、绿泥石、角闪石及阳起石，金属矿物有黄铜矿、辉铜矿、黝铜矿、斑铜矿、铜蓝及氧化次生矿物蓝铜矿、孔雀石、褐铁矿.围岩蚀变强烈，呈现清晰的蚀变分带现象，自接触带向外可分为较强的透闪石-阳起石化带，较强的透闪石-透辉石化带，弱蚀变的大理岩化带.金属元素组合特征明显，为Cu、Pb、Zn、Au、Ag组合.该铜多金属矿的形成与正长花岗岩的侵入作用有关，成矿时代为中侏罗世晚期，与朝阳-赤峰地区金多金属矿成岩成矿时代一致.     

Partial melting and phase relations in high-grade metapelites: an experimental petrogenetic grid in the KFMASH system

Petrogenetic grids are a powerful tool for understanding metamorphic terrains and many theoretical grids have been suggested for the process of granulite formation in metapelitic rocks, via fluid-absent biotite melting reactions. However, application of these grids has been difficult due to the lack of suitable experimental constraints. We present here an experimentally determined and tightly constrained petrogenetic grid for KFMASH system metapelites which extends from 840–1000°C and 5.0–12.5 kbar. Sixty four experiments on three KFMASH, mineral-mix, bulk compositions (X _Mg=0.62, 0.74, 0.86) provide phase composition and assemblage data from which a grid can be derived and constrained. Reversal experiments and consideration of the phase composition data show the experiments to be close to equilibrium. The KFMASH univariant fluid-absent biotite melting reactions occur between 850 and 870°C at 5 kbar and between 900 and 915°C at 10 kbar. These reactions are connected to equilibria beyond the stability of biotite to develop a fixed framework within which the phase assemblage evolution of metapelitic rocks can be interpreted. The effect of minor components on phase equilibria is evaluated using the experimentally determined grid as a simple-system reference. The temperature at which melting occurs in metapelites is strongly controlled by the concentrations of titanium and fluorine in biotite. Pressure-temperature pseudosections presented for each of the experimental compositions show both the univariant and divariant reactions available to a particular bulk composition, clearly illustrating the possible evolution of the phase assemblage. The pseudosections also constrain the stability limits of     

A sapphirine-cordierite-garnet-sillimanite granulite from Enderby Land,Antarctica: implications for FMAS petrogenetic grids in the granulite facies

A quartz-absent magnesian paragneiss layer from Mount Sones, in the Archaean Napier complex of Enderby Land, Antarctica, contains the stable divariant FMAS assemblage sapphirine (X _Mg=78) — cordierite (X _Mg=87) — garnet (X _Mg=51) — sillimanite. Rare green spinel (X _Mg=53.5, ZnO=2.65wt%) occurs as inclusions mainly within sapphirine, but also within sillimanite and garnet. Late thin coronas of cordierite (X _Mg=90.5) mantle sapphirine in contact with extensively exsolved anorthoclase. The mineral textures are interpreted to indicate the former stability of a hypersthene-quartz absent assemblage followed by the development of the FMAS equilibrium assemblage sapphirine-cordierite-garnet-sillimanite (sp, hy, qz) and further divariant reaction involving the consumption of sapphirine. The (sp, hy, qz) assemblage uniquely defines the stable P-T reaction topology appropriate to granulites from the Napier Complex, as this paragenesis is allowed in the grids of Hensen (1971, 1986) but is not possible in other grids which assume the stability of a sapphirine-absent ([sa]) FMAS invariant point involving the phases spinel, garnet, hypersthene, cordierite, sillimanite and quartz. The observed mineral assemblages and textures are consistent with peak metamorphism between the [sp] and [hy] invariant points of Hensen (1971), at temperatures of 930–990° C, followed by cooling on a lower dP/dT trajectory towards the (sp, qz) univariant line. The initial spinel-bearing assemblage was stabilized by Zn and to a lesser extent by Ni and Cr, and hence does not require a marked decrease in temperature and increase in pressure to produce the (sp, hy, qz) assemblage. It is inferred that fO ₂ conditions substantially lower than those used in the experiments of Annersten and Seifert (1981) prevailed in the high-grade metamorphism in the Napier Complex.     

Stability analysis of invariant points using Euler spheres, with an application to FMAS granulites

Alternative assignment of invariant point stabilities in a possible P – T  phase diagram is given by a family of grids that derives from a form of the Euler equation. Invariant points are represented by great circles that divide the surface of a sphere (the Euler sphere) into polygonal regions that correspond to the number of potential solutions or grids in n -component systems with n +3 non-degenerate phases. A particular invariant point is stable in all grids on one side of the great circle and metastable on the other. The advantage of this representation is the ease and efficiency by which all grids consistent with experimental and theoretical constraints can be identified. The method is well suited for systems of n +3 phases in which the thermochemical data necessary for direct calculation of the phase diagram is either uncertain or non-existent for one or more of the phases. The mass balance equations among the n +3 phases of interest define the Euler sphere for any particular system. There is a unique Euler sphere for unary systems, and another for binary systems. Ternary and quaternary systems have four and 11 different types of Euler spheres, respectively. In the ternary case with six phases, the 16 non-degenerate chemographies belong to four groups that are associated with the four Euler spheres. An analysis of those groups shows a close relationship between the topologies of the chemographies and the topologies of the grids represented on the Euler sphere. Euler spheres for degenerate chemographies are characterized by a smaller number of spherical polygons. A useful application of the Euler sphere concept is the systematic derivation of possible FMAS petrogenetic grids from subsystem constraints. Assumption of just one stable invariant point in each of MAS and FAS systems is consistent with seven FMAS grids involving cordierite, garnet, hypersthene, quartz, sapphirine, sillimanite and spinel.     

Petrogenetic grids for metacarbonate rocks: pressure-temperature phase-diagram projection for mixed-volatile systems

Isothermal or isobaric phase diagram sections as a function of fluid composition (X ^F) are widely used for interpreting the genetic history of metacarbonate rocks. This approach has the disadvantages that: (1) the influence of a key metamorphic variable, either pressure (P) or temperature (T), is obscured; (2) the diagrams are inappropriate for systems that are not fluid-saturated. These problems are avoided by constructing phase-diagram projections in which the volatile composition of the system is projected onto a P-T coordinate frame, i.e., a petrogenetic grid. The univariant curves of such P-T projections trace the conditions of the invariant points of isothermal or isobaric phase-diagram sections, thereby defining the absolute stability of high-variance mineral assemblages, with and without a coexistent fluid phase. Petrogenetic grids for metacarbonate rocks are most useful for the study of regional metamorphism and for systems in which fluid composition has not been externally controlled. A calculated example of a P-T projection for the system CaO−MgO−SiO₂−H₂O−CO₂ suggests that many assemblages (e.g., calcite +tale, enstatite+fluid, magnesite+tremolite, antigorite+diopside+dolomite, and calcite+forsterite+tremolite) in mixed-volatile systems have stability fields that make them useful as P-T indicators. Consideration of the principles governing projection topology demonstrates that the univariant curves around a fluid present invariant point cannot be oriented independently with respect to the direction of compositional variation in the fluid phase. This has the interesting predictive implication that if the direction of compositional variation along one univariant curve around an invariant point is known, then the direction of compositional variation along the remaining curves can be determined solely from topologic constraints. The same constraints can be applied to systems containing simple mineral solutions or melts in order to predict compositional variations.     

The stability of sapphirine + clinopyroxene: implications for phase relations in the CaO-MgO-Al2O3-SiO2 system under deep-crustal and upper mantle conditions

Reaction textures and chemographic relations in sapphirine-bearing basic granulites at Finero, Italy, suggest that sapphirine and aluminous diopside were formed in mutual equilibrium from an inferred early spinel-bearing assemblage. Finero appears to be the only known locality where this association has been found in situ, although it is also known from kimberlite and breccia pipe nodules elsewhere. The reactions deduced to have occurred in these rocks suggest the existence of stable invariant points involving the phases sapphirine-spinel-orthopyroxene-clinopyroxene-garnet-anorthite and sapphirine-two pyroxenes-garnetanorthite-kyanite (or sillimanite) in the CMAS end-member system. P-T estimates for the relevant rocks, and the available experimental data, suggest that these points lie at around 800°–900° C, 9–11 kbar. A semi-quantitative petrogenetic grid, incorporating these invariant points with previously determined univariant reactions, is proposed. It is inferred that sapphirine+diopside are stable relative to spinel-bearing assemblages below 900° C. The relatively low temperature explains why sapphirine has not to date been reported from experimental work on the CMAS system. It also suggests that sapphirine may be an important aluminous phase in Mg-rich metagabbros under conditions corresponding to the base of the continental crust, despite the observed rarity of such rocks at the surface.     

Phase relations of osumilite and dehydration melting in pelitic rocks: a simple thermodynamic model for the KFMASH system

 Thermodynamic modelling of (1) osumilite solid solutions and (2) dehydration melting in pelitic compositions within the KFMASH system is quite successful in reproducing the invariant and univariant reactions determined in experimental studies. Even though rather preliminary, such melt thermodynamic models may be very useful in interpolating and extrapolating the limited information available from a small number of experimental runs. These methods allow the compositions of all phases to be monitored as a function of pressure, temperature and equilibrium phase assemblage for any desired bulk composition. Locating the higher variance phase fields (e.g. quadrivariant, quinivariant) is often difficult or impossible by inspection, but is made relatively easy using thermodynamic software such as thermocalc. In the KFMASH system the calculated partition of Fe and Mg between osumilite, garnet, cordierite, orthopyroxene and biotite are shown to be in good agreement with experimental and natural data and allow reliable calculation of mineral compositions coexisting with quartz-saturated and H₂O-undersaturated melts for a variety of bulk compositions. These phase diagram calculations allow quite tight limits to be placed on the pressure, temperature and water activity conditions which accompanied metamorphism of natural osumilite occurrences in Nain, Namaqualand, and Rogaland. At fixed bulk composition, the initial melting of pelites by dehydration of biotite can occur via univariant, divariant or trivariant equilibria depending upon pressure of metamorphism. Of particular interest is that, for low pressures or more magnesian bulk compositions, fluid-absent melting begins by generating liquid from the high-variance assemblage biotite+cordierite+K-feldspar+ quartz. This type of modelling allows investigation, at least qualitatively, of the fine scale details of melt production as a function of changes in pressure, temperature and bulk composition. Received: 29 November 1995 / Accepted: 22 April 1996     

三元体系NaVO3-H2NCONH2-H2O 298K相平衡及平衡液相物化性质研究

采用等温平衡溶解平衡法测定了三元体系NaVO3-H2NCONH2-H2O 298K下的溶解度及平衡液相物化性质(密度、电导率、pH值),绘制出了相应的溶解度等温图和物化性质一组成图。此三元体系等温溶解度图属简单共饱型,未产生新的复盐和固溶体。2个结晶区分别对应为盐NaVO3和H2NCONH2,无变量点平衡液相组成为：ω(NaVO3)3．99％,ω(H2NCONH2)40．17％,对应的平衡固相盐为NaVO3和H2NCONH2。     

Amphibole composition as an indicator of subtle grade variation in epidote-glaucophane schists

Abstract The paragenetic relations of epidote-glaucophane schists are described in terms of the system Al₂O₃-Fe₂O₃-Fe₂O₃-MgO-CaO with excess of quartz, albite and epidote. If alkali-amphibole is free from Ca and Al^IV, its composition when associated with epidote is invariant, univariant or divariant at a given pressure and temperature on Miyashiro's (1957) diagram of alkali-amphibole solid solution if it is also associated, respectively, with three, two or one additional minerals in the system.
Using a group of epidote-glaucophane schists from the Kotu area of the Sanbagawa metamor-phic belt in Shlkoku, Japan (isophysical compositional),univariant boundary lines were determined for the assemblages that, in addition to the ubiquitous quartz + albite + phengitic mica, contain hematite + chlorite, garnet + chlorite and actinolite + chlorite, respectively. The slopes of the univariant boundary lines obtained from petrographical data are in good agreement with those calculated in a model system.
The positions of isophysical univariant boundary lines on the amphibole compositional diagram serve to distinguish the grade of metamorphism among the rocks of the same mineral facies. The hematite-chlorite univariant boundary line can be used to divide the zone of epidote-glaucophane schists of the Sanbagawa metamorphic belt into three, and the garnet-chlorite-paragonite invariant equilibrium can be used to divide the epidote zone of New Caledonia into three.     

Theoretical phase relations involving cordierite and garnet in the system MgO-FeO-Al2O3-SiO2

Theoretical stability relations have been derived between the phases cordierite (Cd), garnet (Ga), hypersthene (Hy), olivine (Ol), sapphirine (Sa), spinel (Sp), sillimanite (Si) and quartz (Qz) in the system MgO-FeO-Al₂O₃-SiO₂. Natural rock data and experimental evidence suggest that the Mg/Mg+Fe²⁺ ratio (X) of coexisting ferromagnesian phases decreases as follows: X _Cd>X _Sa>X _Hy>X _Ol>X _Sp>X _Ga. By use of this information four stable invariant points are proposed involving the phases: Cd, Hy, Sa, Ga, Si, Qz; Cd, Sa, Ga, Sp, Si, Qz; Cd, Hy, Sa, Ga, Sp, Qz; Cd, Ga, Hy, Ol, Sp, Qz. All univariant curves in the system are nonterminal, representing the breakdown of a join rather than the stability limit of an individual phase. A detailed treatment of divariant equilibria involving two and three ferromagnesian solid solutions illustrates the potential of these equilibria as Pressure-Temperature indicators. Interactions between solid-solid reactions and dehydration reactions involving biotite in the system MgO-FeO-Al₂O₃-SiO₂-K₂O-H₂O have been graphically analysed. The addition of biotite to anhydrous divariant assemblages does not affect the composition of coexisting phases at constant P and T but can affect their relative proportions.     

Theoretical phase relations involving cordierite and garnet revisited: the influence of oxygen fugacity on the stability of sapphirine and spinel in the system Mg-Fe-Al-Si-O

The theoreticalP-T grid for stability relations of the phases cordierite (Cd), sapphirine (Sa), hypersthene (Hy), garnet (Ga), spinel (Sp), sillimanite (Si), and quartz (Qz) of Hensen (1971), has proved useful in the interpretation of metamorphic mineral assemblages formed at low oxygen fugacity. Both experimental data and evidence from natural rocks indicate that at high oxygen fugacity compatability relations change as a result of the enlargement of the stability field of spinel, which causes a topological inversion and the stabilisation of the invariant points [Sa], [Ga], and [Cd]. This implies the stable existence of the univariant equilibria (for buffered conditions): Sp+Qz=Ga+Hy+Si+O₂ (Sa, Cd), Cd+Sp+Qz=Hy+Si+O₂ (Sa, Ga) and Sa+Sp+Qz=Hy+Si+O₂ (Ga, Cd) and the divariant reaction: Sp+Qz=Hy+Si+O₂ (Sa, Ga, Cd). These redox equilibria are restricted to conditions of high oxygen fugacity. The proposed theoreticalP-T grids, for both low and high oxygen fugacity, satisfactorily explain all experimental data and metamorphic mineral assemblages so far found in granulites.