泥质变质岩系主要的矿物温度计与压力计

简要介绍泥质变质岩中常用的温度计和压力计,对其可适用性、适用范围、质量优劣等进行了评述。经过对这些温度计和压力计的比较研究,我们发现石榴石-黑云母温度计、石榴石-白云母温度计、石榴石-蓝晶石(夕线石、红柱石)-斜长石-石英(GASP)压力计、石榴石-黑云母-斜长石-石英(GBPQ)压力计、石榴石-白云母-斜长石-石英(GMPQ)压力计、石榴石-黑云母-白云母-蓝晶石(夕线石、红柱石)-石英(GBMAQ)压力计、石榴石-金红石-钛铁矿-斜长石-石英(GRIPS)压力计的准确度较高,可以为地质工作者所采用。二云母温度计、白云母-斜长石温度计准确度还有待大幅度改进。石榴石-金红石-蓝晶石(夕线石、红柱石)-钛铁矿-石英(GRAIL)压力计、石榴石-堇青石温度计和石榴石-堇青石-蓝晶石(夕线石、红柱石)-石英(GCAQ)压力计等温压计的准确度及其可适用性,还有待于进一步研究。     

黑云母-钙质角闪石温度计初步研究

利用新近修正的石榴石- 黑云母温度计(Holdaway,2000)和石榴石- 斜长石- 角闪石- 石英压力计(Kohn and Spear,1990),根据从文献中收集的角闪岩、变泥质岩、变辉长岩和变粒玄岩的相关资料,采用经验标度方法,本文首次标度了适用于560～800℃、0.26～1.4GPa的黑云母- 钙质角闪石温度计。其中,黑云母作为对称的Fe-Mg-A1~Ⅳ -Ti四元固溶体,角闪石简化为对称 Fe Mg二元固溶体。该温度计计算温度与石榴石-黑云母温度计相差不超过±50℃,并且能准确区分     

胶东郭家岭岩体固结冷却轨迹与隆升剥蚀

胶东郭家岭岩体位于华北克拉通东南缘,其详细的冷却固结条件缺乏系统研究,而对岩体形成与金矿之间的关系也存在争议。本文利用岩浆锆饱和温度计、角闪石全铝压力计、角闪石-斜长石温度计、角闪石-黑云母温压计、二长石温度计,结合已有的锆石U-Pb年龄和角闪石、黑云母40Ar-39Ar同位素年龄,探讨郭家岭岩体的冷却固结史及其隆升机制。花岗岩QAb-Or标准矿物共结压力图解、角闪石全铝压力计及角闪石-黑云母压力计均得到岩体就位压力约4~5kbar。锆饱和温度计、角闪石-斜长石温度计、角闪石-黑云母温度计、二长石温度计所获得温度分别为726~800℃、657~717℃、650~700℃、570~580℃,结合角闪石、黑云母的40Ar-39Ar封闭温度确定了郭家岭岩体最有可能的冷却轨迹。郭家岭岩体(130~126Ma)侵位深度约13±1.6km,侵入郭家岭岩体的艾山岩体(116±2Ma)侵位压力在1kbar左右,对应深度约2.7km,表明郭家岭岩体在约10Myr内,隆升剥蚀量达10km左右。这种快速剥蚀的机制可能与华北东部中生代岩石圈减薄导致的地壳浅部隆升有关。     

泥质变质岩系石榴子石-黑云母-白云母-蓝晶石(夕线石、红柱石)-石英压力计

采用天然泥质变质岩石样品,根据经验标定方法,标定了适用于泥质变质岩石的石榴子石-黑云母-白云母-蓝晶石(夕线石、红柱石)-石英压力计(GBMAQ压力计).     

固溶体矿物的性质对计算河南西峡红柱石矿形成条件的影响

自然界的大多数造岩矿的是由两种或多种组分混合而成的固溶体。因此，在矿物，岩石和矿床等地质体的平衡热力学研究中不可避免地要涉及固溶体矿物热力学地为的问题。河南省西峡县杨乃沟红柱石矿泥质片岩中仅出现红柱石而没有蓝晶石和夕线石，红柱石矿物包裹体测温结果为420－600℃。通过矿物共生平衡p-t曲线所计算出的片岩形成温度和压力条件也应位于上述岩石学和岩相学等特征所限定的范围内。以此为基础，通过构筑矿物共生p-t平衡曲线的方法，对片岩中若干固溶体矿物理想性／非理想性问题进行了研究。其中斜长石必须以非理想固溶体处理，而石榴子石，黑云母和白云母视作理想固溶体的计算结果同片岩的岩石学和岩相学等特征相一致。     

变质作用温度与压力极限值的估算方法

变质矿物共生组合中缺少某种矿物,或者变质矿物的原始成分被不同程度地破坏,无法直接应用压力计或温度计算出具体的温度或压力数值时,可以根据实际情况,估算P-T的极限值.估算方法包括如下几类:(1)根据纯相矿物的相变条件,可以确定温度或压力的极限值.例如,根据Al2SiO5矿物相图可知,与红柱石平衡的矿物组合,稳定存在的压力不超过Al2SiO5矿物三相点的压力条件(0.375±0.025GPa);与夕线石平衡的矿物组合,稳定存在的温度不低于Al2SiO5矿物三相点的温度条件(504±20℃);(2)某些特殊矿物组合,其稳定域本身就具有温度或压力极限值的指示意义.例如,高温或超高温变质岩中的紫苏辉石+夕线石+石英组合,稳定存在的压力不低于1.05 GPa;假蓝宝石+石英组合稳定存在的温度不低于1050℃;(3)特殊矿物消失的反应线,可以限定温度极大值.例如,白云母脱水分解的反应,在中等压力条件下,不超过650℃,即与白云母+石英平衡共生的矿物组合一般不会高于650℃;(4)对于矿物固溶体发生出溶的情况,根据新生出溶页片和残留基体矿物,采用溶线温度计(二长石温度计、二辉石温度计、方解石-白云石温度计)计算出的温度数值,代表原先成分均匀的矿物固溶体出溶之前的温度极小值;(5)如果岩石中发生了退变质反应,要恢复变质作用高峰期的P-T条件,需要尽可能恢复高峰期的矿物成分,或者采用接近高峰期的矿物成分;(6)根据压力计模式反应中位于高压侧或低压侧的某种纯相矿物,可以确定压力极限值.例如,金红石位于GRAIL压力计模式反应的高压一侧.当岩石中缺少金红石时,可以人为假定岩石中存在金红石,计算出的压力为真实压力的极大值.由于压力计模式反应大多为纯转变反应,因此根据实际矿物组合和压力计模式反应,一般可以估算变质作用压力的极小值或极大值.估算变质作用P-T的极限值,方法并不限于本文所述的例子.     

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。     

广西拉么Zn—Cu矿床中夕卡岩的主要矿物特点与Ca—Fe—Si—C—O体系T—lgfO2图解

用电子探针数据研究了该矿床中主要夕卡岩矿物特点及其变化规律。采用理想结晶溶液固溶体位置混合模式,计算了石榴子石和辉石矿物对中钙铁榴石和钙铁辉石的摩尔分数。在计算纯固相和流体相参加的反应抵达平衡时的边界方程中,加上理想结晶固溶体中端元组分摩尔分数的修正项后,我们便可以计算出有纯固相、流体相和理想结晶溶液固溶体参加反应的在给定T、P条件下的lgfo_2值。计算结果表明:无变度点4周围的Hed+And+Wol组合对本矿区具有重要的地质地球化学意义。     

河南桐柏北部秦岭群中麻粒岩相变质作用及变质矿物温压条件的研究

本文提供了桐柏北部秦岭群麻粒岩中石榴石、辉石和黑云母的部分电子探针分析数据，根据岩石中矿物共生组合特征和4种地质温度计、地质压力计的计算，确定了麻粒岩相变质作用的温度、压力条件为790-850℃和0.8-1.0GPa，变质时的地热梯度为28-33℃/km，属中压麻粒岩相。     

地幔岩矿物压力计评述

适用于地幔岩石的矿物压力计有石榴石-斜方辉石压力计、石榴石-单斜辉石压力计、橄榄石-单斜辉石压力计、铬尖晶石压力计、二辉石压力计等有限几类.本文通过将这些压力计应用于岩石学相平衡实验数据,检验了其精确度.再将它们应用到天然地幔岩石样品,包括石榴石相二辉橄榄岩、尖晶石相二辉橄榄岩、石榴石-尖晶石过渡相二辉橄榄岩、含金刚石和石墨的地幔岩石,检验了其准确度.初步结论是,现有的石榴石-斜方辉石压力计(Nickel and Green,1985;Taylor,1998;Brey et al．,2008)质量相对最优,石榴石-单斜辉石压力计(Nimis and Taylor,2000;Simakov and Taylor,2000)次之.在应用这些压力计时,可配合二辉石温度计(Brey et al．,1990;Taylor,1998)或石榴石-橄榄石温度计(Wu and Zhao,2007),来同时估算平衡压力和温度.其余的压力计精确度和准确度都还很不够,需要更精确深入的实验研究来标定质量优良的压力计.     

Calibration of the garnet–biotite–Al2SiO5–quartz geobarometer for metapelites

A garnet–biotite–Al₂SiO₅–quartz (GBAQ) geobarometer was empirically calibrated using more than 700 natural metapelites with a broad compositional range of garnet and biotite under P–T conditions of 450–950°C and 1–17 kbar. In the calibration, activity models of garnet and biotite identical to those in the garnet–biotite (GB) geothermometer of Holdaway [American Mineralogist 2000, 85: 881–892] were used. Therefore, the GBAQ geobarometer and the GB geothermometer can be simultaneously applied to iteratively estimate metamorphic P–T conditions. Successful applications of the GBAQ geobarometer to natural metapelites certify its validity. Most importantly, when plagioclase is absent or CaO components in garnet and/or plagioclase are deficient, this geobarometer may prove useful for estimating metamorphic pressures. The random error of the present GBAQ geobarometer is inferred to be around ±1.8 kbar. An electronic spreadsheet is available as Table S4 to apply the GBAQ geobarometer in combination with the GB geothermometer.     

Revised empirical garnet–biotite–muscovite–plagioclase geobarometer in metapelites

The garnet–biotite–muscovite–plagioclase (GBMP) barometer was empirically revised for P–T conditions of 1–14 kbar and 450–840 °C, using 263 metapelitic rock samples from all over the world. This barometer is based on activity models for garnet, biotite and plagioclase identical to those of the well‐calibrated garnet–biotite thermometer and the garnet–aluminosilicate–plagioclase–quartz (GASP) barometer. The GBMP barometer is less temperature dependent than the GASP barometer and can be applied to either Al₂SiO₅‐absent or Al₂SiO₅‐bearing metapelites. The total error of the GBMP barometer is estimated to be about ±1.2 kbar on considering input temperature error and analytical errors of chemical compositions of the phases involved. The random error of the GBMP barometer is evenly distributed with respect to pressure, temperature and mineral composition. Simultaneous application of the GBMP barometer and the garnet–biotite thermometer identifies the correct stability field for Al₂SiO₅‐bearing metapelites. Application of the GBMP barometer to metapelitic rocks within the same geological terranes or thermal contact aureoles yielded similar pressures within error. A spreadsheet for implementing the proposed GBMP geobarometer is supplied on the journal's website.     

A model for garnet and plagioclase growth in pelitic schists: implications for thermobarometry and P-T path determinations

Numerical models of the progressive evolution of pelitic schists in the NCMnKFMASH system with the assemblage garnet + biotite + chlorite ± staurolite + plagioclase + muscovite + quartz + H₂O are presented with the goal of predicting compositional changes in garnet and plagioclase along different P-T paths. The numerical models support several conclusions that should prove useful for interpreting the P-T paths of natural parageneses: (i) Garnet may grow along P-T vectors ranging from heating with decompression to cooling with compression. P-T paths deduced from garnet zoning that are inconsistent with these growth vectors are self-contradictory. (ii) There is a systematic relation between garnet and plagioclase composition and growth such that for most P-T paths, garnet growth requires plagioclase consumption. Furthermore, mass balance in a closed system requires that as plagioclase is consumed the remaining plagioclase becomes increasingly albitic. Inclusions of plagioclase in the core of garnet should be more anorthitic than those near the rim and zoned matrix plagioclase should have rims that are more albitic than the cores. Complex plagioclase textures may arise from the local variability of growth and precipitation kinetics. (iii) A decrease of Fe/(Fe + Mg) in a garnet zoning profile is a reliable indicator of increasing temperature for the assemblage modelled. However, there is no single reliable ΔP monitor and inferences about ΔP can only be made by considering plagioclase and garnet together. (iv) Consumption of garnet during the production of staurolite removes material from the outer shell of a garnet and may make recovery of peak metamorphic compositions and P-T conditions impossible. Low ‘peak’temperatures typically recorded by staurolite-bearing assemblages may reflect this phenomenon. (v) Diffusional homogenization of garnet affects the computed P-T path and results in a clockwise rotation of the computed P-T vector relative to the true P-T path.     

Equilibria within the mineral assemblage quartz + muscovite + biotite + garnet + plagioclase,and implications for the mixing properties of octahedrally-coordinated cations in muscovite and biotite

Interaction parameters derived using empirical calibration methods indicate strong non-ideality in the mixing of octahedrally-coordinated cations in muscovite and biotite. The data set used for calibration comprises mineral compositions from 49 samples containing quartz, muscovite, biotite, garnet, plagioclase and Al₂SiO₅ (kyanite or sillimanite). Pressures and temperatures in the data set were determined through the simultaneous application of geothermometry based on the garnet-biotite FeMg₁ exchange equilibrium and geobarometry based on the anorthite-breakdown equilibrium. Two equilibria yielded simple expressions from which binary interaction parameters for octahedrally-coordinated cations in biotite could be directly determined. A four-component (Fe²⁺, Mg, Al, Ti) regular symmetric mixing model was assumed for biotite. One equilibrium yielded a simple expression from which an interaction parameter for the mixing of the MgAl-celadonite component in muscovite could be directly determined. Two sets of calculations were performed utilizing different calibrations of the garnet-biotite geothermometer and the anothite-breakdown geobarometer and different garnet activity models. Both placed samples within or near the stability field of the Al₂SiO₅ phase present in each sample and both yielded similar values for the interaction parameters within narrow uncertainties, indicating that the values are insensitive to differences in the underlying methods. Using the derived interaction parameters, activity models were formulated for the annite, phlogopite, eastonite, and siderophyllite components of biotite, and for the MgAl-celadonite component of muscovite. These were utilized for the empirical calibration of 45 fluid-independent equilibria involving unique combinations of phase components from the mineral assemblage garnet + plagioclase±biotite±muscovite±quartz. Forty-three of the equilibria may be applied as geobarometers to equilibrium assemblages of quartz + muscovite + biotite + garnet + plagioclase when care is taken to insure that applications are restricted to valid compositional ranges. For these, the calibrations yielded multiple correlation coefficients ranging from 0.953 to 0.998 and standard deviations of the residuals ranging from 597 to 118 bars.     

A Report on a Biotite-Calcic Hornblende Geothermometer

This paper presents a biotite-calcic hornblende geothermometer which was empirically calibrated based on the gamet-biotite geothermometer and the gamet-plagioclase-hornblende-quartz geobarometer, in the ranges of 560-800℃ (T) and 0.26-1.4 GPa (P) using the data of metadolerite, amphibolite, metagabbro, and metapelite collected from the literature. Biotite was treated as symmetric Fe-Mg-AlVI-Ti quaternary solid solution, and calcic hornblende was simplified as symmetric Fe-Mg binary solid solution. The resulting thermometer may rebuild the input garnet-biotite temperatures well within an uncertainty of ±50℃. Errors of ±0.2 GPa for input pressure, along with analytical errors of ?% for the relevant mineral compositions, may lead to a random error of ±16℃ for this thermometer, so that the thermometer is almost independent of pressure estimates. The thermometer may clearly discriminate different rocks of lower amphibolite, upper amphibolite and granulite facies on a high confidence level. It is assume     

Empirical Garnet-Biotite-Plagioclase-Quartz (GBPQ) Geobarometry in Medium- to High-Grade Metapelites

On the basis of the net transfer reactions among garnet, biotite,plagioclase and quartz (for both Mg and Fe end-member models),the garnet–biotite–plagioclase–quartz (GBPQ)geobarometer was empirically calibrated under physical conditionsof P = 1·0–11·4 kbar and T = 515–878°C,based on the input garnet–biotite temperatures and garnet–aluminosilicate–plagioclase–quartz(GASP) pressures of 224 natural aluminosilicate-bearing metapeliticsamples collated from the literature. The calibrations are internallyconsistent with the asymmetric quaternary solid solution modelof garnet, the symmetric quaternary solid solution model ofbiotite, and the Al-avoidance ternary solid solution model ofplagioclase in calibrating the garnet–biotite geothermometerand the GASP geobarometer. The resulting two GBPQ barometerformulae reproduce the input GASP pressures well within ±1·0kbar (mostly within ±0·5 kbar). For both aluminosilicate-bearingand aluminosilicate-absent metapelites, the two GBPQ barometryformulae yielded identical pressures, whether the sample wasincluded or not included in calibrating the GBPQ barometry.The random error of the GBPQ barometry may be expected as ±1·2kbar. The dP/dT slopes of these two GBPQ formulae are closeto that of the GASP barometer in P–T space. Applicationsof the GBPQ barometry of aluminosilicate-absent metapelitesto the rocks within a thermal contact aureole, or rocks withina limited geographical area without post-metamorphic structuraldiscontinuity, show no obvious pressure change. It may be concludedthat the two GBPQ barometry formulae derived in this study maybe used as practical tools for metamorphic pelites under theconditions of 515–878°C and 1·0–11·4kbar, in the composition range of X_gros >3% in garnet, X_an>17% in plagioclase, and     

Empirical garnet-muscovite-plagioclase-quartz geobarometry in medium- to high-grade metapelites

Based on the net transfer reactions among garnet, muscovite, plagioclase, and quartz (for both Mg and Fe end-member models), the garnet-muscovite-plagioclase-quartz (GMPQ) geobarometry was empirically calibrated under the physical conditions of P=1.0-11.4 kbar and T=505-745 °C for 128 natural metapelitic rock samples collected from the literature. The input temperatures and pressures were simultaneously determined by the garnet-biotite thermometer and the garnet-aluminosilicate-plagioclase-quartz (GASP) barometer. The GMPQ calibrations adopted the same asymmetric quaternary solid solution model of garnet and the same Al-avoidance asymmetric ternary model of plagioclase as the calibrations of the garnet-biotite geothermometer and the GASP geobarometer. A symmetric Fe-Mg-Al^VI ternary solid solution model of muscovite was adopted, and the Margules parameters of muscovite were obtained through regression. The Mg and Fe model reactions, along with the assumption of whether the ferric iron content in muscovite is 0% or 50%, resulted in four GMPQ barometry formulae. The GMPQ barometry formulae reproduce the input GASP pressures well within ±1.0 kbar (mostly within ±0.5 kbar). For both aluminosilicate-bearing and aluminosilicate-absent samples, the GMPQ barometry formulae yield identical pressures for every sample, whether the sample was included or not in calibrating the barometers. For each of the Mg or Fe model reaction, the formulae gave identical pressures within ±40 bars. The random error of the GMPQ barometry may be expected as ±1.4 kbar. The dP/dT slopes of these GMPQ formulae are close to that of the GASP barometer in the P-T space. Applications of the GMPQ barometry to aluminosilicate-absent metapelites within a limited geographic area without postmetamorphic structural discontinuity generally show no pressure difference. It may be concluded that the GMPQ barometry formulae derived in this work may be used as practical tools for metamorphic pelites under the conditions of 505-745 °C and 1-11.4 kbar, in the composition range of X_gros>3% in garnet and X_An>17% in plagioclase.     

Empirical calibration of six geobarometers for the mineral assemblage quartz+muscovite+biotite+plagioclase+garnet

Six equilibria among quartz, plagioclase, biotite, muscovite, and garnet were empirically calibrated using mineral composition data from 43 samples having the assemblage quartz+muscovite+biotite+garnet+plagioclase+Al₂SiO₅ (sillimanite or kyanite). Pressures and temperatures in the data set used for calibration were determined through the simultaneous application of garnet-biotite geothermometry and garnet-quartz-plagioclase-Al₂SiO₅ geobarometry. Thermodynamic expressions for four of the six equilibria incorporate interaction parameters that model non-ideality in the mixing of cations in the octahedral sites of both muscovite and biotite. With pressure chosen as the dependent variable, multiple regression was used to solve for unknowns in the equilibrium thermodynamic expressions. The regressions yielded multiple correlation coefficients ranging from 0.983 to 0.999, with corresponding standard deviations of 338 and 92 bars in the residuals. The standard deviations in the residuals may be explained largely or entirely by the propagation of errors associated with electron microprobe analysis. These equilibria enable the determination of pressures from equilibrium assemblages of quartz+garnet+plagioclase+muscovite+biotite, and give results closely comparable to the experimentally calibrated garnet-quartz-plagioclase-Al₂SiO₅ geobarometer. Geobarometric applications should be restricted to rocks in which equilibrium constants and compositional variables fall within the same ranges as those used for calibration.     

Cordierite-bearing schists and gneisses from Timor, eastern Indonesia: P-T conditions of metamorphism and tectonic implications

In the Boi Massif of Western Timor the Mutis Complex, which is equivalent to the Lolotoi Complex of East Timor, is composed of two lithostratigraphical components: various basement schists and gneisses; and the dismembered remnants of an ophiolite. Cordierite-bearing pelitic schists and gneisses carry an early mineral assemblage of biotite + garnet + plagioclase + Al-silicate, but contain no prograde muscovite; sillimanite occurs in a textural mode which suggests that it replaced and pseudomorphed kyanite at an early stage and some specimens of pelitic schist contain tiny kyanite relics in plagioclase. Textural relations between, and mineral chemistries of, ferro-magnesian phases in these pelitic chists and gneisses suggest that two discontinuous reactions and additional continuous compositional changes have been overstepped, possibly with concomitant anatexis, as a result of decrease in P_load during high temperature metamorphism. The simplified reactions are: garnet and/or biotite + sillimanite + quartz + cordierite + hercynite + ilmenite + excess components. P-T conditions during the development of the early mineral assemblage in the pelitic gneisses are estimated to have been P + 10 kbar and T > 750°C, based upon the plagioclase-garnet-Al-silicate-quartz geobarometer and the garnet-biotite geothermometer. P-T conditions during the subsequent development of cordierite-bearing mineral assemblages in the pelitic gneisses are estimated to have been P + 5 kbar and T + 700°C with X_H2O < 0.5, based upon the Fe content of cordierite occurring in the assemblage quartz + plagioclase + sillimanite + biotite + garnet + cordierite coexisting with melt. Final equilibration between some of the phases suggests that conditions dropped to P > 2.3 kbar and T > 600°C. A similar exhumation P-T path is suggested for the pelitic schists with early metamorphic conditions of P > 6.2 kbar and T > 745°C and subsequent development of cordierite under conditions in the range P = 3-4 kbar and T = 600-700°C. The tectonic implications of these P-T estimates are discussed and it is concluded that the P-T path followed by these rocks was caused by decompression during rifting and synmetamorphic ophiolite emplacement resulting from processes during the initiation and development of a convergent plate junction located in Southeast Asia during late Jurassic to Cretaceous time.     

The metapelitic garnet–biotite–muscovite–aluminosilicate–quartz (GBMAQ) geobarometer

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 Al^VI 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.