Effects of progressive mylonitization on Ar retention in biotites from the Santa Rosa mylonite zone,California, and thermochronologic implications

The Santa Rosa mylonite zone developed predominantly from a granodiorite protolith in the eastern margin of the Peninsular Ranges batholith. A wide variation in K−Ar biotite dates within the zone is shown to reflect the times of cooling through closure temperatures whose variability is chiefly a result of deformation-induced reduction in grain size. We suggest that such variation generally may be exploited to place constraints on the timing of deformation episodes. Previous workers have shown that deformation in the Santa Rosa mylonite zone involved the formation of mylonites and an imbricate series of low-angle faults which divide the area into structural units. Field work, petrographic studies, and TEM analysis of deformation mechanisms in biotite show that the granodiorite mylonite, the lowermost structural unit, formed below the granodiorite solidus. The granodiorite mylonite varies from protomylonite to ultramylonite, with regions of high strain distributed heterogeneously within the zone. Previously reported biotite and hornblende K−Ar dates from the granodiorite protolith below (82–89 Ma) and the Asbestos Mountain granodiorite above (61–68 Ma) the mylonite zone indicate dramatically dissimilar thermal histories for the lowermost and uppermost structural units. Other workers' fission track dates on sphene, zircon, and apatite from the granodiorite mylonite and the Asbestos Mountain granodiorite suggest thermal homogenization and rapid cooling to ∼100° C by ca 60 Ma. Within and adjacent to the mylonite zone, K−Ar dates on 5 samples of biotite from variably deformed granodiorite range from 62–76 Ma; dates are not correlated with structural depth but clearly decrease with degree of deformation and concomitant grain size reduction. ⁴⁰Ar/³⁹Ar incremental heating analyses of biotite from the granodiorite protolith reveals no evidence of excess argon and produces a relatively flat age spectrum. ⁴⁰Ar/³⁹Ar incremental heating analysis of biotite from the granodiorite mylonite discloses discordance consistent with ³⁹Ar recoil loss. K analysis of samples, allowing K−Ar dates to be calculated, is therefore recommended as an adjunct to ⁴⁰Ar/³⁹Ar step heating analysis in rocks that have experienced similar deformation. During mylonitization, biotite grain size reduction through intracrystalline cataclasis results in estimated grain dimensions as small as 0.05 μm locally within porphyroclasts as large as 1 mm. Because biotite compositions are relatively Uniform (Fe/[Fe+Mg+Mn+Ti+Al^VI]=0.47–0.52) and show no systematic variation with grain size, compositional dependence of activation energy and diffusivity can be eliminated as sources of variation in Ar retention. Ar closure temperatures, calculated with appropriate diffusion parameters for the observed grain sizes, are in the range ∼220–280° C and define a cooling curve consistent with a thermal history intermediate between those of the granodiorite protolith below and the Asbestos Mountain granodiorite above the mylonite zone. Changes in the slope of the cooling curve indicate that the main deformation episode initiated at or above ca 330° C (∼80 Ma), above the closure temperature for thermally activated diffusion of argon in biotite, and continued to a minimum of ca 220–260° C (∼62 Ma).     

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     

Chlorine,titanium and barium-rich biotites: factors controlling biotite composition and the implications for garnet-biotite geothermometry

Barium-, Cl- and Ti-rich biotite occurs together with garnet, plagioclase and amphibole within narrow shear zones in 1800 Ma old noritic granulites in the Flakstadøy Basic Complex, Lofoten, north Norway. The granulite facies assemblage, plagioclase, clinopyroxene, orthopyroxene, biotite and ilmenite, was replaced by an amphibolite facies mineral assemblage including Ba-, Cl- and Ti-rich biotite during ductile deformation. Biotite shows complex compositional variations with respect to the contents of Ba, K, Cl, Ti, Al, Fe, Mg and Si. There are correlations between Si, Al^IV, K, Ba and Cl and between Al^VI and Ti. Titanium and Cl are uncorrelated. The Fe and Mg are correlated to both Cl and Ti. Multivariate analysis shows that most of the compositional variation of biotite can be described by two exchange reactions. This indicates that most of the variation in biotite composition was controlled by two chemical variables of the system. The content of the first exchange component (Ti_1.0 Fe_0.6 Al _-1.1 ^VI Mg_-0.8) in biotite can be related to the original distribution of Ti-bearing minerals in the igneous protolith. The content of the second exchange component (Al _0.4 ^IV Fe_0.8 Ba_0.5 Cl_1.0 Si_-0.4 Mg_-1.0 K_-0.5 OH_-1.0) is related to compositional variations of an externally derived Ba- and Cl-bearing fluid in equilibrium with biotite.The initially low Cl-content of the externally derived fluid was increasing during bioite forming reactions, because OH was preferentially incorporated, relative to Cl, into biotite. Continued hydration/chloridisation reactions resulted in a gradual consumption of the free fluid phase, resulting in local fluid-absent conditions. The composition of biotite reflects the composition of the last fluid in equilibrium with the mineral, i.e. the composition of the fluid immediately before the grain boundaries were fluid-undersaturated. Thus, the variations in biotite composition reflect how the fluid was gradually consumed throughout the shear zone rock. The correlations between Fe, Mg, Ba, K and Cl can be attributed to differences between the structure of the crystal lattices and the sizes of the cation sites of OH-phlogopite and Cl-annite. The dependency of the Fe/Mg ratios of biotite on the Cl-and Ti-content has a strong effect on the Fe–Mg partitioning between biotite and garnet. The relationship between lnK_D, X _Ti ^Bt and X _Cl ^Bt can be expressed by the regression equation: lnK _D =-1.82+2.60X _Ti ^Bt +5.67X _Cl ^Bt     

Actinolite-hornblende pairs in metamorphosed gabbros,Hidaka mountains,Hokkaido

Actinolite-actinolitic hornblende and actinolitic hornblende-hornblende pairs are described from gabbroic amphibolites and epidote amphibolites formed by dynamic metamorphism during uplift of gabbroic rooks in the Hidaka Metamorphic Belt, Hokkaido. Electron microprobe analyses indicate that coupled substitutions involved in the transition from actinolite to hornblende are essentially those of edenite and tschermakite-ferritschermakite together with smaller amounts of glaucophane-riebeckite, i.e. Al^IV, Al^VI, Fe³⁺, A-site occupancy and Na^M4 increase with replacement of Mg by Fe²⁺+ Mn and Si by Al^IV. During metamorphism the amount of deformation due to shearing has affected the degree of compositional discontinuity in the actinolite-hornblende series and the compositional gap is most pronounced in the epidote amphibolite. The coexisting actinolite-hornblende do not represent an equilibrium pair as textural relations indicate that the actinolitic amphiboles are relics. It is suggested that shearing deformation during uplift has caused an overstepping of the changing physico-chemical conditions of metamorphism so that compositional readjustment of amphiboles was not achieved. Equilibrium-disequilibrium actinolite-hornblende pairs are discussed from other localities where rocks of basaltic composition have been metamorphosed.     

西藏日土县拉热拉新花岗岩体特征及构造环境

拉热拉新花岗岩体属于唐古拉-东达山花岗岩带西段羌塘花岗岩区的组成部分,岩石类型主要为黑云角闪闪长岩、黑云角闪石英闪长岩、黑云角闪石英二长岩、黑云花岗闪长岩、黑云二长花岗岩、黑云花岗岩。其地球化学特征表现为：富Si,K,贫Ti,Mg,Fe,Ca,铝饱和指数ACNK〈1．1,稀土元素球粒陨石标准化图式呈右倾斜的“V”字型,LREE富集,HREE相对亏损,铕亏损。其形成时代为早白垩世,为多期次同碰撞的同熔型花岗岩。     

Processes of regional metamorphism and ultrametamorphism and some problems of the geology of the deep-lying sections of folded zones

On the basis of 135 pairs of chemical analyses of coexisting hornblendes and biotites, we have established a relationship between the contents of Al^IV, Al^VI, Fe³⁺, Mg, Ti, Na, and K and the overall iron index in the hornblendes and the depth of granitoid formation. This relationship has been emphasized by the R-method of factor analysis. We have examined the strength and nature of the correlations between the elements in the hornblendes and have considered the types of Isomorphism in the amphiboles according to depth, from the viewpoint of crystal chemistry. A regular increase in the amounts of A^IV in hornblende from <0.8 to > 1.6 formula units; of (Al^VI + Fe³⁺ + Ti) from <0.5 to >1. 0 formula units; of (K + Na) from <0.35 to >0. 64 formula units; and of Group A from <0.24 to >0.51 formula units has been recorded from the near-surface granitoids to the ultra-abyssal types. Biotites In this respect display no adequately clear and reliable information.—Authors.     

A Comparison of amphiboles from medium- and low-pressure metabasites

A comparison of published metabasite amphibole analyses from medium and low-pressure metamorphic terrains reveals that there is no systematic variation in Na, Na^M4, Al or Al^VI as a function of pressure. This may be due to blurring of the differences by variation in oxidation state, or by analytical differences between laboratories. It is not due to variable Mg/Fe in whole rocks. Differences that can be recognised are generally higher Ti/Al ratios in the low-pressure amphiboles, and a very poorly developed compositional gap between actinolite and hornblende compared with a well-developed gap at medium pressures. These features, together with the relatively low-grade appearance of calcic plagioclase at low pressures, provide the best means of distinguishing metabasites from the two facies series.All three features can be explained by the configuration of cation-exchange equilibria at the greenschist/amphibolite facies boundary. Enrichment in Ti at low-pressures is due to the positive slope of reactions partitioning Ti into the amphibole. The composition gap in amphiboles at medium-pressure is due to overstepping of the tschermakite-enriching equilibrium. At low pressures this overstepping still occurs, but the equilibrium tschermakite-content in the amphibole is much lower for a given amount of overstepping. The relatively low-grade appearance of oligoclase at low pressures is due to convergence of the tschermakite and anorthite-enriching equilibria with decreasing pressure.     

Evolution of Hutti-Maski greenstone belt of the Eastern Dharwar Craton: Evidence for metamorphic and hydrothermal phases from the Hira-Buddini deposit,Raichur district,Karnataka, India

The Hira-Buddini gold deposit is located along the steeply dipping ENE trending sheared contact of felsic and mafic rocks of strike length of about 600 m with mylonitic foliation parallel to the S1 schistosity in amphibolites. Second-generation open folds with axial planes (S2) marked by fractures that are often filled by later calcite veins are observed in surface and underground exposures. Garnetiferous amphibolites occur in patches on the footwall side of the shear in the western part of the deposit. This rock shows garnet porphyroblasts, coarse second-generation hornblende and large grains of biotite that grow over an early S1 fabric which is made up of early hornblende, plagioclase, ilmenite and retrograde first-generation chlorite. Second-generation hornblende and biotite grains make high angles to S1 schistosity and are sub-parallel to S2. Late hydrothermal alteration is marked by an albite-epidote-chlorite-zoisite assemblage. Geothermometric estimates based on garnet-biotite, and garnet-hornblende pairs, as well as Ti in biotite, show that temperatures during D2 deformation that led to the growth of the porphyroblasts were \(530{\pm }20^{\circ }\hbox {C}\). The fabric and mineralogy of the rock indicate that porphyroblastic growth of garnet, hornblende and biotite was preceded and succeeded by stages of hydrothermal alteration. Primary gold mineralization is inferred to be associated with the early stage of hydrothermal ingress.     

Amphiboles and biotites of the hornblende gneisses in an area to the North-East of the fjord Qagssit,Frederikshåb district,South-West Greenland

Summary The chemical characteristics of coexisting biotites and hornblendes and host rocks are examined and discussed. The field data indicate that biotite derives from hornblende at decreasing metamorphism. A chemical equilibrium has been reached for Fe²⁺, Mg and Mn and approached, but not reached, for tetrahedral ions, Fe³⁺, Al^vi and Ti. The disequilibrium is mainly dependent on biotite and is tentatively ascribed to the oxidizing conditions of the environment, at the time of biotite crystallization. The composition of biotite is thus partly related to that of hornblende and partly to the environment, while intracrystalline variations of hornblende are related to the chemical composition of the host rock and to varying metamorphic grade.

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Amphibole und Biotite der Hornblendegneise in einem Gebiet nordöstlich des Qagssit-Fjordes, Frederikshåb-Distrikt, Südwest-Grönland

Zusammenfassung Der Chemismus koexistierender Biotite und Hornblenden sowie von deren Muttergesteinen wird untersucht und diskutiert. Die Felddaten weisen darauf hin, daß der Biotit aus Hornblende bei abnehmender Metamorphose entsteht. Ein chemisches Gleichgewicht wurde für Fe²⁺, Mg und Mn erreicht; für die tetraedrisch koordinierten Ionen sowie Fe³⁺, Al^vi und Ti fand eine Annäherung in seine Richtung statt, es wurde aber nicht erreicht. Das Ungleichgewicht hängt im wesentlichen mit dem Biotitgehalt zusammen, und es wird versuchsweise den Oxidationsbedingungen der Umgebung bei der Biotitkristallisation zugeschrieben. Die Zusammensetzung des Biotits hängt folglich teilweise mit jener der Hornblende und teilweise mit der Umgebung zusammen, während die intrakristallinen Variationen der Hornblende von der chemischen Zusammensetzung des Muttergesteins und dem variierenden Metamorphosegrad abhängen.

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With 7 Figures     

Authigenic layer silicate minerals in borehole Elmore 1, Salton Sea Geothermal Field,California, USA

A combined petrographic/X-ray/electron microprobe and energy dispersive system investigation of sandstone cuttings from borehole Elmore # 1 near the center of the Salton Sea Geothermal Field has revealed numerous regular variations in the composition, texture, mineralogy and proportions of the authigenic layer silicate minerals in the temperature interval 185° C (411.5 m depth) to 361° C (2,169 m). At temperatures near 190° C, dolomite/ankerite+calcite-bearing sandstones contain an illite/mixed layer phase with 10% expandable layers (dolomite/ankerite zone). In shale, the percentage of expandable layers in the mixed layer phase changes from 10–15% at 185° C to 5% at 210° C (494 m). In the interval 250° C (620 m) to 325° C (1,135 m), the calcite+pyrite+epidote-bearing sandstones contain a layer silicate assemblage of chlorite and illite (chlorite-calcite zone). In the shallower portions of this metamorphic zone, the illite contains 0–5% expandable layers, while at depths greater than 725 m (275° C) it is completely free of expandable layers. On increasing temperature, the white mica shows regular decreases in Si^IV, Mg and Fe, and increase in Al^IV, Al^VI, and interlayer occupancy, as it changes gradually from fine-grained illite (=textural sericite) to coarse-grained recrystallized phengitic white mica. In the same interval, chlorite shows decreases in Al^VI and octahedral vacancies and an increase in total Mg+Fe. The sandstones range from relatively unmodified detrital-textured rocks with porosities up to 20% and high contents of illite near 250° C to relatively dense hornfelsic-textured rocks with trace amounts of chlorite and phengite and porosities near 5% at 325° C. Numerous complex reactions among detrital (allogenic) biotite, chlorite, and muscovite, and authigenic illite and chlorite, occur in the chlorite-calcite zone.Biotite appears, and calcite disappears, at a temperature near 325° C and a depth of 1,135m. The biotite zone so produced persists to 360° C in sandstone, at which temperature orthoclase disappears and andradite garnet appears at a depth near 2,155 m. Throughout the biotite zone and into the garnet zone, the biotite undergoes compositional changes that are very similar to those observed in illite/phengite in the chlorite-calcite zone, including increases in interlayer occupancy, Al^IV, Al^VI, and Ti, and decreases in F^–, Si^IV, and Mg/Fe_t+Mg, on increasing temperature. Biotite thus changes from a siliceous, K-deficient biotite at the biotite isograd to a typical low-grade metamorphic biotite at temperatures near 360° C. Minor amounts of talc appear with biotite at the biotite isograd in sandstone, while actinolite appears in both sandstone and shale at temperatures near 340° C (1,325 m). Chlorite completely disappears from sandstone at temperatures of approximately 350° C (1,500 m), and diminishes abruptly in amount in the more chloritic shales at the same depth.     

Coarsening kinetics of the exsolution microstructure in alkali feldspar

Actinolite, hornblende and biotite coexisting in greenschist mafic metagreywackes have been analysed with the electron microprobe to obtain information on their chemical relationship during metamorphism. As in some other parts of the world, the two calcic amphiboles coexist in the greenschist facies because of a miscibility gap between them which is observed under conditions of low-pressure regional metamorphism; it is thought that the two amphiboles are in equilibrium, or at least that the actinolite participated in hornblendeforming reactions. Contact metamorphism by granitic intrusives of these metagreywackes has converted them to hornblende hornfelses with the assemblage hornblende, andesine, quartz, biotite±cummingtonite; the hornblendes of the hornfelses are found to have compositions between actinolite and hornblende of the greenschists, and frequently show fine exsolution lamellae of cummingtonite as a result of oversaturation in this component. The distribution of Fe-Mg between hornblende and biotite changes from the greenschist to the hornblende hornfels facies, and the K _D is probably dependent on Al^VI in the hornblende.     

Relationship between the compositions and unit-cell parameters of biotites from the Taiping-Huangshan batholith in southeast China

The Taiping-Huangshan batholith in southeast China is a polygenetic composite granitoid consisting of two parts. The northern part is known as the Taiping granodiorite, genetically designated to the Syntexis-type granitoids of southeast China^[1] or I-type^[4]. Based on whole-rock Rb-Sr isochron dating, its age is 22 3 m. y. with a relatively low initial Sr ratio (0.7069), suggesting that it was emplaced during the Indosinian period. The southern part, the Huangshan multi-staged granite, is genetically ascribed to the transformation type of southeast China, or S-type, which includes two bodies known as the Huangshan and Shizilin granites. Both were formed in the late Yenshanian period as indicated by their ages: 131 m.y. for the Huangshan granite and 123 m. y. for the Shizilin granite, and high initial Sr ratios (0.714 for the Huangshan granite and 0.718 for the Shizilin granite) determine using whole-rock Rb-Sr isochron dating method. The biotite in the Taiping granodiorite is Mg-biotite with a high Mg/(Mg +ΣFe + Mn +^VIAl³⁺) ratio (> 0.42) in its octahedral layers, and has a longerc-axis (10.220–10.224 Å) and a larger unit-cell volume (499.5 ų), as calculated from X-ray powder diffraction data But the biotites in the Huangshan and Shizilin granies occur mostly as siderophyllite, subsequently as Fe-biotite and occasionally as zoned mica consisting of siderophyllite and musco-vite. The Mg/(Mg +ΣFe + Mn +^VIAl³⁺) ratios are low, while^VIAl³⁺/(Mg+ΣFe+Mn+^VIAl³⁺) ratios high in their octahedral layers. They have a shorterc-axis (10.128–10.196A) and a small nit-cell volume (494.5 ų on average). Comparison of their chemical compositions and corresponding unit-cell parameters for 13 samples from the batholith indicates that thec-axis length increases with increasing Mg content with a noticea ble positive correlation between them, but decreases with increasing^VAl³⁺ content, showing an obvious negative correlation. The changes of theb-axis lengths with the compositions of biotites are much less pronounced than that of thec-axis length.     

福建紫金山矿田罗卜岭铜钼矿化斑岩锆石LA-ICP- MS U-Pb年龄及成矿岩浆高氧化特征研究

罗卜岭斑岩铜钼矿床是紫金山Cu-Au-Mo浅成低温-斑岩矿田内新近发现的大型斑岩铜钼矿床,本文在岩芯及光薄片系统观察的基础上,分析了矿化斑岩锆石LA-ICP-MS U-Pb年龄及锆石Ce4/Ce3+比值.罗卜岭赋矿斑岩体可分为两期,早期为角闪黑云母花岗闪长斑岩及黑云母花岗闪长斑岩,晚期为黑云母花岗闪长斑岩.早期角闪黑云母花岗闪长斑岩和黑云母花岗闪长斑岩锆石LA-ICP-MS U-Pb年龄分别为103.7±1.2Ma,MSWD=0.33和103.0±0.9Ma,MSWD=1.00;晚期黑云母花岗闪长斑岩锆石LA-ICP-MS U-Pb年龄为97.6±2.1Ma,MSWD=6.00.罗卜岭成矿斑岩基质普遍发育硬石膏,两期成矿斑岩锆石都具较高的Ce4 +/Ce3平均值,在630 ～770之间,高于区内非成矿花岗岩锆石的Ce4+/Ce3+平均值(182 ～577),显示罗卜岭斑岩矿床成矿岩浆具有高氧逸度的特征.据罗卜岭斑岩矿床的形成时代、高氧逸度岩浆特征,结合华南地区中生代构造背景,我们初步认为罗卜岭斑岩矿床的形成可能和中生代古太平洋向北西西方向俯冲有关.     

Coexisting hornblendes and biotites from Precambrian gneisses of the south coast of Western Australia

The compositions of coexisting hornblendes and biotites from amphibolite and granulite facies gneisses from the south coast of Western Australia were controlled by host rock composition, paragenesis, metamophic grade, pressure, and oxygen fugacity. The Mg/(Mg + Fe²⁺) and Mn/Fe²⁺ ratios in both minerals and possibly the Al^vi contents of the hornblendes are related to host rock compositions. Metamorphic grade appears to influence, perhaps only indirectly, the Ti, Mn, and Fe³⁺ contents of both minerals and possibly the hornblende Ca content. The higher Ti and lower Mn contents of the granulite facies hornblendes and biotites are attributed to their coexistence with pyroxenes, whereas their lower Fe³⁺/(Fe²⁺ + Fe³⁺) ratios are probably due to lower oxygen fugacity in the granulite facies environment. Grade-related colour variations in both minerals were controlled by their Ti/Fe²⁺ and Fe³⁺/(Fe²⁺ + Fe³⁺ ratios. The relatively low Al^vi contents of the hornblendes suggest low- to moderate-pressure metamorphism.Variations in element distribution coefficients are related to variations in mineral compositions rather than metamorphic grade. Thus K_{D(Al^iv ?Si)} is related to the Al^iv andedenite alkali contents of the hornblendes, K_{D(Fe²⁺ ?Mg)} to the distributions of Al^iv ?Si and Al^vi + Ti + Fe³⁺, K_D(Mn) to the Mn contents of both minerals, and K_D(Al^vi) to the Al^vi contents of the biotites.     

Proterozoic anticlockwise P–T path of the Lewisian Complex of South Harris, Outer Hebrides, NW Scotland

The Leverburgh Belt and South Harris Igneous Complex in South Harris (northwest Scotland) experienced high-pressure granulite facies metamorphism during the Palaeoproterozoic. The metamorphic history has been determined from the following mineral textures and compositions observed in samples of pelitic, quartzofeldspathic and mafic gneisses, especially in pelitic gneisses from the Leverburgh Belt: (1) some coarse-grained garnet in the pelitic gneiss includes biotite and quartz in the inner core, sillimanite in the outer core, and is overgrown by kyanite at the rims; (2) garnet in the pelitic gneiss shows a progressive increase in grossular content from outer core to rims; (3) the Al^VI/Al^IV ratio of clinopyroxene from mafic gneiss increases from core to rim; (4) retrograde reaction coronas of cordierite and hercynite+cordierite are formed between garnet and kyanite, and orthopyroxene+cordierite and orthopyroxene+plagioclase reaction coronas develop between garnet and quartz; (5) a P–T path is deduced from inclusion assemblages in garnet and from staurolite breakdown reactions to produce garnet+sillimanite and garnet+sillimanite+hercynite with increasing temperature; and (6) in sheared and foliated rocks, hydrous minerals such as biotite, muscovite and hornblende form a foliation, modifying pre-existing textures. The inferred metamorphic history of the Leverburgh Belt is divided into four stages, as follows: (M1) prograde metamorphism with increasing temperature; (M2) prograde metamorphism with increasing pressure; (M3) retrograde decompressional metamorphism with decreasing pressure and temperature; and (M4) retrograde metamorphism accompanied by shearing. Peak P–T conditions of the M2 stage are 800±30 °C, 13–14 kbar. Pressure increasing from M1 to M2 suggests thrusting of continental crust over the South Harris belt during continent–continent collision. The inferred P–T path and tectonic history of the South Harris belt are different from those of the Lewisian of the mainland.     

Symplectite breakdown of Ca-rich almandines in upper amphibolite-facies Skagit Gneiss,North Cascades,Washington

In upper amphibolite-facies Skagit Gneiss, certain rocks, usually carrying hornblende, display post-kinematic breakdown of almandine-rich garnets to symplectite consisting of plagioclase plus biotite and/or hornblende. Other almandine-rich garnets, widespread in hornblende-free schists and gneisses, remained stable to the end of the metamorphic cycle. Analysed garnets with symplectite coronas have 14 to 30 mol.% grandite. A set of garnets free of such coronas have 6 to 9% grandite and contain relatively more pyrope.In all symplectite-bearing rocks selected for mineral analyses, symplectite plagioclase is more calcic than the earlier main-fabric plagioclase. Fe and Mn are higher, and Mg and Ti lower, in symplectite than in main-fabric biotite and hornblende, except where late re-equilibration has been locally accomplished. Main-fabric biotite, hornblende, and cummingtonite partly preserve a record of original element partitioning between these phases and garnet, indicating that equilibrium was approached during the essentially syn-kinematic main stage of the metamorphism. Between the main-stage phases and their post-kinematic symplectite counterparts, equilibrium has rarely been attained, despite high T and presence of H₂O. Instead, there are sequences of arrested chemical exchanges and of highly incomplete attainments of successive equilibria. The principles revealed by this study are thought to be more broadly applicable to questions of equilibrium vs. disequilibrium in regional-metamorphic sequences.Combined with petrographic-petrologic data, the mineral analyses permit one to calculate model equations that quantitatively describe symplectite-forming reactions, including the amounts of materials added to and removed from garnets transformed into symplectites. Na, K, minor Ba, H₂O, and part of the Ti present were added; major portions of the garnets' Fe and of their subordinate Mn were removed. Transfers of Mg, Al, and Si during symplectite formation were minor, if any. Source and disposal of added and removed substances are discussed. The mass balance of symplectite formation ranges from significant losses in all hornblende-bearing reactions examined to minor gains in some only-biotite-bearing reactions. V is positive for all reactions examined and ranges from a few per cent for subordinate only-hornblende-bearing symplectites to somewhat over 50% for certain only-biotite-bearing symplectites; intermediate values are obtained for symplectites carrying both hornblende and biotite.     

Zr contents of glaucophane-bearing meta-basalts of Western Crete,Greece

Chemical analyses of hornblendes from different regional metamorphic terrains and from rocks of different metamorphic grade have been compared. Hornblendes of lowpressure type are distinguished from hornblendes of high-pressure type by their Al^VI and Si contents. The Ti content of hornblende is shown to increase with metamorphic temperature from the greenschist-amphibolite transition facies to the hornblende-granulite facies.     

对混合花岗岩的质疑——论中国大别山地区天堂寨片麻状花岗岩的成因

天堂寨片麻状花岗岩,侵位于大别群变质岩系。本文从岩相学、岩石化学、稀土元素地球化学、矿物化学、捕虏体以及矿物之间平衡关系论证了该岩体并非混合交代成因,而是壳源岩浆结晶产物,属I′_u型花岗岩。片麻构造形成与应力作用和塑性变形有关。     

Biotite and garnet compositional variation and mineral equilibria in Grenville gneisses of the Otter Lake area, Quebec

Garnet-biotite gneisses, some of which contain sillimanite or hornblende, are widespread within the Otter Lake terrain, a portion of the Grenville Province of the Canadian Shield. The metamorphic grade is upper amphibolite to, locally, lower granulite facies. The atomic ratio Fe²⁺/(Fe²⁺+ Fe³⁺) in biotite ranges from 0.79 to 0.89 (ferrous iron determinations in 10 highly pure separates), with a mean of 0.86. Mg and Fe²⁺ atoms occupy 67–78% of the octahedral sites, the remainder are occupied by Fe³⁺, Ti, and Al, and some are vacant. Mg/(Mg + Fe²⁺), denoted X, in the analysed samples ranges from 0.32 to 0.65. Garnet contains 1–24% grossular, 1–12% spessartine and X ranges from 0.07 to 0.34. Compositional variation in biotite and garnet is examined in relation to three mineral equilibria: (I) biotite + sillimanite + quartz = garnet + K-feldspar + H₂O; (II) pyrope + annite = almandine + phlogopite; (III) anorthite = grossular + sillimanite + quartz. Measurements of X (biotite) and X (garnet) are used to construct an illustrative model for equilibrium (I) which relates the observed variation in X to a temperature range of 70°C or a range in H₂O activity of 0.6; the latter interpretation is preferred. In sillimanite-free gneisses, the distribution of Mg and Fe²⁺ between garnet (low in Ca and Mn) and biotite is adequately described by a distribution coefficient (KD) of 4.1 (equilibrium II). The observed increase in the distribution coefficient with increasing Ca in garnet is ln K_D= 1.3 + 2.5 × 10^?2 [Ca] where [Ca] = 100 Ca/(Mg + Fe²⁺+ Mn + Ca). The distribution coefficient is apparently unaffected by the presence of up to 12% spessartine in garnet. In several specimens of garnet-sillimanite-plagioclase gneiss, the Ca contents of garnet and of plagioclase increase in unison, as required by equilibrium (III). The mean pressure calculated from these data (n= 17) is 5.9 kbar, and the 95% confidence limits are ±0.5 kbar.     

The Garnet+Hornblende Isograd in Calcic Schists from an Andalusite-Type Regional Metamorphic Terrain, Panamint Mountains, California

Calcic schists in the andalusite-type regional metamorphic terrainin the Panamint Mountains, California, contain the low-varianceassemblage quartz+epidote+muscovite+biotite+calcic amphibole+chlorite+plagioclase+spheneat low grade. Near the sillimanite isograd, chlorite in thisassemblage is replaced by garnet. The low variance in many calcicschists allows the determination of the nature of the reactionthat resulted in the coexistence of garnet+hornblende. A graphicalanalysis of the mineral assemblages shows that the reactioncan not be of the form biotite+epidote+chlorite+plagioclase+quartz=garnet+hornblende+muscovite+sphene+H₂Obecause garnet+chlorite never coexisted during metamorphismand the chlorite-bearing and garnet-bearing phase volumes donot overlap. The compositions of the minerals show that withincreasing grade amphibole changed from actinolite to pargasitichornblende with no apparent miscibility gap, the partitioningof Fe and Mg between chlorite and hornblende changed from K_D(Mg/Fe, chl&amp) < 1 to K_D > 1, the partitioning betweenbiotite and hornblende changed from K_D (Mg/Fe, bio/amp) <1 in chlorite-zone samples to K_D > 1 in garnet + hornblende-zonesamples, and the transition to the garnet-bearing assemblageoccurred when the composition of plagioclase was between An₅₅and An₈₀. Both the graphical analysis and an analytical analysisof the compositions of the minerals using simplified componentsderived from the natural mineral compositions indicate thatat the garnet+hornblende isograd the composition of hornblendewas colinear with that of plagioclase and biotite, as projectedfrom quartz, epidote, muscovite, and H₂O. During progressivemetamorphism, chlorite+biotite+epidote+quartz continuously brokedown to form hornblende+muscovite+sphene until the degeneracywas reached. At that point, tie lines from hornblende couldextend to garnet without allowing garnet to coexist with chlorite.Thus, the garnet+hornblende isograd was established throughcontinuous reactions within the chlorite-grade assemblage ratherthan through a discontinuous reaction. In this type of isograd,the low-grade diagnostic assemblage occurs only in Mg-rich rocks;whereas the high-grade assemblage occurs only in Fe-rich rocks.This relation accounts for the restricted occurrence of garnet+hornblendeassemblage in low-pressure terrains. In Barrovian terrains,garnet+chlorite commonly occurs, and the first appearana ofgarnet+hornblende can simply result from the continuous shiftof the garnet+chlorite tie line to Mg-rich compositions.