Calculated Phase Relations in the System NCKFMASH (Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O) for High-Pressure Metapelites   总被引：5，自引：0，他引：5  

WEI  CHUNJING; POWELL  ROGER 《Journal of Petrology》2006,47(2):385-408

Petrogenetic grids in the system NCKFMASH (Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O)and the subsystems NCKMASH and NCKFASH calculated with the softwareTHERMOCALC 3.1 are presented for the P–T range 7–30kbar and 450–680°C, for assemblages involving garnet,chloritoid, biotite, carpholite, talc, chlorite, kyanite, staurolite,paragonite, glaucophane, jadeite, omphacite, diopsidic pyroxene,plagioclase, zoisite and lawsonite, with phengite, quartz/coesiteand H₂O in excess. These grids, together with calculated compatibilitydiagrams and P–T and T–X_Ca and P–X_Ca pseudosectionsfor different bulk-rock compositions, show that incorporationof Ca into the NKFMASH system leads to many of the NKFMASH invariantequilibria moving to lower pressure and/or lower temperature,which results, in most cases, in the stability of jadeite andgarnet being enlarged, but in the reduction of stability ofglaucophane, plagioclase and AFM phases. The effect of Ca onthe stability of paragonite is dependent on mineral assemblageat different P–T conditions. The calculated NCKFMASH diagramsare powerful in delineating the phase equilibria and P–Tconditions of natural pelitic assemblages. Moreover, contoursof the calculated phengite Si isopleths in P–T and P–X_Capseudosections confirm that phengite barometry in NCKFMASH isstrongly dependent on mineral assemblage. KEY WORDS: phase relations; metapelites; NCKFMASH; THERMOCALC; phengite geobarometry  相似文献   

Ar / Ar dating of high-pressure rocks from the Ligurian Alps: Evidence for a continuous subduction–exhumation cycle     

Laura Federico  Giovanni Capponi  Laura Crispini  Marco Scambelluri  Igor M. Villa   《Earth and Planetary Science Letters》2005,240(3-4):668-680

We present ³⁹Ar–⁴⁰Ar dating of phengite, muscovite and paragonite from a set of mafic and metasedimentary rocks sampled from the high-pressure (HP) metaophiolites of the Voltri Group (Western Alps) and from clasts in the basal layer conglomerates from the Tertiary molasse which overlie the high-pressure basement. The white mica-bearing rocks display peak eclogitic and blueschist-facies parageneses, locally showing complex greenschist-facies replacement textures. The internal discordance of age spectra is proportional to the chemical complexity of the micas. High-Si phengites from eclogite clasts record a ³⁹Ar–⁴⁰Ar age of ca. 49 Ma for the eclogite stage and ca. 43 Ma for the blueschist retrogression; phengites from a blueschist basement sample yield an age of ca. 40 Ma; low-Si muscovite from a metasediment dates the formation of the greenschist paragenesis at ca. 33 Ma. Our data indicate that the analyzed samples reached high-pressure conditions at different times over a time-span of c.a. 10 Ma. Subduction was continuing during exhumation and blueschist retrograde re-equilibration of higher-pressure, eclogite-facies rocks. This process kept the isotherms depressed, allowing the older HP-rocks to escape thermal re-equilibration. Our results, added to literature data, fit a tectonic model of a subduction–exhumation cycle, with different tectonic slices subducted at different times from Early Eocene until the Eocene–Oligocene boundary.  相似文献   

Strain-related differences in the crystal growth of white mica and chlorite: a TEM and XRD study of the development of metapelitic microfabrics in the Southern Uplands thrust terrane, Scotland   总被引：2，自引：0，他引：2  

R. J. MERRIMAN  B. ROBERTS  D. R. PEACOR  S. R. HIRONS 《Journal of Metamorphic Geology》1995,13(5):559-576

Abstract TEM and XRD techniques were used to study crystal growth characteristics of the fabric-forming phyllosilicates which developed in response to low-grade metamorphism and tectonic imbrication in part of the Southern Uplands thrust terrane. Prograde regional metamorphism, ranging from late diagenesis through the anchizone to the epizone, was accompanied by the development of a slaty cleavage which is commonly bedding-parallel. TEM-measured mean thicknesses of white mica and chlorite crystallite populations increase with advancing grade and correlate with XRD-measured crystallinity indices. Analytical TEM data show that prograde changes in composition lead to a net loss of Si, Ca and minor Fe from the fabric-forming phyllosilicates. White micas are paragonite-poor phengites with a mean b lattice parameter of 9.037 Å, and indicate an intermediate pressure series of metamorphism with a field gradient of <25° C km^-1. Chlorite compositions evolved from diabantite (with intergrown corrensite) to ripidolite over an estimated temperature range of 150–320° C. Field gradient and temperature estimates suggest that crystal growth and fabric development occurred at burial depths ranging from 6 km to at least 13 km in the thrust terrane. During late diagenesis, crystal growth of white mica and chlorite was predominantly a consequence of polytypic and phase transitions, and resulted in similar size distributions which resemble typical Ostwald ripening curves. Under anchizonal and epizonal conditions, white mica grew more rapidly than chlorite because of its greater ability to store strain energy and recover from subgrain development; as a result crystal thickness distributions are not typical of Ostwald ripening. In contrast, chlorite crystals which grew under these conditions developed subgrain boundaries at high strain rates which were only partially recovered at low strain rates; these retained dislocations reduce the crystallite thicknesses detected by TEM and XRD, compared with those of white mica. These differences in strain-induced crystal growth indicate that white mica (illite) and chlorite crystallinity indices are likely to show significant differences where low-grade metamorphism is closely associated with tectonic fabric development.  相似文献   

The assemblage stilpnomelane–chlorite–phengitic mica: a geothermobarometer for blueschist and associated greenschist terranes     

Currie  & Van Staal 《Journal of Metamorphic Geology》1999,17(6):613-620

Two anhydrous equilibria can be written among the components of stilpnomelane, chlorite, white mica and quartz, namely 89 daphnite+131 Fe-celadonite+190 quartz=96 stilpnomelane+71 muscovite, and amesite+Mg-celadonite=muscovite+clinochlore. We assume that the free energy change of reaction, ΔG=ΔG^o+ΣRT lna_i^j, is approximated by ΔG=A?BT +C(P?1)+ΣRT lna_i^j, where ΔG^o is the free energy change of the end-member components at temperature T and pressure P, a_i is the activity of component i whose coefficient in the equilibrium is j, and A, B and C are constants to be determined. Values of C can be approximated by the change in volume on reaction, namely C=406.517 J/bar for the first reaction and C=0.613 J/bar for the second reaction. Constants A and B were determined by using six occurrences of the assemblage stilpnomelane–chlorite–white mica for which P–T have been otherwise estimated. Using solution models from the literature, linear regression gives for the first equation A=?6118.269 kJ, B=?4584.09 J/K, and for the second equation A=19.397 kJ, B=66.72 J/K. These values predict P–T within 0.5 kbar and 25 K for all occurrences, and appear reasonably robust relative to probable analytical errors. P–T are determined by intersection of the curves generated by given compositions in P–T space. Fine-grained and/or zoned chlorite and white mica make application of the geothermobarometer difficult in some instances, but our work in the Bathurst region of New Brunswick suggests that, with patience and care, useful analyses can be obtained, and the database for the geothermobarometer greatly expanded.  相似文献   

庐山变质核杂岩的变质变形及构造意义     

杨帆  宋传中  任升莲  李加好  刘欢  黄鹏  王微 《地质论评》2015,61(4):752-766

庐山出露的变质核杂岩的构造层次具有典型的三层结构,即最上面的未变质沉积盖层、中间韧性流变层、核部的变质基底,其间分别以上剥离断层和拆离断层为分界线。上部沉积盖层以脆性变形为主,中间韧性流变层变形程度不一,但远离拆离带变质变形逐渐减弱,而核部的变质基底以强塑性变形为特征。对变质核杂岩南北两条地质剖面的分析,得到三个构造层次的岩石变质变形及岩性变化特征。区域动力学分析反映变质核杂岩东部原拆离带在五里正断层附近。拆离带中多硅白云母压力计得出的变质压力为0.6~1.1 GPa;锆石U-Pb年龄测试得到变质核杂岩的隆升时代为早白垩世。  相似文献   

White mica K–Ar geochronology of Sanbagawa eclogites from Southwest Japan: implications for deformation-controlled K–Ar closure temperature     

Tetsumaru Itaya  Tatsuki Tsujimori 《International Geology Review》2015,57(5-8):1014-1022

White mica (phengite and paragonite) K–Ar ages of eclogite-facies Sanbagawa metamorphic rocks (15 eclogitic rocks and eight associated pelitic schists) from four different localities yielded ages of 84–89 Ma (Seba, central Shikoku), 78–80 Ma (Nishi-Iratsu, central Shikoku), 123 and 136 Ma (Gongen, central Shikoku), and 82–88 Ma (Kotsu/Bizan, eastern Shikoku). With the exception of a quartz-rich kyanite-bearing eclogite from Gongen, white mica ages overlap with the previously known range of phengite K–Ar ages of pelitic schists of the Sanbagawa metamorphic belt and can be distinguished from those of the Shimanto metamorphic belt. The similarity of K–Ar ages between the eclogites and surrounding pelitic schists supports a geological setting wherein the eclogites experienced intense ductile deformation with pelitic schists during exhumation. In contrast, phengite extracted from the Gongen eclogite, which is less overprinted by a ductile shear deformation during exhumation, yielded significantly older ages. Given that the Gongen eclogite is enclosed by the Higashi-Akaishi meta-peridotite body, these K–Ar ages are attributed to excess ⁴⁰Ar gained during an interaction between the eclogite and host meta-peridotite with mantle-derived noble gas (very high ⁴⁰Ar/³⁶Ar ratio) at eclogite-facies depth. Fluid exchange between deep-subducted sediments and mantle material might have enhanced the gain of mantle-derived extreme ⁴⁰Ar in the meta-sediment. Although dynamic recrystallization of white mica can reset the Ar isotope system, limited-argon-depletion due to lesser degrees of ductile shear deformation of the Gongen eclogite might have prevented complete release of the trapped excess argon from phengites. This observation supports a model of deformation-controlled K–Ar closure temperature.  相似文献   

Reconsideration of the age of blueschist facies metamorphism on the Seward Peninsula, Alaska, based on phengite ⁴⁰Ar/³⁹Ar results     

K. A. HANNULA  M. O. McWILLIAMS 《Journal of Metamorphic Geology》1995,13(1):125-139

Abstract ⁴⁰Ar/³⁹Ar step-heating and single-grain laser fusion ages from phengites from the polydeformed and polymetamorphosed blueschist-greenschist facies Nome Group fall into two groups. Samples from the upper part of the structural section that have experienced a relatively weak metamorphic and deformational post-blueschist facies overprint and one sample from the Cape Nome orthogneiss yield plateau ages of 116-125 Ma. More intensely overprinted samples yield hump-shaped spectra with minimum ages of 123 Ma and maximum ages of 334 Ma. Samples with hump-shaped spectra are derived from a greater structural depth than most samples with plateau ages. Unreasonably old maximum ages from some of the disturbed spectra suggest that the hump-shaped spectra result from the incorporation of excess ⁴⁰Ar. This interpretation conflicts with previous interpretations of similarly disturbed spectra from the Brooks Range, which have been argued to provide minimum ages for blueschist facies metamorphism. Since the maximum temperatures achieved by all samples were probably above the blocking temperature of Ar in phengite, the 116-125 Ma plateau ages are a minimum age for blueschist facies metamorphism on the Seward Peninsula, Alaska.  相似文献   

低温高压微地质体中微米纳米级矿物相研究——以大冶葡萄石中绿纤石和多硅白云母为例     

赵文俞  王勤燕  孙振亚  叶先贤  刘祥文  刘嵘 《矿物岩石地球化学通报》1999,(3)

低温高压地质体因与地壳碰撞、俯冲等构造活动密切相关,长期以来一直是国内外地学界关注的研究对象。但由于受研究手段、分辨能力所限,对于呈微米纳米级微粒形式存在于低温高压微地质体（即矿物）中的成分、物相及其成因等问题的研究,至今仍是地学研究中的薄弱环节。随...  相似文献   

张宣地区大白阳金矿床包裹体特征及矿床剥蚀保存     

徐渴鑫  申俊峰  李胜荣  黄绍峰  张士全  王业晗 《矿床地质》2020,39(6):1030-1048

华北克拉通破坏及其成矿事件已引起地学界的广泛关注,其中张宣地区是华北克拉通的重要金矿集中区之一,通过对张宣地区大白阳金矿成矿类型、成矿深度及剥蚀保存的探讨,为区内深部找矿提供了重要的参考依据。文章通过流体包裹体研究对大白阳金矿成矿温度及成矿深度进行计算,利用矿物压力计对矿区周边岩体侵位深度进行估算。流体包裹体研究结果显示大白阳金矿成矿温度160~220℃、盐度w（NaCl_eq）=9%~15%,密度0.78~0.97 g/cm³、压力46.3~104.6 MPa,成矿流体主要属于NaCl-H₂O体系,成矿过程中流体温度和盐度都呈现明显的降低趋势。采用黑云母压力计,针对大白阳金矿周边谷嘴子、杨家营以及前坝口花岗质岩体的侵位深度进行估算,并对大白阳金矿自形成以来的剥蚀保存情况进行了探讨,估算结果显示,谷嘴子岩体（236.0 Ma）成岩深度7.02 km、杨家营岩体（138.6 Ma）成岩深度2.66 km、前坝口岩体（140.2 Ma）成岩深度3.13 km。结合前人的裂变径迹结果,认为张宣地区自中生代以来剥蚀速率为0.022~0.029 km/Ma,剥蚀量为3 km。按照前述成矿压力46.3~104.6 MPa换算,大白阳金矿成矿深度6.93 km,剥蚀量小于矿床成矿深度,说明该矿深部仍具有良好的找矿前景。  相似文献   

Deformation fabrics of glaucophane schists and implications for seismic anisotropy: the importance of lattice preferred orientation of phengite     

《International Geology Review》2012,54(6):720-737

ABSTRACT

Strong seismic anisotropy is observed in many subduction zones. This effect is attributed partly to subducting oceanic crust that is transformed into blueschist facies rocks. Because blueschist facies constituents such as glaucophane, epidote, and phengite show strong anisotropic elasticity, seismic anisotropy in subducting oceanic crust can be attributed to the lattice preferred orientation (LPO) of these minerals. We studied the deformation fabrics and seismic properties of phengite-rich epidote–glaucophane schists from the Franciscan Complex of Ring Mountain, California. The samples are composed mainly of glaucophane, epidote, and phengite. Some samples contain abundant phengite, the maximum being 40%. The LPOs of glaucophane showed that the [001] axes are aligned subparallel to lineation, and both (110) poles and [100] axes are aligned subnormal to foliation. The epidote [001] axes are aligned subnormal to foliation, with both (110) and (010) poles aligned subparallel to lineation. The LPOs of phengite are characterized by the maxima of [001] axes subnormal to foliation, and both (110) and (010) poles and [100] axes are aligned in a girdle subparallel to foliation. The phengite showed substantially strong seismic anisotropy (AV_P = 42%, max.AV_S = 37%). The glaucophane schist with abundant phengite showed significantly stronger seismic anisotropy (AV_P = 30%, max.AV_S = 23%) than the epidote–glaucophane schist (AV_P = 13%, max.AV_S = 9%). When the subduction angle of phengite-rich glaucophane schist is considered, the polarization direction of the fast S-waves for vertically propagating S-waves changed to a nearly trench-parallel direction for the subduction angle of 45?60°, and the S-wave anisotropy became stronger for vertically propagating S-waves with increasing subduction angles. Our data showed that phengite-rich blueschist facies rock can therefore contribute to the strong trench-parallel seismic anisotropy occurring at the subducting oceanic crust and at the slab–mantle interface in many subduction zones.  相似文献