摘要:

AbstractContact metamorphism adjacent to a porphyritic quartz‐monzodiorite at Kentucky, New South Wales, Australia has produced hornfelses in porphyritic leucogranite at a peak temperature of about 650–700° C and a maximum confining pressure of about 2 kbar (200 MPa). A gradation appears to exist from normal slightly peraluminous to modified strongly peraluminous metagranite hornfelses, which have also been enriched in sulphur. The strongly peraluminous hornfelses, containing cordierite, andalusite, sillimanite, biotite, pyrite and pyrrhotite, retain residual porphyritic igneous microstructures. These rocks appear to have been formed by leaching of base cations, during and possibly just before the contact metamorphism. Folia of fibrous sillimanite anastomose between lenticular grains of quartz and feldspar and truncate igneous zoning in plagioclase grains, suggesting that cation leaching and solution transfer occurred during growth of the sillimanite. Fibrous sillimanite also grew in grain boundaries of polygonal aggregates formed by the contact metamorphism. Therefore, at least some of the cation leaching appears to have occurred at the highest metamorphic grade. Metasandstones that are locally strongly peraluminous adjacent to the monzodiorite stock also, have probably undergone similar leac

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00395.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractCordierite‐anthophyllite rocks and related cordierite‐rich, talc‐rich and chlorite‐rich rocks occur in the Rosebud Syncline, north‐west Queensland, Australia, as part of a Proterozoic metasedimentary sequence. Field relations and rock compositions attest the sedimentary origin of these rather unusual metamorphic rocks. Their chemical composition is comparable to that of unmetamorphosed, alkali‐ and Ca‐poor pelites, which are associated with some evaporite deposits.Other occurrences of cordierite‐anthophyllite rocks have commonly been interpreted as metamorphosed chloritic alteration products derived from mafic or felsic volcanics. A comparative chemical study, using analyses of cordierite‐anthophyllite rocks from such alteration zones and analyses of unmetamorphosed magnesian pelites, demonstrates the general chemical similarity between these two rock groups of entirely different origin. However, distinct differences in major element relations help to distinguish these two genetic groups. Particularly useful are Al2O3–FeO–MgO plots, in which evaporitic pelites occupy the Fe‐poor side.The highly magnesian metamorphic rocks from the Rosebud Syncline fall entirely into the compositional field of evaporitic clays and shales. Furthermore, analyses of relatively immobile trace elements give supporting evidence for the sedimentary origin of these cordierite‐anthophyllite rocks. The correlation with trace element ranges of clays and shales is very good. However, the correlation with trace element ranges of mafic and felsic volcanics is poor, and major discrepancies occur with Cr, Ni, Co, Nb, Sc, Th and Ti.Thus, the magnesian metamorphics of the Rosebud Syncline appear to be derived from evaporitic clays rich in magnesian clay minerals, such as palygorskite, sepiolite, chlorite or corrensite. The complete metamorphic rock assemblage of interlayered calcareous, aluminous and magnesian rocks is interpreted as a metamorphosed carbonate

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00396.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractThe Cretaceous‐Eocene basic to intermediate marine volcanic rocks of the Mucuchi Formation constitute the Western Cordillera in northern Ecuador. Their chemical features mostly correspond to those of tholeiitic basalts with some calc‐alkaline affinities and suggest an oceanic island arc setting. The Macuchi rocks are affected by low‐grade, non‐deformative metamorphism, characterized by zeolite, prehnite‐pumpellyite and lower greenschist facies assemblages. Depth‐zonation is suggested by the downward mineral sequence: (i)laumontite+ (pistacitic epidote, pumpellyite + prehnite); (ii)pumpellyite+ prehnite + pistacitic epidote; (iii)actinolite+biotite+ pistacitic epidote + chlorite. This broad zonation and the chemistry of individual minerals point to an interaction between the volcanic rocks and sea‐water under a moderate to high thermal gradient (= 75° C/km?). Alteration appears to have been dependent primarily on fluid control (volume, pressure, composition), temperature and reaction kinetics which together partly overshadow the role of load‐pressure. Compositional variations of a mineral species at the scale of a contiguous flow or even at the scale of a thin section show that intensity of alteration was spatially uneven depending on rock permeability and consequently, metastable equilibrium commonly exists. However, a progressive approximation to equilibrium as a result ofP–Tcontrol is shown by the mineralogy. A highfo2of the fluid phase is evident from the mineral chemistry. The metamorphism of the Macuchi volcanics is similar to the hydrothermal‐burial type produced during the development of a volcanic arc where lavas and volcanoclastics accumulated in a shallow marine environment. However, some of its characteristics point to a transition toward systems defined by a higherT/Pratio such as those found in ocean‐floor metamorphism.A model is proposed in which the Macuchi volcanics are assigned to an oceanic island arc generated contemporaneously with a marginal basin which has opened as the outcome of progressive north‐south attenuation of the continental crust

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00397.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractComparison of the stability relations of Fe‐piemontite, Pm17Ps17Cz66(Ca2Al2Mn0.5Fe0.5Si3O12(OH)), with similar experimentally determined results on Pm33Cz67suggests that, at moderatefo2, addition of Fe to piemontite raises its thermal stability and slightly decreases its sensitivity to oxygen fugacity. Reversal study of the reaction: Fe‐piemontite + quartz = garnet (Gr33Sp27And40) + anorthite + fluid atPfluid= 2 kbar in cold‐seal apparatus using solid oxygen buffers was completed. The data indicated breakdown of Pm17Ps17Cz66at 645°± 10° C along the Cu2O–CuO buffer, 477°± 10° C along the Cu–Cu2O buffer, and 365°± 10° C for the hematite‐magnetite buffer. Mn‐free clinozoisite and epidote have been shown to be stable to much higher temperatures than piemontite at moderatefo2(

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00398.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractThe Paikon Series is considered to be a volcanic arc sequence with a mainly neritic sedimentary sequence and bimodal tholeiitic volcanism of early Mesozoic age. The metamorphic assemblages are syn‐ to post‐kinematic with respect to a pre‐Tithonian tectonic phase and range from the lawsonite‐chlorite‐albite facies through transitional Na‐amphibole‐greenschist facies to the chlorite sub‐zone of the greenschist facies. The metamorphic imprint of the Paikon Series corresponds to a temperature range from less than 330° C to ± 450° C under a total pressure from 3 kbar to 6–7 kbar. The overprinting of these facies on an earlier blueschist assemblage, related either to a subduction zone or to a tectonic overpressure caused by thr

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00399.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractTextural evolution and compositional variation of clinopyroxenes in Ward Creek metabasites are described. Pyroxenes change, with increasing grade, from finegrained aggregates through fan‐shaped medium‐grained prisms to blocky coarse crystals. Characteristic features of metamorphic pyroxenes include: (1) the occurrence of coexisting pyroxene pairs, the compositions of which are used to delineate compositional gaps; (2) the existence of large compositional variations of pyroxenes, within a single specimen, which record a considerable span ofPand/orTfor crystallization; and, (3) the development of compositional trends in single specimens and in three metamorphic zones which are progressive in nature.The first formed clinopyroxene (Jd20Aug65Ac15) in the lower lawsonite zone mimics the composition of relict igneous augite. It changes continuously, with increasing grade, at nearly constant lowXJdcontent towards acmite. At a composition around Jd20Aug30Ac50, the trend turns towards jadeite and intersects a solvus to form two coexisting clinopyroxenes in the middle lawsonite zone. At higher grade, the compositional gap becomes restricted towards the jadeite‐omphacite join and clinopyroxene increases inXJdtoward jadeite. A reversed compositional trend occurs at higher grade; clinopyroxenes decrease in jadeite component at nearly constant Aug/Ac ratio of 50/50 and finally become omphacite in the uppermost pumpellyite and epidote zones. The Na–Ca pyroxenes, close to the binary join Jd–Ac, occur in the lawsonite‐ and pumpellyite‐zones, ranging fromXJd= 1.0–0.30 together with Ab and Qz. The ubiquitous occurrence of aragonite at temperature estimates of 170–240° C by Taylor&Coleman (1968) for these zones does not support the low‐temperature extrapolation of the Jd–Ab–Qz curve by Holland (1980).The estimated metamorphic field gradient indicates an inflection point at 7 kbar, 200° C. Below this, blueschist facies metamorphism proceeded under dominant pressure‐increase from 4 to 7 kbar at nearly constant temperature, about 150–200° C, whereas at higher grade recrystallization, above the inflection point, the metamorphic temperature increased from 200 to 350° C at nearly constant pressure, about 7–8 kbar. Such an inflection point suggests the depth of underplating of either seamounts or accretion

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00400.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00401.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00402.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00403.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1987.tb00404.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY