摘要:

AbstractEndogreisens which replace K‐feld‐spar‐quartz dykes in a Devonian (360 Ma) tin deposit at Mt Bischoff, north‐west Tasmania, formed from the interaction of unusual solutions, probably derived from an underlying leucogranite pluton, porphyry dykes and limited quantities of local dolomitic country rock components. The intensity of greisenization and pH of the solutions increase inward to the greisenized dykes’cores and downward. The following types of greisen assemblages indicate increasing degrees of greisenization: ‘sericite’muscovite + quartz ± tourmaline ± fluorite, topaz + quartz ± tourmaline ± fluorite, weberite, prosopite, ralstonite, Ca‐ralstonite; and quartz ± topaz ± fluorite. Where the solutions interacted with dolomite, exogreisens consisting of topaz‐ or tourmaline‐bearing assemblages were formed. The greisens were subsequently overprinted to varying degrees by siderite, sulphides and hydrous silicates (talc, serpentine, chlorite, micas).The temperature during greisenization ranged from 180 to 414°C, based on fluid inclusions in topaz, quartz, fluorite, sellaite and cassiterite. The main greisen‐forming event occurred at temperatures of 360±20°C. The fluids boiled intermittently. Their salinities ranged from 31.5 to 38.9 wt% total dissolved salts, consisting of Ca–K–Na–Fe–Cl±hydrocarbon species. Fluid inclusion data indicate that only 0.5–1.5 km of cover were present above this deposit at the time of formation.The greisenized dykes were intruded by and intrude different stages of breccias. The breccias consist mainly of country rock and greisenized dyke fragments, with rock‐flour and later tourmaline alteration. The Mt Bischoff greisen system is possibly part of a ‘porphyry tin’style deposit

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00445.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractIn the contact metamorphic aureole of the Tinaroo Batholith (north Queensland, Australia), mylonitic rocks were metamorphosed during a regional folding/crenulation event (D2) synchronous with the emplacement of muscovite‐bearing granitoids. Prismatic and skeletal andalusite porphyoblasts grew in carbonaceous schists, mainly from the dissolution of staurolite. Muscovite, quartz and biotite played a dual role in this reaction, acting in a catalytic capacity as well as reactants or products. Staurolite was replaced by coarse‐grained muscovite ± biotite, whereas andalusite locally replaced quartz ± muscovite ± biotite, with diffusion of H, Al, Si, Mg, Fe and K ionic species linking sites of dissolution and growth.Graphite contributed to the reaction mechanism in a number of ways. Accumulations of graphite in front of advancing andalusite crystal faces led to skeletal growth and the formation of chiastolite structure, where incremental growth occurred on adjacent {110} faces, with subsequent filling in and inclusion of graphite along the diagonal zones. The presence of graphite in some layers in the schist matrix prevented recrystallization of strained muscovite grains. The muscovite grains in these layers, in contrast to adjacent thin non‐graphitic layers, were preferentially replaced by quartz. This resulted in muscovite‐depletion haloes in graphitic layers around andalusite porphyroblasts. Somewhat arcuate zones of graphite, concentrated during dissolution of quartz along a crenulation cleavage, occur on some andalusite faces. Reactivation of the mylonitic foliation during the formation ofD2crenulations led to a preferential dissolution of quartz in zones of progressive shearing localized near andalusite porphyroblasts and hence the accumulation of graphite.Lack of deflection of the pre‐existing mylonitic foliation and anastomosing of the axial planes ofD2crenulations around andalusite porphyroblasts demonstrate not only the timing of growth, but also that growing porphyroblasts do not push aside existin

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00446.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThe textural and compositional features of phlogopites in a contact‐metamorphic dolomite marble inclusion in the Bergell intrusion (central Alps) and in a metasomatic reaction vein cutting through this marble suggest different origins for vein phlogopites:(a) High‐Al vein phlogopite represents former marble phlogopite which has been compositionally modified by reaction with the vein forming fluid.(b) Low‐Al vein phlogopite represents phlogopite precipitated from the vein forming fluid.As both types of vein phlogopite were in contact with the same vein forming fluid at the same time, low‐Al phlogopite most likely represents an equilibrium phlogopite composition, whereas high‐Al phlogopite does not. High‐Al vein phlogopite retained its Al‐content from the contact‐metamorphic marble parent phlogopite and only underwent Fe‐Mg exchange with the metasomatic fluid.All the vein phlogopites studied are strongly enriched in Fe relative to marble phlogopite. The data may suggest in general that phlogopite Al/Si ratios may be retained from the conditions under which the phlogopites first formed, whereas the Mg/Fe‐ratios may be substantially modified by exchange with other ferromagnesian solid phases and/or a metamorphic fluid at later stages in their metamorphic history. This may have significant effects on calculated pressures and temperatures from thermobarometer

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00447.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractIn the northeastern part of the Grenville Province, along the gulf of St Lawrence, cordierite is widespread in the migmatites of Baie Jacques Cartier (BJC) and Baie des Ha! Ha! (BHH). In the BJC area, rafts of mesosome occur in a pervasive network of leucosome consisting of cordierite‐bearing pegmatite. In BHH, however, the mesosome and leucosome are well segregated and locally separated by thin biotite –hornblende melanosomes.Leucosomes in the BJC area record the highest temperatures (oxide thermometry = 900°C), whereas leucosomes of BHH and mesosomes of both areas indicate peak temperatures around 800°C (oxide thermometry; biotite–garnet thermometry with fluorine‐rich biotite). Peak pressures were constrained at 720 MPa using the Ilm‐Sil–Qtz–Grt–Rt (GRAIL) equilibrium.The area is thought to have undergone extensive melting under relatively modest pressures. The highest temperatures recorded in the BJC area are probably related to a pervasive impregnation of this terrane by aluminous granitic melts.Most post‐peakP–Testimates for the mesosomes fall on a nearly isobaric, clockwise,P–Tpath (0.6 MPa/°C) with the exception of the high‐temperature leucosomes of BJC, which fall about 100°C away from this path; this is additional evidence for the external origin of these leucosomes. The ultimate source of heat that generated the migmatites is thus though to be an underlying plut

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00448.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThe Saint‐Martin des Noyers Formation is interpreted as a slice of an island‐arc system of Lower to Middle Palaeozoic age, located in the internal part of the Variscan orogen in Vendée (Armorican Massif, France). Metamorphosed igneous rocks range in composition from ultramafic to rhyolitic. The regular increase in the FeO/(FeO+MgO) ratio, from mafic to silicic samples, results in a systematic variability in the nature and composition of the metamorphic phases. In basaltic samples, the occurrence of relict garnet‐barroisite assemblages suggests relatively high‐pressure conditions for the peak of metamorphism. During a subsequent retrograde evolution, the primary barroisitic hornblendes recrystallized to texturally complex mixtures of actinolite and hornblende. Despite this complication, it is possible to decipher aP–T‐tpath based on amphibole chemistry. TheP–Ttrajectory deduced is dominated by the effect of pressure and consistent with early underthrusting and subsequent tectonic uplift of the ancient arc of Saint‐M

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00449.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

THE YOUNG EARTH: AN INTRODUCTION TO ARCHAEAN GEOLOGY. By E.G. Nisbet. Allen and Unwin, Boston, 1987. pp. 402.

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00450.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00451.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00452.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY