摘要:

AbstractFluids, some of which are CO2‐rich (up to 40 mol.% CO2) and some of which are highly saline (up to 18 wt% NaCl equivalent), are trapped as fluid inclusions in quartz‐calcite (∼ metallic minerals) veins which cross‐cut the pumpellyite‐actinolite to amphibolite facies rocks of the Alpine Schist. Fluids were commonly trapped as immiscible liquid‐vapour mixes in quartz and calcite showing open‐space growth textures. Fluid entrapment occurred at fluid pressures near 500 bars (possibly as low as 150 bars) at temperatures ranging from 260 to 330° C. Saline fluids may have formed by partitioning of dissolved salts into an aqueous phase on segregation of immiscible fluids from a low‐density CO2‐rich fluid. Calcite deposited by these fluids has δ13C ranging from – 8.4 to – 11.5 and δ18O from + 4 to + 13. Isotopic data, fluid compositions and mode of occurrence suggest that the fluids are derived from high‐grade metamorphic rocks. Fluid interaction with wall‐rock has caused biotite crystallization and/or recrystallization in some rocks and retrogression of biotite to chlorite in other rocks.Fluid penetration through the rock is almost pervasive in many areas where permeability, probably related to Alpine Fault activity, has focussed fluids on a regional scale into fractured rocks. The fluid flow process is made possible by high uplift‐rates (in excess of 10 mm/year) bringing

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00405.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractThe Tal y Fan Intrusion is a 110 m thick sub‐concordant metabasite sheet intruded into volcaniclastic and pyroclastic rocks of Ordovician age in North Wales. Despite low grade metamorphism, primary textural zones resulting from initial cooling of the sheet are preserved and retain primary mineralogical and chemical variations which influenced the nature and extent of metamorphic recrystallization. This has resulted in a vertical sequence of secondary mineral assemblages through the intrusion. During early hydrothermal alteration K‐feldspar replaced plagioclase micropheno‐crysts in the marginal and contact zones, and olivine in the central zone was replaced by saponite. Subsequent regional metamorphism resulted in the development of (metastable) prehnite‐pumpellyite‐epidote assemblages in two sub‐zones characterized by high Fe2O3. Elsewhere the assemblage prehnite‐actinolite‐epidote developed except in the contact and marginal zones where activity of CO2suppressed both prehnite and pumpellyite. Both assemblages contain excess albite, quartz and chlorite and, on the basis of uniform mineral compositions over the area of an individual thin section, are considered to represent buffered equilibrium assemblages indicative of prehnite‐pumpellyite and prehnite‐actinolite facies conditions. A metamorphic temperature of 310° C at 1.85 kbar is obtained using theP‐T‐Xgrid of Liou, Maruyama&Cho (1985), which implies a field gradient of ∼ 44° C km‐1. Assuming that metamorphism relates to burial, an overburden thickness of ∼ 7 km is indicated. Total maximum thicknesses, however, of Ordovician, Silurian and Lower Devonian strata, in the area, do not exceed 6 km indicating a field gradient of 52° C km‐1. These relatively high gradients may possibly be related to concealed late Caledonian intrusions, or alternatively may result from high heat flow as a consequence of crustal thinning, rapid sedimentation and intense magmatic ac

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00406.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractAn inverted metamorphic gradient associated with the northern mylonite zone of the Cheyenne belt, a deeply eroded Precambrian suture in southern Wyoming, has been documented within metasedimentary rocks of the Early Proterozoic Snowy Pass Supergroup. Metamorphic grade in the steeply dipping supracrustal sequence increases from the chlorite through the biotite, garnet, and staurolite zones both stratigraphically and structurally upward toward the northern mylonite zone. A minimum temperature increase of approximately 100° C over a km‐wide zone is required for this transition. Parallelism of inverted isograds with the trace of the northern mylonite zone implies a genetic relationship between deformation associated with that zone and the inverted metamorphic gradient within the Snowy Pass Supergroup.Field evidence together with microstructural and petrofabric analysis indicate northward thrusting of amphibolite‐grade rocks over rocks of the Snowy Pass Supergroup along the northern mylonite zone. Mineral equilibria and garnet‐biotite geothermometry on synkinematic mineral assemblages within the Snowy Pass metasedimentary rocks indicate deformation atminimumtemperatures of 480° C and pressures of 350–400 MPa (3°5–4°0 kbar). This implies tectonic burial or upper plate thickness of 13–15 km.The narrow character of metamorphic zonation and microtextures within the Snowy Pass Supergroup which indicate late synkine‐matic growth of garnet and staurolite, preclude rotation of pre‐existing isograds by folding as a mechanism for development of the inverted gradient. Conductive transport of heat from the upper into the lower plate across the originally low‐angle thrust is insufficient to produce the necessary temperatures in the lower plate. Shear heating is considered insufficient to produce the observed metamorphic transition unless high shear stresses are postulated. Up‐dip advection of metamorphic fluids is a feasible, but unproven, mechanism for heat transport. The possibility that rapid uplift due to stacking of several thrust sheets may have played a role in preserving the inverted metamorphic gradient cannot

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00407.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractBiotite and cordierite occur in a 1‐km wide zone of pelitic hornfelses around the McGerrigle pluton. These phases display systematic changes inXFethat can be attributed to continuous reactions involving chlorite or andalusite in the system KFMASH. Through much of the zone biotite and cordierite were products of the ‘breakdown’of chlorite. Close to the pluton this continuous reaction was terminated by a discontinuous reaction that introduced andalusite. Pelites which interdigitate with apophyses of the intrusive at the pluton margin contain assemblages that record a continuous reaction between biotite, cordierite, andalusite, muscovite, and quartz or, alternatively, the discontinuous breakdown of muscovite and quartz to K‐feldspar and andalusite.The mole fraction of Fe in biotite and cordierite increased significantly with the progress of the first continuous reaction and apparently decreased during the second continuous reaction. TheKDof Fe‐Mg between the minerals decreased and apparently increased, respectively, during the two reactions.Biotite‐cordierite‐chlorite assemblages are interpreted to have been stable at temperatures between 525° C and 615° C and biotite‐cordierite‐andalusite assemblages stable at temperatures between 615° C and 635° C. The confining pressure was estimated to have been<2 kbar.The results of this study suggest that theKDof Fe‐Mg between biotite and cordierite is a function of temperature, the Fe‐Mg exchange characteristics of the controlling continuous reaction and non

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00408.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractPartial melting of tonalitic gneisses in the 2.7 Ga Badcallian granulite facies metamorphic episode in the Scourian complex of north‐west Scotland produced a suite of granitic to trondhjemitic liquids. On cooling and excavation of the complex, these melts underwent fractional crystallization and the residual liquids eventually became water saturated. Comparison with experimental data suggests that water saturation would have occurred in these melts at around 620–700°C. From the retrogradeP–T‐time path followed by the complex it is estimated that H2O‐dominated fluids were exsolved from these melts atc.2.5 Ga. It is proposed that these fluids were the cause of the 2.5 Ga Inverian retrogression of the Scourian complex and that water‐saturated melts formed during the crystallization of the leucogneisses were intruded as a suite of pegmatites. The timing of pegmatite intrusion is consistent with this proposition as are the temperature estimates, timing, distribution and nature of the Inverian phase of metamorphism. It is likely that the crystallization of melts is an important process in bringing about hydrous retrogressive metamorphic episodes in a number of other basement terrains, such as West Greenland a

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00409.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

AbstractMineralogical and geochemical evidence indicates that partial melting and desulphidation have occurred in the Big Bell gold deposit. Decarbonation may also have occurred, to account for the lack of a carbonate alteration halo; this is compatible with the present data, but difficult to test.The Big Bell deposit consists of auriferous sulphide‐bearing (‘lode’;) schists with muscovite and K‐feldspar, and surrounding biotite schists, all derived by intense premetamorphic alteration of rocks of mafic composition. Assemblages which include cordierite‐sillimanite‐K‐feldspar‐garnet‐biotite‐quartz suggest peak metamorphic conditions of 4–5 kbar, and 650–700° C, based on phase relations, geobaro‐meters and garnet‐biotite Fe‐Mg exchange partitioning. Partial melting occurred at peak metamorphism, particularly in the altered mafic rocks in and around the deposit, and its occurrence may have been essential to the preservation of the deposit. Melting greatly limited the importance of devolatilization reactions, resulting in negligible aqueous fluids and no means of removing appreciable gold. Minor gold loss may have accompanied desulphidation. A diversity of complex metamorphic assemblages occurs around the mine, compared to the assemblages developed regionally; variable bulk rock composition influences this contrast, but there is no evidence of higher metamorphic grades at the mine, nor that this might have been the prime control on the different assemblages in this narrow belt.It is suggested that the Big Bell and Hemlo deposits are the higher metamorphic grade equivalents of the more abundant greenschist facies gold deposits within Archaean greenstone belts. This interpretation is favoured by the host rock setting and geochemical characteristics of Big Bell. Alternative models that suggest that this class of deposit is a new type must account for the absence of high‐grade equivalents of the greenschist facies deposits and also the lack of low‐grade equivalents of the Big Bell/Hemlo type.Archaean gold deposits in high‐grade metamorphic terrains have undergone a series of processes that are not recorded in the more typical gol

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1988.tb00410.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.tb00411.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY