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Metamorphic Studies: Research in Progress Downloaded from http://jgs.lyellcollection.org/ by guest on September 26, 2021 Journal of the Geological Society, London, Vol. 147, 1990, pp. 561-562, Printed in Northern Ireland Conference Report Metamorphic studies: research in progress G. T. R. DROOP Department of Geology, University of Manchester, Oxford Road, Manchester M13 9PL, UK A Metamorphic Studies Group meeting on 'Metamorphic Studies: fluid inclusions in charnockites and associated gneisses from Research in Progress' and the eighth Annual General Meeting of the Kabbaldurga andother localities in S. India.From the Group were held on 8 March 1989. It was attended by 81 delegates observation that the inclusions in incipient charnockites are and 13 papers were presented on a range of topics, including an more CO,-rich and have higher 6 I3C values than those in invited contribution from C. Chopin (Ecole Normale Supkrieure, adjacent biotite-gneisses, it was inferred that the fluids were Park). Two poster contributions were alsopresented. The meeting trapped during charnockitization. Current estimates of peak was organized by G. T. R. Droop. metamorphic temperature (c. 750 "C at 5-6 kbar) lie above the wet solidus of biotite gneiss, and this was taken as The morning session was concerned mainly with high-grade evidence against the existence of water-rich fluids. The metamorphismand fluid-rock interactions. The first talk conclusion was that an influx of externally (mantle?) derived was given by R. L. Oliver who gave an account of some CO, was instrumental in lowering water activities enough to hithertoundescribed high-T, high-P AI-rich ferruginous suppress melting. A. J. Baker then examined petrological granulites from the western margin of the Gawler Craton, and stable isotopic evidence of the presence or absence of SouthAustralia. Extreme metamorphicconditions of c. open-system behaviour in lower crustal marbles of the Ivrea 10 kbar and 900-1000 "C were inferred from the presence of Zone.Here, the amphibolite-facies marblescontain high the assemblage hypersthene + sillimanite + quartz.The variance assemblages that equilibrated with relatively coexistence of magnetite and Ti-hematite was taken as CO,-poor fluids, suggesting influx of water. The granulite- evidence for a relatively high oxygen fugacity. The rocks are facies marbles on the other hand, show more evidence of apparently of much higher grade than those in the interior buffering. The large ranges in observed 6 13C and 6 ''0 of the Gawler craton, but show similarities with granulites at values of calcite were ascribed to varying degrees of the edge of the Yilgarn Block further west. D. J. Waters interaction with externallyderived fluid that had not (with J. M. Moore) described an unusualgranulite-facies equilibrated with adjacent pelites. Reaction-enhanced assemblage of boron-rich aluminosilicate minerals permeability was suggested as a mechanism for promoting (kornerupine + werdingite + grandidierite + sillimanite) from high rates of fluid flow along individual marble layers. the highest gradepart of the Namaqualandmetamorphic B. W. D. Yardley (with R. A. CM) described an complex (the spine1 + quartzzone). They noted thatthe andradite-richskarn of regional metamorphic origin from mobility or otherwise of boron in high-grade metasediments the DalradianBoulder Bed of Connemara. Thistype of could have important implications for partial melting in view metasomatism, they argued, required the influx of high-Ca of the effect of this elementon the temperature of the fluid. Sr-isotope data were presentedto support the idea granite solidus. The confinement of the assemblage to a that the aqueous fluid originated by dehydration of nearby single asymmetric stratum within a sequence of pelites and pelitic schist, and scavenged Ca when it was channelled the absence of secondaryB-mineralization were taken as along one particular diopside-rich stratum in the intervening evidence thatthe boronenrichment was of sedimentary Connemara Marble. Specifically, the metasomatic diopside- origin. During metamorphism, it was concluded, the boron rocks had evidently equilibrated isotopically with the pelites was essentially immobile and hence unavailable as a flux to but the normal marbles had not. Again, reaction-enhanced aid crustalanatexis. S. Inger thenreported onthe permeability was implicated. The age of the event was petrogenesis of migmatites and cross-cutting leucogranites in constrained by U-Pb dating of titanites from the diopside the units overlying the Main CentralThrust, Nepal. He rocks. presentedthermometric data for rocks in the sillimanite The first fourtalks of the afternoonconcentrated on zone (the area of partial melting) and argued that, at the regional metamorphism in medium- to high-pressure recorded temperatures (c. 700 "C), substantial infiltration of terrains. The session was opened by an invited paper by C. water must have occurred to generate the high degrees of Chopin who reviewed the petrology, geochronology and partial melting (>30%) necessary for magma migration and structural history of the Western Alps. He drew attention to intrusion. The provenance of the fluids was notresolved, the age and P-T contrastsbetween the external but one hypothesis is that theywere produced by (meso-Alpine, blueschist-facies) andinternal (eo-Alpine, dehydration of tectonically buriedmetasediments beneath eclogite-facies) zones. Emphasis was placed on the stability the Main Central Thrust. limits of key minerals and mineral assemblages (e.g. In the first of two papers on fluid-rock interactions in lawsonite, carpholite in the externalzones; Mg-chloritoid, granulites, D. H. Jackson (with D. P. Mattey, N. B. W. talc + kyanite + phengite, almandine + kyanite + jadeite, py- Hams & M. Santosh) presented carbon isotope data from rope + coesite in the internal zones) andthe crystallo- 56 1 Downloaded from http://jgs.lyellcollection.org/ by guest on September 26, 2021 562 G. T. R. DROOP graphy of some rare high-P phases (e.g. ellenbergerite) was The last three talks were devoted to theinterpretation of discussed. The preservation of carpholite requires a cooling metamorphictextures. A. J. Barker presentedresults of path of low dPldT;the discovery of oil-bearing fluid somecareful textural work on porphyroblast-matrix inclusions in lawsonite- and carpholite-bearing rocks is also relations in staurolite/kyanite-grade hornblendegarben- indicative of an unusualP-T-t history forthe external schists from the Caledoniannappes of N. Norway. The zones. Pervasive E-W stretching lineations in the internal observed straightness of inclusion trails and their variable zones are associated with retrograde micas that yield degree of discordance with the external schistosity were mineralages of c. 40 Ma.,and are ascribed to Tertiary used as evidence that the porphyroblasts grew rapidly, at continental collision. Structural evidence forCretaceous different times, during an extended period of simple shear. subduction is scanty but this event may be responsible for The traditional interpretation of porphyroblasts with straight poorly preservedearly N-S trendinglineations. M. P. inclusion trails (viz. slow growth in an interkinematic phase) Williams (with P. J. Treloar & D. C. Rex) discussed the was rejectedin this case. In ajoint paper with D. G. timing of, and processes responsible for, the exhumation of Pearson, S. Reddy discussed the processes responsible for thrusted amphibolite-facies rocks in the Pakistan Himalaya. kelyphitization of pyrope-rich garnets with reference to Evidence was presented for a 3-stage deformation history: lherzolites and pyroxenites from the Beni-Bousera massif, (i)syn-metamorphic SSE-directed shear related to over- N. Morocco. SEM micrographs were presented to illustrate thrusting of the Kohistan Arc over the Indian Plate at c. the fine structure of kelyphites, which consist of a series of 50 Ma, (ii) post-metamorphicthrusting, resulting in concentriczones each containing clinopyroxene f ortho- inversion of metamorphic zones, and (iii) E-W compression pyroxene f spine1 in different proportions and textures. The producing folds and domes. K-Ar mineral and fission-track absence of interstitial glass and the independence of ages document decreasing cooling and uplift rates with time, kelyphite presence on precursor mineralogy, it was argued, and high uplift rates (c. 3 mm per year)between 35 and ruled out an origin by either partial melting or reaction with 45 Ma. Unroofing was synchronous with post-metamorphic olivine. Infiltration of hydrous fluid was the favoured thrusting. A puzzling feature to emerge from the data was mechanism. The last talk was given by J. Appleyard who the very short time lapse between continental collision and presentedsome elegant diffusion modelling of coronas in regional metamorphism in the area. N. F. C. Hudson then metagabbros from the Jotun Nappe, Norway. The models described suite a of low-K, low-Ca, gedrite-bearing were erected using published near-equilibrium,steady- amphibolites from the fault-bounded Pulanesi and Osmanki state thermodynamic theory. It was shown that the observ- Blocks in the Raahe-Ladoga Zone of central Finland. The ed layer sequence plagioclase I hornblende 1 hornblende + protolithswere probably volcanic rocks thathad been quartz I actinolite + quartz I clinopyroxene could not be ex- chemically altered by interaction with seawater. Textural plained by a closed-system model. Successful solutions could observations were combined with constraints from petroge- only be
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