Phase Equilibria in the Metamorphic Rocks of St. Paul Island and Cape North, Nova Scotia

A suite of schists—one from the garnet zone, 19 from thestaurolite zone, 2 from the kyanite isograd, and one from thekyanite zone—were separated into their constituent minerals.Chemical analyses of one chlorite and of 23 sets of coexistingbiotites and garnets were carried out by photometric and titrametricprocedures. Plots of garnet-biotite tie-lines from divariantassemblages on appropriate phase diagrams result in intersectingtie-lines which cannot be ascribed to experimental error. Theoreticalconsiderations argue that at equilibrium, at the same pressureand temperature or at constant pressure and varying temperature,tie-lines of divariant assemblages should not intersect. Possibleexplanations require that diffusion equilibrium of Fe and Mgbe restricted to volumes smaller than that of a hand specimenor that P as well as T varies considerably. Emission spectrographicdeterminations of Fe and Mg in biotite indicate that the Fe/Mgratio varies among biotites little more than a centimeter apart.Such a variation would argue more in favor of a lack of diffusionequilibrium.     

Stratigraphy and Petrology of the Mulcahy Lake Layered Gabbro: An Archean Intrusion in the Wabigoon Subprovince, Ontario

The Mulcahy Lake gabbro is an Archean layered intrusion of tholeiiticbulk composition located in the Wabigoon subprovince. The intrusionis 6 km thick at the thickest and is exposed over an area of63 km². It intrudes basaltic to siliceous volcanics of the CrowLake-Savant Lake greenstone belt and is intruded by the Atikwabatholith. Zircon U-Pb data indicate crystallization at 27322+1·0/–0·9m·y. Principal phases are plagioclase, orthopyroxene, augite (andpigeonite in iron-rich rocks), olivine, hornblende and magnetite.Olivine is confined to several horizons. Apatite and then zirconare prominent accessory phases at advanced stages of fractionation.Plagioclase, pyroxenes and olivine are cumulate phases. Hornblendeis invariably an intercumulus phase. Magnetite is ubiquitousthroughout the intrusion, generally as a cumulate phase, andforms centimeter thick layers in fractionated rocks. Fractionationfollowed a tholeiitic trend with iron enrichment in the liquid. The intrusion is divided into lower, mixed, middle, upper andmarginal zones. The lower and middle zones are 2·0 and2·5 km thick respectively. The upper zone is approximately1 km thick, and the marginal zone is measured in hundreds ofmeters. A 200 m thick mixed zone is interposed between the lowerand middle zones. The base of the lower zone consists of ultramaficunits containing olivine of Fo₈₂. The top of the zone has olivineof Fo₂₈. Fractionation of the lower zone, from the floor up,was interrupted by the introduction of pristine liquid whichmixed with more dense and cooler residual liquid in the chamberto form the mixed zone. Further introduction of several minorpulses of liquid constructed the lower part of the middle zone.The upper part of the middle zone was constructed from a majorpulse of liquid plus several minor pulses each of which is representedby reversals in cryptic layering. The upper zone consists ofultramafic to iron-rich gabbro cumulates formed by cooling throughthe roof plus horizons formed by influx of pristine liquid.Marginal zone rocks represent cooling through the walls of theintrusion. Rhythmic layering is well developed in lower and middle zonecumulates. Petrofabric data show that orthopyroxene has a lineationin the plane of layering and parallel to structures suggestiveof flow. Plagioclase laths also have a preferred orientationin many cumulates and in unlayered gabbros as well. Flow, possiblylaminar, of liquid-crystal material is suggested and may belinked to the ultimate development of layering. Pressure during the course of crystallization probably was greaterthan 2 and less than 5 kb. Temperatures estimated from pyroxenesvaried from approximately 1200 to 1000 °C.f_o², is not wellconstrained but was sufficient to allow the formation of thinlocal magnetite cumulates late in the crystallization. The primarymelt was hydrous as indicated by the presence of hornblende.It is very unlikely that the melt was saturated with water duringcrystallization of the cumulate phases.     

Anorthosites and Related Megacrystic Units in the Evolution of Archean Crust

Archean anorthositic complexes occur in essentially all Archeancratons and contain large equidimensional plagioclase crystals(up to 30 cm. diam.) with highly calcic compositions (An₈₀ toAn₉₀) but are not readily amenable to determination of theirparent melt compositions. However, insight into petrogenesisof the complexes is provided by megacrysts of plagioclase thatare identical to those in the complexes and occur in many Archeanflows, sills, and dikes whose matrices display REE and fractionationpatterns that indicate tholeiitic trends and are compatiblewith prior subtraction of plagioclase during earlier evolutionof the melts. Included blocks of anorthosite and megacrystswith very thin rims that approach the more sodic compositionof lathy plagioclase in the matrices indicate an earlier stageof cryst formation under different conditions of crystallizationthan the matrices. The megacrystic units occur both in greenstonebelts that have oceanic affinities and stable cratonic dikeswarms that have continental affinities. Both major and traceelement contents of the matrices of the megacrystic units differbetween greenstone and cratonic dike environments; the dikesbeing higher in Si0₂, TiO₂ FeO, Na₂, K₂O, and light REEs butlower in Al₂O₃ and CaO. The matrices of both environments followseparate but parallel tholeiitic fractionation with high Fe-enrichmenttrends similar to Skaergaard liquids suggesting relatively lowvolatiles and fo₂. Experimental data and projections in CMAFspace suggest a multistage petrogenesis involving a relativelyhigh-pressure fractionation of olivine and/or orthopyroxenefrom a primitive mafic melt followed by ascension of the fractionated,less-dense melt, probably in several pulses, to a low-pressurechamber, probably at 1 to 2 kb. The depressurization accompaniedby cooling could easily place the melt composition in the plagioclasefield and significantly below the liquidus resulting in severalcrystallization cycles of plagioclase in the low pressure chamber.The melts would crystallize as anorthositic complexes and periodicallyexpel pulses that would form the observed megacrystic flows,sills, and dikes.     

Granulite-Facies Metamorphism, Granite Falls-Montevideo Area, Minnesota

In the Granite Falls-Montevideo area, Minnesota, granulite faciesmineral assemblages were collected from three major lithologicunits—hornblende-pyroxene gneiss, garnet-biotite gneiss,and granitic gneiss. Mineral assemblages most commonly observedare: plagioclase-hornblende-orthopyroxene-clinopyroxene-magnetite-ilmenite-(biotite-quartz);quartz-plagioclase-biotite-garnet-orthopyroxene-(orthoclase);quartz-plagioclase-biotite-garnet-(orthoclase); quartz-plagioclase-microcline-(garnet-hematite);and quartz-plagio-clase-microcline-biotite-(garnet-hematite-rutile).Partial analyses of isomorphous phases from the hornblende-pyroxenegneiss and the garnet-biotite gneiss were determined with anelectron microprobe. Negligible compositional variation within single grains andof a particular mineral within a given specimen, regular distributionof Fe and Mg between coexisting hornblende, ortho-pyroxene,and clinopyroxene, obedience of the mineral assemblages to theGibbs Phase Rule, and lack of empirically determined incompatiblephases indicate a close approach to chemical equilibrium duringmetamorphism. Plots of coexisting biotite, garnet, and orthopyroxeneon an appropriate phase diagram result in some crossing tielines which cannot be adequately explained by temperature orpressure differences, but suggest that H₂O and/or O₂ were notperfectly mobile components during metamorphism. This is alsoindicated by interlayering of hornblende assemblages with pyroxeneassemblages and by different iron oxide phases in essentiallythe same mineral assemblage. Textural and chemical relationships of retrograde metamorphicassemblages suggest that some retrograde reactions are a resultof cooling following the granulite-facies metamorphism, butthat others may have resulted from recrystallization duringa stage of thermal metamorphism that is reflected in the potassium-argonand rubidium-strontium biotite ages of the metamorphic rocks.