Mineral Facies Research Articles

US 5,030,671 Stabilizers for chlorine-containing polymers

The 1.9–1.86 Ga ensialic belt of western Bergslagen, Sweden, comprizes a > 12 km thick felsic volcano-sedimentary sequence intruded by syn-volcanic granitic plutons and mafic dykes and sheets, developed in a rifted continental setting. Regional, low-temperature alteration of the supracrustal sequence produced greenschist to low amphibolite facies mineral assemblages. Localized, cross-cutting zones of intense alteration are developed in felsic volcanics which have been affected by the regional alteration, producing drastic changes in both mineralogy and chemistry. These zones are found close to the Hjulsjö granite-granophyre intrusive complex, most probably the heat source for the hydrothermal system, and related to the exhalative Sirsjöberg-Osjöberg Fe-oxide deposits.The Hjulsjö zones are subdivided into inner and outer zones, based on differences in mineralogy and chemistry resulting from the variable intensity of alteration. While the degree to which most elements are leached increases from the outer into the inner zones, Mg increases with alteration intensity, and Si and Al stay unchanged, although they were mobilized during alteration. Initially K, Rb, Fe and Mn increase in the outer zones, but are depleted in the inner zones. Rare earth elements (REE) are depleted, with the light REE being more depleted than the heavy REE. A pronounced negative Eu anomaly is generally present. The hydrothermal fluid was enriched in ore-forming metals during alteration and leaching. REE patterns of exhalative minerals indicate that the Fe-oxide deposits were derived from these hydrothermal fluids.

Emplacement of deep crustal and mantle rocks on the west median valley wall of the MARK area (MAR, 23°N)

Exposures of deep crustal and mantle rocks within the axial rift valley characterize accretion processes in the slow-spreading Mid-Atlantic Ridge. Nine dives of the submersible Nautile explored the western axial valley wall in the northern cell of the MARK area (Mid-Atlantic Ridge/Kane fracture zone), where peridotite and gabbro outcrops had been previously reported, in order to constrain the structure and determine emplacement mechanisms. The ridge/transform intersection massif on the western wall shows a section of gabbros from 6000 to 2500 mbsl, locally overlain by metabasalts and metadolerites, capped by slightly weathered basalts. The morphology is controlled by ridge-parallel faults, dipping moderately (40–65°) to the east. Transform-parallel scarps, present in the northernmost dives, become rare toward the south. Brittle deformation, along moderately dipping fault scarps, produced a dense microcrack network filled with greenschist facies minerals (chlorite, actinolite, epidote, quartz), locally overprinting high-temperature ductile deformation fabrics. The small hill located at 23°20′ in the western wall shows good exposures of serpentinized peridotite between 3700 and 3100 mbsl. Above 3100 mbsl, the summit of the hill is composed of pillow-basalts and sediments. The peridotite outerops display a strong schistosity dipping 20–40° to the east, parallel to striated normal fault planes. Some steeper east-facing fault scarps truncate the lower-dipping fault surfaces. The serpentinites, deriving from a harzburgitic protolith, are cut by rare dikelets of highly differentiated gabbro. These new data, combined with previous results of Alvin dives, are used to draw a generalized geological map of the western axial valley wall. This map suggests a variation in the thickness of crustal units and composition along strike in the northern cell of the MARK area: the gabbro body disappears toward the south where basalts appear to directly overlie the mantle peridotites which are cut by isolated gabbroic dikelets. Low-angle stretching affecting this heterogeneous lithosphere is probably responsible for the exposure of gabbros in the intersection massif and of peridotites further south.

Deposition and early diagenesis of playa glauberite in the Karinga Creek drainage system, Northern Territory, Australia

Glauberite [Na 2Ca(SO 4) 2], in the form of discrete crystals, nodules and lenses, represents the second most common sulphate mineral type (after gypsum) in the chain of modern playa lakes located in the Karinga Creek drainage system, Northern Territory. These modern glauberite deposits exhibit textures and fabrics which reflect both syndeposition with gypsum within the playa phreatic zone, and early diagenetic replacement of host gypsite in the playa vadose zone. Because the distribution of hydrological zones in individual playa lakes vary significantly, the extent of glauberite facies development is highly irregular. Factors controlling glauberite deposition include the nature and extent of groundwater discharge from local and regional aquifers, playa surface evaporation rates, porewater geochemistry and the depth to basal aquicludes. Geochemical and petrographic evidence indicate that the paragenesis of sulphate minerals in the playa lakes of the study area is primarily dictated by the solubility of gypsum, which in turn reflects the extent of dilution and concentration of playa porewaters. On a regional scale, brine dilution and concentration can be related to seasonal and longer-term groundwater recharge-discharge patterns; but, on a local scale, the proximity to springs and ephemeral seepage creeks emanating from local calcrete aquifers is the dominating influence. Hence, a thorough knowledge of groundwater hydrodynamics, particularly the recharge-discharge pattern in playa-marginal areas, must be an essential part of any detailed assessment of the role of depositional and early diagenetic processes in the genesis of glauberite facies in similar arid drainage basins. The close relationship between glauberite facies development and the playa groundwater level has significant implications for interpretations of variation in the water table and other environmental conditions in the shallowing-upwards sequences of ancient lacustrine deposits. The presence of lacustrine glauberite facies may also mark a transition from depositional to early diagenetic evolution of the basinal sulphate mineral facies.

Two‐stage decompression in orthopyroxene–sillimanite granulites from Forefinger Point, Enderby Land, Antarctica: implications for the evolution of the Archaean Napier Complex

Magnesian metapelites of probable Archaean age from Forefinger Point, SW Enderby Land, East Antarctica, contain very‐high‐temperature granulite facies mineral assemblages, which include orthopyroxene (8–9.5 wt% Al2O3)–sillimanite ± garnet ± quartz ± K‐feldspar, that formed at 10 ± 1.5 kbar and 950 ± 50°C. These assemblages are overprinted by symplectite and corona reaction textures involving sapphirine, orthopyroxene (6–7 wt% Al2O3), cordierite and sometimes spinel at the expense of porphyroblastic garnet or earlier orthopyroxene–sillimanite. These textures mainly pre‐date the development of coarse biotite at the expense of initial mesoperthite, and the subsequent formation of orthopyroxene (4–6 wt% Al2O3)–cordierite–plagioclase rinds on late biotite.The early reaction textures indicate a period of near‐isothermal decompression at temperatures above 900°C. Decompression from 10 ± 1.5 kbar to 7–8 kbar was succeeded by biotite formation at significantly lower temperatures (800–850°C) and further decompression to 4.5 ± 1 kbar at 700–800°C.The later parts of this P–T evolution can be ascribed to the overprinting and reworking of the Forefinger Point granulites by the Late‐Proterozoic (c. 1000 Ma) Rayner Complex metamorphism, but the age and timing of the early high‐temperature decompression is not known. It is speculated that this initial decompression is of Archaean age and therefore records thinning of the crust of the Napier Complex following crustal thickening by tectonic or magmatic mechanisms and preceding the generally wellpreserved post‐deformational near‐isobaric cooling history of this terrain.

Prograde–retrograde P–T–t–deformation path of Austroalpine micaschists during Variscan continental collision (Eastern Alps)

A complete prograde P–T path, defined by 10 calculated P–T fields in succession, is recognized from metapelites by using geothermobarometry on garnet‐bearing assemblages with microstructural control. Overstacking of several tectonic units during an early Variscan continental collision explains the complex prograde P–T history. Isostatic uplift and deformation controlled the retrograde P–T path.Deformation with changing character acted continuously during all stages of the evolution of the Austroalpine basement complex. After the intrusion of Caledonian granitoids, metapelites and magmatic rocks suffered a shearing deformation D1–D2, which produced sheath folds as well as the main foliation S2. Spessartine‐rich first‐generation garnets, situated in microlithons enclosed by S2, record the onset of shearing under increasing high‐pressure–low‐temperature conditions (7 kbar/380°C). Geothermobarometry on second‐generation garnets which have been rotated during growth indicates isothermal decompression from 9 kbar to 5 kbar/500°C and subsequent recompression/heating during continuing shearing. This is explained by overthrusting of a tectonic unit (unit 2) from NE to SW upon the micaschist unit (unit 1), followed by isostatic uplift and further overstocking of a third unit (unit 3). The resulting Pmax of 12 kbar at 650°C and further increasing temperatures up to 680°C accompanied by decompression have been calculated using a third generation of garnets. These high‐pressure–high‐temperature conditions may explain the occurrence of eclogitic metabasites in adjacent regions.Staurolite and kyanite first appeared under decreasing pressures at the last stage of prograde P–T evolution. Shortening deformation D3 and simultaneous growth of typical amphibolite facies minerals (staurolite 2, kyanite 2, sillimanite, andalusite) occurred during the retrograde path. A final step of Variscan evolution was marked by an oppositely directed shearing D4 (at T > 300°C and P > 3 kbar), possibly indicating backthrusting or extension.Apart from acid intrusions, no signs of a previous Caledonian thermotectonic history were found in the area to the south of the Defereggen–Antholz–Vals Line.

Temperature‐time history of subducted continental crust, Mount Olympos Region, Greece

In the Mt. Olympos region of northeastern Greece, continental margin strata and basement rocks were subducted and metamorphosed under blueschist facies conditions, and thrust over carbonate platform strata during Alpine orogenesis. Subsequent exposure of the subducted basement rocks by normal faulting has allowed an integrated study of the timing of metamorphism, its relationship to deformation, and the thermal history of the subducted terrane. Alpine low‐grade metamorphic assemblages occur at four structural levels. Three thrust sheets composed of Paleozoic granitic basement and Mesozoic metasedimentary cover were thrust over Mesozoic carbonate rocks and Eocene flysch; thrusting and metamorphism occurred first in the highest thrust sheets and progressed downward as units were imbricated from NE to SW. 40Ar/39Ar spectra from hornblende, white mica, and biotite samples indicate that the upper two units preserve evidence of four distinct thermal events: (1) 293–302 Ma crystallization of granites, with cooling from >550°C to <325°C by 284 Ma; (2) 98–100 Ma greenschist to blueschist‐greenschist transition facies metamorphism (T∼350–500°C) and imbrication of continental thrust sheets; (3) 53–61 Ma blueschist facies metamorphism and deformation of the basement and continental margin units at T<350–400°C; (4) 36–40 Ma thrusting of blueschists over the carbonate platform, and metamorphism at T∼200–350°C. Only the Eocene and younger events affected the lower two structural packages. A fifth event, indicated by diffusive loss profiles in microcline spectra, reflects the beginning of uplift and cooling to T<100–150°C at 16–23 Ma, associated with normal faulting which continued until Quaternary time. Incomplete resetting of mica ages in all units constrains the temperature of metamorphism during continental subduction to T≤350°C, the closure temperature for Ar in muscovite. The diffusive loss profiles in micas and K‐feldspars enable us to “see through” the younger events to older events in the high‐T parts of the release spectra. Micas grown during earlier metamorphic events lost relatively small amounts of Ar during subsequent high pressure‐low temperature metamorphism. Release spectra from phengites grown during Eocene metamorphism and deformation record the ages of the Ar‐loss events. Alpine deformation in northern Greece occurred over a long time span (∼90 Ma), and involved subduction and episodic imbrication of continental basement before, during, and after the collision of the Apulian and Eurasian plates. Syn‐subduction uplift and cooling probably combined with intermittently higher cooling rates during extensional events to preserve the blueschist facies mineral assemblages as they were exhumed from depths of >20 km. Extension in the Olympos region was synchronous with extension in the Mesohellenic trough and the Aegean back‐arc, and concurrent with westward‐progressing shortening in the external Hellenides.

Pseudotachylite from the Main Zone of the Hidaka metamorphic belt, Hokkaido, northern Japan

Pseudotachylite veins have been found in the mylonite zone of the Hidaka metamorphic belt, Hokkaido, northern Japan. They are associated with faults with WNW‐ESE to ENE‐WSW or NE‐SW trends which make a conjugate set, cutting foliations of the host mylonitic rocks with high obliquity. The mylonitic rocks comprise greenschist facies to prehnite‐pumpellyite facies mineral assemblages. The mode of occurrence of the pseudotachylite veins indicates that they were generated on surfaces of the faults and were intruded as injection veins along microfractures in the host rocks during brittle deformation in near‐surface environments. An analysis of the deformational and metamorphic history of the Hidaka Main Zone suggests that the ambient rock temperature was 200–300° C immediately before the formation of the Hidaka pseudotachylite.Three textural types of veins are distinguished: cryptocrystalline, microcrystalline and glassy. The cryptocrystalline or glassy type often occupies the marginal zones of the microcrystalline‐type veins. The microcrystalline type is largely made up of quench microlites of orthopyroxene, clinopyroxene, biotite, plagioclase and opaque minerals with small amounts of amphibole microlites. The interstices of these microlites are occupied by glassy and/or cryptocrystalline materials. The presence of microlites and glasses in the pseudotachylite veins suggests that the pseudotachylites are the products of rapid cooling of silicate melts at depths of less than 5 km.The bulk chemical composition of the pseudotachylite veins is characterized by low SiO2 and a high water content and is very close to that of the host mylonitic rocks. This indicates that the pseudotachylite was formed by virtual total melting of the host rocks with sufficient hydrous mineral phases. Local chemical variation in the glassy parts of the pseudotachylite veins may be due to either crystallization of quench microlites or the disequilibrium nature of melting of mineral fragments and incomplete mixing of the melts. Pyroxene microlites show a crystallization trend from hypersthene through pigeonite to subcalcic augite with unusually high Al contents. The presence of pigeonite and high‐Al pyroxene microlites, of hornblende and biotite microlites and rare plagioclase microlites may indicate the high temperature and high water content of the melt which formed the pseudotachylite veins. The melt temperatures were estimated to be up to 1100° C using a two‐pyroxene geothermometer. Using published data relating water solubilities in high‐temperature andesitic magmas to pressure, a depth estimate of about 4 km is inferred for the Hidaka pseudotachylites.Evidence derived from pseudotachylites in the Hidaka metamorphic belt supports the conclusion that pseudotachylite is formed by frictional melting along fault surfaces at shallow depths from rocks containing hydrous minerals.

Rb-Sr Systematics of a Gardar-Age Layered Alkaline Monzonite Suite in Southern Norway: A Reply

Previous articleNext article No AccessDiscussion and ReplyRb-Sr Systematics of a Gardar-Age Layered Alkaline Monzonite Suite in Southern Norway: A ReplyP. C. SmalleyP. C. Smalley Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 98, Number 1Jan., 1990 Article DOIhttps://doi.org/10.1086/629381 Views: 1Total views on this site Citations: 4Citations are reported from Crossref Copyright 1990 The University of ChicagoPDF download Crossref reports the following articles citing this article: Michael A. Cosca , Klaus Mezger , and Eric J. Essene The Baltica‐Laurentia Connection: Sveconorwegian (Grenvillian) Metamorphism, Cooling, and Unroofing in the Bamble Sector, Norway Michael A. Cosca et al., The Journal of Geology 106, no.55 (Jul 2015): 539–552.https://doi.org/10.1086/516040L.M Heaman, P.C Smalley A U-Pb study of the Morkheia Complex and associated gneisses, southern Norway: Implications for disturbed Rb-Sr systems and for the temporal evolution of Mesoproterozoic magmatism in Laurentia, Geochimica et Cosmochimica Acta 58, no.88 (Apr 1994): 1899–1911.https://doi.org/10.1016/0016-7037(94)90423-5Lars Kullerud, Sven H. Dahlgren SmNd geochronology of Sveconorwegian granulite facies mineral assemblages in the Bamble Shear Belt, South Norway, Precambrian Research 64, no.1-41-4 (Dec 1993): 389–402.https://doi.org/10.1016/0301-9268(93)90089-KP.C. Smalley, D. Field REE, Th, Hf, Ta in Bamble gabbros (southern Norway) and their amphibolitized equivalents: implications for gabbro tectonic setting, Precambrian Research 53, no.3-43-4 (Nov 1991): 233–242.https://doi.org/10.1016/0301-9268(91)90073-J

Rb-Sr Systematics of a Gardar-Age Layered Alkaline Monzonite Suite in Southern Norway: A Discussion

Previous articleNext article No AccessDiscussion and ReplyRb-Sr Systematics of a Gardar-Age Layered Alkaline Monzonite Suite in Southern Norway: A DiscussionIan C. StarmerIan C. Starmer Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 98, Number 1Jan., 1990 Article DOIhttps://doi.org/10.1086/629380 Views: 1Total views on this site Citations: 9Citations are reported from Crossref Copyright 1990 The University of ChicagoPDF download Crossref reports the following articles citing this article:Tom Andersen, Trine-Lise Knudsen Crustal contaminants in the Permian Oslo Rift, South Norway: constraints from Precambrian geochemistry, Lithos 53, no.3-43-4 (Sep 2000): 247–264.https://doi.org/10.1016/S0024-4937(00)00028-1 Michael A. Cosca , Klaus Mezger , and Eric J. Essene The Baltica‐Laurentia Connection: Sveconorwegian (Grenvillian) Metamorphism, Cooling, and Unroofing in the Bamble Sector, Norway Michael A. Cosca et al., The Journal of Geology 106, no.55 (Jul 2015): 539–552.https://doi.org/10.1086/516040I.A. Munz, B.W.D. Yardley, D.A. Banks, D. Wayne Deep penetration of sedimentary fluids in basement rocks from southern Norway: Evidence from hydrocarbon and brine inclusions in quartz veins, Geochimica et Cosmochimica Acta 59, no.22 (Jan 1995): 239–254.https://doi.org/10.1016/0016-7037(94)00322-DL.M Heaman, P.C Smalley A U-Pb study of the Morkheia Complex and associated gneisses, southern Norway: Implications for disturbed Rb-Sr systems and for the temporal evolution of Mesoproterozoic magmatism in Laurentia, Geochimica et Cosmochimica Acta 58, no.88 (Apr 1994): 1899–1911.https://doi.org/10.1016/0016-7037(94)90423-5Lars Kullerud, Sven H. Dahlgren SmNd geochronology of Sveconorwegian granulite facies mineral assemblages in the Bamble Shear Belt, South Norway, Precambrian Research 64, no.1-41-4 (Dec 1993): 389–402.https://doi.org/10.1016/0301-9268(93)90089-KDiederik Visser, Antony Senior Mg-rich dumortierite in cordierite-orthoamphibole-bearing rocks from the high-grade Bamble Sector, south Norway, Mineralogical Magazine 55, no.381381 (Jul 2018): 563–577.https://doi.org/10.1180/minmag.1991.055.381.09 Timo G. Nijland , and Antony Senior Sveconorwegian Granulite Facies Metamorphism of Polyphase Migmatites and Basic Dikes, South Norway, The Journal of Geology 99, no.44 (Sep 2015): 515–525.https://doi.org/10.1086/629514Ingrid Anne Munz, Randi Morvik Metagabbros in the Modum Complex, southern Norway: an important heat source for Sveconorwegian metamorphism, Precambrian Research 52, no.1-21-2 (Jul 1991): 97–113.https://doi.org/10.1016/0301-9268(91)90015-3 P. C. Smalley Rb-Sr Systematics of a Gardar-Age Layered Alkaline Monzonite Suite in Southern Norway: A Reply, The Journal of Geology 98, no.11 (Sep 2015): 123–127.https://doi.org/10.1086/629381

Synvolcanic and syntectonic gold mineralization at the Bousquet Mine, Abitibi greenstone belt, Quebec

The Bousquet pyrite gold deposit, consisting of disseminated- and vein-type mineralization in sheared and altered greenschist facies Archean volcanic rocks, is hosted by two vein sets: foliation-parallel veins in ore zone 3 and foliation-oblique veins in zones 1, 2, 4, and 5. Foliation-parallel veins are composed of pyrite and lesser amounts of chalcopyrite, sphalerite, pyrrhotite, arsenopyrite, galena, bornite, rutile, tellurides, and native gold. The foliation-oblique veins of zones 1 and 2 are composed of pyrite, chalcopyrite, and sphalerite, with subordinate amounts of pyrrhotite, arsenopyrite, galena, stannite, gudmundite, rutile, ilmenite, and gold. Quartz and muscovite are the principal gangue minerals in both vein types in ore zones 1, 2, and 3. In zone 4, foliation-oblique veins are composed of pyrite, magnetite, pyrrhotite, and minor amounts of chalcopyrite and gold. The gangue minerals are calcite, ankerite, white mica, chlorite, and tourmaline. The veins of zone 5 contain pyrite, chalcopyrite, sphalerite, bornite, magnetite, rutile, and gold in a gangue of quartz, carbonate, white mica, and chlorite. Disseminated auriferous pyrite occurs in zones 3 and 5.Mineralogical and textural features indicate that gold emplacement and associated alteration occurred during two events, one predating peak regional metamorphism and another postdating peak metamorphism. Early, premetamorphic mineralization and related synvolcanic (?) hy-drothermal alteration produced initial auriferous sulfides and an advanced argillic alteration. Subsequent deformation, recrystallization, and metamorphic remobilization of early sulfides resulted in the currently observed, structurally controlled vein-type gold deposits. The vol-canogenic event is suggested by highly aluminous rocks closely associated spatially with the mineralized zones, by textures preserved through the overprint of metamorphic textures, and by the occurrence of stannite in ore zones 1 and 2.The late, postmetamorphic gold mineralization is considered to be contemporaneous with retrograde alteration of metamorphic minerals formed during late deformation. This alteration produced a pervasive kaolinite-pyrophyllite-diaspore gangue mineral assemblage involving the replacement of syntectonic andalusite porphyroblasts in felsic rocks. In mafic host rocks, retrograde alteration is characterized by carbonate, chlorite, muscovite, pyrite, and gold. The formation of a white mica-diaspore-carbonate-chlorite assemblage and the emplacement of associated gold are interpreted as epigenetic metamorphic events because these late retrograde minerals overprint the greenschist facies mineral assemblage.

Reef‐controlled Mineral Deposits in Southern China

Abstract Reef‐controlled mineral deposits in southern China are divided into three types in this paper based on the relative importance of organic reefs in the ore‐forming processes; they are: the direct type, indirect type, and passive type. The case histories of several typical deposits are discussed. In studying reef‐controlled deposits, more attention should be paid to the function of synsedimentary faults in reef construction and mineralization and the importance of oil‐gas palaeoreservoirs in reef rocks for the formation of sulfide deposits, in addition to analysing the favourable sedimentary facies of mineralization.

Reaction history of garnet‐sapphirine granulites and conditions of Archaean high‐pressure granulite‐facies metamorphism in the Central Limpopo Mobile Belt, Zimbabwe

ABSTRACT Sequential reaction textures in Archaean garnet‐corundum‐sapphirine granulites from the Central Zone of the Limpopo Belt document a progression from early, coarse‐grained, high‐pressure (P > 9.5 kbar) granulite‐facies assemblages (M1) to late, low‐pressure (P <6 kbar) granulite‐facies sub‐assemblages (M2).The stable M1 assemblage was garnet (57% pyrope; Mg/(Mg + Fe) = 62) + sapphirine + corundum + gedrite + phlogopite + rutile. Late‐M1 boron‐free kornerupine grew at the expense of garnet and corundum, and coexisted with garnet, sapphirine and gedrite. Partial or complete breakdown of coarse garnet and kornerupine during M2 resulted in the development of pseudomorphs and coronas consisting of fine‐grained symplectic intergrowths of cordierite, gedrite and sapphirine (later, spinel).The majority of reaction textures can be explained in terms of a stable reaction sequence, and a model time‐sequence of mineral facies can be constructed. When compared with a qualitative petrogenetic grid of (Fe, Mg)‐discontinuous reactions in the FMASH multisystem sapphirine‐garnet‐corundum‐spinel‐cordierite‐gedrite‐kornerupine, the facies‐sequence indicates decompression at essentially constant T assuming constant a(H2O).Exhumation of M1 corundum inclusions during M2 breakdown of kornerupine resulted in production of metastable spinel by a disequilibrium reaction with gedrite. A second disequilibrium reaction of the spinel with cordierite produced sapphirine. The operation of such reaction while pressure was decreasing (the opposite dP from that implied by the texture if assumed to be the product of an equilibrium reaction) has serious implications for the use of reaction textures in the construction of P‐T vectors.Garnet‐biotite thermometry on garnet interiors and phlogopite inclusions in corundum yields temperatures of ca. 850°C for the M1 stage. A minimum late‐M1 pressure of ca. 7 kbar is indicated by the former association of kornerupine and corundum. Relict M1 kyanites reported by other workers indicate a minumum early‐M1 pressure of 9.5 kbar, implying metamorphism at depths of at least 33 km (probably 38km). The high‐pressure granulite‐facies metamorphism was followed by an almost isothermal pressure decrease of > 5 kbar, indicative of rapid uplift. The P‐T path is interpreted as the product of a single metamorphic cycle which probably took place in response to tectonic thickening of the crust. Such a process contrasts with the extensional origin recently proposed for isobarically cooled granulite‐facies terranes.

Subduction and accretion of the permanente terrane near San Francisco, California

Previous studies of the Calera limestone (Permanente terrane, Franciscan complex) of California have shown that deposition occurred at approximately 25°N paleolatitude atop an oceanic plateau or rise, and that it was subsequently transported northeast and accreted to the North American continental margin in the early Tertiary. The Permanente terrane in the study area along the Pacific coast south of San Francisco contains a suite of rock types present in north dipping imbricate thrust slices possibly related to the subduction and accretion of an oceanic crustal sequence. Geochemical studies of greenstones, thought to represent fragments of the dismembered oceanic crust, indicate affinities with mid‐ocean ridge basalts and within‐plate basalt. These greenstones may represent fragments of the rise upon which the Calera limestone was deposited. Metamorphic minerals in the greenstones indicate both sea‐floor metamorphism and locally, high‐pressure, low‐temperature metamorphism. It is concluded tentatively that the entire Permanente terrane in the study area underwent high‐pressure, low‐temperature metamorphism but that temporal or geologic conditions prevented widespread recrystallization or development of blueschist facies minerals. Structures in the deformed rocks can be related to underthrusting beneath the continental margin and attachment to the overriding plate. Studies of veins indicate several stages of dewatering. It is likely that dewatering was a major process in the attachment event, as shown by veins associated with this phase of deformation. Structurally interleaved mélanges and broken formation contain sedimentary rock clearly derived from a continental margin; these rocks probably represent trench and trench slope facies that were underthrust with the accreting oceanic crustal sequence.

Depositional environment and metamorphism of early Proterozoic iron formation in the Lüliangshan region, Shanxi Province, China

The early Proterozoic iron formation of the Lüliangshan region is 18 km long and has undergone different degrees of metamorphism at different localities. Within this belt, the Yuanjiacun deposit is characterized by low grade metamorphism, diverse iron sedimentation facies, a well-exposed stratigraphic section and simple structure. It is an ideal case for studying the depositional environment and the metamorphic processes of Precambrian banded iron ores. Formed in a shallow water basin of a geosyncline, the Yuanjiacun iron formation some 1200 m thick comprises three sedimentation-ore formation cycles, each of which begins with coarse- to medium-grained clastic rocks (quartz sandstone or arkose), passes through fine-grained siltstone and argillitic rocks, and grades into pure colloidal chemical deposits represented by thin-bedded ferruginous quartzites. In each cycle from the bottom upwards, in general, there successively occur pyrite, siderite, iron silicates (including Fe-rich chlorite, minnesotaite and stilpnomelane), magnetite and haematite. These iron mineral facies are products of sedimentation and diagenesis to very low grade metamorphism stages. The spatial distribution of iron facies in stratigraphic sequence shows an increasing oxygen fugacity of the depositional environment during the course of each cycle. The main factors controlling the types of iron sedimentation facies and the valence state of iron are considered to be the proportion of sandy-argillitic component to colloidal chemical materials during deposition and the relative amount of organic matter in them. The abundance of organic matter results in the creation of a reducing environment and the formation of principally ferrous facies, whereas the lack of organic matter leads to the development of oxidizing conditions and the formation of ferric facies. Under a transitional oxidation-reduction medium, mixed ferrous-ferric facies dominate. The fracture-related regional metamorphism yielded progressive metamorphic zonation in the Lüliangshan region: the chlorite zone and the biotite zone of greenschist facies, and the almandite zone, staurolite zone and kyanite zone of the amphibolite facies. These metamorphic zones cross-cut banded iron ores and produce corresponding changes in the iron formation.

COBERTURAS METASSEDIMENTARES DO PROTEROZÓICO MÉDIO: OS GRUPOS ARAÍ E PARANOÁ NA REGIÃO DE NIQUELÂNDIA - COLINAS, GOIÁS

In the Niquelândia-Colinas, Goias, Middle Proterozoic Arai Group nonconformably overlies granite-gneiss basement and remains of the possibly Early Proterozoic Ticunzal Formation. The Arai rocks comprise the lower Arraias Formation of quartzite, feldspathic quartzite, and metaconglomerate, and the upper Trairas Formation of carbonatic metapelite, calcschist, chlorite schist, and marble lenses, bearing greenschist facies mineral parageneses. Proeminent deformation structures are the domal shape outlined by the structural high of basement rocks and the large 5-20° S20W plunging asymmetric folds. The latter have not been recognized within the younger Paranoa Group metasediments which unconformably overly the Arai Group. The Paranoa Group is divided into two sequences. The lower is made of five psamitic and pelitic units, with the discontinuous Sao Miguel metaconglomerate at the base. The upper sequence comprises four mostly pelitic and carbonatic units, with less important psamitic layers. The rocks have been metamorphosed to the biotite zone of the greenschist facies. Deformation is represented by an earlier phase of tight to isoclinal folding with axial-plane schistosity, possibly coeval with large nearly N-S trending thrusts verging towards the east. These structures are refolded by asymmetric to reverse chevron folds with north-south axial-plane crenulation cleavage. The latest phases are of SW plunging open folds, and large vertical folds with sub-horizontal E- W trending axes. The lower psamo-pelitic sequence correlates with the unmetamorphosed Paranoa Group outcropping near Alto Paraiso de Goias, to the east of the study area. The dominantly pelitic and carbonatic upper sequence is equivalent to the Minacu Formation described further to the northwest. Stratigraphic relations between these sequences and their natural division in easily recognized units allow their characterization as groups of formations conferring supergroup rank to the Paranoa.

3.5 Ga old amphibolites from eastern Hebei Province, China: Field occurrence, petrography, Sm-Nd isochron age and REE geochemistry

Amphibolite blocks frequently occur in association with grey gneisses within a small area near the villages of Caozhuang and Huangbaiyu in eastern Hebei. They are interpreted as remnants of an earlier bimodal suite now engulfed in late Archaean K-rich granitic magma and are further cut by pegmatite dykes. They are considered together as giant enclaves within late Archaean granite plutons (U-Pb zircon ages = 2.5–2.6 Ga). The amphibolites have been dated by the Sm-Nd isochron method. The results yielded: T = 3470 ± 107 Ma, I Nd = 0.50827 ± 11, and ϵ Nd (T) = +2.7 ± 0.6. This age represents the time of protolithic basalt eruption and possibly suggests a period of formation of primitive mafic crust. This is the oldest reliable age so far obtained for the Archaean rocks of China. The Sm-Nd results of grey gneiss samples yielded model ages ( T DM) of about 3.24 Ga. It is not clear whether this ‘younger’ age in comparison with that of the amphibolites indicates the intrusive nature of these rocks or is a result of contamination by later granite about 2.5 Ga ago. The Rb-Sr and common Pb isotopic systems have been highly disturbed, and no isochron ages could be established. The common Pb data show retarded growth of radiogenic Pb and hence imply reduction of U-Pb ratios in the past. This is consistent with the petrographic study, which suggests that the amphibolites have undergone an earlier period of granulite facies metamorphism and were subsequently retrogressed to the present amphibolite facies mineral assemblages. The positive ϵ Nd ( T) value of +2.7 for the amphibolites implies that the protolithic basalt magmas were generated from mantle sources which had undergone long-term depletion in light rare earth elements (LREE). The depletion event must have taken place prior to 4 Ga ago if their source had a chemical nature similar to the modern oceanic mantle; it could have occurred as early as 4.5 Ga ago if their source was less impoverished in LREE. If the mantle depletion was related to extraction of continental or sialic material, the Sm-Nd data of the Caozhuang amphibolites lend support to the existence of sialic crust, if not of a continental size, before 4 Ga ago.

Taconic deformation and metasomatism in Proterozoic rocks of the easternmost Adirondacks

Proterozoic gneiss exposed in readcuts between Fort Ann and Whitehall, New York, is close to the Precambrian/Paleozoic unconformity. It exhibits several features not previously described in Adirondack rocks. These (features include (1) extensive slickensides (azimuth 120°–150°, plunge 0°–30°, southeast-over-northwest sense of movement) on preexisting foliation surfaces; (2) coexistence of calcite marbles that contain granulite facies mineral assemblages with dolomite marbles that contain dolomite, quartz, microcline, and serpentine in unreacted mutual contact; (3) ferroan dolomite that fills veins, faults, and fractures and occurs in thin, foliation-parallel lamellae in gneiss; and (4) minerals of probable low-temperature origin (adularia, celadonite, and cherty silica) that fill veins and interstitial spaces in gneiss and marbles. We suggest that these features can best be explained by minor rotational shear deformation during the Taconic orogeny, accompanied by local metasomatism caused by Mg-rich connate brines expelled from Cambrian–Ordovician sedimentary rocks by overriding Taconic thrust slices.

Multiple alteration events in the East Bull Lake anorthosite-gabbro layered complex, NE Ontario, Canada: evidence from fracture mineralogy and 40Ar- 39Ar dating

The East Bull Lake anorthosite-gabbro layered complex contains a variety of alteration minerals. Some of the more common ones are calcic amphiboles, biotite, epidote, adularia, quartz, chlorite, calcite, prehnite, pumpellyite, laumontite, gypsum, iron hydroxides and clays. The mode of occurrence and the data related to the stability of the alteration minerals suggest that they were formed under pressure-temperature conditions of: (1) epidote-amphibolite/greenschist facies; (2) prehnite-pumpellyite facies; (3) zeolite facies; and (4) low-temperature mineral facies. 40Ar- 39Ar data of hornblende and adularia indicate that the pluton is affected by distinct alteration events. Two mafic dyke intrusions, that overlap the alteration events, are recognised in the pluton. Synthesis of available radiometric ages suggests that the pluton intruded at 2472 ± 70Ma, and was subjected to alteration as late as the Paleozoic and Cenozoic Eras.

Amphibole composition as an indicator of subtle grade variation in epidote‐glaucophane schists

Abstract The paragenetic relations of epidote‐glaucophane schists are described in terms of the system Al2O3‐Fe2O3‐Fe2O3‐MgO‐CaO with excess of quartz, albite and epidote. If alkali‐amphibole is free from Ca and AlIV, its composition when associated with epidote is invariant, univariant or divariant at a given pressure and temperature on Miyashiro's (1957) diagram of alkali‐amphibole solid solution if it is also associated, respectively, with three, two or one additional minerals in the system.Using a group of epidote‐glaucophane schists from the Kotu area of the Sanbagawa metamor‐phic belt in Shlkoku, Japan (isophysical compositional),univariant boundary lines were determined for the assemblages that, in addition to the ubiquitous quartz + albite + phengitic mica, contain hematite + chlorite, garnet + chlorite and actinolite + chlorite, respectively. The slopes of the univariant boundary lines obtained from petrographical data are in good agreement with those calculated in a model system.The positions of isophysical univariant boundary lines on the amphibole compositional diagram serve to distinguish the grade of metamorphism among the rocks of the same mineral facies. The hematite‐chlorite univariant boundary line can be used to divide the zone of epidote‐glaucophane schists of the Sanbagawa metamorphic belt into three, and the garnet‐chlorite‐paragonite invariant equilibrium can be used to divide the epidote zone of New Caledonia into three.

Geochemistry, mineralogy, and geology of the Jason Pb-Zn deposits, Macmillan Pass, Yukon, Canada

Three separate stratiform Pb-Zn deposits occur on the Jason property in the Macmillan Pass area of the Yukon, Canada. The deposits are contained within chert conglomerates and finer grained clastic rocks of Middle to Late Devonian age. The deposits are on the eastern margin of the Cambro-Ordovician Selwyn basin and are associated with a fault-bounded, second or third-order subbasin, referred to as the MacPass graben.Three different facies of mineralization and associated sedimentation have been defined on the basis of field relationships, drill core descriptions and correlations, and detailed petrography, Facies A comprises massive to thick-bedded galena and sphalerite with abundant pyrrhotite and pyrite. This facies is extensively crosscut by galena-bearing quartz siderite and quartz ankerite veins and has the highest combined Pb-Zn-Ag concentrations. Facies A is proximal to the hydrothermal source represented by the veins. Facies B has a mineralogic assemblage and textures common to facies A but is considerably thinner. Abundant soft sediment deformation features indicate facies B formed from slumping of unstable accumulations of material from facies A. Facies C contains well-bedded sphalerite, chert, and barite laminae and is distal to the hydrothermal source. Secondary alteration and pore-filling minerals found in facies C include celsian, kaolinite, ankerite, and a complex suite of rare Ba carbonates.Fluid inclusions in quartz, siderite, and ankerite within veins crosscutting facies A indicate that the ore-forming fluids were NaCl brines (9 equiv. wt %) with temperatures of approximately 250 degrees C. Sulfur isotope data indicate that the sulfides within the three separate deposits had a common sulfur source, probably reduced seawater sulfate. Barite in the laminated facies C has delta 34 S values consistent with coeval Devonian seawater. The sulfide minerals are approximately 10 per mil lighter than coeval seawater.A maximum water depth of 420 m for venting of hydrothermal fluids was determined by combining fluid inclusion data with boiling curves for H 2 O-NaCl. Brine densities indicate the hydrothermal plume may have reached the seawater surface and dispersed laterally to form the barite in facies C. Accumulations, on unstable slopes, of thick deposits of sulfide slumped to cause thinning and soft sediment deformation of facies B. Since facies B is a sedimentary feature, not necessarily proximal or distal to the vent, it may or may not contain barite.