Volcanic-sedimentary Sequence Research Articles

Types of massive sulphide deposits in the Urals

Massive sulphide deposits in the Urals are found within volcanic and volcanic-sedimentary sequences of Ordovician to Middle Devonian ages. Four types of economic sulphide deposits have been recognized: Cyprus, Besshi, Urals and Baimak. The Cyprus-type copper sulphide deposits are hosted by mafic volcanites that occur in the basal parts of Palaeozoic volcanic sequences. The Besshi-type copper-zinc deposits are located within clastic sedimentary rocks intercalated with basalts and andesites. Zinc-copper deposits of the Urals-type are hosted by bimodal rhyolite-basalt assemblages, which occur at a higher stratigraphic level than those of Cyprus- and Besshi-types. The Baimak-type zinc-copper-barite deposits are associated with intrusive quartz porphyries which occur in the upper parts of bimodal volcanic successions. In addition there are some sulphide deposits of zinc-lead-barite and zinc-copper composition hosted by Ordovician terrigenous sequences which occur within depressions in Precambrian blocks. These types of sulphide deposits have been formed at various stages of divergence and convergence of the Earth's crust during the orogenic history of the Urals.

The early tectono-magmatic evolution of the Southern Province: implications from the Agnew Intrusion, central Ontario, Canada

The Palaeoproterozoic Southern Province comprises a thick, continental rift related volcanic-sedimentary sequence along the southern margin of the Archaean Superior Province. The Agnew Intrusion (50 km2), which is a member of the East Bull Lake suite of layered intrusions, occurs adjacent to the Superior Province - Southern Province boundary in central Ontario, Canada, and provides an opportunity to examine the early tectono-magmatic evolution of a Palaeoproterozoic rifting event. The Agnew Intrusion is a well-exposed, 2100 m thick, layered gabbronoritic to leucogabbronoritic pluton. It was the product of at least four recognizable, but chemically similar, high-Al2O3 and low-TiO2 magma pulses. Structural data, coupled with excellent stratigraphic correlations between the Agnew Intrusion and other East Bull Lake suite layered intrusions, suggest that these plutons are erosional remnants of one or more sill-like bodies that may originally have formed an extensive, subhorizontal mafic sheet. We argue on the basis of field evidence that the early evolution of the Southern Province was characterized by a large, mantle plume induced magmatic event that gave rise to a Palaeoproterozoic continental flood basalt province. However, the incompatible trace element characteristics of the Agnew Intrusion parental magma (i.e., large ion lithophile and light rare earth element enrichment and high field strength element depletion) are more typical of modern subduction-modified subcontinental lithospheric mantle. Given that this is a prevailing geochemical signature of mafic rocks in the Archaean-Palaeoproterozoic, we suggest that there was a fundamental difference in both the composition and structure between the ancient and more modern mantle. "Subduction-like" geochemical signatures may have been imparted to the entire developing mantle during early Earth differentiation.

Epizonal I- and A-type granites and associated ash-flow tuffs, Fogo Island, northeast Newfoundland

Magmatic activity of Silurian–Devonian age is widespread in the Appalachian–Caledonian Orogen. A marked characteristic of this magmatism is the composite nature of the igneous suites, which range from peridotite to granodiorite in single plutonic bodies. The origin of these suites is still enigmatic, and the assumption that all are the same not proven. Such a suite of intrusive rocks, ranging in composition from minor peridotite to granodiorite, intrudes an openly folded sequence of Silurian volcanogenic sandstones and ash-flow tuffs on Fogo Island, northeast Newfoundland. Two units, the Rogers Cove and Hare Bay microgranites, consist of fine-grained hastingsite granites with spherulitic and flow-banded textures, and exhibit drusy cavities and microfractures that contain the mineral assemblage hastingsitic hornblende + plagioclase + magnetite + zircon. These rocks are characterized by elevated high field strength element contents (e.g., Zr = 74–672 and Y = 21–103 ppm), very high FeO*/MgO ratios (FeO*/MgO = 2.4–93.5), and 10 000 Ga/Al ratios of 1.67–10.52, indicating an A-type granitoid affinity. A third and the most voluminous granitic unit, the Shoal Bay granite, is an alkali-feldspar-phyric, medium-grained, equigranular biotite–hastingsite granite with hastingsite and annitic biotite interstitial to euhedral plagioclase, anhedral quartz, and perthite crystals. The Shoal Bay granite exhibits mineral parageneses similar to the microgranites, but chemical characteristics more typical of calc-alkaline, I-type granitoids. Volcanic–sedimentary sequences spatially associated with the granitic rocks include dense, welded, high-silica, hastingsite-bearing ash-flow tuffs with compositions that suggest they represent erupted equivalents of fractionated end members of the Shoal Bay granite. The rocks making up the Fogo Island batholith have been directly equated with the bimodal, calc-alkaline Mount Peyton batholith of northeast Newfoundland, but the specialized A-type nature of the Fogo granites suggests differing source conditions for the two suites.

Relationship between the Aspy and Bras d'Or "terranes" in the northeastern. Cape Breton Highlands, Nova Scotia: Discussion

The author, S. Lin, has made major contributions to the understanding of the complex geology in the Cape Breton Highlands through his detailed mapping in the vicinity of the Eastern Highlands shear zone (see also Lin 1992). However, some generalizations made in his paper do not accurately portray the geology, and imply an unrealistic degree of certainty about some interpretations. The purpose of this discussion is to emphasize that many problems remain unresolved in the vicinity of the Eastern Highlands shear zone, contrary to the impression given by him. The author asserted that the late Precambrian rocks of the Bras d'Or terrane are overlain by the Clyburn Brook formation, a volcanic sedimentary sequence of Silurian age. However, the age of the Clyburn Brook formation is uncertain because it is based on a Rb-Sr isochron; such ages have been shown to be unreliable in the Cape Breton Highlands and elsewhere (e.g., Dunning et al. 1990). In addition, the reported isochron (Keppie and Halliday 1986) is based (with one exception) on Rb Sr data for samples from rhyolite on Ingonish Island, not on samples from the Clyburn Brook formation. Unlike rhyolite in the Clyburn Brook formation, the Ingonish Island rhyolite is unmetamorphosed and lacks associated basaltic, volcaniclastic, and sedimentary units (Raeside and Barr 1992); hence, correlation between the Ingonish Island rhyolite and the Clybum Brook formation is speculative. The statement in the abstract of the paper that the Clyburn Brook formation is Silurian is premature. It may be, but that has yet to be demonstrated. The author (p. 1773) implied that amphibolite-facies rocks occur only in the part of the McMillan Flowage Formation within the Eastern Highlands shear zone, with the further implication that there is a connection between the amphibolitefacies metamorphism and the shear zone. Amphibolite-facies metamorphism in the McMiiian Flowage Formation persists for 16 km southeast of (away from) the shear zone. Furthermore, titanite in the McMillan Flowage Formation has yielded ages of Ca. 540 Ma (Raeside and Barr 1992; G.R. Dunning, personal communication, 1990), showing that peak metamorphism was late Precambrian to Early Cambrian, much older than the deformation associated with the Eastern Highlands shear zone as documented by the author. In describing the Aspy terrane, the author (p. 1774) implied that the Cheticamp Lake Gneiss is the only supracrustal rock unit in the Aspy terrane. As documented by Raeside and Barr (1992), as well as numerous other workers (e.g., Currie 1982; Jarnieson et al. 1987, 1990), the Aspy terrane includes various

Fluid evolution and mineral formation in the Beregovo gold-base metal deposit, Transcarpathia, Ukraine

The Beregovo vein-type gold-base metal deposit is hosted by volcanic sedimentary sequences of the Pannonian basin in the Carpathian Neogene volcanic belt. Four stages of mineralization were distinguished: (1) sulfide, (2) quartz-barite, (3) carbonate-quartz, and (4) carbonate-goethite. Homogenization temperature, salinity, and chemical and isotopic composition of fluid inclusions in sphalerite, galena, barite, quartz, and carbonates, combined with the delta 18 O values of quartz, carbonates, and barite, suggest the presence of three major fluid types: (1) deep, relatively high temperature (250 degrees C) and moderate salinity (about 8 wt % NaCl equiv) Na-K-Cl fluid with relatively high delta 18 O (5ppm), delta D (-60ppm), and delta 13 C (-5ppm) values; (2) high-salinity (up to 14 wt % NaCl equiv) Ca-Mg-Na-Cl formation water with higher delta D values (-40ppm); and (3) near-surface steam-heated (about 200 degrees C) sulfate-bicarbonate-dominated fluid delta 18 O from -5 to -1ppm, and delta D from -110 to -56ppm). Low delta 13 C values (-25ppm) for CO 2 in type 2 and 3 fluids reflect a significant contribution of carbon from organic-rich sedimentary wall rocks.The delta 34 S values of sulfide minerals range from -2 to +2.5 for pyrite, -1 to +6 for sphalerite, and -2 to +8 per mil for galena. Two sources of sulfur are identified for the Beregovo sulfide ores. Ores with delta 34 S values close to 0 per mil may have been magmatic and those with higher delta 34 S values (up to 8ppm) perhaps resulted from organic matter (5ppm) in the wall rocks.Two main isotopically and genetically different barite generations were found: hypogene barites with delta 34 S values from 13 to 26 per mil, and supergene barites with values from 4.0 to 5.5 per mil. None of the barites had isotopically equilibrated with the sulfide minerals. If equilibrium in the isotopic exchange between sulfate and water were realized, the delta 18 O values of the fluid would range from -5.0 to -2.0 and -10.5 to -5.9 per mil for the two generations, respectively. For comparison, oxygen isotope composition for present-day precipitation is about -10 per mil.

Geochemistry and tectonic significance of the Ongarbira metavolcanic rocks, Singhbhum District, India

The volcanic-sedimentary sequence of Ongarbira occurs to the south of the Singhbhum Shear Zone and rests uncomformably on a basement topographic-low between the Chakradharpur Granite and the Singhbhum Granite. Structural analysis reveals that the Ongarbira synform and other adjoining structures are kilometre-scale first-generation folds on E-W striking axial planes. These structures are in marked contrast with the corresponding kilometre-scale first-generation folds typical of the Iron Ore Group synclinorium towards the south of the Ongarbira belt. The Ongarbira metavolcanic rocks exhibit a limited range of tholeitic basalt compositions. The basalts are grouped according to their Mg# and the details of their REE patterns. Groups I and II are relatively unevolved and very similar to LREE- and LIL-depleted ocean ridge tholeiites. The most evolved Group III with LREE enrichment is most similar to LIL-enriched ocean floor basalts, but may be contaminated with crustal materials. In general, the Ongarbira volcanics evolve upward stratigraphically. Petrogenetic major element modelling of the Ongarbira basalts suggests that the primary melt was generated by batch melting (30%) of lherzolite at pressures of at least 1.5 GPa. Sequential fractionation of Ol, O1+Cpx+Pl and Cpx+Pl followed. Comparison of incompatible and compatible element distributions indicates a very similar batch m melting-fractional crystallization sequence for Ongarbira Groups I and II basalts. Group III basalts may have been derived from a batch melt initially more enriched in incompatible elements. Trace element discrimination of the Ongarbira basalts suggests that they were generated in an extensional environment. Primordial mantle normalized element distributions indicate that the basalts are similar to continental rift basalts but with oceanic affinities. The chemical data along with the associated sediments indicate that the Ongarbira basalts were probably emplaced in a mature continental rift, although a back-arc origin cannot be completely disregarded. It is concluded that the Ongarbira volcanics were emplaced as part of the same rifting event that resulted in outpourings of the Proterozoic Dalma and Dhanjori volcanics, implying that the Ongarbira suite is Proterozoic and not part of the Archaean Iron Ore Group. The structural geometry of the Ongarbira rocks is more conformable to that of the Singhbhum Mobile Belt than to the Iron Ore Group synclinorium in the Singhbhum Craton. It is likely that the southern limit of the Singhbhum Mobile Belt is defined by the interface of its E-W structures with the NNE structures of the Iron Ore Group rocks. Thus, the Singhbhum Shear Zone does not mark the interface between the Archaean Singhbhum Craton and the Proterozoic Singhbhum Mobile Belt in the eastern Indian Shield.

An interpretation of gravity anomalies over the Midcontinent Rift, Lake Superior, constrained by GLIMPCE seismic and aeromagnetic data

Cross sections of the Midcontinent Rift in Lake Superior, derived from GLIMPCE seismic reflection images, provide unprecedented structural details of the rift and a new constraint for modelling associated gravity anomalies. In turn, gravity modelling, constrained also by new high-resolution aeromagnetic data, has permitted critical examination of the seismic models. The latter generate gravity anomalies having limited agreement with observed anomalies when appropriate rock densities are assigned. Good agreement may be achieved, generally, by making comparatively local changes to the models, while retaining their larger-scale attributes. Gravity modelling thus enhances and supports GLIMPCE seismic models.Modifications to seismic models include revisions of initial densities within the geometrical framework of the models, leading to a redefinition of lithologies. For example, in some segments of the rift, mafic volcanics are substituted for Keweenawan sedimentary and sedimentary–volcanic sequences and for Lower Proterozoic sediments, and a felsic igneous body is modelled within a mafic volcanic unit. Positions of some unit boundaries and faults, or segments thereof, have also been modified.Gravity modelling traces the paths of the Keweenaw, Isle Royale, Thiel, Douglas, and Michipicoten Island faults deep into the crust, generally supporting the configurations outlined by seismic images and, thereby, arguments for rift development controlled by growth faults. Modelling also indicates a requirement for large, buried masses of mafic (plutonic?) igneous rocks of presumed Keweenawan age along the northern margin of the rift. This imparts an asymmetry to the rift, with northern and southern margins dominated by plutonic and volcanic igneous rocks, respectively.

Guerrero terrane of Mexico: Its role in the Southern Cordillera from new geochemical data

The Guerrero terrane makes up most of the western part of Mexico, is one of the largest terranes of the North American Cordillera, and is characterized by an Upper Jurassic-Lower Cretaceous volcanic-sedimentary sequence of are affinity. Metamorphic rocks that crop out in the western area of the terrane (Arteaga complex) may represent its basement. They are mostly composed of terrigenous sediments (Varales Formation) with minor basaltic pillow lavas, chert, tuff, and limestone. Initial ϵ Nd values (+13) and rare earth element (REE) values for pillow lavas of the Arteaga complex are characteristic of mid-ocean ridge basalts (MORB). In contrast, the Varales Formation sedimentary rocks from the Arteaga complex have negative initial ϵ Nd (-6.2 and -7.2) and are enriched in light REEs. These data indicate that the sediments of the Varales Formation were supplied from an evolved continental crust. The overlying Jurassic(?)-Cretaceous arc-related rocks have initial ϵ Nd (+7.9 to +3.9) and REE patterns similar to those of evolved intraoceanic island arcs. These data show that the evolution of the Guerrero terrane had an early pre-Cretaceous(?) stage, which consisted of an crust receiving sediments from a continental source, and a Cretaceous stage, which was the development of an island arc.The oceanic-continental isotopic signature of the Arteaga complex is different from other western North American Cordilleran terranes (e.g., Alexander, Wrangelfia) that are more completely oceanic in affinity. Nevertheless, the extensive Jurassic(?).Cretaceous arc represents additions of juvenile material to the western North American Cordillera.

Guerrero terrane of Mexico: Its role in the Southern, Cordillera from new geochemical data

Research Article| May 01, 1993 Guerrero terrane of Mexico: Its role in the Southern, Cordillera from new geochemical data Elena Centeno-García; Elena Centeno-García 1Department of Geosciences, University of ,Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Joaquín Ruíz; Joaquín Ruíz 1Department of Geosciences, University of ,Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Peter J. Coney; Peter J. Coney 1Department of Geosciences, University of ,Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar P. Jonathan Patchett; P. Jonathan Patchett 1Department of Geosciences, University of ,Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar Fernando Ortega-Gutiérrez Fernando Ortega-Gutiérrez 2Instituto de Geología, Universidad Nacional Autónoma de México, México D.F. 04510, México Search for other works by this author on: GSW Google Scholar Geology (1993) 21 (5): 419–422. https://doi.org/10.1130/0091-7613(1993)021<0419:GTOMIR>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Elena Centeno-García, Joaquín Ruíz, Peter J. Coney, P. Jonathan Patchett, Fernando Ortega-Gutiérrez; Guerrero terrane of Mexico: Its role in the Southern, Cordillera from new geochemical data. Geology 1993;; 21 (5): 419–422. doi: https://doi.org/10.1130/0091-7613(1993)021<0419:GTOMIR>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Guerrero terrane makes up most of the western part of Mexico, is one of the largest terranes of the North American Cordillera, and is characterized by an Upper Jurassic-Lower Cretaceous volcanic-sedimentary sequence of are affinity. Metamorphic rocks that crop out in the western area of the terrane (Arteaga complex) may represent its basement. They are mostly composed of terrigenous sediments (Varales Formation) with minor basaltic pillow lavas, chert, tuff, and limestone. Initial ϵNd values (+13) and rare earth element (REE) values for pillow lavas of the Arteaga complex are characteristic of mid-ocean ridge basalts (MORB). In contrast, the Varales Formation sedimentary rocks from the Arteaga complex have negative initial ϵNd (-6.2 and -7.2) and are enriched in light REEs. These data indicate that the sediments of the Varales Formation were supplied from an evolved continental crust. The overlying Jurassic(?)-Cretaceous arc-related rocks have initial ϵNd (+7.9 to +3.9) and REE patterns similar to those of evolved intraoceanic island arcs. These data show that the evolution of the Guerrero terrane had an early pre-Cretaceous(?) stage, which consisted of an oceanic crust receiving sediments from a continental source, and a Cretaceous stage, which was the development of an island arc.The oceanic-continental isotopic signature of the Arteaga complex is different from other western North American Cordilleran terranes (e.g., Alexander, Wrangelfia) that are more completely "oceanic" in affinity. Nevertheless, the extensive Jurassic(?).Cretaceous arc represents additions of juvenile material to the western North American Cordillera. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Tectonic implications of 40Ar/39Ar mineral ages from late Precambrian – Cambrian plutons, Avalon composite terrane, southern New Brunswick, Canada

Within the Avalon composite terrane exposed in southern New Brunswick, late Precambrian, low-grade volcanic–sedimentary sequences are juxtaposed against late Precambrian gneisses (Brookville Gneiss) and older platformal metasedimentary rocks (Green Head Group) along the Caledonia Fault. Both assemblages host petrographically similar suites of calc-alkalic dioritic and granodioritic plutons. Those intruding volcanic–sedimentary sequences (Caledonia terrane) record ca. 615–625 Ma crystallization ages typical of arc-related magmatism throughout the Avalon composite terrane. However, 40Ar/39Ar age data from stocks intruding gneisses and platformal metasedimentary rocks (Brookville terrane) suggest significantly younger crystallization ages.36Ar/40Ar versus 39Ar/40Ar isotope correlation ages recorded by hornblende are interpreted to closely date postmagmatic cooling within six plutons: Fairville Granite (547 ± 1 Ma); French Village Quartz Diorite (539 ± 2 and 537 ± 1 Ma); Rockwood Park Granodiorite (529 ± 2 and 523 ± 3.5 Ma); Musquash Granite (526 ± 2 Ma); Milkish Head Granite (Red Bridge pluton, 520 ± 1.5 Ma); Lepreau Diorite (Talbot Road pluton, 519 ± 2 Ma and Hansen Stream pluton, 518 ± 1.5 Ma. A hornblende isotope correlation age of 530 ± 2 Ma from penetratively foliated amphibolite within the French Village Quartz Diorite suggests that the magmatic activity was locally accompanied by ductile shear. Muscovite within granitic pegmatite in the Brookville Gneiss records a 40Ar/39Ar plateau age of 510 ± 1 Ma interpreted to date final phases of associated magmatic activity.Arc-related magmatism extending into the Cambrian contrasts with the characteristic tectono-stratigraphic record in the Avalon composite terrane where late Precambrian igneous rocks are overstepped by Cambrian–Ordovician shallow-marine strata with only a local and minor record of rift-related volcanic activity. Although the Brookville terrane shows affinities with the Avalon composite terrane during the late Precambrian, the 40Ar/39Ar age data suggest that it was isolated as a distinct tectono-stratigraphic element by the Early Cambrian.

Stacking of disparate volcanic and sedimentary units by thrusting in the Archean Favourable Lake greenstone belt, central Canada

In the Favourable Lake greenstone belt, older volcanic-sedimentary units have been stacked concordantly on younger units by three thrust faults. Units above and below thrusts differ in age and/or lithology, and unlike many thrust terranes elsewhere, there is no stratigraphic repetition; the lack of repetition may reflect rapid lateral facies changes. The lowest thrust sheet is at least 6 km thick and includes rock units of two distinct ages: 2925 Ma ultramafic to intermediate flows and intermediate fragmental rocks in the lower part and 2725 Ma mafic flows and a conglomerate-greywacke fan in the upper part. This sheet was thrust northward over a > 2734 Ma felsic to intermediate stratovolcano at least 4 km thick; this implies at least 7 km of vertical movement of the thrust sheet. The stratovolcano is the facies equivalent of, and probable provenance for, the greywacke fan in the upper part of the thrust sheet. The two upper thrust sheets are relatively thin and comprise rock units of 2870-2858 Ma age; rocks of this age were not found in underlying units. The thrust faults apparently developed in a relatively undeformed volcanic-sedimentary sequence; they predated regional isoclinal folding, many discordant faults, and emplacement of bordering granitoid batholiths. The root zone for the thrust sheets is hidden in an area now occupied by granitoid batholiths, but thrusts could have been generated by either horizontal compression at colliding plate margins and/or gravity sliding triggered by vertical uplift associated with emplacement of granitoid plutons.

The Crixas gold deposit, Brazil; thrust-related, postpeak metamorphic gold mineralization of possible Brasilino Cycle age

The Crixas gold deposit is located in central Goias State, approximately 400 km northwest of the capital Brasilia. It is owned in joint venture by INCO Gold Ltd. and Anglo American Ltd. The indicated reserves are 7 million metric tons with a grade of 10 to 12 g/metric ton. The discovery was made in the late 1970s with mining commencing in the fall of 1988.The hosting Crixas greenstone belt is underlain by a typical Archcan metamorphosed volcanic-sedimentary sequence and is surrounded by granite-granite gneiss terrane. The Crixas gold deposit is located at the contact of the metavolcanic and metasedimentary sequences. Gold ore occurs within an upper and lower ore zone. The upper ore zone consists of an outer shell of ferroan dolomite-chlorite-biotite schist, an inner shell of sericite-chlorite schist, and magnetite-chlorite schist which in turn surrounds a core of massive arsenopyrite-pyrrhotite-chalcopyrite. The lower ore zone consists of a quartz-cemented graphitic pelite breccia.The upper and lower ore zones are interpreted to represent metasomatically altered parent lithologies. Metasomatism postdates a maximum regional metamorphism of epidote-amphibolite facies or biotite-garnet grade. Carbonatization, sericitization (base leaching), silicification, and sulfidization are indicated to have operated during the development of the ore deposit. Gold deposition is believed to have occurred as the result of reduction of f (sub S 2 ) through the formation of Fe-rich carbonate, silicate, and sulfide phases.The style of deformation within the deposit is typical of a high-strain environment. Regional structural data provide strong evidence that the high strain is related to low-angle thrusting. The age of the thrusting or the time of mineralization is not known; however, low-angle thrusting is related to a Brasiliano cycle event (550-450 Ma).

Brioverian volcanism and Cadomian tectonics, Baie de St Brieuc, Brittany: stages in the evolution of a late Precambrian ensialic basin

Abstract During late Precambrian times, the Northern Brittany and Lower Normandy region was the site of extensive submarine basaltic volcanism. On the southern side of the Baie de St Brieuc, two volcanic formations have been identified. In the east the Erquy Volcanic Formation (EVF), over 2 km thick, is a sedimentary-volcanic sequence in which the basalts occur mainly as pillowed, massive or composite massive-pillowed flows, interdigitated with intrusive sheets. Whole- and broken-pillow breccias, hyaloclastites, and peperites are locally common. Further west, the Hillion Volcanic Formation (HVF) has a similar thickness and attitude, and exhibits the same range of lithologies as the EVF, though in a more deformed state and metamorphosed to a higher grade. While assigned by some workers to the Ordovician, several lines of evidence are presented which show that the basic volcanics of the EVF form part of the same late Precambrian volcanic suite as the HVF. The suite was erupted in a marginal basin (the Lanvollon basin) environment founded upon a slightly older crust of calc-alkaline plutonic rocks, now represented in part by the adjacent Penthièvre massif. Basic sheets (the Dahouët dykes), cutting this basement, are considered co-genetic with the volcanics of the EVF and HVF. Chemically, the metabasic lavas, sub-volcanic sills and dyke rocks are subalkaline, tholeiitic basalts and basaltic andesites. The Lanvollon basin was subsequently filled with clastic sediments and then subjected to Cadomian deformation and metamorphism, the grade of which increases westwards, varying from prehnite-pumpellyite to lower amphibolite facies.

Geobarometry, differential block movements, and crustal, structure of the southwestern Abitibi greenstone belt, Canada

The Southern volcanic zone of the Archean Abitibi greenstone belt comprises several fault-bounded tectonic blocks with distinct volcanic-sedimentary sequences, pressure-temperature-time ( P-T-t ) histories, granite compositions, and metallogenic provinces. Pressure distributions have been mapped out from an extensive new database, derived from Al in hornblende and other geobarometers. In the composite crustal structure of the Southern volcanic zone, determined from the pressure-time data, the Lacorne block and Pontiac subprovince represent deeper crustal levels, overlain by rocks exposed at the surface in the Round Lake or Lac Abitibi-type blocks, which are in turn capped by those exposed at the surface in a Blake River-Rouyn-type block. Exposure of the deeper crustal levels and juxtaposition of these distinct tectonic blocks are interpreted to have occurred by differential block uplift caused by strike-slip faulting, which was related to oblique collisional tectonics during late-tectonic development of the Superior province in late Archean time.

Contrasting geochemistry of two pegmatite fields in Manitoba: Products of juvenile Aphebian crust and polycyclic Archean evolution

The geochemistry of the Proterozoic Wekusko Lake pegmatite field, in the Flin Flon-Snow Lake greenstone belt of the Churchill Province, is remarkably primitive, compared with the Kenoran Cat Lake-Winnipeg River pegmatite field in the Superior Province and its analogs elsewhere in Archean cratons. Comparable pegmatite types at Wekusko Lake are slightly depleted in Li, considerably impoverished in Rb, Cs, Be, P and F, and poor in Sn, Nb and Ta relative to those in the Cat Lake-Winnipeg River terrane. Eliminating differences in plutonic evolution, level of erosion and, to a degree, in the effectiveness of volatile components, the principal reason for the difference in the geochemical signature of the pegmatite fields is the nature of the crust and its evolution in the two regions. The Proterozoic greenstone belt resulted from a single-cycle metamorphism and melting of juvenile volcanic-sedimentary sequences derived from depleted mantle ( ϵ Nd = +5) with minimal, if any, contribution from pre-existing crust. Consequently, the source rocks of the granite-pegmatite evolution were highly primitive. In contrast, the host terrane of the Kenoran pegmatite field contains pre-2800 Ma crustal components of several generations, and a 2780 Ma tonalitic basement. The greenstone belt lithologies and associated batholithic granitoids are derived in part from a heterogeneous, near-chondritic to REE- and LILE-enriched mantle ( ϵ Nd = +2), in part from the pre-existing crust. This polycyclic crustal evolution generated highly enriched protoliths from which the fertile granites and their pegmatite aureoles evolved. The low level of geochemical evolution in the Wekusko Lake field is not necessarily characteristic of other Proterozoic rare-element pegmatites. In the Svecofennian terrane of Sweden and Finland, developed in part over Archean basement and with considerable Archean components in its lithologies, the fertile granites and their pegmatite aureoles reached the same fractionation levels as in the Kenoran provinces.

Scandian ixiolite, pyrochlore and bazzite in granite pegmatite in terdal, Telemark, Norway. A contribution to the mineralogy and geochemistry of scandium and tin

In Tordal granite pegmatite veins cut through a volcano-sedimentary sequence which overlies an older, gneissic basement and younger granite from which they originate. Positive co-variations of scandium, lithium, beryllium and tin, which were documented through a recent prospecting program for tin, indicate a possible enrichment of scandium and tin during the passage of the pegmatitic material through the volcanic sedimentary sequence. A description is given of scandium-rich ixiolite, (Ta,Nb,Sc,Sn,Fe,Mn,Ti)2O4 in association with pyrochlore, (Ca,Sc,Y,Sn,U)2(Ta,Nb,Ti)2O6(O,OH,F) and bazzite, Be3Sc2Si6O18 from a cleavelandite-amazonite pegmatite occurring as a part of a larger network of granitic pegmatite veins. Alteration from ixiolite to pyrochlore appears as a late magmatic-hydrothermal event.

The regional extent of the ca. 600 Ma Virgilina deformation: Implications for stratigraphic correlation in the Carolina terrane

We present a regional stratigraphic and structural architecture of the southern Appalachian Carolina terrane, based on new field studies in central North Carolina. The Virgilina deformation is interpreted to be a regional feature. Stratigraphic units include an older (ca. 700 to 600 Ma), calc-alkalic volcanic (Hyco and Virgilina Formations) and intra-arc rift(?) epiclastic (Aaron Formation) sequence, folded and faulted by the Virgilina event. Unconformably overlying the older stratigraphy, there is a younger (ca. 600 to 540 Ma, bimodal volcanic-sedimentary sequence (Uwharrie Formation and Albemarle Group). Some present models imply that the effects of the Virgilina deformation are not wide-spread throughout the Carolina terrane. This misconception arose because the northern slate belt stratigraphy (Hyco, Aaron, and Virgilina Formations) was miscorrelated with central slate belt units (Uwharrie Formation and Albemarle Group). A re-evaluation of the physical stratigraphic similarities and temporal constraints of Ediacarian fauna in these units indicates that the two sequences are not equivalent. Stratigraphic and structural data from the study area document the Virgilina deformation and its associated unconformity in the central North Carolina portion of the Carolina terrane. Stratigraphic and map evidence indicates that the Uwharrie Formation unconformably overlies the Hyco Formation. The unconformity between the older and younger stratigraphic units is a boundary between discordances in lineations, fold axial traces, and fold tightness. The Virgilina deformation appears to be coeval with the Monian and Cadomian events of Europe and with the Pan-African orogeny of west Africa. The stratigraphic and tectonic sequence presented herein provides a terrane characterization for the Carolina terrane that may be of use in reassembling Avalon, if Avalon was ever a single entity.

Early Proterozoic bimodal volcanism, hydrothermal activity, and massive sulfide deposition in the Boliden-Langdal area, Skellefte District, Sweden

The Boliden-Laangdal area in the eastern part of the Skellefte district consists of an early Proterozoic volcanic-sedimentary sequence which includes massive sulfide deposits. Both the ores and their country rocks have been strongly folded and metamorphosed in the lower amphibolite facies and intruded by synorogenic (1,890 m.y.) and late orogenic (1,750 m.y.) granitoids.The calc-alkaline/tholeiitic volcanic suite displays petrological and geochemical features similar to Phanerozoic subduction-related volcanic arcs. Its bimodal character suggests that extensional forces dominated in the arc during long periods, probably producing incipient rifting. In connection with this tensional stress large volumes of felsic pyroclastic and subordinate lava flows were extruded and large-scale tectono-volcanic depressions formed (e.g., calderas). This event, with predominantly felsic volcanism, culminated with the extrusion of genetically unrelated mafic and ultramafic rocks. Most of this activity occurred as submarine volcanism.The convergence in space and time of a thick volcanic sequence with high permeability, synvolcanic faulting, and a high geothermal gradient produced by the magmatic activity favored the starting of a convective hydrothermal system where the fluid was modified seawater. The hydrothermal activity produced extensive alteration in the volcanic rocks under low water/rock mass ratio conditions, changing their original chemical compositions and producing ore-bearing hydrothermal solutions. The geochemical data suggest that most of the Cu and Zn were leached from the volcanic rocks whereas the provenance of the Pb is more uncertain because it is normally enriched in these rocks. When the ore-bearing hydrothermal solutions reached the sea floor, they caused the deposition of massive sulfide deposits within local tectono-volcanic depressions.

Two Stage Felsic Volcanism in the Lower Proterozoic Upper Aillik Group, Labrador, Canada: Its Relationship to Syn- and Post-Kinematic Plutonism

Summary The Aillik Group, situated in the Lower Proterozoic Makkovik Province, Labrador, Canada, is a bimodal volcanic suite consisting of a lower mafic volcanicsedimentary sequence and an upper felsic volcanic-volcaniclastic association. These rocks have been intruded by syn-and postkinematic granitoid rocks. Previous mapping and stratigraphic interpretations suggest that the upper Aillik Group is itself divisible into ‘early’ and ‘late’ parts separated by a mafic volcanic/tuffaceous unit. Both involve dacitic to rhyolitic volcanism, but the earlier one has a much higher proportion of metasedimentary material and the later one is dominated by rhyolitic volcanic rocks. Major and trace element data suggest that these two sequences are compositionally distinct. The late sequence is compositionally similar to the postkinematic granites, but data are inadequate to establish a link between the early sequence and synkinematic granites, yet do not preclude it. Although geochronological data suggest c. 1800 Ma and 1660 Ma events in both the upper Aillik Group and granitoid plutons, a straightforward model linking early upper Aillik Group with synkinematic granites and late upper Aillik Group with postkinematic granites is complicated by c. 1800 Ma ages from some felsic volcanic rocks assigned as late upper Aillik Group. Inadequate stratigraphic control, insufficient petrochemical data and (or) inherited ages could account for the contradictions; or the model may be invalid. Detailed stratigraphic analysis and careful geochemical studies are required to resolve the conflicts.

A new look at the stratigraphy of the Yellowknife Supergroup at Yellowknife, N.W.T. — implications for the age of gold-bearing shear zones and Archean basin evolution

Based on recent detailed mapping, a revised stratigraphic column is proposed for the rocks of the Archean Yellowknife Supergroup in the Yellowknife greenstone belt. The mafic volcanic rocks of the Kam Formation, previously thought to represent the oldest supracrustal rocks of the belt, overlap remnants of an earlier volcanic–sedimentary sequence, here referred to as the Octopus Formation. As its enormous thickness makes it too unwieldy to be described as a single formation, the Kam Formation is raised to group status and subdivided into four formations. It is proposed that the Kam Group should replace the Beaulieu Group in the Yellowknife area. The Chan Formation, at the base of the Kam Group, consists of multiple gabbroic intrusions that were emplaced into a carapace of pillowed flows. The intrusions locally resemble sheeted mafic dyke complexes in Phanerozoic ophiolites, thought to represent evidence for sea-floor spreading. The Crestaurum Formation, which overlies the Chan Formation, is characterized by massive and pillowed flows interlayered with a number of laterally continuous cherts and felsic tuffs. The Townsite Formation consists of rhyodacite breccias interbedded with felsic tuffs and pillowed dacites. The Yellowknife Bay Formation, at the top of the Kam Group and comprising massive and pillowed flows with pillow breccias and numerous interflow sediments, contains all the important gold deposits mined at Yellowknife. The Banting Formation, directly overlying the Kam Group and consisting of mafic to felsic volcanics, is also given group status and subdivided into two formations. Conglomerates and sandstones of the Jackson Lake Formation, formerly thought to separate the Kam and Banting groups, are considered to represent the youngest rocks of the Yellowknife Supergroup near Yellowknife. Gold-bearing shear zones clearly postdate deposition of the Banting Group, making the rocks of this group a potential target for gold exploration. The presence of remnants of a possible spreading center at the base of the Kam Group suggests that plate-tectonic processes were active during the formation of Archean supracrustal basins in the Slave Province.