Tholeiitic Volcanic Rocks Research Articles

Tholeiitic volcanic rocks of the late Archean Blake River Group, southern Abitibi greenstone belt: origin and geodynamic implications

The late Archean Blake River Group volcanic sequence forms the uppermost part of the southern Abitibi greenstone belt in Quebec. The group is mainly composed of mid-ocean-ridge basalt (MORB)-like tholeiites that show a progressive change of several incompatible trace element ratios (e.g., Nb/Th, Nb/Ta, La/Yb, and Zr/Y) during differentiation. The compositional variations are inferred to be the result of fractional crystallization coupled with mixing–contamination of tholeiites by calc-alkaline magma which produced the mafic–intermediate lavas intercalated with the tholeiites in the uppermost part of the sequence. The MORB-like tholeiites were probably emplaced in a back-arc setting.

Massive sulfide deposits of the Noranda area, Quebec. III. The Ansil mine

The Ansil massive sulfide deposit occurs at the contact of the underlying Northwest Rhyolite and the overlying Rusty Ridge Andesite, in the lower part of the Central Mine sequence of the Blake River Group. The orebody, which is roughly ellipsoidal in outline and up to 200 m × 150 m across, contained reserves of 1.58 Mt of massive sulfide grading 7.2% Cu, 0.9% Zn, 1.6 g/t Au, and 26.5 g/t Ag. Production began in 1989. Least-altered host rocks are low-K basaltic andesites and low-K rhyolites. These rocks have Zr/Y ratios of ~5 and LaN/YbN ratios of ~2.3, typical of tholeiitic volcanic rocks, although their major-element chemistry is transitional between tholeiitic and calc-alkaline volcanic rocks.The Ansil deposit, which dips ~50° east, is a single orebody comprising two main massive sulfide lenses (up to ~35 m thick) connected laterally via a thinner blanket of massive sulfides, with thin discontinuous but conformable massive magnetite units at the base and top of the orebody. Sulfide ore consists of massive to banded pyrrhotite–chalcopyrite. In the downplunge lens, up to 10 m of massive magnetite are capped by up to 10 m of massive sulfide. Finely banded cherty tuff, with sphalerite–pyrite–chalcopyrite, forms a discontinuous fringe to the deposit.The two main lenses of massive sulfide have the highest contents of Cu, Ag, and Au and are thought to have formed in areas of major hydrothermal input. Altered feeder zones contain either chlorite + chalcopyrite + pyrrhotite ± magnetite, or chlorite + magnetite ± sulfides. Footwall mineralization forms semiconformable zones ~5–10 m thick that directly underlie the orebody and high-angle pipelike zones that extend at least 50 m into the footwall. Ti–Zr–Al plots indicate that almost all altered footwall rocks were derived from a homogeneous rhyolite precursor. Hanging-wall andesites were also altered. Despite some severe alteration, all initial volcanic rock compositions can be readily identified, and thus mass changes can be calculated. Silica has been both significantly added or removed from the footwall, whereas K has been added except in feeder pipes. Oxygen-isotope compositions up to at least 50 m into the hanging wall and footwall are typically depleted in δ18O by 2–6‰. These rocks have gained Fe + Mg and lost Si. Altered samples in general range from light-rare-earth-element (REE) depleted to light-REE enriched, although some samples exhibit little REE modification despite strong alkali depletion. Mineralized volcanic rocks immediately below the orebody are enriched in Eu (as are some Cu-rich sulfides in the orebody).Contact and petrographic relations generally suggest that the main zone of massive magnetite formed by replacement of cp–po-rich sulfides, although local relations are ambiguous. Magnetite formation may reflect waning hydrothermal activity, during which fluids mixed with seawater and became cooler and more oxidized. Cu-rich feeder pipes that cut magnetite-rich footwall indicate a renewal of Cu-sulfide mineralization after magnetite deposition. Chloritic zones with disseminated sulfides occur up to a few hundred metres above the orebody, attesting to continuing hydrothermal activity.

The Early Proterozoic structural and tectonic history of the south central Lake Superior Region

The early Proterozoic tectonic evolution of the south central Lake Superior region is complex, owing to the presence of four tectonostratigraphic terranes, which were affected by six phases of deformation. The four terranes are the passive margin of the Superior craton, two paraautochthonous passive margin terranes (Crystal Falls and Florence‐Niagara terranes), and a southern magmatic arc complex which is probably allochthonous with respect to the other terranes. Four of the six deformational episodes accompanied subhorizontal shortening, while two were caused by subvertical shortening. The first and the most penetrative phase of deformation is marked by subhorizontal shortening in a NNE‐SSW direction. The second and fourth deformations were characterized by subvertical shortening and did not significantly modify the structural orientations from previous events in the study area. The third, fifth, and sixth deformations mostly caused open folding, and shortening directions were NW, NE, and W, respectively. Because all the terranes in the south central Lake Superior region share parallel deformational histories, it is suggested that the accretion of these terranes occurred during the first deformational episode. After removal of younger deformational effects, including open folding of the suture zone, the tectonostratigraphic assemblages in this region show the following sequence from NNE to SSW: a platformal assemblage overlying sialic basement, a basinal assemblage of tholeiitic volcanic rocks overlain by deep‐water turbidites, an assemblage of basin floor deposits (Crystal Falls terrane) with apparently no demonstratably underlying crystalline basement, a fault‐bounded terrane with highly strained passive margin strata (Florence‐Niagara terrane), and a calc‐alkaline magmatic arc assemblage. Such an arrangement of tectonostratigraphic assemblages is comparable with cross sections through Phanerozoic accretionary continental margins and therefore supports an arc‐continent collision model.

Geochemistry and origin of Archean felsic metavolcanic rocks, central Noranda area, Quebec, Canada

Felsic magma petrogenesis was studied by analyzing 24 stratigraphically controlled Archean andesite-to-rhyolite lava flows of both tholeiitic and calc-alkalic affinity from the upper Noranda Subgroup, Quebec, using instrumental neutron activation and X-ray fluorescence techniques. The lavas have moderate values of [La/Yb]N (0.9–3.8) and low values of 100 × Th/Zr (~1). According to calculations following batch partial melting and Rayleigh fractional crystallization models, both the calc-alkalic and tholeiitic felsic volcanic rocks are probably products of shallow-level fractional crystallization of mafic parental magmas formed respectively by lower (~7 % for calc-alkalic) and higher (~14% for tholeiitic) degrees of partial melting of a primitive mantle source.Contribution to the magma genesis from plausible crustal materials was negligible. A back-arc-type diapirism is geochemically suggested for the tectonic model of origin of Noranda felsic magmas, in conformity with geological observations. Felsic volcanic rocks with compositions analogous to the studied samples exist in several other Archean terrains of the Canadian Shield, suggesting thereby that the late Archean sialic crust was at least in part produced by volcanic rocks ultimately derived from the primitive mantle.

Platinum-group element distribution in komatiitic and tholeiitic volcanic rocks from Munro Township, Ontario

Platinum, Pd, Ir, Au, MgO, Cr, S, Ni, Co, Cu, and Zn were determined in volcanic rocks from Pyke Hill, Fred's flow, and Theo's flow in Munro Township, Ontario, to establish the fractionation trends of the platinum-group elements in these rocks and to evaluate the relevant processes controlling such fractionation. Distinctive and relatively consistent fractionation trends are characteristic of Pt, Pd, and Ir. Platinum and Pd are usually concentrated in the upper units of flows (spinifex-textured rocks) relative to the lower olivine and pyroxene-rich cumulates, whereas the reverse is true for Ir. Theo's flow does not show these patterns in that Pt, Pd, and Ir behave in a similar fashion. The platinum-group element fractionation trends formed during the crystallization of these flows suggest that sulfide immiscibility has an insignificant role. It is concluded that, in general, Ir must be coprecipitated with olivine or pyroxene whereas Pt and Pd are rejected from the structures of these minerals. The Ir ferromagnesian association probably does not involve a substitution of Ir in the mineral structure of olivine or pyroxene.

Discussion on the Cretaceous tholeiitic volcanic rocks from the Western Cordillera of Colombia

Drs W. J. McCourt and J. A. Aspden write: The geological arguments used by Millward et al. 1984 to dismiss an island-arc origin for the Diabase Group (DG) are not convincing. The statement ‘the intimate association with basinal sediments necessitates the location of such an arc almost on top of the trench’ is unjustified; or are the authors suggesting that basinal (? deep water) sediments are restricted to trench environments? Similarily, ‘the insignificant amount of pyroclastic material’ is applicable only within the Cali area, whilst sizeable volumes of such rocks are present both to the north and south. Equally the objection to ‘the abundance of dolerite’ is curious; would one not expect to find coarser grained equivalents within the lower levels of a volcanic pile? ‘The presence of continentally derived clasts’ within the interbedded Dagua Group indicates that the sequence was deposited sufficiently close to a continental edge to receive such material; it definitely does not ‘preclude the origin of arc as an allochthonous terrain’. In fact, Millward et al. point out that the sequence ‘could have been translated considerable distances along the S. American continent’, if this is so, then why is it not allochthonous?. Millward et al. correctly point out that ‘the area described is small compared with the total Mesozoic belt of Colombia and Ecuador’. In fact, within Colombia alone it accounts for only 2.5% of the Western Cordillera. Of greater significance, however, is the fact that the area sampled forms part of a major elongate tectonic

茨城県大子地域に産するホルトノライト安山岩

Hortonolite andesites of middle Miocene age have been found in the Daigo district of the northeastern part of Ibaraki Prefecture, Northeast Japan. These andesites (the Nantaisan andesites) occur as subaqueous autobrecciated lavas and massive lavas which are also erupted in water bottom. The Nantaisan andesites are mostly aphyric, but some andesites contain small amount of phenocrysts of hortonolite, ferrohypersthene and plagioclase. An andesite specimen has ferropigeonite, hortonolite and ferrohortonolite in the groundmass. Ferropigeonite of the groundmass is lower in Fe/Mg ratio than the coexisting hortonolite and ferrohortonolite. The latter minerals bear no reaction relation to the former mineral. Bulk chemical compositions of the Nantaisan andesites are similar to those of icelandites from eastern Iceland. Differences in petrologic characters in the Quaternary and Miocene tholeiitic volcanic rocks occurring near the Quaternary volcanic front and its neighbouring Miocene volcanic field in the northeast Japan arc are described in the paper.

Cretaceous tholeiitic volcanic rocks from the Western Cordillera of Colombia

The Western Cordillera of Colombia, underlain by crust of probable oceanic character, is constructed almost entirely of Cretaceous sedimentary and volcanic rocks. The Diabase Group consists of tholeiitic basalts, dolerites and gabbros, with thin intercalations of chert and shale. Two major sedimentary sequences, one predominantly arenaceous, the other argillaceous, are conformable with the volcanic rocks. Diabase Group rocks possess a limited compositional range and show only minor magmatic evolution. The main geochemical characteristics include low concentrations of LIL elements, light-depleted to slightly light-element-enriched chondrite-normalized REE patterns, and high Nb and Ta contents relative to N-type MORB. Detailed studies of minor and trace elements indicate that the rocks have a transitional nature, similar to T-type MORB. The Western Cordilleran sequence probably formed in an extensive oceanic flood basalt province, similar to the Caribbean or Nauru Basin, adjacent to the margin of southAmerica

Rb–Sr isochron ages, magmatic 87Sr/86Sr initial ratios, and oxygen isotope geochemistry of the Proterozoic lava flows and intrusions of the East Arm of Great Slave Lake, Northwest Territories, Canada

Two Rb–Sr isochrons from the oldest (Wilson Island Group) and one of the youngest (Pearson Formation) Proterozoic volcanic units in the East Arm of Great Slave Lake give dates and initial ratios of 1810 ± 19 Ma, 0.7051 ± 0.0008, and 1809 ± 30 Ma, 0.7089 ± 0.0004, respectively. These dates restrict the Great Slave Supergroup entirely to the Aphebian. The Pearson Formation date is interpreted as magmatic, but it is considered to be rapidly followed by a significant metamorphic and tectonic event within the area. Both the above suites and the volcanic formations of intermediate age have undergone metamorphism up to and including epidote–amphibolite facies. A study of remnant clinopyroxene grains from these formations has indicated general increases in δ18O of whole rocks during epidote–actinolite facies metamorphism of 0.4–2.0‰. The tholeiitic volcanic rocks and intrusions all indicated δ18O values typical of modern continental tholeiites (5.9–7.0‰). The highest δ18O came from those suites that were suspected of having been contaminated by crustal material. The estimated initial 87Sr/86Sr ratios of the magmas, both from clinopyroxene separates and isochrons, indicate a mantle origin for early tholeiitic mid-Aphebian diabase (0.7016–0.7017), Union Island Group diabase (0.7021–0.7030), and Sosan Group alkali volcanics (0.7017). The later Jackson gabbro (0.7050–0.7054) and especially the Pearson Formation tholeiitic basalt (0.7089) both show the effects of significant crustal contamination. The evidence for the Wilson Island Group is less decisive but appears to indicate a mantle origin.

Parallel evolution of Archaean low- and high-grade terrane: A view based on relationships between the Abitibi, Wawa and Kapuskasing belts

Abstract The Kapuskasing structural zone (KSZ) is a region of high-grade Archaean gneiss transverse to the regional east—west structural trends of the adjacent lower-grade Abitibi, Wawa and Quetico belts. The structural zone is made up of granulite and amphibolite facies paragneiss, tonalitic gneiss and anorthosite-suite gneiss, whereas adjacent regions are metavolcanic—metasedimentary belts, with gneissic and plutonic components. Foliations in the KSZ strike northeast and dip moderately northwest. To account for the development of contrasting lithological, structural and metamorphic characteristics in adjacent areas, a unique Archaean history is postulated for the KSZ. Psammitic sedimentary rocks, possibly deposited in a fault-bounded basin, were intruded by anorthosite which may have had its erruptive equivalents in the tholeiitic volcanic rocks in the adjacent Abitibi belt. During the Kenoran orogeny the KSZ was metamorphosed and intruded at infrastructural levels while low-grade regions outside underwent sub-greenschist to amphibolite metamorphism at higher levels. Late ductile shear in the KSZ reoriented gneissic structural elements into northeast directions. Proterozoic fault movements juxtaposed low and high-grade regions.

Geochemistry of volcanic rocks from the Tetagouche Group, Bathurst, New Brunswick, Canada

The volcanic rocks of the Tetagouche Group are predominantly dacitic to rhyolitic pyroclastics and lavas; mafic alkaline and tholeiitic volcanic rocks are less abundant. Lavas representing the intermediate range (such as andesites) are uncommon.As a consequence of intense Na2O and K2O metasomatism, the mafic volcanic rocks have been classified on the basis of relatively immobile elements such as Ti, Y, Zr, Nb, Ni and Cr.By reference to volcanic suites described elsewhere for varying geologic and tectonic environments, the Tetagouche Group appears to represent two geologic environments. It is proposed that the deposition of tholeiitic and alkaline basalts accompanied the rifting associated with the opening of the Proto-Atlantic, which began during Hadrynian times. However the calc-alkaline felsic volcanic rocks were deposited on the top of the basaltic sequence along a mature island arc system that developed with the closing of the Proto-Atlantic during Middle Ordovician time.

Geochemistry of volcanics from the Ninetyeast Ridge and its vicinity in the Indian Ocean

The basalts and oceanic andesites from the aseismic Ninetyeast Ridge display trachytic, vesicular and amygdaloidal textures suggesting a subaerial volcanic environment. The normative composition of the Ninetyeast Ridge ranges from olivine picriteto nepheline-normative alkaline basalt, suggesting a wide range of differentiation. This is further supported by the fractionation—differentiation trends displayed by transition metal trace elements (Ni, Cr, V and Cu). The Ninetyeast Ridge rocks are enriched in rare earth (RE) and large ion lithophile (LIL) elements and Sr isotopes (0.7043–0.7049), similar to alkali basalts and tholeiites from seamounts and islands, but different from LIL-element-depleted tholeiitic volcanic rocks of the recent seismic mid-Indian oceanic ridge. The constancy of 87 Sr 86 Sr ratios for basalts and andesites is compatible with a model involving fractional crystallization of mafic magma. The variation of ( La SM ) e.f ratios between 0.97 and 2.79 may possibly be explained in terms of a primordial hot mantle and/or chemically contrasting heterogeneous mantle source layers relatively undepleted in LIL elements at different periods in the geologic past. In general, the Sr isotopic data for rocks from different tectonic environments are consistent with a “zoning—depletion model” with systematically arranged alternate alkali-poor and alkali-rich layers in the mantle beneath the Indian Ocean.

Komatiitic and Iron-rich Tholeiitic Lavas of Munro Township, Northeast Ontario

Munro Township, in the Archean Abitibi greenstone belt of northeast Ontario, contains volcanic and hypabyssal rocks of two magma series: (1) an Fe-rich tholeiitic series of basaltic to dacitic lava flows (3–10 m thick), layered peridotite-pyroxenite-gabbro flows (120 m thick), and layered sills (700 m thick); (2) an ultramafic-mafic komatiitic series, comprising discrete lava flows of peridotitic to andesitic composition (1–17 m thick), layered peridotite-gabbro flows (120 m thick), and layered sills (500 m thick). The komatiitie lavas form a succession about 1000 m thick that is both underlain and overlain by thicker successions of tholeiitic volcanic rocks. Three types of komatiite are recognized: peridotitic, pyroxenitic, and basaltic komatiites. The most ultramafic are peridotitic cumulates rich in forsteritic olivine (Fo89–94), at the bases of flows and sills. Many less mafic peridotitic komatiites (MgO: 20–30 per cent), which typically form the upper parts of flows and the marginal parts of small intrusions, exhibit spinifex textures indicative of their formation from ultrabasic liquids. Pyroxenitic komatiites (MgO: 12–20 per cent) also may contain olivine, but are dominated by clinopyroxene, usually in spinifex textures. Basaltic komatiites (MgO <12 per cent) are composed mainly of clino-pyroxene and plagioclase or devitrified glass, rarely in spinifex texture and more commonly in equigranular textures. Peridotitic komatiite with roughly 30 per cent MgO appears to represent a parental liquid from which the more ultramafic komatiites formed by accumulation of olivine, and the less mafic types were derived by fractionation of olivine, joined and finally succeeded in later stages by clinopyroxene and plagioclase. Komatiites of Munro Township share many of the characteristics of the komatiites from the Barberton Mountain Land, South Africa (Voljoen & Viljoen, 1969a and b), but lack the high CaO/Al2O3 ratios that distinguish the Barberton rocks. The Munro komatiites are identical in this respect to ultramafic volcanic rocks in Australia, Canada, Rhodesia, and India. It is proposed that the definition of the term komatiite be broadened so that it includes all members of this ultramafic-mafic rock series, not only those from Barberton Mountain Land. The proposed criteria are: (1) highly ultramafic compositions in noncumulate lavas; (2) unusual volcanic structures such as spinifex texture and polyhedral jointing; (3) low Fe/Mg ratios at given Al2O3 values or high CaO/Al2O3 ratios; low TiO2 at given SiO2; and high MgO, NiO, and Cr2O3.