Rb-Sr geochronology of the Coldwell Complex, northwestern Ontario, Canada: Reply (original) (raw)
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Canadian Journal of Earth Sciences, 1988
The Flavrian pluton is a sill-shaped intrusion in the Blake River Group (BRG) volcanic rocks in the Noranda region of the Abitibi greenstone belt. The pluton is dominated by trondhjemites and tonalites, with minor peripheral quartz gabbro and hybrid phases. The BRG volcanic rocks consist of a bimodal suite of basalt-andesite and rhyolite. The Flavrian trondhjemites are geochemically identical to the rhyolitic lavas of the BRG (SiO, r 72%, LaISm = 3.4, LaIYb = 3.6, ZrIY = 3.9, YINb = 3.1), and the Flavrian gabbroic and dioritic rocks are identical to the BRG basalts and andesites (SiO, < 58%, LalSm = 3.0, LaIYb = 5.5, ZrIY = 4.2, YINb = 3.3). However, the tonalitic rocks of the Flavrian pluton have no extrusive equivalents in the BRG. The different compositional spectra of the extrusive and intrusive rocks are interpreted as being a result of a transition in magma-chamber evolution from a zoned open system that was active during the evolution of the volcanic rocks to closed-system plutonic crystallization. The latter destroyed the compositional bimodality of the magma chamber and resulted in the evolution of intermediate compositions (tonalites) generated by both fractional crystallization and magma mixing.
Canadian Journal of Earth Sciences, 2011
The Roy Group in the Chibougamau area of the Abitibi subprovince, Southeastern Superior craton, Quebec, exemplifies Archean volcanic evolution. The Roy Group formed in two volcanic cycles, with voluminous basal mafic tholeiitic volcanic rocks being capped by a thin succession of calc-alkaline felsic volcanic rocks, banded iron formations, and cherts that represent regional stratigraphic markers. The Gilman Formation is thought to represent the mafic base of the 2nd Roy Group volcanic cycle. Our new mapping reveals that the Gilman Formation (North of the Lac Doré Layered intrusive complex) can be subdivided into a tholeiitic lower part (David member), a calc-alkaline extrusive sequence (Allard member), and a tholeiitic, more mafic upper part (Bruneau Formation, newly defined here). New U-Pb zircon ages on Allard member tuffs (2726.6 ± 0.7 Ma) are within error of those obtained from rhyolites of the Waconichi Formation (2728.2-2728.7 Ma), which underlies the Gilman Formation. These ages, along with geochemical and lithological similarities, imply that the Allard member belongs to the Waconichi Formation and that the David member occupies the same stratigraphic position as the Obatogamau Formation. The upper part of the former Gilman Formation is reassigned to the new Bruneau Formation. A thin transitional calc-alkaline basalt unit at the base of the Allard member is geochemically similar to basalts occurring at the top of the Lemoine member of the Waconichi Formation near the Lemoine mine, suggesting that the Allard member is slightly younger than the Lemoine member, and that its base (also associated with iron formation, breccia, and chert) represents a regional stratigraphic break favourable for volcanogenic-hosted massive sulphide (VHMS) exploration.
Canadian Journal of Earth Sciences, 2010
The late Neoproterozoic Frog Lake pluton, in the Avalon terrane of the Cobequid Highlands, Nova Scotia, consists predominantly of hornblende gabbro. It shows petrographic similarities to water-rich mafic intrusions known as appinites that are present in some collisional orogens. This study aims to further understanding of the origin of appinitic intrusions. In the field, the main hornblende gabbro was intruded between screens of metasedimentary country rock that is of upper greenschist metamorphic grade. The contacts appear to have been pathways for magma of gabbroic, tonalitic-granodioritic, and granitic composition that carried enclaves of gabbroic lithologies. Some of these magmas had a high volatile content, resulting in abundance of hydrous mineral phases, pegmatites, and diffuse felsic segregations. These varied rocks in the contact zones experienced progressive shear resulting in syn-magmatic deformation. Low-Ti hornblende gabbros have trace-element abundances similar to subduction-related low-K mafic rocks, including some enrichment in large-ion lithophile elements and marked relative depletion in Nb and Y. High-Ti hornblende gabbros and pyroxene-mica gabbro show more alkaline characteristics, with higher amounts of Nb, Y, P 2 O 5 , and high-field-strength elements. Tonalite and granite veins are geochemically similar to volcanic-arc granite. Comparison with appinites in the literature suggests that the Frog Lake pluton represents a deeper structural level than most appinites. The Frog Lake appinites were part of the feeder system to back-arc volcanic rocks of the Jeffers Group. Comparison with other appinites also leads to the conclusion that there is not a single type of ''appinitic magma'': different appinitic plutons range in composition from low-K calc-alkaline to shoshonitic. The essential characteristic is a water-rich mafic magma. Appinites occur in settings undergoing crustal-scale strike-slip shear, where the faults allow rapid rise of mafic magma to shallow crustal levels.
1991
Geochemistry and Nd isotopic compositions are used to characterize mantle and crustal sources and to provide constraints on petrogenetic models for tholeiitic, calc-alkalic, and lamprophyric suites in the Kamiskotia-Montcalm area. The Kamiskotia gabbroic complex (KGC) and cogenetic, bimodal volcanic rocks have eNd(t) = +2.2 to +2.6, consistent with a direct derivation from a long-term, light rare-earth element (LREE)-depleted mantle. The Montcalm gabbroic complex has decreasing eNd(t) upsection from +2.8 to + 1 .O, consistent with contamination by long-term, LREE-enriched (with respect to the long-term, LREE-depleted Abitibi mantle) crust during fractionation. Two calc-alkalic lamprophyre samples, characterized by large-ion lithophile element (LILE) and LREE enrichment and high MgO, Ni, and Cr contents, have eNd(t) of +2.5 and +2.8, indicating a derivation from a depleted mantle source that had undergone recent trace-element enrichment. A different lamprophyre suite is extremely LILE and LREE enriched and has an eNd(t) of + 1 .O, indicating a derivation from a slightly different source that had earlier LREE enrichment. Granitoid rocks internal and external to greenstone belt rocks have eNd(t) = +2.5 to +3.8 and +0.6 to-0.4, respectively. The lower values provide additional evidence for the existence of LREE-enriched crust in this area. Considering these data along with other radiogenic isotope studies, a petrogenetic and tectonic model is suggested for the crustal development of the southern Abitibi Subprovince. From >2740 to 2698 Ma-the major period of volcanic activitykomatiitic and tholeiitic suites and one lamprophyre suite were derived from a uniformly LREE-depleted mantle reservoir with eNd(t) = + 2 to +3. Calc-alkalic granitoids were emplaced generally after 2700 Ma. Their long-term, LREE-depleted and LREE-enriched Nd isotopic signatures are similar to signatures in continental-arc settings (e.g., the Coastal Batholith of Peru). Form 2690 to 2670 Ma, when transpressional tectonism prevailed, mantle-derived magmatism was represented by long-term, LREE-enriched (eNd(t) = + 1 to +2) lamprophyric and alkalic volcanic suites. The Kamiskotia suite has a seven-point, whole-rockmineral isochron Sm-Nd age of 2710 2 30 Ma, identical to U-Pb zircon ages for the suite, indicating closed-system behavior. An Rb-Sr mineralwhole-rock isochron age from one KGC sample is 2450 2 30 Ma, identical to U-Pb ages for the Hearst-Matachewan dike swarm, a prominent feature in the KGC area. Regression of whole-rock and mineral-isotope data for one granitoid sample with a U-Pb zircon age of 2696 * 1.5 Ma gives identical ages of 2530 2 30 Ma in the Sm-Nd and Rb-Sr systems. The latter data add to an increasing body of evidence for cryptic, late thermal events after granitoid-greenstone belt development in the southern Abitibi Subprovince. La gCochimie et les compositions isotopiques du Nd servent a caractkriser les magmas d'origine crustale ou mantellique, et pour ttablir les bornes dans les modeles p6trogCnetiques des suites tholCiitiques, calco-alcalines et lamprophyriques dans la rCgion de Kamiskotia-Montcalm. Le complex gabbroique de Kamiskotia (CGK) et les roches volcaniques bimodales, cogCnCtiques, fournissent les valeurs de eNd(t) = +2,2 a +2,6 qui sont en accord avec une dCrivation directe de magma mantellique appauvri, a long terme, en terres rares 1Cgeres. Dans le complexe gabbroique de Montcalm les valeurs de eNd(t) diminuent du bas vers le haut de la coupe de +2,8 a + 1,0, ce qui tCmoigne d'une contamination par une crotite enrichie, a long terme, en terres rares ICgeres (relativement au manteau d9Abitibi appauvri long terme en t e r i s rares 1Cgkres) durant le fractionnement. Deux Cchantillons de lamprophyre calco-alcalin, caractCrisCs par un enrichissement des ClCments lithophiles a grands rayons ioniques (LILE) et des terres rares ICgeres, et des teneurs ClevCes en MgO, Ni et Cr, possedent des valeurs de eNd(t) de +2,5 et +2,8 qui indiquent une origine d'une source de magma mantellique appauvrie, laquelle aurait subi un enrichissement rtcent en ClCments traces. Une suite diffbrente de lamprophyre est trks fortement enrichie en LILE et terres rares Itgbres, son eNd(t) ttant + 1,O rCvkle une source magmatique 1Cgbrement diffkrente, anterieurement enrichie en terres rares ltgbres. Les roches granitoides qui se trouvent B I'intCrieur et 2 1 I'extCrieur de la ceinture de roches vertes fournissent eNd(t) = +2,5 a +3,8 et +0,6 a-0,4, respectivement. Les valeurs les plus faibles corroborent l'existence d'une croQte enrichie en terres rares ltgbres dans cette rCgion. Ces donnees jointes aux rtsultats de d'autres Ctudes d'isotopes radiogkniques, suggkrent un modkle pCtrogCnCtique et tectonique pour expliquer le dCveloppement de la croQte de la sous-province d'Abitibi sud. De >2740 Ma a 2698 Ma-la ptriode d'activite volcanique la plus importante-les suites komatiitiques et tholtiitiques et une suite de lamprophyre furent dkrivtes d'un rkservoir de magma mantellique appauvri uniformkment en terres rares ICgkres, avec eNd(t) = + 2 a +3. Les granitoides calco-alcalins furent mis en place gCnCralement apres 2700 Ma. Leurs signatures de Nd isotopique, identifiant un magma appauvri a long terme en terres rares 1Cgeres ou un magma enrichi a long terme en terres rares lCgbes, sont analogues aux signatures dCvoilCes dans des contextes d'arc continental (ex., le batholite CBtier du PCrou). De 2690 a 2670 Ma, pCriode dominCe par un tectonisme de transpression, le magmatisme dCrivC du manteau Ctait reprCsentC par des suites lamprophyritiques et de volcanites alcalines enrichies, a long terme, en terres rares lCgirres avec eNd(t) = + I a +2. Les analyses gCochronologiques de la suite de Kamiskotia, sur roche totale et sur minCraux, ont permis de tracer avec sept points une isochrone Sm-Nd fournissant un dge de 2710 2 30 Ma, identique aux dges U-Pb sur zircon de la mCme suite, ce qui indique un comportement en systeme fermt. Une isochrone Rb-Sr sur minCrauxroche totale d'un Cchantillon du CGK a fourni un dge de 2450-+ 30 Ma, identique aux dges U-Pb obtenus pour I'essaim de dykes d'Hearst-Matachewan, une particularit6 gtologique importante dans la rCgion du CGK. Une courbe de rkgression des donnees isotopiques sur roche totale et sur
Major intra-continental shear zones developed during the later stages of continental collision in a back-arc setting are sites of prolonged magmatism. Mantle metasomatism results from both melting of subducted sediments and oceanic crust. In the Cobequid Fault Zone of the northern Appalachians, back-arc A-type granites and gabbros dated ca. 360 Ma are locally intruded by lamprophyric dykes dated ca. 335 Ma. All the lamprophyres are kersantites with biotite and albite, lesser ilmenite, titanite and fluorapatite, and minor magmatic calcite, allanite, pyrite, magnetite, quartz and K-feldspar in some samples. The lamprophyres show enrichment in Rb, Ba, K, Th and REE and classify as calc-alkaline lamprophyre on the basis of biotite and whole rock chemistry. Pb isotopes lie on a mixing line between normal mantle-derived gabbro and OIB magma. Nd isotopes range from 1.3–3.5 εNd t , a little lower than in local gabbro. Most lamprophyres have δ 18 O = 3.8–4.4‰. Country rock is cut by py-rite-(Mg)-chlorite veins with euhedral allanite crystals that resemble the lamprophyres mineralogically, with the Mg-chlorite representing chloritized glass. Early Carboniferous unenriched mafic dykes and minor volcanic rocks are widespread along the major active strike-slip fault zones. The lamprophyres are geographically restricted to within 10 km of a small granitoid pluton with some sodic amphibole and widespread albitization. This was displaced by early Carboniferous strike-slip faulting from its original position close to the large Wentworth Pluton, the site of mantle-derived sodic amphi-bole granite, a major late gabbro pluton, and a volcanic carapace several kilometres thick, previously demonstrated to be the site of mantle upwelling and metasomatism. The age of the lamprophyres implies that enriched source material in upper lithospheric mantle or lower crust was displaced 50 km by crustal scale strike-slip faulting after enrichment by the mantle upwelling before lamprophyre emplacement. This indicates a multi-stage process to emplace lamprophyric magma. Crown
Canadian Journal of Earth Sciences, 2007
The ca. 1110-1120 Ma diabase sills of the Nipigon Embayment represent the oldest phase of the Midcontinent Rift recognized to date. The sills can be subdivided into five geochemically distinct suites: Nipigon, McIntyre, Inspiration, Shillabeer, and Jackfish. Sm-Nd, Rb-Sr, and Pb-Pb isotope data for the sills can be used to evaluate the source regions and emplacement histories of these sills. The εNd T values for all the sill suites are consistently negative (-0.5 to -6.6) but show coherent variations both within and between suites. The negative εNd T values can be interpreted as the result of contamination of a plume-related mantle source by older crustal material. The initial strontium (Sr i ) values (0.7032-0.7068) for the sills indicate that at least two distinct contaminants are required: (1) a source with strongly negative εNd T and lower Sr i , likely Archean metasediment or granite, and (2) one with elevated Sr i , likely sedimentary rocks of the Proterozoic Sibley Group. The Pb data for the Nipigon and Inspiration sills form distinct linear arrays consistent with variable contamination of a source comparable to that of other igneous suites of the Midcontinent Rift. The radiogenic isotopes preserve a complex contamination history best interpreted as the result of <5% contamination by a variety of Archean and Proterozoic sources. The ultramafic Jackfish sill shows the least evidence of contamination and is interpreted to have been emplaced along a crustal-scale fault with little interaction with crustal material. In contrast, the other sill suites have undergone complex contamination histories requiring variable crustal residence times and assimilation of material both at depth in large magma chambers and during emplacement.
Atlantic Geology, 2006
A steeply west-dipping, north-northwest-trending, biotite-bearing quartz-feldspar porphyry dyke is associated with the Lake George granodiorite, a cupola of the Pokiok Batholith. These Early Devonian intrusions are spatially and temporally related to W-Mo-Au and Sb-Au mineralization in the area. The porphyry dyke yielded an age of 420.8 +5.9/-4.0 Ma (U-Pb zircon) and is inferred to be coeval with or slightly older than the Lake George granodiorite (412 +5/-4 Ma, U-Pb zircon). The variably altered dyke contains subhedral to anhedral (rounded) quartz, plagioclase, and orthoclase phenocrysts (0.5 to 30 mm; 30 to 60 vol. %), and numerous angular to subrounded xenoliths of altered intrusion carapace and quenched dyke rocks, as well as local Kingsclear Formation metasedimentary rocks. The dyke is similar to the Lake George granodiorite in phenocryst population, volcanic arc (I-type) affi nity, average Zr (157 ppm), TiO 2 (0.54 %), La/Yb ratio (13), and REE (129 ppm). The average Au content is lower in the dyke (20 ppb) than in the granodiorite (32 ppb), the abundance of gold is related to minor magmatic hydrothermal chalcopyrite and pyrrhotite (< 2 %). Based on textural evidence, including autobrecciation, coupled with rheological and thermal modeling calculations for the 5 m-wide dyke, it is suggested that fl uidization (vapour exsolution) of the magma was key to its rapid emplacement as a quenched glass-gas mixture (tuffi site). The S abundance and S isotopic signature of the dyke, relative to Au and Cu abundance, support reduced I-type magmatic volatiles involved in dyke emplacement and cogenetic autometasomatic alteration.
Canadian Journal of Earth Sciences, 1994
Four types of igneous clasts from Timiskaming conglomerates of the Kirkland Lake area are identified: calc-alkaline porphyry, trachyte (K,O + N 2 0 >7.25 wt.%), trondhjemite, and tholeiitic basalt. Clasts derived from quartz-carbonate veins and carbonatized porphyries and komatiites indicate C02-rich hydrothermal activity along the Larder Lake -Cadillac Fault before sedimentation. Calc-alkaline porphyry, the predominant clast, is similar in appearance to feldspar porphyry intrusions widely exposed in the area, but the two are not related. The porphyry clasts are cogenetic with diorite-granodiorite intrusions and volcanic rocks of the greenstone belt. Andesite clasts are only observed near the base of the assemblage, and the ratio of basalt to andesite increases up stratigraphic sections. The change suggests progressive erosion of arc-related volcanic rocks during sedimentation. Composition, texture, and mineralogy of the trondhjemite clasts are consistent with their derivation from the marginal phases of the Round Lake batholith. Intrusive rocks of the Timiskaming Group (coarse-grained holocrystalline syenitic -monzonitic rocks, biotite-bearing feldspar porphyries, and mica-rich lamprophyres) were not observed in the clasts, which suggests sedimentation prior to unroofing of these rocks. Sedimentation of the conglomerates postdated unroofing of underlying plagioclase porphyry ( -2685 Ma), but predated unroofing of Timiskaming intrusions. The distribution of alkaline clasts only in proximity to the fault implies that alkaline magmatism was confined along the fault. Derivation of clasts from both sides of the fault and proximity of inferred sources support a pull-apart basin for sedimentation with minor strike-slip movement during and after the sedimentation.
The Canadian Mineralogist, 2000
Dykes of lamprophyre and alkaline olivine diabase cut the Cambro-Ordovician Goldenville Formation and the Carboniferous Wedgeport Pluton near Plymouth, southwestern Nova Scotia. 40 Ar/ 39 Ar geochronology yielded hornblende-biotite ages ranging between 231 ± 3 and 222 ± 3 Ma (mean 227 Ma) for the lamprophyres, and biotite ages of 209 ± 6 and 203 ± 15 Ma for the olivine diabase. Minerals present in the dykes include olivine (Fo 83), augite with a rim of titanaugite, kaersutite and titaniferous phlogopite. The whole-rock composition of both the lamprophyre and olivine diabase shows greater LILE and HFSE enrichment in the lamprophyres, but the relative abundances of these elements are remarkably similar in the two rock types, suggesting that they are comagmatic. Values of Nd (~+4) and Sr (~+5) are much more primitive than values for the widespread early Jurassic tholeiites. Pb isotope compositions for the lamprophyres fall near the Northern Hemisphere Reference Line; those of the olivine diabase are intermediate between the lamprophyres and more radiogenic Pb compositions typical of the Triassic alkaline dykes at Seabrook, New Hampshire, and the early Jurassic tholeiites; they may have experienced some contamination in the lower crust. The lamprophyres resemble, in age and chemical composition, other alkaline rocks of the Coastal New England province. The olivine diabases, apparently synchronous with the main Eastern North American tholeiites, represent renewed partial melting of the same mantle source, triggered by the thermal effect of the regional tholeiitic magma.
Precambrian Research, 2003
A suite of metaplutonic rocks representing a mafic to intermediate calc-alkaline batholith has been identified in the Voisey's Bay area of northern Labrador. The suite represents the southern extension of similar metaplutonic rocks occurring along some 500 km of the Paleoproterozoic Torngat Orogen. The rocks have been metamorphosed at upper amphibolite to granulite facies conditions that only mildly disturbed their igneous characteristics. Two magma series are present, one comprising gabbros, diorites and quartz diorites, and another composed of tonalites. The gabbro to quartz diorite series evolved by fractional crystallization of a mantle-derived mafic magma. Two samples of the metaplutonic suite yield U-Pb zircon thermal ionization mass spectrometry (TIMS) ages of 1893 ± 1 and 1890 ± 2 Ma. Based on U-Pb zircon laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS) geochronology, one of these samples contains inherited cores, some 50-100 Ma older than the igneous population, but no Archean cores were found. Metatonalite rocks formed from a separate tonalitic magma during a slightly younger magmatic event dated at 1883 ± 5 Ma. Both the gabbro to quartz diorite and tonalite series rocks are enriched in Rb, Ba, Sr, Zr and light rare earth elements relative to primitive mantle; and have negative Nb-Ta anomalies, which are thought to reflect subduction processes. They have fractionated REE patterns, with the extent of fractionation greatest for the tonalites (chondrite normalized La/Yb = 30-50), suggesting that their parent magmas left more residual garnet in their source regions than did the gabbro to quartz diorite magmas. Sr-Nd-Pb isotope compositions of the metaplutonic suite indicate significant involvement of a lower crustal component in their genesis. When combined with previous data for the suite, and excluding data thought to be disturbed by metamorphism, 87 Sr/ 86 Sr (1890 Ma) = 0.7036-0.7048, εNd (1890 Ma) = +4.7 to −6.9, and µ * (time-integrated 238 U/ 204 Pb of the mantle and crustal sources) = 7.66-7.98. A model for the metaplutonic suite is presented in which the gabbro to quartz diorite series formed by partial melting of the mantle wedge in a subduction zone and interacted with a lower crustal contaminant, possibly in the source region. The tonalites were derived by partial melting of a mixture of subducting oceanic crust and lower crustal detritus. An important role for lower crustal recycling is thus indicated. Crustal genesis by both mantle wedge and slab melting in the Paleoproterozoic may mark the transition from Archean to post-Archean subduction processes.