Tectonized actinolite-albite rocks from the Outokumpu district, Finland: Field and geochemical evidence for mafic extrusive origin

Rehtijärvi, Pentti; Saastamoinen, J.

doi:10.17741/bgsf/57.1-2.004

Cited by 10 publications

(8 citation statements)

References 11 publications

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“…The fields of mantle sequences for some ophiolites are shown for reference (after Roberts and Neary, 1993). dikes in the peridotites. Although an extrusive origin has been proposed for some of the mafic rocks (Park and Bowes, 1981;Park, 1983a;Rehtijärvi and Saastamoinen, 1985), it is important to emphasize that none of the mafic dikes or gabbroic bodies intrude the turbidites enclosing the serpentinite lenses. This is consistent with an allochthonous and exotic origin for the ultramafic bodies, particularly given, as noted above, that the turbidites contain detrital zircons as young as 1.92 Ga (Claesson et al, 1993), whereas a pegmatoid gabbro intruding serpentinite at Horsmanaho yielded a zircon U-Pb age of 1.972 ± 0.018 Ga (Huhma, 1986).…”

Section: Geological Settingmentioning

confidence: 99%

Metamorphism and Chromite in Serpentinized and Carbonate-Silica-Altered Peridotites of the Paleoproterozoic Outokumpu-Jormua Ophiolite Belt, eastern Finland

Säntti

Kontinen

Sorjonen-Ward

et al. 2006

International Geology Review

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The 1.95 Ga Outokumpu-Jormua ophiolite belt of eastern Finland contains numerous mafic-ultramafic, predominantly peridotitic bodies which, despite amphibolite-facies metamorphism and pervasive deformation, retain compelling evidence of a residual mantle origin. These rocks therefore currently represent the oldest documented examples of exhumed mantle lithosphere, so information concerning their primary igneous mineral assemblages and textures and chemical and isotopic characteristics is of considerable scientific value. Although several earlier studies have argued for the preservation of primary mineral assemblages, field and petrographic evidence presented here show that the protolith peridotites had already experienced pervasive low-T serpentinization prior to Svecofennian orogenic deformation, during which they were progressively deserpentinized via antigorite metaserpentinites to olivine-talc-anthophyllite-enstatite-bearing metaperidotites. Evidence is also presented to show that the premetamorphic serpentinization event was closely followed by extensive low-T (< 250°C) metasomatic alteration of the marginal parts of the ultramafic bodies to carbonate-silica rocks which, during the subsequent prograde metamorphism, were converted to the distinctive chromite-bearing carbonate-skarn-quartz rocks comprising the Outokumpu rock assemblage. Because these quartz rocks are intimately associated with the Outokumpu Cu-Co-Zn-Ni deposits and have generally been regarded as metamorphosed siliceous seafloor exhalative deposits, the revised interpretation presented here has important implications for ore formation as well. Equilibrium mineral assemblages in the interior parts of the ultramafic bodies (low XCO 2 ) define four regional metamorphic zones, expressed as an east to west increase in the peak dehydration temperatures from 500° to 775ºC, at 3-5 kbar. Large ultramafic bodies commonly show core to margin zoning from talc via anthophyllite-to enstatite-bearing assemblages, reflecting synmetamorphic core-margin gradients in XCO 2 , attributed to infiltration of CO 2 released by decarbonation reactions in previously formed talc-carbonate and carbonate-silica alteration zones.The only primary igneous phase identified in this study was chromite, which occurs as scattered relict cores within large altered grains. These are relatively common in metaserpentinites, but also occur in metaperidotites and even in carbonate-skarn-quartz rocks. Even though Mg, Fe 2+ , and Zn abundances in these relict chromites may have been somewhat modified, absolute concentrations and ratios of Cr and Al appear to be essentially unmodified. The lobate to amoeboid morphology and observed Cr# range (0.41-0.67) of the best-preserved grains are more consistent with residual depleted lherzolite to harzburgite textures than a cumulate origin. However, during regional metamorphism, most mantle chromites were either pervasively altered to ferrian chromites and Cr-magnetites or (where CO 2 and S fugacities were high, such as within smaller ultramafic bod...

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Section: Geological Settingmentioning

confidence: 99%

Metamorphism and Chromite in Serpentinized and Carbonate-Silica-Altered Peridotites of the Paleoproterozoic Outokumpu-Jormua Ophiolite Belt, eastern Finland

Säntti

Kontinen

Sorjonen-Ward

et al. 2006

International Geology Review

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“…For comparative purposes, data from the Miihkali metabasites are included here, as these rocks appear to be the least altered representatives of the volcanic component of the Outokumpu assemblage (data from Rehtijärvi and Saastamoinen, 1985;see discussion in Park, 1988a). The Rantala amphibolite falls within the range of composition for the Miihkali metalavas, consistent with the interpretation of this rock as representing a minor intrusion related to this volcanism.…”

Section: -I-i-i-i-i-m-i-•-i-i-i-rmentioning

confidence: 99%

“…It consists of a distinctive group of serpentinized ultramafic bodies (dunite and saxonite, Haapala, 1936;Park, 1983), mafic metavolcanic amphibolites and greenschists (Park, 1984;Rehtijärvi & Saastamoinen, 1985), chromium-rich calc-silicate skarns and dolomites, quartz rock (metachert) and metalliferous black schists (carbonaceous sulphidic pelites) (Huhma & Huhma, 1970;Gaål et al 1975;Koistinen, 1981;Fig. 1).…”

Section: Introductionmentioning

confidence: 99%

Metagabbros, metasyenites and related rocks in the Outokumpu assemblage, east Finland

Park¹

1992

Bull Geol Soc Finland

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PARK, ADRIAN F. 1992: Metagabbros, metasyenites and related rocks in the Outokumpu assemblage, east Finland. Bull. Geo!. Soc. Finland 64, Part 1, A diverse group of minor intrusive rocks in the early Proterozoic Outokumpu assemblage of eastern Finland include metagabbro, hornblendite, amphibolite, pyroxenite, syenite and chloritized dykes. All are deformed and/or extensively altered and recrystallized. Intrusive relationships with serpentinite and metavolcanic rocks suggest they include the youngest pre-tectonic rocks in the Outokumpu assemblage. Geochemistry and rare relict mineral assemblages identify one, possibly two suites of minor intrusions that are alkaline in affinity, but not consistently peralkaline, with all but the amphibolites being derived from cumulates. They range from ultramafic diopside + olivine rocks, through quartz-dolerite, to diopside + plagioclase + microcline ± olivine syenites with normative nepheline and leucite. Elevated levels of Na, K and Ca are characteristic, giving the syenitic rocks an apparent kinship with shoshonites, and the most extreme compositions a resemblance to shonkinite. Pervasive carbonation and phlogopitization preceded deformation, and tracc element patterns suggest this was a late stage magmatic process with the bulk addition of C0 2 , rather than bulk assimilation of the carbonate metasediments of the Outokumpu assemblage. The presence of alkaline rocks in the Outokumpu assemblage has profound implications for the original tectonic setting of these rocks, requiring, in particular, modification of the »ophiolite» model. These rocks show broad affinities with undersaturated calc-alkaline rocks of the shoshonite trend towards shonkinites (particularly Group III ultrapotassic igneous rocks), reinforcing an intraoceanic island arc interpretation for part, if not all, the Outokumpu assemblage.

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“…The serpentinites were originally dunites with some peridotites and pyroxenites; the quartzite is a colloidal silica precipitate in parentage (A. Huhma 1975). The skarns, which are calc-silicate rocks, include metavolcanics (Park 1984;Rehtijärvi and Saastamoinen 1985) Fig. 10.…”

Section: Early-proterozoic Volcaniclastic Komatiitesmentioning

confidence: 99%

Komatiitic explosive volcanism and its tectonic setting in Finland, the Fennoscandian (Baltic) Shield

Saverikko¹

1990

Bull Geol Soc Finland

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The variously explosive komatiitic volcanism in Finland was virtually confined to the Archaean continental segment, where it occurred in the Lapland and Kuhmo-Suomussalmi greenstone belts between 3.0 Ga and 2.5 Ga, in the continental borderland at 1.97 Ga, and in the continental shelf break 1.91-1.88 Ga ago. The Archaean continent comprised the Saamian craton, which stabilized at 3.1-3.0 Ga before splitting into an array of sialic megablocks. Except for the NWtrending rifts, the linear crustal openings of the Fennoscandian greenstone belts constituted a radial swarm of aulacogens caused by domal uplift in connection with shield-wide mantle upwelling. Diapirism resulted in the Lapponian pyroclastic komatiite zone of mantle-activated rifting and in the large Solovetski mantle plume in the USSR, along which continental breakup reached no more than an embryonic stage of divergence. The Finnish greenstone belts underwent three periods of supracrustal evolution from cratonic deposition and/or continental rifting to oceanic or mantle-activated rifting. The extensional stage of the greenstone-belt genesis is recognizable in Lapland but not in the Kuhmo-Suomussalmi area, in the ductile? periphery of the continental plate, where sagduction obscured the geological history. In Lapland, the komatiitic central-vents opened at rift-aulacogen intersections. Some signs of volcanic bursting, albeit obscure, can be traced in the earlier ultramafics, namely in the initial komatiitic greenstones beneath the cratonic metasediments and in those associated with bimodal metavolcanics. Pyroclastics formed in all the komatiitic rock types but the pyroxene peridotitic komatiite (MgO 18-30 wt%, anhydrous basis) was the predominant substance to extrude in enormous magmatic explosions during the terminal stage of mantle upwelling. After the Karelian anorogenic period, mantle activity produced two ophiolitic suites from the mantle plumes 1.97-1.96 Ga ago. The one at Outokumpu brought about volcaniclastic komatiites under shallow-water to terrestrial conditions. The preferred environment was the Raahe-Ladoga marginal rift, of basin-range nature. At the peak of the Svecofennian orogeny (1.90-1.87 Ga), some minor komatiites with negligible ejecta discharged at the continental edge, forming the extreme member of Bothnian volcanism.

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Tectonized actinolite-albite rocks from the Outokumpu district, Finland: Field and geochemical evidence for mafic extrusive origin

Cited by 10 publications

References 11 publications

Metamorphism and Chromite in Serpentinized and Carbonate-Silica-Altered Peridotites of the Paleoproterozoic Outokumpu-Jormua Ophiolite Belt, eastern Finland

Metamorphism and Chromite in Serpentinized and Carbonate-Silica-Altered Peridotites of the Paleoproterozoic Outokumpu-Jormua Ophiolite Belt, eastern Finland

Metagabbros, metasyenites and related rocks in the Outokumpu assemblage, east Finland

Komatiitic explosive volcanism and its tectonic setting in Finland, the Fennoscandian (Baltic) Shield

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