Nature of the early proterozoic Outokumpu assemblage, Eastern Finland

Park, Adrian F.

doi:10.1016/0301-9268(88)90088-5

Cited by 17 publications

(8 citation statements)

References 30 publications

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“…Moreover, isotopic data imply that the serpentinite protoliths consisted at least in part, of late Archean subcontinental lithospheric mantle, exhumed from beneath the Karelian cratonic margin during rifting and continental breakup at 1.97-1.95 Ga. (Peltonen et al, 2003). Although the OJOB ultramafic rocks could potentially provide rare insights into the composition and character of ancient lithosphere, we must concede, in contrast to a number of other studies (Park, 1983b(Park, , 1988Vuollo and Piirainen, 1989;Liipo et al, 1995b), that the serpentinites are devoid of primary minerals, apart from scattered relict pristine cores in chromite grains. Local remnants of pre-antigorite mesh-textured serpentine and bastite, and the existence of metarodingites imply derivation of the metaserpentinites by recystallization of pervasively low-T serpentinized protholiths, whereas the invariably metamorphic textures of the metaperidotites attest to subsequent dehydration.…”

Section: Summary Of Important Results and Interpretationsmentioning

confidence: 85%

“…The Cu-Co-Zn-Ni deposits of the Outokumpu district have long been interpreted as either Cyprusor Besshi-type exhalative sulfides issuing from seafloor hydrothermal vents, and the quartz rocks of the host Outokumpu assemblage have consequently been regarded as cogenetic, inhalative-exhalative cherts (Koistinen, 1981;Treloar, 1987;Park, 1988;Gaál and Parkkinen, 1993).…”

Section: Origin and Significance Of Carbonate And Quartz Rocks In Thementioning

confidence: 97%

“…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). The incomplete nature of the Outokumpu ultramafic-mafic bodies as ophiolites leaves the question of their exact origin open to debate (Park, 1988;Peltonen and Kontinen, 1997). We hold the view that these bodies represent fragments of a dominantly ultramafic oceanic floor.…”

Section: Geological Settingmentioning

confidence: 97%

“…18). These selvages constitute the Outokumpu association or assemblage (Gaál et al, 1975;Park, 1988), which is widely known for hosting several economically important Cu-Co-Zn sulfide deposits (Gaál and Parkkinen, 1993). The Outukumpu assemblage is typically most voluminous around serpentinite bodies enclosed in thick layers of sulfide-rich black schist.…”

Section: Geological Settingmentioning

confidence: 99%

“…The Outukumpu assemblage is typically most voluminous around serpentinite bodies enclosed in thick layers of sulfide-rich black schist. Carbonate and quartz rocks of the Outokumpu assemblage have traditionally been considered carbonate and cherty (chemical-exhalative) metasediments, respectively (e.g., Vähätalo, 1953;Huhma and Huhma, 1970;Park, 1983bPark, , 1988. Recently, Auclair et al (1993) and Kontinen (1998) reinterpreted them as carbonated and silicified alteration products derived from the ultramafic rocks, analogous to the listwaenite (carbonate-quartz-talc rocks) and birbirite (quartzcarbonate rocks) assemblages found in association with many serpentinized harzburgites and dunites (e.g., Leblanc and Billaud, 1982;Buisson and Leblanc, 1985;Stanger, 1985;Peabody and FIG.…”

Section: Geological Settingmentioning

confidence: 99%

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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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