Metamorphism and Chromite in Serpentinized and Carbonate- Silica-Altered Peridotites of the Paleoproterozoic Outokumpu-Jormua Ophiolite Belt, eastern Finland
Jaakko Säntti, Asko Kontinen, Peter Sorjonen-Ward, Bo Johanson & Lassi Pakkanen
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.
FIG. 4. Thin section photomicrographs (A–B) and photographs (C–F) of diamond-sawn rock slabs illustrating petrographic
features typical of the OJOB metaserpentinites and metaperidotites. A. Mantle-foliated chromite, altered to
ferrian chromite, in an antigorite metaserpentinite from Sola, zone A (lower amphibolite facies). B. Olivine porphyroblast
in an antigorite metaserpentinite from Sola, zone A (lower amphibolite facies). The olivine porphyroblast is altered to
lizardite + magnetite and embedded in non-pseudomorphic serpentine consisting of interpenetrating antigorite blades
typical of the OJOB metaserpentinites. C. Massive, dark-green talc-olivine rock from Losomäki, zone B (middle amphibolite
facies). The olivine in this rock is completely altered to lizardite, which is stained dark by magnetite dust. Talc is
evident as pale grey flakes and their aggregates. D. Coarse, roundish olivine porphyroblasts completely altered to lizardite
and magnetite (black) in an talc-carbonate-olivine rock from Miihkali, zone A/B (middle amphibolite facies). E and F.
Lizarditic pseudomorphs after prismatic olivine porphyroblasts (black) in talc-olivine ± carbonate rocks from Pirttimäki
(25 km north of Alanen) and Losomäki, respectively, zone B (middle amphibolite facies). Photographs D, E, and F by Jari
Väätäinen.
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 XCO2) 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 XCO2, attributed to infiltration of CO2 released by decarbonation reactions in previously formed talc-carbonate and carbonate-silica alteration zones.
KAYNAK:
To link to this article: http://dx.doi.org/10.2747/0020-6814.48.6.494
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 XCO2) 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 XCO2, attributed to infiltration of CO2 released by decarbonation reactions in previously formed talc-carbonate and carbonate-silica alteration zones.
KAYNAK:
To link to this article: http://dx.doi.org/10.2747/0020-6814.48.6.494
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