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

Abstract

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...

Peridotite enclaves hosted by Mesoarchaean TTG-suite orthogneisses in the Fiskefjord region of southern West Greenland

“…This is supported by the trends that are seen in Fig. 7b, which are typical of amphibolite-facies metamorphic disturbance of chromite in metaperidotites [31,88,7,99,9]. Therefore, it does not appear likely that the highly fosteritic olivine compositions are a primary igneous feature, but it may rather reflect the interplay between the olivine-rich refractory bulk-rock composition of their host rocks and metamorphic and subsolidus re-equilibration.…”

Zoned Cr-spinel and ferritchromite alteration in forearc mantle serpentinites of the Rio San Juan Complex, Dominican Republic

“…These relatively buoyant serpentinites probably contributed to the exhumation of HP and UHP rocks in the subduction channel (Gorczyk et al, 2007;Hattori et al, 2010a (Ishii et al, 1992) and the abyssal peridotite field is after Dick and Bullen (1984). The field of metamorphic Cr-spinels was redrawn from Säntti et al (2006) after Evans and Frost (1975).…”

“…Paktunc and Cabri, 1995;Barnes, 2000;Säntti et al, 2006). However, the general lack of a metamorphic overprint on the Cr-spinel cores in the Cuaba samples suggests that a metamorphic origin for the high Zn contents is unlikely.…”

Delineating ophiolite-derived host rocks of massive sulfide Cu-Co-Zn deposits with 2D high-resolution seismic reflection data in Outokumpu, Finland

“…The ophiolitic rocks are 1.96 Ga old, and the thrusting took place at about 1.9 Ga ago (Säntti et al, 2006;Peltonen et al, 2008). The ophiolitic rocks originated from a narrow oceanic basin opened on the western margin of the Archaean complex about 2.0-1.96 Ga ago.…”

“…Metamorphic grade increases from east to west with peak temperatures of about 500°C-775°C and pressures of 3-5 kbar. Metamorphic alteration has changed the originally depleted upper-mantle rocks into serpentinite, skarn-carbonate rock, and quartz rock (Säntti et al, 2006;Peltonen et al, 2008).…”