Asko Kontinen

An early proterozoic ophiolite — the jormua mafic-ultramafic complex, Northeastern Finland

Abstract The 1.96 Ga old Jormua complex comprises a fault-bounded unit (2–5 × 20 km) in the centre of the Kainuu Schist Belt at the eastern margin of the early Proterozoic Svecokarelides. The complex was dismembered and metamorphosed to lower amphibolite facies during the Svecokarelian orogeny. In the intact complex the basal unit was formed by serpentinites, the middle unit of mafic dykes, and the uppermost extrusive unit of basaltic pillow lavas. Gabbroic rocks are present as intrusive bodies in the serpentinites and narrow interdyke screens within the dyke unit. Trondhjemitic rocks occur as minor segregations in the gabbroic rocks or as cross-cutting narrow dykes in the mafic dyke unit. The dyke unit is locally composed exclusively of subparallel metabasalt and metadolerite dykes; in places interdyke metagabbro and serpentinite screens are abundant. Well-preserved chilled margins are common and many dykes are split and intruded by younger dykes. The chemical composition of the serpentinites indicates derivation from harzburgitic-dunitic mantle peridotites. The gabbroic rocks, which represent obvious tholeiitic cumulates, show extreme differentiation similar to gabbros of the high-Ti ophiolites and gabbros dredged from present midocean ridges. In chemistry, the trondhjemitic rocks are close to granitic rocks from mid-ocean ridges and high-Ti ophiolites. The similar chemical composition of the dykes and lavas of the complex indicates that they are cogenetic, both being tholeiitic metabasalts with chondrite-normalized REE patterns ranging from slightly LREE depleted to slightly LREE enriched ones. Compared with modern rocks, the Jormua metabasalts most closely resemble (transitional) midocean ridge basalts. It is suggested that the Jormua complex and related mafic-ultramafic bodies in eastern Finland originated as divergent-margin ophiolites concomitantly with the break-up of the Archaean craton in the extensional stage of the Svecokarelian orogeny, which culminated in the opening of a major ocean after 1.97 Ga. The tectonic emplacement of these ophiolites into their present locations occurred in connection with thrusting during the compressional stage of the orogeny about 1.9 Ga ago.

Re–Os isotopic systematics of the 1.95 Ga Jormua Ophiolite Complex, northeastern Finland

Whole rock serpentinites, disseminated oxides separated from serpentinites, and podiform chromitites, all from the mantle portions of the circa 1.95 Ga Jormua ophiolite complex (JOC), were analyzed for their Re–Os systematics. The concentrations of Os in the serpentinites are generally consistent with concentrations assumed for the modern convecting upper mantle. Re abundances, however, are highly variable. Most samples have concentrations that are lower than that assumed for the convecting upper mantle, although some are enriched. The enrichments probably reflect recent additions of Re. Indeed, most of the whole rock samples and some of the oxide separates examined show clear evidence of open-system isotopic systematics dominated by recent additions of Re. There is no apparent relation between open-system behavior and the major element compositions of the oxides. Because of their generally homogeneous calculated initial Os isotopic compositions and extremely low Re/Os ratios, the disseminated oxides from the Antinmaki block of the JOC probably reflect closed-system Re–Os isotopic behavior. These samples have an average calculated initial γOs of −5.1±0.8. In addition, several whole rock and oxide samples from different tectonic blocks within the JOC have very depleted present-day of <0.110, requiring a maximum γOs of −4 at 1.95 Ga. The presence of such -depleted materials in the JOC requires the contribution of Os from a mantle reservoir that evolved with a significantly subchondritic Re/Os for at least 1 billion years prior to the formation of the JOC. Today, such strongly negative γOs values are observed only in ancient subcontinental lithospheric mantle. Consequently, the new results may indicate the incorporation of late Archean subcontinental lithospheric mantle during the formation of the JOC. In contrast to the results for the oxides from the Antinmaki block, multiple samples from two chromitite boulders found in the central portion of the JOC have Re–Os systematics that are each consistent with closed-system behavior and calculated initial γOs of approximately 0 and +3. Thus, it is possible that the JOC incorporated both ancient subcontinental lithospheric mantle and a more MORB-like (chondritic) mantle. These results demonstrate that large Os isotopic heterogeneities were well established in the upper mantle by ca. 2 Ga, and that these isotopically disparate reservoirs became intermingled during cratonic rifting (which is the inferred setting of the JOC).

Archean zircons from the mantle: The Jormua ophiolite revisited

The Jormua ophiolite represents seafloor from an Early Proterozoic ocean to continent transition zone that mainly consisted of Archean subcontinental lithospheric mantle. These mantle rocks were exhumed as a result of extreme crustal thinning and detachment faulting in association with the opening of the Svecofennian Sea. At the prerift stage of continental breakup, residual lithospheric peridotites became intruded by alkaline melts that formed a diverse suite of clinopyroxenite and hornblendite-garnetite dikes and veins. These Proterozoic dikes contain Archean zircon xenocrysts inherited from deeper sources of the continental mantle. The relatively large spread of 2 0 7 Pb/ 2 0 6 Pb ages between 3106 and 2718 Ma suggests that the zircons are derived from a variety of source rocks. Some xenocrysts have U and Th abundances comparable to zircon in common alkali basalts, whereas a population of Archean high-U and high-Th zircons is similar to those described from mica-amphibole-rutile-ilmenite-diopside-bearing mantle xenoliths. These are the oldest zircons found from upper-mantle rocks anywhere and imply that the Fennoscandian cratonic root was relatively cool and strongly metasomatized by ca. 3.1 Ga.

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

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 775°C, 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. 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, Fe2+, 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 CO2 and S fugacities were high, such as within smaller ultramafic bodies, or at the margins of larger bodies) to high-Cr (Cr2O3 = 50-70 wt%; Cr# = 0.7-1.0) chromites; with further increase in metamorphic grade, the latter typically recrystallized to mostly spongy/chessboard-textured grains. This interpretation contrasts with previously held views, where high-Cr chromites were considered as residual chromites in ultradepleted residual peridotites. Our study demonstrates that Cr-spinel textures and compositions in amphibolite-facies ultramafic rocks may, to a large degree, be influenced by the metamorphic and metasomatic history of the enclosing host rocks. Clearly, valid application of Cr-spinel as a petrotectonic indicator requires, at least for medium and higher-grade ultramafic rocks, a thorough understanding of the metamorphic and hydrothermal history of the host rocks. Interpretation of the quartz rocks of the Outokumpu assemblage as silicified peridotites demands a reappraisal of the widely accepted concept of the Outokumpu-type sulfide ores as a type example of Precambrian ophiolite—related seafloor hydrothermal sulfide deposits. As an alternative, we tentatively propose a syntectonic hydrothermal origin.

The Archaean Karelia and Belomorian Provinces, Fennoscandian Shield

The Archaean bedrock of the Karelia and Belomorian Provinces is mostly composed of granitoids and volcanic rocks of greenstone belts whose ages vary from c. 3.50 to 2.66 Ga. Neoarchaean rocks are dominant, since Paleoarchaean and Mesoarchaean granitoids (> 2.9 Ga) are only locally present. The granitoid rocks can be classified, based on their major and trace element compositions and age, into four main groups: TTG (tonalite-trondhjemite-granodiorite), sanukitoid, QQ (quartz diorite-quartz monzodiorite) and GGM (granodiorite-granite-monzogranite) groups. Most ages obtained from TTGs are between 2.83–2.72 Ga, and they seem to define two age groups separated by a c. 20 m.y. time gap. TTGs are 2.83–2.78 Ga in the older group and 2.76–2.72 Ga in the younger group. Sanukitoids have been dated at 2.74–2.72 Ga, QQs at c. 2.70 Ga and GGMs at 2.73–2.66 Ga. Based on REE, the TTGs fall into two major groups: low-HREE (heavy rare earth elements) and high-HREE TTGs, which originated at various crustal depths. Sanukitoids likely formed from partial melting of subcontinental metasomatized mantle, whereas the GGM group from partial melting of pre-existing TTG crust.

Some new constraints on hydrothermal alteration and deformation of the Paleoproterozoic serpentinite-hosted Outokumpu Cu-Co-Ni-Zn-Au deposits, Finland

Although the Outokumpu ore deposit has been long regarded as having affinities with ocean floor exhalative deposits, recent studies have shown that the host serpentinites are depleted mantle harzburgites and that the putative silicous exhalites are in fact leached serpentinite. The intense structural reworking and metamorphic overprint precludes further assessment of the original nature of the Cu enrichment, although limited Pb isotope data militate against an interaction with continental crust.