A. N. Konilov

Mesoarchean subduction processes: 2.87 Ga eclogites from the Kola Peninsula, Russia

The nature of tectonic processes on the early Earth is still controversial. The scarcity of high-pressure metamorphic rocks such as eclogite (the high-pressure equivalent of basalt) in Archean cratons has been used to argue that plate tectonics did not operate until Earth had cooled to a critical point, perhaps around the 2.5 Ga Archean-Proterozoic transition. However, eclogites occur as meter- to kilometer-sized lenses enclosed in Archean gneisses of the Belomorian Province of the Fennoscandian shield. Geochemistry and internal features suggest that the protoliths of the eclogites were interlayered olivine gabbros, troctolites, and Fe-Ti oxide gabbros. Greenschist facies mineral parageneses are enclosed in prograde-zoned eclogite garnets, and peak metamorphic conditions define an apparent thermal gradient (12–15 °C/km), consistent with metamorphism in a warm Archean subduction zone. We show here that these eclogites represent the oldest known high-pressure metamorphic rocks. U-Pb dating and Hf isotope analyses of zircons from the eclogites and a crosscutting felsic vein define a minimum age of 2.87 Ga for the Uzkaya Salma eclogite; a 2.70 Ga age for the Shirokaya Salma eclogite is interpreted as the age of a granulite facies overprint. Thermal overprinting and growth of new zircon also occurred during the Svecofennian (1.9–1.8 Ga) orogeny. These new data imply that plate tectonic processes operated at least locally in late Mesoarchean time. The adakitic nature of the felsic vein suggests that partial melting of hydrated eclogites could produce Archean tonalite-trondhjemite-granodiorite–type magmas.

Multistage Metamorphic Evolution of the Trivandrum Granulite Block, Southern India

Abstract The Trivandrum Granulite Block (TGB), southern India records evidence for three distinct stages of evolution (M1–;M3) during the Pan-African high grade metamorphism, with possible temperature gradient from north to south of the terrain as detected from mineral phase equilibria thermobarometry in three classic localities, namely Nuliyam, Kunnanpara and Nellikkala. The charnockites, both incipient and massive, were formed during the first stage (M1) at temperatures higher than their host rocks, and at appreciably lower pressures. Charnockite formation was dominantly controlled by an increase in partial pressure of CO 2 , along structural locales during subisothermal decompression, although an increase of potash activity could have also been an important factor in this process. The charnockites at Nellikkala in the northern margin of TGB were formed under appreciably more H 2 O-rich conditions (X H 2 O = 0.53±0.03) than those at Nuliyam (X H 2 O = 0.25±0.02) in the southern margin. It is inferred that during the period between the metamorphic stages M1 and M2, the terrain experienced subisobaric cooling. Comparison of results from thermobarometry with data on absolute age determinations from geochronology of the metamorphic rocks in TGB allows the interpretation that the M1 metamorphic event took place during 540–;600 Ma, M2 at about 530 Ma and M3 in the interval of 440–;470 Ma. Mineralogic and thermobarometric evidence for earlier high-grade metamorphic processes, if any, have been erased from these rocks. The processes of charnockite formation and post-peak retrograde metamorphism in the TGB took place under high geothermal gradients (40–;150°/km). This probably testifies to the existence of a local heat source, either magmas at depth or mantle (plume) beneath the region. The general metamorphic cycle in the TGB is estimated to be ca . 100–;160 Ma, which is much shorter in time span than that in the other regions of southern Peninsular India such as the Karnataka Craton and the Eastern Ghats Mobile Belt. During this period, the terrain experienced rapid exhumation of approximately 6–;7 cm/year, with the total amplitude of vertical movements estimated to be about 16–;17 km.

The Belomorian eclogite province: Unique evidence of Meso-Neoarchaean subduction and collision

The Belomorian eclogite province (BEP) recently revealed in the eastern part of the Fennoscandian shield is a unique Archaean object. The age of the crust eclogites known in the world outside the BEP does not exceed two billion years [1], which corresponds to the middle of the Paleoproterozoic. Eclogites with an age not less than 2.72 Ga [2] were found for the first time within the Belomorian province. The studies of the BEP open principally new opportunities, first, for the reconstruction of the geodynamic processes in the his� tory of the Early Precambrian crust of the region, and second, for a more correct concept about the geody� namics of the Early Precambrian as a whole, because the lack of reliable findings of Archaean eclogites is one of the arguments against the reality of subduction and application of plate tectonics to the modeling of the Archaean geodynamics.

Dating of key events in the Precambrian polystage complexes: An example from Archaen Belomorian Eclogite Province, Russia

The identification of discrete tectono-thermal events, their sequence, and absolute age is a difficult problem in the highly deformed polymetamorphic Precambrian complexes. The Archean and Paleoproterozoic eclogites are distinguished in the Gridino eclogitebearing melange [1]. Archean eclogites are small equant and elongated boudins and lenses in the granite gneiss matrix and have a U–Pb age of ~2.7 Ga [2]. They are considered a metamorphic product of the Archean oceanic protolith [1]. The Paleoproterozoic eclogites are represented by undeformed and deformed (folded, boudined) eclogitized dikes of different age complexes. They are compositionally similar to the Archean eclogites and, in addition to the Archean zircons, contain Paleoproterozoic zircons with an age ~2.45–2.42 Ga. All mafic bodies (dikes of different generations and boudins) were subsequently overprinted by eclogites, high-grade granulite, and amphibolite-facies events (Fig. 1).

New geochronological data on metamorphic and igneous rocks from the Gridino Village area (Belomorian eclogitic province)

In the Belomorian eclogitic province, eclogites aretraceable in the meridional direction from SalmaStrait (Imandra Lake) via the islands and coast of theKrasnaya Estuary of the White Sea [1] and the GridinoVillage area [2] to the Onezhskaya Estuary coast. Thestudy area is located in the wellexposed coastal zoneand adjacent islands of the White Sea near the villageof Gridino. An eclogitebearing complex approximately 50 km long (from the northwest southeastward) and 10 km wide was described in [2] as anArchean eclogitebearing melange and thoroughlyillustrated in [3] using rocks from Stolbikha Island asan example. The complex includes eclogites andeclogitized mafic dikes [2, 4]. The mafic rocks demonstrate distinct successive stages of metamorphic transformations under eclogite (maximal pressure 15–16 kbar),elevatedpressure granulite (

The Salma Eclogites of the Belomorian Province, Russia: HP/UHP Metamorphism Through the Subduction of Mesoarchean Oceanic Crust

Publisher Summary Eclogite-facies mafic rocks occur within gray gneisses of TTG affinity in the northeastern part of the Belomorian Province, Kola Peninsula. These are characterized by widespread omphacite-breakdown textures and locally preserved relics of omphacite. Thermobarometry indicates a clockwise PT path. Garnet inclusions suggest a prograde path passing from surface-weathering conditions through the low-grade green schist facies (pumpellyiteactinolite facies) before reaching the eclogite facies. Peak metamorphic conditions are estimated to be about 700‑750°C and > 14‑15 kbar. Needle-shaped inclusions (rods) of quartz in omphacite suggest that the peak P-T conditions of studied eclogites could reach significantly higher pressure than estimated in the present study. The retrograde path passed through granulite facies to upper amphibolite facies by near-isothermal decompression. The results of UPb dating and Hf-isotope analysis of zircons from the eclogites and cross-cutting felsic vein can be used to infer an approximate 2.89 Ga age for the oceanic crust, which was subducted and underwent eclogite-facies metamorphism between 2.87 and 2.82 Ga. The granulite-facies overprint is likely to have occurred at 2.72 Ga. Thermal overprinting and growth of new zircon also occurred during the Svecofennian (1.9‑1.8 Ga) orogeny. These new data imply that plate tectonic processes (“hot subduction”) operated at least locally in the late Mesoarchean. The petrology and geochemistry of the Salma eclogites and related TTG rocks can be best explained by subduction of Archean oceanic crust. The adakitic nature of the felsic vein inside the Salma eclogites suggests that partial melting of hydrated eclogites could produce Archean TTG-type magmas.

Metamorphic Evolution of the Gridino Mafic Dyke Swarm (Belomorian Eclogite Province, Russia)

Publisher Summary Data for the Gridino area allow to regard the Belomorian eclogite province as an Archean paleosubduction zone. Age and P‑T conditions of the main metamorphic events in the two regions examined, Gridino and Salma, are similar. In contrast to the Salma eclogites, the protolith of the Gridino Archean eclogites is continental crust. Igneous zircon from metagabbro with an age of 2.82 Ga probably records the time of mafic dyke intrusion. Neoarchean isochron ages about 2.71 Ga of high-pressure granitoids are the upper age limit of mafic dyke intrusion and eclogite metamorphism in the Gridino area. Thus, the continental complexes of the region have undergone high-pressure metamorphism in eclogite and subsequently granulite facies in the Archean. Hot mafic melt (1100‑1200°C) intruded an upper level of the Mesoarchean (about 3.0 Ga) crust under amphibolite conditions (5 kb and 600°C). Burial began after cooling and solidification of the mafic intrusions. Prograde transformation of the igneous minerals is attested by the growth of orthopyroxeneclinopyroxene (omphacite) coronas around them in silica-poor gabbro and gabbronorite, and by garnet coronas with kyanite and omphacite inclusions at the contact of igneous pyroxene and plagioclase in quartz-bearing gabbronorite.

Experience of chemical Th-U-Pb chemical dating of zircon from metasomatic felsic veins of the Mridino area, Belomorian eclogite province

For the first time in Russia, the Th-U-Pb isochron and point ages of zircons with anomalous U, Th, Pb, Y, and REE contents were determined by chemical dating. Zircons were extracted from metasomatic veinlets, which cut the eclogitized dike of olivine gabbronorites. The Paleoproterozoic ages (∼2400, 2100, and 1890 Ma) are quite consistent with the previously determined U-Pb SHRIMP II age of these zircons.

Dumortierite- and corundum-bearing quartz–feldspar–mica rocks of the Belomorian eclogite province: An example of melting of phengite + quartz

In the Salma eclogite of the Belomorian eclogite province, a dumortierite–phengite–corundum–bearing quartz–feldspar rock has been studied: its primary HP mineral paragenesis included garnet, phengite, and quartz. The phengite–quartz rocks were formed during dehydration and/or melting of boroncontaining rocks when they were dipped in the Meso- Neoarchaean subduction zone to a depth of not less than 70 km. As a result of the subsequent superimposed high-temperature metamorphic events under PT conditions of high-pressure granulite facies, the phengite in quartz underwent incongruent dehydration melting with formation of complex polymineral pseudomorphs, consisting of feldspars, biotite, newly formed muscovite, kyanite, corundum, and dumortierite. New estimates of the metamorphic temperature (850–900°C according to the melting reactions of phengite and the dumortierite field of stability; about 1000°C by the reintegrated composition of feldspar–mesoperthite) that affected the HP parageneses of Salma eclogitized rocks are at least 50–100°C (or even more) higher than them estimated earlier.

Olivine-bearing rocks of the Lapland granulite belt (Baltic Shield)

Olivine-bearing varieties of garnet–clinopyroxene crystalline schists of the Lapland granulite belt have been studied in detail for the first time. Two types of olivine (iron mole fractions of 27 and 38%) are distinguished. Olivine with lower Fe content occurs as inclusions in clino- and orthopyroxene and in terms of СаО and Cr contents is close to magmatic minerals. Olivine with high Fe content presumably suffered highand moderate-temperature metamorphism. The olivine-bearing rocks contain several grains of omphacite with 30–37 mol % jadeite and garnet with 44–50 mol % pyrope, which can be regarded as relict assemblages of the early stage of eclogitization of a magmatic protolith. The presence of symplectites indicates their retrograde transformation during decompression. The protoliths of the studied rocks could be olivine gabbronorites and pyroxenites. It was found that the rocks contain high-alumina minerals: corundum, spinel, and sapphirine. In addition, Al2O3 content in some amphibole grains is as high as 19 wt %. This indicates that the ascent of the deep-seated rocks was accompanied by interaction with Al-rich fluid. The positive Eu anomaly in the olivine-bearing rocks and some of their minerals is indicative of the reducing character of fluid. Activation of fluid reworking leading to the formation and transformation of the olivine-bearing rocks, transfer of alumina and its precipitation at different depths are related to the processes at the base of the Paleoproterozoic rift system of Karelides.