No abstract

The Precambrian Salma eclogites on the Kola Peninsula, Russia, represent some of the oldest eclogites in the world; however, there has been much debate regarding whether the timing of their eclogite facies metamorphism is Archean (2.72-2.70 Ga) or Paleoproterozoic (1.92-1.88 Ga). New microstructural observations, pressure-temperature (P-T) analyses, zircon inclusion analyses, and U-Pb zircon dating performed in this study suggest that eclogite facies metamorphism occurred at ca. 1.87 Ga under P-T conditions of 16-18 kbar and 750-770 °C. Metamorphic zircons with the age of 1.87 Ga have inclusions of garnet (Grt) + omphacite (Omp) + Ca-clinopyoxene (Cpx) + amphibole (Amp) + quartz (Qz) + rutile (Rt) ± biotite (Bt), as well as flat heavy rare earth element (HREE) patterns due to the presence of abundant amounts of garnet during peak eclogite facies metamorphism. The Paleoproterozoic ages (1.92-1.88 Ga) presented in previous studies are reinterpreted to represent prograde ages, rather than peak ages, because these ages have been inferred from U-Pb dating in zoisite-bearing zircon and Sm-Nd and Lu-Hf geochronologic analyses of garnet showing growth zoning. In contrast, the 2.73-2.72 Ga unzoned zircons with dark cathodoluminescence contain inclusions of Grt + Amp + plagioclase (Pl) + Qz + rutile (Rt) ± Bt and are relatively enriched in HREEs, suggesting that an initial amphibolite facies metamorphic event occurred during the Archean. This study also proposes that the Salma eclogites underwent granulite facies retrograde metamorphism at 10-14 kbar and 770-820 °C, with rapid decompression occurring soon after peak metamorphism ca. 1.87 Ga. The final period of retrograde amphibolite facies metamorphism occurred at 8-10 kbar and 590-610 °C. Whole-rock chemical analyses indicate that the Salma eclogites were originally tholeiitic basalts formed at a mid-ocean ridge. The occurrence of eclogite facies metamorphism ca. 1.87 Ga suggests that the collision between the Kola and Karelian continents occurred during the Paleoproterozoic, rather than the Archean. These results, as well as those of previous studies, imply that the subduction required to form eclogites may have begun during or before the Paleoproterozoic.

Рассмотрены палеопротерозойские магматические комплексы западной части Волго-Донского орогена. Донской террейн развивался как вулканическая дуга на континентальной коре, а Лосевский -на океанической. Об этом свидетельствуют гранитоиды с разными геохимическими и изотопно-геохимическими характеристиками, которые формировались в результате постколлизионного растяжения коры орогена. К л ю ч е в ы е с л о в а: Донской террейн; Волго-Донской ороген; магматизм Д л я ц и т и р о в а н и я:

In a study based on Pb isotopes, it has been shown that the formation of gold-sulfide mineralization in the Early Proterozoic gabbros of the Northern Ladoga region is associated with different tectonic phases. This conclusion is based on a comparative study of isotopic data for feldspars and lead sulfides in two Early Proterozoic intrusions; Velimäki and Alattu-Päkylä. The investigated intrusions have similar geological settings at the junction of the Svecofennian accretionary complex and the Karelian craton. Both massifs are characterized by manifestations of noble metal sulfide mineralization associated with zones of local shear deformations. The formation of the studied ore massifs is attributed to the Svecofennian and Caledonian tectonic stages. Feldspars of Velimäki intrusion have parameters and model age of Pb significantly older than U-Pb age of zircon (1.9 Ga), although they correspond to the Svecofennian time of magmatic crystallization of gabbro and clinopyroxenites, while sulfide pyrite-pyrrhotite mineralization is associated with gold in these rocks with the Caledonian stage of fluid-thermal processing of the Early Proterozoic magmatic mineral paracenteses. Feldspars and sulfides of the Alattu-Päkylä intrusion have similar lead isotopic parameters and Pb model age (~ 2 Ga), also somewhat older than the U-Pb age of zircon, but at the same time indicating the formation of rock-forming and ore sulfide associations with gold during the Svecofennian (Early Proterozoic) plutonic and tectonic-thermal events. The source of lead sulfides of the Velimäki massif is characterized by the parameters of the upper crust with high µ2 = 238U / 204Pb, while the µ2 parameter of the minerals of the Alattu-Päkylä massif is slightly lower, which indicates the participation of the mantle-lower crustal reservoir material in the formation of magma.

Kaalamo and Velimyaki gabbroid massifs are located in the south-eastern part of the Raahe-Ladoga suture zone of the Karelian craton and the Sveсofennian mobile belt. These massifs were formed almost simultaneously 1.89 Ga ago, occupy the same tectonic position and are located in a single unit of the Early Proterozoic metamorphosed rocks: the intrusions are contained by the Early Proterozoic supracrustal stratas: garnet-biotite gneisses and micaceous shales of the Ladoga series, as well as amphibolites of the Sortavala series. Despite this, the gabbroids of the compared massifs differ sharply in their material composition, especially the content of alkalis in a differentiated series of rocks. The results of thermodynamic modeling of magmatic crystallization (Magma Chamber Simulator, MELTS packages) showed that a differentiated sub-alkaline series of rocks of the Velimyaki massif cannot be obtained from magma of the Kaalamo massif by slightly changing such parameters as pressure, water content in the melt, and oxygen fugacity. The assumption of a significant difference in these parameters for magmas of the Kaalamo and Velimyaki massifs is not confirmed at the mineralogical-petrographic level and other characteristics of the rocks of the compared ones. The effect of contamination at the level of the magma chamber also does not significantly shift the trend of rock compositions from the calc-alkaline region to the sub-alkaline region. Analysis of the results of thermodynamic modeling of magmatic mineral formation in the two massifs allows us to make it possible to conclude that the Kaalamo and Velimyaki massifs were formed either from various parent melts, or their initial melt was one, but the magma of the Velimyaki massif underwent contamination with enrichment with alkalis along the migration route of magma until the level of the upper crust was reached.