First Lu-Hf garnet ages of eclogites from the Belomorian Mobile Belt (Baltic Shield, Russia)

Abstract

Eclogites from the north eastern border of the Bel morian Mobile Belt (BMB) have not only been used for regional geodynamic reconstructions [1], but also as first direct evidence for plate tectonic processes in the Mesoarchean [2]. M.V. Mints and coworkers (2010) dated zircons from eclogites of the Salma area related to the so called Belomorian Meso-Neoar chean eclogite province and interpreted the age of 2.87 Ga obtained for some zircons as the age of meta morphism of the eclogite facies [2]. However, rocks of the Salma area undoubtedly underwent eclogite meta morphism with an age of ~1.9 Ga in the Paleoprotero zoic [2][3][4]; for this reason, there is a question of whether or not zircon datings of 2.87 Ga correspond to the age of Mesoarchean eclogite metamorphism [1, 2], or whether this is the age of the magmatic protolith (gabbro) of eclogites [3,4], or if it represents a low P metamorphic event. In this study we applied the LuHf garnet clinopyroxene geochronology, which are rock forming minerals of apo gabbro eclogites of the BMB. Consequently, the Lu-Hf datings of garnet provide unambiguous evidence for the age of meta morphism of the eclogite facies, provided that the blocking temperature was not exceeded (see discus sion below).We studied four eclogite samples from the Salma area (Samples 46 and 21) and Gridino (Samples 102 and 108), which were preliminarily dated using the U-Pb method by zircons providing evidence for the Svekofennian (~1.9 Ga) age of eclogites [3,4]. According to the specific geochemical characteristics, the studied grains or rims of zircons correspond to zir cons precisely of the eclogite facies [3,4]. Eclogitized gabbro (Samples 46 and 102) are the most abundant eclogite type in the BMB. These are homogeneous fine granular bimineral rocks mainly composed of omphacite partially replaced by decompression sym plectites of plagioclase and clinopyroxene with a very low portion of the jadeitic end member and numerous garnet porphyroblasts. Eclogitized ultrabasic rocks (Samples 21 and 108) form coarse granular garnetclinopyroxene bodies (with a thickness up to 20 cm) and aggregates in the central parts of metaultrabasite bodies transformed mainly into secondary tremo lite-actinolite schist (Sample 21) or amphibolite (Sample 108) along the perimeter.Mineral monofractions were sorted according to standard methods using heavy liquids at the Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, and finally manually separated under a binocular microscope. For each sample three garnet monofractions were independently analyzed (Table 1). Both whole rock powders and mineral sepa rates were dissolved using a selective digestion proce dure, aiming to dissolve the major silicate phases, while leaving behind refractory phases, such as rutile and zircon [5]. This approach circumvents a potential age bias due to zircon and other mineral inclusions [6] and frequently results in high precision Lu-Hf ages (e.g., [5,7,8]). These authors also provide detailed methodological sections, ...

Age and P–T conditions of the Gridino‐type eclogite in the Belomorian Province, Russia

“…Eclogite facies metamorphism of the Gridino-type eclogites is also thought to have occurred at c. 2.82-2.70 Ga, although two zircon overgrowths with ages of c. 1.9 Ga have also been reported in these studies Li et al, 2015;Mints et al, 2014), and an age of c. 1.9 Ga for the metamorphism has been proposed by Skublov, Astaf'ev, et al (2011) and Skublov, Berezin, and Mel'nik (2011). In addition, there are Lu-Hf garnet ages of c. 1.89 and 1.94 Ga from the Gridino-type eclogites (Herwartz, Skublov, Berezin, & Melnik, 2012).…”

“…In addition, there are Lu–Hf garnet ages of c . 1.89 and 1.94 Ga from the Gridino‐type eclogites (Herwartz, Skublov, Berezin, & Melnik, ).…”

Quartz and orthopyroxene exsolution lamellae in clinopyroxene and the metamorphic P–T path of Belomorian eclogites

“…In most cases, ancient (>2.0–1.9 Ga) high‐grade metamorphic terranes rarely preserve (pristine) eclogite—examples are the Hengshan granulite complex (North China Craton, 1.9–1.8 Ga, Zhao, Cawood, Wilde, & Lu, ), the Snowbird granulite–eclogite complex (Canada, 1.9 Ga, Baldwin, Bowring, & Williams, ), the Lapland granulite complex (Russia, 1.9 Ga, Daly et al., ) and the Ubendian/Usagaran eclogite‐bearing complex (Tanzania, 2.0–1.9 Ga, Collins, Reddy, Buchan, & Mruma, ). Overall, there are three scenarios invoked for the formation and evolution of the Belomorian eclogites (see Perchuk & Morgunova, ): scenario I, a single Mesoarchean eclogite facies metamorphism (Dokukina & Konilov, ; Dokukina et al., ; Mints, Belousova, et al., ; Mints, Konilov, et al., ); scenario II, at least two eclogite facies events in the Archean and the Palaeoproterozoic (Balagansky et al., ; Slabunov, Stepanova, Bibikova, Babarina, & Matukov, ; Slabunov, Volodichev, Skublov, & Berezin, ; Volodichev, Slabunov, Sibelev, Skublov, & Kuzenko, ); and, scenario III, a single Palaeoproterozoic eclogite facies metamorphism at 1.9 Ga (Herwartz, Skublov, Berezin, & Mel'nik, ; Mel'nik, Skublov, Marin, Berezin, & Bogomolov, ; Skublov, Berezin, & Mel'nik, ; Skublov et al., ; Travin & Kozlova, ). The Shirokaya Salma eclogites have previously been considered as having a Meso‐Neoarchean age (e.g.…”

Provenance of “Svecofennian” zircons in the Belomorian mobile belt, Baltic shield, and some geodynamic implications

“…However, it was specially mentioned (Herwartz et al, 2012) that obtained Lu-Hf mineral dates on the Belomorian eclogites could be considered as the true ages if Paleoproterozoic zircon crystallized together with garnet. As demonstrated above, such evidence does not exist, while Paleoproterozoic zircon is of fluid origin.…”

“…It is important that the pegmatite veins are rimmed by thick amphiboliza tion zones, which completely replace older garnet bearing metamorphic assemblages of mafic rocks. Although U-Pb dating of zircons from the Kuru-Vaara pegmatite yields an age of 1841 ± 12 Ma (Skublov et al, 2012), the true range of their formation remain unknown, because there is at least one older phase of non economic pegmatite, which is clearly cut by the veins of the main pegmatite phase. However, it is note worthy that zircons from dated pegmatite vein have the same low Th/U ratios (<0.1) and high Hf concentra tions as the modified Paleoproterozoic zircons from eclogites.…”