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

Abstract

The provenance of Archean and Paleoproterozoic zircons in the eclogites and related rocks from the Belomorian Belt of the Baltic Shield is considered on the basis of criteria for their equilibrium with melt, garnet and fluid. It is shown that the Paleoproterozoic zircon with low Th/U ratios (<0.1) and extremely low HREE and MREE contents does not match criteria for its equilibrium crystallization with garnet. This zircon crystallized from Zr and Hf rich fluid that was in disequilibrium with previously formed eclogite assem blages, but equilibrated with amphibole and plagioclase, thus indicating amphibolite rather than eclogite facies conditions. The input of fluid and its long term (~100 Ma) circulation in the deep levels of the Archean juvenile crust of the Belomorian belt was triggered by the Lapland-Kola orogeny. After its peak, the rocks of the belt experienced very slow uplift at rates an order of magnitude lower than those in the Phanerozoic oro gens. It is found that Late Archean zircon population contains metamorphic zircon, which matches criteria for simultaneous crystallization with garnet, and oceanic protolithic zircon, which was in equilibrium with high Mg basalt/gabbro of the Archean oceanic crust. This indicates a subduction origin of the Archean eclog ites of the Belomorian belt.

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

“…We believe a more sophisticated examination should be undertaken, with focus on the petrology of pristine eclogite (sensu stricto), to derive more reliable P–T paths of their metamorphic evolution. For this work, we selected representative rock samples, a pristine eclogite, two moderately retrograded eclogites that contain omphacite relicts, and an intensely retrograded eclogitic rock (the apoeclogite of the Russian literature) from Shirokaya Salma (a subcomplex of the Salma Complex), which is less studied than the adjacent Uzkaya Salma and Kuru‐Vaara localities (see Konilov et al., ; Li, Yu, Zhang, Wei, & Bader, ; Liu et al., ; Mints, Belousova, et al., ; Mints, Konilov, et al., ; Mints et al., ; Shchipansky, Khodorevskaya, Konilov, & Slabunov, ; Shchipansky, Khodorevskaya, & Slabunov, ; Shchipansky & Slabunov, ).…”

Garnet and zircon geochronology of the Paleoproterozoic Kuru-Vaara eclogites, northern Belomorian Province, Fennoscandian Shield

“…Ti-in-zircon thermometry (Watson et al, 2006) mainly yields temperatures of 620-700°C for these zircons (Table S8), that are partly overlapping with the temperature estimates for prograde epidote-amphibolite facies metamorphism of Salma eclogites (610-660°C, Imayama et al, 2017;Yu et al, 2019b). The reported geochemical signature and mineral inclusion assemblage disagree with Balagansky et al (2015) and Shchipansky and Slabunov (2015), who linked the ca. 1.9 Ga zircon with fluid infiltration during exhumation following the Lapland-Kola orogeny.…”

“…For this reason, the eclogitized dike exhibits no clear evidence for prograde metamorphism in contrast to the eclogite boudin. In previous studies (Balagansky et al, 2015;Shchipansky et al, 2012bShchipansky et al, , 2012aShchipansky and Slabunov, 2015), the appearance of the mineral coronas in the Kuru-Vaara gabbro-norite dike was ascribed to amphibolite facies metamorphism as the eclogite assemblage had not been identified. This can be explained by considering that omphacite may not uniformly spread in coronitic gabbro-norites since its occurrence depends on local rock chemistry (Korikovsky, 2005).…”

“…1.9 Ga was suggested to peak eclogite facies metamorphism (Herwartz et al, 2012;Mel'nik et al, 2013;Skublov et al, 2010Skublov et al, , 2011b or prograde eclogite facies stage (Liu et al, 2017) based on Sm-Nd and Lu-Hf dating and zircon U-Pb geochronology. However, Balagansky et al (2015) and Shchipansky and Slabunov (2015) proposed that the obtained ca. 1.9 Ga zircon age in Kuru-Vaara eclogites is related to fluid infiltration during the Lapland-Kola collisional orogeny.…”