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RESEARCH Paleoproterozoic High-Pressure Metamorphic History

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RESEARCH Paleoproterozoic High-Pressure Metamorphic History RESEARCH Paleoproterozoic high-pressure metamorphic history of the Salma eclogite on the Kola Peninsula, Russia Takeshi Imayama1, Chang-Whan Oh2, Shauket K. Baltybaev3, Chan-Soo Park4, Keewook Yi4, and Haemyeong Jung5 1RESEARCH INSTITUTE OF NATURAL SCIENCES, OKAYAMA UNIVERSITY OF SCIENCE, 1-1 RIDAI-CHO, KITA-KU, OKAYAMA 7000005, JAPAN 2DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, AND EARTH AND ENVIRONMENTAL SCIENCE SYSTEM RESEARCH CENTER, CHONBUK NATIONAL UNIVERSITY, 567 BACKJAEDARO, DUCKJIN-GU, JEONJU 54896, REPUBLIC OF KOREA 3INSTITUTE OF PRECAMBRIAN GEOLOGY AND GEOCHRONOLOGY, RUSSIAN ACADEMY OF SCIENCES, 2, MAKAROVA, ST. PETERSBURG, 199034, RUSSIA 4DIVISION OF EARTH AND ENVIRONMENTAL SCIENCE RESEARCH, KOREA BASIC SCIENCE INSTITUTE, 161, YEONGUDANJI, OCHANG-EUP, CHEONGWON-GU, CHEONGJU, 28119, OCHANG 34133, REPUBLIC OF KOREA 5SCHOOL OF EARTH AND ENVIRONMENTAL SCIENCES, SEOUL NATIONAL UNIVERSITY, 1 GWANAK-RO, GWANAK-GU, SEOUL 08826, REPUBLIC OF KOREA 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 cathodolumines- cence 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 granu- lite 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. LITHOSPHERE; v. 9; no. 6; p. 855–873; GSA Data Repository Item 2017317 | Published online 14 September 2017 https://doi.org/10.1130/L657.1 INTRODUCTION evolution of this early style of subduction prior to the Neoproterozoic remains unclear. One of the most important questions in Earth sciences involves the ini- Because eclogite is a typical high-pressure rock formed within subduc- tiation and evolution of subduction during the Precambrian (e.g., Cawood tion zones, unraveling the metamorphic evolution of Precambrian eclogites et al., 2006; van Hunen and Moyen, 2012, and references therein). Many with pre-Neoproterozoic ages is important in order to study the evolution researchers have inferred that subduction began during the Archean of the early style of subduction occurring prior to the Neoproterozoic. (e.g., Komiya et al., 1999; Brown, 2006, 2009; Cawood et al., 2006; Van Precambrian eclogites are rare worldwide. The Paleoproterozoic eclogites Kranendonk et al., 2007), based on the presence of indicators of plate from Tanzania (2.0 Ga in the Usagaran belt; 1.89–1.86 Ga in the Ubendian tectonics, such as accretionary prisms, orogens, and paired metamorphic belt) are well-known examples that are considered to represent remnants belts. Brown (2006) suggested that the first appearance of Neoarchean of the subducted Paleoproterozoic oceanic lithosphere (e.g., Möller et al., high-pressure granulite reflects the initiation of subduction, which has 1995; Collins et al., 2004; Boniface et al., 2012). However, the existence a geothermal gradient higher than that observed in modern subduction of Archean eclogites remains controversial. Mints et al. (2010, 2014) characterized by blueschist. Blueschist facies metamorphism is believed suggested that the eclogite in the Salma area of the Kola Peninsula is an to have begun during the Neoproterozoic (Maruyama et al., 1996; Stern, Archean eclogite, based on the 2.87–2.82 Ga zircon age recorded in the 2005; Tsujimori and Ernst, 2014). These occurrences indicate that an early eclogite. The 2.72–2.70, 2.4, and 1.9 Ga zircon ages obtained from the style of subduction without blueschist may have dominated between the Salma eclogite were interpreted to be retrograde metamorphic ages (Mints Neoarchean and the Neoproterozoic (Brown, 2006, 2009). However, the et al., 2010, 2014). In contrast, Skublov et al. (2010a, 2011) suggested that LITHOSPHERE© 2017 Geological | Volume Society 9 of| America.Number 6For | www.gsapubs.orgpermission to copy, contact [email protected] 855 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/6/855/3992376/855.pdf by guest on 27 September 2021 IMAYAMA ET AL. the Salma eclogite is a Paleoproterozoic eclogite, based on the presence of originally formed. We suggest that Paleoproterozoic subduction zones metamorphic zircons recording ages of ca. 1.92–1.88 Ga, low Th/U ratios, were relatively warmer than Phanerozoic subduction zones but colder and flat heavy rare earth element (HREE) patterns. Lu-Hf and Sm-Grt ages than Neoarchean subduction zones. (1.90–1.88 Ga) obtained from eclogite and eclogitized ultrabasite were also interpreted to reflect eclogite facies metamorphic ages (Skublov et al., GEOLOGICAL BACKGROUND 2010b; Herwartz et al., 2012; Mel’nik et al., 2013). However, garnet from massive eclogite (sample 46, Table 1) exhibits prograde zoning (Skublov et The Fennoscandian shield records a general trend in which the age of al., 2011), implying that these ages may represent prograde metamorphic geological activity decreases toward the southwest. The northern part of ages rather than ages of peak metamorphism. These previous studies pre- the shield is dominated by Archean rocks, whereas the major part of the sented no direct evidence with which to determine which zircons formed shield comprises the Paleoproterozoic 2.0–1.8 Ga Svecofennian Province during eclogite facies metamorphism, such as omphacite inclusions in and the 1.8–1.65 Ga Transscandinavian Igneous Belt. The 1.2–0.9 Ga zircon. Therefore, the timing of the eclogite facies metamorphism of the Sveconorwegian Province is farther to the southwest (Daly et al., 2006; Salma eclogite is still uncertain, and it is necessary to confirm whether Fig. 1A). In the northern part of the shield, the Kola-Karelian orogen is the Salma eclogite is Archean or Paleoproterozoic in age based on direct located between the Kola and Karelian cratons. The Kola-Karelian orogen evidence. In addition, the study of the pressure-temperature (P-T) condi- mainly consists of three Paleoproterozoic tectonic belts (the Kola suture tions of eclogites in the Salma area is necessary, because only minimum belt, the Tanaelv belt, and the Lapland and Umbra granulite belts), the Neo- pressure conditions have been determined using geothermobarometry archean Inari microcontinent, and the Belomorian mobile belt (Fig. 1B). (Mints et al., 2010, 2014; Shchipansky et al., 2012). Determining the The Belomorian mobile belt is principally composed of 2.9–2.6 Ga age and petrogenesis of the Salma eclogite in the Kola Peninsula is thus tonalite-trondhjemite-granodiorite (TTG) gneisses (Hölttä et al., 2008; essential for understanding Precambrian geodynamics and the tectonic Mints et al., 2014) and includes a ca. 2.9 Ga paragneiss complex and evolution of the Fennoscandian shield. 2.9–2.8 Ga greenstone belts (Slabunov et al., 2006). The available geologi- In this paper we provide direct evidence for Paleoproterozoic eclogite cal, isotopic, and geochemical data from the mafic-ultramafic rocks of the based on zircon U-Pb dating coupled with analyses of REEs and inclu- greenstone complex are compatible with their interpretation as the tectoni- sions in zircon. The P-T conditions of the different metamorphic stages of cally disrupted and metamorphosed remnants of a Mesoarchaean ophiolitic the Salma eclogite were estimated using conventional geothermobarom- association (Slabunov et al., 2006). This belt underwent multiple deforma- etry and pseudosection modeling. In addition, whole-rock chemistry was tion and metamorphic events during both the Archean and Paleoprotero- analyzed to characterize the tectonic setting in which these eclogites zoic (Daly et al., 2001, 2006; Mints et al., 2014). The Paleoproterozoic TABLE 1. AGE CONSTRAINTS AND INTERPRETATIONS FROM THE SALMA ECLOGITES AND RELATED ROCKS Lithology (sample identification) Method Age (Ma) Interpretation of source givenReferences Eclogite (S-198/107) U-Pb Zrn 2703 ± 9Retrograde granulite facies metamorphism Mints et al. (2010) Fe-Ti eclogite (S-204-2B) U-Pb Zrn 2820 Magmatic protolith age Mints et al. (2010) 1913 Partially reset age Garnetite (S-204-23B) U-Pb Zrn1891 ±17 Metamorphic event Mints et al. (2010) Plagiogranite vein (S-204-28) U-Pb Zrn 2866 ± 10 Eclogite facies metamorphism to ca. 2.87 Ga or older Mints et al. (2010) 2781 ± 15 Eclogite (S-198/107) U-Pb Zrn 2724 ± 35 Granulite facies metamorphism Kaulina et al. (2010) Eclogite (Ex198) U-Pb Zrn 2917 ± 360 Magmatic protolith age Kaulina et al.
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