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 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.

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

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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. (2010) 2939 ± 81 Magmatic protolith age 1820 ± 180 Metamorphic event Garnetite (S-204-23) U-Pb Zrn1891 ± 17 Metamorphic eventKaulina et al. (2010) Plagiogranite (S-204-28) U-Pb Zrn 2866 ± 36 Magmatic protolith age Kaulina et al. (2010) 2778 ± 23 Magmatic protolith age 1874 ± 29 Metamorphic event Massive eclogite (sample 46) U-Pb Zrn 2865 ± 35 Magmatic protolith event Skublov et al. (2010a) (2879 ± 34) Eclogite facies metamorphism Skublov et al. (2011) 1923 ± 75 (1878 ± 36) Eclogitized ultrabasic (sample 21) U-Pb Zrn1907 ± 11 Eclogite facies metamorphism Skublov et al. (2010a) Pegmatite vein (sample 62) U-Pb Zrn1841 ± 12 Retrograde amphibolite facies metamorphism Skublov et al. (2010a) Massive eclogite (sample 46) Lu-Hf Grt1901 ± 5 Eclogite facies metamorphism Herwartz et al. (2012) Eclogitized ultrabasite (sample 21) Lu-Hf Grt1894 ± 4 Eclogite facies metamorphism Herwartz et al. (2012) Massive eclogite (sample 46) Sm-Nd Grt1897 ± 16 Eclogite facies metamorphism Mel’nik et al. (2013) Garnetites (sample 48) U-Pb Zrn 2864 ± 43 Magmatic protolith age Mel’nik et al. (2013) Sm-Nd Grt 1927 ± 50 Metamorphic event 1887 ± 19 Metamorphic event (1839 ± 11) Massive eclogite (sample 46) Sm-Nd Grt1789 ± 23 Retrograde metamorphic event Skublov et al. (2010b) Eclogitized ultrabasic (sample 21) Sm-Nd Grt1878 ± 12 Metamorphic event Skublov et al. (2010b) Eclogite (RPB1B) U-Pb Zrn 2716 ± 10 Amphibolite facies metamorphism This study 1865 ± 15 Eclogite facies metamorphism Eclogite (RPB3A) U-Pb Zrn 2727 ± 8 Amphibolite facies metamorphism This study 1868 ± 17 Eclogite facies metamorphism 1720 ± 79 Retrograde amphibolite facies metamorphism Note: Ages in parentheses are data from Skublov et al. (2011). Zrn—zircon; Grt—garnet.

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reworking was associated with collisional events, resulting in granulite (2014) inferred that the protolith of the layered mafic body was originally facies metamorphism (Balagansky et al., 2014, and references therein). a series of normal gabbro norite, olivine gabbro, and Fe-Ti oxide gabbro In the Belomorian mobile belt, several eclogite exposures occur (Fig. 1) intercalated with local troctolite, which resembled the gabbroic suite from in the Salma area (Kaulina et al., 2010: Mints et al., 2010; Skublov et al., the modern oceanic crust of the slow-spreading Southwest Indian Ridge 2010a, 2010b, 2011), in the Kuru-Vaara quarry (Shchipansky et al., 2012; (Dick et al., 2000). The different degrees of retrograde metamorphism may Balagansky et al., 2014), and in the Gridino area in the northeastern region be due to compositional differences or varying amounts of water infiltration. of Karelia (Volodichev et al., 2004, 2012; Dokukina et al., 2014). The The retrograde eclogite includes a few leucosomes (Figs. 2B, 2D) consist- Salma and Kuru-Vaara eclogites are related to the high-pressure metamor- ing of quartz, plagioclase, K-feldspar, and relict garnet, thus indicating that phism of oceanic lithosphere and exhibit peak P-T conditions of >13–14 partial melting occurred during metamorphism. kbar and 700–750 °C (e.g., Mints et al., 2010, 2014; Shchipansky et al., 2012). The Gridino eclogites were originally gabbroic dike swarms that PETROGRAPHY AND MINERAL CHEMISTRY intruded felsic gneiss and were metamorphosed under peak P-T conditions of 17–18 kbar (probably to 22 kbar) and 740–865 °C (Volodichev The mineral compositions of the five samples were analyzed using a et al., 2004; Dokukina et al., 2014). Shimadzu electron probe microanalyzer (EPMA-1600) at the Jeonju branch In this study we focus on eclogite and granulite samples collected from of the Korea Basic Science Institute (South Korea). Multiple eclogite sam- two outcrops in the Salma area of the Belomorian mobile belt, which is ples (RPB3A, RPB1A, and RPB1B) and garnet-clinopyroxene granulite located near the Tanaelv belt (Fig. 1B). In the Salma area, mafic bodies samples (RPB3B and RPB3C) were collected. The operating conditions (including eclogite) occur within TTG gneisses, which mainly consist of used for the analyses included an accelerating voltage of 15 kV and a beam quartz diorite and trondhjemite. In the mafic bodies, eclogites occur as layers current of 20 nA. The probe diameter for the mineral composition spot that are intercalated with garnet granulite, garnet amphibolite, amphibolite, analyses was 3 µm. Natural and synthetic silicates and oxides were used and garnetite (Fig. 2A). Although some eclogite layers are fresh and coarse as standards. The ZAF method was employed for matrix correction. Rep- grained, most eclogite layers are medium grained and have retrograded into resentative mineral compositions are listed in Data Repository Table DR11. granulite and amphibolite (Figs. 2B–2D). In the coarse-grained eclogites, garnets occur within a bright green matrix, which mainly consists of clino- Retrograded Eclogite pyroxene (omphacite and calcic clinopyroxene) with minor amphibole (Fig. 2C). In the medium-grained eclogite, less garnet occurs within a matrix Sample RPB3A is a retrograde eclogite mainly consisting of gar- that is dark green, due to the presence of abundant amphiboles that formed net, clinopyroxene, amphibole, biotite, plagioclase, quartz, and rutile. during retrograde metamorphism (Figs. 2B, 2D). The interlayered granu- Garnet is characterized by an inclusion-rich core and an inclusion-free lite and amphibolite can be considered to represent strongly retrograded eclogite. During retrograde metamorphism, omphacite was first retrograded 1 GSA Data Repository Item 2017317, which includes four tables and one figure, into calcic clinopyroxene, thus forming symplectite around garnet and in is available at http://www.geosociety.org/datarepository/2017, or on request from the matrix; then, clinopyroxene was retrograded to amphibole. Mints et al. [email protected].

30o 36o B ▲ A ▲ Kola-Karelia Kola 0 100km Orogen Craton Eclogite samples ▲ ▲ ▲ ▲ ▲ ▲ ▲ Barents Sea ▲ ▲

Karelia ▲ White o Inari unit Craton ●Sea 69 ▲ ▲ 69o ▲ ▲ ▲ Svecofennian ▲ Lapland belt Murmansk Caledonian Belt Orogen terrane ▲ ▲

▲ ▲ ▲ ▲ Kola Province ▲ Transscandinavian ▲ terrane Tanaelv belt ▲ Sveconorwegian Igneous belt Belomorian Belt Baltic

Salma Mobile belt 3A, 3B, 3C ▲ Sea ▲ ▲ Kuru-Vaara ▲ 1A, 1B ▲ Gridino ● Reworked Archean TTG Neoarchan

gneiss & greenstone belt ▲ ▲ Paleoproterozoic ▲

Exposed Archean microcontinent ▲ Kola basement igneous belt Meso-Neoarchean TTG Paleoproterozoic suture

gneiss & greenstone belt Paleoproterozoic granulite belts ▲ belt

▲ ▲ o Reworked accretional orogen Reworked Archean TTG 67

▲ Archean complex Tectonic melange ▲ ▲ Mesoproterozoic gneiss & greenstone belt ▲ Paleoproterozoic orogen Paleoproterozoic Paleoproterozoic collisional orogen Paleozoic orogen volcanics & sediments post-orogenic granites White Sea 36o Umba belt

Figure 1. (A) Simplified geological map of the Fennoscandian shield showing the locations of the eclogites in the Belomorian mobile belt. (B) The tectonic division of the northeastern Fennoscandian shield region. Sample locations: 3A, 3B, and 3C: N67°28′32″/E32°22′39″; samples 1A and 1B: N67°31′07″/ E32°21′03″. Modified from Berthelsen and Marker (1986), Zhao et al. (2002), and Daly et al. (2006). TTG—tonalite-trondhjemite-granodiorite.

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A B Granulized eclogite

amphibolite Retrograded eclogite

C D

Figure 2. Outcrop photographs of the Salma eclogites from the Kola Peninsula, Russia. (A) Eclogite outcrop in which layered-type eclogite is interlayered with amphibolite and granulite. (B) Eclogite that has retrograded into granulite or amphibolite, with relict eclogite. (C) Rather fresh eclogite consisting of garnet and omphacite with minor amphibolite. (D) Thin leucosome in eclogite.

rim (Fig. 3A). The main inclusions in the garnet core are amphibole, and amphibole + plagioclase formed around garnet due to the breakdown clinozoisite, quartz, and rutile (Figs. 3B, 3C), with minor biotite. Most of omphacite during granulite facies metamorphism (Fig. 3F). In contrast, clinopyroxene crystals in the matrix are symplectic with plagioclase (Fig. the formation of coronitic plagioclase is related to the replacement of 3D) and are calcic clinopyroxene (commonly diopside and minor augite). garnet rims during amphibolite facies metamorphism (Fig. 3F). In sample Omphacite is found as relics (Fig. 3D). Omphacite records jadeite contents RPB1A, the jadeite contents of the clinopyroxene in the garnet and matrix of as much as 21%, whereas the symplectic diopside records low jadeite range from 11% to 18% and 15% to 21%, respectively, whereas the jadeite contents (3%–10%; Fig. 4A). Garnet is characterized by homogeneous content of symplectic clinopyroxene is 9% (Fig. 4C). In sample RPB1B,

cores [almandine (Alm41–44), pyrope (Prp33–34), grossular (Grs20–26), spes- clinopyroxene in garnet records jadeite contents ranging between 14%

sartine (Sps1)] and Mg-rich inner rims (Alm39–40Prp36–39Grs22–23Sps1) and and 27%, whereas the clinopyroxene in the symplectite records jadeite

outer rims (Alm41–42Prp35Grs22–23Sps1; Fig. 5A). A decrease in XFe from 0.54 contents ranging between 5% and 10% (Fig. 4C). The garnets in both

to 0.55 to 0.50–0.52 is observed toward the inner rim, and XFe increases samples have homogeneous cores (Alm41–43Prp36–38Grs18–21Sps1–2 with XFe

to 0.54 at the outer rim. All amphiboles within garnets and the matrix plot values of 0.52–0.54 in sample RPB1A; Alm44–46Prp29–32Grs23–25Sps1 with

within the compositional field of pargasite (Fig. 4B) with an FeX ratio of XFe values of 0.59–0.61 in sample RPB1B) and relatively Fe-rich outer-

0.08–0.26. Plagioclase has an anorthite content (Xan) of 0.21–0.35. Cli- most rims (with XFe ratios of 0.57–0.58 in sample RPB1A and XFe ratios of 3+ 3+ nozoisite records low Fe /(Fe + Al) ratios (0.02–0.03). 0.63–0.64 in sample RPB1B) (Fig. 5B). The XFe ratio in garnet increases Samples RPB1A and RPB1B are also retrogressed eclogites that are toward the rim, thus reflecting retrograde metamorphism (Fig. 5B). The

relatively dark in color and consist of garnet, clinopyroxene, amphibole, amphibole inclusions in garnet are magnesiohornblende with XFe contents

plagioclase, quartz, and rutile with minor biotite. The retrogressed eclog- of 0.11–0.12 in sample RPB1A and pargasite with XFe values of 0.23–0.29 ites include many subhedral garnet porphyroblasts ranging in size from 3 in sample RPB1B (Fig. 4D). In contrast, amphibole in contact with garnet to 10 mm. The main inclusions in the garnet are amphibole, clinopyroxene, records a wide compositional range, from magnesiohornblende to par-

quartz, and rutile. Subhedral clinopyroxene is dominant within the matrix, gasite, with XFe contents of 0.26–0.38 in sample RPB1A and 0.21–0.29

and plagioclase lamellae were found within the clinopyroxene in sample in sample RPB1B (Fig. 4D). The Xan values of plagioclase are 0.18–0.31 RPB1A (Fig. 3E). The symplectites of calcic clinopyroxene + plagioclase in sample RPB1A and 0.23–0.30 in sample RPB1B.

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A B C BtBt Amp Qz Qz Czo Amp Grt -

Fig.3c Czo Grt Czo Grt Amp Fig.3b Rt Czo Amp Qz Bt

1.0 mm 100 μm 150 μm D E F

Ca-Cpx Amp Ca-Cpx + Pl Amp + Pl Omp Grt Relict Qz Omp Pl lamellae

Ca-Cpx Grt Amp+Pl + Pl Coronitic Pl 0.5 mm 100 μm Grt 400 μm

Figure 3. Photomicrographs of the Salma eclogites. (A) Inclusion-rich core and inclusion-free rim in garnet in sample RPB3A. (B) Backscattered electron (BSE) image of clinozoisite and quartz in garnet in sample RPB3A. (C) BSE image of amphibole, rutile, clinozoisite, and quartz in garnet in sample RPB3A. (D) Plane-polarized light photomicrograph of symplectite of Ca-clinopyroxene + plagioclase and relict omphacite in sample RPB3A. (E) BSE image of plagioclase lamellae in Ca-clinopyroxene in sample RPB1A. (F) BSE image of omphacite inclusion in garnet and Ca-clinopyroxene + plagioclase and amphibole + plagioclase symplectites surrounding garnet porphyroblasts in sample RPB1B. Abbreviations: Amp—amphibole, Grt—garnet, Czo—clinozoisite, Cpx—clinopyroxene, Omp—omphacite, Pl—plagioclase, Rt—rutile, Qz—quartz.

Garnet-Clinopyroxene Granulite slightly lower than those in amphiboles in contact with garnet (0.26–0.38). The amphibole in sample RPB3C is magnesiohornblende (Fig. 4B) with

Samples RPB3B and RPB3C are garnet-clinopyroxene granulites XFe values of 0.19–0.30. The Xan values of plagioclase in samples RPB3B consisting of garnet, clinopyroxene, amphibole, plagioclase, and quartz. and RPB3C are 0.43–0.65 and 0.33–0.36, respectively. Clinopyroxene crystals in the 2 granulites are diopside with jadeite contents of <5% (Fig. 4A). In sample RPB3B, the composition of the garnet P-T ESTIMATES

core is Alm39–42Prp31–33Grs25–27Sps1; the almandine component increases

at the rim (Alm44–48Prp23-29Grs25–27Sps2), thus exhibiting retrograde zoning. Metamorphic Stages Based on Petrography The garnet in sample RPB3C also exhibits retrograde zoning. The garnet

core has a composition of Alm42–43Prp27–30Grs26–28Sps1, whereas the gar- On the basis of microstructural observations and mineral relationships,

net rim has an Fe-rich composition of Alm46–49Prp23–24Grs26–29Sps1–2. The several metamorphic stages have been recognized from the Salma eclog-

values of XFe increase toward the rim from 0.59 to 0.62 and to 0.66–0.68 ites. (1) Evidence of prograde metamorphism is preserved within garnet in RPB3B and RPB3C. The amphiboles in sample RPB3B plot on the in sample RPB3A (Fig. 3A). The prograde assemblage is clinozoisite boundary between the compositional fields of magnesiohornblende and (Czo) + amphibole (Amp) + garnet (Grt) + biotite (Bt) + quartz (Qz) +

pargasite (Fig. 4B). The XFe values of amphibole in garnet (0.11–0.22) are rutile (Rt) (Whitney and Evans, 2010). (2) The omphacite in garnet in

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A WoEnFs C WoEnFs

20 20 20 20

omphacite aegirine- aegirine- 40 omphacite 40 40 augite 40 augite

Jd Aeg Jd Aeg RPB3A RPB3B RPB3C RPB1A RPB1B In grt In grt Symplectite Symplectite Relict Matrix B D 1 1

Pargasite Pargasite Edenite Sadanagaite Edenite Sadanagaite

0.5 0.5 + 2Ca) in A site Tremolite + 2Ca) in A site Tremolite

RPB3A RPB3B RPB3C Tschermakite Tschermakite

(Na+K In grt (Na+K RPB1A RPB1B contact Mgnesio- In grt Mgnesio- matrix Hornblend contact Hornblend 0 0 0 0.5 11.5 2 0 0.511.52 (Al + Fe3+ + 2Ti) in C site (Al + Fe3+ + 2Ti) in C site Figure 4. Clinopyroxene compositions plotted on the wollastonite + enstatite + ferrosilite (WoEnFs)–jadeite–aegirine (Jd-Aeg) diagram of Morimoto (1988) (grt—garnet) for (A) samples RPB3A–RPB3C and (B) samples RPB1A, RPB1B. Classification of Ca-amphiboles according to Hawthorne et al. (2012) based on a (Na + K + 2Ca) in A site (Al + Fe+3 + 2Ti) in C site diagram for (C) samples RPB3A–RPB3C and (D) samples RPB1A, RPB1B.

sample RPB1B (Fig. 3F) and the relict omphacite in the matrix of sample producing the mineral assemblage Ca-Cpx + Amp + Grt + Qz + Rt +Pl + RPB3A (Fig. 3D) formed during eclogite facies metamorphism. The Mg- melt ± Bt. The symplectite of Ca-clinopyroxene + plagioclase observed rich garnet rim in sample RPB3A and the omphacite-bearing garnet core in all samples (e.g., Figs. 3D, 3F), as well as the plagioclase lamellae in sample RPB1B grew during this stage. The omphacite-bearing garnets observed in the Ca-clinopyroxene of sample RPB1A (Fig. 3E), developed from sample RPB1B also include amphibole, quartz, and rutile. The leu- during this stage. (4) The secondary amphiboles replacing garnet rims cosome associated with eclogite within the outcrop (Fig. 2D) indicates (Figs. 3A, 3F) and surrounding the symplectites (Fig. 3D) represent an that melt existed during metamorphism. The inferred peak assemblage amphibolite facies overprint that formed during cooling. The coronitic consists of omphacite (Omp) + Amp + Grt + Qz + Rt + melt ± Bt. (3) The plagioclase surrounding the garnet rims also formed during this stage, first overprint resulted in the growth of plagioclase and Ca-clinopyroxene, along with secondary amphibole.

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A the estimated temperatures may be high, due to the possibility of com- A RPB3A A’ positional changes occurring during peak or retrograde metamorphism. 0.60 Sample RPB3A records Mg-rich inner rims in garnet and relict ompha- XFe cite, and the garnet core in sample RPB1B includes omphacite. Thus, we 0.50 infer that these garnet and omphacite compositions are related to eclogite facies metamorphism. They yield peak eclogite-stage temperatures rang- 0.40 Xalm ing from 730 to 810 °C, assuming a pressure of 17 kbar, as obtained using compositional isopleths in the pseudosection (see later section). The P-T conditions of the first overprint (11.5–12.5 kbar and 770 °C), which indi- 0.30 Xpyp cate granulite facies metamorphism, were inferred from the compositions of garnet rims and the Ca-clinopyroxene and plagioclase located near the 0.20 garnet in sample RPB1A. The retrograde P-T conditions of amphibolite Xgrs facies metamorphism (8.0–10.0 kbar and 590–610 °C) were calculated 0.10 using the compositions of garnet rims and the plagioclase and amphibole surrounding the garnet in samples RPB3A, RPB1A, and RPB1B. Xsps 0.00 P-T Pseudosection 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Distance (mm) The P-T pseudosections for samples RPB3A and RPB1B were cal-

culated in the modal chemical system NCKFMASHTO (Na2O-CaO- B RPB1B K O-FeO-MgO-Al O -SiO -H O-TiO -O ) using the Perplex X program B B’ 2 2 3 2 2 2 2 0.70 (Connolly, 1990) with an internally consistent thermodynamic data set (Holland and Powell, 1998; updated in 2002). The following solid-solution XFe 0.60 models were used in these calculations: garnet (White et al., 2000), biotite (Tajcmanová et al., 2009), plagioclase (Newton et al., 1980), K-feldspar (Waldbaum and Thompson, 1968), clinopyroxene and orthopyroxene 0.50 Xalm (Holland and Powell, 1996), phengite (parameters from thermodynamic 0.40 dataset of Powell and Holland, 1988), amphibole (Dale et al., 2005), and melt (White et al., 2001). All fluid was assumed to be H O; its content Xpyp 2 0.30 was obtained from the values of weight loss on ignition. The ferrous/ferric (Fe2+/Fe3+) ratio was determined by using the titration of FeO to calculate Xgrs 0.20 the O2 component. The haplogranitic melt of White et al. (2001) may not always be appropriate for modeling partial melting in metabasites. As a 0.10 result, it is possible that the assemblages containing melt in the calculated pseudosections may be metastable. However, the topology of the phase Xsps 0.00 relationship between amphibolite facies and granulite facies metabasites 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 does not significantly change when mineral assemblages coexist with Distance (mm) fluids or melts (Pattison, 2003). The pseudosection approach is useful for inferring mineral assemblages including melt for the Salma eclogites. Figure 5. Representative compositional zoning profiles of garnet in The bulk-rock compositions used in the pseudosection calculations were (A) sample RPB3A and (B) sample RPB1B. Abbreviations: alm—alman- analyzed using inductively coupled plasma–mass spectrometry (ICP-MS; dine; pyp—pyrope; grs—grossular; sps—spessartine. Perkin Elmer Optima 3000) at Activation Laboratories Ltd. (Canada). The effective bulk composition was possibly modified by the growth of zoned garnet due to crystal fractionation (e.g., Evans, 2004). Because Conventional Geothermobarometry garnet in sample RPB3A displays prograde zoning with homogeneous cores and Mg-rich inner rims, its effective bulk composition was calcu- The P-T conditions of the different metamorphic stages of the metaba- lated. First, the pseudosection used to estimate prograde P-T conditions sites were estimated using the garnet-clinopyroxene and garnet-amphibole was constructed using the bulk composition determined from the ICP-MS geothermometers (Ellis and Green, 1979; Graham and Powell, 1984) analyses (Fig. 6). Second, the modal percentage of garnet cores within and the garnet-clinopyroxene-plagioclase-quartz and garnet-amphibole- the rock was estimated (~10 vol%), and the composition of the garnet plagioclase-quartz geobarometers (Powell and Holland, 1988; Kohn and cores was subtracted from the bulk chemical data (Konrad-Schmolke Spear, 1990). The P-T results obtained using conventional thermobarom- et al., 2008). The recalculated bulk composition was used for the pseu- etry are listed in Table 2. dosection in order to estimate the peak and retrograde P-T conditions The homogeneous garnet cores from sample RPB3A include clinozo- (Fig. 7). The molar proportion of the unfractionated and effective bulk isite; their compositions are relatively Mg poor compared to their inner rock composition used for the pseudosection modeling is shown in the rim compositions. These data imply that the homogeneous cores formed captions for Figures 6 and 7. during the prograde stage. The metamorphic temperatures estimated using The P-T pseudosection constructed using the actual measured bulk the compositions of the amphibole inclusions in garnet and the adjacent composition indicates that the prograde assemblage of zoisite (Czo) +

garnet cores in sample RPB3A are ~610–660 °C, which represent the P-T Amp + Grt + Bt + Qz + Rt + H2O occurs at P-T conditions of 13–18 kbar conditions during prograde metamorphism. However, the uncertainties of and 640–720 °C (Fig. 6). This P-T range can be considered reasonable

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TABLE 2. PRESSURE-TEMPERATURE RESULTS OF THERMOBAROMETRIC CALCULATIONS

Metamorphic grade Sample Grt Loc. XFe Xgrs Amp Loc. XFe Cpx Loc. XFe XNa Pl Loc. Xan GH GC GHPQ GCPQ T T P P (ºC) (ºC) (kbar) (kbar) Epidote-Amphibolite RPB3A 42 core 0.564 0.205 31 inc 0.250 660 RPB3A 12 core 0.524 0.206 10 inc 0.182 610 Eclogite RPB3A 15 near 0.509 0.21451relic 0.1410.233 730 rim RPB1B 10 core 0.604 0.245 3inc 0.2320.278 810 Granulite RPB1A 1rim 0.543 0.2113cont 0.1810.198 27 cont 0.297770 11.5–12.5 Amphibolite RPB3A 1rim 0.542 0.21919 cont 0.173 18 cont 0.314590 8.5–10.0 RPB1B 33 rim 0.632 0.228 31 cont 0.243 32 cont 0.237610 8.0–9.0 RPB1A 60 rim 0.596 0.21062 cont 0.21061cont0.313590 8.5–10.0 Note: Amp—amphibole; Grt—garnet; Cpx—clinopyroxene; Pl—plagioclase; Loc.—location; inc—inclusion in garnet; cont—contact to garnet; P—pressure; T—tem-

perature. The numbers in the columns below the mineral abbreviations represent analytical spot numbers. XFe = Fe/(Fe + Mg), Xgrs = Ca/(Fe + Mg + Ca + Mn), Xan = Ca/(Ca + Na) (grs is grossular, an is anorthite). GH and GC—garnet-hornblende and garnet-clinopyroxene Fe-Mg exchange thermometers; GHPQ and GCPQ—garnet- hornblende-plagioclase-quartz and garnet-clinopyroxene-plagioclase-quartz barometers.

23 7 9 RPB3A Bt Cpx Grt Ky NCKFMASHTO (+ Amp, Rt) 1 Bt Pl Ilm Opx H2O Cpx Ph Grt Qz H2O 29 30 Cpx Melt Grt 2 Bt Pl Opx H2O 10 H2O (-Amp) Cpx Chl Ph 8 Ky Zo H2O Cpx Ph 3 Bt Pl Grt Opx H2O 28 Bt Cpx Melt 4 Grt Zo Grt Ky H2O Bt Cpx Bt Melt Pl Ilm Opx H2O H2O Grt Ky Qz Grt H2O (-Amp) 5 Bt Melt Pl Grt Opx H2O Cpx Ph Grt Cpx Melt Bt Cpx Melt 6 Bt Pl Ilm Grt Opx Qz H2O Chl Ph Ph Grt H2O 27 Zo H2O Ky Qz H2O Grt H2O Grt (-Amp) Grt Zo 7 Bt Cpx Ph Grt Ky H2O Bt Cpx Ph 20 H2O 8 Cpx Ph Grt Zo H2O 12 Grt Ky 22 Bt Cpx Melt 9 Bt Cpx Ph Grt Ky Qz H2O (-Amp) Ph Grt Ky Qz H2O 11 25 Zo H2O 23 Grt H2O 10 Bt Cpx Ph Grt Ky Qz H2O 20 26 24 Bt Cpx 11 Bt Chl Ph Grt Zo 21 18 19 Grt Qz 12 Bt Cpx Chl Ph Grt Zo 15 13 H2O Bt Cpx Melt Bt Cpx Melt Grt 13 Chl Ph Grt Zo 17 Bt Ph Grt Grt Qz H2O 14 Chl Ph Grt Zo Qz 16 Ky Zo Qz H2O 14 15 Chl Ph Grt Ky Zo H2O Bt Grt Ky Bt Cpx 17 16 Chl Ph Grt Zo Q H2O Zo Qz H2O Grt Zo Qz Bt Cpx Melt Bt Chl 17 Bt Grt Ky Zo H2O Bt Chl Grt H2O Grt Qz Ph Grt Bt 18 Bt Ph Grt Ky Zo H2O Ky Zo Qz H2O Zo Qz Chl 19 Ph Grt Ky Zo Qz H2O Grt Bt Grt 20 Ph Grt Zo Qz H2O Zo Qz Zo Qz H2O Bt Cpx Melt 21 Bt Ph Grt Zo Qz H2O H2O Bt Cpx Melt 22 Cpx Ph Grt Ky Zo Qz H2O Bt Melt Grt Opx Pl Grt Qz 23 Cpx Ph Grt Zo Qz H2O Pressure (kbar) 14 Grt Qz 24 Bt Cpx Ph Grt Zo Qz H2O Bt Chl Bt Cpx Melt Grt Zo Pl Grt 25 Cpx Ph Grt Qz H2O Qz Bt Melt 26 Bt Cpx Ph Grt Qz H2O Bt Grt Grt Qz H2O 27 Bt Cpx Melt Grt Ky Qz H2O Bt Cpx Melt Qz H2O 28 Bt Cpx Melt Grt Ky Qz H2O (-Amp) Bt Melt Pl Grt Opx Pl Grt 29 Bt Cpx Melt Grt Qz H2O (-Amp) Bt Melt Bt Cpx Melt 30 Bt Cpx Grt Qz H2O (-Amp) Pl Grt Qz Pl Ilm Grt Opx 11 Bt Chl Bt Melt 31 Bt Melt Pl Ilm Grt Opx H2O (-Rt) Grt Qz Bt Melt Pl Grt Opx Bt Cpx Melt Pl 32 Bt Chl Ilm Grt Qz H2O 33 Bt Pl Grt Opx Qz H2O H2O Pl Grt Qz Bt Melt Pl Bt Melt Pl Bt Melt Pl IlmIlm Grt Opx (-Rt) Bt Chl Grt Opx Ilm Grt Opx H2O Qz Grt Opx (-Rt) Grt Qz Bt Cpx Bt Pl Grt 31 Bt Melt Pl Qz H2O 4 Melt Pl Bt Chl 6 5 Bt Melt Pl Ilm Opx (-Rt) 3 Ilm Opx Ilm Grt Qz 33 2 Bt Pl Ilm Ilm Opx H2O (-Rt) Opx H2O (-Rt) (-Rt) 8 32 1 600 660 720 780 840 900 Temperature (oC)

Figure 6. Pressure-temperature (P-T) pseudosection of sample RPB3A calculated in the NCKFMASHTO (Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-

O2) system. The bulk compositions (mol%) used are SiO2 (45.15), TiO2 (0.33), Al2O3 (8.95), FeO (7.46), MgO (16.85), CaO (10.59), Na2O (2.01), K2O (0.13),

O2 (1.03), and H2O (7.49). The prograde assemblage is shown in italics. Bt—biotite, Chl—chlorite, Ilm—ilmenite, Opx—orthopyroxene, Zo—zoisite, Ky— kyanite, Ph—phengite. Abbreviations as in Figure 3.

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1 Bt Chl Grt Zo H2O Cpx Phe NCKFMASHTO (+ Amp, Rt) RPB3A 5 2 Cpx Ph Grt Zo H2O 2 3 Grt Ky H2O Bt Cpx Grt 20 Bt Ph (effective bulk composition) 3 Ky Qz H2O Bt Cpx Melt Cpx Ph Grt Ky Zo H2O 1 Grt Zo Cpx Grt Ph Grt 4 7 Grt Ky Qz 4 Cpx Ph Gt Ky Zo Qz H2O Bt Chl H2O 100*XNa 100*XFe Zo H2O 6 H2O Ph Grt 16 5 Bt Cpx Ph Grt Ky Qz H2O (-Amp) 11 24 Bt Cpx Melt 58 Zo H2O Bt Cpx Phe 17 Grt H2O 6 Cpx Ph Grt Zo Qz H2O Grt Zo Qz H2O 10 7 Cpx Ph Grt Ky Qz H2O 9 Bt Phe Grt 31 Bt Cpx Grt Bt Cpx Melt 8 12 Zo Qz H2O Bt Chl Ph Grt Zo Qz 13 Zo Qz Grt 14 20 9 Bt Ph Grt Ky Zo H2O 2 17 8 H O Bt Grt Ky 56 10 Ph Grt Ky Zo H2O Bt Chl 54 Zo Qz H2O 52 50 Bt Cpx 11 Ph Grt Zo Qz H2O Grt Zo 30 58 48 15 Qz H2O Melt Grt Opx 60 Bt Cpx Melt 12 Chl Ph Grt Zo H2O Bt Grt Zo 29 62 Grt Qz 13 Chl Ph Grt Ky Zo H2O Qz H2O Bt Melt Grt Zo 14 Bt Chl Ph Grt Zo Qz H2O 16 28 16 Qz H2O Bt Cpx 15 Bt Chl Grt Zo Qz 14 Melt Pl Grt Qz 16 Bt Chl Zo Qz 17 Bt Cpx Grt Qz H2O

Pressure (kbar) Bt Chl Bt Melt 12 Bt Cpx 18 Bt Melt Pl Grt Qz H2O Zo Qz H2O Grt Qz Melt Pl Grt 19 Bt Pl Opx Qz H2O Bt Zo Bt Melt Bt Cpx Melt 20 Bt Pl Grt Opx Qz H2O Qz H2O Pl Grt Qz Pl Grt Opx Bt Grt Bt Melt 27 21 Bt Melt Pl Opx Qz H2O Qz H2O Grt Qz Bt Cpx Melt Pl Ilm H2O Bt Melt 22 Bt Melt Pl Opx Qz 11 Grt Opx (-Rt) Bt Qz Pl Grt Opx Bt Melt Pl 23 Bt Melt Pl Opx H2O H2O Qz Grt Opx Bt Cpx Melt Pl 24 Bt Pl Opx H2O Bt Pl Bt Melt Ilm Grt Opx (-Rt) 25 Bt Melt Pl Ilm Opx H2O Grt Qz Pl Ilm Bt Chl Qz H2O Opx Bt Cpx Melt 26 Bt Melt Pl IIm Opx H2O (-Ru) H2O 18 22 Br Melt Pl Opx Bt Melt Pl Pl Ilm Opx (-Rt) 20 21 27 Bt Melt Pl Grt 19 Ilm Opx (-Rt) 23 25 26 28 Bt Melt Grt Zo Qz 8 24 600 660 720 780 840 900 29 Bt Cpx Melt Grt Zo Qz 30 Bt Cpx Melt Grt Zo Qz H2O o Temperature ( C) 31 Bt Cpx Melt Grt Qz H2O

Figure 7. Pressure-temperature (P-T) pseudosection of sample RPB3A calculated using the effective composition after 10% garnet fractionation. NCK-

FMASHTO is Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2. The bulk compositions (mol%) used are SiO2 (45.39), TiO2 (0.36), Al2O3 (8.36), FeO (6.19),

MgO (17.13), CaO (10.72), Na2O (2.23), K2O (0.14), O2 (1.14), and H2O (8.32). The peak and retrograde assemblages are shown in bold and white letters,

respectively. Compositional isopleths of garnet for XFe and clinopyroxene for XNa from sample RPB3A are also shown. The bold circle represents the peak P-T conditions. Abbreviations are as in Figures 3 and 6.

because it matches with the temperature estimate (610–660 °C) in the composition (Fig. 8). This bulk composition has relatively higher SiO2,

conventional geothermobarometry section and overlaps that of the epidote- Na2O, and CaO contents than sample RPB3A. The calculated P-T pseu- amphibolite facies on the metamorphic facies diagram delineated by Oh dosection shows an increase in the stability of plagioclase, which is stable and Liou (1998). The eclogite facies assemblage of Omp + Amp + Grt + at P-T conditions below 12–16 kbar at 700–850 °C (Fig. 8). The melt Bt + melt + Qz + Rt yields P-T conditions of 13–20 kbar and 730–820 stability field occurs at temperatures above ~680–700 °C. The eclogite

°C (Fig. 7). The compositional isopleths of XNa in clinopyroxene and XFe facies assemblage of Omp + Grt + Rt + Amp + Qz + melt is stable at the in garnet are shown in Figure 7. The isopleths of the inner rims of garnet P-T conditions of 15–18.5 kbar and 740–790 °C (Fig. 8). The isopleths of

(XFe = 0.50–0.52) and relict omphacite (XNa = 0.21–0.23) constrain the XNa = 0.27–0.28 for omphacite in garnet and XFe = 0.58–0.60 for garnet peak P-T conditions to 17–18 kbar and 750–770 °C, which are within the cores yield peak P-T conditions of 16–17 kbar and 750–770 °C (Fig. 8), range inferred from the mineral phase assemblages and within the tem- consistent with those of sample RPB3A. The granulite facies assemblage perature estimate (730–800 °C) in the conventional geothermobarometry of Ca-Cpx + Amp + Grt + Qz + Rt + Pl + melt plots within the P-T condi-

section. The plagioclase-forming reaction occurred at conditions of 9–14 tions of 10–15 kbar and 720–800 °C. The isopleths of the XFe (0.63–0.64)

kbar and 700–850 °C, thus indicating that the growth of plagioclase with of garnet rims and the maximum XNa (0.10) of clinopyroxene symplectite Ca-clinopyroxene occurred during decompression under granulite facies yield P-T conditions of 10–11 kbar and 790–820 °C, indicating that they conditions. The granulite facies assemblage of Ca-Cpx + Amp + Grt + reflect a rapid uplifting P-T path (Fig. 8). Qz + Rt + plagioclase (Pl) + melt + Bt constrains the P-T conditions to 12–14 kbar and 780–820 °C (Fig. 7). As a result, a clockwise P-T path WHOLE-ROCK CHEMISTRY including rapid uplift was estimated. The garnet core in sample RPB1B is homogeneous, and the pseudo- The major and trace element contents of the five studied metabasites section modeling was done without considering the effective bulk-rock were analyzed using an ICP-MS (Perkin Elmer Optima 3000, Activation

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23 NCKFMASHTO (+ Amp, Rt) 1 RPB1B Cpx Ph Grt Cpx Melt Pl Ilm Grt Opx Qz Cpx Ph Cpx Grt 2 Qz H2O (-Amp) Cpx Melt Pl Ilm Grt Qz Grt Ky Qz 100*XNa 100*XFe 3 Bt Cpx Melt Pl Ilm Grt Opx Qz (-Rt) H2O Bt Cpx Ph 16 58 4 Ph Grt Zo Qz Cpx Ph Grt Qz H2O (-Amp) Cpx Melt Grt Grt Ky Zo 5 Cpx Ph Qz H2O (-Amp) Bt Cpx Melt Grt Zo Qz H2O Qz H2O Grt Qz H2O 6 Bt Melt Pl Grt Zo Qz Bt Cpx Grt Qz H2O 7 Bt Melt Pl Grt Qz H2O 20 Bt Cpx Ph Cpx Ph Grt (-Amp) 8 Bt Melt Pl Ilm Grt Qz H2O (-Rt) Grt Qz H2O Bt Cpx Melt Zo Qz H2O 9 Bt Pl Ilm Grt Opx Qz H2O (-Rt) Bt Cpx Grt Qz H2O (-Amp) 10 Bt Cpx Ph Grt Qz Cpx Melt Grt Qz (-Amp) Bt Melt Pl Ilm Grt Opx Qz H2O (-Rt) 11 Ph Grt Grt Zo Qz H2O H2O Bt Melt Pl Ilm Grt Opx Qz (-Rt) Zo Qz Cpx Melt 12 Melt Pl Ilm Grt Opx Qz (-Rt) H2O Grt Qz 13 Melt Pl Ilm Opx Qz (-Rt) Bt Cpx Bt Cpx Melt 14 4 Bt Phe Grt Zo Bt Cpx Melt Pl Ilm Grt Qz (-Rt) Grt Qz H2O 17 Qz H2O Grt Zo Qz 28 15 Cpx Melt Pl Grt Opx Qz H2O

Bt Cpx Melt Bt Grt Grt Qz 24 Zo Qz H2O 56 5 Cpx Melt Pl Bt Melt 20 Grt Qz (-Amp) Grt Qz H2O 14 Bt Melt Pressure (kbar) Grt Zo Qz 16 Cpx Melt H2O Bt Melt Pl Grt Qz Bt Grt Zo Qz Grt Qz Bt Cpx Melt 60 Pl Grt Qz Bt Melt 64 Cpx Melt Pl Grt Grt Zo Qz 6 12 Bt Melt Opx Qz (-Amp) Bt Pl Grt Pl Grt Qz Bt Cpx Melt Cpx Melt Pl Ilm 11 Zo Qz Pl Ilm Grt Opx Qz (-Amp) Bt Melt Pl Bt Grt Qz 7 Grt Qz Bt Pl Ilm Grt Qz 2 1 Cpx Melt Pl Ilm Grt Bt Pl 14 3 Cpx Melt Pl Opx Qz (-Rt, -Amp) Grt Qz Grt Qz H2O Bt Melt Pl Ilm Grt Opx Qz Bt Pl Ilm Grt Qz (-Rt) Cpx Melt Pl Bt Pl 8 Ilm Grt Cpx Melt Pl Ilm Opx Qz Ilm Grt Bt Pl Ilm Grt Qz 11 12 Ilm Opx Qz (-Rt) (-Rt, -Amp) Qz H2O 10 8 9 13 600 660 720 780 840 900 Temperature (oC)

Figure 8. Pressure-temperature (P-T) pseudosection of sample RPB1B calculated in the NCKFMASHTO (Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-

O2) system. The bulk compositions (mol%) used are SiO2 (52.81), TiO2 (1.14), Al2O3 (8.46), FeO (9.87), MgO (11.14), CaO (10.45), Na2O (1.46), K2O (0.04),

O2 (0.86), and H2O (3.77). The peak and retrograde assemblages are shown in bold and white letters, respectively. Compositional isopleths of garnet

for XFe and clinopyroxene for XNa from sample RPB1B are also shown. The bold and dashed circles represent the peak and retrograde P-T conditions, respectively. Abbreviations are as in Figures 3 and 6.

Laboratories Ltd., Canada). The whole-rock data are listed in Data Reposi- the origin of the Salma eclogites is likely subducted oceanic crust that tory Table DR2. The five metabasites have basaltic compositions, with originated at a spreading center. This interpretation agrees well with those

SiO2 contents of 47.0–52.4 wt% and low alkali concentrations in terms of previous studies (Mints et al., 2010, 2014; Konilov et al., 2011).

of Na2O (1.49–2.16 wt%) and K2O (0.02–0.33 wt%). These metabasites were originally low-alkali tholeiitic basalts, as shown in Figure 9A. They ZIRCON U-Pb AGES AND GEOCHEMISTRY plot in the island arc basalt (IAB) and mid-oceanic ridge basalt (MORB) fields on the Ti/100-Zr-Y and Nb-Zr/4-Y diagrams (Figs. 9B, 9C). Analytical Procedure The REE patterns normalized using C1 chondritic values are basically depleted in light REEs, thus yielding flat patterns showing an affinity to Zircon grains from the two eclogite samples (RPB3A and RPB1B) normal (N) MORB (Figs. 10A, 10B). Sample RPB1B exhibits a negative were separated using the standard heavy liquid technique and were then Eu anomaly. The incompatible trace element abundances normalized using hand-picked under a binocular microscope. Cathodoluminescence (CL) primitive mantle values are shown in Figures 10C and 10D. The elements and backscattered electron (BSE) images were obtained using the JEOL ranging from Nd to Yb produce low and flat trends in most samples, except 6610LV scanning electron microscope at the Korea Basic Science Insti- for sample RPB1B. In contrast, the elements from Rb to Sr, which are tute (KBSI, Ochang, South Korea). The CL and BSE images from sam- highly incompatible, record variable values, which were likely produced ples RPB3A and RPB1B are shown in Figures 11 and 12, respectively. by disturbances during subduction. These whole-rock data indicate that Microinclusions in zircons were identified using the scanning electron

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FeOt Ti/100 Nb*2 RPB3A RPB3B RPB3C

ABRPB1A RPB1B C

Tholeiitic AI

AII WPB IAT B

CAB MORB C Calc-Alkaline D

Na2O+K2OMgO Zr Y*3 Zr/4 Y

Figure 9. The whole-rock compositions of retrograded eclogite and granulite in the Salma area. (A) Plotted on the FeOtotal-(Na2O + K2O)-MgO classification diagram. (B) Plotted on the Ti/100-Zr-Y*3 tectonic discrimination diagram. IAT—island-arc tholeiites; CAB—calc-alkaline basalts; WPB—within-plate basalts; MORB—mid-oceanic ridge basalt. (C) Plotted on the Nb*2-Zr/4-Y tectonic discrimination diagram. AI—within-plate alkali basalts; AII—within- plate alkali basalts and within-plate tholeiites; B—enriched-type MORB; C—within-plate tholeiites and volcanic-arc basalts; D—normal-type MORB and volcanic-arc basalts (Rollinson, 1993, and references therein).

300 300 OIB B OIB e

e A it it 100 100

E-MORB E-MORB

N-MORB N-MORB 10 10

1 1 ample/C1 Chondr ample/C1 Chondr S S RPB1A RPB1B RPB3A RPB3B RPB3C

.1 .1 La CePr Nd SmEuGdTbDyHoErTmYbLu La CePr Nd SmEuGdTbDyHoErTmYbLu

300 300 RPB3A RPB3B RPB3C RPB1A RPB1B 100 C 100 D OIB OIB e Mantle e Mantle 10 10 E-MORB imitiv E-MORB imitiv

ORB 1 1 N-M

ample/ Pr N-MORB ample/ Pr S S increasing incompatibility increasing incompatibility .1 .1 Rb Th Nb Ce Nd Zr Eu Tb Y Yb Rb Th Nb Ce Nd Zr Eu Tb Y Yb Ba U Ta Sr Sm Hf Gd Dy Er Ba U Ta Sr Sm Hf Gd Dy Er Figure 10. Chondrite-normalized rare earth element patterns. MORB—mid-oceanic ridge basalt (N—normal; E—enriched); OIB— oceanic island basalts for (A) samples RPB3A–RPB3C and (B) samples RPB1A, RPB1B. Primitive mantle-normalized trace element patterns for (C) samples RPB3A–RPB3C and (D) samples RPB1A, RPB1B. Data are normalized using the values of chondrite and primitive mantle from Sun and McDonough (1989).

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A CL B CL C BSE 7.1 12.2 2733 ± 14 ± 2811 8 8.1 12.1 1776 ± 48 1845 ± 21 50μm 50μm 50μm D 17.2 CL E CL F CL 1904 ± 16 15.1 Qz 17.1 1783 ± 60 Grt 1855 ± 37 15.2 9.2 1892 ± 23 1855 ± 46 50μm 50μm 50μm

Figure 11. (A–F) Representative cathodoluminescence (CL) and backscattered electron (BSE) images of dated zircon crystals in sample RPB3A. The analyzed spots are shown with their 207Pb/206Pb ages and spot numbers. Abbreviations as in Figure 3.

CL CL A 3.1 1799 ± 82 BCBSE

3.2 2.1

2703 ± 14 2757 ± 7

50μm 50μm 50μm D CL E CL F CL 13.2 20.2 2729 ± 10 18.1 20.1 1833 ± 67 Qz 1914 ± 28 13.1 1932 ± 44 1769 ± 101

50μm Cpx 50μm 50μm

G CL H CL I 17.2 CL 1850 ± 36 1924 ± 34 11.1 2a.2 Omp ± 1848 31 2a.1 17.1 ± 1863 ± 56 1720 79 50μm 50μm 50μm

Figure 12. (A–I) Representative cathodoluminescence (CL) and backscattered electrons (BSE) images of dated zircon crystals in sample RPB1B. The analyzed spots are shown with their 207Pb/206Pb ages and spot numbers. Abbreviations as in Figure 3.

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microscope with energy-dispersive X-ray spectroscopy (SEM-EDX) TABLE 3. MINERAL INCLUSIONS IN ZIRCONS FROM THE SALMA ECLOGITES detector at the KBSI and the Thermo Scientific DXR micro-Raman Sample Domain Inclusion type microscope equipped with a 532 nm laser at the Tectonophysics Labora- Qz Grt Amp Omp Ca-Cpx Bt Rt Pl K-Fsp Ap tory in the School of Earth and Environmental Sciences (Seoul National RPB3Adark CL core •• • ••• r University). The EDX spectra obtained from the inclusions in the zircon pale gray domain •• •• were used to identify mineral inclusions by comparing them with those of bright CL rim minerals in thin sections within the same samples. The mineral inclusion RPB1Bdark CL core • assemblages in the zircons are listed in Table 3, and the EDX spectra of pale gray domain •••• •• garnet, omphacite, and Ca-clinopyroxene inclusions in zircons are shown bright CL rim in Data Repository Figure 1. Note: CL is cathodoluminescence. Mineral abbreviations: Qz—quartz; Grt—garnet; Amp—amphibole; Omp—omphacite; Cpx—clinopyroxene; Bt—bitote; Rt— The REE composition of zircon was analyzed using laser ablation rutile; Pl—plagioclase; K-Fsp—potassium feldspar; Ap—apatite. Tr iangle represents (LA)-ICP-MS at the KBSI. The LA-ICP-MS system consists of a laser mineral formed by later alteration. ablation system (213 nm Nd-YAG [neodymium-doped yttrium aluminum garnet laser] UP213, New Wave Research, a division of ESI), ICP, and a quadrupole mass spectrometer (X2 series, Thermo Scientific). The ana- 10000 lytical procedures for the REE analyses of the zircons followed those of (A) RPB3A Yuan et al. (2004) and Liu et al. (2007). Ablation signals were collected by ICP-MS using a time-resolved analysis of 45 s. The Nd-YAG laser 1000 was operated at a repetition rate of 10 Hz, a spot size of 55 mm, and an energy level of 80% (27 J/cm2). NIST 612 glass was used as an external 100 calibration standard, and each analysis was normalized to the silicon 29 content ( Si) as an internal standard. GLITTER software (http://www 10 .glitter-gemoc.com/) was used for data reduction. Zircon REE data are given in Data Repository Table DR3. 1 The zircon U-Pb ages were analyzed using the SHRIMP (sensitive

high-resolution ion microprobe) IIe ion microprobe at the KBSI. The Zircon/Chondrite analytical procedures for SHRIMP dating were mainly based on those 0.1 proposed by Williams (1998). A spot size of 15–20 µm and a 1.5–2 nA − 0.01 Dark CL core negative ion oxygen beam (O2 ) were used for all analyses. The measured 206Pb/238U ratio was calibrated using the FC1 zircon standard (ca. 1099 Pale gray domain Ma; Paces and Miller, 1993). The SL13 zircon standard was also used 0.001 for the calibration of U concentrations (238 ppm; Hoskin, 1998). Data La Ce Pr*NdSmEuGdTbDyHoErTmYbLu reduction, age calculations, and common Pb corrections were conducted using SQUID 2.50 (Ludwig, 2009) and Isoplot 3.6 software (Ludwig, 10000 2008). The zircon U-Pb ages are listed in Data Repository Table DR4. (B) RPB1B 1000 Results 100 Sample RPB3A The zircon grains from sample RPB3A are subhedral and have sub- 10 rounded edges (Figs. 11A–11F). Most zircon grains have dark CL cores surrounded by pale gray CL mantles with sector or patchy zoning (Figs. 1 11A, 11B). In the BSE images of these grains, the cores are brighter than

the mantles (Fig. 11C). However, some zircons have pale gray CL cores Zircon/Chondrite 0.1 with sector or patchy zoning that are similar to the mantles surrounding the dark CL cores (Figs. 11D, 11E). A few grains have bright CL cores 0.01 Dark CL core surrounded by pale gray CL mantles (Fig. 11F). Thin, bright CL rims Pale gray domain are locally observed surrounding pale gray CL cores and mantles (Figs. 0.001 11E, 11F). The dark CL cores contain inclusions of garnet, amphibole, La Ce Pr*NdSmEuGdTbDyHoErTmYbLu plagioclase, quartz, biotite, and rutile; the pale gray CL cores and mantles Figure 13. Chondrite-normalized rare earth element patterns for dif- contain inclusions of garnet (Fig. 11F), amphibole, biotite, and quartz ferent zircon domains. CL—cathodoluminescence. (A) Sample RPB3A. (Table 3; Data Repository Item). K-feldspar occurs along fractures in (B) Sample RPB1B. Data are normalized using the chondritic values zircons and is likely associated with later alteration. On the chondrite- of Sun and McDonough (1989). normalized diagram (Fig. 13A), the dark CL cores are enriched in HREEs,

with LuN/GdN ratios of 8.6–33.6. Significant negative Eu anomalies can 207 206 be observed in these zircons (Eu/Eu* = 0.07–0.20). The Pb/ Pb ages patterns (LuN/GdN ratio = 0.6–1.2) and small negative Eu anomalies (Eu/ of the dark CL cores are 2875–2387 Ma. The weighted mean 207Pb/206Pb Eu* = 0.40–0.61; Fig. 13A). They yield 207Pb/206Pb ages of 2378–1776 age of 10 nearly concordant ages from the dark CL cores is 2716 ± 10 Ma, with a major cluster at 1900–1780 Ma. The weighted mean 207Pb/206Pb Ma (mean square of weighted deviates, MSWD = 2.7, n = 10, 2σ). In age of the concordant data is 1865 ± 15 Ma (MSWD = 1.7, n = 14, 2σ; contrast, the REE patterns of the pale gray domains exhibit flat HREE Fig. 14A). The U-Pb zircon data from sample RPB3A produce a discordia

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(A) RPB3A

Dark CL core 0.22 Pale gray domain

2800 Interceptsat 1820 ± 70 & 2905 ± 85 Ma 0.18 MSWD = 3.6 * Pb 6 20 2400 Pb* / 0.14 20 7

2000 2000

1920 0.10 1840 1600

1760 1865 ± 15 Ma

1680 MSWD = 1.7 0.06 1.21.6 2.02.4 2.83.2 3.64.0 4.4 238U/ 206Pb* (B) RPB1B 0.22 Dark CL core 0.20 Pale gray domain 2800 Bright CL rim 0.18 Intercepts at 1881 ± 38 & 2764 ± 33 Ma 0.16 MSWD = 1.3 * 2400 Pb

206 0.14

Pb*/ 2000 207 0.12 2050

1950 0.10 1850 1600

1750

0.08 1650 1868 ± 17 Ma MSWD = 1.4 1550 0.06 1.41.8 2.22.6 3.03.4 3.8 238U/ 206Pb* Figure 14. Concordia diagrams for the sensitive high-resolution ion microprobe (SHRIMP) U-Pb analyses of zircon. The dashed line represents the discordia line. All error ellipses are quoted at the 1σ level. MSWD—mean square of weighted deviates. The mean and discordia ages are shown at the 2σ level. CL—cathodoluminescence. (A) Sample RPB3A. (B) Sample RPB1B.

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line, yielding an upper intercept age of 2905 ± 85 Ma and a lower intercept and felsic veins at 2.82–2.78 Ga. Mints et al. (2014) believed that high- age of 1820 ± 70 Ma (Fig. 14A; MSWD = 3.6, 2σ), indicating that a Pb- pressure metamorphism, which formed the Salma eclogite, may have loss event occurred during the period of Paleoproterozoic metamorphism. occurred during the subduction stage; however, the Archean age of the The brighter CL rims were not analyzed due to their insufficient thickness. eclogite facies metamorphism in the Salma area is uncertain, because they did not provide direct evidence for it. Sample RPB1B The 2.73–2.72 Ga ages obtained from the dark CL zircons in samples The zircon grains from sample RPB1B are subhedral with rounded or RPB3B and RPB1B are interpreted to represent an Archean amphibolite subrounded edges (Figs. 12A–12I). Three domains are observed on the facies metamorphic event. The unzoned regions present in the dark CL basis of CL and BSE images. Most crystals have dark CL cores, which zircons are generally produced by metamorphism. We also found a rep- are surrounded by pale gray CL mantles (Figs. 12A, 12B, 12D). The resentative metamorphic mineral assemblage (i.e., garnet) in the dark CL BSE images show brighter cores and relatively darker mantles (Fig. 12C). zircons from sample RPB3A. Although high to moderate Th/U ratios Several zircons have pale gray CL cores with sector or patchy zoning that in the dark CL zircons (0.2–5.1 for RPB3A, 0.2–0.3 for RPB1B) may are similar in their CL brightness to the mantles surrounding the dark CL indicate their magmatic origins (cf. Skublov et al., 2010a), metamorphic cores (Figs. 12E, 12F). In some zircons, the internal structure of the pale zircons with high Th/U ratios have also been reported in high-grade rocks gray cores features a cloudy zoning pattern (Figs. 12G–12I). These pale (e.g., Harley et al., 2007). gray cores are mostly surrounded by brighter CL outer rims, which vary Some researchers reported 2.72–2.70 Ga (retrograde) granulite facies in thickness from narrow (Figs. 12E–12H) to broad (Fig. 12I). Apatite metamorphism (Kaulina et al., 2010; Mints et al., 2010). Although there occurs as inclusions in the dark CL cores, whereas Ca-clinopyroxene (Fig. is no direct petrological evidence to link these ages to granulite facies 12D), omphacite (Fig. 12H), quartz, amphibole, rutile, and apatite occur in metamorphism, the REE pattern with HREE enrichment in zircons was the pale gray CL domains (Table 3; Data Repository Fig. 1). The dark CL explained by zircon growth in equilibrium with melt during granulite cores display HREE-enriched patterns with a steep slope from the middle facies metamorphism (Kaulina et al., 2010). However, melt can be pro-

REEs to the HREEs (LuN/GdN = 22.3–34.3). They also record negative duced from temperatures of 680–700 °C, which correspond to conditions Eu anomalies (Eu/Eu* = 0.21–0.30; Fig. 13B). The 207Pb/206Pb ages of the of upper amphibolite facies metamorphism in bulk-rock composition dark CL cores are 2757–2342 Ma. The weighted mean 207Pb/206Pb age of of the Salma eclogites (Figs. 7 and 8). Metamorphic zircons have been the concordant data is 2727 ± 8 Ma (MSWD = 0.6, n = 8, 2σ). The pale reported in amphibolites in the orogeny (e.g., Oh et al., 2014), thus imply- gray domains have flat HREE patterns with moderate to shallow slopes ing that (upper) amphibolite facies metamorphism can produce abundant

from the middle REEs to the HREEs (LuN/GdN = 1.1–8.8) and slightly zircons. In this study, the mineral inclusions within the dark CL zircons smaller negative Eu anomalies (Eu/Eu* = 0.32–0.56) compared to the with 2.73–2.72 Ga ages are characterized by an amphibolite facies mineral dark CL zircons (Fig. 13B). Their 207Pb/206Pb ages are Paleoproterozoic assemblage (Grt + Amp + Pl + Qz + Rt ± Bt). The relatively enriched and range from 1967 to 1750 Ma, with the exception of 2 analytical spots HREE patterns and remarkably negative Eu anomalies observed in the that yield ages of 1687–1636 Ma. The weighted mean 207Pb/206Pb age of dark CL zircons, compared to the pale gray zircons, can be explained by the pale gray domains is 1868 ± 17 Ma (MSWD = 1.4, n = 22, 2σ; Fig. the presence of less garnet and abundant plagioclase. These indicate that 14B). The omphacite-bearing zircon domains yield ages of 1863 ± 56 Ma amphibolite facies metamorphism occurred at ca. 2.73–2.72 Ga. and 1850 ± 36 Ma (Fig. 12H). These U-Pb zircon data produce a discordia line yielding upper and lower intercept ages of 2764 ± 33 Ma and 1881 Interpretation of Paleoproterozoic Zircon Ages ± 38 Ma, respectively (MSWD = 1.3, 2σ; Fig. 14B), thus indicating that a Pb-loss event occurred during the period of Paleoproterozoic meta- The pale gray CL zircons from samples RPB3A and RPB1B that morphism. These upper and lower intercept ages are consistent (within contain garnet and omphacite yield 207Pb/206Pb age mean ages of 1865 ± error) with the weighted mean ages of the dark CL cores and pale gray 15 Ma and 1868 ± 17 Ma, respectively. Direct evidence for Paleoprotero- domains, respectively. One analytical spot from the brighter CL rim yields zoic eclogite is provided by 207Pb/206Pb ages of 1863 ± 56 and 1850 ± 36 an apparent 207Pb/206Pb age of 1720 ± 79 Ma (Fig. 12I). Ma from omphacite-bearing zircon domains. These results indicate that eclogite facies metamorphism (16–18 kbar and 750–770 °C) occurred ca. DISCUSSION 1.87 Ga. The flat HREE patterns indicate that these metamorphic zircons formed in equilibrium with garnet during eclogite facies metamorphism Meaning of Archean Zircon Ages (e.g., Rubatto, 2002; Whitehouse and Platt, 2003; Imayama et al., 2012). Metamorphic zircons characterized by flat HREE patterns have been Petrographic, geochemical, and geochronological data from the Salma obtained from many eclogites around the world (e.g., northeast Green- eclogites in the Kola Peninsula reveal the polymetamorphic history of land, Gilotti et al., 2004, McClelland et al., 2006; South Korea, Kim et this area. The ages of the magmatic protoliths of the Salma eclogites are al., 2006; Central Alps, Liati et al., 2009; Bohemian Massif, Bröcker et known to be 2.94–2.92 Ga (Kaulina et al., 2010) and 2.88–2.82 Ga (Mints al., 2010). High HREE abundances in garnet produce flat HREE pat- et al., 2010; Skublov et al., 2010a, 2011; Mel’nik et al., 2013). However, terns in zircon. In addition, the very weak Eu anomalies in the pale gray the 2.88–2.87 Ga zircon age obtained from Skublov et al. (2010a, 2011) CL zircons, compared to the dark CL zircons, indicate that plagioclase- should be interpreted as a metamorphic age, rather than as a magmatic free mineral assemblages exist in the eclogite. The presence of small Eu age, because these zircons do not record zoning patterns that are typical anomalies and Ca-clinopyroxene (instead of only omphacite) in the pale for igneous zircons, such as concentric or banded zoning (Hoskin and gray CL zircons may indicate that zircon growth continued to granulite Schaltegger, 2003); instead, they exhibit unzoned patterns, which are char- facies during decompression. Nevertheless, the absence of plagioclase acteristic of metamorphic zircons. Mints et al. (2014) also interpreted 2.82 inclusions in the pale gray CL zircons means that the zircons yielding Ga to be the earliest age of metamorphism based on 176Hf/177Hf isotopic ages of ca. 1.87 Ga mainly grew during eclogite facies metamorphism. ratios, and suggested that subduction occurred in the Salma area between Skublov et al. (2010a, 2011) reported that eclogite facies metamor- 2.87 and 2.82 Ga based on the intrusions of mafic dikes at 2.86–2.83 Ga phism occurred ca. 1.92–1.88 Ga, based on the analysis of metamorphic

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zircon rims surrounding Archean magmatic zircon cores (ca. 2.88–2.87 in this study supports the model of Paleoproterozoic collision between the Ga) within the massive eclogite (sample 46, Table 1). However, the ca. Kola and Karelian cratons suggested by Berthelsen and Marker (1986), 1.92–1.88 Ga metamorphic zircons include zoisite and quartz (Skublov Zhao et al. (2002), and Daly et al. (2006). et al., 2010a, 2011), which appear to represent prograde metamorphism, Eclogites that formed within a transitional eclogite-granulite facies rather than peak eclogite facies metamorphism, which is represented by P-T range could have formed in a deep continental crustal root zone (e.g., the presence of garnet and omphacite. Garnet in the analyzed eclogite De Paoli et al., 2009). However, the whole-rock chemistry of the Salma records prograde zoning, with increasing pyrope contents from core to eclogites in this study is characterized by depleted light REEs, which rim (Skublov et al., 2011). Studies of Lu-Hf and Sm-Nd garnet geochro- reflects their origins as N-MORB and is consistent with the results of pre- nology from the same eclogite yielded garnet–whole-rock isochron ages vious studies (Konilov et al., 2011; Mints et al., 2014). It is likely that the of 1901 ± 5 Ma and 1897 ± 16 Ma, respectively (Herwartz et al., 2012; 2.94–2.93 Ga ages inferred from the magmatic zircon in the Salma eclog- Mel’nik et al., 2013). Although these ages were interpreted to reflect the ite represent the protolith age of the N-MORB (Kaulina et al., 2010). This timing of peak eclogite facies metamorphism, the Lu-Hf and Sm-Nd study also indicates that the Salma eclogites underwent an amphibolite ages of garnets with growth zoning are generally interpreted to represent facies metamorphic event ca. 2.73–2.72 Ga. During the Paleoproterozoic prograde metamorphic ages, due to the low diffusivities of REEs (Baxter subduction of the unit including the protolith of the Salma eclogites, these and Scherer, 2013). The omphacite-bearing metamorphic zircons that rocks underwent progressive metamorphism from epidote-amphibolite formed ca. 1.87 Ga found in this study provide the first clear age of peak facies ca. 1.92–1.88 Ga to eclogite facies ca. 1.87 Ga. This subduction eclogite facies metamorphism in the Salma eclogite. stage was followed by the collision of the Kola and Karelian cratons; the unit including the Salma eclogites was rapidly uplifted and recorded P-T Path During Paleoproterozoic Metamorphism strong overprinting produced first by granulite facies metamorphism and then by amphibolite facies metamorphism during or after the collision. In the Salma eclogites, the identification of epidote-amphibolite facies prograde metamorphism was based on the presence of clinozoisite and Secular Changes in the Geothermal Gradients of Subduction amphibole inclusions in garnet cores and prograde zoned garnets with Zones homogeneous cores and Mg-rich inner rims in sample RPB3A. Because the garnet in sample RPB1A only contains amphibole inclusions but lacks Subduction in the Precambrian may have proceeded differently than epidote (Fig. 4D), some Salma eclogites may have undergone amphibo- modern subduction, due to the hotter conditions in the mantle of the lite facies metamorphism prior to eclogite facies metamorphism. The early Earth (e.g., Davies, 1992); however, it is debatable when ancient P-T conditions of the prograde stage are estimated to be 13–18 kbar and subduction changed into modern subduction (e.g., Cawood et al., 2006; 640–720 °C. These results closely match with the boundary between the van Hunen and Moyen, 2012, and references therein). Determining the amphibolite, epidote-amphibolite, and eclogite facies on the metamor- changes in the patterns of metamorphism at plate boundary zones allows phic facies diagram developed by Oh and Liou (1998). This prograde us to understand the evolutions and geodynamics of subduction zones metamorphism likely occurred ca. 1.92–1.88 Ga (Skublov et al., 2010a, (e.g., Brown, 2006, 2009). Determining the timing of the first appearances 2011; Herwartz et al., 2012; Mel’nik et al., 2013), as mentioned herein. of high-pressure granulite, eclogite, and blueschist is thus very important The granulite facies overprinting (10–14 kbar and 770–820 °C) for understanding the changes in the patterns of metamorphism and the occurred during the subsequent exhumation stage from 17 to 18 kbar, lead- geothermal gradient in Precambrian subduction zones over time. ing to the breakdown of omphacite to Ca-clinopyroxene and plagioclase. The oldest high-pressure granulite is Neoarchean (ca. 2.5 Ga) and is Because the zircon growth ca. 1.87 Ga could have continued to undergo present in the Jianping complex; it was produced by a subduction-collision granulite facies metamorphism, this probably occurred soon after the event in the eastern region of the North China Craton (Wei et al., 2001; eclogite facies metamorphism, thus implying that rapid decompression O’Brien and Rötzler, 2003; Liu et al., 2011; Lu et al., 2017). The P-T occurred. Upon cooling, the amphibolite facies overprint occurred at conditions of this high-pressure granulite metamorphic event were 10–13 conditions of 8–10 kbar and 590–610 °C. kbar and 780–850 °C (Wei et al., 2001; Wang and Cui, 1992; Lu et al., 2017; Fig. 15). Eclogite facies metamorphism occurred at ca. 2.0 Ga in Tectonic Implications the Usagaran orogen of Tanzania, with peak metamorphic conditions of ~750 °C and 18 kbar (Möller et al., 1995; Collins et al., 2004). The peak Some researchers have interpreted the eclogites in the Belomorian metamorphic conditions and P-T paths of the Tanzanian eclogites are mobile belt to represent evidence of Archean subduction followed by col- similar to those of the Salma eclogites (Fig. 15). These rocks underwent lision, leading to the amalgamation of the Karelia craton, the Kola craton, rapid decompression after peak metamorphism and were then retrograded and the microcontinent between them (Mints et al., 2010, 2014). However, into first granulites and then amphibolites. Because the eclogite facies in this study, an age of ca. 1.87 Ga for eclogite facies metamorphism was metamorphism in the Salma eclogite and the Usagaran eclogite occurred obtained from zircons with omphacite inclusions and flat HREE patterns during the Paleoproterozoic, subduction accompanying the development within the Salma eclogite. The Paleoproterozoic zircons collected from of eclogite likely began during or prior to the Paleoproterozoic. The oldest the eclogites from the Kuru-Vaara quarries also exhibit flat HREE pat- reported blueschist (ca. 750–730 Ma) is from the Aksu Group of western terns (Skublov et al., 2011) and the P-T paths of the eclogites from the China (Liou et al., 1989, 1996; Nakajima et al., 1990; Maruyama et al., Grindino and Salma areas are similar (Mints et al., 2014), indicating that 1996; Zhu et al., 2011; Yong et al., 2013); the P-T conditions of this blue- the eclogites in the Belomorian mobile belt formed during the Paleopro- schist facies metamorphism were 4–10 kbar and 300–400 °C (Liou et al., terozoic; these findings do not support the Archean subduction-collision 1989, Zhang et al., 1999; Fig. 15). The geothermal gradient required for a model. The Archean subduction-collision model is not able to explain the formation of high-temperature eclogite (>~750 °C) is higher than that of regional occurrence of Paleoproterozoic granulite facies metamorphism blueschist and lower than that of high-pressure granulite. Consequently, (i.e., the Lapland and Umba belts) in the Kola-Karelian collisional zone the first appearance of high-pressure granulite, eclogite, and blueschist (cf. Daly et al., 2006). The 1.87 Ga metamorphic age of the Salma eclogite in the Neoarchean, Paleoproterozoic, and Neoproterozoic, respectively,

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(e.g., Oh and Liou, 1998; Rubatto and Hermann, 2001; Fig. 15). More- over, eclogites that formed via prograde metamorphism from amphibolite WA (UHP) facies are almost absent in Phanerozoic subduction zones (Oh and Liou, 1998). However, the prograde mineral assemblage of the Neoarchean 30 granulite in the Jianping complex is Amp + Pl ± Qz ± Bt, which represents amphibolite facies metamorphism (Wang and Cui, 1992; Liu et al., 2011). These data imply that Paleoproterozoic subduction zones were EG SA (UHP) relatively warmer than Phanerozoic subduction zones but colder than Neoarchean subduction zones.

CONCLUSIONS 20 BS TZ WA (HP) Based on the petrologic, thermobarometric, geochemical, and geochro- HG nological data presented in this study and previous studies, the following conclusions are proposed for the tectonothermal evolution of the Salma

Pressure (kbar) NC This eclogites in the Kola Peninsula. (HP) study JC 1. The source rocks for the Salma eclogites formed in a mid-ocean 10 EA ridge ca. 2.94–2.93 Ga and first underwent amphibolite facies metamor- AG phism ca. 2.73–2.72 Ga. This amphibolite facies metamorphism is con- SA TZ LG (HP) AM firmed by inclusions of Grt + Amp + Pl + Qz + Rt ± Bt in 2.73–2.72 Ga previous study unzoned dark CL zircons, which are characterized by enriched HREE this study patterns and remarkably negative Eu anomalies. GS P-T path 2. The Salma eclogites may have undergone epidote-amphibolite facies 0 or amphibolite facies prograde metamorphism at ca. 1.92–1.88 Ga. The 200 400 600 8001000 ca. 1.87 Ga peak eclogite facies metamorphism can be inferred from the Temperature (oC) U-Pb age dating of pale gray CL metamorphic zircons with inclusions of Figure 15. Pressure-temperature (P-T) diagram showing schematic P-T-time Grt + Omp + Ca-Cpx + Amp + Qz + Rt ± Bt. These zircons record flat path (red arrow) of the Salma eclogites determined in this study and that HREE patterns and nearly lack Eu anomalies. The peak metamorphic P-T (blue arrow) of the Usagaran eclogite facies rocks in Tanzania (TZ; Collins conditions were ~16–18 kbar and 750–770 °C. Soon after this peak meta- et al., 2004). Solid, dashed, and dotted red rectangles indicate the P-T morphism, granulite facies metamorphism occurred after decompression conditions of the eclogite, granulite, and amphibolite facies metamorphic at 10–14 kbar and 770–820 °C and was followed by amphibolite facies stages, respectively, of the Salma eclogite. Orange and purple rectangles overprinting at 8–10 kbar and 590–610 °C during cooling. represent the P-T conditions of Neoarchean high-pressure granulites in the 3. The Paleoproterozoic subduction and subsequent continent-conti- Jianping Complex (JC) of the North China Craton and Neoproterozoic blueschist in the Aksu Group (AG) of western China. Dotted and dashed green nent collision between the Kola and Karelian continents is supported by curves represent schematic fieldP -T curves for high-pressure (HP) and the occurrence of eclogite facies metamorphism at ca. 1.87 Ga. ultrahigh-pressure eclogites (UHP) of the representative Phanerozoic sub- 4. The oldest appearances of high-pressure granulite, eclogite, and duction zones: western Alps (WA), New Caledonia (NC), and Sambagawa blueschist occurred in the Neoarchean, Paleoproterozoic, and Neopro- (SA). The P-T curves and petrogenetic grid are from Oh and Liou (1998), terozoic, respectively, which may reflect a decrease in the geothermal Rubatto and Hermann (2001), and Itaya et al. (2011). BS—blueschist facies, gradients of Precambrian subduction zones due to the cooling of the GS—greenschist facies, EG—eclogite facies, HG—high-pressure granulite facies, LG—low-pressure granulite facies, EA—epidote-amphibolite facies, Earth. The prograde metamorphism from epidote-amphibolite facies or AM—amphibolite facies. amphibolite facies to eclogite facies in the subduction zone during the Paleoproterozoic also implies that the Paleoproterozoic subduction zone was relatively warmer than the Phanerozoic subduction zone but was colder than the eclogite-free Neoarchean subduction zones. may reflect a decrease in the geothermal gradients of subduction zones from the Neoarchean to the Neoproterozoic due to the cooling of the Earth. ACKNOWLEDGMENTS Changes in metamorphic facies during prograde metamorphism in the We thank K. de Jong, Seoul National University, Korea, for helpful discussion; Y. Park, Seoul National University, Korea, for assistance with micro-Raman analyses; and Juhn G. Liou Phanerozoic to Paleoproterozoic eclogites and Neoarchean high-pressure and two anonymous reviewers for constructive and critical reviews that significantly helped granulites can provide clearer information about changes in the geothermal to improve the manuscript. We also thank R. Damian Nance for careful editorial handling. gradients at subduction zones from the Neoarchean to the Phanerozoic. This work was supported by National Research Foundation of Korea (NRF) grants 657 NRF- 2013R1A1A2058525, NRF-2014R1A2A2A01003052, and NRF-2017R1A2B2011224. This study provides evidence that there was a prograde metamorphic event that involved epidote-amphibolite facies and/or amphibolite facies REFERENCES CITED during the formation of the Paleoproterozoic Salma eclogites. 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