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Petrology of Chromitites in the Higashi-Akaishi Ultrahigh-Pressure (UHP) Peridotite Complex, Japan: Toward Understanding of General Features of the UHP Chromitites

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Petrology of Chromitites in the Higashi-Akaishi Ultrahigh-Pressure (UHP) Peridotite Complex, Japan: Toward Understanding of General Features of the UHP Chromitites minerals Article Petrology of Chromitites in the Higashi-Akaishi Ultrahigh-Pressure (UHP) Peridotite Complex, Japan: Toward Understanding of General Features of the UHP Chromitites Makoto Miura 1,2,*, Shoji Arai 1, Tomoyuki Mizukami 1, Vladimir R. Shmelev 3 and Satoko Ishimaru 4 1 Department of Earth Sciences, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan; [email protected] (S.A.); [email protected] (T.M.) 2 Gemological Institute of America (GIA) Tokyo Godo Kaisha, Tokyo 110-0016, Japan 3 Zavaritsky Institute of Geology and Geochemistry, Ural Branch, Russian Academy of Sciences, Yekaterinburg 620075, Russia; [email protected] 4 Department of Earth and Environmental Sciences, Faculty of Advanced Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-76-264-6513; Fax: +81-76-264-6545 Received: 9 October 2018; Accepted: 6 November 2018; Published: 11 November 2018 Abstract: Ultrahigh-pressure (UHP) chromitites containing UHP minerals such as coesite and diamond have been reported from some ophiolites in Tibet and the Polar Urals. Their nature, i.e., origin, P-T path and abundance, however, are still controversial and left unclear. Here we describe chromitites in the Higashi-akaishi (HA) ultramafic complex in the Cretaceous Sanbagawa metamorphic belt, Japan, which experienced UHP condition (up to 3.8 GPa) at the peak metamorphism via subduction, in order to understand the nature of UHP chromitites. The HA peridotites typically contain garnets and are associated with eclogites, and their associated chromitites are expected to have experienced the UHP metamorphism. The Higashi-akaishi (HA) chromitites show banded to massive structures and are concordant to foliation of the surrounding peridotite. Chromian spinels in the chromitite and surrounding peridotites were sometimes fractured by deformation, and contain various inclusions, i.e., blade- and needle-like diopside lamellae, and minute inclusions of pyroxenes, olivine, and pargasite. The peculiar UHP minerals, such as coesite and diamond, have not been found under the microscope and the Raman spectrometer. Spinels in the HA chromitites show high Cr#s (0.7 to 0.85), and low Ti contents (<0.1 wt %), suggesting a genetic linkage to an arc magma. The HA chromitites share the basic petrographic and chemical features (i.e., diopside lamellae and arc-related spinel chemistry) with the UHP chromitites from Tibet and the Polar Urals. This suggests that some of the characteristics of the UHP chromitite can be obtained by compression, possibly via deep subduction, of low-P chromitite. Keywords: chromitite; ultrahigh-pressure metamorphism; the Higashi-akaishi peridotite complex; diopside lamella; arc-related magmatism; subduction 1. Introduction Podiform chromitites, which are mainly composed of chromian spinels, have been interpreted as an igneous cumulate precipitated from a spinel-oversaturated melt formed by melt/peridotite reaction at low-pressure (low-P) conditions [1,2]. Podiform chromitites from ophiolites, such as the Oman ophiolite, commonly show various lines of evidence for low-P origin, such as the frequent presence of Minerals 2018, 8, 525; doi:10.3390/min8110525 www.mdpi.com/journal/minerals Minerals 2018, 8, 525 2 of 18 Minerals 2018, 8, x FOR PEER REVIEW 2 of 18 presencepargasite of inclusions pargasite in inclusions chromian spinel in chromian [3,4]. However, spinel [3,4]. peculiar However, minerals peculiar indicating minerals ultrahigh-pressure indicating ultrahigh(UHP) and‐pressure highly reduced (UHP) and conditions, highly reduced such as diamond, conditions, moissanite, such as diamond, and alloys, moissanite, have been and found alloys, from havechromitites been found and surroundingfrom chromitites peridotites and surrounding in several ophiolites peridotites such in several as Luobusa ophiolites ophiolite such of as Tibet Luobusa [5–9] ophioliteand Ray-Iz of Tibet ophiolite [5–9] ofand the Ray Polar‐Iz ophiolite Urals [7 ].of Origin the Polar of suchUrals UHP [7]. Origin mineral-bearing of such UHP chromitites mineral‐bearing (UHP chromititeschromitites) (UHP is still chromitites) very controversial is still very because controversial their petrographic because andtheir petrologic petrographic features and arepetrologic unclear, featuresalthough are several unclear, models although were several proposed models [5,7,10 were–12]. proposed One of serious [5,7,10–12]. problems One in of constraining serious problems the origin in constrainingof UHP chromitites the origin is a of lack UHP of chromitites information is on a theirlack of P-T information trajectory. on their P‐T trajectory. ChromititesChromitites from from the the Higashi Higashi-akaishi‐akaishi complex complex will will give give us a us clue a clueto clear to clearthis problem. this problem. The HigashiThe Higashi-akaishi‐akaishi (HA) peridotite (HA) peridotite body in body the Sanbagawa in the Sanbagawa metamorphic metamorphic belt, Southwest belt, Southwest Japan (Figure Japan 1), has(Figure been1 ),interpreted has been interpreted to have originally to have formed originally at low formed‐P conditions at low-P conditions such as the such uppermost as the uppermost mantle in amantle mantle in wedge, a mantle and wedge, subsequently and subsequently experienced experienced UHP metamorphic UHP metamorphic condition condition up to 3.8 up GPa, to 3.8 due GPa, to downgoingdue to downgoing mantle mantle flow induced flow induced by subduction by subduction [13–18]. [13– 18The]. The P‐T P-T trajectory trajectory has has been been established established basedbased on on petrological petrological and and petrofabric petrofabric studies studies of of peridotites peridotites and and related related rocks, rocks, although although UHP UHP minerals minerals havehave not not been been found found [15]. [15]. Clear Clear evidence evidence for for their their high high-P‐P conditions conditions is is the the existence existence of of pyrope pyrope-rich‐rich garnetgarnet in in peridotites peridotites [13]. [13]. We We expect expect that that the the HA HA chromitites chromitites also also have have experienced experienced the the UHP UHP metamorphismmetamorphism together together with with the the associated associated peridotites. peridotites. The The advantage advantage of the of HA the chromitite HA chromitite is that is itsthat P‐ itsT history P-T history has hasbeen been independently independently determined determined [13–18]. [13–18 The]. The HA HA chromitites chromitites will, will, therefore, therefore, provideprovide us us with with unrivaled unrivaled information information on on the the behavior behavior of of low low-P‐P chromitite chromitite upon upon compression compression via via UHPUHP metamorphism. metamorphism. We We present present here here petrographic petrographic and and petrological petrological features features of chromitites of chromitites from from the HAthe HAperidotite peridotite complex complex (Figure (Figure 1) in1) inorder order to tounderstand understand the the nature nature of of chromitites chromitites in in the the UHP metamorphicmetamorphic belt, belt, or or the the UHP UHP chromitites chromitites from from ophiolites. ophiolites. FigureFigure 1. 1. LocalityLocality and and geological geological sketch sketch maps maps of of the the Higashi Higashi-akaishi‐akaishi ultramafic ultramafic complex complex located located on on thethe Sanbagawa Sanbagawa high high-P‐P metamorphic metamorphic zone. zone. (a) ( Locationa) Location of the of theHigashi Higashi-akaishi‐akaishi ultramafic ultramafic complex. complex. (b) General(b) General geological geological sketch sketch of the of the Higashi Higashi-akaishi‐akaishi ultramafic ultramafic complex complex and and surrounding surrounding metamorphic metamorphic rocks.rocks. Modified Modified from from Hattori Hattori et et al. al. [17]. [17]. Open Open star, star, sampling sampling localities. localities. 2. Geological Background 2. Geological Background The Sanbagawa belt, a Cretaceous regional metamorphic belt, extends over 800 km along The Sanbagawa belt, a Cretaceous regional metamorphic belt, extends over 800 km along southwest southwest Japan arc (Figure1a), and is mainly composed of pelitic and basic schists with small numbers Japan arc (Figure 1a), and is mainly composed of pelitic and basic schists with small numbers of of metagabbro and ultramafic bodies [14,19,20]. The Higashi-akaishi (HA) ultramafic complex is one of metagabbro and ultramafic bodies [14,19,20]. The Higashi‐akaishi (HA) ultramafic complex is one of the largest ultramafic complexes in the Sanbagawa metamorphic belt (Figure1). Ultramafic rocks from the largest ultramafic complexes in the Sanbagawa metamorphic belt (Figure 1). Ultramafic rocks the HA complex are dominated by dunites, which contain layers of clinopyroxenite and websterite from the HA complex are dominated by dunites, which contain layers of clinopyroxenite and websterite with small amounts of chromitite (Figure1). A unit of eclogite exists along the contact (the Gongen-goe with small amounts of chromitite (Figure 1). A unit of eclogite exists along the contact (the Gongen‐ goe area) between the HA peridotite and surrounding garnet amphibolite (Figure 1). Ultramafic rocks have experienced eclogite facies metamorphism because of the presence of pyrope‐rich garnet [14,19]. Minerals 2018, 8, 525 3 of 18 area) between the HA peridotite and surrounding garnet amphibolite
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