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New Finding of Paragonite–Clinozoisite Association

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New Finding of Paragonite–Clinozoisite Association Journal of Mineralogical and Petrological Sciences, Volume 110, page 71–75, 2015 LETTER New finding of paragonite–clinozoisite association in garnet from the type locality of Sanbagawa belt (Kanto Mountains, Japan) Atsushi MIYASHITA Seikei High School, 3–10–13 Kichijoji–kitamachi, Musashino, Tokyo 180–8633, Japan Paragonite–clinozoisite associated within garnets (pyrope2–7 almandine60–64 grossular10–28 spesartine1–10) was newly found in the type locality of the Sanbagawa schist in the Kanto Mountains, Japan. The composite in- clusions were confirmed in prograde–zoned garnet porphyroblasts in garnet–zone metapelites that have a typical pelitic whole–rock composition (Mn/Fe = 0.019, Mg/Fe = 0.381). The garnets also contain mineral inclusions of quartz, albite, phengite, chlorite, rutile, calcite, apatite, and zircon. Phengite in the metapelite yielded a K–Ar age of 67.3 ± 1.6 Ma as the younger part of the Sanbagawa schist in this region. This paragonite–clinozoisite association provides a mineralogical index of high pressure (P)–temperature (T ) metamorphism of the type locality of the Sanbagawa schist. Moreover, the clinozoisite–paragonite composite inclusions within prograde– zoned garnets suggest a prograde P–T path from the stability of lawsonite and albite. Keywords: Sanbagawa belt, Type locality, Metapelite, Paragonite, Clinozoisite, Kanto Mountains INTRODUCTION 90 Ma from Shimanto HP schists are significantly young- er than the ~ 120 Ma peak metamorphic age of the San- The Sanbagawa belt (sensu lato; s.l.), one of the best– bagawa HP belt (s.s.) (Aoki et al., 2008; Tsutsumi et al., studied high–pressure (HP) metamorphic belts, trends 2009). According to this new subdivision, schists in the roughly E–W over 800 km in central to SW Japan. It Kanto Mountains, including the type locality of ‘Sanba- has been considered that the Sanbagawa belt (s.l.) con- gawa’ schist, belong to the Shimanto HP belt, although sists of a Mesozoic metamorphosed accretionary complex Koto (1888) named the schists ‘Sambagawan’ after the subjected to HP metamorphism (1.5–2.2 GPa/500–750 name of a small valley in this region. In the present study, °C) at around 120 Ma and retrograde metamorphism the term ‘Sanbagawa schist’ is used rather than ‘Shiman- (0.7–1.1 GPa/460–510 °C) at around 86 Ma (e.g., Isozaki to schist’. and Itaya, 1990; Aoki et al., 2011; Itaya et al., 2011). This study reports the occurrence of a paragonite– The regional metamorphic mapping shows a systematic clinozoisite association in garnets from the type locality change of the appearance of characteristic Fe–Mg silicate of the Sanbagawa belt, which provides a mineralogical minerals in metapelites (e.g., Banno and Sakai, 1989; index of HP metamorphism of this type locality. Higashino, 1990); metamorphic grades of chlorite, gar- Mineral abbreviations throughout this paper are after net, and biotite, oligoclase–biotite zones increase in as- Whitney and Evans (2010). cending order. Based on geochronology, the Sanbagawa belt (s.l.) OUTLINE OF GEOLOGY is subdivided into two HP belts: the Sanbagawa belt (sen- su stricto; s.s.) and the Shimanto HP belt (Aoki et al., The Amabiki–gawa area in the Sanbagawa metamorphic 2008, 2011; Itaya et al., 2011); the Shimanto HP belt is belt is located in the northern extreme end of the Kanto a newly defined petrotectonic belt that differs from the Mountains (Fig. 1). The Sanbagawa River as the type Shimanto accretionary complex. Detrital zircons of 80– locality of the Sanbagawa belt is located 5 km south from the Amabiki–gawa area. The western part of Sanbagawa doi:10.2465/jmps.140725 schist, including the type locality in the Kanto Moun- A. Miyashita, [email protected] Corresponding author tains, is characterized by the existence of a homocline 72 A. Miyashita (B) (A) Okhotsk Plate R. Amabiki-gawa Ushibuse Formation (Miocene) 40° N Ushibuse RC A Fault Eurasia Plate N Pacific Plate 378 JAPA T.T.L. I.S.T.L. E Central Shikoku N 50 an ap W J Kanto Mountains Shimo-doya e of S on RC N A nner z PA I JA SW 424 M.T.L. Ryoke Belt Outer zone ofPhilippine SW Japan Sea Plate Sanbagawa Belt 468 B.T.L. Chichibu Belt Rock type ° 62 130° E 140 E 428 N Metapelite 465.0 Metabasite Shimonita AM73P Kami-doya Type Locality (Sanbagawa R.) Sampling locality Metapelite (chlorite only) 535 Metapelite (garnet bearing) Metabasite 579 46 0 200m Chichibu 42 Ogose Ryoke Belt Atokura Formation Pultono-metamorphic Complex (Paleo-Ryoke Belt) Mikabu Greenstones Sanbagawa Metamorphic Rocks Sanchu Group Chichibu Sedimentary Complex Kurouchi Ultramafic Rocks Fault Figure 1. (A) Geological map of the Kanto Mountains. (B) Route map of Amabiki–gawa area showing the sampling site of AM73P. AM73P belongs to the Middle Garnet Zone (Miyashita, 1998; Tsutsumi et al., 2009). structure toward the south and is divided into four units nozoisite/epidote 0.2%, titanite 1.3%, calcite 1.5%, apa- including Mikabu, Southern, Middle, and Northern units tite 0.2%, and tourmaline 0.1%. Rutile, zircon, allanite, in ascending order of age. These divisions were deter- and opaque minerals also occur as accessory minerals. mined on the basis of their lithofacies, mineral assemb- The opaque minerals have been identified as ilmenite, lages, phengite K–Ar ages, and U–Pb ages of the detrital pyrite, chalcopyrite, sphalerite, galena, carbonaceous ma- zircons (Miyashita, 1998; Miyashita and Itaya, 2002; terials, and thorite using ore microscopy and energy– Tsutsumi et al., 2009). dispersive X–ray spectroscopy (EDS) analyses. Typical The metamorphic sequence of the Amabiki–gawa higher–grade mineral assemblages of the Sanbagawa belt area belongs to the Middle Unit and consists of thick (s.l.) such as biotite, barroisite, and oligoclase were not metabasites and metapelites with metapsammite schists observed. and a thin layer of metaquartzites (Fig. 1). The whole–rock major element composition of the sample shows a typical metapelite (Mn/Fe = 0.019 and SAMPLE DESCRIPTION Mg/Fe = 0.381) of the Sanbagawa belt (s.l.) (Goto et al., 1996; Miyashita, 1997). It contains 1.48 wt% CaO. The The investigated sample (AM73P) was collected from a Mg–Fe partition coefficient between garnet and chlorite site along the Amabiki–gawa area (Fig 1). The sample in the matrix of the metapelite is 0.131 (Miyashita, 1998). exhibits well–developed schistosity with abundant albite The lattice parameter and crystallite size value of carbo- porphyroblasts several millimeters in diameter. The mo- naceous materials are d002 = 3.361 Å and Lc (002) = dal abundance of matrix minerals are quartz 46.5%, albite 371, respectively (Miyashita, 1997; Miyashita, 1998). 15.2%, phengite 30.1%, chlorite 2.3%, garnet 1.9%, cli- These values showing metamorphic grade are within New finiding paragonite in the Sanbagawa belt 73 Table 1. Representative chemical compositions of paragonite and clinozoisite included in the garnet Compositions of garnet and matrix phengite in addition to that of epidote are also shown. the ranges of the garnet zone in the central Shikoku area ined to determine inclusion mineralogy. The paragonite– (Higashino, 1975; Itaya, 1981). The K–Ar phengite age is clinozoisite association was confirmed from 20 grains 67.3 ± 1.6 Ma (7.88 wt% potassium), which is about 5 (Fig. 2). Clinozoisites have Fe3+/(A l + Fe3+) = 0.20. Ma older than the 60–Ma middle unit rocks around the No paragonite or clinozoisite were observed in the ma- AM73P (Miyashita and Itaya, 2002). This observable fact trix. Matrix epidote is relatively more Fe+3–rich as com- was also reported by Hirajima et al. (1992) on the San- pared to the inclusions in the garnet (Table 1). Allanite is bagawa schist in the Kanto Mountains. found in both the inclusions and the matrix. The representative chemical compositions of para- gonites and clinozoisites within the garnet and phengites, GEOLOGICAL SIGNIFICANCE epidotes, and a garnet in the matrix are shown in Table 1. Phengites occur generally as aggregates of fine–grained The mineral assemblage Pg + Czo is an index of HP crystals parallel to the schistosity exhibiting Na–rich metamorphism (Turner, 1980) and has been described cores and Na–poor rims (Table 1; Miyashita and Itaya, from HP Ca–bearing metapelites such as New Caledonia. 2002). The Na–rich core is generally less celadonitic than This assemblage can be formed from the breakdown of that of the Na–poor rim. Epidotes show tabular anhedral lawsonite–bearing mineral assemblages. It is possible that shapes with 0.1–mm–long grains, and are weekly zoned the paragonite–clinozoisite association within the pro- (Fe3+/(A l + Fe3+) = 0.12–0.19). Garnets occur as euhe- grade–zoned garnet is a reaction product of the former dral crystals, 1–3 mm in diameter, with Mn–rich cores Lws + Ab assemblage (cf. Tsujimori and Ernst, 2014). and Mn–poor rims and show normal zoning (Table 1 The sample may have had a lawsonite–bearing mineral and Fig. 2). assemblage prior to garnet growth that transformed to Garnets contain mineral inclusions of paragonite, Pg + Czo + Qz during prograde metamorphism. Figure clinozoisite, quartz, albite, phengite, chlorite, rutile, zir- 3 shows a P–T diagram of the stable fields of both Lws + con, calcite, apatite, and pyrite. In this study, up to 200 Ab and Pg + Czo assemblages (Heinrich and Althaus, grains of garnets extracted from the sample were exam- 1988). 74 A. Miyashita P(GPa) Heinrich & Althaus (1988) (B) (C) (A) 2.0 s+Jd +Zo+Qz w L Pg 1.5 Prograde pass of AM73P Qz Ab Pl Pg+Czo+ 1.0 Jd + Qz Lws+ Qz Low-Ab Pg+Zo+ Mrg+Pl (C) 0.5 Czo Pg Pg 0.0 Pg ° 200300 400500 600 T( C) Czo Czo Figure 3.
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