New Finding of Paragonite–Clinozoisite Association

Journal of Mineralogical and Petrological Sciences, J–STAGE Advance Publication, March 20, 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 inclusions 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 younger 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 (sensu 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.140725Advance Publicationschist, including the type Article locality in the Kanto Moun- A. Miyashita, [email protected] Corresponding author tains, is characterized by the existence of a homocline 2 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 abundanceAdvance of matrix minerals are quartz 46.5%,Publication albite 371, respectively (Miyashita, Article 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 3

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,Advance apatite, and pyrite. In this study, Publication up to 200 Ab and Pg + Czo assemblages Article (Heinrich and Althaus, grains of garnets extracted from the sample were exam- 1988). 4 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. Pressure (P)–temperature (T ) diagram in the Na2O– CaO–Al2O3–SiO2–H2O (NCASH) system showing reaction curves to constrain the P–T stability of the Pg + Zo assemblage Grt (Franz and Althaus, 1977; Heinrich and Althaus, 1988). The thick arrow represents the inferred prograde P–T path of the investigated paragonite–bearing garnets.

Figure 2. X–ray maps and back–scattered electron (BSE) image phengites suggests that Na–rich phengite may have coex- showing occurrence of paragonite and clinozoisite in garnet. isted with the paragonite prior to the back–reaction. (A) Mn distribution map of the core–mantle region of a por- phyroblastic garnet showing normal zoning. The color variation Preserved paragonite and clinozoisite within garnet represents the relative abundance of elements. (B) Ca distribu- have also been reported in Sanbagawa schist in the Ase- tion map showing the distribution of clinozoisite inclusions. (C) mi–gawa area of central Shikoku (Taguchi and Enami, – BSE image of the boxed area in (B) showing paragonite clino- 2014). Therefore, prograde–zoned garnets can be poten- zoisite association. tial rigid capsules retaining prograde mineral assemblages (Maruyama et al., 2010). More systematic studies fo- In the Sanbagawa belt, lawsonite has been reported cusing on the inclusion mineralogy of the prograde from chlorite zone Ca–bearing metapelites (e.g., Wata- garnets in the Kanto Mountains will provide new oppor- nabe and Kobayashi, 1984; Goto et al., 1996; Ueno, tunities for characterizing the prograde metamorphic evo- 2001). Lawsonite does not occur in garnet and biotite lution of the type locality of the Sanbagawa schist. zones; however, a recent finding of relict lawsonite preserved in the core of garnet from eclogites has been re- ACKNOWLEDGMENTS ported (Tsuchiya and Hirajima, 2013). For the other ex- ample of the prograde lawsonite–bearing assemblage in The author would like to thank M. Aoki (Okayama Uni- Japan, Miyakawa (1982) reported that lawsonite occurs versity of Science) and S. Omori (The Open University in garnet–bearing metapelite of Renge metamorphic of Japan) for discussion on prograde metamorphic evo- rocks in a Paleozoic high P–T metamorphic belt. In that lution of the Sanbagawa schist. K. Yokoyama and M. locality, lawsonite occurs in both the matrix and inclu- Shigeoka of the National Museum of Nature and Science sions within the garnet. are appreciated for their assistance in EDS analysis. The The paragonite–clinozoisite association within the author is also grateful to T. Tsujimori, H.U. Rehman, and garnet suggests that the schist experienced prograde P– Associate Editor M. Enami for their helpful reviews of T evolution exceeding the reaction curve between the two this manuscript. points of P = 0.8 GPa, T = 400 °C and P =1.2 GPa, T = 460 °C (Fig. 3). REFERENCE The absence of paragonite in the matrix may be the fi – result ofAdvance a signi cant back reaction to erase Publication the prograde Aoki, K., Itaya, T., Shibuya, T.,Article Masago, H., Nishizawa, M., Tera- features. The presence of compositionally heterogeneous bayashi, M., Omori, S., Yokoyama, T., Takahara, N., Sano, Y. New finiding paragonite in the Sanbagawa belt 5

and Maruyama, S. (2008) The youngest blueschist belt in SW Miyakawa, K. (1982) Low–grade metamorphic rocks of the Hida Japan: implication for the exhumation of the Cretaceous San- marginal belt in the upper Kuzuryu river area, central Japan. bagawa high–P/T metamorphic belt. Journal of Metamorphic Journal of Japan Association Mineralogy, Petrology and Eco- Geology 26, 583–602. nomic Geology, 77, 256–265 (in Japanese with English ab- Aoki, K., Maruyama, S., Isozaki, Y., Otoh, S. and Yanai, S. (2011) stract). Recognition of the Shimanto HP metamorphic belt within the Miyashita, A. (1997) Electron microprobe analyses of rock–form- traditional Sanbagawa HP metamorphic belt: New perspec- ing minerals in the Sanbagawa metamorphic rocks from the tives of the Cretaceous–Paleogene tectonics in Japan. Journal Ayukawa–Sanbagawa Area, Kanto Mountains, Central Japan. of Asian Earth Science, 42, 355–369. Bulletin of National Science Museum, Tokyo, Ser. C, 23, 1– Banno, S. and Sakai, C. (1989) Geology and metamorphic evolu- 25. tion of the Sanbagawa belt. In Evolution of Metamorphic Miyashita, A. (1998) Re–examintaion of “Geologic structure Belts (Daly, J.S., Cliff, R.A., and Yardley, B.W.D. Eds.). Pro- formed by tectonic stacking of sliced layer” in the Sanbagawa ceedings of the 1987 Joint Meeting of the Metamorphic Stud- metamorphic rocks, Kanto Mountains. Journal of the Geolog- ies Group and IGCP Project 235: Geological Society, London, ical Society of Japan, 104, 731–746 (in Japanese with English Special Publication, 43, 519–532. abstract). Franz, G. and Althaus, E.K. (1977) The stability relations of para- Miyashita, A. and Itaya, T. (2002) K–Ar age and chemistry of genesis paragonite–zoisite–quartz. Neues Jahrbuch für Miner- phengite from the Sanbagawa schists in the Kanto Mountains, alogie–Abhandlungen, 130, 159–167. Central Japan, and their implication for exhumation tectonics. Goto, A., Higashino, T. and Sakai, C. (1996) XRF analyses of San- Gondwana Research, 5, 837–848. bagawa pelitic schists in central Shikoku, Japan. Memoirs of Taguchi, T. and Enami, M. (2014) Compositional zoning and in- Faculty of Science, Kyoto University, Series of Geology and clusions of garnet in Sanbagawa metapelites in the Asemi– Mineralogy, 58, 1–19. gawa route, central Shikoku, Japan. Jounal of Mineralgical Heinrich, W. and Althaus, E. (1988) Experimental determination of and Petrological Sciences, 109, 1–12. reactions 4 lawsonite + 1 albite = 1 paragonite + 2 zoisite + 2 Tsuchiya, S. and Hirajima, T. (2013) Evidence of the lawsonite quartz + 6H2O and 4 lawsonite + 1 jadeite = 1 paragonite + 2 eclogite facies metamorphism from an epidote–glaucophane zoisite + 1 quartz + 6H2O. Neues Jahrbuch für Mineralogie eclogite in the Kotsu area of the Sanbagawa belt, Japan. Jou- Monatshefte, H11, 516–528. nal of Mineralgical and Petrological Sciences, 108, 166–171. Hirajima, T., Isono, T. and Itaya, T. (1992) K–Ar age and chemistry Tsujimori, T. and Ernst, W.G. (2014) Lawsonite blueschists and of white mica in the Sanbagawa metamorphic rocks in the lawsonite eclogites as proxies for palaeo–subduction zone Kanto Mountains, central Japan. Journal of Geological Soci- processes: a review. Journal of Metamorphic Geology, 32, ety of Japan, 98, 445–455 (in Japaneses with English ab- 437–454. stract). Tsutsumi, Y., Miyashita, A., Terada, K. and Hidaka, H. (2009) Higashino, T. (1975) Biotite zones of the Sanbagawa metamorphic SHRIMP U–Pb age of detrital zircons from the Sanbagawa terrain in the Shiragayama area, central Shikoku. Journal of Belt, Kanto Mountains, Japan: need to revised the framework the Geological Society of Japan, 81, 653–670 (in Japanese of the belt. Jounal of Mineralgical and Petrological Sciences, with English abstract). 104, 12–24. Higashino, T. (1990) The higher grade metamorphic zonation of Turner, F.J. (1980) Metamorphic Petrology. pp. 524, McGraw–Hill, the Sambagawa metamorphic belt in central Shikoku, Japan. NewYork. Journal of Metamorphic Geology, 8, 413–423. Ueno, T. (2001) Lawsonite–bearing pelitic schists from the Sanba- Isozaki, Y. and Itaya, T. (1990) Chronology of Sanbagawa meta- gawa metamorphic belt in the Ise area, eastern Kii Pennisula, morphism. Journal of Metamorphic Geology, 8, 401–411. central Japan. Japanese Magazine of Mineralogical and Petro- Itaya, T. (1981) Carbonaceous material in pelitic schists of the San- logical Sciences, 30, 255–264 (in Japanese with English ab- bagawa metamorphic belt in central Shikoku, Japan. Lothos, stract). 14, 215–224. Watanabe, T. and Kobayashi, H. (1984) Occurrence of lawsonite in Itaya, T., Tsujimori, T. and Liou, J.G. (2011) Evolution of the San- pelitic schists from the Sanbagawa metamorphic belt, central bagawa and Shimanto high–pressure belts in SW Japan: In- Shikoku, Japan. Journal of Metamorphic Geology, 2, 365– sights from K–Ar (Ar–Ar) geochronology. Journal of Asian 369. Earth Sciences, 42, 1075–1090. Whitney, D.L. and Evans, B.W. (2010) Abbreviations for names of Koto, B. (1888) On the so–called crystalline schists of Chichibu rock–forming minerals. American Mineralogist, 95, 185–187. (The Sambagawan Series). Journal of the College of Science, Imperial University of Tokyo, Japan, 2, 77–141. Maruyama, S., Masago, H., Katayama, I., Iwase, Y., Toriumi, M., Manuscript received July 25, 2014 Omori, S. and Aoki, K. (2010) A new perspective on meta- Manuscript accepted January 29, 2015 morphism and metamorphic belts. Gondwana Research, 18, 106–137. Manuscript handled by Masaki Enami

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