DOCSLIB.ORG

Archive of SID

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Archive of SID Archive of SID IJST (2014) 38A3: 243-252 Iranian Journal of Science & Technology http://ijsts.shirazu.ac.ir An unusual assemblage of talc-phengite-chlorite-K-feldspar in quartz schists from the Nahavand area, Sanandaj-Sirjan zone, Iran J. Izadyar*, S. Mojab, O. Kuroshi and M. Zare Department of Geology, University of Zanjan, University Blvd., Zanjan, Iran E-mail: izadyar@znu.ac.ir Abstract For the first time, an unusual assemblage of talc-phengite-chlorite-K-feldspar was found in quartz schists from the Sanandaj-Sirjan zone in the Nahavand area in western Iran. The talc-bearing quartz schists occur as small bodies or lenses within pelitic schist layers and contain talc, phengite, chlorite, K-feldspar and quartz as major mineral constituents with subordinate amounts of calcite and graphite. Textural analysis revealed that talc, phengite, chlorite and K-feldspar are in sharp contact and no reaction rims between them were observed. Constructed petrogenetic gird in the K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH) model system containing talc, phengite, chlorite, K-feldspar, phlogopite and kyanite with excess quartz and H2O shows that divariant assemblage of talc- phengite-chlorite-K-feldspar is stable over a wide P-T range defined by the following two univariant reactions: phengite + talc + quartz = chlorite + K-feldspar + kyanite + H2O and chlorite + phlogopite + quartz = talc + phengite + K-feldspar + H2O. Constructed Al2O3-KAlO2-MgO+FeO (AKM) compatibility diagrams predict that phengite (XPh = 0.280, YPh = 0.860), chlorite (XChl = 0.570, YChl = 0.640), talc (XTlc = 0.160, YTlc = 0.02) and K- feldspar are stable at P = 11 kbar and T = 400°C. This relatively high-pressure assemblage could be formed during the subduction of the Neo-Tethys oceanic plate under Iranian microcontinent. Keywords: High-pressure metamorphism; Nahavand; Sanandaj-Sirjan zone; Talc-phengite-K-felds parassemblage 1. Introduction and Schreyer (1976) regarding the high-pressure origin for talc-phengite assemblages. This In natural rocks, talc-phengite assemblage was first assemblage was subsequently recognized in the found in the piemontite-quartz schist in Serbia western Alps (Chopin, 1981) and thereafter in a (Abraham and Schreyer, 1976). Abraham and number of areas around the world in high-pressure Schreyer (1976) suggested that talc-phengite metamorphic terrains such as the Sanbagawa assemblage is an indicator of high-pressure metamorphic belt, Japan (Izadyar et al., 2000; metamorphism based on theoretical grounds in the Hatanaka and Hirajima, 1999; Ubukawa et al., K2O-MgO-Al2O3-SiO2-H2O (KMASH) model system, 2007). Talc-kyanite and talc-omphacite although they also proposed an alternative assemblages have also been identified from ultra hypothesis that this pair could be formed due to the high-pressure metamorphic rocks (Reinecke, 1991; presence of small amounts of additional non- Mattinson et al., 2004; Liou and Zhang, 1995). KMASH components such as Fe2O3-MnO-CuO- However, the natural parageneses of talc-phengite NiO in phyllosilicates under low-pressure and talc-kyanite usually coexist with Fe-bearing conditions. Massonne and Schreyer (1989) assessed mineral phases such as garnet, biotite, chloritoid the stability field of talc-phengite assemblage by and chlorite (Chopin, 1981; Frank et al., 1987) and the synthetic work in the KMASH model system do not belong to the constituents of the simple containing kyanite, chlorite, talc, phlogopite and KMASH model system. Wei and Powell (2003) muscovite with excess quartz and H2O. They calculated the KFMASH model system for high- proposed that the following two reactions: chlorite+ pressure metapelites with mineral phases of garnet, phlogopite + quartz = talc + muscovite + H2O and chloritoid, chlorite, biotite, carpholite, talc, talc + muscovite = phlogopite + quartz + H2O staurolite, kyanite with phengite, quartz/coesite and intersect at an invariant point at 10.8 kbar and H2O in excess. They documented the typical 610°C, which supported hypothesis 2 of Abraham parageneses of talc-phengite and talc-kyanite for a variety of bulk compositions from several high- pressure terrains. Wei and Powell (2004) also *Corresponding author calculated the stable mineral assemblages of Na- Received: 4 February 2013 / Accepted: 7 May 2014 bearing metapelites in high-pressure conditions as www.SID.ir Archive of SID IJST (2014) 38A3: 243-252 244 they also occur widely in high-pressure types of the complexly deformed sub-zone are schist, metamorphic terrains (Chopin, 1981; Izadyar et al., marble, amphibolite, quartzite, dolomite-marble and 2000; Meyre et al., 1999; Theye et al., 1992). By metasandstone. All sub-zones are imbricated in a using Schreinemakers' analyses and thermodynamic complicated thrust system with out-of-sequnce thrusts calculation based on the dataset of Holland and that locally juxtaposes the younger units over the older Powell (1998), Izadyar et al. (2000) indicated that ones (Mohajjel et al., 2003). The Nahavand area the talc-phengite-albite assemblage from belongs to the complexly deformed sub-zone and is piemontite-quartz schists of the Sanbagwa located near the main Zagros reverse fault, which is metamorphic belt is stable at 580-600°C and 11.6- proposed as a suture zone between the Arabian plate and Eurasia (Sengör, 1984; Agard et al., 2006; 12 kbar. However, Ubukawa et al. (2007) obtained Ghasemi and Talbot, 2006) (Fig. 1B). Apart from a temperatures of 565-580°C and pressures of 9.5-10 thin ophiolite unit, the lithologic units in the Nahavand 3 kbar for talc-phengite assemblage in NCKFe region can be generally subdivided into two groups of MASH model system in talc-Na pyroxene metamorphosed and non-metamorphosed rocks (Alavi piemontite-quartz schist in the Sanbagawa and Mahdavi, 1994) (Fig. 2). The metamorphic rocks metamorphic belt. None of these studies, either include five different successions that are in fault experimental or calculated, have addressed a talc- contact with both the overlying and underlying rocks. phengite-K-feldspar assemblage. In fact, only The oldest one (PTrm) is a sequence of massive Massonne and Schreyer (1989) proposed high- marbles of Permian-Triassic age that are strongly pressure stability field for the talc-phengite-K- sheared and mylonitized near the contact with the feldspar assemblage purely on theoretical grounds underlying rocks. The next succession (TrJmv) but it is not reported in natural rocks from high- consists of an assemblage of Middle to Upper Triassic-Lowermost Jurassic metamorphosed basaltic pressure metamorphic terrains. During a and andesite-basaltic lava flows (with tholeitic petrological investigation in the Nahavand area of affinities). This assemblage contains locally occurring the Sanandaj-Sirjan zone, an unusual assemblage of pillow structures interlayered with well-foliated talc-phengite-chlorite-K-feldspar was recognized in marbles that have thin, upward increasing slate layers. quartz schists. This paper is the first to describe this Small plutons of gabbro and diorite (now assemblage, to characterize its mineralogy and metamorphosed) have locally intruded this assemblage texture and to explain its equilibrium condition (Alavi and Mahdavi, 1994) (Fig. 2). The third unit based on the petrogenetic grid in the KFMASH (TrJm) consists entirely of a sequence of fossiliferous model system containing K-feldspar. marbles of Late Triassic to Early Jurassic. The fourth succession (Jsv) is a sequence of Lower Jurassic 2. Geological setting slates, phyllites and metagreywakes that have intercalations of marbles and metamorphosed volcanic The Sanandaj-Sirjan zone, which is 1500km long and rocks in the lower parts. The fifth lithological unit (Jh) up to 200km wide, stretches from the northwest to the consists of an Upper Liassic succession of cordierite- southwest of Iran and is located between the central bearing hornfels which is the product of contact Iran block and Zagros Fold-thrust belt (Stoclin, 1968; metamorphism due to the intrusion of Late Cretaceous Berberian and King, 1981; Alavi, 1994) (Fig. 1A). The granitoid plutons (Alavi and Mahdavi, 1994) (Fig. 2). Sanandaj-Sirjan zone consists of mafic-ultramafic rocks, metamorphic complexes and Paleozoic- Mesozoic sequences that underwent amphibolite/greenshist-facies metamorphism during the Cretaceous-Tertiary continental collision between the Afro-Arabian continent and the Iranian microcontinent (Sengör and Natalin, 1996; Mohajjel and Fergusson, 2000; Mohajjel et al., 2003). From the southwest to the northeast it consists of several elongated sub-zones (Fig. 1B): (1) a radiolarite sub- zone consisting of Triassic-Cretaceous shallow marine limestone and dominant deep-marine radiolarite; (2) a Bisotun sub-zone that includes Late Triassic to Late Cretaceous limestone; (3) an ophiolite sub-zone that is the location of the Zagros suture zone; (4) a marginal sub-zone that contains abundant Late Jurassic-Early Cretaceous volcanic rocks in addition to shallow Fig. 1. (A) Simplified structural map of Iran showing the marine strata in the Cretaceous unit and (5) a location of the Sanandaj-Sirjan zone. (B) Tectonic map of complexly deformed sub-zone (Mohajjel et al., 2003). southwestern Iran showing the divisions of the Sanandaj- The last sub-zone is distinguished from the other sub- Sirjan zone (modified after Mohajjel et al., 2003) zones by abundant metamorphic rocks. The main rock www.SID.ir Archive of SID 245 IJST (2014) 38A3: 243-252 are observed (Figs. 4A, 4B, 4D and 4E). Chlorite is one of the major constituent
Load more

Recommended publications
  • List of Abbreviations
    List of Abbreviations Ab albite Cbz chabazite Fa fayalite Acm acmite Cc chalcocite Fac ferroactinolite Act actinolite Ccl chrysocolla Fcp ferrocarpholite Adr andradite Ccn cancrinite Fed ferroedenite Agt aegirine-augite Ccp chalcopyrite Flt fluorite Ak akermanite Cel celadonite Fo forsterite Alm almandine Cen clinoenstatite Fpa ferropargasite Aln allanite Cfs clinoferrosilite Fs ferrosilite ( ortho) Als aluminosilicate Chl chlorite Fst fassite Am amphibole Chn chondrodite Fts ferrotscher- An anorthite Chr chromite makite And andalusite Chu clinohumite Gbs gibbsite Anh anhydrite Cld chloritoid Ged gedrite Ank ankerite Cls celestite Gh gehlenite Anl analcite Cp carpholite Gln glaucophane Ann annite Cpx Ca clinopyroxene Glt glauconite Ant anatase Crd cordierite Gn galena Ap apatite ern carnegieite Gp gypsum Apo apophyllite Crn corundum Gr graphite Apy arsenopyrite Crs cristroballite Grs grossular Arf arfvedsonite Cs coesite Grt garnet Arg aragonite Cst cassiterite Gru grunerite Atg antigorite Ctl chrysotile Gt goethite Ath anthophyllite Cum cummingtonite Hbl hornblende Aug augite Cv covellite He hercynite Ax axinite Czo clinozoisite Hd hedenbergite Bhm boehmite Dg diginite Hem hematite Bn bornite Di diopside Hl halite Brc brucite Dia diamond Hs hastingsite Brk brookite Dol dolomite Hu humite Brl beryl Drv dravite Hul heulandite Brt barite Dsp diaspore Hyn haiiyne Bst bustamite Eck eckermannite Ill illite Bt biotite Ed edenite Ilm ilmenite Cal calcite Elb elbaite Jd jadeite Cam Ca clinoamphi- En enstatite ( ortho) Jh johannsenite bole Ep epidote
    [Show full text]
  • CHAPTER 4 - MINERAL CHEMISTRY and METAMORPHIC CONDITIONS A
    - 217 - CHAPTER 4 - MINERAL CHEMISTRY AND METAMORPHIC CONDITIONS a - 218 - Mineral chemistry and metamorphic conditions The aims of this chapter are to present mineral chemical data, to describe the mineral chemistry in relation to parageneses in selected lithologies (with particular reference to high-pressure metamorphic types) and to evaluate the metamorphic conditions under which these parageneses formed in the light of published experimentally and theoretically derived phase equilibria. Finally, a synthesis of pressure-temperature-time (P-T-t) relationships will be presented. Analytical methods and mineral recalculation schemes are described in Appendices 1 and 2. Results of geothermometry calculations are sumarised in Tbj 4.20. SECTION 4.01 The Eclogites 4.01.1 Summary of petrography Detailed petrographic descriptions appear in Chapter 2, the salient features of which are summarised here. The most common parageneses in the eclogites are:- garnet + omphacite +1- amphibole +1- rutile garnet + omphacite + phengite +1- quartz +1- rutile garnet + omphacite + phengite + zoisite +/_ quartz +/_ rut i e. S - 219 - garnet + omphacite + phengite + zoisite + kyanite +1- quartz ^1- rutile Segregations of zoisite + kyanite + quartz occur in some more aluminous types. Occasionally, Na-poor rocks have pale green or blue-green amphibole in textural equilibrium with omphacite and garnet (figure 2.18) which may become an essential or dominant phase, to give:- garnet + amphibole + clinopyroxene +/_ rutile. The phases listed above will be referred to as 'matrix' phases. Solid inclusions are common in the matrix phases (figures 2.19, 2.45). They are not associated with fractures in the garnets and commonly occur in samples where no alteration of garnet rims or matrix grains has occurred.
    [Show full text]
  • Genesis and Solvus Relations of Submicroscopically Intergrown
    Contrib Mineral Petrol (1991) 106:367-378 Contributions to Mineralogy and Petrology Springer-Verlag 1991 Genesis and solvus relations of submieroseopieally intergrown paragonite and phengite in a blueschist from northern California* Yen-Hong Shau, Melanie E. Feather**, Eric J. Essene, and Donald R. Peacor Department of Geological Sciences, The University of Michigan, Ann Arbor, MI 48109, USA Received February 12, 1990/Accepted July 13, 1990 Abstract. Electron microbeam techniques have been used Introduction to examine submicroscopically intergrown paragonite, phengite and chlorite from the South Fork Mountain Coexisting paragonite and muscovite (or phengite) have Schist of the Franciscan Terrane of northern California, been analyzed in prograde metamorphic assemblages in which was subjected to blueschist facies metamorphism. the anchizone to epizone (Frey 1970, 1978; Livi et al. The sample also contains quartz, albite, lawsonite, and 1988), in the pnmpellyite facies (including prehnite-pum- rutile. The subassemblage albite-lawsonite-rutile re- pellyite and pumpellyite-actinolite facies, term from Es- quires metamorphic conditions on the low-temperature sene 1989; occurrences from Kramm 1980; Merriman side of the equilibrium albite + lawsonite + futile = para- and Roberts 1985; Pc-Piper 1985), the albite-epidote gonite+sphene+quartz+H20 (T<200 ~ and P< hornfels facies (Okuyama-Kusunose 1985), the green- 7.4 kbars based on thermodynamic data of Holland and schist facies (H6ck 1974; Katagas and Baltatzis 1980; Powell 1990). The white micas appear to be optically Ashworth and Evirgen 1984; Baltatzis and Sideris 1985), homogeneous, but back-scattered electron images can the amphibolite facies (Burnham and Radoslovich 1964; distinguish two different micas by their slight difference Zen and Albee 1964; Henley 1970; Baltatzis and Wood in contrast.
    [Show full text]
  • Retrogression of Eclogites to Blueschists: a Record of Fluid
    Retrogression of eclogites to blueschists: a record of fluid infiltration during uplift in subduction zones Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakult¨at der Christian–Albrechts–Universit¨at zu Kiel vorgelegt von Fran¸cois van der Straaten Kiel, 2008 Eklogit-Blauschiefer Umwandlung: Hinweise auf Fluidinfiltrierung w¨ahrend des Aufstiegs in Subduktionszonen Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakult¨at der Christian–Albrechts–Universit¨at zu Kiel vorgelegt von Fran¸cois van der Straaten Wappen der Familie van der Straaten Kiel, 2008 Referent: Prof. Dr. Volker Schenk Korreferent/in: Prof. Dr. Astrid Holzheid. Tag der m¨undlichen Pr¨ufung: 04. November 2008 Zum Druck genehmigt: Kiel, . Der Dekan Vorwort Die vorliegende Arbeit ist eine monographische Dissertation und beinhaltet drei von einan- der getrennte Kapitel mit einer vorangestellten Einleitung. Die Gesamtheit aller Kapitel spiegelt das Thema der Dissertation wieder. Jedes Kapitel f¨ursich kann oder wurde bereits in Fachzeitschriften publiziert. Daher sei f¨urden Leser angemerkt, dass jedes Kapitel eine Zusammenfassung, Einleitung, Diskussion und Literaturangabe enth¨alt. M¨ogliche thema- tische Uberschneidungen¨ oder Wiederholungen sind daher zu ber¨ucksichtigen. Kiel, September 2008 vi Man setzt der Finsternis ein Ende und durchforscht bis zur ¨außerstenGrenze das Gestein der Dunkelheit und Finsternis. Buch Hiob: 28,3 Und er ¨offneteden Schlund des Abgrundes; und ein Rauch stieg auf aus dem Schlund. und die Luft wurden von dem Rauch des Schlundes verfinstert. Offenbarung: 9,2 Der tiefe Fluss ist ruhig – der wis- sende Mensch ist bescheiden. aus der Mongolei In Wirklichkeit erkennen wir nichts; denn die Wahrheit liegt in der Tiefe. Demokrit Contents Zusammenfassung xi Abstract xiii 1 Introduction 1 1.1 The slab-mantle interface and subduction channel .............
    [Show full text]
  • Blueschist Facies Metaconglomerates: Catalina Schist, CA
    Altered dioritic clasts in lawsonite- blueschist facies metaconglomerates: Catalina Schist, CA Natalie Elizabeth Sievers GEOL 394 Advisors: Dr. Sarah Penniston-Dorland Dr. William McDonough Dr. Philip Piccoli 0 Altered dioritic clasts in metaconglomerates: Catalina Schist, CA Table of Contents Abstract ...................................................................................................................................................... 2 I. Introduction ................................................................................................................................. 3 II. Geologic Setting ......................................................................................................................... 5 III. Objectives of Research ............................................................................................................... 8 IV. Discussion of Subduction Zone Chemistry ........................................................................... 8 V. Experimental Design and Approach ...................................................................................... 9 VI. Results ........................................................................................................................................ 14 A. Petrographic Results ...................................................................................................14 B. Major Element Compositions ...................................................................................16 C. Whole Rock Results ....................................................................................................17
    [Show full text]
  • Tracing Fluid Transfers in Subduction Zones: an Integrated 18 2 Thermodynamic and Δ O Fractionation Modelling Approach
    https://doi.org/10.5194/se-2019-140 Preprint. Discussion started: 23 September 2019 c Author(s) 2019. CC BY 4.0 License. 1 Tracing fluid transfers in subduction zones: an integrated 18 2 thermodynamic and δ O fractionation modelling approach 3 Alice Vho1, Pierre Lanari1, Daniela Rubatto1,2, Jörg Hermann1 4 1 Institute of Geological Sciences, University of Bern, CH-3012 Bern, Switzerland 5 2 Institut de Sciences de la Terre, University of Lausanne, CH-1015 Lausanne, Switzerland 6 Correspondence to: Alice Vho (alice.vho@geo.unibe.ch) 7 Abstract. Oxygen isotope geochemistry is a powerful tool for investigating rocks that interacted with fluids, to assess fluid 8 sources and quantify the conditions of fluid-rock interaction. We present an integrated modelling approach and the computer 9 program PTLOOP that combine thermodynamic and oxygen isotope fractionation modelling for multi-rock open systems. The 10 strategy involves a robust petrological model performing on-the-fly Gibbs energy minimizations coupled to an oxygen 11 fractionation model both based on internally consistent databases. This approach is applied to subduction zone metamorphism to 12 predict the possible range of δ18O values for stable phases and aqueous fluids at various pressure-temperature (P-T) conditions in 13 the subducting slab. The modelled system is composed by a sequence of oceanic crust (mafic) with sedimentary cover of known 14 initial chemical composition and bulk δ18O. The evolution of mineral assemblage and δ18O values of each phase is calculated 15 along a defined P-T path. Fluid-rock interactions may occur as consequence of (1) infiltration of an external fluid into the mafic 16 rocks or (2) transfer of the fluid liberated by dehydration reactions occurring in the mafic rocks into the sedimentary rocks.
    [Show full text]
  • High-Si Phengite Records the Time of Greenschist Facies Overprinting: Implications for Models Suggesting Mega-Detachments in the Aegean Sea
    J. metamorphic Geol., 2004, 22, 427–442 doi:10.1111/j.1525-1314.2004.00524.x High-Si phengite records the time of greenschist facies overprinting: implications for models suggesting mega-detachments in the Aegean Sea M. BRO¨ CKER1 ,D.BIELING1 , B. HACKER2 AND P. GANS2 1Institut fu¨r Mineralogie, Zentrallaboratorium fu¨r Geochronologie, Corrensstr. 24, 48149 Mu¨nster, Germany (brocker@nwz.uni-muenster.de) 2Department of Geological Sciences and Institute for Crustal Studies, University of California, Santa Barbara, CA 93106-9630, USA ABSTRACT In the lower main unit of the Attic-Cycladic crystalline belt (Greece), white mica geochronology (Rb–Sr, K–Ar, 40Ar–39Ar) has established the timing of at least two metamorphic events: well-preserved high- pressure/low-temperature (HP/LT) rocks yielded Eocene ages (c. 53–40 Ma) and their greenschist facies counterparts provided Oligocene–Miocene dates (c. 25–18 Ma). Marbles from Tinos Island contain high-Si phengite with Rb–Sr (phengite–calcite) and 40Ar–39Ar white mica ages between 41 and 24 Ma. All Ar age spectra are disturbed and 40Ar–39Ar total fusion ages generally are 3–6 Ma older than corresponding Rb–Sr ages. Due to the polymetamorphic history, we consider inheritance from the HP stage as the most likely cause for the complex Ar age spectra and the older 40Ar–39Ar dates. This concept also suggests that the Rb–Sr system is more sensitive to modification during overprinting than the Ar isotope system, because resetting of the Sr isotope system can be accomplished more quickly by Sr exchange with other Ca-rich phases, whereas lack of pervasive deformation and/or restricted availability of synmetamorphic fluids has favoured partial inheritance of the Ar isotope system.
    [Show full text]
  • Nomenclature of Common Metamorphic Rocks
    Appendix Nomenclature of Common Metamorphic Rocks Magmatic rocks are usually named after some locality. Only in rare cases does the rock name give any indication about the fabric and mineralogical composition of the rock. The names of magmatic rocks have to be memorized like words of a foreign language. Fortunately, this difficulty is not encountered in the nomenclature of most metamorphic rocks. It is only necessary to learn a few names of rock groups, which are characterized by a certain fabric and/or mineralogical composition. Furthermore, the presence of the main or critical minerals is indicated by placing their names in front of the group name. For instance, there is the group of marbles, all of which contain well­ crystallized carbonates as their main constituent. A particular marble may be designated as dolomite marble, diopside-grossularite marble, tremolite marble, etc. Thus the nomenclature of most metamorphic rocks is clear and easily understood. A more elaborate nomenclature based on quantitative mineralogical composition was proposed by Austrian petrographers after a discussion with colleagues from other countries.1 This no­ menclature is recommendable and is to a large extent adopted here. Names of Important Rock Groups Phyllite. Fine-grained and very finely schistose rock, the platy minerals of which consist mainly of phengite. Phengite sericite gives an overall silky sheen to the schistosity planes. The grain size is coarser than in slates but finer than in mica schists. In phyllites the amount of phyllosilicates (phengite + some chlorite ± biotite) exceeds 50%. The other most abundant constituent l"Ein Vorschlag zur quantitativen und qualitativen Klassifikation der kristallinen Schiefer" (a symposium).
    [Show full text]
  • Titanium in Phengite: a Geobarometer for High Temperature Eclogites
    View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by RERO DOC Digital Library Contrib Mineral Petrol (2010) 159:1–24 DOI 10.1007/s00410-009-0412-7 ORIGINAL PAPER Titanium in phengite: a geobarometer for high temperature eclogites Estelle Auzanneau Æ M. W. Schmidt Æ D. Vielzeuf Æ J. A. D Connolly Received: 4 July 2008 / Accepted: 13 May 2009 / Published online: 12 June 2009 Ó Springer-Verlag 2009 Abstract Phengite chemistry has been investigated in phases in HT-(U)HP metamorphic rocks, and are often experiments on a natural SiO2–TiO2-saturated greywacke preserved from regression by entrapment in zircon or garnet, and a natural SiO2–TiO2–Al2SiO5-saturated pelite, at 1.5– thus providing an opportunity to determine the T–P condi- 8.0 GPa and 800–1,050°C. High Ti-contents (0.3–3.7 wt tions of crystallization of these rocks. Two applications on %), Ti-enrichment with temperature, and a strong inverse natural examples (Sulu belt and Kokchetav massif) are correlation of Ti-content with pressure are the important presented and discussed. This study demonstrates that Ti is a features of both experimental series. The changes in com- significant constituent of phengites that could have signifi- position with pressure result from the Tschermak substitu- cant effects on phase relationships and melting rates with tion (Si ? R2? = AlIV ? AlVI) coupled with the decreasing P or increasing T in the continental crust. substitution: AlVI ? Si = Ti ? AlIV. The latter exchange is best described using the end-member Ti-phengite Keywords Phengite Á Titanium Á Thermodynamic (KMgTi[Si3Al]O10(OH)2, TiP).
    [Show full text]
  • Metamorphic Chemical Geodynamics of Subduction Zones
    Earth and Planetary Science Letters 260 (2007) 373–393 www.elsevier.com/locate/epsl Frontiers Metamorphic chemical geodynamics of subduction zones Gray E. Bebout Lehigh University, USA Received 13 March 2006; received in revised form 8 March 2007; accepted 31 May 2007 Available online 12 June 2007 Editor: A.N. Halliday Abstract Study of metamorphic suites directly representing the deep subduction of altered oceanic crust and sediments can help elucidate the geochemical evolution of the forearc-to-subarc slab–mantle interface, the nature of slab-derived fluids added to arc lava source regions, and the chemical changes in subducting rocks potentially contributing to the geochemical heterogeneity of the deeper mantle. The stage is set for more focused geochemical study of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic suites incorporating knowledge of mineral chemistry and reactions, kinetics and disequilibrium, prograde and exhumation-related P–T paths, fluid flow and fluid–rock interactions, and experimental evidence for the physical and chemical properties of fluids at the pressures and temperatures experienced by subducting slabs. Detailed studies of HP and UHP metamorphism published thus far provide some insight regarding the geochemical effects of devolatilization and the element transport capabilities of metamorphic fluids in forearc regions of subduction zones (b100 km). Future research in this rapidly developing field should prioritize study of UHP suites metamorphosed at depths N100 km, which have potentially experienced partial melting, with the goal of better characterizing fluid additions to arcs from subducting slabs and sediments and the compositions of dehydration and melting residues that could impact geochemical compositions of the mantle including source regions for ocean island basalts.
    [Show full text]