The Pressure–Temperature Path and the Origin of Phlogopite in Spinel–Garnet Peridotites from the Blanský Les Massif of the Moldanubian Zone, Czech Republic

JOURNAL OF PETROLOGY VOLUME 50 NUMBER 10 PAGES 1795^1827 2009 doi:10.1093/petrology/egp052 The Pressure^Temperature Path and the Origin of Phlogopite in Spinel^Garnet Peridotites from the Blansky¤ Les Massif of the Moldanubian Zone, Czech Republic KOSUKE NAEMURA1*, TAKAO HIRAJIMA1 AND MARTIN SVOJTKA2 1 DEPARTMENT OF GEOLOGY AND MINERALOGY, GRADUATE SCHOOL OF SCIENCE, KYOTO UNIVERSITY, KYOTO 606-8502, JAPAN 2 INSTITUTE OF GEOLOGY, ACADEMY OF SCIENCES OF THE CZECH REPUBLIC, V.V.I., ROZVOJOVA¤ 269, 16502 PRAHA 6, CZECH REPUBLIC RECEIVED MAY 29, 2006; ACCEPTED JULY 23, 2009 A new lithotype of peridotite, phlogopite- and apatite-bearing spinel^ garnet peridotite, associated with leucocratic granulite, has been recognized at the Ples› ovice quarry in the Gfo« hl Unit within the Moldanubian Zone of the Bohemian Massif, Czech Republic. There are three equilibrium stages in the Ples› ovice peridotite. The existence of Stage I, the precursor spinel  garnet peridotite stage, is supported by the presence of an aluminous (Al2O3  30 wt %) orthopyroxene megacryst in the matrix. The minimum temperature of Stage I was estimated to be 1020 158C. Stage II is defined by the cores of relatively large (53 mm long) grains of olivine, low-Al orthopyroxene (Al2O3  13^17 wt %), clinopyroxene, and chromian spinel [Cr/(Cr þ Al) ¼ 050^057], along with relatively small (51mm long) Ba-rich phlogopite (BaO ¼10^40 wt %), Sr-rich apatite (SrO 17 wt %) and rare potassic (K2O 09^ 12 wt %) amphibole. Garnet generally occurs as large spheroidal grains (up to 20 mm in diameter). It contains inclusions of olivine, orthopyroxene, chromian spinel, and phlogopite, all of which have similar compositions to their matrix counterparts. Therefore, garnet appears to be in equilibrium with the matrix phases at Stage II. Application of appropriate geothermobarometers to the assemblage at Stage II yielded temperatures of 850^10308C and pressures of 23^35 GPa. Stage III is defined by aluminous orthopyroxene (Al2O3  21^40 wt %), aluminous clinopyroxene and aluminous spinel along with pargasitic amphibole and Ba-rich phlogopite in kelyphite; temperature conditions at this stage were estimated to be *Corresponding author. E-mail: 730^770 ( 27)8C at 08^15 GPa. Multiphase solid inclusions, mainly composed of phlogopite, dolomite, apatite and calcite with minor amounts of chlorite and magnesiohornblende, are present only within large grains of chromian spinel, which are surrounded by kelyphites. The idiomorphic outline of the multiphase solid inclusions suggests that frozen remnants of carbonatite melts or supercritical fluids were trapped in the spinel. The mineral assemblage in the multiphase solid inclusions suggests relatively low-P and low-T conditions (T57508C; P516 GPa) for its crystallization. Furthermore, the timing of the crystallization of the multiphase solid inclusions appears to predate Stage II, as most multiphase solid inclusions are completely surrounded by the host chromian spinel. These data suggest that the Ples› ovice peridotite experienced cooling after Stage I and was then transformed to spinel^garnet peridotite by subsequent subduction processes. KEY WORDS: garnet; metasomatism; peridotite; phlogopite; P^T conditions I N T RO D U C T I O N The association of Mg- and Cr-rich orogenic peridotites and felsic metamorphic rocks in orogenic belts suggests that subducted felsic rocks have interacted with mantle ß The Author 2009. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@ oxfordjournals.org JOURNAL OF PETROLOGY VOLUME 50 materials during collisional events (e.g. Brueckner & Medaris, 2000). These spinel- and/or garnet-bearing orogenic peridotites record some important aspects of crust^ mantle interactions. First, they generally record pressure and temperature (P^T) conditions that are more extreme than those of their country rocks. Furthermore, they often contain polyphase mineral assemblages that allow reconstruction of their P^T paths. The resultant P^T paths for the peridotites provide information about their provenance and the mechanism of their emplacement into the country rocks. Another important aspect of orogenic peridotites is that they sometimes contain metasomatic phases such as phlogopite, apatite (Zanetti et al., 1999; Morishita et al., 2003) and amphibole (Rampone & Morten, 2001), and in more extreme cases dissakisite (Mg end-member of allanite) (Tumiati et al., 2003) and thorianite (Th oxide) (Naemura et al., 2008). These minerals have been considered as key products of crust^mantle interaction. Furthermore, recent studies of orogenic peridotites have reported the occurrence of multiphase solid inclusions consisting of phlogopite, amphibole, apatite, dolomite, and magnesite in spinel and garnet (Zaccarini et al., 2004; Carswell & van Roermund, 2005; Malaspina et al., 2006). These multiphase solid inclusions are considered to be possible remnants of fluids or melts that have been entrapped by mantle rocks in a subduction zone setting (e.g. Ferrando et al., 2005; Malaspina et al., 2006). To investigate these issues, we focused on a spinel^garnet peridotite body hosted by leucocratic granulites within the Blansky¤ les granulite massif, which is the largest granulite body of the Gfo«hl Unit in the Bohemian Massif. We present a thermobarometric study of the Ples› ovice peridotite complemented by detailed petrographic descriptions that give special attention to the textures and chemical compositions of phlogopite and multiphase solid inclusions. Our results reveal that the Ples› ovice peridotite is a new type in the context of P^T paths for the garnet peridotites in the Bohemian Massif and that it was metasomatized several times during its development. GEOLOGIC A L S ET T I NG The Moldanubian Zone of the Bohemian Massif (Fig. 1a) is divided into three tectonostratigraphic units (Fig. 1b): (1) the basal Monotonous Unit; (2) the Varied Unit; (3) the overlying Gfo«hl Unit (Fuchs, 1986; Matte et al., 1990). The Gfo«hl Unit mostly consists of garnet  kyanite^ mesoperthite-bearing felsic granulites, subordinate mafic pyroxene-bearing granulites, garnet pyroxenites, garnet/ spinel peridotites and eclogites. The garnet peridotites occur sporadically both in granulites and migmatitic gneisses at many localities in the Gfo«hl Unit, and have been classified into three types by Medaris et al. (2005). Type I peridotites, which generally occur within the NUMBER 10 OCTOBER 2009 granulites, show a low-P/high-T regime and may represent asthenospheric mantle materials; large grains of garnet enclosing Cr-rich spinel are common. Type II peridotites may represent disrupted fragments of a mafic^ultramafic cumulate complex. They form decimeter- to meter-scale boudins in the granulites. Type III peridotites, which occur within host migmatitic gneiss, show a medium P/T regime and may represent fragmen (...truncated)