Characteristics and origins of diverse Variscan peridotites in the Gföhl Nappe, Bohemian Massif, Czech Republic

An ultra-high-pressure metamorphic condition and isothermal decompression path obtained from a garnet-pyroxenite in the Horní Bory granulite of the Bohemian Massif

“…The peak P -T conditions estimated for the studied Grt -pyroxenite (~ 4.3 GPa) are slightly higher than those estimated for the Cr -rich Type II -Mg peridotite [3.8 GPa and 905 -910 °C of Medaris et al (2005) and Ackerman et al (2008)], and those estimated for the Type II -Fe peridotites and associated pyroxenites [2.1 -3.6 GPa of Medaris et al (2005) and Ackerman et al (2008)]. Medaris et al (2005) suggested that the Gföhl granulite was originally a high P melt, which subsequently ascended from the deep upper mantle regions and entrapped the mantle wall rocks at the deep upper mantle regions (Type II -Mg peridotites) and at the relatively shallow upper mantle regions (Type II -Fe peridotites). However, Ackerman et al (2008) recently proposed that SiO 2 -undersaturated melts of basaltic composition invaded the mantle at a level of ~ 100 to 110 km and then formed the Type II -Fe peridotites and associated pyroxenites during the subsequent uplift.…”

“…These P -T conditions were calculated using the following geothermometers: Cain orthopyroxene (Opx) thermometer of Brey and Köhler (1990) [BK90thermometer], garnet (Grt) -Opx thermometer of Harley (1984) [HR84], single clinopyroxene geothermobarometer of Nimis and Taylor (2000) [NT00], and amphibole -plagioclase thermometer of Blundy and Holland (1990) [BK90] in conjunction with the following geobarometers: Grt -Opx barometer of Brey and Köhler (1990) [BK90barometer] and of Nickel and Green (1985) [NG85]. Dotted areas represent P -T conditions for Type II -Mg peridotites, Type II -Fe peridotites (85GM8B), and garnetpyroxenite (HB4) at the Horní Bory quarry reported by Medaris et al (2005) and Ackerman et al (2008).…”

“…Many researchers reported ultra -high -pressure metamorphic (UHPM) conditions within coesite -and/or diamond -stability fields from Mg -and Cr -rich ultramafic rocks (e.g., Medaris et al, 2005), Fe -dunite/wehrlite (Ackerman et al, 2008), and eclogite layers and garnet -bearing pyroxenite layers within such ultramafic rocks (e.g., Nakamura et al, 2004;Medaris et al, 2006;Ackerman et al, 2008) in the Bohemian Massif. Some UHPM evidence was found from minerals/rocks such as pyrope/orthopyroxene -bearing Mg -Al rich metasedimentary rock (Kotková et al, 1997), clinopyroxene with K -feldspar lamellae in Lower Austria (Becker and Altherr, 1992), garnet with small but significant amounts of Na 2 O (= 0.18 wt%) and TiO 2 (= 0.5 wt%) in the Gföhl granulite (Vrána and Frýda, 2003), and SiO 2 inclusions in zircons with 521 cm −1 Raman spectrum extracted from the Gföhl gneiss (Kobayashi et al, 2008).…”

“…1), is considered to be a member of the disrupted fragments of the Gföhl granulite, and it contains various kinds of mafic and ultramafic rocks, such as harzburgites, lherzolites, dunites, wehrlites, olivine -pyroxenites, pyroxenites, and eclogites (Misař and Jelínek, 1981). Medaris et al (2005) classified ultramafic rocks into the following two groups: Type II -Mg peridotites have Mg -and Cr -rich bulk compositions typical for mantle -derived rocks, and Type II -Fe peridotites have Fe -and Ti -rich bulk compositions. Medaris et al (2005) interpreted such rocks as the disrupted fragments of mafic -ultramafic cumulate complex.…”

“…Medaris et al (2005) classified ultramafic rocks into the following two groups: Type II -Mg peridotites have Mg -and Cr -rich bulk compositions typical for mantle -derived rocks, and Type II -Fe peridotites have Fe -and Ti -rich bulk compositions. Medaris et al (2005) interpreted such rocks as the disrupted fragments of mafic -ultramafic cumulate complex. On the other hand, Ackerman et al (2008) proposed that Type II -Mg peridotites represent the intact fragments of the upper mantle from a depth of ~ 120 km.…”