Phase relations during peak metamorphism and decompression of the UHP kyanite eclogites, Pohorje Mountains (Eastern Alps, Slovenia)

Vrabec, Mirijam; Janák, Marian; Froitzheim, Nikolaus; Hoog, Jan C.M. De

doi:10.1016/j.lithos.2012.04.004

Cited by 36 publications

(27 citation statements)

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“…Peak metamorphic P–T conditions of different UHP lithologies in the Pohorje Mountains: GN ‐gneiss (Janák et al ., ; this study), EC ‐eclogite (Janák et al ., ; Vrabec et al ., ), GP ‐garnet peridotite (Janák et al ., ) and exhumation P–T path of diamond(Dia)‐bearing gneiss based on this study (dashed arrow) and previous studies (full arrows, Janák et al ., ; Hurai et al ., ). Transition from diamond to graphite (Gr) according to Day ().…”

Section: Discussionmentioning

confidence: 97%

“…Identification of in situ metamorphic diamond in metasedimentary crustal rocks following previous microtextural and thermobarometric evidence from eclogites and garnet peridotites (Janák et al ., , ; Vrabec et al ., ) provides compelling evidence for UHP metamorphism in the Pohorje Mountains. Thermobarometric data on Pohorje rocks record peak‐pressure conditions of up to 3.5–3.7 GPa at 800–850 °C in eclogites (Vrabec et al ., ) and 4.0 GPa at 900 °C in garnet peridotites (Janák et al ., ).…”

Section: Discussionmentioning

confidence: 99%

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Diamond in metasedimentary crustal rocks from Pohorje, Eastern Alps: a window to deep continental subduction

Janák

Froitzheim

Yoshida

et al. 2015

Journal Metamorphic Geology

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We report the first finding of diamond and moissanite in metasedimentary crustal rocks of Pohorje Mountains (Slovenia) in the Austroalpine ultrahigh‐pressure (UHP) metamorphic terrane of the Eastern Alps. Microscopic observations and Raman spectroscopy show that diamond occurs in situ as inclusions in garnet, being heterogeneously distributed. Under the optical microscope, diamond‐bearing inclusions are of cuboidal to rounded shape and of pinkish, yellow to brownish colour. The Raman spectra of the investigated diamond show a sharp, first order peak of sp3‐bonded carbon, in most cases centred between 1332 and 1330 cm−1, with a full width at half maximum between 3 and 5 cm−1. Several spectra show Raman bands typical for disordered graphitic (sp2‐bonded) carbon. Detailed observations show that diamond occurs either as a monomineralic, single‐crystal inclusion or it is associated with SiC (moissanite), CO2 and CH4 in polyphase inclusions. This rare record of diamond occurring with moissanite as fluid‐inclusion daughter minerals implies the crystallization of diamond and moissanite from a supercritical fluid at reducing conditions. Thermodynamic modelling suggests that diamond‐bearing gneisses attained P–T conditions of ≥3.5 GPa and 800–850 °C, similar to eclogites and garnet peridotites. We argue that diamond formed when carbonaceous sediment underwent UHP metamorphism at mantle depth exceeding 100 km during continental subduction in the Late Cretaceous (c. 95–92 Ma). The finding of diamond confirms UHP metamorphism in the Pohorje Mountains, the most deeply subducted part of Austroalpine units.

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Section: Discussionmentioning

confidence: 97%

Section: Discussionmentioning

confidence: 99%

Diamond in metasedimentary crustal rocks from Pohorje, Eastern Alps: a window to deep continental subduction

Janák

Froitzheim

Yoshida

et al. 2015

Journal Metamorphic Geology

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“…The formation of sapphirine, spinel and corundum as a breakdown products of kyanite is commonly reported from decompressed kyanite eclogites (e.g. Liati and Seidel, 1996;Godard and Mabit, 1998;Elvevold and Gilotti, 2000;Janák et al, 2012;Vrabec et al, 2012). These reactions require also an input of Fe and Mg from breakdown of other (U)HP phases such as garnet and omphacite, formation of orthopyroxene is considered to be evidence of high-temperature, granulite facies overprint (e.g.…”

Section: P-t Evolution Of Kyanite Eclogitementioning

confidence: 99%

UHP metamorphism recorded by kyanite-bearing eclogite in the Seve Nappe Complex of northern Jämtland, Swedish Caledonides

et al. 2013

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“…Pohorje massif was subjected to metamorphism in the Late Cretaceous (ca. 95-92 Ma; Thöni, 2002;Miller et al, 2005;Janák et al, 2009) when these units reached conditions within stability field of diamond (Janák et al, 2004(Janák et al, , 2006(Janák et al, , 2015Vrabec et al, 2012). The major exhumation of the Pohorje nappe, from ultrahigh pressure depth to mid crustal levels most probably occurred already during the Upper Cretaceous, similarly to the Koralpe area, the north-westward extension of the Pohorje nappe, where Upper Cretaceous (75-70 Ma) cooling ages were determined (Schuster et al, 2004).…”

Section: Geological Setting and Sample Locationmentioning

confidence: 99%

Calcite deformation twins in Pohorje marbles

Nastja¹,

Vrabec²

2018

Geologija

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Marbles in Pohorje occur in lenses and smaller bodies in the southern and southeastern part of the massif. Marbles are very pure, predominantly calcitic and rarely calcitic-dolomitic, containing a maximum of 5 % of non-carbonate mineral phases. The latter comprise pyroxenes (diopside), amphiboles (tremolite), olivines (forsterite) in places replaced by serpentine, quartz, feldspars (potassium feldspars and plagioclases), epidote, zoisite, vesuvianite, scapolite, muscovite, biotite partly replaced by chlorite, phlogopite, rare grains of titanite, rutile, zircone, apatite, and small grains of ferric oxides and sulfides. Calcite exhibits intensive deformational e-twinning whereas dolomite is undeformed and untwined. All four known types of mechanical twins in calcite were recognized: thin Type I twins, straight thick Type II twins, curved, lensoid and tapered thick Type III twins, and thick patchy Type IV twins. Type III twins are the dominant mechanical twins in the Pohorje marbles indicating the temperature of deformation somewhat above 200 °C. Since they lack signs of grain boundary recrystallization, we assume that the twinning was followed by a decrease temperature during exhumation. With increasing temperature the process of recrystallization along calcite grain becomes pronounced. Small individual untwined calcite crystals are progressively replacing bigger calcite grains. In few examples second generation of Type I deformational twins develop in recrystallized calcite grains, which also implies lowering of temperature due to exhumation. IzvlečekPohorski marmor se pojavlja v lečah in manjših telesih v južnem in jugovzhodnem delu pogorja. Marmorji so po sestavi zelo čisti, skoraj izključno kalcitni, izjemoma kalcitno-dolomitni in vključujejo največ do 5 % nekarbonatnih mineralnih faz. Te vključujejo piroksene (diopsid), amfibole (tremolit), olivine (forsterit) mestoma nadomeščen s serpentinom, kremen, glinence (kalijeve glinence in plagioklaze), epidot, zoisit, vezuvianit, skapolit, muskovit, biotit deloma nadomeščen s kloritom, flogopit, redka zrna titanita, rutila, cirkona in apatita ter majhna zrna železovih oksidov in sulfidov. Kalcitni kristali so močno deformirani z mehanskim dvojčenjem, dolomitni kristali pa so nedeformirani in brez dvojčkov. Našli smo vse štiri znane tipe mehanskih dvojčkov v kalcitu: tanke dvojčke tipa I, ravne in debele dvojčke tipa II, zakrivljene, lečaste in zašiljene dvojčke tipa III, ter debele krpaste dvojčke tipa IV. Prevlada dvojčkov tipa III v pohorskem marmorju nakazuje temperaturo deformacije nekoliko nad 200 °C. Ker ti dvojčki ne kažejo znakov rekristalizacije na robovih zrn, sklepamo, da je dvojčenju sledilo znižanje temperature zaradi ekshumacije kamnin. Z višanjem temperature postane bolj izrazit proces rekristalizacije vzdolž robov kalcitnih zrn. Velika kalcitna zrna so pri tem vedno bolj nadomeščena z majhnimi kristali brez mehanskih dvojčkov. V nekaj primerih so v rekristaliziranih kalcitnih zrnih nastali mehanski dvojčki tipa I druge generacije, kar tudi na...

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Phase relations during peak metamorphism and decompression of the UHP kyanite eclogites, Pohorje Mountains (Eastern Alps, Slovenia)

Cited by 36 publications

References 84 publications

Diamond in metasedimentary crustal rocks from Pohorje, Eastern Alps: a window to deep continental subduction

Diamond in metasedimentary crustal rocks from Pohorje, Eastern Alps: a window to deep continental subduction

UHP metamorphism recorded by kyanite-bearing eclogite in the Seve Nappe Complex of northern Jämtland, Swedish Caledonides

Calcite deformation twins in Pohorje marbles

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