Shah Wali Faryad

Mechanism of formation of atoll garnet during high-pressure metamorphism

Abstract Atoll garnet has been found in metabasites and quartz- and mica-rich rocks that have experienced low to medium-temperature, high-pressure eclogite facies metamorphism in the Krušné Hory (Erzgebirge). They occur in several localities but are restricted to thin, texturally distinct zones, even on a thin-section scale. The mechanism of atoll garnet formation is documented by a series of micrographs and compositional maps and profiles of atoll garnet in combination with textural relations to other phases in the rocks. The core of full garnet or its relics in the atoll garnet have larger Ca and Fe, but smaller Mg contents, compared with the thin rim (ring). In addition to quartz, Na-Ca amphibole and phengite, the atoll cores are filled by a new garnet that has a composition similar to the outer rim. Formation of the atoll garnet is interpreted as resulting from fluid infiltration and element exchange between the garnet core and matrix, a process facilitated by a temperature increase during eclogite facies metamorphism. In addition to fluid access, the primary textures, mainly grain size, were also effective for the atoll garnet formation. Small grain fractions with thin rims were easily infiltrated by fluid, which used the short distance for element exchange between core and matrix. The core garnet was gradually dissolved and replaced by new garnet having the same crystallographic orientation as the rim or relics in the core.

1 Frontiers of Ultrahigh-Pressure Metamorphism: View from Field and Laboratory

Ultrahigh-pressure metamorphism (UHPM) is a relatively new but fast-growing discipline related to the deep subduction of continental and/or oceanic crust into the Earth’s mantle and their return toward the surface as fragments incorporated within orogenic belts bordering active continental margines. The discipline was established ∼25 years ago after the discoveries of high-pressure minerals, coesite, and diamond in the rocks of the continental affinities, a place where such minerals were “forbidden” according to the existing mainstream geological concepts at that time. Since then several new directions in studying UHPM terranes were created, and new approaches for studies of microstructures of minerals “transformed” by decompression were formulated, discovered at first-time nanometric fluid inclusions in microdiamonds, developed diamond synthesis from unusual combination of C+H 2 O at high P – T conditions, made discoveries of new UHPM terranes, and new minerals, developed new applications of advanced analytical instruments to study solid and fluid inclusions of nanometric scale with high-resolution electron microscopy, nanoscale secondary ion mass spectrometry (nanoSIMS), X-ray in situ , and infrared (IR) spectroscopy assisted with synchrotron technologies and others. This chapter provides summary of main achievements in UHPM researches and formulates further directions.

High-pressure polymetamorphic garnet growth in eclogites from the Mariánské Lázně Complex (Bohemian Massif)

The Marianske-Lazně Complex is a Cambro-Ordovician terrane with metabasites of oceanic-crust affinity and tectonically emplaced between the Saxothuringian Zone and the Tepla-Barrandian Unit (western part of the Bohemian Massif). It is formed by amphibolite (retrogressed after eclogite), serpentinized peridotite, coronitic metagabbro with paragneiss, and felsic orthogneiss. Available geochronological data support Variscan ages for the eclogite-facies metamorphism and subsequent amphibolite-facies reequilibration. Garnet crystals with multi-stage growth features in eclogite were studied to analyse the relation of their chemical zoning with pressure–temperature ( P – T ) changes and/or reactions among coexisting phases during metamorphism. The core garnet has highly contrasted compositions with high Mg and low Ca compared to the surrounding host-garnet crystal. Compositional zoning and crystallographic orientation of the garnet showed that the cores and rims were formed during two different metamorphic events. In addition to conventional geothermobarometry, pseudosection and garnet isopleths were used to estimate P T conditions for both metamorphic events. The older core (garnet I) indicates high-pressure amphibolite facies and the younger rim (garnet II) indicates eclogite-facies conditions. Garnet (II) contains inclusions of Na–Ca amphibole and omphacite, suggesting a prograde metamorphism from blueschist- to eclogite-facies conditions. Full multicomponent-diffusion modelling of compositional zoning at the interface of amphibolite-/eclogite-facies garnets in conjunction with the retrieved P – T paths were used to evaluate the average heating/cooling rates during the eclogite-facies event. These two metamorphic events deciphered from different garnet generations are consistent with available geochronological data and bring new insight into the subduction history of the Saxothoringian and/or Rehic oceanic basins during the Variscan orogeny in the Bohemian Massif.

Two contrasting mineral assemblages in the Meliata blueschists, Western Carpathians, Slovakia

Abstract Low-grade metamorphic rocks from the Meliata unit (Western Carpathians) are characterized by the presence of typical blueschist-facies minerals. In metabasalt, an early low-pressure assemblage (<0.5 GPa at 350°C), characterized by muscovite and zoisite, is followed by high-pressure glaucophane, phengite, Na-pyroxene, chlorite, clinozoisite and Al-poor titanite, indicating pressures of >1.2 GPa at 450°C. Na-pyroxene shows strong compositional variations between the end-members Jd4-70, Aeg10-49 and Q17-49, respectively. Phengite has high Si content of 3.5 a.p.f.u. The zoisite with Al2Fe (100[Fetot/(-2+Altot+Fetot)]) = 3-5%, is rimmed by clinozoisite, with a maximum of 75% Al2Fe, as well as being enclosed by glaucophane. The occurrence of clinozoisite, rimming zoisite, suggests that the transformation of orthorhombic to monoclinic epidote depends not only on the temperature but also on the pressure. In the studied metabasalt, retrograde phases reflecting greenschist-facies conditions are albite and chlorite. Some neighbouring metabasites may additionally contain actinolite and biotite.