A Clockwise P-T Path from the Variscan Basement of the Tisza Unit, Pannonian Basin, Hungary

The polymetamorphic basement of the Tisza Unit forms a detached fragment of the Variscan European foreland of the Neotethyan realm. A clockwise evolution path of a gneiss-amphibolite complex of the Tisza Unit was reconstructed, investigating the polymetamorphic rocks of the borehole Baksa-2, SE Transdanubia, Hungary. The results obtained by microstructural and mineral paragenetic observations, mineral chemical analyses, and thermobarometric calculations define a P-T loop which suggests a complex Variscan polyphase model rather than a pre-Variscan - Variscan polycyclic one. The early part of the prograde path with kyanite is characterized by T-P conditions of 480±50°C and 470±70 MPa, respectively. The metamorphism reached its peak at 660±25°C and 750±50 MPa, when both kyanite and staurolite were stable. This metamorphic climax was followed by a nearly isothermal decompression to 440±20 MPa at 650±40°C. This event is marked by the presence of sillimanite and a second generation of garnet, and is closely related to the collisional Variscan granitoid magmatism observed in considerable parts of the Tisza Unit. In amphibolites intercalated with gneisses, only this last event was preserved, providing T-P estimates of ca. 650-690°C/400-500 MPa. The present paper provides the first demonstration of a continuous, clockwise P-T path from the metamorphic basement of the Hungarian part of the Tisza Uni

Very low-grade metamorphism of sedimentary rocks of the Meliata unit; Western Carpathians, Slovakia: implications of phyllosilicate characteristics.

International audienc

Temperature and age constraints on the metamorphism of the Tethyan Himalaya in Central Nepal: A multidisciplinary approach

International audienceMetasediments of Devonian to Triassic age of the Tethyan Himalaya (TH) from several areas in central and western Nepal, between western Dolpo and Marsyandi Valley, were sampled for thermo-metamorphic studies (illite and chlorite "crystallinity", vitrinite reflectance, calcite-dolomite and chlorite-chloritoid geothermometers, K/Ar dating on illite-rich fractions and zircon fission track thermochronology). This paper reports, for the first time, the occurrence of chloritoid in the TH out of the range of granite contact aureoles. It also presents the first zircon fission track dating performed on TH metasediments. The peak temperatures of metamorphism have been around 250-300 °C, 320-350 °C, 330-370 °C and 400-450 °C in the western Dolpo, Hidden Valley, Manang and Marpha areas, respectively. In the Manang and Hidden Valley areas, illite K/Ar data are interpreted as ages of recrystallized K-white micas newly formed during metamorphism at around 25-30 Ma. In the Marpha area, illite K/Ar and zircon fission-track ages (12-15 Ma) are consistent representing cooling ages after metamorphic overprinting of higher grade than in the other areas. The joint investigation of the organic maturation and zircon FT chronology yields insight on the thermal calibration of zircon reset. The Rmax of 5.7% to ∼8.0% indicates a temperature range of ca. 315-325 °C which is in the partial annealing zone of the zircon FT thermochronometer. Our results might be explained by the presence of a thick thrust sheet once existing above the study area

Strain-related differences in the crystal growth of white mica and chlorite: a TEM and XRD study of the development of metapelitic microfabrics in the Southern Uplands thrust terrane, Scotland

TEM and XRD techniques were used to study crystal growth characteristics of the fabric-forming phyllosilicates which developed in response to low-grade metamorphism and tectonic imbrication in part of the Southern Uplands thrust terrane. Prograde regional metamorphism, ranging from late diagenesis through the anchizone to the epizone, was accompanied by the development of a slaty cleavage which is commonly bedding-parallel. TEM-measured mean thicknesses of white mica and chlorite crystallite populations increase with advancing grade and correlate with XRD-measured crystallinity indices. Analytical TEM data show that prograde changes in composition lead to a net loss of Si, Ca and minor Fe from the fabric-forming phyllosilicates. White micas are paragonite-poor phengites with a mean b lattice parameter of 9.037 Å, and indicate an intermediate pressure series of metamorphism with a field gradient of <25° C km -1 . Chlorite compositions evolved from diabantite (with intergrown corrensite) to ripidolite over an estimated temperature range of 150–320° C. Field gradient and temperature estimates suggest that crystal growth and fabric development occurred at burial depths ranging from 6 km to at least 13 km in the thrust terrane. During late diagenesis, crystal growth of white mica and chlorite was predominantly a consequence of polytypic and phase transitions, and resulted in similar size distributions which resemble typical Ostwald ripening curves. Under anchizonal and epizonal conditions, white mica grew more rapidly than chlorite because of its greater ability to store strain energy and recover from subgrain development; as a result crystal thickness distributions are not typical of Ostwald ripening. In contrast, chlorite crystals which grew under these conditions developed subgrain boundaries at high strain rates which were only partially recovered at low strain rates; these retained dislocations reduce the crystallite thicknesses detected by TEM and XRD, compared with those of white mica. These differences in strain-induced crystal growth indicate that white mica (illite) and chlorite crystallinity indices are likely to show significant differences where low-grade metamorphism is closely associated with tectonic fabric development.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74144/1/j.1525-1314.1995.tb00243.x.pd