Marschall et al, 2003 simplectitas comparación

Marschall et al, 2003 simplectitas...

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JOURNAL OF PETROLOGY VOLUME 44 NUMBER 2 PAGES 227–253 2003 P T Evolution of a Variscan Lower-Crustal Segment: a Study of Granulites from the Schwarzwald, Germany H. R. MARSCHALL , A. KALT† AND M. HANEL MINERALOGISCHES INSTITUT, UNIVERSITA ¨ T HEIDELBERG, IM NEUENHEIMER FELD 236, D-69120 HEIDELBERG, GERMANY RECEIVED NOVEMBER 27, 2001; REVISED TYPESCRIPT ACCEPTED AUGUST 2, 2002 Pressure–temperature–time ( P T t ) paths of orogenic granulites INTRODUCTION provide important information on the thermal and chemical structure General background of the lower continental crust through time, and constraints on tectonic Granulites are typical rocks of the Earth’s middle to processes. We present the first detailed petrological investigation of lower crust under high-temperature conditions. They are granulites from the Variscan Schwarzwald. Pelitic granulites from the found as xenoliths in basaltic volcanic rocks, mainly Central Schwarzwald Gneiss Complex (CSGC) are characterized by within continental rifts, but most granulites occur as the peak assemblage garnet + rutile + kyanite + antiperthite complexes or terranes in orogenic settings. Orogenic ± quartz. Felsic to intermediate granulites from the Southern granulites display a wide compositional range (Harley, Schwarzwald Gneiss Complex (SSGC) exhibit di ff erent peak as- 1989, 1998) and are known from a variety of collisional semblages with clinopyroxene, orthopyroxene, ternary feldspar, garnet, belts that formed during di ff erent episodes since the quartz and sillimanite, and manifold retrograde reaction textures. Archaean (e.g. Harley, 1998; Clarke et al. , 2000; Mo ¨ller Peak P–T conditions were calculated by two-feldspar thermometry, et al. , 2000). Determination of bulk-rock and mineral garnet orthopyroxene thermometry and various geobarometers. Mini- compositions, calculation of peak equilibration con- mum estimates for peak conditions are 950 1010 ° C and 1·4 1·8 ditions, and dating of orogenic granulites thus provide us GPa for the granulites of the CSGC, which followed a clockwise with important constraints on the thermal and chemical P–T path. The retrograde path is characterized by initial isothermal structure of the Earth’s continental crust at di ff erent decompression, associated with partial melting, followed by isobaric geological times. cooling. Peak conditions for the SSGC are 1015 ° C and 1·5 GPa Harley (1989) showed that equilibration conditions (minimum temperature, maximum pressure). No prograde relics are deduced from natural orogenic granulites cover a wide preserved, and isothermal decompression was less pronounced than range. In particular, pressures are highly variable, and in the CSGC. Other Variscan HP HT granulites from Central the granulite field may be divided into a low-pressure Europe show similar lithologies, equilibration temperatures and (LP), a medium-pressure (MP) and a high-pressure (HP) ages (340 335 Ma). The heat for widespread high-temperature facies according to Green & Ringwood (1967). Peak
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