Permian A-type rhyolites of the Muráň Nappe, Inner Western Carpathians, Slovakia: in-situ zircon U–Pb SIMS ages and tectonic setting

Three representative A-type rhyolitic rock samples from the Muráň Nappe of the inferred Silicic Unit of the Inner Western Carpathians (Slovakia) were dated using the high-precision SIMS U–Pb isotope technique on zircons. The geochronological data presented in this paper is the first in-situ isotopic dating of these volcanic rocks. Oscillatory zoned zircon crystals mostly revealed concordant Permian (Guadalupian) ages: 266.6 ± 2.4 Ma in Tisovec-Rejkovo (TIS-1), 263.3 ± 1.9 Ma in Telgárt-Gregová Hill (TEL-1) and 269.5 ± 1.8 Ma in Veľká Stožka-Dudlavka (SD-2) rhyolites. The results indicate that the formation of A-type rhyolites and their plutonic equivalents are connected to magmatic activity during the Permian extensional tectonics and most likely related to the Pangea supercontinent break-up.

Existence de filons ignimbritiques au sud de la caldera de Maure-Vieille, Esterel oriental (Alpes-Maritimes)

The southern flank of the Maure-Vieille caldera of the Esterel massif of the Maritime Alps in southeastern France is remarkable for the large number of ignimbrite dikes in the series of alternating sedimentary and volcanic rocks of Permian age. Three types of dikes occur. The first is a rhyolitic rock poor in quartz and sanidine phenocrysts, but with abundant oriented laths consisting of a spherulitic quartz-potassic feldspar nucleus enclosed in an aureole of feldspar dusted with iron oxide. These 'fiammes' are devitrified fragments of pumice. The second type is of similar material but is brecciated in fragments cemented by a rhyolitic matrix. The third type is vitroclastic in structure with iron oxide inclusions, but completely devitrified. The rhyolite of the area is the same chemically as the ignimbrite, and the dikes of ignimbrite represent pyromagmatic fragmentation occurring in the conduits through which the rhyolite was erupted.

AN EXAMPLE OF THE INFLUENCE OF REMANENT MAGNETIZATION ON MAGNETIC INTENSITY MEASUREMENTS

Near Ironton, Missouri, measurements of the relative vertical intensity of the earth’s magnetic field were made along an 1,800‐ft line over an intrusive rhyolitic rock unit. Sharp variations of the order of 2,500 gammas were obtained along the sample line. These variations cannot be explained on the basis of induced magnetization alone. However, paleomagnetic measurements made on samples collected coincident to the line of magnetic measurements reveal great inhomogeneities in the magnetic properties: the magnetic susceptibility varies between [Formula: see text] and [Formula: see text] units; the ratio of the intensity of remanent magnetization to the intensity of induced magnetization varies from 0.21 to 26.91; the direction of remanent magnetization varies from sample to sample and is significantly different from the direction of the present geomagnetic field; and the polarity of the samples is both normal and reversed. A theoretical magnetic profile, calculated on the basis of the direction of total magnetization in the individual samples along the sample line, has the same gross features as the measured profile. The improved correlation which results upon consideration of the total magnetization properties of the rock unit emphasizes the necessity for considering the effects of remanent magnetization when interpreting magnetic intensity measurements.