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  • Author or Editor: Zsolt Bendő x
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The Csódi-hegy laccolith belongs to the Middle Miocene Börzsöny and Visegrád Andesite Formation, and is built up by fluidal amphibole biotite dacite. This is a small, typical laccolith as shown by its size (Dl = 11000-1200 m, D2 = 900-1000 m, A ~ 150 m), shape, areal extent of 1 km2 and aspect ratio of 0.08 to 0.14. The fluidal structure of the laccolith is the result of alternation of dark and light bands. The thickness of the bands is in cm-dm scale. This structure and the feeding channels are very well exposed here thanks to quarrying activity. The time span-calculation was performed by counting of the individual bands, by volume estimations and by detailed magnetostratigraphic study. For the calculation of magma transport and growing-time the following assumptions were made: 6-8 km depth of the magma chamber, pressure of 8-10 kbars, temperature of 850-900 °C, a rate of ascent 1 mm/20-36 hours, 15 cm as the average width of a single band, a relative depth for emplacement about 450 m and the volume of the emplaced body of 0.05 km3. A total of 3150 bands were counted along the 330 m-long key section of the laccolith, from the center of the main feeding channel toward both flanks of the body. In this way the calculated total emplacement time for the formation of the Csódi-hegy laccolith varies between 1,100 and 1,900 years. The final episodic kink stage may range from 3 to 6 years and the cooling stage have lasted about 100 years. Volume estimations resulted in similar time spans, varying between 1,800 and 3,300 years. The rapid change from transitional to reverse magnetic polarity may cover some hundreds to some thousands of years.

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Ringwoodite, produced by shock metamorphism, is common in and adjacent to melt veins in highly shocked chondrites. Although ringwoodite can be crystallized from the silicate melt in the shock-veins or pockets, a major part of the easily observed ringwoodite in shock veins is formed by the transformation of olivine in host-rock fragments entrained in the melt or olivine along shock-vein margins. In this paper we examine the microstructures and textures of ringwoodites from NWA 5011 L-chondrite in order to better understanding the transformation mechanisms of ringwoodite by optical microscope. Finally, we attempt to locate the source region of L-type chondrites in three different impact scenarios of the L parent body.

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