1. Monazite in lithoclasts of suevite impact breccia in the Nördlinger Ries (Bavaria, Germany) and its Th-U-Pb dating by electron probe microanalysis.
- Author
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Schulz, Bernhard, Lange, Jan-Michael, Krause, Joachim, and Czygan, Dana
- Subjects
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ELECTRON probe microanalysis , *MONAZITE , *BRECCIA , *CRYSTALLINE rocks , *SCANNING electron microscopes , *QUARTZ , *ELECTRONIC probes , *GARNET , *LEAD oxides - Abstract
In the Lehenberg (Lehberg or Limberg North) quarry in the NW part of the Megablock Zone of the Nördlinger Ries impact crater, granite and micaschist lithoclasts occur in a polymict suevite impact breccia. The lithoclasts display the petrographic characteristics of the shock metamorphism scale, as cavities filled with diaplectic glass, decorated planar elements in quartz grains, and severely kinked mica. In petrographic thin sections, monazite grains were detected by scanning electron microscope based automated mineralogy methods of spectral mapping. In backscattered electron imaging (BSE), monazite revealed the typical crystal shapes, and internal Th zoning and distribution pattern as known from igneous and metamorphic crystallisation. Intragrain signs of shock metamorphism in a minority of monazite grains are strictly straight and parallel crack pattern resembling lamellae structures. Monazite mineral chemistry and bulk chemical Th-U-Pb ages were investigated by electron probe microanalyser (EPMA). The igneous and metamorphic monazites display contrasting and typical mineral-chemical properties. Metamorphic monazites follow strictly the cheralite substitution trend in Th + U vs Ca coordinates. Igneous monazite in alkali feldspar granite has the highest Y2O3 contents (~2 wt%) among all studied samples. In ThO2* vs PbO coordinates the monazite data define isochrones. Micaschist lithoclasts yielded 328 ±3 Ma, 326 ±6 Ma and 324 ±5 Ma, interpreted to represent the thermal peak and post-peak age of metamorphic monazite crystallisation. The 328 ±5 Ma age of igneous monazite in the alkali feldspar granite in contact to micaschist is interpreted to date the crystallisation of a synmetamorphic anatectic melt. This contrasts the 313 ±3 Ma monazite crystallisation age in a post-tectonic monzogranite. No indications of bulk Pb loss in monazite by shock metamorphism have been observed. The EPMA Th-U-Pb monazite ages from the lithoclasts match data from granites and meta-psammopelites in the outcropping pre-Mesozoic basement in the Western Bohemian Massif and the Black Forest. They confirm that the bottom of the Nördlinger Ries impact crater is situated in crystalline basement rocks belonging to the Moldanubian Zone. [ABSTRACT FROM AUTHOR]
- Published
- 2023
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