Zeilmarken Im Variszischen Kollisionsbereich Des
Total Page:16
File Type:pdf, Size:1020Kb
Nr. 7S Dirk M arheine Zeilmarken im variszischen Kollisionsbereich des Rhenoherzynikums - SaxOlhuIingikums zwischen Harz und Sächsischem Granulitmassiv - Ergebni sse von K1Ar-Altersbeslimmungen GÖTTINGER ARBEITEN ZUR GEOLOGIE UND PALÄONTOLOGIE Nr. 75 Dirk Marheine Zeitmarken im variszischen Kollisionsbereich des Rhenoherzynikums - Saxothuringikums zwischen Harz und Sächsischem Granulitmassiv - Ergebnisse von K/Ar-Altersbestimmungen 1997 Im Selbstverlag der Geologischen Institute der Georg-August- Uni versität Göttingen Göttinger Arb. Geo!. Paläont. 97 S., 50 Abb., 8 Tab. Göttingen, 17. 11.1997 ••• 75 Dirk Marheine Zeitmarken im Kollisionsbereich des Rhenoherzynikums - Saxothuringikums zwischen Harz und Sächsischem Granulitmassiv - Ergebnisse von KIAr-AItersbestimmungen Als Dissertation eingereicht am 15.05.1997 bei den Mathematisch-Naturwissenschaftlichen Fakultäten der Georg-August-Universität erscheinen in unregelmäßiger Folge im Selbstverlag der Geologischen Institute der Georg-August-Universität Gättingen: Institut und Museum für Geologie und Paläontologie Institut für Geologie und Dynamik der Lithosphäre are issued irregularily by the Geological Institutes (until 1985: Geol.-Paläont. Inst.) of Gättingen University: Institute and Museum for Geology und Palaeontology Institute of Geology and Dynamics of the Lithosphere Redaktion Dr. Helga Uffenorde Institut und Museum für Geologie und Paläontologie Goldschmidt-Str. 3 D - 37077 Gättingen ISS N 0534-0403 @ Geologische Institute, Universität Gättingen. Offsetdruck KINZEL, Gättingen MARHEINE, D. (1997): Zeitmarken im variszischen Kollisionsbereich des Rhenoherzynikums - Saxothu- ringikums zwischen Harz und Sächsischem Granulitmassiv - Ergebnisse von KlAr-Altersbestirnmungen. [Timemarks in the Variscan collision zone of the Rhenohercynian - Saxothuringian in the region of the Harz Mountains and the Saxonian Granulite Massif - Results of KlAr-age determinations.] - Gättinger Arb. Geol. Paläont., 75: 97 S., 50 Abb., 8 Tab.; Gättingen. KlAr dating on fine mineral fractions of very low to low grade, pre- to synorogenic sediments in the border area of the Rhenohercynian - Saxothuringian yield informations about the tectonometamorphic evolution during the Variscan orogeny. Combined use of illite crystallinity and illite polytype determinations made it possible to prove and separate polyphase overprinting. Supplementary KlAr-age data of detrital white micas indicate potential provenances for the Variscan sediments. KlAr dating of some crystalline complexes were carried out in order to discriminate these units as potential source areas. The tectonometarnorphic evolution in the investigated area spans aperiod of time from 350 Ma to about 310 Ma. A Permian post-kinematic thermal event in the Harz Mountains is dated at about 275 Ma. geodynamic evolution, geochronology, illite crystallinity, illite polytypes, low grade metamorphism D. Marheine, URA CNRS 1763, Lab. Geochron., Univ. 11,Place E. Bataillon, F - 34095 Montpellier SUMMARY The border between the Rhenohercynian and Saxothuringian in the Mid-European Variscides is considered to be a former micropiate boundary. The collision zone of both of these micropiates is represented by the Northern Phyllite Zone, which is the northern margin of the Mid-German Crystalline Rise. The aim of this study was to set timemarks for the tectonometamorphic evolution of both crustal segments during the collisional stage of the Variscan orogeny in Central Europe. For this purpose two profiles from the Saxonian Granulite Massif to the upper Harz Mountains and the block of Flechtingen-Rosslau were investigated, respectivly. Essential data for the metamorphic evolution in the border area of the Rhenohercynian - Saxothuringian were ascertained by means of K1Ar-age determinations on white micas in fine mineral fractions of very low to low grade metamorphic, pre- to synorogenic sediments. Because of the combination of white mica polytype determinations in the fine mineral fractions it was possible for the first time to prove and separate polyp hase overprinting. Combined use of illite crystallinity measurements with polytype determinations made it possible to estimate the influence of detrital white micas on the KI Ar-ages of the fine fractions, thus leading to a better interpretation of these ages. K1Ar-age determinations on detrital white micas from Cambrian to Upper Carboniferous sediments were carried out, for example, to draw conclusions about the eroded crustal units. The age data indicates potential provenances for the Variscan sediments. Furthermore, age determinations of mica K1Ar-cooling ages of some crystalline complexes were carried out on outcropping crystalline units in order to diccriminate these units as potential source areas. The K1Ar-age determinations on white micas in fine mineral fractions «211m, <0,2Ilm) yield the following timemarks for the development of the very low to low grade metamorphic overprints: - The earliest, very low, Variscan overprint in the Cambrian and Ordovician sediments southeast (Cambrian series of Delitzsch) and northwest (Pakendorf Zone) of the Mid-German Crystalline Rise was shown and dated at 350 Ma. - In the Wippra Metamorphic Zone (Northern Phyllite Zone), the southwestward prolongation of the Pakendorf Zone, the ages indicate a span of overprinting between 350-320 Ma. - In the region of the Harz Mountains the ages of overprinting were set for the Lower Harz Mountains at 335-328 Ma, the northern Middle Harz Mountains at 320-310 Ma and for the Upper Harz Mountains at 309 :!: 9 Ma. - For the first time a Permian, post-kinematic thermal event in the Upper Harz Mountains Devonian Anticline (Oberharzer Devonsattel) was narrowed down to an age of about 275 Ma. - The northwestern epizonal "Schiefermantel" (slaty cover) of the Saxonian Granulite Massif shows ages of metamorphism at 321-316 Ma. - The units of the Phyllite Zone of Rosslau (Northern Phyllite Zone) and Phyllite Zone of Bitterfeld-Drehna (Southern Phyllite Zone), which are in direct contact with the Mid-German Crystalline Rise, yield ages of metamorphism at 310-290 Ma. II The dating of detrital white micas and therefore the assignment to potential provenances gave the following results: - Detrital white micas from Cambrian sediments which were dated at 590 Ma shows a Cadomian/Panafrican overprint. The source area assigned is Gondwana or a related terrane. The input of Silurian detrital material into the Rhenohercynian basin in Mid-Devonian times could be reconfirrned. Recent research suggests the provenance could be the Old Red continent. A change of provenances was shown for the sediments in the Rhenohercynian basin in the late Upper Devonian. From this period until the uppermost Lower Carboniferous early-Variscan metamorphic white micas are present in the sediments. A former extensively distributed crystalline nappe complex located towards the south is favored as the source area for these white micas. Abruptly, without transition late-Variscan detrital micas with ages of about 320 Ma appear in Upper Carboniferous sediments. The Bohemian Massif may represent the source area. An important exception is the Silurian detrital age of 435 Ma from the Kamm quartzite (cd II-III) of the Acker- Bruchberg zone. During Lower Carboniferous times the adjoining areas were supplied with early-Variscan detritus from southerly regions, whereas the sediments of the Kamm quartzite were derived from the northerly situated Old Red continent. This confirms the exceptional facies position of the Acker-Bruchberg zone during the Lower Carboniferous. The dating of mica cooling ages from crystalline complexes yield: - for the crystalline complexes belonging to the Mid-German Crystalline Rise (Kyffhäuser, Dessau, Hohnsdorf, Prettin) mica cooling ages from 337-329 Ma. - for biotites of the Ramberg pluton and the Delitzsch plutonite massif 295 and 303 Ma, respectively. The K/Ar-age data presented above shows evidence for a tectonometamorphic evolution of the border area Rhen ohercynian -Sax oth uringian: The oldest early- Variscan, very low metamorphic overprint appears in Cambrian sediments of the Saxothuringian basin and also in Ordovician sediments of the Northern Phyllite Zone with an early Lower Carboniferous age of 350 Ma. In the Northern Phyllite Zone (here: Wippra Metamorphic Zone), the ages of the metamorphism decrease from SE to NW or 350 to 320 Ma. The exception is the age from the southeasternmost unit of the Wippra Metamorphic Zone at 330 Ma. This succesion is interpreted as a classical "out of sequence"-stacking at the back of a thrustbelt. The different ages of metamorphism in the various series of the Wippra Metamorphic Zone prove that they were not in the present geotectonic position at the time of their individual overprinting. Therefore, the final stacking of the Wippra Metamorphic Zone took place after the youngest overprint with an age of 320 Ma. The age data obtained from the Wippra Metamorphic Zone suggest a metamorphic overprint prior to the SW -part of the Northern Phyllite Zone (S-Hunsrück, S-Taunus). The different ages of metamorphism from Hunsrückffaunus and the Wippra Metamorphic Zone allows two possible interpretations: Either the data from the Wippra Metamorphic Zone indicate an earlier beginning of collision, and therefore, of metamorphism in the SE-Harz region or at least parts of the Wippra Metamorphic Zone do not belong to the Rhenohercynian but to the accretionary wedge of the overriding Saxothuringian plate.The latter could be the proof of an