Zusammenfassung
34 Gerölle magmatischer Gesteine vorwiegend rhyolithischer Zusammensetzung aus dem Parishholm- und dem Hareshaw-Konglomerat (Silur, Hagshaw Hills inlier, Midland Valley, Schottland) wurden auf Haupt- und Spurenelementgehalte hin analysiert. Die geochemische Zusammensetzung der Gerölle deutet auf verschiedene Liefergebiete hin. Es handelt sich einmal um Gerölle von Rhyolithen aus einem Intraplatten-Gebiet (Peralkali-Rhyolithe ozeanischer Inseln oder kontinentaler Gräben) und zum zweiten um Gerölle von kalkalkalischen und kaliumreichen Rhyolithen. Solche Rhyolithe werden vorzugsweise über konvergierenden Plattengrenzen gefördert, sie treten aber auch in kontinentalen Dehnungsgebieten auf. Beide Konglomerate wurden aus der Richtung der Southern Uplands in das Midland Valley geschüttet. Da kalkalkalische Rhyolithe in den Southern Uplands unbekannt sind, dürfte im Silur ein vulkanisches Terraine in dem Gebiet, in welchem sich die Southern Uplands heute befinden, gelegen haben. Der geotektonische Charakter dieses vulkanischen Terraines lä\t sich mit Hilfe der Spurenelement-Daten der Gerölle nicht eindeutig festlegen. Es werden deshalb mehrere Modelle diskutiert.
Abstract
34 igneous clasts of predominantly rhyolitic composition of the Parishholm- and Hareshaw-Conglomerate (Silurian, Hagshaw Hills inlier, Midland Valley, Scotland) have been analysed for major and trace element contents. The geochemical composition of the clasts indicates different source areas. Firstly, some clasts are derived from rhyolites of a within-plate setting (peralkaline rhyolites of oceanic islands or continental rifts) and secondly, other clasts are derived from calc-alkaline to high-K rhyolites. Such rhyolites erupt mainly above convergent plate boundaries, but occur also in continental areas of crustal extension. Both conglomerates were shed from the direction of the Southern Uplands into the Midland Valley. Since no calc-alkaline rhyolites are known in the Southern Uplands, a volcanic terrane should have been situated during the Silurian in the area where the Southern Uplands lie now. The geotectonic character of this terrane cannot unequivocally be identified with the help of the trace element data of the clasts. Therefore several models are discussed.
Résumé
Les auteurs ont analysé les éléments majeurs et en traces de 34 galets de roches magmatiques, en majorité rhyolitiques, prélevés dans les conglomérats de Parishholm et de Hareshaw (Silurien, Hagshaw Hills inlier, Midland Valley, Ecosse). Leur composition géochimique indique différentes provenances. D'une part, il s'agit de galets rhyolitiques d'une région intra-plaque (rhyolites peralcalines d'Îles océaniques ou de fossés continentaux), d'autre part, il s'agit de galets de rhyolites calco-alcalines et de rhyolites riches en potassium. Des rhyolites de ce type sont émises d'ordinaire au-dessus de sutures de plaques convergentes, mais elles se rencontrent aussi dans des régions continentales en extension. Les deux conglomérats proviennent du Sud, de la région des Southern Uplands. Comme des rhyolites calco-alcalines sont inconnues dans les Southern Uplands, on est amené à supposer l'existence, au Silurien, d'une accrétion («terrane») de nature volcanique dans la région occupée aujourd'hui par les Southern Uplands. Les teneurs des éléments en trace ne permettent pas de déterminer sans équivoque le caractère géotectonique de cette accrétion volcanique. Dans ces conditions, plusieurs modèles sont discutés.
кРАткОЕ сОДЕРжАНИЕ
пРОВЕлИ АНАлИж 34 гАлЕк МАгМАтИЧЕскИх пОРОД пРЕИМУЩЕстВЕННО РИОлИтОВОгО сОстАВА Иж кОНглОМЕРАтОВ Parishholm И Hareshaw, сИлУР, Hagshaw Hills inlier, Midland Valley, шОтлАНДИь/, НА ОсНОВ АНыЕ И РАссЕьННыЕ ЁлЕМЕНты. гЕОхИМИЧЕс кИИ сОстАВ гАлЕк УкАж ыВАЕт НА сУЩЕстВОВАНИЕ РАжлИ ЧНых ОБлАстЕИ сНОсА. Ё тО. ВОпЕРВых,гАлькА РИОлИтОВ Иж Intraplatten-Gebiet / пЕРЩЕ лОЧНыЕ РИОлИтОВыЕ ОкЕАНИЧЕ скИЕ ОстРОВА, ИлИ МАтЕ РИкОВыЕ гРАБЕНы/ И гАлькА ИжВЕ сткОВО-ЩЕлОЧНых И БОгАтых кАльцИЕМ РИО лИтОВ. тАкИЕ РИОлИты п ОДНИМАУтсь гл. ОБР. НАД кОНВЕРгЕНт НыМИ гРАНИцАМИ плИт, НО ВстРЕЧАУтсь тАкжЕ И НА МАтЕРИкОВых жОНА х РАстьжЕНИь. ОБА кОНтИНЕНтА ВыНОс ИлИсь Иж УжНОгО НАгОР ьь И ОтлОжИлИсь В ДОлИНЕ МИДлЁНД. ИжВЕсткОВО-Щ ЕлОЧНыЕ РИОлИты В УжНОМ НАгОР ьЕ НЕ ИжВЕстНы, ВОжМОж НО, ЧтО В ОБлАстИ, гДЕ сЕгО ДНь НАхОДИтсь УжНОЕ Н АгОРьЕ В сИлУРскОМ пЕРИОДЕ И МЕлА МЕстО ВУлкАНИЧЕ скАь ДЕьтЕльНОсть. гЕОтЕк тОНИЧЕскИИ хАРАктЕР ЁтОИ ОБлАстИ ВУлкАНОВ ОЧЕРтИть с п ОМОЩьУ ДАННых пО РАсс ЕьННыМ ЁлЕМЕНтАМ В гАлькЕ ОД НОжНАЧНО НЕ УДАЕтсь. ОБсУжДАУтсь РАжлИЧН ыЕ МОДЕл.
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Heinz, W., Loeschke, J. Volcanic clasts in Silurian conglomerates of the Midland Valley (Hagshaw Hills inlier) Scotland, and their meaning for Caledonian plate tectonics. Geol Rundsch 77, 453–466 (1988). https://doi.org/10.1007/BF01832391
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DOI: https://doi.org/10.1007/BF01832391