Domainal variations of U-Pb monazite ages and Rb-Sr whole-rock dates in polymetamorphic paragneisses (KTB Drill Core, Germany): influence of strain and deformation mechanisms on isotope systems

Krohe, A; Wawrzenitz, N

dc.contributor.author	Krohe, A	en
dc.contributor.author	Wawrzenitz, N	en
dc.date.accessioned	2014-03-01T01:49:51Z
dc.date.available	2014-03-01T01:49:51Z
dc.date.issued	2000	en
dc.identifier.issn	0263-4929	en
dc.identifier.uri	https://dspace.lib.ntua.gr/xmlui/handle/123456789/25940
dc.subject	deformation mechanisms	en
dc.subject	discordant monazite	en
dc.subject	Europe	en
dc.subject	Rb-Sr whole-rock dating	en
dc.subject	U-Pb dating	en
dc.subject.classification	Geology	en
dc.subject.other	NW BOHEMIAN MASSIF	en
dc.subject.other	SHEAR ZONE	en
dc.subject.other	GEOCHRONOLOGICAL CONSTRAINTS	en
dc.subject.other	TECTONOMETAMORPHIC EVOLUTION	en
dc.subject.other	CONTINENTAL LITHOSPHERE	en
dc.subject.other	WESTERN MARGIN	en
dc.subject.other	METAMORPHISM	en
dc.subject.other	BOUNDARY	en
dc.subject.other	CREEP	en
dc.subject.other	FLUID	en
dc.title	Domainal variations of U-Pb monazite ages and Rb-Sr whole-rock dates in polymetamorphic paragneisses (KTB Drill Core, Germany): influence of strain and deformation mechanisms on isotope systems	en
heal.type	journalArticle	en
heal.language	English	en
heal.publicationDate	2000	en
heal.abstract	Polyphase metamorphic paragneisses from the drill core of the continental deep drilling project (KTB; NW Bohemian Massif) are characterized by peak pressures of about 8 kbar (medium-P metamorphism) followed by strain accumulation at T > 650 degrees C, initially by dislocation creep and subsequently by diffusion creep. U-Pb monazite ages and Rb-Sr whole-rock data vary in the dm-scale, indicating Ordovician and Mid-Devonian metamorphic events. Such age variations are closely interconnected with dm-scale domainal variations of microfabrics that indicate different predominant deformation mechanisms. U-Pb monazite age variations dependent on microfabric domains exceed grain-size-dependent age variations. In 'mylonitic domains' recording high magnitudes of plastic strain, dislocation creep and minor static annealing, monazite yields concordant and near concordant Lower Ordovician U-Pb ages, and the Rb-Sr whole-rock system shows isotopic disequilibrium at an mm-scale. In 'mineral growth/mobilisate domains', in which diffusive mass transfer was a major strain-producing mechanism promoting diffusion creep of quartz and feldspar, and in which static recrystallization (annealing) reduced the internal free energy of the strained mineral aggregates, concordant U-Pb ages are Mid-Devonian. Locally, in such domains, Rb-Sr dates among mm(3)-sized whole-rock slabs reflect post-Ordovician resetting. In 'transitional domains', the U-Pb-ages are discordant. We conclude that medium-P metamorphism occurred at 484 +/- 2 Ma, and a second metamorphic event at 380-370 Ma (Mid-Devonian) caused progressive strain in the rocks. Dislocation creep at high rates, even at high temperatures, does not reset the Rb-Sr whole-rock system, while diffusion creep at low rates and stresses (i.e. low epsilon/D-eff ratios), static annealing and the presence of intergranular fluids locally assist resetting. At temperatures above 650 degrees C, diffusive Pb loss did not reset Ordovician U-Pb monazite ages, and in domains of overall high imposed strain rates and stresses, resetting was not assisted by dynamic recrystallization/crystal plasticity. However, during diffusion creep at low rates, Pb loss by dissolution and precipitation ('recrystallization') of monazite produces discordance and Devonian-concordant U-Pb monazite ages. Hence, resetting of these isotope systems reflects neither changes of temperature nor, directly, the presence or absence of strain.	en
heal.publisher	BLACKWELL SCIENCE INC	en
heal.journalName	JOURNAL OF METAMORPHIC GEOLOGY	en
dc.identifier.isi	ISI:000086981600004	en
dc.identifier.volume	18	en
dc.identifier.issue	3	en
dc.identifier.spage	271	en
dc.identifier.epage	291	en