Metamorphic evolution of the Sesia-Lanzo Zone, Western Alps; time constraints from multi-system geochronology

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doi: 10.1007/s004100050241
Authors:Inger, S.; Ramsbotham, W.; Cliff, R. A.; Rex, D. C.
Author Affiliations:Primary:
University of Leeds, Leeds, United Kingdom
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 126(1-2), p.152-168. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1996
Note:In English. 38 refs.; illus., incl. 4 tables, geol. sketch map
Summary:This zone is a polymetamorphic unit containing Hercynian granulite relics overprinted by eclogite- and greenschist-facies metamorphism and deformation during the Alpine orogeny. Different parts record different evolutionary stages, allowing multi-system isotopic studies to unravel the metamorphic history. New Rb-Sr white mica and U-Pb sphene data constrain the age of eclogite-facies metamorphism and deformation to 60-70 m.y. This substantially alters the common view of early- to mid-Cretaceous eclogite-facies metamorphism in the unit. The new results are more consistent with the established geotectonic framework for the Alpine orogeny since they do not need a prolonged period of depressed geothermal gradient at a time when the region was in extension, and also agree better with recent studies of other units that suggest post-Cretaceous high-P metamorphism. Step-heated 40Ar/39Ar analysis of phengites yields good plateaux giving ages older than the corresponding Rb-Sr age. Such anomalously high ages suggest the presence of radiogenic Ar-rich fluids in the grain boundary network in the fluid/P conditions acting during this event. The U-Pb sphene ages are variable in polymetamorphic rocks, and show inheritance of older Pb or sphene crystals into the high-P event. Two monometamorphic assemblages yield concordant ages of 66 ± 1 m.y., reflecting crystallization of the eclogite facies assemblage. The Gneiss Minuti complex (GMC) lies structurally below the eclogitic micaschists (EMC), and its pervasive greenschist facies fabric yields tightly clustered Rb-Sr white mica ages at 38-39 m.y. This greenschist event did not affect most of the EMC. The 40Ar-39Ar ages of micas formed at this time were very disturbed, whereas micas survivng from an earlier higher-P assemblage had their 40Ar-39Ar system reset. The greenschist event did not srongly affect U-Pb systematics in Hercynian age sphenes, suggesting that the GMC did not suffer eclogite-facies metamorphism uniformly during the Alpine cycle but was juxtaposed against the EMC later in the orogeny. [R.E.S.]
Subjects:Absolute age; Alkali metals; Alkaline earth metals; Alpine Orogeny; Ar/Ar; Argon; Dates; Facies; Granulites; Greenschist facies; Isotope ratios; Isotopes; Kinzigite; Metals; Metamorphic rocks; Metamorphism; Noble gases; P-T conditions; Radioactive isotopes; Rb/Sr; Rubidium; Sr/Sr; Stable isotopes; Strontium; Tectonics; Variscan Orogeny; Alps; Europe; Italy; Sesia-Lanzo Zone; Southern Europe; Western Alps
Abstract Numbers:97M/3452
Record ID:1997055691
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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