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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001 1997055691
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040 |a ViAlAGI  |c ViAlAGI 
072 7 |a 05A  |2 georeft 
072 7 |a 03  |2 georeft 
100 1 |a Inger, S.  |e analytic author  |u University of Leeds, Leeds 
245 1 0 |a Metamorphic evolution of the Sesia-Lanzo Zone, Western Alps; time constraints from multi-system geochronology 
300 |a p. 152-168 
500 |a In English. 38 refs. 
500 |a Abstract number: 97M/3452 
500 |a Abstractor: R.E.S. 
500 |a Affiliation: University of Leeds; Leeds; GBR; United Kingdom 
500 |a Key title: Contributions to Mineralogy and Petrology 
500 |a Source note: Contributions to Mineralogy and Petrology, 126(1-2), p.152-168. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999 
500 |a Publication type: journal article 
504 |b 38 refs. 
510 3 |a GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom 
520 |a 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 >40`Ar/>39`Ar 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 >40`Ar->39`Ar ages of micas formed at this time were very disturbed, whereas micas survivng from an earlier higher-P assemblage had their >40`Ar->39`Ar 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. 
650 7 |a Absolute age  |2 georeft 
650 7 |a Alkali metals  |2 georeft 
650 7 |a Alkaline earth metals  |2 georeft 
650 7 |a Alpine Orogeny  |2 georeft 
650 7 |a Ar/Ar  |2 georeft 
650 7 |a Argon  |2 georeft 
650 7 |a Dates  |2 georeft 
650 7 |a Facies  |2 georeft 
650 7 |a Granulites  |2 georeft 
650 7 |a Greenschist facies  |2 georeft 
650 7 |a Isotope ratios  |2 georeft 
650 7 |a Isotopes  |2 georeft 
650 7 |a Kinzigite  |2 georeft 
650 7 |a Metals  |2 georeft 
650 7 |a Metamorphic rocks  |2 georeft 
650 7 |a Metamorphism  |2 georeft 
650 7 |a Noble gases  |2 georeft 
650 7 |a P-T conditions  |2 georeft 
650 7 |a Radioactive isotopes  |2 georeft 
650 7 |a Rb/Sr  |2 georeft 
650 7 |a Rubidium  |2 georeft 
650 7 |a Sr/Sr  |2 georeft 
650 7 |a Stable isotopes  |2 georeft 
650 7 |a Strontium  |2 georeft 
650 7 |a Tectonics  |2 georeft 
650 7 |a Variscan Orogeny  |2 georeft 
651 7 |a Alps  |2 georeft 
651 7 |a Europe  |2 georeft 
651 7 |a Italy  |2 georeft 
651 7 |a Sesia-Lanzo Zone  |2 georeft 
651 7 |a Southern Europe  |2 georeft 
651 7 |a Western Alps  |2 georeft 
700 1 |a Ramsbotham, W.,  |e analytic author 
700 1 |a Cliff, R. A.,  |e analytic author 
700 1 |a Rex, D. C.,  |e analytic author 
773 0 |t Contributions to Mineralogy and Petrology  |d Heidelberg-New York : Springer International, Dec. 1996  |x 0010-7999  |y CMPEAP  |n Contributions to Mineralogy and Petrology, 126(1-2), p.152-168. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999 Publication type: journal article  |g Vol. 126, no. 1-2  |h illus., incl. 4 tables, geol. sketch map 
856 |u urn:doi: 10.1007/s004100050241