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Comparison of 40Ar-39Ar and Rb-Sr Data on Phengites from the UHP Brossasco-Isasca Unit (Dora Maira Massif, Italy): Implications for Dating White Mica |
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| Authors: | DI VINCENZO GIANFRANCO; TONARINI SONIA; LOMBARDO BRUNO; CASTELLI DANIELE; OTTOLINI LUISA |
| Institution: | 1 ISTITUTO DI GEOSCIENZE E GEORISORSE–CNR VIA MORUZZI 1, I-56124 PISA, ITALY 2 ISTITUTO DI GEOSCIENZE E GEORISORSE–CNR, SEZIONE DI TORINO VIA VALPERGA CALUSO 35, I-10125 TORINO, ITALY 3 DIPARTIMENTO DI SCIENZE MINERALOGICHE E PETROLOGICHE VIA VALPERGA CALUSO 35, I-10125 TORINO, ITALY 4 ISTITUTO DI GEOSCIENZE E GEORISORSE–CNR, SEZIONE DI PAVIA VIA FERRATA 1, I-27100 PAVIA, ITALY |
| Abstract: | Different lithologies (impure marble, eclogite and graniticorthogneiss) sampled from a restricted area of the coesite-bearingBrossasco–Isasca Unit (Dora Maira Massif) have been investigatedto examine the behaviour of 40Ar–39Ar and Rb–Srsystems in phengites developed under ultrahigh-pressure (UHP)metamorphism. Mineralogical and petrological data indicate thatzoned phengites record distinct segments of the P–T path:prograde, peak to early retrograde in the marble, peak to earlyretrograde in the eclogite, and late retrograde in the orthogneiss.Besides major element zoning, ion microprobe analysis of phengitein the marble also reveals a pronounced zoning of trace elements(including Rb and Sr). 40Ar–39Ar apparent ages ( 35–62Ma, marble; 89–170 Ma, eclogite; 35–52 Ma, orthogneiss),determined through Ar laserprobe data on phengites (step-heatingand in situ techniques), show wide intra-sample and inter-samplevariations closely linked to within-sample microchemical variations:apparent ages decrease with decreasing celadonite contents.These data confirm previous reports on excess Ar and, more significantly,highlight that phengite acted as a closed system in the differentlithologies and that chemical exchange, not volume diffusion,was the main factor controlling the rate of Ar transport. Conversely,a Rb–Sr internal isochron from the same eclogite yieldsan age of 36 Ma, overlapping with the time of the UHP metamorphicpeak determined through U–Pb data and thereby corroboratingthe previous conclusion that UHP metamorphism and early retrogressionoccurred in close succession. Different phengite fractions ofthe marble yield calcite–phengite isochron ages of 36to 60 Ma. Although this time interval matches Ar ages from thesame sample, Rb–Sr data from phengite are not entirelyconsistent with the whole dataset. According to trace elementvariations in phengite, only Rb–Sr data from two wet-groundphengite separates, yielding ages of 36 and 41 Ma, are internallyconsistent. The oldest age obtained from a millimetre-sizedgrain fraction enriched in prograde–peak phengites mayrepresent a minimum age estimate for the prograde phengite relics.Results highlight the potential of the in situ 40Ar–39Arlaser technique in resolving discrete P–T stages experiencedby eclogite-facies rocks (provided that excess Ar is demonstrablya negligible factor), and confirm the potential of Rb–Srinternal mineral isochrons in providing precise crystallizationages for eclogite-facies mineral assemblages. KEY WORDS: 40Ar–39Ar dating; Rb–Sr dating; phengite; SIMS; UHP metamorphism |
| Keywords: | : 40Ar– 39Ar dating Rb– Sr dating phengite SIMS UHP metamorphism |
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