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71.
DI VINCENZO GIANFRANCO; TONARINI SONIA; LOMBARDO BRUNO; CASTELLI DANIELE; OTTOLINI LUISA 《Journal of Petrology》2006,47(7):1439-1465
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 相似文献
72.
The Sanbagawa high-pressure schists from central Shikoku in Southwest Japan have experienced high-strain ductile deformation during exhumation and cooling. This study examines the effects of high-strain ductile deformation on K–Ar ages of phengites on the basis of fabric, chemistry and K–Ar ages of phengites from the pelitic, psammitic and quartzose (or albitic) schists collected from the same outcrop in the albite–biotite zone. Phengites in the pelitic and psammitic schists generally occur forming aggregates consisting of fine-grained phengite crystals and are extremely fine-grained in domains close to relatively rigid garnet and albite porphyroblasts, indicating that deformation-induced grain-size reduction had taken place in phengite during the ductile deformation accompanying the exhumation of host schists. We suggest that the grain-size reduction of phengite is due to strain-induced recrystallization or dynamic recrystallization. The matrix phengites in schists are chemically heterogeneous on the thin-section scale but the phengites from pelitic and psammitic schists from the same outcrop have similar chemical range. Phengite included in garnet has a high Si value and its Na/(Na + K) and Mg/(Mg + Fe) ratios are significantly low in comparison with those in matrix. The phengite included in garnet records the chemistry in equilibrium with other major silicate phases during the higher pressure stage of the P–T–t history of the schists. In contrast, the matrix phengites having low Si values are likely to have been formed during retrograde metamorphism in extremely restricted equilibrium domains. The two or three different types of schists from the same outcrop, which have a similar grain size of phengite, have similar K–Ar ages, suggesting that the closure temperature does not depend on chemistry. However, the hematite-rich quartzose schist with strong grain-size reduction of both phengite and quartz has a significantly younger K–Ar phengite age than the pelitic and quartzose schists in the same outcrop that do not show grain-size reduction. We suggest that the exhumation tectonics of the schists, which have experienced strong ductile deformation at temperatures less than ~350°C, played an important role resulting in the observed variation in age. 相似文献
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