The U-Pb dating of zircon domains by ion probe microanalysis in combination with a thorough investigation of the evolution of zircon crystals by cathodoluminescence imaging is a powerful approach to the chronometry of geological processes in high-grade metamorphic terrains. Various processes (e. g. anatectic melting, magmatic crystallization, fluid infiltration) produce morphologically distinct domains within zircon crystals.
In the Ivrea Zone (Southern Alps), an upended section through parts of the lower continental crust, a recent zircon study comprises anatectic metapelites from the upper amphibolite to the granulite facies part of the section, a metagabbro and a metaperidotite from near the base of the section. The anatectic zircon domains in the metapelites and the magmatic domains in the mafic/ultramafic rocks are
all analytically concordant with apparent 206Pb/238U ages scattering between 300 and 200 Ma, due to differential Pb loss. The oldest ages of each of the 7 samples agree to within their analytical errors at the mean of 297 ± 3 Ma (95% c. l.). This age agrees with the mean of a distinct cluster of 10 ages in one of the metasediment samples (292 ± 2.5 Ma). The age of 297 ± 3 Ma is considered as the best estimate for a major pulse of regional anatexis caused by mafic/ultramafic magmatism in the lower crust. The morphology of zircon overgrowths on detrital cores indicates that anatectic processes were different in the granulite facies and upper amphibolite facies part of the Ivrea Zone. In the granulite facies, a typically isometric overgrowth formed during prograde formation of anatectic melt and became partly redissolved at the anatectic climax. Only traces of overgrowth crystallized after the climax, indicating that the anatectic melt was efficiently extracted, in accordance with the restitic chemistry of the granulitic rocks. The situation is reversed in the amphibolite facies, where the prograde (isometric) overgrowth is absent and zircon grew after partial dissolution of crystals, from anatectic melt which crystallized in place. There is no detectable age difference between these different processes.
The present observations and data exclude any regional granulite facies metamorphism and anatexis in the Ivrea Zone prior to 300 Ma. Amphibolite facies conditions may have been attained earlier, as their is growing evidence from our zircon study that pre-300 Ma anatexis and magmatism occurred locally. A pre-300 Ma generation of zircon growth is present in an amphibolite facies metapelite. It occurs as a distinct high-U prismatic overgrowth below the 297 Ma overgrowth. Taking partial Pb-loss into account, the analytically concordant data provide a minimum age of 310 2.5 Ma (95 % c. l.) for the early anatexis in this rock. This is significantly older than the 297 3 Ma regional anatectic event. During the early anatexis, the growth of the {110}-prism
of zircon was inhibited, which indicates that the melt was probably H2O-saturated.
In several of the investigated samples, including a typical metapelite, a garnetite and a metagabbro, the granulite zircons became partly recrystallized and rejuvenated during the Upper Triassic. The recrystallized parts of zircon have characteristics pointing to an origin under fluid attack. They are enriched in common Pb relative to unrecrystallized domains. They are adjacent to crystal rims and associated with rough surfaces and irregular channels within the
crystals (visible in cathodoluminescence). In accordance with an origin during fluid circulation, the amount of zircon recrystallization is highly variable at localities beeing only a few meters apart. Noteworthy, extreme zircon recrystallization is observed in a sample from a 5 m thick garnetite layer, where 11 ages from recrystallized zircon domains agree to within analytical errors at the mean of 219.5 ± 3 Ma (95 % c. l.). Apart from this recrystallization, zircons in the garnetite closely resemble zircons in the typical granulite facies metapelites with anatectic overgrowth at ca. 297 Ma. Thus,the garnetite originated from a metapelite and its transformation to a nearly monomineralic lithology occurred, most probably, under Upper Triassic fluid circulation which also severely attacked the zircons. The fluid circulation may have been associated with coeval alkaline magmatism related to continental rifting. The question whether parts of the Ivrea Zone were under granulite facies conditions in the Upper Triassic is currently approached by a study of monazites from the vicinity of the garnetite.