Cenki-Tok, and 11 co-authors reported in 2011 that this paper addresses the mechanical and chemical behavior of allanite during shear zone formation under high-pressure metamorphism. Understanding physico-chemical processes related to the retention or resetting of Pb isotopes in allanite during geological processes is essential for robust petrochronology. Dating of allanite in meta-granodiorite showing variable amounts of strain (from an undeformed protolith to mylonite) at Monte Mucrone (Sesia Zone, NW Italy) gave surprising results. Based on structural and petrographic observations the shear zones at Mt Mucrone are Alpine, yet allanite located within an eclogite facies mylonite yielded Permian ages (208Pb/232Th average age: 287±7 Ma). These mm-sized allanite grains are rimmed by an aggregate of coarse-grained garnet+phengite, thought to derive from former epidote.
These aggregates were immersed in a weak matrix that experienced granular flow, and they were thus chemically and mechanically shielded during Alpine mylonitization. In undeformed samples (8a and 8b), two populations of epidote group minerals were found. Allanite forms either coronas around Permian monazite or individual grains with patchy zoning. Both types yield Permian ages (208Pb/232Th age: 291±5 Ma). On the other hand, grains of REE-rich clinozoisite of Cretaceous age are found in undeformed rocks. These grains appear as small fragments with embayed surface outlines and minute satellites or rims around Permian allanite. These (re)crystallized grains are Sr-rich and show mosaic zoning.
These results indicate that allanite crystals retained their chemical and isotopic characteristics, and thus their Permian age, as a result of strong strain partitioning between the epidote group porphyroclasts and the eclogite facies matrix in HP-mylonites. The observed partial mobilization of Pb isotopes, which lead to the Cretaceous-aged rims or grains in undeformed samples was facilitated by (re)crystallization of allanite and not by mere Pb diffusion alone under the HP conditions
The authors then concluded that the preservation of Permian allanite within an Alpine eclogite facies shear zone at Mt Mucrone is attributed to mechanical and chemical shielding during mylonite formation. The authors proposed that Permian allanite grains (several millimeters in size) crystallized in late magmatic (or early post-intrusive) fractures, and that these cores were subsequently mantled by clinozoisite/epidote. Alpine shearing reworked the mechanical heterogeneities left by the veins. During the HP metamorphic transformations, the allanite were largely unaffected, whereas their mantle reacted statically, but at conditions very similar to the mylonite, in which they now reside. The resistance of allanite to deformation is remarkable and has implications on the interpretation of allanite ages in deformed rocks.
In this study, deformation did not play a major role in triggering metamorphic recrystallization and isotopic resetting of allanite. However, it is essential to record microstructures and link these to mineral separates (or in situ domains) chosen for age dating. Allanite is a highly retentive geochronometer, not only in terms of its thermal
stability, but also owing to its mechanical strength and thus its reluctance to recrystallize under stress. The composition, notably elevated Sr-contents of recrystallized grains, allows the recognition of metamorphic allanite, which yielded indicative Cretaceous ages.
While these grains occur within little deformed and orthogneissic metagranodiorite, but not the high-strain part of the HP shear zone, well-equilibrated metamorphic assemblages in each of these domains reflect identical P–T conditions. Based on the petrographic and geochemical data from this study, mylonite formation must have occurred at the same conditions as the metamorphic overprint, and shall be Cretaceous in age.
Published as: https://www.sciencedirect.com/science/article/pii/S002449371100017X?via%3Dihub
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