Coronitic microstructures in patchy eclogitised continental crust : the Lago della Vecchia pre-Alpine metagranite (Sesia-Lanzo Zone, Western Italian Alps)

The Lago della Vecchia pre-Alpine coronitic metagranite (Sesia-Lanzo Zone, Western Italian Alps, Valle del Cervo) preserves igneous textures and mineralogy despite the complete eclogitization recorded by surrounding deformed metagranites and country rocks. The coronitic cores are metre to few metres undeformed metagranites preserving igneous cm-sized K-feldspar and mm-sized biotite. Microscale analysis shows that igneous association is partly preserved: biotite, white mica and K-feldspar are only partly replaced by Alpine assemblages, while the original plagioclase is pseudomorphically overgrown by aggregates of albite + phengitic mica + Fe-epidote ± garnet. Metamorphic reactions are also localized at the interfaces biotite – plagioclase and white mica – plagioclase. A continuous garnet corona with a distinctive partitioning of Ca2+ and Fe2+ occurs between biotite and plagioclase, suggesting diffusive mass transfer during a prograde burial path. Microdomains have been described and analysed in terms of their geometry, mineral composition and chemistry patterns using optical microscopy, X-rays compositional maps, chemical analyses and image analysis in order to detail the microstructures and to reconstruct the relative chronology of the reactions within each microdomain. The results allow demonstration of the chemical exchanges that occurred between adjacent igneous microdomains: Biotite microdomain at contact with plagioclase – two coronas are observed: 1) BtII + Phn-WmII + Alm-rich GrtI; 2) Phn-WmII + Grs-rich Grt coronas; White mica microdomain with plagioclase: a single Phn-WmII + Grt + Ep + Ab corona develops; K-feldspar microdomain with plagioclase: a single Ab + Phn-WmII ± Ep ± Grt corona develops; Plagioclase core microdomain: Ab + Phn-WmII (fine-grained) + Ep ± Grt; microdomain: Ab + Phn-WmII (fine-grained) + Ep ± Grt; K-feldspar core microdomain: large igneous grains are partly to completely replaced by Ab from the plagioclase-K-feldspar boundaries or along fractures. The absence of deformation seems to inhibit the complete development of eclogite-facies metamorphic parageneses, allowing the preservation of microstructures, mineral phases and chemistry. This information is commonly hidden by the widespread development of high pressure - low temperature eclogite-facies assemblages within surrounded deformed volumes.