Geochemistry and Ar/Ar geochronology of pseudotachylyte associated with UHP whiteschists from the Dora Maira massif, Italy

In situ UV-laser ablation ⁴⁰Ar/³⁹Ar geochronological and geochemical data, together with rock and mineral compositional data, have been determined from pseudotachylyte and surrounding mylonitic gneiss associated with the UHP whiteschists of the Dora Maira Massif, Italy. Several generations of fresh pseudotachylyte occur as irregular veins up to a few cm thick both parallel and at high angles to the foliation. Whole rock XRF data collected from representative lithologies of mylonitic gneiss are uniformly consistent with a mildly alkalic granitic protolith. Minimal compositional variation is observed between the pseudotachylyte and its surrounding mylonitic gneiss. The pseudotachylyte contains newly crystallized grains of biotite and K-feldspar in a matrix of glass with partially fused grains of quartz, zircon, apatite, and titanite. Electron microprobe analyses of the glass show significant compositional variation that is probably strongly influenced by micrometer-scale changes in mineralogy. UV-laser ablation ICP-MS traverses across the mylonitic gneiss–pseudotachylyte contact are consistent with cataclastic communition of REE carriers such as epidote, monazite, allanite, zircon, and apatite before melting as an efficient mechanism of REE homogenization in the pseudotachylyte. The ⁴⁰Ar/³⁹Ar data from one band of pseudotachylyte indicate formation at 20.1 ± 0.5 Ma, when the mylonitic gneisses were already in a near surface position. The variable effects of top-to-the-west shear deformation within outcrops of the coesite-bearing unit are reflected in localized zones of protomylonite, cataclasite, ultracataclasite, and pseudotachylyte. Preservation of several generations of pseudotachylyte suggests that seismic events may have played a significant role in triggering late unroofing of the UHP rocks. It is speculated that deeper crustal seismic events potentially played a role in the unroofing of the UHP rocks at earlier stages in their exhumation history.     

The ultra‐high pressure coronitic and pseudomorphous reactions in a metagranodiorite from the Brossasco‐Isasca Unit,Dora‐Maira Massif,western Italian Alps: a petrographic study and equilibrium thermodynamic modelling

Metagranodiorite samples from the Brossasco‐Isasca Unit, Dora‐Maira Massif, western Alps, show pseudomorphous and coronitic textures where igneous minerals were partially replaced by ultra‐high pressure (UHP) metamorphic assemblages. The original magmatic paragenesis consisted of quartz, plagioclase, K‐feldspar, biotite and minor phases. During UHP metamorphism, the plagioclase (site P) was replaced by zoisite, jadeite, quartz, K‐feldspar and kyanite, and coronitic reactions developed between biotite and adjacent minerals. At the original igneous biotite–quartz contact (site A), a single corona of poorly zoned garnet is developed, whereas at the biotite–K‐feldspar (site B) and biotite–plagioclase (site C) contacts, composite coronas are formed. Integration of results from petrographic observations, calculations of mineral stoichiometry and thermodynamic calculations of mineral stability has allowed the determination of the metamorphic reactions involved and the estimation of the metamorphic conditions, which reached as high as 24 kbar and 650 °C. Accurate microanalysis by energy‐dispersive spectroscopy (EDS) and statistical analysis of the data allowed us to identify, for the first time in a natural Na‐pyroxene of metagranitoid rocks, the end‐member Ca‐Eskola.