Metasomatic Influx of LILE-Enriched Complex Brines at UHP Conditions In Dora-Maira Whiteschists (western Alps): Petrology and Fluid Inclusion Data

Recent petrological studies on HP-UHP terranes have been directed not only to constrain the P-T path, but also to characterise the fluid-rock interaction during metamorphic evolution, mostly at UHP peak conditions. Dora-Maira whiteschists are the best candidates for this kind of study.

We report SEM-EDS, WDS, micro-Raman, IR-synchrotron-radiation imaging, and LA-ICP-MS data on minerals and multiphase solid inclusions (MSI) in equilibrium at UHP peak in Dora-Maira whiteschists. These rocks consist of pyrope-rich garnet, quartz/coesite, phengite, kyanite, talc, chlorite, and minor Mg-rich minerals. Three generations of pyrope occur: 1) prograde reddish megablasts (10-20 cm across); 2) zoned porphyroblasts (2-10 cm across) with prograde core and peak rim; and 3) peak smaller crystals (<2 cm across). Peak assemblage consists of pyrope (Prp_86-98Alm_2-11), coesite, phengite (Si=3.548-3.580 a.p.f.u.), kyanite, talc, and accessory rutile, apatite, zircon and monazite. Coesite occurs only in the peak pyrope, indicating UHP conditions of 4.0 GPa and 730°C.

The fluid phase present at UHP conditions may be inferred from primary MSI (10-60 microns across) within peak pyrope. SEM-EDS and micro-Raman data reveal that MSI contain Mg-chlorite, Na-phlogopite, Cl-rich apatite, Zn-rich pyrite, and chlorides as daughter minerals, talc and magnesite as step-daughter minerals, and rutile, zircon and monazite as incidentally-trapped minerals. Trace elements and REE patterns collected by LA-ICP-MS reveal substantial enrichment in LILE (LREE, Th, U, Pb, Sr, Zn). Although water was never observed, IR-synchrotron-radiation mapping reveals significant water diffusion from MSI to host pyrope.

Present data suggest that growth of pyrope at peak conditions occurred in presence of a complex brine enriched in Mg, Na, Ca, LREE, Th, U, Pb, Sr, Zn, probably derived from dehydration of serpentine, in agreement with the model for whiteschist genesis envisaging metasomatism along shear zones of precursor granitoids/orthogneiss, by influx of complex brines derived from deeply subducted mantle rocks.