Zircon geochemical constraints on the protolith nature and metasomatic process of the Mg-rich whiteschist from the Western Alps

The Mg-rich silicic crustal rocks characterized by high SiO2 and MgO contents are widespread in the Alps, providing rare targets to investigate the origin of Mg-rich fluids in the continental subduction zone. However, it is a prerequisite to elucidate the protolith nature of such Mg-rich rocks, including the possible alteration process experienced before the Alpine metamorphism. For this purpose, we carried out a detailed study of trace elements, O-Hf isotopes and U-Pb ages for zircons from the coesite-bearing whiteschist and its country rock metagranite at Dora-Maria in Western Alps. The whiteschist is characterized by much higher MgO contents than the adjacent metagranite. Relict magmatic zircons from the two types of rocks give essentially the same U-Pb ages of ~ 262 Ma, similar REE contents and patterns, the same range of εHf(t1) values from − 6.4 to 2.8 and δ18O values from mostly 9 to 11‰. These features provide compelling evidence that the whiteschist and metagranite share the same protolith, i.e., the Permian granite. All the above geochemical compositions are consistent with the argument that the high MgO content of the whiteschist was acquired from high Mg fluids from dehydration of the serpentinized peridotite at the slab-mantle interface in the continental subduction channel. The hypothesis of a pre-Alpine hydrothermal alteration of the granitic protolith to cause the MgO enrichment is not favorable. Zircon in the metagranite does not show any metamorphic overgrowth during the Alpine orogeny, in contrast to the extensive overgrowth of metamorphic rims at ~ 35 Ma in the whiteschist. The metamorphic rims show relatively low REE contents with variably flattened HREE patterns, different from the relict magmatic zircons that exhibit relative high REE contents with steep HREE patterns. This difference suggests that the breakdown of hydrous minerals plays a crucial role in dictating mineral reactions for zircon growth during the continental subduction-zone metamorphism. This interpretation is also consistent with the local occurrence of garnet coronae along the grain margins of biotite in the metagranite. Therefore, this study provides not only the constraints on the protolith nature and metasomatic process of Mg-rich whiteschist from the Western Alps, but also insights into the role of metamorphic fluids in controlling mineral reactions for the metamorphic growth of both zircon and garnet in the continental subduction zone.