Protoliths and phase petrology of whiteschists

Abstract

Whiteschists appear in numerous high-and ultrahigh-pressure rock suites and are characterized by the mineral assemblage kyanite ? talc (?-quartz or coesite). We demonstrate that whiteschist mineral assemblages are well stable up to pressures of more than 4 GPa but may already form at pressures of 0.5 GPa. The formation of whiteschists largely depends on the composition of the protolith, which requires elevated contents of Al and Mg as well as low Fe, Ca, and Na contents, as otherwise chloritoid, amphibole, feldspar, or omphacite are formed instead of kyanite or talc. Furthermore, the stability field of the whiteschist mineral assemblage strongly depends on XCO 2 and fO 2 : already at low values of XCO 2 , CO 2 binds Mg to carbonates strongly reducing the whiteschist stability field, whereas high fO 2 enlarges the stability field and stabilizes yoderite. Thus, the scarcity of whiteschist is not necessarily due to unusual P-T conditions, but to the restricted range of suitable protolith compositions and the spatial distribution of these protoliths: (1) continental sedimentary rocks and (2) hydrothermally and metasomatically altered felsic to mafic rocks. The continental sedimentary rocks that may produce whiteschist mineral assemblages typically have been deposited under arid climatic conditions in closed evaporitic basins and may be restricted to relatively low latitudes. These rocks often contain large amounts of the clay minerals palygorskite and sepiolite. Marine sediments generally do not yield whiteschist mineral assemblages as marine shales commonly have too high iron contents. Sabkha deposits may have too high CO 2 contents. Protoliths of appropriate geochemical composition occur in and on continental crust. Therefore, whiteschist assemblages typically are only found in settings of continental collision or where continental fragments were involved in subduction. Our calculations demonstrate that whiteschists can form by closed-system metamorphism, which implies that the chemical and isotopic composition of these rocks provide constraints on the development of the protoliths.

On yoderite: Using calculated phase equilibria to investigate its rarity in the geological record of whiteschists

“…More recently, Franz et al. () calculated phase diagrams for rock compositions largely conforming to the MASHO system and determined that stable talc–kyanite assemblages can occur above pressures of ~5 kbar, in agreement with the experimental results of Fockenberg and Schreyer (). In naturally occurring whiteschists, P – T estimates from talc–kyanite assemblages have been determined via conventional barometry to be stable at 6–8 kbar in the Modum Complex (Norway; Munz, ).…”

“…Whiteschists are always found in ancient collisional zones where continental rocks were subducted. As a consequence, there is a predominance of whiteschists in HP–UHP terranes (Franz, Romer, & de Capitani, ; Johnson, ).…”

“…Second, the ability to understand the metamorphic evolution of whiteschists is impeded by the inability to calculate reliable thermodynamic models for rocks with compositions closely approximating the MASHO system, largely because of the absence of an activity–composition ( a–x ) model for yoderite (cf. Franz et al., ). Franz et al.…”

“…Franz et al. () provided the only existing study to have performed calculations using an a – x model for yoderite. In their study, they explored controls on yoderite and whiteschist stability, but their yoderite a – x model was of a fixed, single composition.…”

“…This study presents the first attempt at developing a solid‐solution a–x model for yoderite that is calibrated on the basis of matching calculated phase equilibria to experimentally constrained invariant and univariant equilibria, that is P – T grids (cf. Franz et al., ).…”