Labradoritic Hollandite in the Central Uplift of the Manicouagan Impact Structure

English Abstract:

The mineralogy, texture and formation of new phases associated with shock veins developed in the central uplift of the Manicouagan impact structure, Canada, have been studied using micro-Raman spectrometry and analytical field emission scanning electron microscopy. The shock veins were formed in anorthositic and gabbroic gneisses as thin fracture-microfault systems during the contact/compression stage of crater formation at 214 Ma. The shock veins comprise an amorphous to nano-micro crystalline matrix, which may exhibit fluidal textures, hosting wall rock mineral fragments of hornblende, augite, plagioclase and garnet. Here we report the existence of high-pressure polymorphs of plagioclase (labradoritic-hollandite) that have been observed in natural terrestrial rocks. The chemical composition of the polymorph is (Ca_0.54,Na_0.46)Al_1.6Si_2.4O₈, which is compatible with the composition of the host rock labradorite. In many cases the labradoritic-hollandite (Lab-Holl) coexists with stishovite. The Lab-Holl displays intense Raman bands at 695-697 and 376-378 cm^-1, characteristic of a jadeite-structured phase with a broad band at ~997 cm^-1. The spectrum also consists of the superimposed Raman bands at 485 and 523 cm^-1, close to that of host rock labradorite (481 and 509 cm^-1). We suggest that the shock impedance contrast between components, and rarefaction, are responsible for shear failure or fracture and in situ shock melting of local portions of the host rock with associated crystallizing quench phases within the shock veins. The formation of high pressure phases is interpreted to be the result of the solid-state transformation of labradorite and/or direct crystallization from a labradorite melt. The development of mosaic-fractured stishovite crystals and neocrystallites of Lab-Holl with interstitial glass indicate metastable conditions in the melt veins due to the spatially and temporally heterogeneous shock process. The coexistence of the Lab-Holl and stishovite sets an upper bound for shock pressure in the shock vein to near 23 GPa.