Deformation microstructures and fabric development of clinopyroxene in antigorite serpentinite: Insights from Val Malenco, Italian Alps

The (100)[001] clinopyroxene (cpx) fabric, characterized by clustering of the [001] axes along the lineation and the [100] axes normal to the foliation, is a common cpx fabric in lower crustal and mantle rocks. However, the physical processes remain unclear for the formation of this common cpx fabric. This study investigates the deformation microstructures of cpx in an antigorite serpentinite from the Val Malenco, located in the eastern central Italian Alps. The cpx grains can be classified into four types: porphyroclasts cpx, lenticular polyphase aggregate bands, fine-grained polyphase aggregates along olivine grain boundaries, and fine-grained cpx aggregate pockets. Microstructural observations, along with major and trace element analyses, suggest that the porphyroclasts cpx are remnants of primary peridotite, while other cpxs are metasomatic products by fluid activity. Notably, the cpx grains in the lenticular polyphase aggregate bands show a strong (100)[001] fabric, with a significant correlation between shape preferred orientation (SPO) and crystallographic preferred orientation (CPO) in fine-grained cpx aggregates. Intracrystalline deformation features are rare in cpx, indicating the operation of mechanisms other than dislocation creep. Based on the analyses of chemical composition and microstructure, we propose that dissolution-precipitation creep and oriented growth, governed by deviatoric local stress, are key mechanisms for the development of the strong (100)[001] cpx fabric in the Val Malenco serpentinized peridotite. This work also sheds light on the interpretation of strong (100)[001] cpx fabrics observed in lower crustal and upper mantle rocks in fluid rich environments.