Water Content and Deformation Microstructure of Layered Panzhihua Gabbro: Implications for the Origin of Rhythmic Layering

Abstract:

The Panzhihua, Hongge and Baima mafic-ultramafic layered intrusions host three giant Fe-Ti oxide deposites in the Pangzhihua-Xichang (Panxi) region, SW China. Various proportions of silicates (plagioclase, pyroxene and olivine), ilmenite and magnitite form rhythmic layering in the upper part of the intrusions. The origin of rhythmic layering in mafic-ultramafic layered intrusions has been debated for a long time in the literatures. Here we provide detailed FTIR and EBSD studies on the water content and deformation microstructure of gabbros from the Panzhihua intrusion. The FTIR analyses revealed a significant amount of hydroxyls in both clinopyroxene (411-775 ppm) and plagioclase (328-716 ppm), suggesting a high water content mantle plume source. The EBSD analyses show: 1) a weak clinopyroxene fabric of (100) parallel to foliation and [001] parallel to lineation; 2) a strong plagioclase fabric of (010) parallel to foliation and [100] parallel to lineation; 3) a weak ilmenite fabric of (001) parallel to foliation and [hk0] parallel to lieantion; 4) a near random magnitite fabric. These results suggest weak plagioclase is the main strain accommedation phase in lower crustal rocks under hydrous conditions, which is also consistent with previous deformation experiments on plagioclase and clinoproxene. Water plays an important role in deteriming the rheology contrast between plagioclase and clinoproxene. Our results revealed strong layer-parallel shearing deformation during the formation of the Panxi layered intrusions. We propose that the formation of the rhythmic layering is mainly caused by rheological stratification rather than chemical stratification of Fe-Ti oxides and gabbros.