Grain-size reduction of feldspar and flow of deformed granites within the Gaoligong shear zone, southwestern Yunnan, China

• Published in: Science China. Earth sciences 2019-09, Vol.62 (9), p.1379-1398
• Main Authors: Dong, Yanlong, Cao, Shuyun, Cheng, Xuemei, Liu, Junlai, Cao, Hanchen
• Format: Article
• Language: English
• Subjects: Biotite; Composition; Creep strength; Crystal structure; Crystallography; Deformation; Deformation mechanisms; Dislocation; Dislocations; Earth and Environmental Science; Earth Sciences; Fabrics; Feldspars; Grain size; High temperature; Lithosphere; Microstructure; Mineral assemblages; Mineral composition; Minerals; Particle size; Plagioclase; Preferred orientation; Quartz; Recrystallization; Research Paper; Rheological properties; Rock; Rocks; Shear zone; Size reduction; Solifluction; Superplasticity; Trace elements
• ISSN: 1674-7313; 1869-1897
• DOI: 10.1007/s11430-018-9351-8

Mineral deformation and rock flow mechanism in the lithosphere are related to the rheological behavior and weakening mechanism of the continent. Natural deformation behaviors of feldspars are not well understood due to the complexity of their mineral compositions, crystal structures, as well as changing deformation conditions. The refined microstructure, fabric and composition of major minerals in the deformed granitic rocks within the Gaoligong shear zone (GLGSZ), southwestern Yunnan, China, were studied. With increasing mylonitization, two fabric types of end-members have been distinguished (type-I banded granitic mylonite and type-II banded ultramylonite). The two types of deformed granitic rocks have the same mineral assemblage, but different mineral modes. The type-I banded granitic mylonite has a greater proportion of K-feldspar (mostly present as porphyroclasts)>plagioclase>quartz±biotite, however, the type-II banded ultramylonite has a greater proportion of fine-grained plagioclase>K-feldspar>quartz±biotite. The crystallographic preferred orientation (CPO) patterns of quartz combined with two-feldspar geothermometer, confirm that the quartz grains in the type-I and type-II granitic rock have undergone high-temperature dislocation creep deformation. The K-feldspar grains in the matrix of type-II banded ultramylonite show a dominant (100) [010] slip system with dislocation creep recrystallization, while the fine-grained plagioclase grains present a weak CPO pattern with superplastic flow. The K-feldspar porphyroclasts show grain-size reduction associated with mineral composition and fabric transformation. The myrmekite formation with the fine-grained neocrystallization of plagioclase and quartz significally replaced the K-feldspar porphyroclasts. Finally, the fine-grained neocrystallization plagioclases were formed further into the high strain localized ultramylonites with superplastic flow.