Evidence for cool extrusion of the North Indochina block along the Ailao Shan Red River shear zone, a Diancang Shan perspective

Much debate surrounds the temporal-thermal-structural evolution of the >1000-km NW-SE-trending Ailao Shan–Red River (ASRR) shear zone. This mainly reflects the contradictory interpretations of the timing and P-T conditions for the occurrence of left-lateral shearing event due to a diverse data set of mineral ages. Our new microstructural, petrological, and geochronological data from the southern section of the Diancang Shan (DCS) block along the west wall of the ASRR support slow cooling from ∼300°C to ∼150°C from the Middle Eocene to the Late Miocene, accompanied by left-lateral shearing activity. This conclusion is based on the following: (1) The lower greenschist facies assemblages of biotite replaced by muscovite or chlorite, the albitized K-feldspar, and the stable association of K-feldspar + chlorite rather than muscovite + biotite are universally observed for the synkinematic S_n+1 and mylonitic S/C fabrics from both quartzofeldspathic and amphibolite domains. (2) New ⁴⁰Ar/³⁹Ar data for K-feldspar and muscovite of various structural domains such as the K-feldspar phenocrysts (∼20 to ∼40 Ma) from the early Triassic magmatic protolith (zircon U-Pb ages of 230–250 Ma), pre- to synkinematic muscovite (∼17 to ∼23 Ma) in mylonites, and K-feldspar (∼5 to ∼10 Ma) in postmylonitic cross-cutting veins produce an age spectrum that increases from ∼5 to ∼40 Ma under lower greenschist facies. Together these data indicate that the DCS block was slowly extruded along the ASRR shear zone through much of the Tertiary. These results contradict earlier interpretations of rapid cooling from synkinematic amphibolite metamorphic conditions caused by shear heating based in part on the assumption that all Miocene ages were cooling ages rather than crystallization ages.