Diancangshan Complex in SE Tibet: An Oligocene granitic pegmatitic volcanic edifice Open Access

The Cenozoic tectonic record of southeastern Tibet includes WNW-ESE−trending, large-scale ductile shear zones that are inconsistent with the lithospheric structure revealed by seismic data. The Diancangshan Complex is a major segment of the WNW-ESE−trending Ailaoshan−Red River “ductile shear zone” that contains voluminous granitic gneiss, augen mylonite, and sheared schists, which exhibit microtextures resembling those of typical mylonites. We investigated these mylonite-like rocks using TESCAN Integrated Mineral Analyzer, electron backscattered diffractometer, microscopic, and microprobe techniques to reassess the origin of these rocks. The analyses revealed that these mylonite-like rocks are actually granitic pegmatitic volcanic or subvolcanic rocks that did not undergo ductile deformation after solidification. The sequential mineral assemblages, large range of grain sizes for all minerals, and unidirectional solidification textures indicate dynamic crystallization related to a high degree of undercooling of the granitic pegmatitic melt. Dynamic crystallization during magma flow generated mylonite-like textures. Thus, the so-called Ailaoshan−Red River ductile shear zone does not exist. Detailed U-Pb dating revealed that >90% of the zircons in the pegmatitic rocks were inherited from their source rocks, and <10% were newly formed during the Oligocene (31−25 Ma). Heating of high-temperature, H₂O-rich fluids derived from crystallized, mantle-derived magmas, which had accumulated beneath the upper crust, rapidly partially melted the upper-crustal rocks and generated hydrous granitic melts. Both real granitic mylonite and mylonite-like pegmatitic rock are common in most ancient orogenic belts, and their origins are critical to understanding their tectonic evolution.