Record of tectonic transformation in SW Yunnan: Geochemical and zircon U-Pb geochronological evidence from arc-derived, Late Devonian volcanogenic sediments in the southern Lancangjiang zone

摘要

The Longmucuo-Shuanghu and Lancangjiang (including Changning-Menglian) suture zones in northern Tibet and western Yunnan, preserve abundant geological records of the Proto-Tethys (Late Sinian-Silurian) and Paleo-Tethys (Devonian-Triassic Oceans, suggesting a temporal and spatial connection between the two.In order to document their relationship, one must understand the tectonic transformation that took place in the Devonian.In this paper, we present new elemental data as well as LA-ICPMS U-Pb zircon ages on the volcanogenic sediments from the southern Lancangjiang zone in SW Yunnan.Three tuff samples yielded weighted mean 206Pb/238U ages of 366±5 Ma, 378±4 Ma and 382±8 Ma, documenting the Devonian age of these sedimentary rocks.Of 108 analyses, 105 on 101 grains from two sandstone samples have extremely tight age clusters, ranging from 355 Ma to 385 Ma, with mean 206Pb/238U ages of 363±3 Ma (n=63, MSWD=1.7) and 364±4 Ma (n=42, MSWD=1.8), indicating a relatively uniform source with rapid deposition.