Paleo-Tethyan tectonic evolution of Lancangjiang metamorphic complex: Evidence from SHRIMP U-Pb zircon dating and 40Ar/ 39Ar isotope geochronology of blueschists in Xiaoheijiang-Xiayun area, Southeastern Tibetan Plateau

摘要

Blueschists are sporadically exposed as lenses within the Lancangjiang metamorphic complex, and represent unique components of the Paleo-Tethys. In this paper, we present geochemical and geochronological results of blueschists to decipher their origin and tectonic significance. The whole-rock geochemical analyses revealed strong similarities with ocean island basalt (OIB), and further discrimination diagrams confirm an affinity to a within-plate setting. Combined studies on blueschists using cathodoluminescence (CL) imaging, SHRIMP U-Pb dating of zircon domains and40Ar/39Ar dating of phengite and glaucophane provide evidence of their magmatic origin and metamorphic evolution. Slightly oscillatory zoned or compositionally homogeneous zircon grains/domains, as well as structureless zircon rims, yield ages from 231.6±3.7 to 225.3±4.8Ma, recording the blueschist facies metamorphic event. In contrast, the captured zircon grains and cores with a major age peak at ~241Ma as well as several minor older age peaks indicate the multiple provenance of the zircons.40Ar/39Ar step heating analyses on single grains of phengite and glaucophane separated from blueschists yield plateau ages ranging from 242.5±1.4 to 228.7±1.5Ma which are interpreted to reflect high-pressure metamorphism.This study provides geochemical and geochronological constraints on the tectonic evolution of the Paleo-Tethyan ocean, which was closed and subsequently subducted as a result of the collision of the Simao and Baoshan Blocks. During subduction in Trassic (243 to 225Ma), the protoliths of blueschists underwent blueschist facies conditions.