PALEOTETHYAN SUBDUCTION REVEALED FROM TRIASSIC BLUESCHISTS IN THE LANCANGJIANG TECTONIC BELT OF SOUTHWEST CHINA

The subduction of the Paleotethyan Ocean and subsequent continental collision along the Lancangjiang tectonic belt of the southeastern Paleotethyan belt is a major tectonic event in Southwest China, but the events of the subduction preceding the final collision is still not well constrained. The mafic blueschists exposed in the Lancang accretionary complex provide the crucial records of the Paleotethyan subduction process. In this paper, we present a set of new petrological, geochronological and geochemical data for the Suyi mafic blueschists in the Lancang metamorphic zone. The mineral assemblage of these blueschists consists of zoned sodic amphibole (25–30 %), albite (15–20 %), epidote (25–30 %), phengite (5–10 %), chlorite (~5-10 %), and minor amounts of actinolite, apatite, sphene, zircon, ilmenite, quartz and secondary limonite. This suggests a prograde metamorphism from ~0.5 GPa to ~0.9 GPa and retrograde metamorphic overprinting (back to ~0.6 GPa) within the temperature range 300–450 °C. The glaucophane during prograde metamorphism yield a ⁴⁰Ar/³⁹Ar plateau age of 242 ± 5 Ma (MSWD=0.77; P=0.54). The Suyi blueschists give the zircon U-Pb age of 260 ± 4 Ma and are compositionally identical to the subalkaline basalt with typical OIB-type REE and multi-elemental patterns and show ε_Nd(t) values ranging +3.35 from +4.85. Based on available data, it is inferred that the protolith formed at 260 Ma and originated from a basaltic seamount. The basaltic rocks subducted down to 30–35 km depths beneath the Lincang arc to form the epidote blueschists at ~242 Ma. The blueschists were subsequently transported to shallower crustal levels in response to the continuous underthrust of the subducted slab and the continent-continent collision at middle-late Triassic. These results provide the systematic constraint on the tectonic evolution and temporal framework of the southeastern Paleotethys belt in Southwest China.