Identification of Ordovician oceanic island basalt in the Changning-Menglian suture zone and its tectonic implications:Evidence from geochemical and geochronological data

The Changning-Menglian tectonic belt has been traditionally known as one of the Paleo-Tethys suture zones in the Sanjiang orogenic domain, Southwest China. This study focused on the newly found volcano-sedimentary rocks consisting of vesicular and amygdaloidal basalts, andesitic basalt, siliceous rock, and meta-mud siltstone in the Mengyong-Manghong area located to the east of the Tongchangjie ophiolitic rocks in the central Changning-Menglian suture zone. LA-ICP-MS zircon U-Pb dating reveals that the ²⁰⁶Pb/²³⁸U weighted mean age of the amygdaloidal basalt is 449.3±8.4Ma(MSWD=3.5), suggesting that this suite of volcanic rocks was emplaced in Ordovician. The basaltic rocks are rich in Al2O3 (12.11%~15.04%), show high K₂O/N₂O ratios of 0.03~0.44 and high MgO (6.78%~12.34%, 9.43% on average) with high Mg^# of 45.9~61.7, and Cr, Ni, Nb values of 58.6×10^-6~636×10^-6, 57.4×10^-6~410×10^-6 and 18.9×10^-6~32.8×10^-6 (310.3×10^-6, 202.1×10^-6 and 25.06×10^-6 on average) respectively. They have flat chondrite-normalized REE patterns with low (La/Yb)_N (averagely 5.25), (Ce/Yb)_N (4.42 on average) and (Ce/Sm)_N (1.68 on average). The ΣREE value of the basaltic rocks increases with the increasing of the total alkaline, which suggests that the rocks are similar to the Azores-type oceanic island basalts (OIB). Generally, this suite of volcanic rocks is alkaline with OIB affinity, which might have resulted from the melting of the head of the hotspot in the Intra-oceanic plate. The new discovery of the Azores-type OIB in the Changing-Menglian records the product of OIB activity in the Early Paleozoic evolution of Tethys Ocean, which provides important basic information for further understanding of the composite arc-basin system in the Tethyan domain of Southwest China.