Late Paleozoic tectonic evolution of Ganzi-Litang Ocean in Sanjiang region, Southwest China: Constraints from U-Pb geochronology of cumulate gabbro in Litang ophiolitic mélange belt

Located in the northwest of Gaocheng Town, Litang County, the Caganglongwa rock Formation is a set of ophiolitic mélange with basalt as the matrix. LA-ICP-MS zircon U-Pb dating of the cumulate gabbro in the ophiolitic mélange shows an age distribution range from 274 Ma to 349 Ma, indicating two age groups. The first group of baddeleyite ²⁰⁶Pb/²³⁸U has a weighted average age of 346 ± 17 Ma, which represents the age of early magmatism of cumulate gabbro; the second group of baddeleyite ²⁰⁶Pb/²³⁸U has a weighted average age of 286.2 ± 5.1 Ma, which represents the crystallization age for late magmatism of cumulate gabbro. The ultrabasic and basic rocks in the Caganglongwa rock Formation display combined characteristics of N-MORB and E-MORB, indicating that these rocks may be formed in the mid-ocean ridge environment, and the magma material comes from a mixed source region formed by depleted upper mantle source region represented by N-MORB and metasomatically contaminated by the mantle plume. The research finds that the Ganzi-Litang Ocean may begin to stretch as early as the Middle Devonian due to the regional mantle plume activity, resulting in the split of the Zhongza block from the western margin of the Yangtze block and the Formation of an ocean basin in the Early Carboniferous. By the Mid-Late Triassic, the Ganzi-Litang oceanic crust was subducted westward and closed at the end of the Late Triassic, and the region entered the arc-continent collision orogeny stage.