The subduction-collision transition of the Early Devonian southern Tianshan Ocean:Evidences from intrusive rocks in the Burandaban–Baluntai area

In order to explore the petrogenesis, formation ages and tectonic environments of intrusive rocks in the Burandaban-Baluntai area of the Central Tianshan, we have carried out the LA-ICP-MS zircon U-Pb dating and geochemical study of samples from the intrusive rocks (monzonitic granite, biotite granodiorite) in this paper. The results show that the intrusive rocks in the Burandaban-Baluntai area were emplaced in the Early Devonian [(393±4.5) Ma, (401±5.2) Ma], and they are characterized with relatively high contents of SiO2 (59.72%–73.39%), MgO (0.44%–4.97%), and K2O (2.81%–5.24%), with Mg# values ranging from 33 to 53. Especially, the monzonitic granite samples have A/CNK values of 1.00~1.09 (1.1). The biotite granodiorite belongs to the peraluminous high-calcium-potassium-alkaline S-type granite. The chondrite-normalized REE distribution patterns for samples of these granites are characterized by lightly right-declined patterns, with significant negative Eu anomalies ( δEu=0.30–0.72). Samples of these granites are enriched in K, Rb and Th, but relatively depleted in Ba and Sr. Comprehensive studies indicate that the Early Devonian monzonitic granite in the Burandaban-Baluntai area was formed by the primarily dominated magma derived from the partial melting of deep lower crustal material or crust-mantle mixed material, with the addition of a small amount of mantle magma, through a certain degree of crystallization diffrentiation in the magmatic evolution process. The Early Devonian biotite granodiorite in the study area is a product formed by the water-induced melting of metamorphic sandstone. Combining with regional geological data, it is concluded that the Burandaban-Baluntai region underwent the northward subduction of the South Tianshan Ocean Basin which was in the transition stage from the subduction to the collision at that time. The main collision stage was confined to the period following the Early Devonian.