Timing of Early Paleozoic oceanic crust subduction in North Altun: Evidence from plagiogranite and granodiorite

The granitic rocks that were formed from ophiolite or intruded into the ophiolites are important for the precise dating of ophiolites, and some of them can be the powerful evidence for constraining the timing of subduction. Based on detailed studies of field geological background and petrographical features, the authors conducted analysis of whole-rock major, trace and rare earth elements and zircon U-Pb ages as well as Lu-Hf isotopes for the plagiogranite and granodiorite. The weighted mean ages of plagiogranite and granodiorite from Hongliugou-Lapeiquan ophiolite in North Altun Mountains determined by LA-ICP-MS UPb method are 501±3 Ma and 496±2 Ma, respectively, suggesting that the subduction timing of the northern Altun Ocean might have started in the mid-Cambrian or earlier. The zircons from plagiogranite and granodiorite exhibit positive ε_Hf (t) values of 1.6-5.6 and 2.7-6.9, separately, implying that the plagiogranites and granodiorites were all derived from the depleted mantle. Geochemical analysis indicates that the plagiogranites are characterized by high SiO₂, high Sr, low Y and corresponding high Sr/Y, which are similar to features of adakitic rocks, suggesting that they resulted from the small proportion of partial melting of metabasites during the hot oceanic subduction to the depth of 40-50 km, belonging to the amphibolite facies. The granodiorite belongs to the series of high-Na calc-alkaline rocks, and may represent the product of mafic rocks remelting from the lower crustal in an island arc setting. The geochronological analysis indicates that the north Altun Ocean might have experienced both northward and southward subductions, and the northward subduction of the northern Altun Ocean may be slightly earlier than the southward subduction. The comparative study of the subduction time between the northern Altun and northern Qilian shows that the northern Altun Early Paleozoic suture zone is the extension of the North Qilian Early Paleozoic suture zone.