内蒙古阿拉善北缘尚丹岩体的年代学、地球化学特征 及构造意义

The Northern Alxa Orogenic Belt is the southernmost segment of the Central Asian Orogenic Belt. The less investigated Shangdan pluton is a key point to understanding its tectonic evolution. The granodiorites from ages of 331.7 ±0.9Ma, which is the earliest age obtained for the Late Paleozoic plutons in the Zongna and indicates an Early Carboniferous magmatic event The granodiorites from the Shangdan pluton have moderate silicon contents (SiO₂ = 66.07% ~ 68.15%), high Na₂O contents (Na₂O/K₂O > 1.6) and relatively high CaO contents (CaO/Na₂O > 1), and belong chemically to metaluminous series (A/CNK < 1, A/NK > 1) with medium-K and calc-alkaline (σ = 1.21 ~ 1.40, K₂O = 1.55% ~ 2.21%) characteristics. The hornblendes are also found in the rock slices. Both the geochemistry and petrography show am ity to the (I-type granites. The granodiorites have moderate REE contents and moderate to strong LREE to HREE differentiation ((La/Yb)_N = 5.30 ~ 6.62). They are enriched in Rb, Th, K, Zr, Hf and depleted in Ba, Nb, Ta, P, Ti. No negative Eu anomalies have been found in the samples, suggesting that they might be derived from partial melting of middle-lower crust. Meanwhile, two whole-rock samples have slightly negative ε_Nd (t) values of -1.02 and -0.91 with Nd model age t_DM1 of 1.30 ~ 1.28 Ga. One sample yields zircon ε_Hf (t) values between +0.30 and +5.27 with Hf model age t_DM2 of 1.32 ~ 1.00Ga. Combined with Shangdan pluton was likely recycled from the Meso-Neoproterozoic crust. In conclusion, the Shangdan pluton with arc characteristics has been discriminated to be formed in a subduction-related setting, suggesting that after the termination of the Early-Middle Paleozoic orogenesis in the northern Alxa, the Late Paleozoic subduction occurred at least in the Early Carboniferous.