Origin and Tectonic Significance of the Early Paleozoic Magmatism in the Alxa Block

Genetic studies on the Early Paleozoic magmatism in the Alxa block is crucial for understanding the interaction processes between the Alxa block and the Paleo-Asian Ocean. We have newly identified a granite intrusion (the Gashun granite) emplaced in Middle Silurian, with a zircon U-Pb age of 432Ma. The samples collected from this granite are weakly peraluminous, belonging to the medium-high-K calc-alkaline series, with high Sr/Y ratios and epsilon(Hf) (t) = -8.8 similar to -19.4, which suggests that the Gashun granite should be formed by melting of ancient lower crustal rocks. Based on the major and trace element compositions of Late Ordovician to Early Devonian magmatic rocks in the Alxa block, two genetic types of magmatic rocks have been identified: Type I, emplaced during Late Ordovician to Early and Middle Silurian, are typical arc magmatic rocks formed by partial melting of mantle wedge metasomatized by fluids from subducting slab; Type II, emplaced during Middle and Late Silurian to Early Devonian, are characterized by high Sr/Y ratios resulted from partial melting of ancient intermediate to mafic crustal materials. The magmatism in the Alxa block transitioned from mantle-derived typical arc magmatic rocks during Late Ordovician-Early and Middle Silurian to crust-derived high Sr/Y-type granites during Middle and Late Silurian-Early Devonian, and then gradually disappeared, reflecting that the Alxa block transformed from a relatively extensional continental arc to a compressional arc. This magmatic evolutionary process recorded that either a subduction of Paleo-Asian Ocean beneath the Alxa block were gradually weakening and ceasing, or a transition of the subduction angle from steep to flat happened during the Late Ordovician to Early Devonian. In general, the Alxa block was under an overall increasingly compressional dynamic background in Early Paleozoic.