Early to Middle Paleozoic magmatism and metamorphism in the Alxa Block and its northern margin: Implications for the western extension of the Bainaimiao arc

The Early to Middle Paleozoic tectonic evolution of the Alxa block is still unclear due to the sporadic outcropping of magmatic rocks and widespread coverage by the Mesozoic to Cenozoic sedimentary rocks or sediments, which makes it difficult to contrast and connect the northern margin of the Alxa block with the adjacent tectonic units. We address this key issue using new geochronological and geochemical analyses of the Early to Middle Paleozoic igneous and metamorphic rocks from the Alxa Block and its northern margin. Early Paleozoic plutons outcropping on the northern margin of the Alxa Block show slightly enriched to depleted isotopic compositions and arc-related geochemical characteristics, similar to Bainaimiao arc plutons on the northern margin of the NCC, supporting the western extension of the Bainaimiao arc. In contrast, Early Paleozoic intrusive rocks (458–440 Ma) in the Alxa Block exhibit more enriched isotopic compositions and ancient model ages and were generated in a continental arc setting triggered by southward subduction of the Paleo-Asian Ocean (PAO). The meta-mafic rocks documented ca. 419-415 Ma regional metamorphic events in response to an arc-continent collision event in the northern Alxa Block. Our magmatic and metamorphic data combined with published data reveal that the linear Early to Middle Paleozoic orogenic belt is distributed along the northern margin of the Alxa Block. Similar Early to Middle Paleozoic oceanic subduction and subsequent arc-continent collision evolution history support that the Alxa Block was probably a part of the North China Craton during the Early Paleozoic.