Crustal evolution of Ordovician to Devonian subduction and collision zones of eastern Central Asian Orogenic Belt: Insight from igneous rocks of Xing'an Block

Abstract

Different phases of subduction and arc-continent collision commonly influence magmatic flare-ups and lulls, as well as processes of crustal accretion and reworking. Well-preserved Ordovician to Devonian magmatic and sedimentary records in the western Xing'an Block (XAB) provide valuable insights into the Paleozoic oceanic crust subduction phases and their effects on arc magmatism and crustal evolution. Zircon U-Pb dating results reveals newly identified Early Ordovician–Devonian granitoids and dacites in the western XAB, forming at 474–462, 449–446, 421–417, 383, and 367 Ma. Voluminous Ordovician to Devonian igneous rocks in western XAB documented a long magmatic life span (499–367 Ma) with a high-flux (flare-up) event occurring at 449–439 Ma and a magmatic lull from 400 to 370 Ma. Whole-rock geochemical data and zircon Hf isotopes suggest that Late Ordovician biotite monzogranite magma with high temperatures (843–852 °C) features, formed through partial melting of Mesoproterozoic accreted mafic crust with an amphibole-dominated residue. In contrast, fluid-present partial melting of pelitic rocks produced Late Silurian and Early Devonian (421–417 Ma) muscovite monzogranite and two-mica monzogranite, while Late Devonian dacite magma originated from partial melting of juvenile mafic lower crust. The Late Ordovician flare-up event, marked by voluminous high temperature granitoids, coeval bimodal volcanic rocks and Nb-rich gabbro was likely triggered by ridge subduction. A transition from advancing to retreating subduction during the Ordovician drove systematic crustal thickness changes and a shift from dominant crustal reworking and minor juvenile crust melting outboard, to mixing of dominant ancient and minor juvenile crustal materials in western XAB. Late Silurian and Early Devonian collapse extension following arc-continent collision induced extensive remelting of Proterozoic sedimentary rocks, while sustained Middle to Late Devonian extension promoted extensive partial melting of juvenile crust.