Petrogenesis and tectonic significance of Early Paleozoic magmatism in the northern margin of the Qilian block, northeastern Tibetan Plateau

Abundant Early Paleozoic magmatism is preserved in northern Tibet and has important implications for continental crustal growth in response to continental collisions. To better constrain the evolution of the northern margin of the Tibetan Plateau and the resulting closure of the North Qilian Ocean, we conducted an integrated investigation involving U-Th-Pb zircon geochronology, whole-rock geochemistry, and syntheses of existing data sets across the North Qilian orogenic belt. Zircon U-Pb dating indicates that the Early Paleozoic intrusive rocks in the study area can be divided into two stages: 460-480 Ma and 440 Ma. The 478-480 Ma meta gabbro-diorite samples belong to the calc-alkaline series and show a fractional crystallization trend in the Harker diagrams. These samples have relatively high La/Nb (2.08-3.26) and low La/Ba (0.05-0.06) ratios, indicating a subduction-modified continental-lithospheric mantle source. The 460-480 Ma meta-granite samples are I-type and classified as high-K calc-alkaline series. Both the meta gabbro-diorite and meta-granite samples are characterized by negative Nb, Ta, and Ti anomalies and enriched LILEs, and showing arc signatures that may be related to the southward subduction of the North Qilian oceanic crust. The 441 Ma hornblende syenite samples have high contents of alkalis (K2O +Na2O = 780%-12.07%) and belong to the alkaline series, suggesting an extensional geological setting. The 440 Ma syenogranite samples are classified as high Ba-Sr granites and belong to the shoshonitic series. The syenogranite samples were most likely generated by partial melting of lithospheric mantle that had been metasomatized by slab-derived fluids, and mixed by the crust derived granitic magmas during the ascent of the magma. Both the hornblende syenite (441 Ma) and the syenogranite (440 Ma) samples were emplaced in a postcollisional tectonic setting associated with the closure process of the North Qilian Ocean.