Abstract
Abstract In this paper, we present new petrological, zircon U–Pb–Hf isotopic, bulk-rock geochemical, and Sr–Nd isotopic data for the rocks from the Pennsylvanian Liushugou and Qijiagou Formations, Bogda Mountains (BMs), northwest China. The new data help in understanding the petrogenesis and geodynamic background of the two formations, further constraining the evolution of BMs during the Pennsylvanian. The eastern Liushugou Formation is composed mainly of bimodal volcanic rocks, while the western Liushugou Formation is dominated by pillow basalts with interstitial limestones, peperites, and pyroclastic rocks. The Qijiagou Formation consists principally of bioclastic limestones, peperites, and volcanic and volcaniclastic rocks with turbidites. Depositional environment analyses of the Liushugou and Qijiagou Formations reveal subaqueous volcanism and a progressively deepening shallow marine environment with times. Zircon LA-ICP-MS U–Pb dating of felsic volcanic rocks from the Liushugou Formation indicates that the subaqueous volcanism occurred at ca. 310–302 Ma, viz., the Pennsylvanian era. The basaltic rocks from the Liushugou and Qijiagou Formations are high-K calc-alkaline, enriched in light rare earth elements and large-ion lithophile elements, and depleted in high-field-strength elements (Nb, Ta, and Ti). The above characteristics, together with their depleted isotopic signature (εNdt=3.0-8.1, εHft=8.0-15.6, and ISr=0.703-0.707), suggest the derivation from a depleted mantle source metasomatized by slab-derived fluids and sediment-derived melts. Most felsic volcanic rocks of the high-K calc-alkaline to shoshonite series from the Liushugou and Qijiagou Formations show features of the A2-type granites and have similar trace and isotopic composition to the basaltic rocks, which were probably generated from the partial melting of juvenile continental crust. Combining the newly acquired data with the regional geology, we propose that the Pennsylvanian volcanic and sedimentary rocks in the BMs were formed in a series of postcollisional rift basins which were related to local strike-slip faulting. Moreover, the volcanic rocks in the east were derived from a relatively deeper mantle source (thick lithosphere) due to their smaller rifting.
Highlights
The postcollisional setting is distinctive and not integrated into the model of plate tectonics (e.g., [1])
The postcollisional magmatism is so complex, but they still share some common characteristics: (1) They mainly belong to the high-K calc-alkaline to shoshonitic series, of which the felsic volcanic rocks are usually peraluminous and have attributes of A2-type granites [3,4,5,6,7,8]
Three felsic volcanic rocks were collected from the Haxionggou section for zircon LA-ICP-MS U–Pb dating. 16HXG-12 and 16HXG-13 were chosen from the western Liushugou Formation (Haxionggou section). 16QJJ-15 was selected from the eastern Liushugou Formation (Qijiaojing section)
Summary
The postcollisional setting is distinctive and not integrated into the model of plate tectonics (e.g., [1]). How to distinguish the characteristic of postcollisional magmatism remains questionable [1, 2]. Liegeois [1] proposed that geochemical discrimination diagrams cannot characterize the postcollisional tectonic setting due to a variety of magma types generated coevally. The postcollisional magmatism is so complex, but they still share some common characteristics: (1) They mainly belong to the high-K calc-alkaline to shoshonitic series, of which the felsic volcanic rocks are usually peraluminous and have attributes of A2-type granites [3,4,5,6,7,8].
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