Elemental and isotopic perspective on the formation of Permian mafic magmatic rocks from the Xing'an Mongolia Orogenic Belt and northern North China Craton

Abstract

The identification of lithology and characteristics of the mantle sources plays a pivotal role in understanding the origin of mantle-derived magmas. In this study, we present new zircon U-Pb ages, geochemical and Sr-Nd-Pb isotopic data for the Tugurige basalts and gabbros, at the intersection of the Xing'an Mongolia Orogenic Belt (XMOB) and the northern North China Craton (NNCC). We combined this new evidence with previous data on the Permian mafic rocks of XMOB and NNCC to unravel the petrogenesis of these rocks and shed light on the tectonic evolution of the XMOB and NNCC during the Late Paleozoic. Zircon U-Pb isotopic data constrained the magmatic emplacement of the Tugurige basalts and gabbros at ca. 288 and 277 Ma, respectively. The low SiO₂, high MgO, E-MORB-like trace element patterns and isotopic compositions (ε_Nd(t): +3.5 to +3.9, (²⁰⁶Pb/²⁰⁴Pb)_i: 18.00–18.54) of the basalts are comparable to that of the Permian mafic rocks in the XMOB. The various Ba/Nb, La/Nb, La/Ta, Nb/Th, and Th/La ratios of the Permian mafic rocks from the XMOB demonstrate the contributions of two distinct mantle sources beneath the XMOB – asthenosphere and young lithosphere that were metasomatically enriched by the Paleo-Asian Ocean subduction pre-Permian. Conversely, the gabbros exhibit high SiO₂, low MgO, a typical enrichment in light rare earth elements (LREEs), large ion lithophile elements (LILEs), Th, and U relative to the high field strength elements (HFSEs), and an enriched isotopic composition (⁸⁷Sr/⁸⁶Sr_(t): 0.7069–0.7071, ε_Nd(t): −4.2 to −3.6, (²⁰⁶Pb/²⁰⁴Pb)_i: 18.13–18.23), analogous to the Permian mafic rocks of the NNCC. These observations are attributed to the partial melting of pyroxenite and peridotite from the ancient lithospheric mantle beneath the North China Craton, which had been significantly modified by multistage subduction-related metasomatism before the Permian. We suggest that the arc-like trace element signatures of the Permian mafic rocks from the XMOB and NNCC were supposed to be a remarkable degree of inheritance from the enriched lithospheric mantle without requiring the involvement of contemporaneous subduction environment or deep-Earth water cycling process as previously suggested. The mixed, asthenospheric and lithospheric mantle source of the XMOB's Permian mafic rocks suggests that the Permian magmatic activities probably developed in a post-collisional extensional setting. This implies that the final closure of the Paleo-Asian Ocean occurred prior to the Permian, at least in the western part of the XMOB.