Age, Geochemistry and Geological Significance of an Early Neoproterozoic Porphyritic Granite in the Jianshanzi Area of the Beishan Orogenic Belt

The geochronological and geochemical data of an early Neoproterozoic porphyritic granite in the Jianshanzi Area of the Beishan Orogenic Belt has been present. Combined with published geochemical data of contemporaneous granitic rocks, their magma sources and the tectonic affinity between the Precambrian basement in the southern Beishan and Tarim Cration are discussed. Two samples from the porphyritic granite have zircon U-Pb ages of (901±5) Ma and (935±3) Ma, respectively, indicating that it was formed during the early Neoproterozoic. Samples from the porphyritic granite contain K-feldspar phenocrysts with high SiO₂ contents (70.41%～76.05%) and peraluminous characteristics (A/CNK=1.01～1.21), and exhibit similar chondrite-normalized REE patterns with LREE-enrichment and pronounced negative Eu anomalies (δEu = 0.30～0.46), and show positive Rb, Th, U and K anomalies, negative Ba, Nb, Ta, Sr, P and Ti anomalies in the primitive mantle-normalized trace-element diagram, and have evolved zircon ε_Hf(t) values of −5.0 to −1.4 and two-stage Hf model ages of 2.08～1.61 Ga. All these geochemical characteristics suggest that the porphyritic granite is a typical crustal-derived granite, indicating an older crustal reworking event. Together with the reported data, the Precambrian basement of the southern Beishan experienced both Mesoproterozoic crustal growth and early Neoproterozoic crustal reworking events with the zircon ε_Hf(t) values of granitic rocks plotting within the crustal evolution trend defined by 2.1～1.3 Ga crustal material, indicating the absence of an Archean basement in the southern Beishan, and therefore it may not have tectonic affinity with the Tarim craton with an Archean crystalline basement.