The ∼860 Ma mafic dikes and granitoids from the northern margin of the Yangtze Block, China: A record of oceanic subduction in the early Neoproterozoic
Abstract There are voluminous Neoproterozoic arc-related volcano-sedimentary sequences and small intrusions on the northern margin of the Yangtze Block, South China. The understanding the origin of the Sanligang granitoid intrusion and the spatially associated mafic dikes in the region is crucial for unraveling the tectonic evolution and continental crust growth processes in the Yangtze Block. Zircon U–Pb dating suggests that the mafic dikes (ca. 871 Ma) and granitoids (ca. 860 Ma) are contemporaneous. The mafic dikes have low SiO2 (45.37–46.55 wt.%), K2O (0.32–0.82 wt.%) and Na2O (2.01–2.85 wt.%), and are characterized by enrichment in large ion lithophile elements (LILEs) and depletion in high-field strength elements (HFSEs), suggesting that their mantle source was modified by subducted materials. The Sanligang granitoids have intermediate to high SiO2 (60.35–71.38 wt.%), intermediate K2O (1.38–3.67 wt.%) and Na2O (3.97–5.33 wt.%), and high MgO (1.03–3.16 wt.%). They show LREE-enriched REE patterns (La/YbN = 7.2–12.3) with no or minor negative Eu anomalies. Their primitive mantle-normalized trace element patterns are characterized by enrichment of LILEs and depletion of HFSEs. Both the mafic dikes and granitoids share similar zircon ɛHf(t) values (+10.5 to +12.9, +7.9 to +11.7, respectively), whole-rock initial 87Sr/86Sr ratios (0.7051–0.7057, 0.7033–0.7041, respectively) and ɛNd(t) values (+4.0 to +7.1, +3.4 to +4.9, respectively), suggesting that the granitoids were generated by partial melting of juvenile basaltic crust. High Mg# values (49–58) in the granitoids may have resulted from assimilation of residual mafic minerals in their source region. Based on its arc-related geochemical affinity and contemporaneous arc-related magmatism, the Sanligang pluton is proposed to be generated in a Neoproterozoic arc setting during crustal growth and reworking. The early Neoproterozoic assemblage from the Sangligang–Sanyang fault belt provides an important record of oceanic slab subduction in the northern margin of the Yangtze Block.