Prelude to Late Triassic Ni–Cu sulphide mineralization in the eastern Central Asian Orogenic Belt: Geochronological and geochemical constraints from Middle Triassic mafic‐ultramafic magmatism in central and eastern Jilin Province, NE China

Abstract

The Hongqiling‐Piaohechuan region in the east Central Asian Orogenic Belt (CAOB) is an economically important Ni–Cu sulphide mineralization region in China. It is widely accepted that these Ni–Cu sulphide deposits mainly formed during the Late Triassic period. The early Middle Triassic mafic‐ultramafic plutons are therefore key in understanding the mineralization process. Here, we report the laser ablation inductively coupled plasma mass spectrometer (LA‐ICP‐MS) U–Pb dating of zircon and geochemical data for two Middle Triassic mafic‐ultramafic plutons and one felsic pluton in central and eastern Jilin Province, NE China. The mafic‐ultramafic plutons are composed of gabbronorite and olivine websterite. Zircon dating results indicate that these mafic‐ultramafic plutons crystallized at 243 ± 1 to 245 ± 1 Ma in the Middle Triassic. The coeval felsic pluton is mainly quartz diorite. Zircon dating results indicate that the felsic pluton crystallized at 239.7 ± 0.63 Ma in the later period of Middle Triassic. The mafic‐ultramafic plutons are low in SiO2 (43.49–49.66 wt%), depleted in K2O (0.28–2.74 wt%) and Na2O (0.70–1.18 wt%). They have low rare earth element (REE) abundances without Eu anomalies, exhibit depletion in high‐field‐strength elements (HFSEs), and hold various εHf(t) values (−15.3 to +2.9). These findings suggest that the primary magma of mafic‐ultramafic plutons was derived from the partial melting of the depleted lithospheric mantle, metasomatized by subducted‐slab‐derived fluids/melts. Remarkably, these mafic‐ultramafic plutons have extremely high Cu–Ni contents (23.6–669.4 and 419–1,956 ppm, respectively). The coeval quartz diorites have 59.55–64.86 wt% of SiO2, 3.02–4.21 wt% of K2O and 3.97–4.49 wt% of Na2O. They are enriched in light REEs (LREEs) and large‐ion lithophile elements (LILEs); depleted in heavy REEs (HREEs) and HFSEs, and they have εHf(t) values in the range −6.4 to +2.9. Additionally, the felsic rocks exhibit adakitic‐features with high Sr/Y (28.5–58.7) and (La/Yb)N (4.9–12.2) ratios, implying the thickening crustal sources of the North China Craton (NCC) and CAOB, respectively. The above findings, combined with the regional geological data, suggest that the Middle Triassic mafic‐ultramafic plutons are the prelude to the large‐scale Late Triassic mineralization. This was induced by the thickening event of the later period of Middle Triassic felsic rocks, accompanying the final closure of the Palaeo‐Asian Ocean (PAO) in the eastern CAOB.