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

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 epsilon(Hf)(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 epsilon(Hf)(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.