Origin of Late Triassic mafic-ultramafic intrusions in the Hongqiling Ni-Cu sulfide deposit, Northeast China: evidence from trace element and Sr-Nd isotope geochemistry

摘要

There are many Late Triassic mafic-ultramafic intrusions in the Hongqiling magmatic Ni-Cu sulfide deposit, Northeast China. Research on magma evolution leading to formation of these mafic-ultramafic intrusions is of great significance for understanding the mantle beneath Northeast China and associated Ni-Cu mineralization. A trace element study of the No. 1, 3, and 7 intrusions in the Hongqiling deposit reveals that these mafic-ultramafic intrusions are characterized by enrichment of incompatible elements, which however cannot be interpreted by subduction modification. Furthermore, model of batch partial melting of depleted mantle accompanied by upper crustal contamination can simulate the trace element patterns of these mafic-ultramafic intrusions, but partial melting of depleted mantle accompanied by lower crustal contamination model cannot work. In addition, Sr-Nd isotopic compositions of the Hongqiling No. 1, 3, and 7 intrusions also indicate that crustal contamination could have occurred mainly during the magma ascent. Consequently, a possible scenario for the magma evolution is that the primary mafic-ultramafic magma was derived from batch partial melting of a depleted mantle, and then contaminated by Cambrian-Ordovician metamorphic rocks of the Hulan Group during ascent. We conclude that the mantle source contained no significant crustal component in the Late Triassic and was also independent of substantial contribution from subducted material, and therefore the Mesozoic large-scale lithospheric delamination beneath eastern China may happen after a period of time of the Late Triassic.