The origin of the mantle peridotite from ophiolitite in northeast Jiangxi and its geological implications

The ophiolite in northeast Jiangxi is one of rare Precambrian ophiolites in Jiangnan orogenic belt and also a hot topic for the tectonics of South China. The mantle peridotite mainly consists of harzburgite. Compared with primitive mantle, the peridotites have lower total REE values varying from 0.83×10^-6 to 2.62×10^-6 and higher MgO content. On the chondrite-normalized REE patterns, the rocks display enriched LREE patterns. When normalized to primitive mantle, all samples show variable enrichment of LILEs and are characterized by significant Nb negative anomalies. These features imply a depleted mantle source, which was overlapped by fluid alteration in a subduction zone. On the HREE patterns, the calculation shows that the harzburgites exhibit the residues of 10%-30% partial melting of the mantle source. Meanwhile, the spinels from mantle peridotites in northeast Jiangxi can be divided into two types which are Cr^#>60 and Cr^# < 60 in the light of their chemical compositions. It is thus concluded that the spinels are of characteristics of both MOR and SSZ setting. In combination with previous researches, the authors infer that ophiolites in northeast Jiangxi originated in a MOR setting at ca. 1060 Ma, and was modified by melts and fluids in a SSZ mantle wedge at ca. 970 Ma.