赣东北蛇绿岩地幔橄榄岩岩石成因及其地质意义

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

赣东北蛇绿岩作为华南少有的前寒武纪蛇绿岩长期都是华南大地构造研究中的热点。该蛇绿岩地幔橄榄岩以方辉橄榄岩为主,稀土总量为0.83×10-6~2.62×10-6,远低于原始地幔的含量,而Mg O含量高于原始地幔。稀土元素表现为LREE相对富集,微量元素表现为大离子亲石元素（LILE）富集,高场强元素Nb明显亏损的特征,这表明赣东北地幔橄榄岩具有亏损地幔源区特征,同时也有不同程度的俯冲带流体的交代特征。HREE模拟的部分熔融程度为10%~30%;同时,赣东北蛇绿岩地幔橄榄岩中同时包含Cr#〉60和Cr#〈60两种不同成因的铬尖晶石,说明其经历了MOR（mid-ocean ridge）和SSZ（supra-subduction zone）两种构造环境。结合前人资料,笔者推测赣东北蛇绿岩早先可能形成于洋中脊环境（约1060 Ma）,随后在洋内俯冲作用下（约970 Ma）,位于俯冲带上部的地幔橄榄岩受到了来自俯冲带的流体/熔体的交代,经历了SSZ环境的改造。

The ophiolite in northeast Jiangxi is one of rare Precambrian ophiolites in Jiangnan orogenic belt and also a hot topic forthe tectonics of South China. The mantle peridotite mainly consists of harzburgite. Compared with primitive mantle, the peridotiteshave lower total REE values varying from 0.83 ×10- 6 to 2.62×10- 6 and higher MgO content. On the chondrite-normalized REEpatterns, the rocks display enriched LREE patterns. When normalized to primitive mantle, all samples show variable enrichment ofLILEs and are characterized by significant Nb negative anomalies. These features imply a depleted mantle source, which wasoverlapped by fluid alteration in a subduction zone. On the HREE patterns, the calculation shows that the harzburgites exhibit theresidues of 10%-30% partial melting of the mantle source. Meanwhile, the spinels from mantle peridotites in northeast Jiangxi canbe 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.