赣杭构造带中西段中生代基性脉岩地球化学特征

Geochemical Characteristics of the Mafic Dikes from the Gan-Hang Tectonic Belt

赣杭构造带中西段发育的基性脉岩以辉绿岩为主,SiO2质量分数为46.88%~53.69%,从西段至中段总体显示亚碱性至碱性的变化规律。微量元素相对富集大离子亲石元素(LILE)和轻稀土元素(LREE),亏损高场强元素(HFSE)和重稀土元素(HREE)。稀土元素总量具有明显的规律性分布,即呈中段(广丰)—中西段交接部(余江、东乡)—西段(永丰、峡江)的递减趋势。西段永丰地区有明显的负Eu异常。Sr、Nd同位素变化比较大((87Sr/86Sr)i为0.704 635~0.708 769,(143Nd/144Nd)i为0.512 173~0.512 758,εNd(t)为-6.8^+5.6),暗示其源区具有明显的不均一性。具体表现为中西段结合部(东乡、余江)及西段(峡江)为亏损地幔,而西段(永丰)和中段(广丰)为偏向EMⅡ型富集地幔来源。基性脉岩总体表现为地幔部分熔融的产物,西段(永丰)受到较明显的分离结晶作用影响。在古太平洋板块俯冲影响下产生的弧后拉张作用以及造成的构造带重新活化作用对基性脉岩的形成产生了重要影响,地区间岩石地球化学特征上的差异主要与流体对岩浆源区交代程度的不同有关。

The SiO2 content of the mafic dikes from Gan-Hang tectonic belt ranges from 46.88% to 53.69%.The dolerites from middle and west of Gan-Hang（Jiangxi-Zhejiang） tectonic belt belong to sub-alkaline and alkaline series,respectively.The trace elements of the mafic dikes are characterized by enrichment in LREE and LILE（Ba,Rb and K） and depletion in HREE and HFSE（Ta,Nb,Zr,Hf and Ti）.The REE of the mafic dikes decreases gradually from Guangfeng to Yujiang and Dongxiang,and then to Yongfeng and Xiajiang.The mafic dikes from Yongfeng have obviously Eu anomalies.The Sr and Nd isotopic composition of mafic dikes are of a wide range,（^87Sr/^86Sr）i varying from 0.704 635 to 0.708 769,（^143Nd/^144Nd）i from 0.512 173 to 0.512 758,and εNd（t） from-6.8 to ＋5.6,revealing that the mafic dikes were derived from partial melting of different magma sources.The mafic dikes from Yujiang,Dongxiang and Xiajiang were derived from depleted mantle,while the mafic dikes from Guangfeng and Yongfeng were produced by partial melting of EMⅡ-type enriched mantle due to mantle metasomatism.In addition,there occurred fractional crystallization of olivine,clinopyroxenes,plagioclase and Ti-Fe oxide during the magma ascent.The mafic dikes were related with the subduction of the Paleo-Pacific,different tectonic locations may account fro their different petrologic characteristics.