皖南伏川SSZ型蛇绿岩的地球化学特征与构造意义

Geochemical Characteristics of the SSZ-type Ophiolite in the Fuchuan Area, Southern Anhui, China and Their Tectonic Significances

伏川蛇绿岩作为华南极为重要的前寒武纪蛇绿岩之一,本文对其中的橄辉岩和辉长岩样品进行了岩石学、地球化学及同位素特征研究。研究表明,伏川蛇绿岩中橄辉岩含有极高的MgO(39.92%~44.95%)和较低的Al2O3(0.48%~4.35%)、CaO(0.11%~0.63%)和TiO2(0.01%~0.06%),稀土元素总量(7.50×10-6~10.62×10-6)低于亏损地幔,为一套高度亏损的原始地幔熔融残留物,其部分熔融程度介于22%~26%之间。具'U'型的稀土元素配分模式,ISr值显著高于地幔,富集LILE,HFSE含量变化大,既继承了亏损地幔源区的特征,又经历不同程度熔体和流体的交代作用,指示其形成于具有一定规模的弧后小洋盆背景。变辉长岩具高MgO(8.78%~13.26%)、Al2O3(17.25%~18.75%)和CaO(11.98%~15.14%),贫TiO2(0.20%~0.28%)、P2O5(0.01%~0.02%)及富钠低钾的特征,其稀土元素配分模式较为平坦,具有与亏损地幔相当的Sr、Nd同位素组成,富集LILE,亏损HFSE,显示岛弧(IAT)特点,其不活动元素比值与N-MORB相当,兼具IAT和MORB的地球化学特征,判断其形成于弧后盆地的初始扩张阶段。结合前人已有研究,认为新元古时期,皖南地区存在沟-弧-盆体系,随着华夏板块向扬子板块之下俯冲,本区弧后盆地进一步引张形成小型洋盆。

The Fuchuan ophiolite is one of the most important Precambrian ophiolites in South China.An integrated study of petrology,geochemistry and isotopes of the olivine pyroxenite and gabbro from the Fuchuan ophiolite has been carried out in this paper.This study shows that the olivine pyroxenite is characterized with high contents of MgO(39.92%-44.95%)and relatively low contents of Al2O3(0.48%-4.35%),CaO(0.11%-0.63%),and TiO2(0.01%-0.06%),with lowerΣREE contents(7.50×10-6-10.62×10-6)than that of the primary mantle,indicating that the rock was a series of residuals originated from partial melting of highly depleted primary mantle at partial melting degrees of 22%-26%.In addition,the rocks have high contents of large ion lithophile elements(LILE),varied contents of high filed strength elements(HFSE),higher initial ratios of strontium than that of the mantle,and the U-shaped REE distribution patterns,implying the incorporation of the crustal materials into the rocks.Therefore,it is suggested that the olivine pyroxenite was originated from depleted mantle which was altered by fluid in subduction zone,and could be formed in a small back-arc ocean basin.The meta-gabbro is characterized with high contents of MgO(8.78%-13.26%),Al2O3(17.25%-18.75%),and CaO(11.98%-15.14%),low contents of TiO2(0.20%-0.28%)and P2O5(0.01%-0.02%),and rich in Na2O and poor in K2O.It has relatively flat REE patterns and similar Sr and Nd isotopic compositions to those of the depleted mantle.In addition,it is enriched in large ion lithophile element(LILE)and depleted in high filed-strength element(HFSE),showing characteristics of the rock formed in island-arc setting(IAT).The ratios of inactive elements of these samples are similar to that of the normal mid-ocean ridge basalt(N-MORB).It exhibits both MORB and IAT geochemical characteristics,indicating that it could be formed in the initial extensional stage of the back-arc basin.Integrated with previous studies,we propose that there was a trench-arc-basin system in southern Anhui Province between the Yangtze and Cathaysia blocks during Neo-proterozoic period.With the subduction of Cathaysia block down to the Yangtze block,the back-arc basin was furtherly extended to form a small ocean basin.