长江中下游宁镇地区早白垩世岩浆岩地球化学研究

Numerous studies reveal that adakite has important meaning for tectonics and deposits. Lower Yangtze River Cu- polymetallic metallogenic belt(LYRB) makes up one of the most important metallogenic belts in easten China. Field investigations and chronological studies reveal that these deposits are closely associated, both spatially and temporally, with early Cretaceous adakitic rocks.The petrogenesis of these ore-bearing adakites and the tecotonic setting of LYRB are hot debated. Ningzhen district is located in eastern part of LYRB. Ningzhen adakite was the first reported adakites in China, and was explained by delamination model. However, the geochemistry of ningzhen adakite is totally different from those other adakites in LYRB, espacially in Pb isotopes. Previous geochronology study indicated adakites from Ningzhen were formed 100-109Ma, younger than the adakites from Lower Yangtze River belt. Systematic study on the petrogenesis of the ore-bearing adakites in Ningzhen district is necessary to reveal the sources and their relationship to Cu-Au mineralization in LYRB. This study will go further to systematically study the geochemistry of three ore-bearing porphyries in Ningzhen area: Anjishan, Tongshan and Xiangshan to constrain the ore-bearing adakites sources and forming processes. Our study will give more constrains on the spatial and temporal of adakites in LYRB and the tecotnic setting on east China in early Cretaceous.

埃达克岩有着重要的构造学和矿床学指示意义，受到广大地质学者的关注。长江中下游是中国东部最重要的铜多金属成矿带，与这些铜多金属矿床相关的岩石也具有埃达克岩特征。对于这些埃达克岩的成因以及长江中下游中生代的构造背景，目前还存在极大的争议。宁镇地区位于长江中下游成矿带的最东部，离太平洋俯冲带最近，也是最早提出陆内埃达克岩成因模型的地区。然而，从地球化学特征上分析，宁镇安基山地区埃达克具有低放射成因铅，明显不同于长江中下游其他地区埃达克岩。预研表明宁镇地区岩浆岩形成于100-109Ma。解决长江中下游埃达克岩的成因特征问题，宁镇地区埃达克岩可能是一个突破点。本项目拟通过对宁镇地区出露的三个含矿岩体：安基山岩体、铜山岩体和象山岩体做系统的地球化学和锆石学工作，确定岩石源区和形成过程，进一步限定长江中下游埃达克岩源区、时空分布以及中国东部早白垩世构造背景。

长江中下游是中国东部最重要的铜多金属成矿带，与这些铜多金属矿床相关的岩石也具有埃达克岩特征。对于这些埃达克岩的成因以及长江中下游中生代的构造背景，目前还存在极大的争议。宁镇安基山地区埃达克具有低放射成因铅和较年轻的形成时代（100-109Ma），明显不同于长江中下游其他地区埃达克岩。宁镇地区中生代岩浆岩主要组成为花岗闪长（斑）岩和共生的基性辉长岩,其主要形成于110Ma-100Ma之间。闪长斑岩锆石落在长江中下游含矿埃达克质岩与郯庐南段不含矿埃达克质岩O-Hf同位素数据之间，锆石Ce、Eu异常指示其形成于高氧逸度、富水环境下。闪长斑岩具有低SiO2，高Mg#，富Na、K，有明显的高场强元素（Nb-Ta-Zr-Hf）亏损，大离子亲石元素（Ba、Pb、Sr）富集，稀土元素配分图解上，表现为富集LREE，亏损HREE，无Eu*异常等特征。同位素具有高Sr、低Nd，低放射成因Pb同位素特征。而基性岩部分具有低SiO2、高Mg、Mg#，富K2O、Na2O的特征，同时具有更低的K2O/Na2O值和与中酸性岩体相似的微量元素和稀土元素特征。同位素上显示亏损Nd，Hf同位素，和与酸性岩相似的Sr-Pb同位素特征。宁镇地区中生代岩浆岩为岩浆混合：为古老下地壳与地幔物质熔融熔体在中上地壳发生混合形成。. 花岗闪长斑岩具有明显的Nd-Hf同位素解耦特征（deltaEHf=-4.2~-1.4），通过计算认为花岗闪长斑岩可能华南新元古代埃达克质岩石重新熔融的结果。. 长江中下游A型花岗岩地球化学特征显示其具有不同的Pb同位素组成，其中苏州岩体具有极低的Pb同位素组成，明显不同于大龙山和黄梅尖A型花岗岩Pb同位素组成，表明长江中下游A型花岗岩源区存在较大差异，长江中下游地区中生代130-125Ma可能经历大规模的地幔上涌导致不同壳层均发生熔融，形成A型花岗岩。