CCSD HP-UHP变质岩中磷灰石稀土元素(REE)地球化学及其示踪意义  

Rare earth elements (REE) geochemistry of apatites in HP-UHP metamorphic rocks from Chinese Continental Scientific Drilling (CCSD) project and their implications.

作　　者：汤倩[1] 孙晓明[1,2,3] 梁金龙[1] 徐莉[1] 翟伟[1] 梁业恒[1] 

机构地区：[1]中山大学地球科学系,广州510275 [2]中国科学院同位素年代学和地球化学实验室,广州510640 [3]中山大学-有色金属华东地勘局隐伏矿床勘查研究院,南京210007

出　　处：《岩石学报》2007年第12期3255-3266,共12页Acta Petrologica Sinica

基　　金：国家自然科学基金(编号:40399142);中国科学院同位素年代学和地球化学实验室合作基金(GIGIso-0504);国家重点基础研究发展规划项目(973项目)(编号:2003CB716501);中国地质大学地质过程与矿产资源国家重点实验室开放基金(编号:2003015)联合资助

摘　　要：磷灰石是一种能在UHP变质峰期稳定存在并富含稀土元素(REE)的常见副矿物,其REE组成变化可以对变质过程进行地球化学示踪。本文利用激光剥蚀等离子光谱仪(LA-ICP-MS)对中国大陆科学钻探(CCSD)钻井中及其附近出露的不同变质程度的HP-UHP变质岩(榴辉岩、角闪(片)岩和片麻岩)中的磷灰石进行了REE组成原位测定,结果显示不同围岩中磷灰石的REE组成特征及其相关系数变化很大,其球粒陨石标准化配分曲线可以分为3大类:Ⅰ.轻稀土(LREE)富集型,其REE总量(∑REE)很高,可达n×1000×10^(-6);Ⅱ.中稀土(MREE)富集型,其(La/Sm)_N<1,Eu异常变化较大;Ⅲ.重稀土(HREE)富集型,其配分曲线呈明显的左倾形式,∑REE总量很低,仅为9.228×10^(-6),且具明显的Eu负异常。磷灰石颗粒原位分析显示从边部到中心∑LREE有逐渐升高的趋势,表明了在俯冲折返的过程发生过短时增温作用,并极有可能发生过部分熔融。部分熔融过程中磷灰石中的LREE将与其它大离子半径元素一起优先释放。Apatite is one of the major REE-bearing accessory minerals which can keep stable during peak ultra-high pressure(UHP) metamorphism, its REE compositional variation can be used as an indicator of metamorphic conditions and process. In situ REE compositions of apatites in various degrees retrograde eclogites, amphibolite, amphibolite schist and paragneiss from of CCSD and nearby outcropped locations were determined by laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS), the results indicate that the REE contents and related parameters of the apatites vary considerably. Chondrite-normalized distribution patterns of apatites can be divided into three types: I, LREE-enriched type, its Sigma REE reaches a high concentration to n x 1000 x 10(-6); II, MREE-enriched type, its La/Sm ratios less than 1 and delta Eu vary greatly; III, HREE-enriched type, its Sigma REE is very low, only 9.228 x 10(-6), the chondrite-normalied patterns are left-leaning with marked negative Eu anomalies. Besides, in situ REE analyses on apatite grains show that Sigma LREE decline gradually from core to the margins, suggesting that the apatite-bearing rocks might have suffered a shortlived heating and even partial melting during exhumation. During the partial melting, LREE were selectively released with other large radius ions.

关 键 词：LA-ICP-MS 稀土元素 磷灰石 高压-超高压变质岩 中国大陆科学钻探(CCSD) 

分 类 号：P597.3[天文地球—地球化学] P595[天文地球—地质学] P578.922P588.348