摘要
沿鲜水河断裂分布的折多山花岗岩由早期的中细粒花岗闪长岩、二长花岗岩和略晚的主体似斑状二长花岗岩、少量的伟晶岩和细晶岩构成。早期中细粒二长花岗岩的SHRIMP锆石U-Pb同位素定年表明其侵位结晶于18±0．3Ma,使折多山花岗岩岩浆侵位结晶年龄和鲜水河断裂开始活动的时间提前近6Ma。岩石中保存有818±47Ma和156±8Ma的继承锆石．表明存在扬子西缘元古宙和中生代陆壳物质的再循环。在K_2O对SiO_2分类图上,早期的中细粒花岗闪长岩和二长花岗岩落在钙碱性系列区域,似斑状二长花岗岩落在橄榄玄粗岩系列区域,两个系列岩石总体上为铝饱和到过饱和。中细粒花岗闪长岩具有中等的稀土总量(162×10^(-6)～224×10^(-6))、高的(La/Yb)_N(74～118)和明显的正Eu异常,大离子亲石元素富集,Nb、Ta、P和Ti亏损,形成于岛弧钙碱性火山岩低度部分熔融。中细粒二长花岗岩与花岗闪长岩的地球化学特征相似,只是稀土总量和(La/Yb)n较低,形成于杂砂岩部分熔融。主体岩性粗粒似斑状二长花岗岩具有很宽的稀土总量(最高达533×10^(-6)),(La/Yb)_N随着稀土总量增加而增加(最高达523),明显负Eu异常,大粒子亲石元素强烈富集,Nb、Ta、Sr、P和Ti亏损,(^(87)Sr/^(86)Sr)_0=0．7084～0．7133,ε_(Nd)(t)=-5．67～-8．69,形成于元古代上部陆壳物质—砂页岩的较低程度的部分熔融。各类岩石的地球化学特征表明,部分熔融源区的物质组成继承了元古代大陆边缘的某些地球化学特性,经历了高压变质作用,部分熔融残留相主要为石榴石等。部分熔融过程发生于鲜水河断裂早期活动的剪切熔融过程,岩浆作用之前可能发生过强烈挤压而产生了高压变质作用。
Zheduoshan granitoids emplaced along Xianshuihe fault belt consist of earlier medium- to fine-grained granodiorite and monzogranite, slightly later predominated medium- to coarse-grained porphyry-like monzogranites and the latest granitoid pegmatites and aplites. The earlier medium- to fine-grained monzogranites have been dated by SHRIMP zircon U-Pb isotopes and an age of 18±0.3 Ma is obtained. This SHRIMP zircon U-Pb age makes the time of emplacement of Zheduoshan granitoids and starting activation of the Xianshuihe fault belt, be predated for 6 Ma. Inherited zircons with 818±47 Ma and 156±8 Ma ages, respectively, are preserved in this sample. The earlier granodiorite and monzogranite samples were ploted into calc-alkaline and high-potassium calc-alkaline series domains and later porphyry-like monzogranite samples into shoshonite series domain in the K2O versus SiO2 diagram. All of these samples exhibit aluminium saturation to supersaturation. Earlier granodiorite samples display medium total REEs （162×10^-6 -224 × 10^-6）, higher （La/Yb）n （ = 74-118 ） and positive Eu anomalies, and enrichment in LILEs, but depletion in the Nb, Ta, P and Ti in the primitive mantle normalized spider diagram, indicating that the granodioritic magma derived from lower degree of partial melting of calc-alkaline basaltic volcanic rocks. The earlier monzogranite samples show very similar geochemical characteristics to the earlier granodiorites, excepted that their total REEs and （LMYb）N are lower than these of granodiorites, suggesting that the earlier monzogranite magma was most probably derived from lower degree partial melting of graywackes. Predominant later porphyry-like monzogranite samples display a wider total REEs （the highest arrived to 533×10-6）, the （La/Yb）N （the highest to 523 ） , obvious negative Eu anomalies, and intensive enrichment in LILEs, but depletion in Nb, Ta, Sr, P and Ti in the primitive mantle normalized spider diagram. These porphyry-like monzogranites also exhibit （ ^87 Sr/^56 Sr）0 = 0. 7084 - 0.7133 and εNd （t） = -5. 67～ -8. 69, suggesting that the later monzogranitic magma was most probably derived from lower degree partial melting of Proterozoic upper continental materials of siltstones and/or shales. These geochemistry features in various kind of granitoids in Zheduoshan intrusion indicate that the partial melting source materials have inherited some geochemisty characteristics of Proterozoic active continental margin and underwent high-pressure metamorphism, and partial melting residuals should dominate garnet. The partial melting was probably produced with early shear melting process of Xianshuihe fault belt, also occurred strong compression before the magmatism to produce the high-pressure metamorphism.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2006年第2期343-352,共10页
Acta Petrologica Sinica
基金
国家重点基础研究发展规划项目资助
项目名称:印度与欧亚大陆主碰撞带成矿作用(项目编号:2002CB412608)
