北阿尔金蛇绿混杂岩带中斜长花岗岩的成因及其地质意义  

The origin and geologic significance of plagiogranite in ophiolite belt at North Altyn Tagh

作　　者：盖永升 刘良[1] 康磊[1] 杨文强[1] 廖小莹[1] 王亚伟[1] 

机构地区：[1]大陆动力学国家重点实验室、西北大学地质学系,西安710069

出　　处：《岩石学报》2015年第9期2549-2565,共17页Acta Petrologica Sinica

基　　金：中国地质调查局项目(12120113044500);国家自然科学基金重点项目(41430209);国家重点基础研究发展规划“973”项目(2015CB856103)联合资助

摘　　要：北阿尔金蛇绿混杂岩带恰什坎萨依剖面南端的斜长花岗岩,呈不规则脉状和透镜状产出于蚀变辉绿岩之中。该斜长花岗岩具有高的SiO2含量（70.42%~78.29%）,Al2O3含量低（9.51%~11.75%）,高Na2O低K2O,K2O/Na2O=0.02~0.09,属于低钾准铝质岩石;其∑REE偏低（80.15×10-6~104.1×10-6）,具中等Eu异常（δEu=0.68~0.73）以及轻稀土轻微富集的特点,微量元素显示相对富集Th、U而亏损Nb、Ta、Ti的特征。以上特征与产于蛇绿岩中的斜长花岗岩的基本特征相符合。蚀变辉绿岩主量元素显示低SiO2、Al2O3、K2O、TiO2,高CaO、Na2O以及富集Th、U、La、Ce,相对亏损Nb、P、Ti的特征,部分蚀变辉绿岩样品SiO2、Na2O含量的升高应与岩石遭受海水蚀变有关。稀土元素显示弱的Eu异常（δEu=0.86~0.94）以及轻稀土轻微富集的特点。蚀变辉绿岩Mg#=51.3~54.8,Ti/V=11~22,La/Nb=1.67~2.07,Zr/Nb=10.3~27.1的特点与区内蛇绿岩地球化学特征一致,应为区内蛇绿岩的组成部分。斜长花岗岩LA-ICP-MS锆石U-Pb年龄为518.5±4.1Ma,Hf同位素εHf（t）=4.25~9.90。在La-SiO2及TiO2-SiO2图解上,斜长花岗岩均显示部分熔融成因,结合岩石非常低Sr高Yb的特点,认为其应形成于洋壳在运移过程中发生的部分熔融。全岩Sr、Nd、Pb同位素显示蛇绿岩源区具有EMII型富集地幔特征,与前人对区内洋中脊型玄武岩的同位素研究具有一致性,而洋壳源区富集地幔特征暗示此时的洋壳形成于洋盆裂解的早期。结合以上资料分析,本次研究的斜长花岗岩和蚀变辉绿岩可能形成于北阿尔金有限洋盆裂解的早期,形成时代为518.5±4.1Ma。The plagiogranite,which outcropped at the south part of Qiashikansayi profile in North Altyn ophiolite belt,output among the altered diabase as an irregular intrusive stock. The plagiogranite is characterized with high SiO2（ 70. 42% ~ 78. 29%）,low Al2O3（ 9. 51% ~ 11. 75%） and low ratio of K2O / Na2O（ 0. 02 ~ 0. 09）,belongs to quasi-aluminous low-K calc-alkaline series. The plagiogranite is enriched in Th,U and depleted in Nb,P,Ti. REE patterns show features of low ∑REE（ 80. 15 × 10- 6~ 104. 1 ×10- 6）,moderate negative Eu anomaly（ δEu = 0. 68 ~ 0. 73） and relative enrichment of LREE. All these features consist with the basic characteristics of oceanic plagiogranites. Altered diabase is low in SiO2,Al2O3,K2O,TiO2,high in Ca O and Na2 O,while enriched in Th,U,La,Ce and depleted in Nb,P,Ti. The high Na2O and SiO2 values in particular samples can be observed in the altered diabase,which are supposed to be influenced by seawater dominated alteration. The REE patterns of altered diabase show weak negative anomaly of Eu and relative enrich in LREE. Mg#for the altered diabase ranges from 51. 3 ~ 54,Ti / V varies from 11 to 22,La / Nb ranges from 1. 7 to 2. 1,and Zr / Nb ranges from 10 to 27. These geochemical features of altered diabase are consistent to the ophiolite that have been studied in this belt. LA-ICP-MS U-Pb age of plagiogranite is 518. 5 ± 4. 1Ma for zircons characterized by oscillatory zoning,εHf（ t） vary from 4. 25 to 9. 90. The La-SiO2 and TiO2-SiO2 diagrams show that this plagiogranite is formed by partial melt. Furthermore,in combination with the characteristics of low Sr and high Yb content,the plagiogranite is interpreted to be origined from partial melting of the ocean crust which is beneath the altered diabase in the ocean crust section during migration. Whole rock Sr-Nd-Pb isotopes of plagiogranite show that the sources of these rocks are EMII enriched mantle,suggesting they formed mostly at the early stage of the oceanic crust splitting. Intergrating with regional geol

关 键 词：斜长花岗岩 锆石LA-ICP-MS U-Pb定年及Hf同位素 全岩Sr-Nd-Pb同位素 富集地幔 北阿尔金蛇绿混杂岩带 

分 类 号：P588.121[天文地球—岩石学] P597.3[天文地球—地质学]