大别造山带天柱山燕山期中酸性岩浆岩元素和同位素地球化学研究  

Element and isotope geochemistry of Mesozoic intermediate-felsic rocks at Tianzhushan in the Dabie orogen.

作　　者：徐小军[1] 赵子福[1] 郑永飞[1] 魏春生[1] 

机构地区：[1]中国科学院壳幔物质与环境重点实验室

出　　处：《岩石学报》2005年第3期607-622,共16页Acta Petrologica Sinica

基　　金：国家自然基金项目(40303002和40334036)资助成果

摘　　要：大别-苏鲁造山带中生代岩浆岩的物质来源和成因机制,是大陆碰撞造山带研究的热点和前沿问题之一。本文通过对大别造山带天柱山燕山期中酸性岩进行全岩主量和微量元素以及放射成因同位素、单矿物氧同位素分析,指示这些碰撞后岩浆岩是俯冲陆壳物质再循环的产物。研究发现,这些中酸性岩表现出明显的轻稀土富集、高场强元素Nb、Ta和Ti负异常以及Pb正异常。它们都具有较高的Sr同位素初始比值(0．7062-0．7083)和较低的εNd(t)值(-17．1--19．7),与北大别杂岩的变化范围一致。单矿物氧同位素变化范围较大(石英:7．48-9．16‰,钾长石:3．76-7．80‰,斜长石:2．06- 6．96‰,锆石:4．16-5．83‰),大多数样品锆石具有与正常地慢锆石(5．3±0．3‰)一致的δ18O值。大多数样品的石英-锆石之间达到并保存了岩浆结晶时的氧同位素平衡分馏,而其它矿物(如钾长石、斜长石、黑云母和角闪石)与锆石之间大多表现为明显的氧同位素不平衡分馏,表明它们受到了岩浆期后亚固相水-岩相互作用的扰动。元素、Sr-Nd同位素和锆石氧同位素特征表明,大别山中生代中性岩可能是由加厚的扬子板块基性下地壳脱水部分熔融并经结晶分异形成的,花岗岩是由化学组成类似于北大别TTG片麻岩的中性下地壳脱水部分熔融形成的。Major and trace elements, Sr-Nd isotope and oxygen isotopes of whole-rock and mineral separates were determined for post-collisional intermediate-felsic rocks at Tianzhushan in the Dabie Mountains. The results show that these Early Cretaceous intrusive rocks are characterized by strong LREE enrichment, negative HFSE anomalies （ Nb, Ta and Ti） but positive Pb anomalies. Their（^87Sr/^86Sr）, ratios vary from 0. 7062 to 0. 7083 and εNd （t） values from - 17. 1 to - 19.7. There is a large variation in mineral δ^18O values, with quartz from 7.48 to 9.16‰, K-feldspar 3.76 to 7.80‰, plagioclase 2.06 to 6.96‰, and zircon 4.16 to 5.83‰. Most of the zircons have δ^18O values similar to the normal mantle zircon δ^18O values of 5. 3±0. 3‰. Oxygen isotope equilibrium fractionations between quartz and zircon are preserved for most of the samples; the other minerals such as K-feldspar, plagioclase,biotite and amphibole show obvious disequilibria when paired with zircon, indicating alteration by post-magmatic subsolidus water-rock interaction. The element and isotope results suggest that the post-collisional intermediate rocks were generated by amphiboledehydration melting of mafic rocks in the thickened lower crust, coupled with fractional crystallization during magma emplacement; the felsic rocks were derived from partial melting of middle crust which is similar in chemical compositions to TFG orthogneisses in North Dabie.

关 键 词：中酸性岩 氧同位素 锆石 中生代 大别山 

分 类 号：P588.121[天文地球—岩石学] P597.2[天文地球—地质学]