华北克拉通北部古-中元古代富碱侵入岩的Nd、Sr、Pb同位素地球化学:岩石圈地幔富集的证据  

Nd,Sr and Pb isotopic geochemistry of Paleo-Mesoproterozoic alkaline-rich intrusions from the northern part of the North China Craton:evidence of the lithospheric mantle enrichment

作　　者：任康绪[1] 阎国翰[1] 蔡剑辉[1] 牟保磊[1] 李凤棠[1] 储著银[2] 

机构地区：[1]北京大学地球与空间科学学院 [2]中国科学院地质与地球物理研究所,北京100029

出　　处：《岩石学报》2006年第12期2933-2944,共12页Acta Petrologica Sinica

基　　金：国家自然科学基金(项目编号:40072024;49373170)的资助。

摘　　要：华北克拉通北部古-中元古代富碱侵入岩主要分布在燕辽三叉裂堑系和辽吉拗拉谷中及附近,为一套高碱高钾的基性-中性岩,以正长岩类为主。本文通过开展系统的 Nd、Sr、Pb 同位素研究,发现所有岩体岩石均以高负ε_(Nd)(t)为特征,ε_(Nd)(t)=-3.4～-7.5,平均值为～4.8,这与研究区古-中元古代基性-超基性岩石的 Nd 同位素特征(ε_(Nd)(t)=-4～-8) 一致,与燕辽裂堑系发育的偏碱性火山岩 Nd 同位素特征(ε_(Nd)(t)=-4.3～-8.9)也比较相似,表明它们的物质来源相似,都与富集地幔有关。Sr 同位素初始比值(^(87)St/^(86)Sr)_i比较低,主要变化在0.7028～0.7053之间,平均值为0.7041。钾长石铅同位素组成普遍较低,^(206)Pb/^(204)Pb、^(207)pb/^(204)Pb、^(208)Pb/^(204)Pb 比值变化范围(梁屯-矿洞沟岩体除外)分别为14.500～15.70l、14.887～15.150和34.178～36.537,平均值分别为14.968、14.984和35.057。在ε_(Nd)(t)-ε_(Sr)(t)图解上,所有岩体的投影点均在地幔演化趋势线附近,比较接近 EMI 型富集地幔端员,暗示它们的物质来源与 EMI 型富集地幔有关;钾长石 Pb 同位素模式图也说明这些岩体物质来源与地幔和下地壳有关。通过两端员混合模拟,揭示了岩浆演化过程中存在少量的下地壳物质混染,平均约14%左右。从2.5Ga 左右开始华北克拉通岩石圈地幔的亏损程度逐渐变小,在2.2Ga 左右局部呈现富集性特征,1.85Ga时整个岩石圈地幔已经完全转变为富集性,之后富集程度越来越高。推测早期俯冲携带的壳源物质以及后期地幔流体的交代作用可能是岩石圈地幔逐渐转变为富集性的原因。The Paleo-Mesoproterozoic alkaline-rich intrusive rocks, mostly distributed in or near Yan-Liao triple-junction aulacogen system and Liao-Ji aulacogen along the north part of the North China Craton （ NCC）, are composed of basic to intermediate rocks dominated with syenite suites with elevated contents of total alkali and potassium. In this paper a systematic geochemical study on Nd, Sr and Pb isotopic characteristics is presented. All the εNd （t） values of these alkali-rich rocks are negative and the range （ -3.4 ～ -7.5） is in a small scale with an average of - 4. 8. These are consistent with the Nd isotopic characteristics of the Paleo- Mesoproterozoic basic-ultrabasic rocks in this area（ εNd （t） = -4 ～ -8）, and also similar to the subalkaline extrusive in Yan-Liao aulacogen（εNd（t） = -4.3 ～ -8.9 ）, which indicate that these alkaline-rich intusives are of enriched mantle origin. Their initial ^87Sr/^86Sr values have a relatively restricted range from 0. 7028 to 0. 7053,with an average of 0. 7041. The ^206Pb/^204 Pb（ 14. 500 ～15. 701,14.968 average） ,^207Pb/^204Pb（ 14. 887～ 15. 150, 14. 984 average） and ^208Pb/^204 Pb（ 34. 178 ～ 36. 537, 35. 057 average） ratios for K-feldspar show that most of these alkali-rich rocks have very low ratios except for those from the Liangtun-Kuangdonggou complex. On the εNd （t）- εSr（t） diagram, all the spots are near the mantle array line and close to the EMI endmember,which indicates that these rocks have a close connection to the materials from the EMI enriched mantle. Their lead-isotope evolution also illustrates that they have relation with mantle and lower crust. Furthermore, a simulative calculation of the two endmembers reveals that the average ratio of the lower crust contamination is about 14%. According to this study, the depletion degree of lithospheric mantle of NCC has become weak and weak since ca. 2.5Ga, and the mantle enrichment appeared partly at about 2.2Ga, all the lithospheric mantle was enriched at

关 键 词：古-中元古代 华北克拉通 富碱侵入岩带 Nd Sr PB同位素 富集地幔 

分 类 号：P588.12[天文地球—岩石学] P597.3[天文地球—地质学]