塔西南玄武岩年代学和地球化学特征及其对二叠纪地幔柱岩浆演化的制约  

Chronology and geochemistry of Taxinan basalts from the Tarim basin:evidence for Permian plume magmatism.

作　　者：厉子龙[1,2] 杨树锋[1] 陈汉林[1] C．H．Langmuir 余星[1] 林秀彬[1] 励音骐[1] 

机构地区：[1]浙江大学地球科学系,杭州310027 [2]哈佛大学地球行星科学系,美国麻省02138

出　　处：《岩石学报》2008年第5期959-970,共12页Acta Petrologica Sinica

基　　金：国家重点基础研究发展规划项目(2007CB411303,2001CB409801);国家自然科学基金(40472040和40472020)资助

摘　　要：本文通过对塔西南达木斯剖面中玄武岩进行 K-Ar 同位素定年,获得年龄为289.6Ma,并结合 Ar-Ar 坪年龄结果(290.1Ma)和古生物以及沉积特征,认为290Ma 的年龄代表了塔西南玄武岩形成于早二叠世,对应于盆地内下二叠统库普库兹满组下段层位的年龄。地球化学特征显示塔西南熔岩为分异的碱性玄武岩并含45%SiO_2和4%MgO 含量。塔西南玄武岩与盆地内柯坪玄武岩具有相近的主量元素含量和稀土配分与微量元素蜘蛛网图型、无 Eu 异常、富集轻稀土元素、较高的其它不相容元素(如高场强元素)。但塔西南玄武岩比柯坪玄武岩具有较高的 Al_2O_3和 CaO 含量及稀土总量(288×10^(-6)～358×10^(-6)),偏低的 Na,O,P_2O_5和 FeO 含量。K、Rb 和 Cs 丰度的无系统性变化主要受这些元素丰度本身变化的影响。对其它不活动组分,塔西南玄武岩具有高 Ti(Ti/Y=522～624)和 Nb 含量(30×10^(-6)～40×10^(-6))及低 Zr/Nb 比值,暗示其来自富集的地幔源区。其 Nb 含量相对 La 含量无显著变化以及相对低的 Nb/U(近30)和 Ce/Pb 比值(近15),指示塔西南玄武熔岩来自大陆岩石圈或受一定程度的地壳混染。塔里木盆地大规模的火山喷发以及富集不相容元素的地球化学特征支持这样一种假设,即塔西南玄武岩来自地幔柱火山作用,或由于地幔柱的供热和上升导致富集的岩石圈地幔部分熔融而形成。且岩浆作用过程以部分熔融为主,结晶分异作用较弱。基于塔西南玄武岩和柯坪玄武岩相近的时代、源区成分和/或岩浆作用过程以及处于陆内稳定构造环境,笔者认为塔里木二叠纪玄武岩的分布范围可以从塔里木盆地内的塔中、柯坪一带一直延伸到塔西南地区。A new K-Ar age of 289.6Ma has been obtained for Taxinan basalt in the southwest Tarim basin of the northwest China. This age is consistent with the Ar-Ar age of 290.1 Ma from the same sample. This dating confirms the formation age of the basalt is the early Permian, contemporaneous with the Kupukuziman formation of the lower part of the early Permian. Geochemically, the analyzed Taxinan basaltic lavas are differentiated alkaline basalts, with -45% SiO2 and 4% MgO contents. The Taxinan basalts and Keping basalts in the inner Tarim basin have similar major element contents, rare earth element and trace elements spider patterns with enriched light rare earth elements, and elevated contents of other incompatible elements including the high field strength elements. However, the Taxinan basalts have higher A1203 and CaO contents and total rare earth elements （288 × 10^ -6 - 358 × 10 -4 ）, and lower Na2O, P2O5 and FeO contents compared to the Keping basahs. Abundances of K, Rb, and Cs are variable and non-systematic, suggesting an important influence of alteration on the abundances of these elements. For more immobile elements, the Taxinan basalts have high Ti （Ti/Y = 522 - 624） and low Zr/Nb ratios with high Nb contents （30 × 10^6 -40 × 10^-6）, indicating derivation from enriched mantle sources. While there is not a pronounced Nb anomaly relative to La, the lavas have slightly low Nb/U （ - 30） and Ce/Pb （ - 15） suggesting either derivation from continental lithosphere or modest amounts of crustal contamination. The large volumes of eruptives combined with the enriched geochemical signature support the hypothesis that the Taxinan basalt was derived from plume volcanism, either from the plume itself or from melting of enriched lithospheric mantle that was heated and uplifted by ascending mantle plume. Based on the affinity between Taxinan and Keping basahs in time and composition, the areal distribution of the Permian basalts in Tarim basin can be extended to the southwest part from the Tazhong-Ke

关 键 词：K—Ar定年 地球化学特征 地幔柱岩浆作用 玄武岩 塔里木盆地 

分 类 号：P597.3[天文地球—地球化学] P588.145[天文地球—地质学]