西秦岭甘肃夏河县腾布—日周早白垩世玄武岩年代学、地球化学及构造意义

Geochronology,geochemistry and tectonic significance of Tengbu-Rizhou Early Cretaceous basalts in Xiahe,West Qinling,Gansu Province

甘肃省夏河县腾布-日周一带发育早白垩世多禾茂组(K1d)火山岩地层,该套地层属于典型的陆内裂谷沉积组合。岩石组成主要为玄武岩,底部偶见复成分砾岩。岩相学及岩石地球化学特征表明腾布-日周玄武岩为钠质碱性橄榄玄武岩,富集LREE及Nb、Ta、La、Zr等,亏损Rb和K等,稀土元素球粒陨石标准化配分曲线及微量元素原始地幔标准化蛛网图,均与世界典型的OIB型玄武岩相似。但是Th/Nb=0.055~0.060,平均为0.057(OIB为0.08),Zr/Nb=6.9~8,平均为7.25(OIB为5.83),Th/La=0.07(OIB为0.11),又与之不同,表明玄武岩岩浆来自软流圈地幔,同时受到地壳物质混染。LA-ICP-MS锆石U-Pb同位素年龄为(106.27±1.3)Ma,另外麦秀山一带多禾茂组中产以Classopollis-Osmundacidites为主的孢粉组合。认为该玄武岩是西秦岭晚中生代大陆裂谷岩浆作用的产物,但是未发育典型的双峰式火山岩,多禾茂组有陆相红层建造,上部万秀组发育类磨拉石建造。因此裂谷作用很可能夭折于岩石圈拉张的早期阶段,并未发展到陆间裂谷阶段。

The volcanic stratum of the Early Cretaceous Duohemao Formation(K1d),developed in the Tengbu-Rizhou area of Xiahe County,Gansu Province,is a typical sedimentary association of intra-continental rift.The rock assemblage is composed mainly of basalts,with occasional occurrence of polymictic conglomerates at the bottom of the assemblage.Based on petrographic and geochemical analyses,we classified TengbuRizhou basalts as sodium alkaline olive basalt.They are enriched in LREE and Nb,Ta,La and Zr,depleted in Rb and K.They have similar chondrite-normalized rare earth and primitive mantle-normalized trace elemental patterns as that of typical global ocean island basalt(OIB).However,we found great difference in the ratios of Th to Nb(0.055-0.060,averaging 0.057 vs.0.08 for OIB),Zr to Nb(6.9-8,averaging 7.25 vs.5.83 for OIB)and Th to La(0.07 vs.0.11 for OIB).This suggests that the basalt magma may have derived from the asthenospheric mantle and is contaminated with crustal materials.The zircon U-Pb age is 106.27±1.3 Ma.Given that the pollen assemblage,mainly Classopollis-Osmundacidites,is produced in the Duohemao Formation of the Maixiu Mountain area,we propose that Tengbu-Rizhou basalts are the product of the late Mesozoic continental rift magmatism in West Qinling,while rifting likely ended in the early stage of the lithospheric extension,since we found no signs of typical bimodal volcanic rocks except red bed formation in the Duohemao Formation and molasse-like formation in the upper Wanxiu Formation.