摘要
本文报道了郯庐断裂带巢湖—庐江段上白垩纪火山岩的主量元素、微量元素及 Sr-Nd-Pb 同位素特征。研究区火山岩 SiO_2含量为50.2%～75.2%,主要为基性岩和偏酸性岩类。样品富钾、贫钠、偏碱性(σ平均5.36),明显富集 LILEs、LREEs,相对亏损 HFSEs,具有弱的 Eu 负异常(δEu 平均0.70)及富集的 Sr-Nd-Pb 同位素组成(I_(Sr)=0.7054～0.7090,ε_(Nd)(t)=-5.2～-17.6)。地球化学特征表明岩浆具有多源与源区多变的特征,早期基性岩(125Ma)的岩浆源区主要为华北克拉通地幔,兼有下地壳的贡献;流纹岩(120Ma)主要是地热异常背景下华北中、下地壳源区熔融的结果;粗面岩(100Ma)为华北下地壳和富集岩石圈地幔来源岩浆的混合结果;晚期基性岩(93Ma)的源区主要为华北岩石圈地幔,兼有软流圈和下地壳物质的贡献。这些岩浆活动是在华北克拉通东部岩石圈减薄背景下、郯庐断裂带伸展活动中发生的,深部软流圈上涌使断裂带内的地温升高而造成了岩石圈的部分熔融,其具体岩浆过程指示了详细的岩石圈减薄过程。其中120Ma 时期的流纹岩喷发指示断裂带内地壳出现了最高的地温与最浅的等温面,应对应减薄的峰期;而93Ma 时期的基性岩喷发反映了深部开始出现了软流圈的部分熔融。一系列现象均指示,郯庐断裂带是华北克拉通东部的岩石圈强减薄带,是热流与岩浆上升的有利通道,也是发生强烈岩石圈地幔交代与置换的场所。
Geochemical and Sr-Nd-Pb isotopic analyses have been carried out on the Early Cretaceous volcanic rocks from the south segment of the Tan-Lu fault zone. These rocks are rich in K2O and alkali, and relatively depleted in Na2O, with SiO2 contents varied between 50. 2% -75.2%. They belong to calc-alkaline to alkaline series （with a average σ of 5.36）. They are enriched in LILEs, LREEs but depleted in HFSEs, and show some negative Eu anomalies （ average δEu = 0. 70） and enriched Sr-Nd-Pb compositions （ISr =0. 7054 - 0. 7090, εNd （t） = -5.2 - -17.6）. Geochemical data indicate that magma contamination by ascending through middle-upper crust could not play an important role while lower crust was likely an important end member during magma forming. Magmatic mixing is the main magma process and four different types of magma sources are detected. These lavas were formed under an intensively extensional tectonic setting. The magma evolution was ascribed to the increasing of lithospheric heat flow followed sustaining thinning of its lower part, the asthenosphere upwelling and lithosphere extension also are the key factors for the magmatism. Some geological, geochemical and geophysical features suggest a shallower source and a more intensive extensive setting, which indicate a more intensive lithospheric thinning beneath this fault zone, the lithospheric thinning may be trigged by the activity of the fault zone.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2008年第8期1823-1838,共16页
Acta Petrologica Sinica
基金
国家自然科学基金(批准号:90714004
40672131
40603010)
西北大学大陆动力学教育部重点实验室开放课题基金资助
关键词
郯庐断裂带
早白垩纪
火山岩
地球化学
岩石圈减薄
The Tan-Lu fault zone
Volcanic rocks
Geochemistry
Early Cretaceous
Lithospheric thinning
