粤东河源盆地晚白垩世玄武岩地球化学特征及其大地构造意义

Petrogenesis of the Late Cretaceous Basalts in the Heyuan Basin(Eastern Guangdong):Implications for Late Mesozoic Tectonic Transition along the Southern Margin of South China

粤东河源盆地是华南大陆南缘晚中生代伸展体系的重要组成部分,记录着华南大陆南缘从安第斯型大陆边缘向西太平洋型大陆边缘转换的关键信息。本项研究以河源盆地晚白垩世玄武岩(~74 Ma)为研究对象,通过研究其元素地球化学和Sr-Nd-Pb-Hf同位素特征,探究了华南大陆南缘晚白垩世玄武岩的岩石学成因及大地构造意义。研究结果显示河源盆地晚白垩世玄武岩具拉斑玄武岩特征,岩石的稀土元素总量(ΣREE)较低,富集轻稀土元素(LREEs)和大离子亲石元素(LILEs,Ba、Th、Sr等),高场强元素(HFSEs,Nb、Ta、Ti等)并未出现明显亏损,表现出与E-MORB相似的特征。模拟计算显示,河源盆地玄武岩可能来自于尖晶石+石榴石二辉橄榄岩地幔源区约10%的部分熔融。玄武岩的(^(87)Sr/^(86)Sr)_(i)值较低,ε_(Nd)(t)值和ε_(Hf)(t)值为正,La/Nb值与ε_(Nd)(t)值在软流圈地幔与华南大陆岩石圈地幔(SCLM)间,靠近软流圈范围,且呈负相关关系,表明本区玄武岩主要来自于亏损的软流圈地幔。然而,与来自亏损的软流圈地幔的N-MORB相比,河源盆地玄武岩表现出较高的(^(87)Sr/^(86)Sr)_(i)和较低的ε_(Nd)(t)值,显示可能受到了富集地幔的影响。玄武岩(^(207)Pb/^(204)Pb)_(i)和(^(208)Pb/^(204)Pb)_(i)值较高,接近EMⅡ范围,表明其源区的富集组分为EMⅡ型。玄武岩相对E-MORB更富集LREEs和Ba、Th、Pb等元素,Pb同位素比值落入太平洋沉积物区域,表明其源区有俯冲沉积物熔体的参与。玄武岩的微量元素和Sr-Nd同位素特征与日本海弧后盆地玄武岩相似,指示其形成于弧后拉张环境,可能受控于古太平洋板块的后撤,并伴随着古南海弧后洋盆的打开。

The Heyuan Basin in Eastern Guangdong of China is crucial to constrain how the southern margin of South China was transferred from an Andean-type to a Western Pacific-type subduction system in the late Mesozoic.In this paper,we reported new major and trace element and Sr-Nd-Pb-Hf isotopic results of the late Cretaceous basalts(~74 Ma)in the Heyuan Basin and discussed their petrogenesis and tectonic implications.Our results show that the basalts are tholeiitic without obvious Ce anomaly(δCe=0.97-0.98)and slight positive Eu anomaly(δEu=1.06-1.19).The basalts are characterized by enriched light rare earth elements and large-ion lithophile elements,and no obvious depletions of high-field-strength elements such as Nb,Ta and Ti.The overall geochemical and isotopic characteristics of the basalts are similar to those of typical E-MORB.Based on Sm/Yb-La/Sm and Lu/Hf-La/Sm diagrams,we infer that the basalts were likely derived from 10%partial melting of a mantle source of spinel+garnet lherzolite.The basalts have low(^(87)Sr/^(86)Sr)_(i) values,positiveε_(Nd)(t)andε_(Hf)(t)values.Their La/Nb andε_(Nd)(t)values are negatively correlated,and vary in ranges between those of the asthenosphere mantle and SCLM,but closer to that of the asthenosphere,indicating that the basalts in this area are mainly derived from the depleted asthenosphere mantle.However,compared with the N-MORB from the depleted asthenosphere mantle,the basalts from the Heyuan Basin have higher(^(87)Sr/^(86)Sr)_(i) and lowerε_(Nd)(t)values,and relatively high(^(207)Pb/^(204)Pb)i and(^(208)Pb/^(204)Pb)_(i) values,suggestive of a EMⅡtype source.Besides,the basalts are enriched in LREEs,Ba,Th,and Pb in comparison with E-MORB,and the Pb isotope ratios fall into the Pacific sediment region,indicating the involvement of subducted sediments.In conclusion,the trace element and Sr-Nd isotopic characteristics of the basalts are similar to those found in the back-arc basin of the Japan Sea,thus,it can be inferred that the basalts in the Heyuan Basin were formed in a back-arc extensional setting,possibly associated with the opening of the Proto-South China Sea in response to the slab rollback of the Paleo-Pacific plate.