【摘要】 琼南分布有大量白垩纪火山岩,目前对这些火山岩的研究程度仍然较低。本文对出露在三亚六罗地区六罗村组的白垩纪火山岩进行了精细的年代学和元素地球化学分析。LA-ICP-MS锆石U-Pb定年表明,六罗村组上部的流纹岩和下部的玄武安山岩具有一致的形成年龄,均为102±1 Ma。其中流纹岩样品富硅高钾,Si O2含量73.77%~74.79%,K2O含量5.09%~6.77%,K2O/Na2O比值1.71~2.80;属强过铝质(A/CNK=1.17~1.22)、高钾钙碱性岩系。样品具中等负Eu异常(δEu=0.51~0.61),亏损Nb、Ta,富集Rb、Ba等大离子亲石元素(LILE);轻、重稀土元素分馏程度较强,(La/Yb)CN值为15.05~16.97。玄武岩-安山岩样品的Si O2含量在49.82%~57.93%之间,其K2O/Na2O比值(0.32~1.11)明显低于流纹岩样品,Mg#(46~58)明显高于流纹岩样品。样品的轻、重稀土元素分馏程度相对流纹岩样品要弱,其(La/Yb)CN值为9.42~12.24;Eu异常不明显(δEu=0.86~0.94),亏损Nb、Ta,富集LILE,P、Ti元素的负异常相对流纹岩样品要弱。Sr-Nd同位素研究结果显示,流纹岩和玄武岩–安山岩样品的(87Sr/86Sr)i值分别为0.708222~0.708965和0.707532~0.708401;εNd(t)值分别为–2.49~–2.69和–2.35~–4.09,二者具有一致的Sr-Nd同位素组成。综合分析认为这套火山岩的源区可能来自受俯冲组分交代的大陆岩石圈地幔,形成于伸展环境,可能与古太平洋板块俯冲引起的岩石圈伸展有关。更多还原

【Abstract】 A large number of volcanic rocks outcrops have been reported in the southern Hainan Island, but their geochronology, petrogenesis and tectonic implications are still unclear. In this study, we present detailed zircon U-Pb dating results and major and trace elemental compositions of the Liuluocun Formation volcanic rocks in the Liuluo area of Sanya, Hainan Island. LA-ICP-MS zircon U-Pb dating of the rhyolite from the upper Liuluocun Formation yields an age of 102±1 Ma, basaltic andesite from the lower Liuluocun Formation also shows an age of 102±1 Ma, indicating late stage of the Early Cretaceous volcanic activity occurred in southern Hainan Island. The rhyolites are characterized by high contents of silicon(Si O2=73.77%-74.79%) and potassium(K2O=5.09%-6.77%) and K2O/Na2 O ratios(1.71-2.80), and low Mg#(22-28). They show negative Ti, P, Nb and Ta anomalies, obvious depletion of Eu(δEu=0.51-0.61), enrichment in Rb, Ba and LILE and strong fractionation between LREE and HREE(La/YbCN=15.05-16.97). The basalts-andesites have Si O2 ranging from 49.82% to 57.93% and lower value of K2O/Na2O(0.32-1.11) and higher value of Mg#(46-58) than those of the rhyolites. They show slight Eu anomalies(δEu=0.86~0.94), depletion of Nb and Ta, and enrichment of LILE. They have lower degree fractionation between LREE and HREE(La/YbCN=9.42-12.24) and weaker negative P, Ti anomalies than those of the rhyolites. Besides, rhyolite and basalt-andesite samples show similar(87Sr/86Sr)i(0.708222-0.708965 and 0.707532-0.708401, respectively) and εNd(t) values(-2.49--2.69 and-2.35--4.09, respectively), indicating that they may have derived mainly from the same source. Combining with the regional geological data, we propose that these volcanic rocks are most likely generated from continental lithospheric mantle which had been modified by subduction component, and their formation was closely related to the regional lithospheric extension in the South China Craton during the Early Cretaceous, most likely caused by subduction of the Paleo-Pacific plate beneath the continental plate of China.更多还原