摘要
在海南岛南端的三亚地体上发现了一些石榴霓辉石正长岩,其矿物学及岩石地球化学特征表明,它们是硅饱和的过碱性侵入岩,富集大离子亲石元素(Rb ,Th ,U ,K ,Pb)和轻稀土元素,具有较明显的Nb ,Ta ,Ti,Sr,Ba ,Eu ,P负异常,重稀土含量相对较高且分异不明显;Sr,Nd ,Pb同位素组成与EMⅡ型富集岩石圈地幔类似。锆石的SHRIMPU Pb测定给出11个分析点的2 0 6Pb/ 2 3 8U年龄为2 37～2 5 1Ma,加权平均值为2 4 4±7Ma。根据这些资料,揭示了三亚过碱性正长岩是在中三叠世由于岩石圈拆沉、软流圈上涌导致减薄后的富集岩石圈地幔在较浅深度(<6 0km)因增温减压而发生低度的脱水部分熔融而形成。强调三叠纪时海南岛属于后造山伸展环境,是海西期造山事件连续演化的结果,不应划分出独立的印支期造山事件或印支旋回。提出三亚地体可能于晚二叠世通过九所—陵水断裂的大规模右旋走滑而移置到和琼中地体相邻。指出海南东南部存在一条北东向的三叠纪富碱侵入岩带,该岩带可能延伸到广东罗定—福建明溪。
We discovered some garnet-acmite syenite rocks in Sanya terrain at the southern margin of Hainan Island. The mineralogical and geochemical studies demonstrate that these rocks are silica-saturated and peralkaline, enriched in LILE (Rb, Th, U, K, Pb) and LREE, with relatively distinct Nb, Ta, Ti, Sr, Ba, Eu, P negative anomalies. The HREE contents are relatively high, without obvious differentiation. Their Sr, Nd and Pb isotopic compositions are similar to those of EM-Ⅱ lithospheric mantle. 11 zircon SHRIMP analytical data give the ^(206) Pb/^(238)U age ranges of 237～251 Ma, with a weighted average of 244±7 Ma. The above-mentioned information indicates that the Sanya syenite rocks were formed by decompression-dehydration melting of enriched lithospheric mantle at a shallower depth(<60 km, which was in tern caused by Triassic delamination and asthenospheric upwelling. It is emphasized that during the Triassic, the Hainan island was under a post-orogenic extension environment, being the continuous evolution of the Hercynian orogeny. It is not reasonable to divide the Triassic activity as an independent Indosinian orogeny. It is also proposed that the Sanya terrain was placed to present site adjacent to middle Hainan terrain through a Late Permian large scale right-lateral slide-slip along the Jiusuo-Lingshui deep fault. There exists a NE-trending Triassic alkali-rich igneous belt in SE Hainan Island. This belt probably extends to South China continent through Luoding of Guangdong Province to Mingxi of Fujiang Province. It is considered that the South China was probably also under a post-orogenic environment during the Triassic.
出处
《高校地质学报》
CAS
CSCD
北大核心
2005年第1期47-57,共11页
Geological Journal of China Universities
基金
国土资源大调查项目 (编号 :2 0 0 0 13 0 0 0 12 7
2 0 0 113 90 0 0 18
2 0 0 3 13 0 0 0 0 41)
湖北省新世纪高层次人才工程基金
南京大学内生金属矿床成矿作用国家重点实验室开放基金
宜昌地质矿产研究所科技发展基金联合资助
