Determination and Tectonic Significance of Volcanic Rock Series in the Okinawa Trough
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摘要: 冲绳海槽是一个处于弧后扩张作用早期的年轻的弧后盆地，是研究弧后扩张作用早期盆地演化和壳幔过程的天然实验室.随着调查研究工作的逐步展开和深入，也发现了一些新的、重要的、亟待解决的科学问题，而火山岩岩石系列归属的厘定又是其他研究工作的基础.在系统收集和整理迄今已有冲绳海槽火山岩资料的基础上，结合近期分析测试数据, 对冲绳海槽火山岩的岩石系列归属进行了重新厘定，探讨了火山岩的构造环境指示意义和浮岩与玄武岩之间的成因联系.研究结果表明：冲绳海槽火山岩分布具有以基性玄武岩和酸性(流纹)英安岩为主的双峰式特征，中性火山岩稀少，基性的玄武岩属于亚碱性系列的橄榄拉斑玄武岩，酸性浮岩可归属为亚碱性岩系的流纹英安岩或流纹岩；在构造环境判别上，冲绳海槽玄武岩表现出大洋中脊和岛弧构造环境的特点，既有别于大洋中脊扩张中心，也有别于成熟型弧后盆地，呈现出弧后早期扩张阶段盆地独特的构造环境特征；广泛分布于冲绳海槽的酸性浮岩表现出一定的岛弧环境的特点；酸性浮岩与玄武岩具有同源性，酸性岩是基性的玄武质岩浆经不同程度结晶分异和同化混染作用的产物.Abstract: The Okinawa Trough is a young back-arc basin at its early back-arc spreading stage, and is a natural laboratory for understanding the formation and evolution of back-arc basins and crust-mantle interaction processes under this tectonic setting. However, with the gradually deepening of research work, the rock series of volcanic rocks from the Okinawa Trough are becoming an important and urgent need to solve problem. Based on the geochemical compositions of some pumices determined recently by authors and the petrologic and geochemical data of volcanic lavas from the Okinawa Trough in literatures, the rock series of volcanic rocks from the Okinawa Trough have been determined in detail. The tectonic significance of volcanic rocks and the genesis relationship between pumices and basalts have also been discussed. The magmatism in the Okinawa Trough is bimodal, being comprised dominantly of basic basalts and acidic rhyolites, accompanied by minor intermediate rocks. The basalts are mainly olivine tholeiitic basalts, which belong to subalkalic series, and the pumices are mostly rhyodacite or rhyolite, which belong to subalkalic series too. According to the geochemical compositions, basalts are not only similar to those from the mid-ocean ridge but also from the island arc tectonic environment. They are different from those occurring at mid-ocean ridge spreading center or mature back-arc basins, but with distinct geochemical features associated with distinct tectonic environment their occurred, a back-arc basin at early spreading stage. The acid pumice, a widely distributed rock type in Okinawa Trough, is characterized by the geochemical compositions that are typical for volcanic rocks from island arcs. The magma of intermediate-acidic pumices, with same sources as associated basalts, may be formed by fractional crystallization of basalt magma.
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Key words:
- rock series /
- magmatism /
- tectonic environment /
- Okinawa Trough /
- marine geology
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图 2 冲绳海槽火山岩TAS图解
据Rickwood(1989)；冲绳海槽火山岩数据翟世奎和干晓群(1995)，李巍然等(1997)，Shinjo and Kato(2000)，马维林等(2004)，韩宗珠等(2005)，Hoang and Uto(2006)，黄朋等(2006)；包裹体数据于增慧(2000)；玄武岩玻璃数据见国坤(2016)
Fig. 2. TAS diagram of the volcanic rocks in Okinawa Trough
图 3 R1-R2火山岩分类
据 De la Roche et al.(1980)；R1和R2按公式从千阳离子数获得此参数：R1=4Si-11(Na+K)-2(Fe+Ti)；R2=6Ca+2Mg+Al
Fig. 3. R1-R2 diagram of the volcanic rocks
图 4 SiO2-Nb/Y和Zr/TiO2-Nb/Y火山岩分类
Fig. 4. SiO2-Nb/Y and Zr/TiO2-Nb/Y diagram of the volcanic rocks
图 5 SiO2-K2O岩石系列判别
据Peccerillo and Taylor(1976)；Lau海盆玄武岩数据来自Fretzdorff et al.(2006)，Escrig et al.(2009)，Peate et al.(2001)，Hergt and Woodhead(2007)；马里亚纳海槽玄武岩数据来自Volpe et al.(1990)，Stern et al.(1990)
Fig. 5. SiO2-K2O diagram of rock series
图 6 AFM(a)和SiO2-TFeO/MgO(b)岩石系列判别
据Kuno et al.(1968)和Miyashiro(1974)；TH拉斑玄武岩系列；CA钙碱性玄武岩系列
Fig. 6. AFM (a) and SiO2-TFeO/MgO (b) diagram of rock series
图 7 微量元素岩石系列判别
H-K和SHO高K钙碱性系列和橄榄玄粗岩系列；IAT.岛弧拉斑玄武岩系列；B.玄武岩；BA/A.玄武安山岩/安山岩；D/R.英安岩/流纹岩；据Hastie et al.(2007)
Fig. 7. Trace element diagram of rock series
图 8 冲绳海槽火山岩构造环境判别
CAB.钙碱性玄武岩；CABC.大陆边缘钙碱性玄武岩；ORTB.大洋中脊拉斑玄武岩；ITB.岛弧拉斑玄武岩；MORB.大洋中脊玄武岩；IAT.岛弧拉斑玄武岩；OIT.洋岛拉斑玄武岩；OIB.洋岛碱性玄武岩；WPAB/WPB.板内玄武岩；N-MORB.正常型大洋中脊玄武岩；E-MORB.富集型大洋中脊玄武岩；VAB.火山玄武岩；syn-COLG.同碰撞；VAG.岛弧；WPG.板内；ORG.大洋中脊.a~i为玄武岩构造环境判别；j~l为酸性岩构造环境判别
Fig. 8. Tectonic diagram of volcanic rock in Okinawa Trough
图 9 冲绳海槽火山岩微量元素平均值蛛网图
Fig. 9. Trace element distribution pattern of volcanic rocks in Okinawa Trough
图 10 冲绳海槽火山岩稀土元素平均值
Fig. 10. REE distribution pattern of volcanic rocks in Okinawa Trough
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