Regional Differences of Magmatism in the Okinawa Trough
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摘要: 冲绳海槽位于西太平洋活动大陆边缘，是一个目前正处于弧后扩张作用早期的、年轻的弧后盆地，张性断裂发育，火山活动强烈.岩石类型分布和岩石地球化学特征表明冲绳海槽中部和南部的岩浆作用存在明显差异，但迄今为止人们对于导致该差异的原因尚不清楚.对已有的冲绳海槽玄武岩的资料进行了综合对比分析，并依据冲绳海槽岩浆岩的Pb同位素组成特征，指出冲绳海槽及其所在的沟弧盆体系之下的地幔属于印度洋型地幔，相对于海槽中部的玄武岩岩浆，海槽南部的玄武岩岩浆来自地幔更高程度的熔融；冲绳海槽玄武岩岩浆受到了俯冲板块物质加入的影响，且南部玄武岩岩浆受影响的程度要高于中部，这可能受控于俯冲板块的深度(南部：100~150 km；中部：约200 km)；冲绳海槽玄武岩部分属于MORB型，这应是冲绳海槽早期扩张的重要特征和证据之一.在上述分析工作的基础上，提出了今后对冲绳海槽岩浆作用的研究应主要集中在以下几个方面：冲绳海槽基底岩石类型的甄别与分布；岩浆源区地幔的类型及特征；板块俯冲对冲绳海槽岩浆作用的贡献及制约机制；冲绳海槽岩浆作用与海底热液活动的内在联系等.Abstract: Okinawa Trough is a young and active back-arc basin located at active continental margin in the western Pacific, which is in the initial stage of spreading and characterized by widespread extensional faults and intense volcanic activities. Distribution and geochemistry of rocks show distinct differences of magmatism in the central and southern Okinawa Trough, but the reason is unclear so far. To explore this issue, we study comprehensively published petrologic and geochemical data, combined with Pb isotopic composition of basalts in Okinawa Trough in this paper. It is found that mantle source beneath Okinawa Trough and the trench-arc-basin system in which the Okinawa Trough is located belongs to Indian Ocean-type mantle, and basalts derived from southern Okinawa Trough are products of higher degree of mantle melting relative to basalts from middle Okinawa Trough. Basalts derived from Okinawa Trough are affected by the subduction component and extent of the effect of subduction component on magma source in the southern Okinawa Trough is higher than that in the middle Okinawa Trough, and the cause may be subducted depth of plate (southern: 100-150 km; middle: about 200 km); only a few basalts are similar to MORB-type, which is considered to be an important characteristic and evidence of spreading in the Okinawa Trough. On the basis of the above analysis, we suggest that the studies of magmatism in the Okinawa Trough shall be focused on the following aspects in the future: identification and distribution of basement rocks, the types and characteristics of magmatic mantle source, the restriction mechanism and contribution of subduction to magmatism, and the internal relation between magmatism and hydrothermal activities.
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Key words:
- Okinawa Trough /
- basalts /
- magmatism /
- subduction /
- degree of mantle melting /
- tectonics
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图 1 冲绳海槽玄武岩的分布
MRA.琉球岛弧中部；MOT.冲绳海槽中部；SOT.冲绳海槽南部；KBS.Kobisyo；SBS.Sekibisy；下文同.红线为俯冲板块顶面深度等深线(km)，据Shinjo et al.(1999)；黑点和红点为玄武岩取样点，位置据Herman et al.(1979)、张明书(1982)、Kimura et al.(1986)、Sibuet et al.(1987)、Ishizuka et al.(1990)、Ishikawa et al.(1991)、Honma et al.(1991)、翟世奎和干晓群(1995)、李巍然等(1997a, 1997b)、Shinjo et al.(1999)以及马维林等(2004)；KBS和SBS位置据Shinjo(1998)；MRA位置据Shinjo et al.(1999)
Fig. 1. The distribution of basalts in the Okinawa Trough
图 2 冲绳海槽玄武岩的87Sr/86Sr-SiO2(a)和143Nd/144Nd-SiO2(b)关系
Fig. 2. Relationships of 87Sr/86Sr-SiO2 (a) and 143Nd/144Nd-SiO2 (b) for basalts in the Okinawa Trough
图 3 冲绳海槽玄武岩的207Pb/204Pb-206Pb/204Pb (a)和208Pb/204Pb-206Pb/204Pb (b)关系
NTVZ.中国台湾北部火山带，数据来自Wang et al.(2004)；NHRL.北半球参考线；EMI.Ⅰ型富集地幔；EMII.Ⅱ型富集地幔；DM.亏损地幔；九州的数据来自Hoang and Uto(2006)
Fig. 3. Relationships of 207Pb/204Pb-206Pb/204Pb (a) and 208Pb/204Pb-206Pb/204Pb (b) for basalts in the Okinawa Trough
图 4 冲绳海槽及琉球岛弧和KBS和SBS样品的Ta/Yb-Nb/Yb关系
Fig. 4. The relationship of Ta/Yb-Nb/Yb for samples in Okinawa Trough and Ryukyu arc and KBS and SBS
图 5 冲绳海槽玄武岩的Nb/Yb-Nb关系
Fig. 5. Relationships of Nb/Yb-Nb for basalts in the Okinawa Trough
图 6 冲绳海槽玄武岩Na8-Fe8关系
Fig. 6. Relationships of Na8-Fe8 for basalts in the Okinawa Trough
图 7 冲绳海槽玄武岩地幔熔融程度的(Nb/Yb)9-Yb9关系
PUM.原始上地幔；DMM.亏损地幔
Fig. 7. Relationships of (Nb/Yb)9-Yb9 for degree of mantle melting of basalts in the Okinawa Trough
图 8 冲绳海槽玄武岩Ba/Yb-Nb/Yb (a)和Th/Yb-Nb/Yb (b)关系
Fig. 8. Relationships of Ba/Yb-Nb/Yb (a) and Th/Yb-Nb/Yb (b) for basalts in the Okinawa Trough
图 9 冲绳海槽玄武岩Th/Ta-Nb/Ta关系
X-BABB.extreme-BABB，指富集LREE、P和Zr的玄武岩，据Sinton et al.(2003)
Fig. 9. Relationships of Th/Ta-Nb/Ta for basalts in the Okinawa Trough
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