摘要
对南海北缘新生代玄武岩中幔源包体的研究揭示了该地区上地幔的不均一性和复杂的地幔过程。部分橄榄岩中单斜辉石(Cpx)具有极低的Ti含量(<160μg/g),这与这些样品中较高的Cpx含量(8%～12%)不匹配。主元素成分变化趋势以及Cpx中HREE分异现象,暗示该区上地幔经历了变压熔融。地幔熔融始于深部的石榴子石稳定区并延续到浅部的尖晶石稳定区(总熔融程度达23%)。这些熔融残余受到了硅酸盐和含水流体的交代。橄榄岩的微量元素组成指示南海北缘地幔具有大陆裂谷型地幔的特征,而与俯冲带之上的地幔楔或受俯冲作用影响的地幔有很大的差别。包体的平衡温度和橄榄石中Fo之间的负相关关系暗示岩石圈地幔具成分分层结构。该地幔的下部由大洋型橄榄岩组成,而地幔顶部为类似于太古宙-元古宙地幔的富斜方辉石方辉橄榄岩。这一岩石圈结构与该地区岩石圈的减薄和软流圈对老岩石圈的置换有关。深源岩石记录的信息支持南海海盆是陆缘扩张引起的主动盆地的观点。
The mantle heterogeneity and complex mantle processes are revealed on the basis of studies of mantle derived xenoliths in Cenozoic basalts in thenorthern South China Sea. Clinopyroxenes (Cpx) in some peridotites display extremely low Ti contents (< 160 μg/g) inconsistent with their relatively high Cpxmodes (8%～12%). Peculiar major element compositions and fractionated HREEin Cpx requires a polybaric melting to take place in the mantle beneath south China. These patterns can be modeled as residues of 23%fractional melting froma primitive mantle, first in garnet stability field then continuing in spinel stability field after breakdown of garnet to pyroxenes and spinel. These residueshave been further metasomatized by silicate melts and aqueous fluids. The traceelement characteristic of the peridotite xenoliths suggests a rift type mantle under this region, which is significantly different form those above the subduction zone or those affected by subduction related processes. The negative correlation between Fo in olivine and equilibration temperature suggests a stratified mantle beneath South China. The lower part of the upper mantle predominantly consists of fertile and moderately depleted peridotites which plot within the Phanerozoic mantle domain. In contrast, the upper layer contains harzburgite and Cpx poor lherzolite that are strongly affected by metasomatism of melt/fluid ofhighly variable compositions. The anomalously high orthopyroxene mode (up to 47%) rends some of these refractory samples compositionally similar to the Proterozoic/Archean mantle. Such lithosphere architecture may have resulted from partial replacement of the Archean Proterozoic lithosphere by the asthenosphere that rose adiabatically subsequent to lithospheric thinning during the Cenozoic. The xenolith data support the contention that the formation of South China Sea isrelated to southward migration of lithosphere extension, rather than passive back arc basin.
出处
《地球化学》
CAS
CSCD
北大核心
2002年第3期230-242,共13页
Geochimica
基金
国家杰出青年基金(49925308)
中国科学院知识创新工程重要方向项目(KZCX2-209)
国家攀登项目(95-预选-39)
