摘要
大别核部杂岩单元中斜长角闪岩的地球化学特征对于认识大别核部杂岩单元的特征及归属有着十分重要的意义。本文对大别核部杂岩单元中斜长角闪岩的岩石地球化学特征及Sm-Nd 同位素特征进行了较系统研究。斜长角闪岩的 Nd亏损地幔模式年龄(t_(DM))有3个不连续年龄段,它们分别为2.50～2.92Ga(平均2.71Ga);1.79～2.47Ga(平均2.14Ga);1.07～1.57Ga(平均1.38Ga)。模式年龄为中元古代、古元古代的斜长角闪岩的等时线年龄分别为1444±180Ma、2220±210Ma,其模式年龄与等时线年龄在误差范围内一致。表明至少模式年龄为元古宙的斜长角门岩的t_(DM)可代表其原岩的近似形成年龄,从而证明大别核部杂岩单元中存在着古元古代、中元古代两期自亏损地幔分异的基性火山岩变质形成的斜长角闪岩。而模式年龄为新太古代的斜长角闪岩,其模式年龄能否代表其形成年龄还需要进一步研究。各期斜长角闪岩在地球化学特征上的差异表现得不明显。通过对其原岩性质研究表明:原岩主要为拉斑玄武岩,少数为钙碱性玄武岩。大别核部杂岩中斜长角闪岩原岩ε_(Nd)(t)反映出大别核部杂岩地区上地幔自古元古代到中元古代一直沿着亏损地幔 Nd 同位素的演化线演化发展,这种性质及演化趋势与扬子北缘及南秦岭早期上地幔 Nd
This study of the geochemical characteristics of the amphibolites from the Dabie core complex is important for tracing the origin of the Dabie core complex unit. This paper reports the geochemical features including major and trace element contents and Sm-Nd isotope systematic of amphibolites from the Dabie core complex. The analytical results show that the Nd Model ages relative to depleted mantle (TDM) of the amphibolites exhibit three discontinuous ranges that are 2. 50-2. 92 Ga(average 2. 71 Ga);l. 79 - 2. 47 Ga(average 2. 14 Ga);l. 07 -1. 57 Ga (average 1. 38 Ga) respectively. These amphibolites formed in mesoproterozoic and paleoprotorozoic have 1440±180Ma, 2220±210Ma isochron ages which are consistent with their model ages. Therefore, the TDM ages of the amphibolites could represent the formation ages of the original rocks of the amphibolites and there are Neoarchaean, Paleoproterozoic and mesoproterozoic basalts derived from the depleted mantle and metamorphosed to the amphibolites in the Dabie core complex. These amphibolites don't show obvious geochemical difference from each other. The original rocks of amphibolites with different ages are dominantly tholeiites and less calc-alkali-basalts. The variation of εNd(t)of the amphibolites with time from the Dabie core complex shows that the early upper mantle beneath the region evolved along the depleted mantle Nd evolutionary line. The early upper mantle beneath the Northern margin of Yangtze block and Southern Qinling also show the same Nd evolutionary trendancy. This supports that the Dabie core complex was a part of the Yangtze block in the early time.
出处
《地质论评》
CAS
CSCD
北大核心
2004年第1期77-86,共10页
Geological Review
基金
国家自然科学基金重大项目(编号49794043)
