博格达裂谷双峰式火山岩地质年代学与Nd-Sr-Pb同位素地球化学特征

Geochronology and Nd-Sr-Pb isotops of the bimodal volcanic rocks of the Bogda rift.

东天山博格达造山带位于准噶尔和吐鲁番-哈密两个前寒武纪地块之间。已有充分的地质与元素地球化学证据证明东天山博格达陆内碰撞造山带构造属性是属于大陆裂谷,全区早石炭世火山岩的SiO_2含量在55%～64%之间出现明显的间断,其刺梅沟七角井组中玄武岩和流纹岩在时空上紧密拌生,为一典型的双峰式火山岩组合。该双峰式火山岩组合中玄武岩和流纹岩Rb-Sr同位素等时线年龄分别为:342．0±3．2 Ma和340．3±3．4 Ma。七角井组中玄武岩和流纹岩同位素等时线年龄在误差范围内与博格达古生物化石指示的早石炭世时代一致。七角井组玄武岩:ε_(Nd)(t)为+6．4～+6．7,(^(87)Sr/^(86)Sr)_i为0．703261～0．703328,(^(206)pb/^(204)Pb)_i为17．703～17．989、(^(207)pb/^(204)Pb)_i为15．407～15．498、(^(208)Pb/^(204)Pb)_i为37．147～37．825;七角井组流纹岩:ε_(Nd)(t)为+6．4～+6．6,(^(87)Sr/^(86)Sr)_i为0．703368～0．703469,(^(206)Pb/^(204)Pb)_i为17．827～18．114、(^(207)Pb/^(204)Pb)_i为15．460～15．517、(^(208)Pb/^(204)Pb)_i为37．077～37．973,由此可见七角井组玄武岩和流纹岩在Nd-Sr-Pb同位素特征方面具有相当一致。根据Nd-Sr-Pb同位素地球化学和元素地球化学认为其玄武岩是来源于亏损地幔,而伴生的流纹岩则是由玄武岩浆分离结晶作用的产物。一般都认为岩浆分离结晶作用只产生成分连续变化的岩浆系列,但是在特定条件下也是可以产生双峰式火山岩组合。由玄武岩浆分离结晶作用形成的双峰式火山岩并不少见,但是在裂谷环境下双峰式火山岩中流纹岩是由玄武岩浆分离结晶作用形成的这种双峰式火山岩组合至今还未发现。因此,博格达双峰式火山岩是大陆裂谷环境中比较特殊的并是十分少见的或可能是一种新的双峰式火山岩构造成因类型。

The Bogda orogen,which is located between the Junggar and Turpan-Hami Precambrian blocks, has been proved to be a Late Palaeozoic continental rift by many lines of geological and geochemical evidence. SiO2 content of Lower Carboniferous volcanic rocks in this orogen exhibits a gap between 55% and 64%. Bimodal volcanic suite of the Cimeitou formation at Qijiaojing is composed of basalt and a less amount of rhyolite. The Rb-Sr isochron ages of the basalts and rhyolite are 342.0± 3.2 Ma 和11 340.3 ±3.4 Ma, respectively. These rocks were formed during strong extension of the rift. These ages are consistent with those indicated by fossil. Basalts of the Qijiaojing formation are characterized by εNd （t） = ＋ 6. 4 ～ ＋ 6. 7, （ ^87 Sr/^86 Sr） i = 0.703261 - 0. 703328, （ ^206 Pb/^204 Pb） i = 17. 703 - 17. 989, （^207 pb/^204 Pb） i = 15. 407 - 15. 498 and （ ^208 Pb/^204 Pb） i = 37. 147 - 37. 825, while rhyolites of this formation are characterized by ε Nd （t） = ＋ 6.4 - ＋ 6.6, （ ^87 Sr/^86 Sr ） i = 0. 703368 - 0. 703469, （ ^206 pb/^204 Pb） i = 17. 827 - 18.114, （ ^207 pb/^204 Pb ） i = 15. 460 - 15. 517 and （^208 pb/^204Pb） i = 37. 077 - 37. 973. These data indicate that the rhyolites is similar in Nd-Sr-Pb isotopes to basalts and that the basalts were derived from a depleted mantle, while the associated rhyolites were formed by fractional crystallization of the basaltic magma. It is a common idea that fractional crystallisation can only produce a continuously differentiated rock series. However, bimodal volcanic rocks can also be formed in some particular cases. Bimodal volcanic rocks, in which rhyolite is formed by fractional crystallisation of basalt is not rare on earth, but has not been reported from continental rift environments. Therefore, the bimodal volcanic rocks is a very rare case, and is possibly a new genetic type of bimodal suite.