中国兴蒙—吉黑地区岩石圈结构基本特征
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摘要
兴蒙—吉黑地区岩石圈由额尔古纳、兴安、松嫩和佳木斯4个古陆块及完达山中生代大陆边缘增生杂岩构成。Nd同位素模式年龄显示,佳木斯陆块时代最老,1500～2200Ma;额尔古纳陆块次之,1000～1600Ma;兴安和松嫩陆块具有相同的Nd模式年龄,500～1200Ma。地球化学示踪分析表明,该区古生代时表层地壳的Nd同位素模式年龄以中元古代为主,而中生代花岗岩的Nd同位素模式年龄主要为新元古代,表明该区深部地壳的年龄较表层地壳的年龄年轻,显示出该区地壳具有下新上老的年龄结构。Os同位素分析同时证明,该区岩石圈地幔也多表现为年轻性质。地震(Vp)速度结构显示,该区岩石圈结构在垂向上具有两个明显的特征:一是与传统意义上的地震岩石圈概念明显不同,该区岩石圈地幔的低速带没有稳定连续的顶界面,低速异常顶界面深浅不一,与高速异常体犬齿交错,某些构造单元之下的低速异常直达Moho,但底界面却十分稳定,深度为230～240km;二是“立交式”速度结构,表现为在地壳范围内,速度等值线总体呈北东向展布;岩石圈地幔的速度等值线呈北北西-近南北向展布;低速异常圈层的速度等值线为近东西向展布。
The lithosphere beneath the Hingmong-Jihei (Hinggan-Mongolia-Jilin-Heilongjiang) region, northeastern China, is composed of the Ergun, Hinggan, Songneng and Jiamusi continental blocks and Mesozoic Wandashan accretionary complex. Nd isotope model ages indicate that: the Nd model age of the Jiamusi block is oldest, being 1500-2200 Ma; the age of the Ergun block comes next, being 1000-1600 Ma; and the Hinggan block and Songnen block have the same Nd model age, ranging from 500 to 1200 Ma. Geochemical tracing analysis indicates that the Nd isotope model age of Paleozoic supracrustal rocks is dominantly Mesoproterozoic, while that of Mesozoic granites is mainly Neoproterozoic. Therefore, it is concluded that the deep level of the crust is younger than the surficial level of the crust, indicating that the crust in the region has an age structure of being younger in the lower part and older in the upper part. The Os isotope analysis also indicates that the lithospheric mantle in this region also shows the younger character. The seismic (Vp) velocity structure shows that vertically the lithospheric structure in the region has the following two prominent features: (1) notably different from the traditional concept of the seismic lithosphere, the low-velocity zone of the lithospheric mantle has no persistent and continuous top interface which the low-velocity anomalous top is highly varied in depth and interlocks with the high-velocity anomalous bodies, and the low-velocity anomalies below some tectonic units may reach the Moho but the bottom interface occurring at 230-240 km depth is very persistent; and (2) the "overpass-type" velocity structure is manifested by the following: the velocity contours are generally distributed in a NE direction in the crust, in a NNW-NS direction in the lithospheric mantle and in a nearly E-W direction in the low-velocity anomaly asthenosphere.
The lithosphere beneath the Hingmong-Jihei (Hinggan-Mongolia-Jilin-Heilongjiang) region, northeastern China, is composed of the Ergun, Hinggan, Songneng and Jiamusi continental blocks and Mesozoic Wandashan accretionary complex. Nd isotope model ages indicate that: the Nd model age of the Jiamusi block is oldest, being 1500-2200 Ma; the age of the Ergun block comes next, being 1000-1600 Ma; and the Hinggan block and Songnen block have the same Nd model age, ranging from 500 to 1200 Ma. Geochemical tracing analysis indicates that the Nd isotope model age of Paleozoic supracrustal rocks is dominantly Mesoproterozoic, while that of Mesozoic granites is mainly Neoproterozoic. Therefore, it is concluded that the deep level of the crust is younger than the surficial level of the crust, indicating that the crust in the region has an age structure of being younger in the lower part and older in the upper part. The Os isotope analysis also indicates that the lithospheric mantle in this region also shows the younger character. The seismic (Vp) velocity structure shows that vertically the lithospheric structure in the region has the following two prominent features: (1) notably different from the traditional concept of the seismic lithosphere, the low-velocity zone of the lithospheric mantle has no persistent and continuous top interface which the low-velocity anomalous top is highly varied in depth and interlocks with the high-velocity anomalous bodies, and the low-velocity anomalies below some tectonic units may reach the Moho but the bottom interface occurring at 230-240 km depth is very persistent; and (2) the "overpass-type" velocity structure is manifested by the following: the velocity contours are generally distributed in a NE direction in the crust, in a NNW-NS direction in the lithospheric mantle and in a nearly E-W direction in the low-velocity anomaly asthenosphere.
引文
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[4]W ilde S A,Zhang X ingzhou,W u Fuyuan.Extension of a new ly identified500M a m etam orphic terrane in N orth EastC hina:further U-Pb SH R IM P dating of the M ashan C om plex,H eilongjiang Province,C hina[J].T ectonophysics,2000,328:115～130.
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[6]康宝祥,张海.那丹哈达岭饶河蛇绿岩及其地质意义[J].黑龙江地质,1990,(1):3～18.K ang Baoxang,Zhang H airi.The ophiolites and its geological s ignifecentin R aohou,N adanhada[J].G eology ofH eilongjiang,1990,(1):3～18(in Chinesew ith English abstract).
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[8]邵济安,唐克东,王成源,等.那丹哈达地体的构造特征及演化[J].中国科学(B辑),1991,(7)744～751.Shao Jian,Tang K edong,W ang C hengyuan,etal.The N adanhada terrane and evolution[J].Science in C hina(Ser.B),1991,(7)744～751(in C hinese).
[9]水谷伸治郎,邵济安,张庆龙.那丹哈达地体与东亚大陆边缘中生代构造的关系[J].地质学报,1989,63(3):204～215.M izutani S,Shao Jian,Zhang Q inglong.The relationship of the N adanhada terrane and M esozoic structure ofcontinentalm argin in EastAsia[J].Acta G eologica Sinica,1989,63(3):204～215(in C hinese w ith English abstract).
[10]张庆龙,水谷伸治郎,小岛智,等.黑龙江省那丹哈达地体构造初探[J].地质论评,1989,35(1):67～71.Zhang Q inglong,M izutani S,K ojim a S,et al.The N adanhada terrane in H eilongjiang Province[J].G eologicalR eview,1989,35(1):67～71(in Chinese w ith English abstract).
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