【摘要】 利用长江中下游成矿带多学科深部探测剖面于2009年11月至2011年3月间采集的天然地震数据,通过天然地震接收函数成像等分析研究,得到了研究区地壳和上地幔结构的清晰图像。接收函数成像结果显示研究区内Moho面深度存在着明显的起伏变化,在长江中下游成矿带(指剖面穿过的长江中下游成矿带宁芜矿集区,下同)下方存在着"幔隆构造"。在剖面东南端(即扬子克拉通北缘),Moho面相对稳定,深度约为30km;在茅山和江南断裂附近,Moho面存在上下起伏现象;在剖面中部或宁芜矿集区下方,Moho面存在明显隆起,深度只有28km;在郯庐断裂带下方,Moho面明显加深,深度达到36km;进一步向北到华北地台南缘,Moho面深度逐渐恢复到了32km左右的平均深度水平。其次,我们在接收函数成像结果中发现,长江中下游成矿带与其周边下地壳结构存在着明显的差异,成矿带的下地壳具有显著的地震波方位各向异性。扬子克拉通北缘的下地壳呈高速的近水平状结构,地震波各向异性特征不明显;与此相比,长江中下游成矿带的下地壳虽然也呈近水平状结构特征,但是,对于沿成矿带走向方向传播的地震波,其下地壳具有高速特征,而对于垂直于成矿带走向方向上传播的地震波,其下地壳却又表现为低速特征,这意味着成矿带的下地壳存在着平行于成矿带走向(即近北东—南西)方向的地震波各向异性,我们解释其是下地壳熔融并沿成矿带走向水平流动导致矿物晶体定向排列的结果。最后,在郯庐断裂以西的华北地台南缘观测到一条从上地壳延伸到中下地壳的南南东向倾斜的转换震相,我们推测它可能是合肥盆地内地壳伸展构造的反映。此外,我们发现接收函数成像结果中观测到的"幔隆构造"与远震P波层析成像结果在成矿带下方150km深度上显示的上地幔低速异常(江国明等,另文发表)存在着良好的对应关系,我们解释它们是软流圈物质上涌的遗迹。综合天然地震接收函数成像、远震P波层析成像和前人关于岩浆岩等方面的研究成果,我们认为长江中下游成矿带现今的下地壳可能是中生代发生成矿作用的多级岩浆房系统的一部分,成矿带的形成可能是类似MASH过程的产物。首先,软流圈物质上涌导致了长江中下游成矿带及其周边拉张环境的形成,在其上部地壳中形成了一系列伸展构造;然后,软流圈物质通过底侵进入长江中下游成矿带的原下地壳并与原下地壳物质发生同化作用,形成类埃达克质岩浆;接着,类埃达克质岩浆沿着伸展、拆离构造上升到地壳浅部形成不同层次的岩浆房和侵入岩体,并与围岩作用形成矿床。更多还原

【Abstract】 Using the passive-source seismic data collected during November 2009 to March 2011 from a multidisciplinary transect in the mid-lower Yangtze metallogenic belt,we perfomed studies of receiver function imaging and others,and achieved seismic images of the crust and upper mantle structures in the study region with great clearance.First,the receiver function images show the Moho depth varies significantly along the profile in the study region,and a "mantle uplift" is seen just beneath the Yangtze metallogenic belt(to be more precisely,refers to the Nanjing-Wuhu ore district of the Yangtze metallogenic belt in all this paper).In the southeast section of the profile,the Moho topography is relatively stable with depths about 30 km beneath the northern margin of Yangtze craton;it is shown rugged up in the vicinity of the Jiangnan fault and the Maoshan fault;the Moho is shown uplift apparently to a depth of only about 28km beneath the centre part of the cross-section or the Nanjing-Wuhu ore district;it is seen deepened up to a depth of about 36 km below the Tan-Lu fault zone;the Moho depth is shown gradually restored to the average depth in the study region or about 32 km to further north beneath the southern margin of North China block.Secondly,we found the lower crust of the Yangtze metallogenic belt is different from that of its surrounding area in the structures from the receiver function results.The lower crust of the Yangtze metallogenic belt is seen apparently seismic azimuthal anisotropic.The lower crust of the northern margin of Yangtze craton is seen in a nearly layered structure of high velocity but lacks significant seismic anisotropy;Contrastly,the lower crust beneath the Yangtze metallogenic belt is,though in a sub-horizontal layered structure,seen to be of high velocity for seismic waves propagated approximately along the strike of the metallogenic belt,and to be of low velocity for waves propagated approximately normal to the strike,which implies that the lower crust of the metallogenic belt possess belt-parallel(or NE-SW) seismic anisotropy.We interprete this derived from melting,horizontal flow and mineral crystal orientation in the lower crust.Finally,we observed a converted structure extended from the shallow crust to the middle and lower crust on the western side of the TanLu fault in the southern margin of North China block,which dips to southeast and interpreted relevant to the crustal extension within the Hefei Basin.In addition,we found that the "mantle uplift structure" observed in the receiver function result corresponds well with a low velocity zone in the upper mantle at depths of about 150 km beneath the metallogenic belt on the teleseismic P wave tomogram(Jiang Guoming et al.,in another paper).We interpreted both of them to be a consequence of asthenosphere upwelling.Based on the images obtained from the receiver function studies and the teleseismic P wave tomography,as well as previous results on the magmatic rocks etc.,we suggest that the lower crust of the Yangtze metallogenic belt to be one site of the multilevel metallogenic magma system(Chang et al.,1991),and the formation of the metallogenic belt to be a result of MASH process.First,the upwelling asthenosphere led to an extensional environment in the the Yangtze metallogenic belt and its surrounding regions,and resulted in a series of extensional structures in the upper to middle crust;then,upwelling asthenospheric materials underplated into the lower crust of the Yangtze metallogenic belt,and occurred assimilation when mixed with the lower crustal material in situ,resulted in the formation of adakitic-like magma;Then adakitic-like magma rised up along the extensional and the decollemental structures in the shallow crust as intrusive bodies,and reacted with country rocks to form mineral deposits.更多还原