中国东部中生代岩浆活动与板块俯冲的关系——浙闽与日本弧和安第斯弧的对比及其意义

摘要

The Mesozoic-Cenozoic tectonic background of eastern China is the top concern for geologists in China.Mter the plate tectonic theory is introduced into China since the 1970s,the Chinese geologists generally accepted the view that eastern China Mesozoic subduction of the Pacific plate to Eurasian plate result in strong tectonic-magmatic activities and corresponding mineralization,and even become generally cognitive theory remains widespread by Chinese and foreign scholars.But,this paper argues that a lot of problems.As is known to all,island arc predominantly basalt exposed,continental arc are composed dominantly of andesite,basalt and andesite are not developed in eastern China.In this paper,according to the thinking way of big data,Japan island arc and the Andean arc Cenozoic magmatic rocks statistics show that the above understanding is basically right:Japan arc mainly basalt,second is andesite;Andean arc mainly is andesite,followed by basaAnd eastern China (represented by Zhejiang-Fujian area),are mainly granite,the second is basalt,a bimodal distribution characteristics.The tectonic background of eastern China is completely different from Japan and the Andean,and there is not clear evidence of subduction in eastern China.Second,island arc and continental arc have obvious composition and structure zoning,such as Japan arc,magmatic activity is starting from the trench,then front-arc,arc,rear-arc to back-arc.Andean arc is less obviously than Japan,eastwards from the trench to the mainland fore-arc trench complex-arc magmatic rocks-back-arc basin.Where is structure and composition zone associated with subduction in eastern China (including the East China Sea continental shelf,China's eastern coastal)? Our study focused on the Zhejiang-Fujian area 400km width within the scope of the distribution of the Jurassic and Cretaceous magmatic rocks,from age to geochemical (SiO2,MgO style,the change of K2O,the change of the age,etc.),never has the tendency of zonation from east to west,how to link to plate subduction? Island arc magmatic rocks mainly comes from the depleted mantle,oceanic crust,deep sea sediments,and the fluid caused by the subduction zone,therefore,arc magmatic rocks are with obvious oceanic crust features.Continental arc also comes from the mantle,but magma across the continental crust,and brings obvious continental crustal contamination,so the Andean type magmatic rocks have obvious marks of continental crust.If not considering the influence of the subduction zone,Continental magmatic rocks should come from the heat asthenosphere mantle.If the heat asthenosphere stay at the bottom of the lithosphere,partial melting,there should be formed continental flood basalts,and intermediate-acid magmatic rocks is very little;On the contrary,if the heat asthenosphere breakthrough lithospheric block and up to the bottom of the crust,it will heat the bottom of the lower crust of partial melting,forming a lot of acidic granite,basalt and andesite is rarely.Emei Mountain is the former,and eastern China is the latter.What are the differences and similarities of magrnatic rocks in eastern China,Andean and Japan should be petrologists' first proposition,we suggested that China's petrologists and geochemists study not only in eastern China,but also in the Japan arc and the Andean magmatic rocks deeply and in detail.Compared with eastern China's situation,we will obtain new knowledge,perhaps may help solve the problem of eastem China's magmatic rocks background.%中国东部中-新生代的构造背景是中国地质学界最关注的问题之一.自20世纪70年代板块构造学说引入中国后,中国地质学家普遍接受了太平洋板块向欧亚板块俯冲导致中国东部中生代强烈的构造-岩浆活动和相应的成矿作用的观点,乃至成为被中外学者普遍认知的理论,至今仍然广泛流传.但是,本文研究认为问题很多.众所周知,岛弧是以玄武岩出露为主,大陆弧则是以安山岩出露最多,而中国东部玄武岩和安山岩极不发育.本文按照大数据研究思路,对日本和安第斯全部新生代岩浆岩的统计研究表明,上述认识基本上是对的:日本弧主要是玄武岩,其次是安山岩;安第斯弧主要是安山岩,其次是玄武岩;而中国东部(以浙闽地区为代表),主要是花岗岩,其次是玄武岩,出现双峰式分布的特征.看来,中国东部与日本和安第斯的构造背景完全不同,中国东部没有俯冲作用的明显证据.其次,岛弧和大陆弧有明显的成分和结构分带,如日本弧,从海沟开始,岩浆活动是从前弧-岛弧-后弧-弧后(frant-arc,arc,rear-arc,back-arc).安第斯弧不如日本弧明显,从海沟向东到大陆是从弧前杂岩-弧岩浆岩-弧后盆地.中国东部(包括东海大陆架、中国东部沿海)与俯冲有关的结构和成分分带哪里有？我们的研究集中讨论了浙闽地区400km宽度范围内侏罗纪-白垩纪岩浆岩的分布,从年龄到地球化学(SiO2的变化,MgO、K2O的变化,年龄的变化等等),基本上见不到有从东到西分带的趋势,这种情况如何与板块俯冲作用联系起来呢？岛弧岩浆岩主要来源于亏损的地幔、洋壳、深海沉积物,以及由俯冲带带来的流体,因此,岛弧岩浆岩洋壳的特征非常明显.大陆弧也来自地幔,但是,岩浆穿过大陆壳,会带来明显的陆壳混染的影响,因此安第斯型岩浆岩陆壳的印记比较明显.大陆岩浆岩如果不考虑俯冲带的影响,岩浆岩应当来自高热的软流圈地幔.如果高热的软流圈停滞在岩石圈底部,在那里发生部分熔融,形成的应当是大陆溢流玄武岩,而中酸性岩浆岩非常少;相反,如果高热的软流圈突破岩石圈的阻隔而上升到地壳底部,则会加热下地壳底部使之发生部分熔融,形成的则是大量的酸性花岗岩,而玄武岩和安山岩很少.峨眉山是前面的情况,中国东部则是后面的情况.中国东部岩浆岩究竟与日本、安第斯有何异同点？应当是岩石学家研究的首要命题,建议中国的岩石学家和地球化学家不要仅限于中国东部的研究,而将研究的触角延伸一步,深入细致地研究一下日本和安第斯岩浆岩的情况,再对比中国东部的情况,如此可能会得出新的认识,这样的认识也许才可能有益于解决中国东部岩浆岩形成背景的问题.