黄山-温州地球化学剖面及廊区解析

HUANGSHAN-WENZHOU GEOCHEMICAL SECTION AND ITS CORRIDOR AREA OF LITHOSPHERE IN SOUTHEAST CHINA

黄山－温州地学断面及周围地区幔源、壳源与基底岩石的Ｐｂ、Ｓｒ、Ｎｄ同位素，年代学与主、微量元素地球化学研究表明，华南与扬子从元古宙到新生代的幔源与壳源岩石均表现出不同的地球化学特征，反映出两区具完全不同的地质演化历史。应用新发展起来的地球化学填图与剖面研究方法，确定了华南－扬子地幔边界在浙江龙游以北（北纬２９．１°），向东呈东西走向，而华夏（指华南东部）地壳明显向北逆冲１０—３７ｋｍ，存在地壳相对于地幔的左旋滑移；确证了华夏古陆的存在。在剖面线及廊区华夏块体陆壳年龄为１．９５—２．５Ｇａ，而扬子块体为１．２—１．９Ｇａ。地球化学特征表明，扬子块体具华北块体与华南块体的过渡特征。

The Huangshan-Wenzhou section in Anhui-Zhejiang area, Southeast China, is passed through the Yangtze and Cathaysia blocks with a distance of about 350 km. Based on major and minor elements as well as Pb-Sr-Nd isotopic data for variety of rocks derived from mantle and crust along the section corridor area, it's indicated that there existed three kinds of mantles beneath Southeast China, i.e. MORB (in Yangtze),OIB and recycling (in Cathaysia) types, and the granites in Yangtze were derived from partial melting of basaltic ocean crust and showed high crystallization, but those in Cathaysia were derived from magma of crust-mantle mixing. Thus, the mantle and crust in Yangtze and Cathaysia possess extremely different chemical compositions and evolution histories. Ingeochemistry, Yangtze block shows transition features between North China and Cathaysia blocks. TheV 1 mapping of Pb isotopes for Cenozoic basalts and Mesozoic granites indicates that the mantle boundary between Yangtze and Cathaysia was mainly along E-W direction from Longyou to Sanmen, in stead of Jiangshan-Shaoxing Fault with NE direction, which has be regarded as a suture zone by geologists, and the crustal boundary was located in the north of mantle boundary with NEE direction. The V 1 fitting section from Huangshan to Wenzhou for current mantle and crust shows that the crust of Cathaysia has been moved about 10 km towards north with respect to its mantle. Themoved distance of crust in the coast area is about 37 km. The geochemical deduction is well consistent with the seismic velocity section, which has indicated that there existed mantle upwell and a body with high density and low velocity beneath the Longyou area, and a series of crust thrust towards north. Therefore, there were existed an anticlock rotation of the crust with respect to the mantle. Based on V 1 variation on the section, the Cathaysia can be divided into two parts bounded at Lishui, corresponding to Lishui-Zhenghe fault. The southern part undoubtedly showed geochemical features of Gondwana, whereas in the northern part, there existed ancientre cycling mixing of materials from the Yangtze subduction plate in the mantle. The Nd model age mapping of granites and basement rocks shows that there existed a massif nucleus of Archaean crust in the Longquan and Suichang area of Cathaysia and the crustal growth was mainly from Late Archaean to Early Proterozoic (1.9—2.8 Ga). The crust in the eastern Yangtze was mainly formed during Middle Proterozoic (1.3—1.8 Ga). It also can be seen in the Nd model age section that there was an abrupt changing of crustal age in the Xikou area, where the variation of magnetic anomaly also showed a transition from negative to positive. The positive magnetic anomaly usually reflected existance of Archaean basement.