东南沿海基底杂岩的同位素年代学研究

ISOTOPIC GEOCHRONOLOGIC STUDY ON BASEMENT COMPLEX IN SOUTHEAST COAST OF CHINA

系统研究同位素年龄结果表明,中国东南中生代火山岩盖层下普遍存在着前寒武系古老地层,变质杂岩最大年龄约为1900Ma,属早元古界晚期——中元古界早期。双溪坞组“底部”的英安质霏细斑岩的年龄,与上墅组的同位素年龄接近。象山石浦的硅质岩“天窗时代”应属震旦系地层,可与“灯影灰岩”对比。同位素年龄还揭示,华南地区有加里东旋回和印支旋回热事件的信息。基底杂岩的Nd同位素表明,前加里东期变质岩,大多数属大洋火山——硅质建造。它们表现出大的富集因子,和高的^(143)Nd/^(144)Nd.因此,不能说凡是有古老基底杂岩存在,就与克拉通或古陆等同。在中国东南板块内基底杂岩大多数在加里东造山褶皱期之前都处于活动“地槽”期,真正形成大陆地壳是在加里东期之后。Nd同位素资料还证明,不同微板块构造区地幔表现出明显的不均一性。

1. The ages, determined from basement complex of Yangtze subplate and Cathaysia subplate in southeast China plate, indicate that a large number of Precambrian strata exists underthe cover strata. The oldest age of metamorphic complex exposed in such as Dashi, Laodian in Suichang, Dikou in Jianou, and Shenou in Longhai, is early lower Proterozoic-late Lower Proterozoic epoch in Cathaysia subplate. The metamorphic complex exposed in Jianyang-Juke-Masha zone, being similar to lower Longbeixi formation, should be situated above Dikou formation, and belongs to early Middle Proterozoic sequence. Louzhiba group of volcanic-siliceous turbidite fades, being later than aforesaid two formations, belongs to middle Middle Proterozoic epoch.Longbeixi formation, being oceanic island-oceanic hot spot basement volcanic sequence, belongs to Middle-late Middle Proterozoic epoch. Metamorphic complex-volcanic siliceous sequence exposed in Chituzhai belongs to late Middle Proterozoic epoch. The oldest basement complex exposed in two sides of Bingxiang-Xiangshan zone in Zhejiang is Shuangxiwu formation. The dacitic felsite-porphyry in the formation was considered previously as lower Shuangxiwu formation. It is not necessary so . The dacite felsite-porphyry may belong to Upper Proterozoic Shangshu formation. Zhenzhai group in Yujiawu region belongs to Middle and Upper Proterozoic sequence, and shouldn't correlate with and belong to basement metamorphic complex exposed in southern Zhenan. The 'fenster', exposed in Mesozoic volcanic rocks in Shibu town, Xiangshan County, can compare with 'Deng Yeng limeston' in Yangtze region and belong to Upper Shangsu for- mation. 2. The informations from K-Ar, Rb-Sr, U-Pb and Sm-Nd isotopic dating form basement complex of various 'fenster' indicate that differet methods used to date old rocks obtain similar effects, that the date can take effect through their mutual complment. But it is impossible to determine precisely the data of rocks when samples analysed have experienced alteration and metamorphism, especially when K-Ar, Rb-Sr and U-Pb systems in rocks have been seriouly disturbed and detroyed during amphibolite-granulite facies metamorphism. However, the data obtained can be used as the record of thermal events during ploycycle metamorphism. The aforesaid appearance is very clear in Cathaysia subplate. Dikou formation records two obvious thermal-events (190Ma and 260Ma) affected by Yanshan cycle and Indo-Sinian cycle. Longbeixi formation records the affection of thermal-events in Jinning cycle and Caladonian cycle. Metamorphic complex of Mayuan group is Obviously affected by polycycle metamorphism in which the strongest thermal-event took place in Caladonian cycle. The southeastern coastal growth-thermodynamical metamorphic belt shows clearly affection of late the-mal-event (100Ma±). 3. Most of basement complexes belong to oceanic volcanic-siliceous construction, their fSm/Ndconcetration coeficient and 143Nd/144Nd value are bigger than those of magmatite and gneiss in Looten Vesterale of Norway and in craton of Canada, and similar to Nd isotopic composition of oceanic tholeiite. Therefore, all the old basement complexes cann't be considered as the craton or old landmass. For southeast China plate, most of basement complexes existed in the form of active 'geosycline' before Caladonian orogenic folding period, continental crust formed actually after Caladonian period. 4. Nd isotopics indicates that geosyclines of different geotectonic regions in southeast China plate are inhomogeneous, 143Nd/ 144Nd value is higher, fSm/Ndvalue is near 0 in paleo-oceanic basalt of Cathaysia subplate. Rare earth elements and trace elements also display, differetly i.e. the former derives from development magmatic reservoir of depleted oceanic mantle, the latter derives from the richment oceanic island-continental basalt with characteristics of CHUR magmatic reservoir.