摘要
本文根据中国东南部 140个晚中生代火成岩 Rb- Sr等时线年龄匹配的 Isr值 ,勾画了 Isr等值线图。该图与 TDM等值线图有好的对应 ,都在浙闽沿海地区、南岭东段和武夷山北段出现低值区或低值带 ,反映中国东南地壳在晚中生代时期 ,有广泛的地幔物质的加入 ,它们以底侵的形式底垫在中 -下地壳。沿海地区 Isr、TDM的低值区、带受地壳厚度的控制 ,内陆的低值区、带可能与局部应力场导致的拉张减薄有关。浙闽沿海晚中生代火成岩岩石地球化学显示大离子亲石元素 Rb、Sr、Ba、Th和 Ce富集 ,高场强元素 Nb、Ta、Zr、Hf和重稀土 Y、Yb亏损 ,具有明显的消减带岩浆作用的特征。西太平洋地震层析成像显示 ,西太平洋板块俯冲是连续的。在大约 2 0 0 0～ 12 0 0 km深度 ,地幔中正在下沉的古俯冲洋壳明显偏西 ,可能反映那时为低角度俯冲。中国东南部广泛发育的NE- NNE褶皱和断裂构造 ,与古太平洋板块向欧亚大陆之下的俯冲作用有关。大规模晚中生代岩浆活动之前的陆相沉积 ( T3- J2 )是造山磨拉石建造。
Based on 140 Isr values of Late Mesozoic granitoids from southeastern China, the Isr isoline diagram was drawed. The low Isr regions and bands, which are similar to the low TDM regions and bands, appear in the coastal region of Zhejiang and Fujian Provinces, in the northern Wuyi Mountain and the eastern Nanling Mountain, which suggested that extensive mafic magma underplated in the mid lower crust of southeastern China during the Late Mesozoic. The low Isr band occurring in the coastal region of Zhejiang and Fujian Provinces was related to the thickness of crust, and the low Isr regions in the inland were caused by the local pull apart of the continental lithosphere. The geochemical characteristics of Late Mesozoic igneous rocks from the coastal region of Zhejiang and Fujian Provinces enriched large ion lithophile elements (Rb, Sr, Ba, Th) and Ce and depleted high field strength elements (Nb, Ta, Zr, Hf) and heavy rare earth elements (Y, Yb), which were the typical characteristics of subduction zone magmatism. From 2 800 km to surface, the subducted slab is a continuum shown by the seismic tomography map of the western Pacific Ocean. From about 2 000 km to 1 200 km below the surface, the subducted slab shows departure from the normal locus to west, which suggested that the subduction angle was lower in that epoch. The NE-NNE trending folds and faults developed in the extensive region of southeastern China were caused by the paleo Pacific plate subducting under the Eurasia plate with a low dip angle. The Late Triassic to Middle Jurassic terrestrial deposits that predated the generation of numerous Late Mesozoic igneous rocks make up an orogenic molasse formation, suggesting that Indosinian folding and faulting, deposition of the orogenic molasse formation and generation of numerous Late Mesozoic igneous rocks in southeastern China were different stages of a whole orogenic event. All the geological evidences shown above supported the model of lithosphere subduction and mafic magma underplating.
出处
《大地构造与成矿学》
EI
CAS
CSCD
北大核心
2001年第1期55-63,共9页
Geotectonica et Metallogenia
基金
国家自然科学基金!(批准号 :496 72 10 4
496 32 0 80 )资助项目
