华北中元古代火山岩Sin－Nd同位素年代及其构造环境的地球化学依据

Sm-Nd ISOCHRONE AND GEOCHEMICAL EVIDENCE OF TECTONIC SETTING OF MESOPROTEROZOIC BASALTS,NORTH CHINA

由于缺乏合适的测年对象，华北地区中上元古界的年龄值主要是从沉积岩及脉岩中获得。该文据近来从辽北中元古界凡河群中识别出的一套同期喷发玄武岩，获得其Ｓｉｎ－Ｎｄ等时线年龄为１１９９±３９Ｍｒ。ＦｒＯ（全）－Ａｌ２Ｏ３－ＭｇＯ，Ｔｉ／１００－ＺＩ－Ｙ·３，ＴｉＯ２－Ｋ２Ｏ－Ｐ２Ｏ５，ＴｉＯ２－Ｙ／Ｎｂ和ＴｉＯ２－Ｚｒ图解指示了其大陆内部的裂谷构造环境。８７Ｓｒ／８６Ｓｒ＝０．７０３０，＝－９．５～－１１，及无Ｅｕ负异常的轻稀土富集型配分模式表明岩浆源于上地幔及太古界地壳的重熔。综合分析华北板块内中晚元古代沉积层序及火山作用，推测中晚元古代时凡河地区是位于华北板块内部近北北东向裂陷槽北部末端部位的裂谷盆地，该裂陷槽由南向北渐次开裂呈楔形。

The geologic ages of Middle-Upper Proterozoic in North China are obtained mainly from isotopic dating of sedimentary rocks and dykes, because there are few volcanics in that period. Recent works have recognized basalts from the Mesoproterozoic Fanhe Group (PFG) with classic continental rift sequence which is composed of shallow water elastics and carbonate rocks with gabbro sills and dykes. The isotopic dating of PFG basalts gives a Sin-Nd isochronic age of 1199 ± 39 Ma and , which suggests that there did exist volcanism within NCP ca 1200 Ma. Geochemical diagrams of FeO (total)-Al2O3-MgO,TiO2-K2O-P2O5, Ti/100--Zr-Y·3,TiO2-T/Nd and TiO2-Zr indicate that the PFG basalts erupted in the continental rift environment. The Archaean crust-like REE pattern with enrichment of LREE and no negative Eu anomaly and not only demonstrate that the PFG basalts came from the Archaean continental contamination of the partial melting of the crust,but also indicate that there might exist a reworking of crustal structure ca 1200 Ma. From the above information and regional geology,it can be inferred that NCP developed a large NNE-SSW continental rifting system which rifted gradually from S to N and that Fanhe area was just located at the north end of the rifting system during Meso-Neoproterozoic.