龙首山岩群成岩时代探讨

Discussion on the Petrogenic Time of Longshoushan Group, Gansu Province

文中利用颗粒锆石 U- Pb法获得了龙首山岩群奥长花岗岩 2 0 15± 16 Ma的上交点年龄和 4 5 2±16 Ma下交点年龄 ,上交点代表成岩的时代 ,下交点揭示的为广泛发育的加里东期同碰撞热构造事件 ;结合 Sm-Nd、Rb- Sr法的年龄数据和区域地质对比 ,认为龙首山岩群成岩时代属古元古代 ,它反映的是华北陆台除 2 5 0 0 Ma和 180 0 Ma之外 ,又一重要构造

Longshoushan Group(LGr) is located in a narrow zone with NW-SE direction in Longshoushan area (Fig.1), northwestern Gansu Province. It is suspended that LGr belongs to Archean or Preterozoic since there was no precise and reliable isotopic data. Currently an accurate isotopic age of 2015±16 Ma (Fig.3) with single zircon U-Pb was obtained from trondjemite in Longshoushan area. The location of sample D10 is in Shuangjizi, about 30 kilometers northwest to Jinchang City; and the zicorns that we had analyzed were selected from the gray deformed trondjemite in so-called Longshoushan Group. After Fig.4, the under intersection age: 452±16 Ma, stands for the thermal-tectonic event of collision in Longshoushan area in Caledonian; and the upper intersection age: 2015±16 Ma, reflects the petrogenic time of the trondjemite. The trondjemite might intrude the upper crustal rocks although it is adjacent to the rocks of LGr with fault. So the petrogenic time of Longshoushan Group should be more than 2015 Ma. Combined with Sm-Nd, Rb-Sr , other single zircon U-Pb ages (Table 1) and regional geology correlation, we thought that the rocks of LGr mainly contributed to Paleoproterozoic. And these ages also implied that there was another important Precambrian thermal-tectonic event besides two main activities of 2500 and 1800 Ma in North China Platform.