北京密云环斑花岗岩锆石U-Pb年龄和Hf同位素及其地质意义

Zircon U-Pb ages and Hf isotopes and their geological significance of the Miyun rapakivi granites from Beijing,China.Acta Petrologica

密云岩体位于华北克拉通东部,由环斑花岗岩和斑状黑云母花岗岩组成,是典型 A 型环斑花岗岩岩体之一。锆石的 LA-ICPMS U-Pb 年代学研究表明,密云环斑花岗岩形成于1681±10Ma 和1679±10Ma,而围岩片麻岩的原岩形成于2521±14Ma。锆石的 LA-MC-ICPMS Hf 同位素研究表明,太古代片麻岩来源于亏损地幔物质的部分熔融,从而表明2.5 Ca 是华北克拉通地壳重要的生长期;环斑花岗岩中锆石 Hf 同位素组成为δ_(Hf)(t)=～-5,两阶段模式年龄为 T_(DM2)=2.6～2.8Ga,表明它们来源于太古代新生地壳的部分熔融,密云环斑花岗岩侵位于华北克拉通大陆裂解、伸展环境,可能与全球古元古代未期Columbia 超大陆的裂解有关。

The Miyun pluton, a typical A-type rapakivi pluton, is located in the eastern North China Craton and is mainly composed of rapakivi granite and porphyric biotite granites. Zircon LA-ICPMS U-Pb dating gives that the emplacement ages of Miyun rapakivi granites are 1681 ± 10Ma and 1679 ± 10Ma, whereas, their host gneiss formed at 2521 ± 14Ma. The mean εHf（t） value of zircons from Archean gneiss is about ＋4.5, indicating the protolith of gneiss was derived from a depleted mantle source. It suggests that the 2.5 Ga is an important period of crustal growth of the North China Craton. The zircons in the rapakivi granites have eHf （t） value of -5.0, indicating that the rapakivi granites were derived from a crustal source. The two-stage model ages （ TDM2 ） of rapakivi granites are about 2.6 to 2.8 Ga, similar to those of host Archean gneisses, indicating that the rapakivi granites were derived from partial melting of the crust formed during Late Archean. The Miyun rapakivi granites were emplaced in an intra-plate extensional setting, which may be related to breakup of Paleoproterozoic Columbia supercontinent.