Zircon U-Pb ages and Hf isotopes of granites in Longzhen area and their geological implications
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摘要: 龙镇地区花岗岩位于大、小兴安岭的结合部,属于兴安地块与松嫩地块中的A-I型花岗岩带; 该区各岩体岩石类型基本相同,主要为花岗闪长岩和二长花岗岩,地球化学特征显示为高钾钙碱性系列的I型花岗岩。花岗岩中锆石的LA-ICP-MS U-Pb年代学分析结果表明,正达山岩体形成于320~315Ma,模拉布山岩体形成于169±3Ma,朝阳林场岩体形成于187~171Ma。这些花岗质岩浆的就位分别与古生代时期古亚洲洋闭合过程中的块体拼合作用及中生代古太平洋的构造演化有关。Hf同位素成分特征表明,两期花岗岩的源岩均为新元古代-显生宙期间亏损地幔来源的火成岩。结合已发表的Sr-Nd-Hf同位素资料,本文认为兴安地块大陆地壳增生的时间主要为新元古代-显生宙,这与松嫩地块一致,而明显晚于额尔古纳地块的地壳增生时间(中-新元古代),从而揭示了它们不同的地壳演化历史。Abstract: Longzhen granites, which locate in the boundary area of Great Xing'an Range and Lesser Xing'an Range Mts,belong to the A-I granite belt between the Xing'an and Songnen blocks. The rock assemblages of this area are generally similar, mainly comprising granodiorite and monzonite granite, with geochemical characteristics of high-K calc-alkaline I-type granite series. The U-Pb ages obtained by zircon LA-ICP-MS dating indicate that the Zhengdashan, Molabushan and Chaoyanglinchang plutons were emplaced at 320~315Ma,169±3Ma and 187~171Ma, respectively. The emplacement of these granites was related to the block-amalgamation during the closure of the Paleo-Asian Ocean in Paleozoic and the tectonic evolution of Paleo-Pacific in Mesozoic. Zircon Hf isotopic features indicate that the source rocks of the two episodes of granites are igneous rocks which originated from depleted mantle of Neoproterozoic-Phanerozoic time. Combined with previously published data of Sr-Nd-Hf isotopes, it is concluded that the time of crustal growth of Xing'an Block was mainly in the Neo-Proterozoic and Phanerozoic, which is consistent with that of the Songnen Block. However, it is younger than that of Ergunna Block (Middle- to Neo-Proterozoic),that is, these blocks have different evolution histories.
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Key words:
- Granites in Longzhen area /
- Zircon U-Pb age /
- Hf isotopes /
- Geological implications
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