大兴安岭东北部花岗岩类锆石U-Pb年龄、岩石成因及地壳演化

1、大兴安岭东北部花岗岩类锆石U-Pb年龄、岩石成因及地壳演化        本文运用LA-ICPMS、LA-MC-ICPMS等先进实验测试技术对大兴安岭东北部花岗岩类进行了详细的年代学和地球化学研究,对大兴安岭东北部花岗岩的形成时代、地球化学特征、成因和地壳演化等方面取得了一些新的认识。大兴安岭东北部花岗岩主要为早古生代(461500Ma)和侏罗纪花岗岩(171190Ma),少量晚古生代(298±2Ma)和三叠纪(236±1Ma)花岗岩。早古生代花岗岩的主要岩石组合为花岗闪长岩-二长花岗岩-碱长花

2、岗岩,具有后造山I型花岗岩的岩石组合和地球化学特征,形成于额尔古纳地块和兴安地块碰撞拼贴结束后的后造山阶段。本区存在高Sr低Yb型和低Sr高Yb型两类侏罗纪花岗岩,在太平洋板块俯冲导致的加厚地壳背景下,类似于“C”型埃达克岩的高Sr低Yb型花岗岩形成于较高压力的下地壳,而低Sr高Yb型花岗岩形成于较低压力的中地壳。Hf同位素和Nd同位素研究表明,花岗岩主要来源于中元古代(1.11.5Ga)、新元古代-显生宙(0.40.8Ga)从亏损地幔中增生的地壳物质。兴安地块主要是显生宙-新元古代增生的地壳,而额尔古纳地块则以中元古代增生的地壳为主,也有新元古代-显生宙增生的地壳。根据组成物质的属性和增生时

3、代,确定额尔古纳地块和兴安地块的东北部边界在呼玛镇-十四站林场一线Granitoids from northeastern Da Hinggan Range are taken as studied object in this thesis. Based on systematic study of field geology, zircon LA-ICPMS U-Pb dating, petrology, geochemistry, Sr-Nd isotope and Hf isotope of these granitoids, their chronology, petrogenesi

4、s, tectonic setting and geological implications are discussed, and the following conclusions are obtained.1. Zircon U-Pb dating results by laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) indicate that granitoids from northeastern Da Hinggan Range formed mainly in the Early Pa

5、leozoic and Jurassic, and some of Late Paleozoic and Triassic. The emplacement ages of Early Paleozoic granitoids including Shibazhan pluton, Neihe pluton, Baiyinna pluton, Chalabanhe pluton and Halapaqi pluton are 499±1 Ma, 500±1 Ma, 460±1 Ma, 465481 Ma and 461500 Ma, respectively. C

6、ombined with zircon U-Pb dating results of Early Paleozoic granitoids from adjacent regions, the time scale of Early Paleozoic magmatism in the studied area could be determined as 460500Ma. The Jurassic magmatism of this area could be classified into two stages, i. e. , 188190Ma and 171181Ma. The em

7、placing ages of Hanjiayuanzi pluton and Zhengqicun pluton of the early stage are 188±1 Ma and 190±1 Ma, respectively, while Xinghua-Fanshentun pluton, Jiweidianzi pluton and Heihuashan pluton of the late stage emplaced at 178±1Ma, 176181 Ma and 171±2 Ma, respectively. Shierzhan g

8、ranitic pluton of Late Paleozoic formed at 298±2Ma, and Chahayan granitic pluton of 236±1 Ma.2. The Early Paleozoic granitoids in this area mainly consists of granodiorite, monzogranite and syenogranite. The mafic minerals in these rocks are hornblende and biotite, while the accessory mine

9、ral is titanite and without Al-rich minerals. These granitoids are metaluminous and weakly peraluminous, belonging to calc- alkaline-high-K calc-alkaline series, enriched in large ion lithophile elements (LILE) (e. g. , Rb, Th, U, K, La, Nd and so on), depleted in high field strength elements (HFSE)

10、 such as Nb, Ta, P, Ti, etc. . These lines of evidence indicate they exhibit a geochemical characteristics of postorogenic I-type granite, and should formed at postorogenic or postcollisional stage.3. The Jurassic granitoids in this area are mainly composed of diorite, quartz diorite, granodiorite a

11、nd monzodiorite, being metaluminous and weakly peralu- minous, belonging to high-K calc-alkaline series. Based on the enrichment and depletion degree of Sr and Y, they can be separated into two types, i. e. , high-Sr and low-Y type (Sr>500g/g, Y2g/g). The early stage of Jurassic granite are of hi

12、gh-Sr and low-Y type, such as Zhengqicun pluton, Hanjiayuan pluton, Sandaoka pluton and Wudingshan pluton, while late stage of them are of low-Sr and high-Y type, e. g. , Xinghua-Fanshentun pluton, Jiweidianzi pluton and Heihuashan pluton. The two types of granites possess similar abundance and same

13、 evolutional trend of major elements, and similar Sr-Nd and Hf isotope characteristics. Taken together, the two types of granites originated from similar magmatic source of different depth, high-Sr and low-Y type granite being similar to C-type adakites which derived from low crust, while low-Sr and

14、 high-Y type granites derived from middles crust of low pressure.The Jurrasic granitoids of this area have similar rock association and geochemical characteristics to granitoids from active continental margin, belonging to NE-NNE distributed granite belt in northeast China, and their origins are rel

15、ated to the subduction of Paleopacific plate.4. Most of the Early Paleozoic granitoids have high (176)Hf/(177)Hf ratios and positive_(Hf)(t) values, and these spots cluster around the evolutional line of chondrite, implying the source materials of granitoids mainly came from depleted mantle. Some pl

16、utons have low (176)Hf/(177)Hf ratios and negative_(Hf)(t) values (-14.98-8.19), falling within the evolutional scope of Proterozoic crust, indicating that they should be derived from partial melting of newly accreted crustal materials during Mesoproterozoic- Neoproterozoic, moreover, their magmatic

17、 source may possess affinities of enriched mantle. In addition, contamination of old crustal materials may exist during the formation of some rocks.Jurassic granitoids have widely various Hf isotopic composition (_(Hf)(t)= -2.82+11.64 ), attributing to contribution of accreted crustal materials. Zhe

18、ngqicun pluton have (176)Hf/(177)Hf ratios of 0.28260.2827,_(Hf)(t) values of -2.82+2.82, and two stage model age of 1.2Ga in average, indicating its magmatic source are accreted crustal materials during Mesoproterozoic-Neoproterozoic. Heihuashan pluton have highest (176)Hf/(177)Hf ratios for zircon

19、s whose_(Hf)(t) values up to +11.64, and its young two stage model age (0.470.72Ga) indicating the magmatic source are newly accreted crustal materials from depleted mantle during Neoproterozoic-Phanerozoic. Hanjiayuanzi pluton has (176)Hf/(177)Hf ratios,_(Hf)(t) values (all of them are positive), a

20、nd two stage model age between that of Zhengqicun pluton and Heihuashan pluton, implying its magmatic source consist of crustal materials mainly accreted during Mesoproterozoic- Neoproterozoic and some of them accreted during Phanerozoic similar to Heihuashan pluton.5. Hf and Nd model ages of granit

21、oids indicate this area have two crust accreted stages, Mesoproterozoic (1.01.5Ma) and Neoproterozoic-Phanerozoic (0.40.8Ma). Along with age of granitoids from old to young, (176)Hf/(177)Hf ratios and_(Hf)(t) values gradually increased, while model age became young, suggesting decrease of old crustal materials and increase of juvenile accreted crustal materials in the magmatic source, implying the crust growth of Da Xingga