班公湖—怒江俯冲增生杂岩带东段晚古生代辉绿岩锆石U-Pb年龄、Hf同位素特征及其构造意义

doi:10.13745/j.esf.sf.2021.7.27

摘要/Abstract

摘要：

嘉玉桥—同卡混杂岩带位于班公湖—怒江俯冲增生杂岩带东段嘉玉桥至邦达地区,1: 250 000区域地质调查将其主体厘定为晚古生代混杂岩,但缺乏准确的同位素年代学与地球化学约束。本文选择该带邦达岩组中的辉绿岩作为研究对象,开展详细的岩相学、地球化学及年代学工作,揭示班公湖—怒江俯冲增生杂岩带的形成时间及构造环境。结果表明,辉绿岩明显具有N-MORB型的地球化学性质,同时富集大离子亲石元素(Rb,Ba),亏损高场强元素(Nb,Zr),其岩浆源区为尖晶石二辉橄榄岩经过约30%的部分熔融,一致指示岩浆源区可能源于受俯冲流体交代影响的亏损地幔源区。辉绿岩的锆石LA-ICP-MS U-Pb年龄为(330.9±1.6) Ma,代表了辉绿岩的结晶年龄,指示邦达辉绿岩形成于早石炭世,进一步揭示班公湖—怒江俯冲增生杂岩带存在古特提斯洋演化残留地质信息。邦达早石炭世N-MORB型辉绿岩的厘定,为进一步认识班公湖—怒江俯冲增生杂岩带早石炭世的构造环境,探讨班公湖—怒江特提斯洋的时空演化提供了重要依据。

关键词: 班公湖—怒江俯冲增生杂岩带, 嘉玉桥—同卡增生杂岩带, U-Pb年龄, N-MORB, 大地构造

Abstract:

The Jiayuqiao-Tongka mélanges are located within the eastern part of the Bangong Lake-Nujiang subduction-accretion complex belt from Jiayuqiao to Bangda area whose spatial distribution, material composition, tectonic setting and genesis have been discussed to some extent by previous researchers but have not been studied in detail. For example, the formation era of the subduction-accretion complex has been determined to be the Late Paleozoic by geological survey (1: 250000) but lacked precise geochronological and geochemical constraints. In this contribution, the diabase of the Bangda Formation is investigated through detailed petrographic, geochemical and chronologic analyses to shed light on the development time and tectonic setting of the Bangong Lake-Nujiang structural zone. The diabase was found to be enriched in LILEs (Rb, Ba) and depleted in high HFSEs (Nb, Zr) and exhibited the geochemical characteristics of N-MORB. Its LREE pattern indicated a depleted N-MORB mantle source, while geochemical analysis further suggested the diabase is sourced from 30% partial melting of spinel lherzolite formed in a back-arc rift basin. The LA-ICP-MS U-Pb dating of zircon from the diabase yielded an average zircon age of (330.9 ± 1.6) Ma, which represents the crystallization age of diabase, indicating the Bangda diabase is formed in the Early Carboniferous epoch, and supporting the notion that the Bangong Lake-Nujiang structural zone retains geological information of the residual Paleo-Tethys. The results of this study on the Early Carboniferous N-MORB diabase provided an important reference for the reconstruction of the regional tectonic setting as well as for future studies on the spatio-temporal evolution of the Paleo-Tethys in the Bangong Lake-Nujiang region.

Key words: Bangong Lake-Nujiang subduction-accretion complex belt, Jiayuqiao-Tongka mélange belt, U-Pb age, N-MORB, geotectonics

中图分类号:

任飞, 尹福光, 彭智敏, 潘桂棠, 魏栋. 班公湖—怒江俯冲增生杂岩带东段晚古生代辉绿岩锆石U-Pb年龄、Hf同位素特征及其构造意义[J]. 地学前缘, 2022, 29(2): 164-179.

REN Fei, YIN Fuguang, PENG Zhimin, PAN Guitang, WEI Dong. Late-Paleoanthics diabase from the eastern part of the Bangong Lake-Nujiang subduction belt: U-Pb geochronology, Hf isotopic composition and geological characteristics[J]. Earth Science Frontiers, 2022, 29(2): 164-179.

图/表 16

图1 班公湖—双湖—怒江—昌宁—孟连俯冲增生杂岩带展布图

Fig.1 Simplified geological sketch map of the Banggong Lake-Shuanghu-Nujiang-Changning-Menglian subduction-accretion complex belt

图2 研究区地质简图 1—第四系;2—宗白群;3—多尼组;4—甲丕拉组;5—波里拉组;6—阿堵拉组;7—瓦达岩组;8—错绒沟口岩组;9—邦达岩组;10—怒江岩组;11—瞎绒曲岩组;12—大理岩岩块;13—灰岩岩块;14—超基性岩块;15—采样位置。

Fig.2 Geological sketch map of the study area

图3 邦达地区基性岩野外及镜下照片 a—邦达岩组构造变形;b—辉绿岩与基质呈构造接触;c—辉绿岩野外露头;d—辉绿岩显微照片;e—玄武岩与基质呈构造接触;f—玄武岩显微照片。Pl—斜长石;Px—辉石;Cal—方解石;Q—石英。

Fig.3 Outcrops and photomicrographs of basic rocks in the Bangda area

表1 邦达混杂岩辉绿岩主量、微量及稀土分析数据及相关系数

Table 1 Analytical results and relevant coefficients for major elements,trace elements and REE in the Bangda diabases

表2 邦达混杂岩辉绿岩锆石LA-ICP-MSU-Pb年龄.

Table 2 LA-ICP-MS zircon U-Pb dating results for the Bangda diabase

表3 邦达辉绿岩锆石Hf同位素组成表

Table 3 Zircon Hf isotopic compositions of the Bangda diabase

点位	Age/Ma	¹⁷⁶Hf/¹⁷⁷Hf	1σ	¹⁷⁶Lu/¹⁷⁷Hf	1σ	¹⁷⁶Yb/¹⁷⁷Hf	1σ	ε_Hf(0)	ε_Hf(t)	t_DM1	t_DM2	f_Lu/Hf
NJ-7-01	328	0.282 72	0.000 011	0.001 627	0.000 052	0.045 599	0.001 391	-1.84	5.09	766	928	-0.95
NJ-7-02	329	0.282 724	0.000 009	0.001 321	0.000 026	0.036 33	0.000 724	-1.69	5.30	754	916	-0.96
NJ-7-03	326	0.282 710	0.000 009	0.000 592	0.000 021	0.016 125	0.000 558	-2.2	4.95	760	936	-0.98
NJ-7-04	329	0.282 721	0.000 012	0.001 191	0.000 03	0.033 257	0.000 96	-1.8	5.22	756	921	-0.96
NJ-7-05	331	0.282 709	0.000 011	0.001 528	0.000 016	0.043 11	0.000 492	-2.23	4.72	780	949	-0.95
NJ-7-06	331	0.282 728	0.000 011	0.002 725	0.000 097	0.079 218	0.002 953	-1.56	5.13	778	926	-0.92
NJ-7-07	332	0.282 741	0.000 012	0.002 052	0.000 099	0.055 263	0.002 582	-1.1	5.74	745	892	-0.94
NJ-7-08	335	0.282 726	0.000 012	0.003 022	0.000 021	0.082 842	0.000 598	-1.63	4.99	788	933	-0.91
NJ-7-09	334	0.282 737	0.000 011	0.001 064	0.000 023	0.029 42	0.000 608	-1.25	5.80	732	889	-0.97
NJ-7-10	329	0.282 733	0.000 012	0.002 1	0.000 044	0.057 157	0.001 173	-1.37	5.45	757	908	-0.94
NJ-7-11	330	0.282 683	0.000 01	0.001 049	0.000 011	0.030 204	0.000 342	-3.16	3.89	807	995	-0.97
NJ-7-12	333	0.282 731	0.000 01	0.000 807	0.000 06	0.021 568	0.001 555	-1.44	5.66	734	896	-0.98
NJ-7-13	327	0.282 721	0.000 011	0.000 998	0.000 011	0.027 777	0.000 418	-1.82	5.24	753	920	-0.97
NJ-7-14	327	0.282 729	0.000 011	0.001 481	0.000 061	0.039 949	0.001 847	-1.5	5.45	750	908	-0.96
NJ-7-15	332	0.282 733	0.000 01	0.000 93	0.000 013	0.025 509	0.000 357	-1.37	5.71	734	894	-0.97

图4 样品NJ-7锆石阴极发光图像、分析点位黄圈—锆石U-Pb测试点;红圈—锆石Lu-Hf同位素测试点。

Fig.4 Cathodoluminescence images of zircon grains from sample NJ-7, with the sampling spots shown in yellow circles.

图5 锆石U-Pb年龄谐和图及加权平均图

Fig.5 Concordia diagram and weighted mean plot of zircon U-Pb ages

图6 八宿邦达地区辉绿岩Zr/TiO2-Nb/Y(a)和FAM(b)分类图解

Fig.6 Zr/TiO2 vs. Nb/Y (a) and FAM (b) diagrams for the Bangda diabase

图7 邦达辉绿岩稀土元素球粒陨石标准化图解(a)和微量元素原始地幔标准化图解(b) (球粒陨石、原始地幔、OIB、N-MORB和E-MORB数值引自文献[33])

Fig.7 REE and trace element primitive mantle standardization diagrams of dibase in Bangda Area. Chondrite, primitive mantle, OIB, N-MORB and E-MORB data from 33.

图8 邦达辉绿岩Nb/Yb-Th/Yb图解(a)及Nb/Yb-TiO2/Yb图解(b)

Fig.8 Th/Yb mv Nb/Yb and TiO2/Yb vs. Nb/Yb diagrams for glucite from the Bangda area

图9 邦达辉绿岩锆石Hf同位素εHf(t)直方图(a)和εHf(t)与U-Pb年龄图解(b)

Fig.9 Histogram of initial Hf isotope ratio (a) and plot of εHf(t) vs. zircon U-Pb age (b) for the Bangda diabase

图10 邦达辉绿岩La-La/Sm和La-La/Zr判别图解

Fig.10 La/Zr vs. La and La/Sm vs. La diagrams for the Bangda diabase

图11 邦达辉绿岩La/Yb-Dy/Yb和La/Sm-Sm/Yb判别图解

Fig.11 Dy/Yb vs. La/Yb and Sm/Yb vs. La/Sm diagrams for the Bangda diabase

图12 邦达辉绿岩TiO2-FeO*/MgO判别图解 IAT—岛弧拉斑玄武岩;OIB—洋岛玄武岩;MORB—大洋中脊玄武岩。

Fig.12 TiO2-FeO*/MgO diagram for the Bangda diabase

图13 邦达辉绿岩2Nb-Zr/4-Y判别图解(a)和Hf/3-Th-Nb/16判别图解(b) AⅠ—板内碱性玄武岩;AⅡ—板内碱性玄武岩和板内拉斑玄武岩;B—E-MORB;C—板内拉斑玄武岩和火山弧玄武岩;D—N-MORB和火山弧玄武岩;MORB—洋中脊玄武岩;IAT—岛弧拉斑玄武岩;WPT—板内拉斑玄武岩;CAB—钙碱性玄武岩;WPB—板内玄武岩。

Fig.13 2Nb-Zr/4-Y (a) and Hf/3-Th-Nb/16 (b) diagrams for the Bangda diabase

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