西藏日喀则蛇绿岩镁铁质岩石Re-Os同位素特征及意义

doi:10.19657/j.geoscience.1000-8527.2022.06.06

摘要/Abstract

摘要：

通过对日喀则蛇绿岩的镁铁质岩石进行岩石学和地球化学研究,探讨其岩石成因及源区性质,同时根据Re-Os同位素的地球化学约束来探索雅鲁藏布蛇绿岩的形成机制。研究表明,日喀则蛇绿岩镁铁质岩石微量元素的标准化配分型式与洋中脊玄武岩类似,又具有岛弧玄武岩的地球化学特征。结合微量元素和同位素特征,均表明岩石的形成与俯冲作用有关。日喀则蛇绿岩产出于远离大陆地壳的SSZ环境,其形成过程未受到陆壳的混染;同时,Re-Os同位素体系受蚀变作用的影响也不明显。日喀则蛇绿岩镁铁质岩石的Re、Os含量低,¹⁸⁷Os/¹⁸⁸Os同位素比值较高,主要是源区性质和俯冲作用影响的结果。特提斯洋早期发生的多次俯冲作用造成地幔源区不均一。新特提斯洋壳俯冲过程中,上述不均一地幔发生部分熔融产生的镁铁质岩浆上升,经过遭受了早期熔体/岩石作用的纯橄岩通道,发生强烈的Re-Os同位素分馏,使熔体与地幔残余Os同位素组成表现出明显的解耦现象,进而形成现今的日喀则蛇绿岩。

关键词: 岩石成因, 源区特征, Re-Os同位素, 日喀则蛇绿岩

Abstract:

Petrology and elemental geochemistry of mafic rocks from the Xigaze ophiolite are studied so as to investigate the petrogenesis and source characteristics of the ophiolite. At the same time, Re-Os isotope geochemical constraints in formation mechanisms of the Yarlung Zangpo ophiolite is also explored. The result shows that the primitive mantle-normalized multi-element pattern of the mafic rocks is similar to the normal Mid-Ocean Ridge Basalt (N-MORB) with the characteristics of island arc. Combined with the characteristics of the trace elements and isotope, all of these are shown that the petrogenesis is related to subduction. The Xigaze ophiolite was formed in the SSZ environment far away from the continental crust, so it’s not contaminated by the continent crust during its development, also the alteration in Re-Os isotope systematics was not obvious. The reason of Re-Os isotopic characteristics with low Re, low Os and high ratios of ¹⁸⁷Os/¹⁸⁸Os for the mafic rocks is manifested by the source characteristics and the subduction. The multi-stage Tethyan subduction resulted in the mantle heterogeneity. During the subduction of Neo-Tethys, the mafic magma from the above-mentioned heterogeneous mantle went through the peridotite channel influenced by the early melts, leading to the Re-Os fractionation and amplifying the Os-isotope mismatch between the melt and residual mantle, that’s the petrogenesis of the Xigaze ophiolite.

Key words: petrogenesis, source characteristics, Re-Os isotope, Xigaze ophiolite

中图分类号:

P588.12
P597

李文霞, 赵志丹, 王晓丽, 严溶, 路远发. 西藏日喀则蛇绿岩镁铁质岩石Re-Os同位素特征及意义[J]. 现代地质, 2022, 36(06): 1503-1512.

LI Wenxia, ZHAO Zhidan, WANG Xiaoli, YAN Rong, LU Yuanfa. Re-Os Isotopic Characteristics and Geological Significance of Mafic Rocks from the Xigaze Ophiolite,Tibet[J]. Geoscience, 2022, 36(06): 1503-1512.

图/表 10

图1 雅鲁藏布蛇绿岩中西段分布简图(a)和研究区日喀则地质简图(b) ((a)据朱弟成等[25]修改;(b)据王希斌等[24]修改) 1.第四系;2.第三系;3.晚白垩统;4.早白垩统;5.晚侏罗统—早白垩统;6.枕状熔岩;7.角砾状玄武岩;8.块状熔岩;9.席状岩墙群;10.辉绿岩;11.辉长岩;12.斜辉橄榄岩和辉绿岩床群;13.斜辉橄榄岩—二辉橄榄岩;14.蛇纹混杂岩;15.采样位置。

Fig.1 Simplified distribution map of central-western segment of theYarlung Zangpo ophiolite (a) and simplified geological map of the Xigaze ophiolite (b)

图2 日喀则蛇绿岩照片 (a) 枕状玄武质熔岩;(b) 玄武岩(正交偏光);(c) 斑状玄武岩(正交偏光);(d) 粗玄岩(正交偏光);Chl.绿泥石;Cpx. 单斜辉石; Opx. 斜方辉石;Pl. 斜长石。

Fig.2 Photographs for the Xigaze ophiolite

表1 日喀则蛇绿岩Re-Os同位素结果

Table 1 Re-Os isotopic compositions of the Xigaze ophiolite

样品号	Re含量/ (ng/g)	2σ	Os含量/ (ng/g)	2σ	¹⁸⁷Os/¹⁸⁸Os	2σ	¹⁸⁷Re/¹⁸⁸Os	2σ	Re/Os	(¹⁸⁷Os/ ¹⁸⁸Os )₀	γ_Os (125 Ma)	t_DM/Ga
RK1010	0.005 8	0.490 17	0.006 7	0.011 29	0.289 86	0.001 20	4.3	0.360 79	0.865 67	0.280 968	125.2	2.45
RK1012	0.038 5	0.607 31	0.005 6	0.006 58	0.501 49	0.001 36	34.9	0.552 87	6.875 00	0.429 318	289.7	0.65
RK1014	0.003 6	0.428 86	0.019 3	0.046 65	0.345 64	0.000 83	0.9	0.110 31	0.186 53	0.343 779	168.6	21.73
RK1015	0.015 4	0.865 26	0.004 3	0.006 31	0.367 30	0.001 59	17.9	1.004 79	3.581 40	0.330 283	185.4	0.82
RK1018	0.011 6	0.885 91	0.007 8	0.012 38	0.343 92	0.000 96	7.4	0.563 38	1.487 18	0.328 617	167.2	1.83
RK1035	0.056 8	1.596 73	0.003 0	0.004 29	0.671 63	0.003 22	98.7	2.781 76	18.933 33	0.467 521	421.9	0.33
RK1044	0.009 4	0.302 23	0.020 3	0.030 56	0.179 54	0.000 44	2.2	0.072 22	0.463 05	0.174 990	39.5	1.71
RK1047	0.016 7	1.389 64	0.009 2	0.015 74	0.246 53	0.000 90	8.9	0.736 06	1.815 22	0.228 125	91.6	0.83

图3 日喀则蛇绿岩稀土元素球粒陨石标准化配分模式(a)和微量元素原始地幔标准化蛛网图(b) 球粒陨石的稀土元素标准化值引自Boynton [51];原始地幔标准化值和亏损地幔(N-MORB)、富集地幔(E-MORB)、洋岛玄武岩(OIB)数值引自Sun and McDonough [52]。

Fig.3 Chondrite-normalized REE patterns(a) and primitive mantle normalized trace elements spider diagrams (b) for the Xigaze ophiolite

图4 日喀则蛇绿岩Os-187Os/188Os (a)和Os-Rb/Cs(b)图解 SWIR数据引自Peucker-Ehrenbrink等[43];其他数据引自Zhang等[11]。

Fig.4 Plots of Os-187Os/188Os (a) and Os-Rb/Cs (b) for the Xigaze ophiolite

图5 日喀则蛇绿岩Rb-LOI (a)和Cs-LOI (b)图解

Fig.5 Plots of Rb-LOI (a) and Cs-LOI (b) for the Xigaze ophiolite

图6 日喀则蛇绿岩LOI-Re图解

Fig.6 Plot of LOI-Re for the Xigaze ophiolite

图7 日喀则蛇绿岩Nb/Yb-Th/Yb图解(底图据Pearce[55]) SSZ(FA)=SSZ弧前型;SSZ(OBA)= SSZ大洋弧后型;SSZ(BA-FA)= SSZ弧后—弧前型;岩石的成分区域据Dilek and Furnes[54]。

Fig.7 Plot of Nb/Yb-Th/Yb for the Xigaze ophiolite

图8 日喀则蛇绿岩Re/Os-Os图解(底图据史仁灯等[53]) 东波、Yungbwa、普兰和大竹区橄榄岩数据分别引自文献[17??-20]。

Fig.8 Plot of Re/Os-Os for the Xigaze ophiolite

图9 日喀则蛇绿岩Os-(187Os/188Os)i图解(底图据Dale等[58])

Fig.9 Plot of Os-(187Os/188Os)i for the Xigaze ophiolite

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