Diversity of Trace Element Compositions of Volcanic in Different Slabs of Bikou Group: Implications for Tectonics
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摘要: 对碧口群内部大茅坪和筏子坝2个岩片变火山岩微量元素的研究表明, 二者在岩石组合、源区组成及产出的构造环境等方面均存在系统差异.大茅坪岩片主要由偏碱性与非碱性的玄武质岩石组成, 二者分别具有类似OIB和MORB的微量元素组成, 起源于相对富集和相对亏损的地幔, 并受到陆源物质不同程度的混染.偏碱性与非碱性的玄武质岩石分别显示板内火山岩(WPB) 与洋脊玄武岩(MORB) 的属性, 表明大茅坪岩片火山岩主要形成于裂谷-有限洋盆环境.筏子坝岩片由玄武质与中酸性火山岩组成, 岩石化学显示明显的“双峰式”组成特征, 二者分别具有类似MORB和壳源岩石的微量元素组成, 为亏损地幔与壳源岩石部分熔融的产物, 形成于大陆裂谷环境.根据不同火山岩构造属性及组合特征, 推断碧口群主要形成于陆内裂谷—有限洋盆构造环境.Abstract: Bikou Group, a metamorphic terrene with a series of slabs, and formed in the mid-late Proterozoic era, has undergone multi-periods of complex metamorphisms and deformation. The composition of its major and trace elements of volcanic rocks shows there are obvious differences in source and tectonic settings between Damaoping and Faziba slabs. Damaoping slab, with alkalic and non-alkalic meta-volcanic rocks, possesses compositions similar to those of OIB-type and MORB-type source respectively. The systemic variations of the ratios between high incompatibility immobile elements and geochemical twins such as Nb/Ta indicate that derived from relative enrichment mantle for former and depleted mantle, they developed within plate tectonics and similar ridge setting respectively. Also the development of the alkalic rocks involved the participation of the crust material, so Damaoping slab volcanics may have been formed in rift or oceanic setting. Faziba slab with meta-basalt and medium-acid lava shows "double mode" composition characteristics, in which the source composition of meta-basalt is similar to that of mid-ridge basalt. This may clarify that they were derived from relative depleted mantle. The medium-acid volcanic rocks, which are obviously different from meta-basalts, show typical crust source rock characteristics, so Faziba volcanic rocks are thought to have been formed in continental rift setting. According to what is discussed above, Bikou Group volcanics may have possibly developed in continental rift or limited ocean setting.
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Key words:
- slab /
- volcanic /
- magma source /
- tectonics /
- trace elements /
- Bikou Group
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图 1 碧口地体地质略图
1.震旦-石炭系; 2.豆坝群; 3.横丹群; 4.碧口群-筏子坝岩片; 5.碧口群-大茅坪岩片; 6.碧口群-红岩沟岩片; 7.鱼洞子群; 8.花岗岩; 9.花岗闪长岩; 10.闪长岩; 11.断裂或地质界线; 12.超基性岩
Fig. 1. Sketch map of geology of Bikou terrance in adjacent area of Shaanxi, Gansu and Sichuan provinces
图 2 大茅坪岩片变火山岩球粒陨石标准化的稀土元素配分曲线(a) 和原始地幔标准化的微量元素蜘蛛网(b)
Fig. 2. Normalized-chondrite REE distribution patterns (a) and normalized-primitive mantle trace elements spider diagrams (b) of meta-volcanic rocks of Damaoping slab
图 3 筏子坝岩片变火山岩球粒陨石标准化的稀土配分曲线(a) 与原始地幔标准化的微量元素蜘蛛网(b)
Fig. 3. Normalized-chondrite REE distribution patterns (a) and normalized-primitive mantle trace elements spider diagrams (b) of meta-volcanic rocks of Faziba slab
图 4 碧口群变火山岩构造环境判别图解(据Pearce等, 1982)
WPB.板内玄武岩; IAB.岛弧玄武岩; MORB.洋脊玄武岩; fzb.筏子坝变玄武岩; dmp-1.大茅坪偏碱性玄武岩; dmp-2.大茅坪非碱性玄武岩
Fig. 4. Diagnostic diagram of tectonic setting of Bikou Group
图 5 碧口群变火山岩微量元素比值协变关系
fzb.筏子坝变玄武岩; dmp-1.大茅坪偏碱性玄武岩; dmp-2.大茅坪非碱性玄武岩; MORB.洋脊玄武岩; OIB.洋岛玄武岩; DM.亏损地幔; CC.陆壳; PM.原始地幔
Fig. 5. Covariant diagram of minor element ratios of meta-volcanic rocks of Bikou Group
表 1 碧口群火山岩微量元素特征参数
Table 1. Characteristic parameters of trace elements
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