Orogenic Peridotite Origins and Crust-Mantle Interactions in Continental Subduction Zones
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摘要: 俯冲地壳衍生流体交代地幔楔，是产生俯冲带岩浆作用的重要机制.但是，目前人们对俯冲大陆物质改造地幔楔的岩石学过程和机理仍缺乏深入认识，造山带橄榄岩是解析这一问题的直接样品.通过对大别-苏鲁造山带橄榄岩进行系统的矿物学、岩石学和地球化学研究，发现橄榄石Ni/Co比值可有效区分幔源和壳源造山带橄榄岩，揭示幔源造山带橄榄岩起源于华北岩石圈地幔.苏鲁李家屯纯橄岩在进入俯冲带之前就已在地幔内部经历了碳酸盐熔体交代.大别毛屋和苏鲁蒋庄橄榄岩及其交代脉体记录了约170~200 km深度的俯冲带壳幔相互作用过程.深俯冲陆壳释放的富Si-Al质熔体可不同程度地改造地幔楔底部，形成富石榴石和富辉石的交代岩，并引发强烈的Os同位素分馏效应.该过程不仅改变地幔楔岩性和化学组成，还能够改变交代介质成分，为俯冲带各类深部地幔岩浆提供源区物质.因此，大陆深俯冲是导致上地幔不均一的重要途径.Abstract: Metasomatism of the mantle wedge by crust-derived fluids is a crucial mechanism responsible for subduction zone magmatism. However, how crust-mantle interactions proceed in continental subduction zones is poorly understood. Orogenic peridotite is a direct, ideal sample to resolve this issue. Through combined studies of mineralogy, petrology and geochemistry for on Dabie-Sulu orogenic peridotites, it is found that the Ni/Co ratio of olivine can successfully discriminate between mantle and crustal peridotites, and that mantle-derived peridotites originated from the subcontinental lithospheric mantle wedge beneath the North China craton. The Lijiatun dunites (Sulu) derived from such lithospheric mantle wedge were likely modified by carbonatitic melts prior to their incorporation into the subduction channel. The Maowu (Dabie) and Jiangzhuang (Sulu) peridotites and their metasomatic veins record slab-mantle interaction processes at depths of~170-200 km. Deeply subducted continental crust would release Si-Al-rich melts to heterogeneously modify the lower margin of the mantle wedge, which could result in forming various garnet-and pyroxene-rich metasomatites and in generating significant Os isotope fractionation. Such processes can not only modify lithological and geochemical compositions of the mantle wedge but also affect crust-derived melt compositions. Recycled heterogeneous peridotites and metasomatites could contribute to the mantle sources of various subduction-related mantle magmas. Therefore, deep continental subduction is a crucial mechanism responsible for the mantle heterogeneity.
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Key words:
- orogenic peridotites /
- subduction zones /
- crust-mantle interactions /
- metasomatism /
- dunite /
- garnet pyroxenite /
- petrology
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图 1 造山带幔源和壳源橄榄岩中橄榄石成分对比
Fig. 1. Olivine chemistry for discrimination between 'mantle' and 'crustal' origins of orogenic peridotites
图 2 苏鲁李家屯纯橄岩橄榄石脉体取代斜方辉石斑晶结构(a~c)及脉体矿物成分(d、e)图解
图d和e修改自Su et al.(2016b)；金刚石中橄榄石包裹体数据引自Sobolev et al.(2009)
Fig. 2. Microphotographs (a~c) showing typical metasomatic texture of olivine veins (Ol vein) crosscutting orthopyroxene porphyroclasts (Opx-P) and diagrams (d, e) showing the compositions of olivine and clinopyroxene in olivine veins
图 3 大别毛屋石榴纯橄岩与富石榴石(a)和贫石榴石(b)的斜方辉石岩脉体及其Re-Os同位素组成(c, d)
Fig. 3. Microphotographs showing the garnet-rich (a) and garnet poor (b) orthopyroxenite veins cutting the wall garnet dunite from Maowu (Dabie) and diagrams showing their Re-Os isotope compositions (c, d)
图 4 苏鲁蒋庄石榴二辉橄榄岩(a、b)和两类交代脉体及其单斜辉石微量元素成分对比(c、d)
Fig. 4. Garnetite (a) and pyroxenite (b) veins cutting garnet lherzolite from Jiangzhuang (Sulu), and variations in Nb/Th versus La/Nb (c) and Ti/La versus (La/Yb)N (d) for clinopyroxenes from the two type veins and the host lherzolites
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