Re-Os同位素对峨眉山大火成岩省成因制约的探讨

Comment on Re-Os isotopes constrain the formation of the Emeishan Large Igneous Province.

峨眉山大火成岩省(ELIP)主要由玄武岩、玄武质火山碎屑岩及少量的苦橄岩(包括越南的科马提岩)、长英质岩石以及层状岩体和岩墙组成,其物质来源直接关系到其成因是否与地幔柱活动有关。Re-Os 同位素体系是地核、地幔和地壳物质的最佳示踪剂。前人对 ELIP 内的 Re-Os 同位素研究表明,低 Ti 玄武岩的 Os 含量为0.006×10^(-9)～0.400×10^(-9),^(187)Os/^(188)Os 初始值为0.1371～1.403,并提出其与地幔柱活动有关;而高 Ti 玄武岩的 Os 含量为0.004×10^(-9)～0.560×10^(-9),^(187)Os/^(188)Os 初始值为0.1271～5.19,认为起源于大陆岩石圈地幔或地幔柱上升过程中受到大量岩石圈地幔"混染"(Xu JF et al.,2007);科马提岩的 Os 含量为1.24×10^(-9)～7.00×10^(-9),^(187)Os/^(188)Os 初始值为0.1251～0.1261,苦橄岩的 Os 含量为0.32×10^(-9)～2.329×10^(-9),^(187)Os/^(188)Os 初始值为0.1233～0.1266,指示苦橄岩和科马提岩均来自亏损地幔源区(Hanskiet al.,2004;陈雷等,2007)。本文利用 Os 含量最低、^(187)Os/^(188)Os 最高的高 Ti 玄武岩作为地壳端员,用铁质陨石、原始上地幔(PUM)和亏损地幔(DMM)作为地核和各种地幔端员,分别做二元混合计算,结果显示绝大多数玄武岩和所有苦橄岩及科马提岩均落在地壳和 DMM 混合曲线附近,并且邻区特提斯洋地幔岩与 DMM 具有相近的 Os 含量和^(187)Os/^(188)Os 组成,据此推测峨眉山火成岩的形成与特提斯洋的活动有关,主要受控于地壳和亏损地幔的相互作用。

Emeishan Large Igneous Province （ELIP） in Southwest China is composed of major basalts, basaltic pyroclastic rocks with minor picrites, including komatiites in northwest Vietnam, and felsic rocks. Based on the ＂later veneer＂ theory of the earth formation, Re-Os isotopic systematies can trace different possible proposed mantle sources for ELIP, including subcontinental lithospheric mantle, asthenospheric mantle, and deep plume-related mantle sources. The basalts, picrites and komatiites yield distinct Os signatures in terms of ^187Os/^188Os and Os content. Both low- and high-Ti basalts have a radiogenic Os isotopic composition, the value of ^187Os/^188Os for low-Ti basalts ranging from 0. 1371 and 1. 403 with the Os content between 0. 006 × 10^-9 and 0. 400×10^-9, which were considered mainly derived from a mantle plume reservoir, while one high-Ti basalt with highest Os concentration （0. 560× 10^-9） has unradiogenic Os isotopic signatures （^187Os/^188Os value is low down 0. 1271 ）, suggesting that a subcontinental lithosphere mantle （SCLM） component most likely contributed to the generation of these magmas, indicating the high-Ti basaltic magmas were most likely derived from a SCLM reservoir or were contaminated by a significant amount of lithospheric mantle material during plumerelated magma ascent through the SCLM （Xu JF et al. , 2007 ）. Both picrites and komatiites have subehondritic Os isotopic signatures with ^187Os/^188Os initial values ranging from 0. 1233 to 0. 1266, and ranging from 0. 1251 to 0. 1261, respectively, indicating they originated from SCLM （ Hanski et al. , 2004 ; Chen et al. , 2007 ）. We demonstrate that the material source of ELIP mainly controlled by the mixing of crust and depleted mantle, such as Tethys oceanic mantle, including some SCLM, according to the two-end member mixing model using the Os content versus ^187 Os/^188Os initial value, in which one high-Ti basalt with highest 187 Os/lSS Os initial values and lowest Os content in both high-Ti and low-Ti basalts is used as crustal end member, the iron meteorite as enriched core end member, primitive upper mantle （ PUM ） as fertile one, abyssal peridotite as depleted mantle （ DMM ） one. In the mixing model diagrams, most ELIP samples, including basalts, picrites and komatiites, plot near the crust-DMM mixing line. And the Paleo-Tethys mantle has the same Os isotopic signature as the DMM suggesting that the EIJP may be controlled by the crust-mantle interaction during or after the closing of the Paleo-Tethys Ocean.