The role of amphibole in the evolution of arc magmas and crust: the case from the Jurassic Bonanza arc section, Vancouver Island, Canada
The role of amphibole in the evolution of arc magmas and crust: the case from the Jurassic Bonanza arc section, Vancouver Island, Canada
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DOI：
10.1007/s00410-009-0436-z
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发表时间：
2010-04-01
影响因子：
3.5
通讯作者：
Canil, Dante
中科院分区：
文献类型：
--
作者：
Larocque, Jeff;Canil, Dante
不列颠哥伦比亚省温哥华岛上的侏罗纪富矿弧代表了一个出露的岛弧地壳剖面，其成分大致为闪长岩。我们研究了该弧深部的镁铁质和超镁铁质堆晶体，以确定从高镁玄武岩到安山岩和英安岩的条件和分异途径。主量元素趋势结合结构信息表明，在原始堆晶中，作为包裹橄榄石的大的嵌晶的角闪石的晶间结晶控制着液相的成分，直到斜长石开始结晶。这一过程是隐蔽的，仅发生在深成岩部分，并解释了镁铁质弧火山岩中角闪石的缺乏以及许多弧岩套中镝/镱比值随分异作用的变化。在高镁玄武岩的液相线实验中，角闪石中八面体铝与压力的相关性被用作富矿弧角闪石的经验压力计。结果表明，在水饱和的原始玄武质岩浆中，结晶发生在470 - 880兆帕。富矿弧中没有与大陆地壳整体相当的岩浆；无论超镁铁质堆晶体有多少分层，都不会使弧的整体成分转变为大陆地壳整体的高镁指数安山岩成分。从湿岩浆中去除石榴石似乎是形成大陆地壳的关键因素，这需要高压和厚地壳。由于大洋岛弧是在较薄的地壳上形成的，生成大陆地壳整体的长期过程是岛弧向大陆边缘的增生以及伴随的构造增厚。
The Jurassic Bonanza arc, on Vancouver Island, British Columbia, represents an exhumed island arc crustal section of broadly diorite composition. We studied bodies of mafic and ultramafic cumulates within deeper levels of the arc to constrain the conditions and fractionation pathways leading from high-Mg basalt to andesite and dacite. Major element trends coupled with textural information show the intercumulus crystallization of amphibole, as large oikocrysts enclosing olivine in primitive cumulates controls the compositions of liquids until the onset of plagioclase crystallization. This process is cryptic, occurring only in the plutonic section, and explains the paucity of amphibole in mafic arc volcanics and the change in the Dy/Yb ratios in many arc suites with differentiation. The correlation of octahedral Al in hornblende with pressure in liquidus experiments on high-Mg basalts is applied as an empirical barometer to hornblendes from the Bonanza arc. It shows that crystallization took place at 470-880 MPa in H2O-saturated primitive basaltic magmas. There are no magmatic equivalents to bulk continental crust in the Bonanza arc; no amount of delamination of ultramafic cumulates will shift the bulk arc composition to the high-Mg# andesite composition of bulk continental crust. Garnet removal from wet magmas appears to be the key factor in producing continental crust, requiring high pressures and thick crust. Because oceanic island arcs are built on thinner crust, the long-term process generating the bulk continental crust is the accretion of island arcs to continental margins with attendant tectonic thickening.