Accessory minerals and subduction zone metasomatism: a geochemical comparison of two mélanges (Washington and California, U.S.A.)
Accessory minerals and subduction zone metasomatism: a geochemical comparison of two mélanges (Washington and California, U.S.A.)
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DOI：
10.1016/0009-2541(93)90258-k
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发表时间：
1993-11
期刊：
影响因子：
3.9
通讯作者：
S. Sorensen;J. Grossman
中科院分区：
文献类型：
--
作者：
S. Sorensen;J. Grossman
俯冲板片或俯冲沉积物向弧源区贡献许多不相容微量元素的能力可能取决于这些岩石中副矿物的稳定性，这只能间接研究。与此相反，副矿物在古俯冲带内低Tand-P交代过程中的作用可以直接在俯冲带变质岩中进行研究。位于华盛顿州北喀斯喀特的Shuksan变质岩套的Gee Point-Iron Mountain地区是一个变质超镁铁质岩基质中的高温镁铁质岩块。该混杂岩在地质背景和岩石学上与加州南部圣卡塔利纳岛卡塔利纳片岩中一个未命名斜长角闪岩单元的上部相似。两者都被解释为地幔和板块岩石之间的剪切带，形成于俯冲的早期阶段。一些石榴石角闪岩块从Gee点铁山地方显示微量元素富集类似于那些在对应的卡塔利纳片岩。与大洋中脊玄武岩（MORB）和同一单元中的其他石榴石斜长角闪岩块体相比，某些卡塔利纳块体高度富集Th、稀土元素（REE）、高场强元素Ti、Nb、Ta、Zr和Hf（HFSE）、U和Sr。结构和地球化学数据表明，变质成因的副矿物控制了Th、REE和HFSE的富集。与方辉橄榄岩、纯橄榄岩和蛇纹岩相比，块体周围的富镁外皮和来自两种混杂岩的变超镁铁质基质高度富集大量微量元素。Gee Point和卡塔利纳混杂岩的资料表明，副矿物钛铁矿、金红石、磷灰石、锆石和富稀土绿帘石在与俯冲有关的流体-岩石相互作用过程中，对镁铁质岩和超镁铁质岩中微量元素的富集起着重要作用。在俯冲带富流体变质环境中，不相容元素的活动和这些元素在镁铁质和超镁铁质岩石的副矿物中的沉积可能是相当普遍的。
The ability of a subducted slab or subducted sediment to contribute many incompatible trace elements to arc source regions may depend on the stabilities of accessory minerals within these rocks, which can only be studied indirectly. In contrast, the role of accessory minerals in lower-Tand -Pmetasomatic processes within paleo-subduction zones can be studied directly in subduction-zone metamorphic terranes.The Gee Point-Iron Mountain locality of the Shuksan Metamorphic Suite, North Cascades, Washington State, is a high-Tmélange of metamafic blocks in a matrix of meta-ultramafic rocks. This mélange is similar in geologic setting and petrology to the upper part of an unnamed amphibolite unit of the Catalina Schist, Santa Catalina Island, southern California. Both are interpreted as shear zones between mantle and slab rocks that formed during the early stages of subduction. Some garnet amphibolite blocks from the Gee Point-Iron Mountain locality display trace-element enrichments similar to those in counterparts from the Catalina Schist. Some Catalina blocks are highly enriched in Th, rare-earth elements (REE), the high-field-strength elements Ti, Nb, Ta, Zr and Hf (HFSE), U and Sr compared to mid-ocean ridge basalt (MORB), and to other garnet amphibolite blocks in the same unit. Textural and geochemical data indicate that accessory minerals of metamorphic origin control the enrichment of Th, REE and HFSE in blocks from both areas. The Mg-rich rinds around blocks and the meta-ultramafic matrix from both mélanges are highly enriched in a large number of trace elements compared to harzburgites, dunites and serpentinites. Evidence for recrystallization or formation of accessory minerals in the former rocks suggests that these minerals control some of the trace-element enrichments.Data from the Gee Point and Catalina mélanges suggest that the accessory minerals titanite, rutile, apatite, zircon and REE-rich epidote play a significant role in the enrichment of trace elements in both mafic and ultramafic rocks during subduction-related fluid-rock interaction. Mobilization of incompatible elements, and deposition of such elements in the accessory minerals of mafic and ultramafic rocks may be fairly common in fluid-rich metamorphic environments in subduction zones.