Early Cretaceous near‐trench plutonism in southern Alaska: Atonalite‐Trondhjemite Intrusive Complex injected during ductile thrusting along the Border Ranges Fault System
Early Cretaceous near‐trench plutonism in southern Alaska: Atonalite‐Trondhjemite Intrusive Complex injected during ductile thrusting along the Border Ranges Fault System
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阿拉斯加南部早白垩世近海沟深成岩作用：沿边界山脉断层系韧性逆冲过程中注入的无托岩-特隆赫杰米岩侵入杂岩
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发表时间：
1988
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通讯作者：
Michael D. Reason
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作者：
T. Pavlis;D. Monteverde;J. R. Bowman;J. Rubenstone;Michael D. Reason
阿拉斯加楚加奇山脉西部的一组白云母-奥长花岗岩岩体记录了一个沿古阿拉斯加/阿留申俯冲带沿着的早白垩世近海沟岩体活动时期。这些岩体沿着较老的（侏罗纪）弧形火山岩和较年轻的（白垩纪）混杂岩之间的构造连接处侵位，后者是在结晶火山岩下俯冲而增生的。深成岩是在一次主要的逆冲事件中注入的，该逆冲事件将上地幔（？）白垩纪俯冲组合之上的超镁铁质物质。在这个逆冲事件的变形显然进展从韧性到脆性条件下，和深成岩在这两个阶段注入。与韧性变形同步的深成作用的证据包括白云母岩脉的石香肠或褶皱，以及较大深成岩体内特有的弱组构。后者通常与次生矿物生长以及变形诱导的显微结构有关，这些显微结构表明在从较低的角闪岩到亚绿片岩相条件下的温度下变形，这些条件相当于相邻片岩中的结构/变质进程。然而，奇怪的是，岩体局部横切低品位混杂岩组构的下盘，以及脆性断层网络内的结构较高的部分结晶上盘。自相矛盾的领域的关系，部分是由于洗牌由早期第三纪（？）右旋走滑断层作用，但也表明在一段时间内深成侵位。然而，考虑到同构造侵位的历史，深成岩作用的绝对年龄跨度并不一定很长。特别是，变形局部可能已熔体为主，使熔体初始注入后，变形集中在火成岩群众。岩石学上，岩体遵循低钾系列从dialite白云母石榴石奥长花岗岩，这是一种趋势，是不寻常的，在中生代的岩石，通常与太古代的岩石。此外，其成分趋势与Hill等人（1981年）研究的海沟附近岩体的钙碱性趋势不同。海沟附近深成事件的构造机制是难以捉摸的，但斜长角闪岩或变质杂砂岩的浅熔融沿着一个年轻的俯冲带提供了最合理的解释的意见。然而，将岩浆活动与海脊-海沟相遇联系起来的另一种模式也是允许的。尽管如此，在这两种模式中，高温韧性逆冲带内的火成岩的成因是现代俯冲带的非典型。因此，构造背景和岩性组合与太古代系统有许多相似之处，岩石可能代表了一个年轻的类似物，最终可能为太古代系统提供关键的岩石成因约束。
A group of leucotonalite to trondhjemite plutons in the western Chugach Mountains, Alaska, record a period of Early Cretaceous near-trench plutonism along the paleo-Alaskan/Aleutian subduction zone. The plutons were emplaced along the tectonic join between an older (Jurassic) arc massif and a younger (Cretaceous) melange terrane that was accreted by subduction beneath the crystalline massif. The plutonic rocks were injected during a major thrusting event that placed upper mantle(?) ultramafic masses atop the Cretaceous subduction assemblages. Deformation during this thrusting event apparently progressed from ductile to brittle conditions, and plutonic rocks were injected during both phases. Evidence for plutonism synchronous with ductile deformation includes boudinage or folding of leucotonalite dikes as well as a characteristic weak fabric within larger plutons. The latter is commonly associated with secondary mineral growth as well as deformation-induced microstructures that suggest deformation under temperatures that decreased from lower amphibolite to subgreenschist facies conditions, conditions which are equivalent to the structural/metamorphic progression in adjacent schists. Paradoxically, however, the plutons locally crosscut low-grade melange fabrics in the footwall as well as brittle fault networks within structurally higher parts of the crystalline hanging wall. The paradoxical field relationships are partially due to shuffling by early Tertiary(?) dextral strike-slip faulting but are also indicative of plutonic emplacement over a period of time. The absolute age span of plutonism, however, need not have been long considering the syntectonic emplacement history. In particular, the deformation locally may have been melt-dominated such that after initial injection of melts, the deformation was concentrated in the igneous masses. Lithologically, the plutons follow a low-K series from diorite to muscovite-garnet trondhjemite, a trend that is unusual in Phanerozoic terranes and is typically associated with Archean terranes. Moreover, the compositional trends are unlike the calc-alkaline trend of near-trench plutons studied by Hill et al. (1981). The tectonic mechanism for the near-trench plutonic event is elusive, but shallow melting of amphibolite or metagreywacke along a young subduction zone provides the most reasonable explanations for the observations. However, an alternative model relating magmatic activity to a ridge-trench encounter is also allowable. Nonetheless, in either model the genesis of the igneous rocks within a high-temperature ductile thrust zone is atypical of modern subduction zones. Thus structural setting and lithologic association have many similarities to Archean systems, and the rocks may represent a young analog that ultimately may provide key petrogenetic constraints for the Archean system.