The causes of continental arc flare ups and drivers of episodic magmatic activity in Cordilleran orogenic systems
The causes of continental arc flare ups and drivers of episodic magmatic activity in Cordilleran orogenic systems
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DOI：
10.1016/j.lithos.2021.106307
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发表时间：
2021-10
期刊：
影响因子：
3.5
通讯作者：
J. Chapman;J. Shields;M. Ducea;S. Paterson;Snir Attia;K. Ardill
中科院分区：
文献类型：
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作者：
J. Chapman;J. Shields;M. Ducea;S. Paterson;Snir Attia;K. Ardill
科迪勒拉造山系统中的大陆弧显示出岩浆产生速率的幕式变化，在数千万年的时间尺度上交替出现爆发（70-90 km 3 km − 1 Myr −1）和平静（< 20 km 3 km − 1 Myr −1）。弧段或个别岩浆岩套的速率甚至更高，在耀斑爆发期间可达数百km 3 km − 1 Myr −1。这些速率很大程度上取决于弧壳的估计量，但并不反映来自地幔的熔体生产。大部分幔源岩浆在地壳深部分异后通过弧根拆沉作用再循环回地幔。大陆弧的幔源熔体产生速率估计在耀斑上升期间为140-215 km 3 km − 1 Myr − 1，在平静期间≤ 15 km 3 km − 1 Myr − 1。熔体生产率平均在多个岩浆周期是一致的，独立估计部分熔融的地幔楔在俯冲带，然而，在耀斑上升和平静的速度都非常高，非常低，分别。耀斑爆发和平静之间的地幔熔融产物的差异大于岩石学和数值模型的预测，这些模型探索了全球观测到的俯冲参数的范围（例如，收敛速率、地幔楔高度）。这表明，需要其他过程来增加岩浆活动在耀斑上升和抑制岩浆活动在平静。有许多可行的解释，但有一种可能性是，来自软流圈地幔楔的结晶熔体在平静期间暂时储存在岩石圈深处，然后在耀斑爆发期间重新流动。玄武岩熔体可能在弧系统的不活跃部分（如弧后）的地幔岩石圈中停滞，从而使地幔岩石圈重新膨胀，抑制熔体向下地壳的输送。随后的向陆弧迁移（即，向大陆内部）可能会遇到这种再受精的地幔岩石圈岩浆源区，在耀斑期间有助于岩浆活动。对全球大陆弧的回顾表明，耀斑通常与向陆地的弧迁移同时发生，并且这种迁移可能在耀斑发生之前数千万年就开始了。岩浆活动的区域，或弧宽，也可以在耀斑期间显著扩大。岛弧向不同地幔源区的迁移或扩张，以及跨越岩石圈和地壳边界，可引起岩浆活动放射成因同位素组成的时间变化。在没有弧迁移的情况下，时间变化更加温和。地幔捕虏体和深弧地壳暴露的同位素研究表明，在耀斑期间产生的原生幔源岩浆反映了次大陆地幔岩石圈的重大贡献。弧迁移可能是由多种机制引起的，包括地幔过渡带中的板片锚定或板片折叠，这可能会导致板片倾角的变化。幕式板片变浅与科迪勒拉造山系统中的许多构造过程有关，如上板块缩短和伸展之间的交替。弧迁移的研究可能有助于将大陆弧的不规则岩浆生产与造山旋回的地球动力学模型联系起来。
Continental arcs in Cordilleran orogenic systems display episodic changes in magma production rate, alternating between flare ups (70–90 km3km−1Myr−1) and lulls (< 20 km3km−1Myr−1) on timescales of tens of millions of years. Arc segments or individual magmatic suites may have even higher rates, up several 100 s of km3km−1Myr−1, during flare ups. These rates are largely determined by estimating volumes of arc crust, but do not reflect melt production from the mantle. The bulk of mantle-derived magmas are recycled back into the mantle by delamination of arc roots after differentiation in the deep crust. Mantle-derived melt production rates for continental arcs are estimated to be 140–215 km3km−1Myr−1during flare ups and ≤ 15 km3km−1Myr−1during lulls. Melt production rates averaged over multiple magmatic cycles are consistent with independent estimates for partial melting of the mantle wedge in subduction zones, however, the rates during flare ups and lulls are both anomalously high and anomalously low, respectively. The difference in mantle-derived melt production between flare ups and lulls is larger than predicted by petrologic and numerical models that explore the range of globally observed subduction parameters (e.g., convergence rate, height of the mantle wedge). This suggests that other processes are required to increase magmatism during flare ups and suppress magmatism during lulls. There are many viable explanations, but one possibility is that crystallized melts from the asthenospheric mantle wedge are temporarily stored in the deep lithosphere during lulls and then remobilized during flare ups. Basaltic melts may stall in the mantle lithosphere in inactive parts of the arc system, like the back-arc, refertilizing the mantle lithosphere and suppressing melt delivery to the lower crust. Subsequent landward arc migration (i.e., toward the interior of the continent) may encounter such refertilized mantle lithosphere magma source regions, contributing to magmatic activity during a flare up. A review of continental arcs globally suggests that flare ups commonly coincide with landward arc migration and that this migration may start tens of millions of years before the flare up occurs. The region of magmatic activity, or arc width, can also expand significantly during a flare up. Arc migration or expansion into different mantle source regions and across lithospheric and crustal boundaries can cause temporal shifts in the radiogenic isotopic composition of magmatism. In the absence of arc migration, temporal shifts are more muted. Isotopic studies of mantle xenoliths and exposures of deep arc crust suggest that that primary, mantle-derived magmas generated during flare ups reflect substantial contributions from the subcontinental mantle lithosphere. Arc migration may be caused by a variety of mechanisms, including slab anchoring or slab folding in the mantle transition zone that could generate changes in slab dip. Episodic slab shallowing is associated with many tectonic processes in Cordilleran orogenic systems, like alternations between shortening and extension in the upper plate. Studies of arc migration may help to link irregular magmatic production in continental arcs with geodynamic models for orogenic cyclicity.