To sink, or not to sink: The thermal and density structure of the modern northern Andean arc constrained by xenolith petrology
To sink, or not to sink: The thermal and density structure of the modern northern Andean arc constrained by xenolith petrology
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下沉还是不下沉：现代北安第斯弧的热力和密度结构受到捕虏体岩石学的限制
DOI：
10.1130/g50973.1
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发表时间：
2023
期刊：
影响因子：
5.8
通讯作者：
Szymanowski, Dawid
中科院分区：
文献类型：
--
作者：
Zieman, Lisa;Ibañez-Mejia, Mauricio;Rooney, Alan D.;Bloch, Elias;Pardo, Natalia;Schoene, Blair;Szymanowski, Dawid
弧的热结构和成分结构影响着岩浆分选和下地壳沉降，这是影响大陆地壳演化的两个关键过程。尽管许多研究提出了基于对流下流计算的岩石圈循环的时间尺度，但这些模型依赖于下地壳和地幔的组成、密度（ρ）和热结构，而这些在活跃的大陆弧中很难量化。在这里，我们利用来自哥伦比亚Mercaderes的一套独特的下地壳和地幔包体的直接岩石学观察，对安第斯北部火山岩带的这些性质进行了限制。化学磨损-同位素稀释-热电离质谱（CA-ID-TIMS）测定了主凝灰岩中锆石的U-Pb年龄，表明捕虏体喷发时间不早于238（±19）ka，因此捕捉到了弧和次弧地幔的最新快照。81个捕虏体的平衡压力-温度（P-T）估计定义了三个不同的热域，解释为：(1)弧下地壳中陡峭的导电地温梯度；(2)对流地幔楔；(3)靠近俯冲板块的地幔冷却。我们的结果表明存在一个~10 - 14km厚的高密度岩石圈根，其密度比下伏地幔大~0.1 g/cm3。与发掘出的古弧的记录不同，瑞利-泰勒不稳定性计算使用了我们的p - t -ρ约束，对北安第斯山脉来说是不切实际的短。我们认为，在这个热弧根部存在部分熔体，作为浮力的潜在来源，可以防止或显著减缓沉没。
The thermal and compositional structure of arcs influence magmatic differentiation and lower-crustal foundering, two key processes impacting the evolution of the continental crust. Although many studies have proposed time scales of lithospheric recycling based on convective downwelling calculations, these models depend on the composition, density (ρ), and thermal structure of the lower crust and mantle, which are difficult to quantify in active continental arcs. Here, we constrained these properties for the Andean Northern Volcanic Zone using direct petrologic observations from a unique suite of lower-crust and mantle xenoliths from Mercaderes, Colombia. Chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb dates for zircons within the host tuff indicate the xenoliths erupted no earlier than 238 (±19) ka and thus capture a recent snapshot of the arc and subarc mantle. Equilibrium pressure-temperature (P-T) estimates for 81 xenoliths define three distinct thermal domains, interpreted as (1) a steep conductive geothermal gradient in the lower arc crust; (2) a convecting mantle wedge; and (3) cooled mantle in proximity to the subducting slab. Our results indicate the presence of an ~10–14-km-thick, high-density lithospheric root that is ~0.1 g/cm3denser than the underlying mantle. Unlike records from exhumed paleoarcs, Rayleigh-Taylor instability calculations using ourP-T-ρ constraints are unrealistically short for the northern Andes. We suggest the presence of partial melts in this hot arc root as a potential source of buoyancy preventing or significantly slowing down foundering.