In continental arcs, the exposure of primitive eruptive products at the surface is typically a result of rapid magmatic transfer through the crust. As a result, the initially primitive magma experiences minimal crustal residence and thus insignificant differentiation towards more evolved products. This rapid transfer of primitive magma through thickened crust is commonly recorded from smaller, monogenetic cinder cones. Manantial Pelado (35.5° S) is a long-lived stratocone in the Southern Andean Volcanic Zone (SVZ) overlying thick continental crust (45–50 km) that produces almost exclusively mafic material. As Manantial Pelado is surrounded by extensive silicic volcanism, the study of its mafic exposure as a stratocone can be used to further understand magmatic origins of long-lived volcanic systems. Our study uses textural, geochemical, and geochronological data from lavas collected from Manantial Pelado to characterize its magmatic petrogenesis, assess the primitive nature, and explain processes in the crust within the SVZ. A geologic description of the volcano reveals a mostly monotonous eruptive history of basaltic andesites that are now accessible through glacially carved valleys. New 40Ar/39Ar dating constrains most of the volcano’s cone constructing phase to last from ~ 220 to 190 ka. At ~ 30 ka, small-volume activity and different petrography of more intermediate magmas were present reflecting a change in the volcano’s character. A combination of the whole-rock and mineral-scale data reveals that basaltic andesites at Manantial Pelado are among the most primitive magmas in the thickened crust of the SVZ. Evidence for this primitive signature consists of textural and zonation patterns in olivine, the presence of Cr-spinel in olivine cores, and elevated Fo and Ni content within olivine cores. This data combined with elemental diffusion modeling provides evidence for a primitive signature for these lavas. Intermediate Fo olivines with uniform core compositions (Fo80–84) suggest that basaltic andesites reside in the crust in quasi-closed system environments for extended storage prior to eruption (~ 25–6000 years). Diffusive equilibration in those intermediate Fo olivines masks the primitive nature of the magmas. These results suggest that mafic magmas can have a protracted storage history in the crust that does not significantly alter their primitive bulk composition before reaching the surface. We argue that these are important processes in understanding the magmatic origin of long-lived systems and the presence of compositionally homogenous olivines at intermediate Fo content may represent cryptic evidence for recharge with primitive magmas that experienced prolonged crustal storage.

中文翻译：

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加厚地壳中长期平衡后原始岩浆储存和喷发的证据

在大陆弧中，地表原始喷发产物的暴露通常是岩浆通过地壳的快速转移的结果。结果，最初原始的岩浆经历了最小的地壳驻留，因此向更进化的产物的分化微不足道。原始岩浆通过增厚地壳的这种快速转移通常是从较小的单生煤渣锥中记录下来的。Manantial Pelado (35.5° S) 是位于南安第斯火山带 (SVZ) 的一个长寿命平层锥体，覆盖在厚厚的大陆地壳（45-50 公里）上，几乎完全产生基性物质。由于 Manantial Pelado 被广泛的硅质火山活动所包围，因此对其作为平层锥体的镁铁质暴露的研究可用于进一步了解长寿火山系统的岩浆起源。我们的研究使用了质地、地球化学、和从 Manantial Pelado 收集的熔岩的地质年代学数据，以描述其岩浆成因、评估原始性质并解释 SVZ 内地壳的过程。对火山的地质描述揭示了玄武安山岩的大部分单调喷发历史，现在可以通过冰川雕刻的山谷进入。新的 40Ar/39Ar 测年将大部分火山锥构造阶段限制为持续约 220 至 190 ka。在 ~ 30 ka 时，出现了小体积活动和更多中等岩浆的不同岩相，反映了火山特征的变化。全岩和矿物尺度数据的结合表明，Manantial Pelado 的玄武安山岩是 SVZ 加厚地壳中最原始的岩浆之一。这种原始特征的证据包括橄榄石的纹理和分带模式、橄榄石核心中铬尖晶石的存在以及橄榄石核心内升高的 Fo 和 Ni 含量。该数据与元素扩散模型相结合，为这些熔岩的原始特征提供了证据。具有均匀核心成分的中间叶橄榄石 (Fo80-84) 表明玄武质安山岩位于准封闭系统环境中的地壳中，以便在喷发前延长储存时间（约 25-6000 年）。这些中间橄榄石中的扩散平衡掩盖了岩浆的原始性质。这些结果表明，基性岩浆可以在地壳中具有长期的储存历史，在到达地表之前不会显着改变其原始的整体成分。