Role of the subducted slab, mantle wedge and continental crust in the generation of adakites from the Andean Austral Volcanic Zone
Role of the subducted slab, mantle wedge and continental crust in the generation of adakites from the Andean Austral Volcanic Zone
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DOI：
10.1007/s004100050155
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发表时间：
1996-04
影响因子：
3.5
通讯作者：
C. Stern;R. Kilian
中科院分区：
文献类型：
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作者：
C. Stern;R. Kilian
安第斯南部火山带（AVZ; 49-54°S）的所有六个全新世火山中心都只喷发埃达克质安山岩和英安岩，其特征是低Yb和Y浓度以及高Sr/Y比值，这表明源岩含有残余石榴石、角闪石和辉石，但很少或没有橄榄石和斜长石。镁铁质下地壳熔融可能是某些岛弧埃达克岩的源区，但这种源区与AVZ埃达克岩的高Mg#不一致。此外，AVZ出现在地壳相对较薄的区域（<35 km），在该区域内斜长石而不是石榴石是稳定的。AVZ埃达克岩的源区更可能是俯冲洋壳玄武岩，再结晶为石榴角闪岩或榴辉岩。地质模型表明，由于俯冲速率缓慢（2 cm/年）和俯冲洋壳年龄年轻（<24 Ma），俯冲洋壳在南安第斯山脉以下可能发生部分熔融。AVZ埃达克岩的地球化学模型也与俯冲洋壳（35-90%的板片衍生质量），包括沉积物（高达4%的沉积物衍生质量，约占所有俯冲沉积物的15%）的大量物质贡献一致。在AVZ埃达克岩中观察到的可变同位素和微量元素比率涵盖了世界范围内报道的埃达克岩范围，需要多阶段模型，涉及不同比例的俯冲玄武岩和沉积物的熔融，以及上覆地幔楔的重要物质贡献（10-50%质量贡献）和大陆地壳（0-30%质量贡献）。位于AVZ最南端（54°S）俯冲较斜的库克岛火山安山岩具有类MORB的Sr、Nd、Pb和O同位素组成和微量元素比值。这些可以通过榴辉岩MORB的小程度（2-4%）部分熔融来模拟，产生英云闪长岩母岩（中间SiO2，CaO/Na 2 O>1），随后该熔体与上覆地幔的有限相互作用（≥90% MORB熔体，≤10%地幔），但只有很少（≤1%）或没有俯冲沉积物或地壳的参与。与此相反，伯尼（52°S）、雷克卢斯（51°S）和AVZ最北端（49-50°S）安山岩和英安岩的岩浆演化模型要求MORB和俯冲沉积物的混合物熔融，然后这种熔融不仅与上覆地幔相互作用，而且与地壳相互作用。地壳同化和分离结晶（AFC）过程和质量的贡献变得更加显着向北在AVZ的收敛角变得更加正交。
All six Holocene volcanic centers of the Andean Austral Volcanic Zone (AVZ; 49–54°S) have erupted exclusively adakitic andesites and dacites characterized by low Yb and Y concentrations and high Sr/Y ratios, suggesting a source with residual garnet, amphibole and pyroxene, but little or no olivine and plagioclase. Melting of mafic lower crust may be the source for adakites in some arcs, but such a source is inconsistent with the high Mg# of AVZ adakites. Also, the AVZ occurs in a region of relatively thin crust (<35 km) within which plagioclase rather than garnet is stable. The source for AVZ adakites is more likely to be subducted oceanic basalt, recrystallized to garnet-amphibolite or eclogite. Geothermal models indicate that partial melting of the subducted oceanic crust is probable below the Austral Andes due to the slow subduction rate (2 cm/year) and the young age (<24 Ma) of the subducted oceanic lithosphere. Geochemical models for AVZ adakites are also consistent with a large material contribution from subducted oceanic crust (35–90% slab-derived mass), including sediment (up to 4% sediment-derived mass, representing approximately 15% of all sediment subducted). Variable isotopic and trace-element ratios observed for AVZ adakites, which span the range reported for adakites world-wide, require multistage models involving melting of different proportions of subducted basalt and sediment, as well as an important material contribution from both the overlying mantle wedge (10–50% mass contribution) and continental crust (0–30% mass contribution). Andesites from Cook Island volcano, located in the southernmost AVZ (54°S) where subduction is more oblique, have MORB-like Sr, Nd, Pb and O isotopic composition and trace-element ratios. These can be modeled by small degrees (2–4%) of partial melting of eclogitic MORB, yielding a tonalitic parent (intermediate SiO2, CaO/Na2O>1), followed by limited interaction of this melt with the overlying mantle (≥90% MORB melt, ≤10% mantle), but only very little (≤1%) or no participation of either subducted sediment or crust. In contrast, models for the magmatic evolution of Burney (52°S), Reclus (51°S) and northernmost AVZ (49–50°S) andesites and dacites require melting of a mixture of MORB and subducted sediment, followed by interaction of this melt not only with the overlying mantle, but the crust as well. Crustal assimilation and fractional crystallization (AFC) processes and the mass contribution from the crust become more significant northwards in the AVZ as the angle of convergence becomes more orthogonal.