Geochemical Signals and Magmatic Consequences of Arc Migration in the Central Andes

安第斯山脉中部弧形迁移的地球化学信号和岩浆后果

• 批准号: 0126000
• 负责人: Suzanne Kay
• 金额: $ 15.91万
• 依托单位: Cornell University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2002
• 资助国家: 美国
• 起止时间: 2002-01-01 至 2004-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0126000&HistoricalAwards=false
• 关键词: Geochemical; Signals; Magmatic; Consequences; Arc

KayEAR-0126000The Central Andean margin, the classic example of an oceanic slab subducting beneath a continent, is a laboratory for studying processes that typify continental crustal formation, modification and destruction. This study focuses on such a process - the migration of an arc magmatic front towards the backarc - that has reconfigured the forearc, arc and backarc of continental margins since plate tectonics began. Lithospheric scale profiles indicate significant mass balance problems that imply recycling of continental crustal and mantle lithosphere into the mantle. We plan a trace element and isotopic study of magmas erupted before, during and after arc migration as probes to changing magmatic source regions and lithospheric configurations beneath the migrating arc. The focus area from 26.5?S to 28?S latitude overlaps the transition between the modern Central Volcanic Zone (CVZ) and the non-volcanic region above the shallowing dipping subduction zone to the south. In this region, the magmatic arc front migrated ~ 50 km eastward from the Maricunga belt to the southern CVZ and Bonete region between ~7 and 2 Ma. Existing geologic maps, K/Ar ages (~300) and major element and Instrumental Neutron Activation trace element data (~ 400 samples at Cornell) from virtually every center provide a basis for an in-depth ICP-MS and Nd, Sr, Pb and O isotopic study of magmatism in this environment. Chemical data show that lavas erupted in the old dying arc and as the front migrated eastward have extremely steep REE patterns (La/Yb 60) and high field strength (HFS) element depletions (La/Ta to 85) that contrast with lower ratios in both pre-8 Ma and post-4 Ma arc lavas. The magmas with the steepest REE patterns appear better analogues to anomalous Archean (TTG) magmatic suites than do magmas thought to be melts of the subducting slab (adakites). Variable age mafic magmas (58% SiO2) in the region, which include the most mantle-like Central Andean lavas, are critical to deciphering mantle processes whereas silicic magmas are keys to crustal processes. A crucial question is why very steep REE patterns occur in lavas erupted as the arc terminates and migrates, but not before or after. Residual garnet must play a role, but is that garnet associated with melting in the mantle, in situ thickened crust, or in crust removed by subduction erosion from the base of the arc and forearc and cycled through the subduction zone? Tests are to correlate spatial and temporal isotopic patterns with variations expected across the arc region and to look at interrelations between the isotopes and trace elements. Other questions are the origin of La/Ta ratio variations and the history of fluid interaction as the slab shallows and the arc migrates. Oxygen isotopes and expanded trace element data sets can be used with fluid/melt distribution coefficients to constrain spatial and temporal variations. Expanded trace element data sets including Nb, Zr as well as Ta, Hf, and Ti will allow modeling of residual phases and source areas. Longer range questions are the importance of crustal thickening, arc migration and removal of young continental lithosphere in both the modern Central Andes and the Archean, and the extent to which continental lithospheric is recycled in this setting. As Andean analogies are used to understand processes in both ancient and young mountain belts, results of this project will set up Os-Re and Lu-Hf isotopic studies and define geophysical targets addressing fundamental questions in Earth's continental lithospheric evolution.

安第斯山脉中部边缘是大洋板块俯冲到大陆之下的典型例子，是研究大陆地壳形成、变形和破坏过程的实验室。 本研究的重点是这样一个过程-弧岩浆前缘向弧后迁移-自板块构造开始以来，这一过程重新配置了大陆边的弧前、弧和弧后。岩石圈尺度剖面显示了重要的物质平衡问题，这意味着大陆地壳和地幔岩石圈再循环进入地幔。 我们计划进行微量元素和同位素研究的岩浆喷发之前，期间和之后的弧迁移作为探针，以改变岩浆源区和岩石圈配置下的迁移弧。 从26.5的焦点区域？S到28？南纬度与现代中央火山带（CVZ）和南部浅倾俯冲带上方的非火山区之间的过渡重叠。 在该地区，岩浆弧前缘迁移约50公里，从Maricunga带向南CVZ和博内特地区之间的~7和2 Ma。现有的地质图，K/Ar年龄（~300）和主要元素和仪器中子活化微量元素数据（约400个样品在康奈尔大学）从几乎每个中心提供了一个深入的ICP-MS和Nd，Sr，Pb和O同位素研究在这种环境中的岩浆作用的基础。 化学数据表明，熔岩爆发的老垂死弧和作为前东迁移有非常陡峭的稀土元素模式（La/Yb 60）和高场强（HFS）的元素亏损（La/Ta 85），对比较低的比例在前8马和后4马弧熔岩。 最陡峭的稀土元素模式的岩浆出现更好的类似物异常太古代（TTG）岩浆套房比岩浆被认为是熔融的俯冲板（埃达克岩）。可变年龄的镁铁质岩浆（58%SiO2）在该地区，其中包括最像地幔中部安第斯熔岩，是至关重要的破译地幔过程，而岩浆是地壳过程的关键。一个关键的问题是，为什么非常陡峭的稀土元素模式发生在火山喷发弧终止和迁移，而不是之前或之后。 残留的石榴石肯定起了一定的作用，但石榴石是与地幔熔融有关，还是与就地增厚的地壳有关，还是与从弧底和弧前底部被俯冲侵蚀并循环通过俯冲带的地壳有关？ 测试是将空间和时间同位素模式与整个弧区的预期变化相关联，并研究同位素和微量元素之间的相互关系。 其他问题是起源的La/Ta比的变化和历史的流体相互作用的板变浅和电弧迁移。氧同位素和扩展的微量元素数据集可以与流体/熔体分布系数一起使用，以约束空间和时间变化。 包括Nb、Zr以及Ta、Hf和Ti在内的扩展微量元素数据集将允许对残余相和源区进行建模。 更长远的问题是地壳增厚的重要性，弧迁移和去除年轻的大陆岩石圈在现代中央安第斯山脉和太古代，以及在何种程度上大陆岩石圈是在这种情况下回收。由于安第斯山脉的类比被用来了解古老和年轻的山脉带的过程，该项目的结果将建立Os-Re和Lu-Hf同位素研究，并确定地球物理目标，解决地球大陆岩石圈演化的基本问题。