埃达克岩

埃达克岩（英语：Adakite）是具有中性岩到长英质成分的火山岩，其地球化学成分与在火山弧下，玄武岩与部分熔融而成的岩浆类似，故此岩石最初被认为形成于俯冲带^[1]。这种观点认为，俯冲带中的大多数岩浆都来自俯冲板块上方的地幔，当含水矿物中的水从变质玄武岩释放出来，上升到地幔中时，即导致部分熔融。然而德凡特和德拉蒙德认为，埃达克岩产生于岁数小于2500万年的年轻海洋地壳俯冲时的弧形中。他们的假设中埃达克岩的产生过程为，俯冲的年轻海洋地壳通常比其他俯冲的地壳更靠近洋中脊。因此温度更高，故而在俯冲带内洋壳中的变质玄武岩而不是上地幔会被部分熔融。此假说后经过实验工作证实。^[2]

根据德凡特和德拉蒙德的研究，埃达克岩的化学特征是：

二氧化硅₂ 大于 56 wt %
氧化铝₂O_yun3 大于或等于 15 wt %
氧化镁通常小于 3 wt%
锶大于 400 ppm，其中锶86和锶87通常小于 0.7045
钇小于 18 ppm
镱小于 1.9 ppm

二人的继续研究指出埃达克岩与金、铜等多种矿床的存在有关^[3]。

太古宙地壳的争论

这两科学家又指出指出，太古宙构成古代大陆地壳的大部分的奥长花岗岩具有与埃达克岩相似的地球化学特征^[4]。他们认为，整个太古宙地壳可能源自彼时被部分熔融的俯冲洋壳。因在地球早期，地幔的温度比现在高得多，洋壳更常在更年轻时俯冲。目前，该推论一直有争议.

另一种解释是，埃达克岩以及大陆地壳来源于在下地壳中的玄武岩部分熔融。然而，这个假设并没有解释为何埃达克岩只和年轻的地壳俯冲有关，也没有解释埃达克岩中为何钇和镱的含量较低。这种两元素的的低含量表明石榴石在下地壳中稳定，但石榴石仅生成于高温高压环境下，在下地壳中一般不稳定^[5]。

含低量镁的埃达克岩，可能代表俯冲洋壳的玄武岩相对纯净，而高镁埃达克岩或高镁安山岩可能代表其被上覆地幔中的橄榄岩污染^[6]。此外在西藏和小高加索下方的大陆-大陆碰撞带也有发现埃达克岩的报导^[7]^[8]，

参考文献

^ Defant M.J.; Drummond M.S. (1990). "Derivation of some modern arc magmas by melting of young subducted lithosphere" (PDF). Nature. 347 (6294): 662–665. Bibcode:1990Natur.347..662D. doi:10.1038/347662a0. S2CID 4267494.
^ Rapp R.P.; Watson E.B. (1995). "Dehydration Melting of Metabasalt at 8–32 kbar: Implications for Continental Growth and Crust-Mantle Recycling". Journal of Petrology. 36 (4): 891–931. Bibcode:1995JPet...36..891R. doi:10.1093/petrology/36.4.891.
^ Defant M.J.; Kepezhinskas P. (2001). "Evidence suggests slab melting in arc magmas". Eos. 82 (6): 65–69. Bibcode:2001EOSTr..82...65D. doi:10.1029/01EO00038.
^ Drummond M.S.; Defant M.J. (1990). "A model for Trondhjemite-Tonalite-Dacite Genesis and crustal growth via slab melting: Archean to modern comparisons". Journal of Geophysical Research: Solid Earth. 95 (B13): 21503–21521. Bibcode:1990JGR....9521503D. doi:10.1029/JB095iB13p21503.
^ Martin H.; Smithies R.H.; Rapp R.; Moyen J.-F.; Champion D. (2005). "An overview of adakite, tonalite–trondhjemite–granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution" (PDF). Lithos. 79 (1–2): 1–24. Bibcode:2005Litho..79....1M. doi:10.1016/j.lithos.2004.04.048.
^ "R. P. Rapp and N. Shimizu, Arc Magmatism in Hot Subduction Zones: Interactions Between Slab-Derived Melts and the Mantle Wedge, and the Petrogenesis of Adakites and High-Magnesian Andesites (HMA) Conference abstract". Archived from the original on 2006-05-14. Retrieved 2007-09-24.
^ Chung, Sun-Lin, et al. Adakites from continental collision zones: Melting of thickened lower crust beneath southern Tibet, Geology v. 31 no. 11 p. 1021-1024
^ Lebedev, V.A.; Vashakidze, G.; Parfenov, A.V.; Yakushev, A.I. (2019). "The origin of adakite-like magmas in the modern continental collision zone: evidence from Pliocene dacitic volcanism of the Akhalkalaki lava plateau (Javakheti highland, Lesser Caucasus)". Petrology. 27 (3): 307–327. doi:10.1134/S0869591119030056. S2CID 195217070.

[”1”-1] Defant M.J.; Drummond M.S. (1990). "Derivation of some modern arc magmas by melting of young subducted lithosphere" (PDF). Nature. 347 (6294): 662–665. Bibcode:1990Natur.347..662D. doi:10.1038/347662a0. S2CID 4267494.

[”2”-2] Rapp R.P.; Watson E.B. (1995). "Dehydration Melting of Metabasalt at 8–32 kbar: Implications for Continental Growth and Crust-Mantle Recycling". Journal of Petrology. 36 (4): 891–931. Bibcode:1995JPet...36..891R. doi:10.1093/petrology/36.4.891.

[”3”-3] Defant M.J.; Kepezhinskas P. (2001). "Evidence suggests slab melting in arc magmas". Eos. 82 (6): 65–69. Bibcode:2001EOSTr..82...65D. doi:10.1029/01EO00038.

[”4”-4] Drummond M.S.; Defant M.J. (1990). "A model for Trondhjemite-Tonalite-Dacite Genesis and crustal growth via slab melting: Archean to modern comparisons". Journal of Geophysical Research: Solid Earth. 95 (B13): 21503–21521. Bibcode:1990JGR....9521503D. doi:10.1029/JB095iB13p21503.

[”5”-5] Martin H.; Smithies R.H.; Rapp R.; Moyen J.-F.; Champion D. (2005). "An overview of adakite, tonalite–trondhjemite–granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution" (PDF). Lithos. 79 (1–2): 1–24. Bibcode:2005Litho..79....1M. doi:10.1016/j.lithos.2004.04.048.

[”6”-6] "R. P. Rapp and N. Shimizu, Arc Magmatism in Hot Subduction Zones: Interactions Between Slab-Derived Melts and the Mantle Wedge, and the Petrogenesis of Adakites and High-Magnesian Andesites (HMA) Conference abstract". Archived from the original on 2006-05-14. Retrieved 2007-09-24.

[”7”-7] Chung, Sun-Lin, et al. Adakites from continental collision zones: Melting of thickened lower crust beneath southern Tibet, Geology v. 31 no. 11 p. 1021-1024

[”8”-8] Lebedev, V.A.; Vashakidze, G.; Parfenov, A.V.; Yakushev, A.I. (2019). "The origin of adakite-like magmas in the modern continental collision zone: evidence from Pliocene dacitic volcanism of the Akhalkalaki lava plateau (Javakheti highland, Lesser Caucasus)". Petrology. 27 (3): 307–327. doi:10.1134/S0869591119030056. S2CID 195217070.

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