Generation of Palaeocene Adakitic Andesites by Magma Mixing; Yanji Area, NE China
Generation of Palaeocene Adakitic Andesites by Magma Mixing; Yanji Area, NE China
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DOI：
10.1093/petrology/egl077
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发表时间：
2007-04
影响因子：
3.9
通讯作者：
F. Guo;E. Nakamuru;W. Fan;Katsura Kobayoshi;Chaowen Li
中科院分区：
文献类型：
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作者：
F. Guo;E. Nakamuru;W. Fan;Katsura Kobayoshi;Chaowen Li
延吉地区东北中国地区古新世埃达克质安山岩(约55±58 Ma)是典型的含单斜辉石的苏打安山岩，含SiO_2 60 9±62 2%，MgO 4 02±4 36%，高Mg数[100 mg/(mg-Fe)原子比]65 5~70 1。全岩地球化学特征为高铬(128^161 ppm)、镍(86^117 ppm)，极高的锶(2013^2282 ppm)、低Y(10^11 ppm)和重稀土元素(HREE)；Yb1⁄4 0 79^1 0 1ppm)和大洋中脊玄武岩类锶、钕、铅同位素组成[如：锶/锶(I)1⁄4 0 70298 70298^0 70316，END(T)1⁄4塔3 8~塔6 3和铅/铅⁄417 98 18 06]，类似于现代俯冲带中的高镁埃达克岩。然而，来自单斜辉石斑晶和微晶斜长石的矿物学证据清楚地表明，岩浆演化过程中发生了岩浆混合。富铁单斜辉石(辉石)岩芯具有低锶、高Y和重稀土元素含量，稀土配分模式较弱，Eu负异常较大，可能与下地壳长英质岩浆中的低钙斜长石一起结晶。与俯冲带埃达克岩类似，高镁单斜辉石(透辉石和透辉石)地幔和边缘具有较高的锶和较低的HREE和Y含量，高分馏的稀土配分模式(高La/Yb)和可忽略的Eu异常。延吉埃达克质安山岩可解释为低镁和锶的壳源岩浆和高Y和HREE的壳源岩浆和高锶和低Y和HREE的地幔来源的高镁埃达克岩的混合物。在储存和/或上升过程中，混合岩浆经历了进一步的地壳混染，从围岩中捕获了不同年龄的锆石。延吉地区及其周围地区没有同时代的弧形岩浆作用和伸展构造体制，表明这些古新世埃达克质安山岩是在晚白垩世Izanagi、Farallon海脊俯冲后的俯冲伸展过程中形成的。这些埃达克质安山岩的产生不需要像以前的模型所建议的那样，同时俯冲年轻的热洋脊或榴辉岩下地壳的拆沉。
Palaeocene (c. 55^58Ma) adakitic andesites from the Yanji area, NE China, are typically clinopyroxene-bearing sodic andesites containing 60 9^62 2% SiO2 and 4 02^4 36% MgO, with high Mg-number [100Mg/(Mgþ Fe) atomic ratio] from 65 5 to 70 1. Whole-rock geochemical features include high Cr (128^161ppm) and Ni (86^117 ppm) concentrations, extremely high Sr (2013^2282 ppm), low Y (10^11ppm) and heavy rare earth elements (HREE; e.g. Yb1⁄4 0 79^1 01ppm), and mid-ocean ridge basalt (MORB)-like Sr^Nd^Pb isotopic compositions [e.g. Sr/ Sr(i)1⁄4 0 70298^0 70316, eNd(t)1⁄4þ3 8 to þ6 3 and Pb/ Pb1⁄417 98 ^ 18 06], analogous to high-Mg adakites occurring in modern subduction zones. However, mineralogical evidence from clinopyroxene phenocrysts and microcrystalline plagioclase clearly points to magma mixing during magma evolution. Iron-rich clinopyroxene (augite) cores with low Sr, high Y and heavy REE contents, slightly fractionated REE patterns and large negative Eu anomalies probably crystallized along with low-Ca plagioclase from a lower crustal felsic magma. In contrast, high Mg-number clinopyroxene (diopside and endiopside) mantles and rims have higher Sr and lower HREE and Yconcentrations, highly fractionated REE patterns (high La/Yb) and negligible Eu anomalies, similar to those found in adakites from subduction zones.The Yanji adakitic andesites can be interpreted as a mixture between a crust-derived magma having low Mg-number and Sr, and high Y and HREE, and a mantle-derived high Mg-number adakite having high Sr and low Y and HREE concentrations. During storage and/or ascent, the mixed magma experienced further crustal contamination to capture zircons, of a range of ages, from the wall rocks.The absence of coeval arc magmatism and an extensional tectonic regime in the Yanji area and surrounding regions suggest that these Palaeocene adakitic andesites were formed during post-subduction extension that followed the late Cretaceous Izanagi^Farallon ridge subduction. Generation of these adakitic andesites does not require contemporaneous subduction of a young, hot oceanic ridge or delamination of eclogitic lower crust as suggested by previous models.