Geochemistry of Cambrian High-Mg Andesites in North Qilian and Its Geodynamic Implications
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摘要: 北祁连扎柯山高镁安山岩铷锶同位素年龄为早—中寒武世[（516±7）Ma（1σ）]，高镁安山岩类（SiO2=51.45%~59.62%，平均为56.33%），具有比典型岛弧安山岩更高的MgO（4.85%~9.64%，平均为6.2%，Mg#=66.68~74.15，平均为71.44），高Cr（21.6×10-6~476×10-6，平均为196.26×10-6）、高Ni（14×10-6~102×10-6，平均为56.14×10-6），低Al贫Ca（Al2O3=13.22%~17.37%，均值为15.69%，CaO=2.01%~10.33%，均值为5.64%），富Na、贫K（Na2O＝1.57%~5.2%，K2O＝1.04%~2.79%，Na2O/K2O=0.59~5.0，均值为2.36），弱负Eu异常以及富集Rb、Th、K大离子不相容元素（LILE），亏损Sr、Ti、P、Nb、Ta等高场强元素（HFSE），富集LREE，强烈亏损HREE，LREE/HREE=7.19~13.58，Yb=1.47×10-6~3.26×10-6，Y=11.6×10-6~22.5×10-6，含有较低的Sr（119×10-6~278×10-6，平均为193×10-6）。综合区域地质背景等资料研究表明，扎柯山寒武纪高镁安山岩可能是蚀变洋壳和俯冲的沉积物熔融进入地幔楔诱发橄榄岩部分熔融的产物，形成于岛弧环境。Abstract: High-Mg andesites are occurred in Zakeshan area,North Qilian, which were formed in Early and Middle Cambrian, with Rb-Sr isotopic age of (516±7)Ma (1σ).These andesites (SiO2=51.45%~59.62%,averaging in 56.33%)have higher MgO contents (MgO contents are 4.85%~9.64%, averaging in 71.44%, Mg# values are 66.68~74.15, averaging in 71.44)than the ones of typical island arc andesite, with enrichment of Cr (21.6×10-6~476×10-6, averaging in 196.26×10-6), Ni (14×10-6~102×10-6, averaging in 56.14×10-6)and Na elements, but low Al (Al2O3=13.22%~17.37%, averaging in 15.69%), Ca (CaO=2.01%~10.33%, averaging in 5.64%)and K (K2O=1.04%~2.79%, Na2O/K2O=0.59~5.0, averaging in 2.36).Meanwhile, these rocks show weakly negative Eu anomaly, they are enriched in Rb, Th and K (LILE), but they are depleted in Sr, Ti, P, Nb and Ta (HFSE), with LREE/HREE ratios ranging from 7.19 to 13.58 (Yb=1.47×10-6~3.26×10-6, Y=11.6×10-6~22.5×10-6). Combined with regional geological setting, the results suggest that high-Mg andesite was formed in island arc setting and was sourced from the partial melting of the peridotites in mantle wedge, which were metasomated by the fluids from subducted sediments and altered oceanic crust.
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Keywords:
- high-Mg andesite /
- Cambrian /
- geoehemistry /
- geodynamics /
- Zakeshan area /
- North Qilian
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