Highly magnesian olivines and green-core clinopyroxenes in ultrapotassic lavas from western Yunnan, China: evidence for a complex hybrid origin
Highly magnesian olivines and green-core clinopyroxenes in ultrapotassic lavas from western Yunnan, China: evidence for a complex hybrid origin
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DOI：
10.1127/0935-1221/2003/0015-0965
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发表时间：
2003-12
影响因子：
2.1
通讯作者：
Yigang Xu;Xiao-Long Huang;M. Menzies;Rucheng Wang
中科院分区：
文献类型：
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作者：
Yigang Xu;Xiao-Long Huang;M. Menzies;Rucheng Wang
滇西渐新统超古生代岩石中含有丰富的大小不一的橄榄石和绿芯斜长石。橄榄石大晶体（1-5 mm）具有非常高的橄榄石含量（Fo 92-94），高CaO （0.15-0.30 wt%）和cr2o3（高达0.23 wt%）含量。它们被认为是岩浆形成的，而不是来自上地幔的崩解晶体。这些镁质橄榄石与与寄主熔岩整体岩石组成相对应的熔体不处于平衡状态，因此可能是超古典岩浆上升到地表过程中携带的异种结晶。虽然这些异种晶体可能来自于科麻岩、博长岩和橄榄煌斑岩，但其成因与晚二叠世峨眉山大火成岩省（LIP）有关，因为峨眉山洪水玄武岩是研究区唯一已知的可能产生高镁质橄榄石的地质事件。橄榄石异种结晶可能是由高速下地壳（V p = 7.1 ~ 7.8 km/s）夹带而来，认为是由峨眉山洪水玄武岩时期下镀侵入地壳的高镁玄武岩（苦橄岩？）形成的。考虑到云南镁质玄武岩中橄榄石异种结晶和背带斜辉石的存在，表明云南镁质玄武岩具有复杂的杂化成因。前者指向超镁铁质/原生熔体，后者表明演化熔体与相对原生熔体之间的相互作用，从而表明复杂的岩浆混合过程。
Oligocene ultrapotassic rocks from western Yunnan have abundant olivines of varying size and green-core clinopyroxenes. Olivine macrocrysts (1–5 mm) have very high forsterite contents (Fo 92–94 ), and high CaO (0.15–0.30 wt%) and Cr 2 O 3 (up to 0.23 wt%) contents. They are believed to be magmatic rather than disintegrated crystals from the upper mantle. These magnesian olivines are not in equilibrium with melts corresponding to the whole-rock compositions of host lavas and, as such, may represent xenocrysts entrained by ultrapotassic magmas during their ascent to the surface. While komaiites, boninites and olivine lamproites are potential sources of these xenocrysts, a genetic link to the late Permian Emeishan large igneous province (LIP) is favoured, because the Emeishan flood basalt is the only known geologic event in the studied area that could have produced highly magnesian olivines. The olivine xenocrysts may have been entrained from the high velocity lower crust (V p = 7.1–7.8 km/s), which is believed to have formed from high Mg basalts (picrites?) that underplated and intruded the crust during the Emeishan flood basalt episode. Given the presence of olivine xenocrysts and reverse-zoned clinopyroxenes, it is apparent that some magnesian ultrapotassic lavas from Yunnan have a complex hybrid origin. While the former point to ultramafic/primary melts the latter indicate interaction between evolved melts and relatively primary melts thus indicating complex magma mixing processes.