Magmatic recharge in continental flood basalts: Insights from the Chifeng igneous province in Inner Mongolia
Magmatic recharge in continental flood basalts: Insights from the Chifeng igneous province in Inner Mongolia
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DOI：
10.1002/2015gc005805
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发表时间：
2015-07
期刊：
影响因子：
3.7
通讯作者：
Xun Yu;Cin-Ty A. Lee;Li‐Hui Chen;G. Zeng
中科院分区：
文献类型：
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作者：
Xun Yu;Cin-Ty A. Lee;Li‐Hui Chen;G. Zeng
喷发序列可以作为了解岩浆房热演化和化学演化的窗口。研究了内蒙古赤峰晚新生代溢流玄武岩中白岔河玄武岩流的连续剖面。从最古老到最年轻，MgO含量增加，K2 O、轻稀土等不相容元素含量减少，Nb/La和放射成因Pb同位素比值增加，表明随着时间的推移，地壳混染作用的贡献逐渐减小，而地壳混染作用的贡献逐渐增大。可变的铅同位素和不相容的元素比需要一个组成部分的地壳污染，最有可能的下地壳（非放射性铅，低Ce/Pb），在最早期的熔岩。部分结晶可以解释一些元素的系统，但单独不能解释可变的不相容元素比和铅同位素，也不是时间趋势更原始的组成。无论有没有分离结晶，地壳同化也无法解释所有元素的系统学。相反，我们发现，补给的原始岩浆，结合结晶分离和地壳同化率下降，需要解释所观察到的地球化学系统。我们的观察表明，新鲜玄武岩的岩浆房的交付必须增加速度快于地壳可以同化或地壳同化率必须降低。然而，原始岩浆的逐渐增加应该会加热地壳，导致更多的地壳同化。我们认为，在形成岩浆房的初始阶段，岩浆冷却，并发展了镁铁质到超镁铁质堆晶的外部结晶皮。这导致了一个增厚的非对流化学边界层，它用于隔离岩浆房进一步同化地壳和冷却，后者导致结晶速率的降低和缓冲的岩浆成分在更原始的组成。我们发现，其他大型火成岩省的某些部分也显示出更原始的岩浆随着时间的推移，表明岩浆补给可能是一个共同的特点玄武岩浆房的演变。
Eruptive sequences can be used as windows into the thermal and chemical evolution of magma chambers. We examined a continuous vertical section of the Baichahe basalt flow associated with the late Cenozoic Chifeng flood basalt in Inner Mongolia, North China. From oldest to youngest, MgO increases, K2O, light rare earths and other incompatible elements decrease, and Nb/La and radiogenic Pb isotopic ratios increase, all of which indicate increasing primitiveness and decreasing contribution of crustal contamination with time. The variable Pb isotope and incompatible element ratios require a component of crustal contamination, most likely of the lower crust (unradiogenic Pb, and low Ce/Pb), in the earliest lavas. Fractional crystallization can explain some of the elemental systematics, but alone cannot explain variable incompatible element ratios and Pb isotopes, nor the temporal trend to more primitive compositions. Crustal assimilation with or without fractional crystallization also cannot explain all the elemental systematics. We find instead that recharge by a primitive magma, in combination with fractional crystallization and decreasing rates of crustal assimilation, is needed to explain the observed geochemical systematics. Our observations suggest that the delivery of fresh basalt to the magma chamber must increase at rates faster than the crust can be assimilated or that the rates of crustal assimilation must decrease. However, progressive addition of primitive magma should heat up the crust and lead to more crustal assimilation. We suggest that during the initial stages of forming a magma chamber, the magma cools and develops an outer crystalline rind of mafic to ultramafic cumulates. This results in a thickening nonconvecting chemical boundary layer, which serves to insulate the magma chamber from further assimilation of crust and cooling, the latter resulting in the reduction of crystallization rates and the buffering of magma compositions at more primitive compositions. We show that certain segments of other large igneous provinces also display an evolution toward more primitive magmas with time, indicating that magmatic recharge may be a common feature of basaltic magma chambers.