Mineralogical and Geochemical Constraints on the Petrogenesis of Post-collisional Potassic and Ultrapotassic Rocks from Western Yunnan, SW China
Mineralogical and Geochemical Constraints on the Petrogenesis of Post-collisional Potassic and Ultrapotassic Rocks from Western Yunnan, SW China
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滇西碰撞后钾质和超钾质岩岩石成因的矿物学和地球化学约束
DOI：
10.1093/petrology/egq032
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发表时间：
2010-08-01
影响因子：
3.9
通讯作者：
Yang, Qi-Jun
中科院分区：
文献类型：
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作者：
Huang, Xiao-Long;Niu, Yaoling;Yang, Qi-Jun
滇西古近系基性钾质火山岩和超古生代火山岩具有从钾质粗玄武岩到白垩岩的组成谱（MgO = 6中心点24 ~ 21中心点8 wt %， SiO(2) = 44中心点5 ~ 59中心点1 wt %）。这些岩石具有较高的K(2)O(3中心点07-5中心点28 wt %)，相对较低的Na(2)O（0中心点99-4中心点18 wt %）和较高的K(2)O/Na(2)O比率（0中心点91-3中心点89）。它们具有相同的地球化学特征，如Ta、Nb和Ti相对于其他类似不相容元素的枯竭和Sr- nd同位素组成的富集(初始（87）Sr/(86)Sr为0中心点7056-0中心点7101；（Nd）(t)（-0中心导97到-4中心导36）岩石中含有丰富的橄榄石和斜辉石斑晶和异晶。斜辉石本文展示复杂的分区模式(如正常、反向和振荡),都以高Mg-number点中心77 - 0点(0中心90),低TiO(2)中心(0点中心还有点29 wt %),阿尔阿(2)(3)(0中心导73 - 1中心导68 wt %)和Na (2) O(0点22-0中心点42 wt %)具有类似Ti / Al(0点06-0中心点16)中心和convex-upward稀土元素(REE)模式,显然是在平衡与宿主融化。部分斜辉石斑晶绿核具有低mg数（0中心点50 ~ 0中心点74）和低Ti/Al（< 0中心点05）、高Al(2)O(3)（1中心点66 ~ 3中心点63 wt %）、高Na(2)O（0中心点87 ~ 2中心点17 wt %）和高Al(VI)/Al(IV)（0中心点38 ~ 0中心点76）的特征，具有球粒陨石归一化稀土模式，从La向Dy向上凸起，从Dy向Lu向下凸起。我们把这些绿色岩心解释为来自围岩的异晶。所有这些观测结果表明岩浆房过程复杂，包括广泛的分离结晶、斑结晶积累和类似母熔体的多次熔体补充，并伴有可识别但有限的地壳污染。所有橄榄石晶体（Fo = 94 ~ 75%）均具有较高的CaO含量（> 0中心点1 wt %），表明其岩浆成因。NiO随Fo的减少而减少，这与分级结晶的效果一致。高镁橄榄石（即含Fo bbb90的橄榄石）最好解释为来自超古生代岩浆系统（相对于地幔橄榄石），因为它们的尖晶石包裹体具有高cr数和低Al(2)O(3)和TiO(2)，与岩浆（相对于地幔）起源一致。尖晶石的高fO(2)值表明母熔体被氧化，具有高Fe(3+)/Fe（Tot）或低Fe(2+)/Fe(Tot)，这解释了熔体和高Mg橄榄石的高Mg数[= Mg/(Mg + Fe(2+))]。超古典岩浆的高fO(2)可能继承自其交代富集的岩石圈地幔源。交代地幔岩石圈的极低程度部分熔融是这些超古典岩石成因的最好解释。交代作用可能相对较晚，可能在晚古生代峨眉山洪水玄武岩岩浆活动之后。交代介质可能以碳酸盐岩熔体为主，形成了丰富的Sr-Nd-Pb同位素特征和不相容元素富集，并在钾质和超古生代火山岩中出现明显的负Ta-Nb-Ti异常。熔融上升过程中富钛角闪石的分馏也可能放大了这些岩石的地球化学特征。
Paleogene mafic potassic and ultrapotassic volcanic rocks in western Yunnan, China, show a compositional spectrum from potassic trachybasalt to latite (MgO = 6 center dot 24-21 center dot 8 wt %; SiO(2) = 44 center dot 5-59 center dot 1 wt %). These rocks have high K(2)O (3 center dot 07-5 center dot 28 wt %), relatively low Na(2)O (0 center dot 99-4 center dot 18 wt %) and high K(2)O/Na(2)O ratios (0 center dot 91-3 center dot 89). They share geochemical features such as depletion of Ta, Nb and Ti relative to other similarly incompatible elements and enriched Sr-Nd isotopic compositions (initial (87)Sr/(86)Sr of 0 center dot 7056-0 center dot 7101; epsilon(Nd)(t) of -0 center dot 97 to -4 center dot 36). The rocks contain abundant olivine and clinopyroxene phenocrysts and xenocrysts. Clinopyroxene phenocrysts show complex zoning patterns (e.g. normal, reverse and oscillatory) and are all characterized by high Mg-number (0 center dot 77-0 center dot 90), low TiO(2) (0 center dot 13-0 center dot 29 wt %), Al(2)O(3) (0 center dot 73-1 center dot 68 wt %) and Na(2)O (0 center dot 22-0 center dot 42 wt %) with similar Ti/Al (0 center dot 06-0 center dot 16) and convex-upward rare earth element (REE) patterns, which are apparently in equilibrium with the host melts. Green cores in some clinopyroxene phenocrysts are characterized by low Mg-number (0 center dot 50-0 center dot 74) and Ti/Al (< 0 center dot 05), high Al(2)O(3) (1 center dot 66-3 center dot 63 wt %), Na(2)O (0 center dot 87-2 center dot 17 wt %) and Al(VI)/Al(IV) (0 center dot 38-0 center dot 76), and have chondrite-normalized REE patterns convex-upward from La to Dy and convex-downward from Dy to Lu. We interpret these green cores as xenocrysts from the wall-rocks. All these observations indicate complex magma chamber processes including extensive fractional crystallization, phenocryst accumulation and multiple melt replenishment of similar parental melts with discernible, but limited crustal contamination. All the olivine crystals (Fo = 94-75%) have high CaO contents (> 0 center dot 1 wt %), indicative of a magmatic origin. The NiO decrease with decreasing Fo is consistent with the effect of fractional crystallization. High-Mg olivines (i.e. those with Fo > 90) are best interpreted as having crystallized from the ultrapotassic magma system (vs mantle olivines) because their spinel inclusions have high Cr-number and low Al(2)O(3) and TiO(2), consistent with a magmatic (vs mantle) origin. High fO(2) values calculated from the spinels indicate that the parental melts were oxidized, with high Fe(3+)/Fe(Tot) or low Fe(2+)/Fe(Tot), which explains the high Mg-number [= Mg/(Mg + Fe(2+))] of the melts and the high-Mg olivines. The high fO(2) of the ultrapotassic magmas is probably inherited from their high fO(2) metasomatically enriched lithospheric mantle source. Very low-degree partial melting of metasomatized mantle lithosphere best explains the petrogenesis of these ultrapotassic rocks. The metasomatism may have been relatively recent, probably since the Emeishan flood basalt magmatism in the late Paleozoic in the region. The metasomatic agent may be dominated by a carbonatitic melt, which has imprinted the enriched Sr-Nd-Pb isotopic signature and incompatible element enrichments with conspicuous negative Ta-Nb-Ti anomalies seen in the resulting potassic and ultrapotassic volcanic rocks. Fractionation of Ti-rich amphiboles during melt ascent may have also magnified the geochemical signatures of these rocks.