Petrogenesis of Tertiary Hornblende-bearing Lavas in the Rhön, Germany
Petrogenesis of Tertiary Hornblende-bearing Lavas in the Rhön, Germany
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德国罗恩河第三纪含角闪石熔岩的岩石成因
DOI：
10.1093/petrology/egt042
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发表时间：
2013
影响因子：
3.9
通讯作者：
Garbe-Schönberg
中科院分区：
文献类型：
--
作者：
Stracke;Garbe-Schönberg
德国Rhön地区（中欧火山省的一部分）含角闪石玄武岩和碱性玄武岩具有较高的TiO2（3 ~ 4 wt %）、中等高的Mg#（大部分为>0·50）、多变的Cr （400 ~ 30 ppm）和Ni （160 ~ 20 ppm）丰度，并富含不相容的微量元素和稀土元素（REE）。在原始地幔归一化多元素图中，Ba、Rb和K相对于相似不相容的微量元素有较强的耗损。部分碱性玄武岩和分异程度较高的岩石具有较低的Mg#和较低的Ni、Cr丰度，并经历了橄榄石、斜辉石、铁钛氧化物和角闪孔的分选作用。微量元素的限制（如低Nb/U和Ce/Pb以及某些玄武岩的Nd-Sr-Pb同位素组成）表明，下地壳物质的同化改变了原生幔源岩浆的组成。大部分玄武岩和碱性玄武岩的Sr-Nd-Pb同位素组成接近欧洲软流圈储层（EAR）组分。稀土丰度的变化和稀土比值的相关性表明，含角闪石的尖晶石橄榄岩部分熔融，其中含有大量非橄榄岩物质（即辉石岩）。残余角闪洞的存在，由相对于Ba和Nb的K和Rb的枯竭表明，需要在软流圈-岩石圈边界附近或岩石圈地幔内熔化，最有可能是脉状地幔源。温度和压力估计表明，最原始的熔岩的融化深度为~ 80 km，温度为~ 1290°C。基于Sr-Nd同位素和微量元素约束，提出软流圈熔体的组成与在CEVP其他地方观测到的EAR熔体相似，作为岩石圈地幔中的脉体冻结在软流圈-岩石圈热边界。这些岩脉在上升的软流圈熔体的作用下，仅在很短的储存时间内就被重新熔化，在玄武岩上留下了明显的角闪孔特征。相当放射性成因的Pb同位素特征预计来自于富集的、低熔融温度组分的熔融，这些熔融组分合并在枯竭的上地幔（软流圈）中，因此对于Rhön或许多其他具有类似Pb同位素特征的大陆碱性熔岩来说，不需要深部地幔源的上涌。
Hornblende-bearing basanites and alkali basalts from the Rhön area of Germany (part of the Central European Volcanic Province; CEVP) have high TiO2(3–4 wt %), moderately high Mg# (mostly >0·50), variable Cr (400–30 ppm) and Ni (160–20 ppm) abundances, and are enriched in incompatible trace elements and rare earth elements (REE). In primitive mantle-normalized multi-element diagrams they show a strong depletion in Ba, Rb, and K relative to trace elements of similar incompatibility. Some alkali basalts and more differentiated rocks have lower Mg# and lower abundances of Ni and Cr, and have undergone fractionation of olivine, clinopyroxene, Fe–Ti oxides and amphibole. The trace element constraints (e.g. low Nb/U and Ce/Pb and the Nd–Sr–Pb isotope compositions of some basalts) indicate that assimilation of lower crustal material has modified the composition of the primary mantle-derived magmas. Most of the basanites and alkali basalts approach the Sr–Nd–Pb isotope compositions inferred for the EAR (European Asthenospheric Reservoir) component. Variations in REE abundances and correlations between REE ratios suggest partial melting of amphibole-bearing spinel peridotite containing a significant portion of non-peridotitic material (i.e. pyroxenite). The presence of residual amphibole, indicated by depletion of K and Rb relative to Ba and Nb, requires melting close to the asthenosphere–lithosphere boundary or within the lithospheric mantle, most probably of a veined mantle source. Temperature and pressure estimates indicate a depth of melting for the most primitive lavas at ∼80 km at temperatures of ∼1290°C. Based on Sr–Nd isotope and trace element constraints it is proposed that asthenospheric melts similar in composition to EAR melts observed elsewhere in the CEVP froze at the asthenosphere–lithosphere thermal boundary as veins in the lithospheric mantle. These veins were remelted after only short storage times by ascending asthenospheric melts, imposing the prominent amphibole signature upon the basalts. The fairly radiogenic Pb isotope signatures are expected to originate from melting of enriched, low melting temperature components incorporated in the depleted upper (asthenospheric) mantle and therefore do not require upwelling of deep-seated mantle sources for the Rhön or many other continental alkaline lavas with similar Pb isotope signatures.
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DOI：
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