Development of the Long Valley, California, magma chamber recorded in precaldera rhyolite lavas of Glass Mountain
Development of the Long Valley, California, magma chamber recorded in precaldera rhyolite lavas of Glass Mountain
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加利福尼亚州长谷的发育，玻璃山破火山口流纹岩熔岩记录的岩浆室
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发表时间：
1991
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通讯作者：
G. Mahood
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作者：
J. Metz;G. Mahood
加州的玻璃山由大于50 km 3的高硅流纹岩熔岩和相关的火山碎屑沉积物组成，这些熔岩和火山碎屑沉积物在0.73 Ma的主教凝灰岩爆炸性喷发和长谷破火山口形成之前的大于1年的时间内喷发。这些“最小熔融”流纹产生的铁钛氧化物温度为695-718°C，并含有斜长石+石英+磁铁矿+磷灰石±透长石、黑云母、钛铁矿、褐帘石和锆石的稀疏斑晶。不相容的微量元素显示类似或更大的范围内的玻璃山套房比主教凝灰岩，尽管范围小得多的主要元素的浓度，主要是由于变化之间的老熔岩（爆发之间2.1和1.2马）。最不相容的元素的比例有较大的范围在较老的熔岩比在年轻的熔岩（1.2-0.79马），和浓度的不相容的元素跨越宽的范围在几乎恒定的Ce/Yb，这表明，这些元素的最高浓度是不是单独的广泛的分离结晶的结果，而是，他们继承了从父母的岩浆与更大比例的地壳部分熔融。这一地壳成分的性质的证据来自于来自花岗质和绿片岩级变质岩的稀少的微小捕虏晶的存在。较老熔岩的化学和同位素组成范围更广，斑晶模式的范围更大，不同组成的岩浆在几乎同一时间在野外的不同部分喷发，单个熔岩的体积较小，这表明在较老的熔岩喷发过程中，不止一个岩浆体被流出，或者所有熔岩流出的单个岩浆室足够小，其上游易受新加入的地壳熔体的影响。我们解释的相对化学，矿物学和同位素均匀性的年轻玻璃山熔岩反映从一个大的，综合岩浆房喷发。1.4和1.2马之间的小数量的cruptions可能有时间为一个大的岩浆体合并，而且，随着房的增长，其上游变得不太受新的输入地壳熔体，使微量元素的趋势在1.2马后爆发的岩浆在很大程度上控制分离结晶。玻璃山熔岩的极低Sr浓度意味着在体积至少数百立方公里的室中广泛结晶。年轻的玻璃山熔岩和未蚀变的主教凝灰岩之间的Sr，Nd和Pb同位素比值的密切相似性表明，他们利用相同的岩浆体，这已经成为同位素均匀的1.2马，但在此之后继续分化。从1.2到0.79马，体积喷发率可能超过了分化率，年轻的玻璃山熔岩变得稍微少进化随着时间的推移。早期喷发的主教凝灰岩比最年轻的玻璃山熔岩更进化，其特征是微量元素比例略有不同。这表明，虽然岩浆已经存在了0.5百万年，但主教凝灰岩所表现出的成分梯度并不是长谷岩浆房中的长期稳定状态，而是至少部分地在最后一次玻璃山熔岩挤出和高潮喷发之间的0.06百万年的间隙中发展起来的。
Glass Mountain, California, consists of >50 km3 of high-silica rhyolite lavas and associated pyroclastic deposits that erupted over a period of >1 my preceding explosive eruption of the Bishop Tuff and formation of the Long Valley caldera at 0.73 Ma. These “minimum-melt” rhyolites yield Fe-Ti-oxide temperatures of 695–718°C and contain sparse phenocrysts of plagioclase+quartz+magnetite+apatite±sanidine, biotite, ilmenite, allanite, and zircon. Incompatible trace elements show similar or larger ranges within the Glass Mountain suite than within the Bishop Tuff, despite a much smaller range of major-element concentrations, largely due to variability among the older lavas (erupted between 2.1 and 1.2 Ma). Ratios of the most incompatible elements have larger ranges in the older lavas than in the younger lavas (1.2–0.79 Ma), and concentrations of incompatible elements span wide ranges at nearly constant Ce/Yb, suggesting that the highest concentrations of these elements are not the result of extensive fractional crystallization alone; rather, they are inherited from parental magmas with a larger proportion of crustal partial melt. Evidence for the nature of this crustal component comes from the presence of scarce, tiny xenocrysts derived from granitic and greenschist-grade metamorphic rocks. The wider range of chemical and isotopic compositions in the older lavas, the larger range in phenocryst modes, the eruption of magmas with different compositions at nearly the same time in different parts of the field, and the smaller volume of individual lavas suggest either that more than one magma body was tapped during eruption of the older lavas or that a single chamber tapped by all lavas was small enough that the composition of its upper reaches easily affected by new additions of crustal melts. We interpret the relative chemical, mineralogical, and isotopic homogeneity of the younger Glass Mountain lavas as reflecting eruptions from a large, integrated magma chamber. The small number of cruptions between 1.4 and 1.2 ma may have allowed time for a large magma body to coalesce, and, as the chamber grew, its upper reaches became less affected by new inputs of crustal melts, so that trace-element trends in magmas erupted after 1.2 Ma are largely controlled by fractional crystallization. The extremely low Sr concentrations of Glass Mountain lavas imply extensive crystallization in chambers at least hundreds of cubic kilometers in volume. The close similarity in Sr, Nd, and Pb isotopic ratios between the younger Glass Mountain lavas and unaltered Bishop Tuff indicates that they tapped the same body of magma, which had become isotopically homogenous by 1.2 Ma but continued to differentiate after that time. From 1.2 to 0.79 Ma, volumetric eruptive rates may have exceeded rates of differentiation, as younger Glass Mountain lavas become slightly less evolved with time. Early-erupted Bishop Tuff is more evolved than the youngest of the Glass Mountain lavas and is characterized by slightly different trace element ratios. This suggests that although magma had been present for 0.5 my, the composiional gradient exhibited by the Bishop Tuff had not been a long-term, steady-state condition in the Long Valley magma chamber, but developed at least in part during the 0.06-my hiatus between extrusion of the last Glass Mountain lava and the climactic eruption.