Ignimbrites to batholiths: integrating perspectives from geological, geophysical, and geochronological data
Ignimbrites to batholiths: integrating perspectives from geological, geophysical, and geochronological data
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DOI：
10.1130/ges01091.1
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发表时间：
2015-06
期刊：
影响因子：
2.5
通讯作者：
P. Lipman;O. Bachmann
中科院分区：
文献类型：
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作者：
P. Lipman;O. Bachmann
在南落基山脉火山场(SRMVF)，与大重力低有关的第三系点火沸石、源区火山口和花岗岩类侵入体记录了在建造大型次火山岩浆体过程中增量积累、分馏和固化的多阶段历史。地球物理数据结合地质约束，以及与其他地方倾斜的深成岩体和岩浆弧段的比较，与主要SRMVF火山轨迹(Sawatch，San Juan，Questa-Latir)之下垂直分布的(>；20公里)中硅质岩基(侵入：喷出比为10：1或更大)的存在一致。同位素数据需要大量地幔衍生的镁铁质岩浆的参与，其规模等于或大于中等至硅质火山岩和深成岩的规模。与地球物理定义的岩基相比，为圣胡安地区中等成分中心火山提供物质来源的早期蜡化期侵入岩(35-30 Ma)更为广泛；这些岩基可能对地壳进行了加热和加工，为褐煤火山作用(32-27 Ma)和上地壳岩基的大规模生长做好了准备。年龄和成分的相似性表明，SRMVF火成岩与花岗岩侵入体密切相关，但深成岩浆记录了因火山喷发而失去熔融的前岩浆储集层的残留物，这一点一直存在争议。已发表的与褐辉石破火山口有关的深成岩的Ar/Ar长石和U-Pb锆石年龄记录了花岗岩类侵入体的最终结晶，有时与凝灰岩难以区分，年龄则要年轻数百万年。这些年龄还表明，随着岩浆供应的减少，与SRMVF有关的火山口侵入体在锆石结晶后不久就冷却和固化。一些研究人员将这些结果解释为，记录了快速结晶的小增量岩体组装，导致了褐沸石喷发和侵入生长之间的时间脱节。另外，花岗岩的结晶年龄被推断为记录了晚期凝固，经历了长期的开放系统演化，包括大量的地幔输入，长时间(105-106年)的近固相线结晶碎屑，以及断续分离的液体供应火山喷发。演化程度最低的褐辉岩浆的成分往往与与火山口有关的深成岩浆的成分相融合，这表明深成岩浆记录了长期共生岩浆系统的未喷发部分，在最终凝固之前进行了可变的修改。前寒武纪源区的锆石在火山口深成岩体中很稀少，而在SRMVF的一些外围衰退期侵入体中则很丰富，这意味着在褐沸石喷发和次火山岩基建造的漫长岩浆组装过程中，继承的地壳锆石发生了溶解。一些原始岩和侵入岩的个别样品中的锆石年龄跨度很大(至数百万年)，通常被平均和解释为“侵入-侵位年龄”，或者提供了在漫长的增量岩浆体组装过程中间歇结晶的不完整记录，只有当系统开始衰退时才会最终凝固。对整个锆石的分析不能解决晶体生长的后期阶段，而且由于锆石溶解的时期，在长寿命岩浆系统中的早期生长可能记录得很差。总体而言，建造岩基可能需要比锆石结晶年龄记录的时间更长的时间，而分离和浅层聚集喷发岩浆的时间间隔可能要短得多。根据已公布的热模型，岩浆供应估计(根据年龄和火山-深成岩量)得出的重点侵入-集结率足以产生褐煤规模的可喷发岩浆。与SRMVF有关的中第三纪岩基组装过程导致整个岩石圈剧烈的化学和物理重建，可能伴随着软流圈的输入。
Multistage histories of incremental accumulation, fractionation, and solidification during construction of large subvolcanic magma bodies that remained sufficiently liquid to erupt are recorded by Tertiary ignimbrites, source calderas, and granitoid intrusions associated with large gravity lows at the Southern Rocky Mountain volcanic field (SRMVF). Geophysical data combined with geological constraints and comparisons with tilted plutons and magmatic-arc sections elsewhere are consistent with the presence of vertically extensive (>20 km) intermediate to silicic batholiths (with intrusive:extrusive ratios of 10:1 or greater) beneath the major SRMVF volcanic loci (Sawatch, San Juan, Questa-Latir). Isotopic data require involvement of voluminous mantle-derived mafic magmas on a scale equal to or greater than that of the intermediate to silicic volcanic and plutonic rocks. Early waxing-stage intrusions (35–30 Ma) that fed intermediate-composition central volcanoes of the San Juan locus are more widespread than the geophysically defined batholith; these likely heated and processed the crust, preparatory for ignimbrite volcanism (32–27 Ma) and large-scale upper-crustal batholith growth. Age and compositional similarities indicate that SRMVF ignimbrites and granitic intrusions are closely related, but the extent to which the plutons record remnants of former magma reservoirs that lost melt to volcanic eruptions has been controversial. Published Ar/Ar-feldspar and U-Pb-zircon ages for plutons spatially associated with ignimbrite calderas document final crystallization of granitoid intrusions at times indistinguishable from the tuff to ages several million years younger. These ages also show that SRMVF caldera-related intrusions cooled and solidified soon after zircon crystallization, as magma supply waned. Some researchers interpret these results as recording pluton assembly in small increments that crystallized rapidly, leading to temporal disconnects between ignimbrite eruption and intrusion growth. Alternatively, crystallization ages of the granitic rocks are here inferred to record late solidification, after protracted open-system evolution involving voluminous mantle input, lengthy residence (105–106 yr) as near-solidus crystal mush, and intermittent separation of liquid to supply volcanic eruptions. The compositions of the least-evolved ignimbrite magmas tend to merge with those of caldera-related plutons, suggesting that the plutons record nonerupted parts of long-lived cogenetic magmatic systems, variably modified prior to final solidification. Precambrian-source zircons are scarce in caldera plutons, in contrast to their abundance in some peripheral waning-stage intrusions of the SRMVF, implying dissolution of inherited crustal zircon during lengthy magma assembly for the ignimbrite eruptions and construction of a subvolcanic batholith. Broad age spans of zircons (to several million years) from individual samples of some ignimbrites and intrusions, commonly averaged and interpreted as “intrusion-emplacement age,” alternatively provide an incomplete record of intermittent crystallization during protracted incremental magma-body assembly, with final solidification only when the system began to wane. Analyses of whole zircons cannot resolve late stages of crystal growth, and early growth in a long-lived magmatic system may be poorly recorded due to periods of zircon dissolution. Overall, construction of a batholith can take longer than recorded by zircon-crystallization ages, while the time interval for separation and shallow assembly of eruptible magma may be much shorter. Magma-supply estimates (from ages and volcano-plutonic volumes) yield focused intrusion-assembly rates sufficient to generate ignimbrite-scale volumes of eruptible magma, based on published thermal models. Mid-Tertiary processes of batholith assembly associated with the SRMVF caused drastic chemical and physical reconstruction of the entire lithosphere, probably accompanied by asthenospheric input.