Evolution of silicic magma in the upper crust: the mid-Tertiary Latir volcanic field and its cogenetic granitic batholith, northern New Mexico, U.S.A.
Evolution of silicic magma in the upper crust: the mid-Tertiary Latir volcanic field and its cogenetic granitic batholith, northern New Mexico, U.S.A.
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DOI：
10.1017/s0263593300014279
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发表时间：
1988
影响因子：
1.2
通讯作者：
P. Lipman
中科院分区：
文献类型：
--
作者：
P. Lipman
沿着新墨西哥州北方格兰德河裂谷东缘的构造和地形起伏，提供了一个通过26 Ma的奎斯塔破火山口和Alzheur油田的同生火山岩和深成岩的显著横截面。从火成岩杂岩北方部分的中第三纪沉积面到南方3-5 km深处的深成岩，暴露水平随深度增加。侵蚀残余的灰流片弱过碱性流纹岩（阿马利亚凝灰岩）和安山英安质前体熔岩，断裂断裂相关的故障中断，保存远至45公里以外的奎斯塔破火山口的来源。广泛comagmatic 26马岩基花岗质岩石，暴露在20公里的面积35公里，范围从中带花岗闪长岩到浅成斑状花岗岩和细晶岩;较浅和更多的火山口内的阶段。成分和纹理不同的花岗岩定义复活的侵入火山口和不连续的环形岩脉沿着其边缘;岩基质量的花岗闪长岩延伸20公里的破火山口和当地的等级垂直于花岗岩下面的平躺屋顶。负布格重力异常（15-20 mgal）包围了暴露的花岗岩，并与奎斯塔破火山口的边界重合，定义了浅岩基的边界，位于火山序列和下伏前寒武纪岩石的低位。古地磁极位置表明，连续结晶花岗质岩体冷却通过居里温度在破火山口形成的时间，初始区域的扩展，和旋转倾斜的火山岩。大多数侵入体的同位素年龄与火山岩的同位素年龄难以区分。这些关系表明，岩基杂岩广泛地代表了火山岩的源岩浆，其中奎斯塔破火山口崩溃，并在区域构造破坏的岩浆主要是液体。火山岩浆和深成岩浆（1）在破火山口喷发前由早期高钾钙碱性转变为碱性;（2）在26·5 Ma破火山口形成时，分化为弱过碱性流纹岩和相当于钙碱性花岗岩盖层（下伏钙碱性花岗岩）;碱金属和微量元素Rb、Th、U、Nb、Zr和Y的浓度在破火山口阶段达到最大值。火山岩构成间歇淬火样品的上部奎斯塔岩浆体在结晶的早期阶段，相比之下，同源岩浆花岗质岩石保存一个综合记录的长期结晶的岩浆残留物的喷发减少。在奎斯塔岩浆系统的演化过程中，多种分异过程是活跃的：晶体分馏、地幔和下地壳不同化学和同位素特征的熔融物的补充、演化岩浆与更原始岩浆的混合、上地壳同化，以及可能的挥发分转移过程。因此，一个不断发展的岩基集群的合并岩浆房产生了广泛的同源岩石在几百万年内的各种组合。奎斯塔岩浆系统的演化和类似的高层次第三纪花岗质岩基附近的落基山脉南部提供了广泛的见解，在大陆地区的岩浆过程，如整体形状的岩基，时间和成分之间的关系，同生火山岩和深成岩，岩浆与围岩的密度平衡，以及大陆地壳的热演化。
ABSTRACT Structural and topographic relief along the eastern margin of the Rio Grande rift, northern New Mexico, provides a remarkable cross-section through the 26-Ma Questa caldera and cogenetic volcanic and plutonic rocks of the Latir field. Exposed levels increase in depth from mid-Tertiary depositional surfaces in northern parts of the igneous complex to plutonic rocks originally at 3–5 km depths in the S. Erosional remnants of an ash-flow sheet of weakly peralkaline rhyolite (Amalia Tuff) and andesitic to dacitic precursor lavas, disrupted by rift-related faults, are preserved as far as 45 km beyond their sources at the Questa caldera. Broadly comagmatic 26 Ma batholithic granitic rocks, exposed over an area of 20 by 35 km, range from mesozonal granodiorite to epizonal porphyritic granite and aplite; shallower and more silicic phases are mostly within the caldera. Compositionally and texturally distinct granites define resurgent intrusions within the caldera and discontinuous ring dikes along its margins; a batholithic mass of granodiorite extends 20 km S of the caldera and locally grades vertically to granite below its flat-lying roof. A negative Bouguer gravity anomaly (15–20 mgal), which encloses exposed granitic rocks and coincides with boundaries of the Questa caldera, defines boundaries of the shallow batholith, emplaced low in the volcanic sequence and in underlying Precambrian rocks. Palaeomagnetic pole positions indicate that successively crystallised granitic plutons cooled through Curie temperatures during the time of caldera formation, initial regional extension, and rotational tilting of the volcanic rocks. Isotopic ages for most intrusions are indistinguishable from the volcanic rocks. These relations indicate that the batholithic complex broadly represents the source magma for the volcanic rocks, into which the Questa caldera collapsed, and that the magma was largely liquid during regional tectonic disruption. Volcanic and plutonic magmas (1) changed from early high-K calc-alkaline to alkalic prior to caldera eruptions; (2) differentiated to a weakly peralkaline rhyolite and equivalent acmiteartvedsonite granite cap (underlain by calc-alkaline granite) when the caldera formed at 26·5 Ma; then (3) reverted to calc-alkaline compositions. Concentrations of alkalis and minor elements such as Rb, Th, U, Nb, Zr, and Y reached maxima at the caldera stage. The volcanic rocks constitute intermittently quenched samples of upper parts of Questa magma bodies at early stages of crystallisation; in contrast, the comagmatic granitic rocks preserve an integrated record of protracted crystallisation of the magmatic residue as eruptions diminished. Multiple differentiation processes were active during evolution of the Questa magmatic system: crystal fractionation, replenishment by mantle and lower crustal melts of varying chemical and isotopic character, mixing of evolved with more primitive magmas, upper crustal assimilation, and perhaps volatile-transfer processes. As a result, an evolving batholithic cluster of coalesced magma chambers generated diverse assemblages of broadly cogenetic rocks within a few million years. Evolution of the Questa magmatic system and similar high-level Tertiary granitic batholiths nearby in the southern Rocky Mountains provides broad insights into magmatic processes in continental regions such as the overall shapes of batholiths, time and compositional relations between cogenetic volcanic and plutonic rocks, density equilibration of magmas with country rocks, and thermal evolution of continental crust.