Voluminous lava-like precursor to a major ash-flow tuff: low-column pyroclastic eruption of the Pagosa Peak Dacite, San Juan volcanic field, Colorado
Voluminous lava-like precursor to a major ash-flow tuff: low-column pyroclastic eruption of the Pagosa Peak Dacite, San Juan volcanic field, Colorado
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DOI：
10.1016/s0377-0273(99)00185-7
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发表时间：
2000-05-01
影响因子：
2.9
通讯作者：
Lipman, PW
中科院分区：
文献类型：
--
作者：
Bachmann, O;Dungan, MA;Lipman, PW
帕戈萨峰英安岩是一种不同寻常的火山碎屑矿床，在28 Ma时，它就在巨大的鱼峡谷凝灰岩(类似于5000公里(3))从La Garita火山口喷发之前。帕戈萨峰英安岩厚度(1千米)、体积(200千米(3))，高宽比(1：50)，与硅质熔岩流相似。它含有很高比例(40%-60%)的幼年碎屑(至3-4m)，作为粘性岩浆侵位，比典型的浮石泡少。单位底部5-10%以上无意外碎石碎片。尽管富含晶体的岩浆具有很高的粘度，但由于重力扩散，厚密焊接的近端矿床以流变学的形式弯曲，导致宏观外观类似于流动层状硅质熔岩。尽管Pagosa Peak英安岩是一个独立的沉积单元，但在整体岩石化学、斑晶成分和40Ar/39Ar年龄方面，它与上复的鱼峡谷凝灰岩没有什么区别。矿床的不同寻常的特征被解释为低柱状火山碎屑喷发和侧向输送的结果，即密集的、低膨胀的火山碎屑流。推测的喷发方式可能在一定程度上与块状断裂对鱼峡谷岩浆室南缘的同步破坏有关。Pagosa Peak喷发源显然埋藏在La Garita火山口南部，那里观测到的协同断裂向北延伸作为裂隙喷口。累积的喷口截面很大，导致在给定的排放速率下，排放速度相对较低。许多连续的火山碎屑流积累得足够快，足以作为一个厚达1000米的冷却单元进行密集焊接，并充分保持热以允许流变性流动。岩浆在上升过程中通过裂隙管道的脱气作用可能降低了岩浆的爆炸潜力，导致在低孔隙度时以裂缝为主的岩浆碎裂。随后75×35公里(2)La Garita火山口的坍塌和鱼峡谷凝灰岩的喷发可能是由洞顶的不稳定引发的，因为在帕戈萨峰喷发期间岩浆被抽走。(C)2000 Elsevier Science B.V.保留所有权利。Pagosa Peak英安岩是一种不寻常的火山碎屑矿床，在28 Ma时，在距离La Garita火山口的巨大鱼类峡谷凝灰岩(类似于5000公里(3))喷发之前。帕戈萨峰英安岩厚度(1千米)、体积(200千米(3))，高宽比(1：50)，与硅质熔岩流相似。它含有很高比例(40%-60%)的幼年碎屑(至3-4m)，作为粘性岩浆侵位，比典型的浮石泡少。单位底部5-10%以上无意外碎石碎片。尽管富含晶体的岩浆具有很高的粘度，但由于重力扩散，厚密焊接的近端矿床以流变学的形式弯曲，导致宏观外观类似于流动层状硅质熔岩。尽管Pagosa Peak英安岩是一个独立的沉积单元，但在整体岩石化学、斑晶成分和40Ar/39Ar年龄方面，它与上复的鱼峡谷凝灰岩没有什么区别。矿床的不同寻常的特征被解释为低柱状火山碎屑喷发和侧向输送的结果，即密集的、低膨胀的火山碎屑流。推测的喷发方式可能在一定程度上与块状断裂对鱼峡谷岩浆室南缘的同步破坏有关。Pagosa Peak喷发源显然埋藏在La Garita火山口南部，那里观测到的协同断裂向北延伸作为裂隙喷口。累积的喷口截面很大，导致在给定的排放速率下，排放速度相对较低。许多连续的火山碎屑流积累得足够快，足以作为一个厚达1000米的冷却单元进行密集焊接，并充分保持热以允许流变性流动。岩浆在上升过程中通过裂隙管道的脱气作用可能降低了岩浆的爆炸潜力，导致在低孔隙度时以裂缝为主的岩浆碎裂。随后75×35公里(2)La Garita火山口的坍塌和鱼峡谷凝灰岩的喷发可能是由洞顶的不稳定引发的，因为在帕戈萨峰喷发期间岩浆被抽走。(C)2000 Elsevier Science B.V.保留所有权利。
The Pagosa Peak Dacite is an unusual pyroclastic deposit that immediately predated eruption of the enormous Fish Canyon Tuff (similar to 5000 km(3)) from the La Garita caldera at 28 Ma. The Pagosa Peak Dacite is thick (to 1 km), voluminous (>200 km(3)), and has a high aspect ratio (1:50) similar to those of silicic lava flows. It contains a high proportion (40-60%) of juvenile clasts (to 3-4 m) emplaced as viscous magma that was less vesiculated than typical pumice. Accidental lithic fragments are absent above the basal 5-10% of the unit. Thick densely welded proximal deposits Bowed rheomorphically due to gravitational spreading, despite the very high viscosity of the crystal-rich magma, resulting in a macroscopic appearance similar to flow-layered silicic lava. Although it is a separate depositional unit, the Pagosa Peak Dacite is indistinguishable from the overlying Fish Canyon Tuff in bulk-rock chemistry, phenocryst compositions, and 40Ar/39Ar age. The unusual characteristics of this deposit are interpreted as consequences of eruption by low-column pyroclastic fountaining and lateral transport as dense, poorly inflated pyroclastic flows. The inferred eruptive style may be in part related to synchronous disruption of the southern margin of the Fish Canyon magma chamber by block faulting. The Pagosa Peak eruptive sources are apparently buried in the southern La Garita caldera, where northerly extensions of observed syneruptive faults served as fissure vents. Cumulative vent cross-sections were large, leading to relatively low emission velocities for a given discharge rate. Many successive pyroclastic flows accumulated sufficiently rapidly to weld densely as a cooling unit up to 1000 m thick and to retain heat adequately to permit rheomorphic flow. Explosive potential of the magma may have been reduced by degassing during ascent through fissure conduits, leading to fracture-dominated magma fragmentation at low vesicularity. Subsequent collapse of the 75 x 35 km(2) La Garita caldera and eruption of the Fish Canyon Tuff were probably triggered by destabilization of the chamber roof as magma was withdrawn during the Pagosa Peak eruption. (C) 2000 Elsevier Science B.V. All rights reserved.The Pagosa Peak Dacite is an unusual pyroclastic deposit that immediately predated eruption of the enormous Fish Canyon Tuff (similar to 5000 km(3)) from the La Garita caldera at 28 Ma. The Pagosa Peak Dacite is thick (to 1 km), voluminous (>200 km(3)), and has a high aspect ratio (1:50) similar to those of silicic lava flows. It contains a high proportion (40-60%) of juvenile clasts (to 3-4 m) emplaced as viscous magma that was less vesiculated than typical pumice. Accidental lithic fragments are absent above the basal 5-10% of the unit. Thick densely welded proximal deposits Bowed rheomorphically due to gravitational spreading, despite the very high viscosity of the crystal-rich magma, resulting in a macroscopic appearance similar to flow-layered silicic lava. Although it is a separate depositional unit, the Pagosa Peak Dacite is indistinguishable from the overlying Fish Canyon Tuff in bulk-rock chemistry, phenocryst compositions, and 40Ar/39Ar age. The unusual characteristics of this deposit are interpreted as consequences of eruption by low-column pyroclastic fountaining and lateral transport as dense, poorly inflated pyroclastic flows. The inferred eruptive style may be in part related to synchronous disruption of the southern margin of the Fish Canyon magma chamber by block faulting. The Pagosa Peak eruptive sources are apparently buried in the southern La Garita caldera, where northerly extensions of observed syneruptive faults served as fissure vents. Cumulative vent cross-sections were large, leading to relatively low emission velocities for a given discharge rate. Many successive pyroclastic flows accumulated sufficiently rapidly to weld densely as a cooling unit up to 1000 m thick and to retain heat adequately to permit rheomorphic flow. Explosive potential of the magma may have been reduced by degassing during ascent through fissure conduits, leading to fracture-dominated magma fragmentation at low vesicularity. Subsequent collapse of the 75 x 35 km(2) La Garita caldera and eruption of the Fish Canyon Tuff were probably triggered by destabilization of the chamber roof as magma was withdrawn during the Pagosa Peak eruption. (C) 2000 Elsevier Science B.V. All rights reserved.