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| Titel |
Relationships between rates of silicic magma generation, eruption and extensional tectonics: Insights from the Bolanos Graben, Southern Sierra Madre Occidental, Mexico |
| VerfasserIn |
Luca Ferrari, Scott Bryan, Aldo Ramos Rosique, Charlotte Allen, Margarita López, Andrew Rankin |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250034835
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| Zusammenfassung |
| Recent studies have realised potential differences in the rates of eruption of large-volume
(>102 km3) silicic magmas that bear on connections and relationships between silicic
plutonic systems and their volcanic counterparts. Large-volume rhyolite eruptions may
represent either: 1) the end product of prolonged steady-state or episodic growth of a
melt-rich magma bodies or large-volume, crystal-rich, melt-poor mushes, or 2) rapid
accumulations of melt bodies formed shortly prior to each eruption, either through crustal
melting, or extraction from crystal-rich mushes. In addition, active rifting processes and
tectonic disruption of mush piles are increasingly being invoked as an important trigger
mechanism for eruptions. Two important consequences of eruption timing linked to active
rifting processes are that: 1) gaps between the eruption age and peak model crystallisation
ages can develop (Charlier & Wilson, 2009; J Petrol.); and 2) batholithic volumes of
crystal-rich (~50% vol.) and apparently uneruptable magma or mush piles can be
evacuated (Gottsman et al., 2009; EPSL). The 140 km long, N-S trending Bolaños
graben, in the southeastern part of the Sierra Madre Occidental silicic large igneous
province, was cited as an example by Gottsman et al. (2009) where large-volume silicic
eruptions tapping semi-molten batholiths were triggered by concurrent extensional
faulting, and produced crystal-rich, fissure-fed ignimbrites and ‘graben calderas’.
Here we present new field, U-Pb zircon geochronologic and chemical data from the
central Bolaños graben that are at odds with this ‘graben caldera’ model and provide
important insights into source regions and mechanisms for crystal-poor rhyolite
generation. An ~1km thick Oligocene succession of moderate- to high-grade rhyolitic
ignimbrites interbedded with resedimented pyroclastic units form the walls to the central
Bolaños graben but in detail, both crystal-poor (30%)
ignimbrites are present. Capping the graben wall succession and partly infilling the
graben is a ~24-23 Ma suite of very crystal-poor (-¤5%) rhyolite to high-silica
rhyolite ignimbrites and domes that are interbedded with, and overlain by, basaltic
lavas. The Chimal Tuff (18.4 ± 0.4 Ma, 40Ar/39Ar) is the youngest ignimbrite
deposited within the graben. Rhyolite dome emplacement was focussed around a
major NE-trending transfer fault zone to the graben, as well as along bounding N-S
faults during graben initiation. The ~23 Ma (40Ar/39Ar) Alacrán ignimbrite is the
best candidate for a graben-caldera-related ignimbrite, however, several aspects
contradict eruption(s) from a pre-existing molten or semi-molten batholith. The
Alacrán ignimbrite is very crystal poor ( |
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