HR: 08:00h
AN: V41F-01 INVITED    [Abstracts]
TI: The Growth of Magma Bodies by Amalgamation of Discrete Sheet Intrusions: Implications for the Formation of Magma Chambers
AU: * Annen, C
EM: catherine.annen@terre.unige.ch
AF: Section des Sciences de la Terre, University of Geneva, 13 rue des Mara\ⁱchers, Genève, 1205, Switzerland
AB: Until recently, igneous bodies (plutons and magma chambers) were commonly considered to be approximately spherical bodies, rapidly emplaced into the crust. However, field, structural, geophysical, and geochronological studies indicate that many plutons are low aspect-ratio tabular bodies (sills) that are formed by the amalgamation of successive discrete magma pulses. The thermal evolution of an igneous body that grows by accretion of thin magma sheets is fundamentally different from the evolution of a rapidly emplaced magma sphere or of a single thick magma sill. In thin sheet intrusions, the heat loss is through the walls of the sheets and the temperatures within the intrusions do not depend on the volumes injected but on the one-dimension sheets emplacement rate. The first sheets injected in a cold crust rapidly cool down and solidify. The ability of successive intrusions to stay at high temperature and eventually build up a long-lived magma chamber is controlled by the emplacement rate. Heat transfer modeling applied in the context of a volcanic arc shows that average emplacement rates of at least several centimeters per year and an incubation time of tens thousands of years are needed for a persistent magma chamber to form. During the incubation time, the intrusions solidify and when a chamber of high melt fraction magma eventually grows, the volume of eruptible magma only form a small part of the total intruded volume. The emplacement rate of plutons is controversial. Geochronological data suggest that some plutons may be emplaced over millions years. For a pluton that is assembled at a slow rate of a few millimeters per year, millions of years are needed, over which kilometric thicknesses are intruded, before a volume of magma larger than the size of a single intrusion becomes mobile and eruptible. In many cases, volcanic products may come from a deep source without being associated with a long-lived upper crust magma chamber. If volcanism is associated with the assembly of plutons over millions years, an active magma chamber only represents a small part of the total intruded volume. The formation of the magma chamber occurs at a late stage in the growth of the pluton unless transient accelerations of the emplacement rate affect the pluton assembly.
DE: 3618 Magma chamber processes (1036)
DE: 8035 Pluton emplacement
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting