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Magma chamber formation by dike accretion and crustal melting: 2D thermal model with emphasis on zircon record
  • Oleg Eduardovich Melnik,
  • Ivan S Utkin,
  • Ilya N. Bindeman
Oleg Eduardovich Melnik
Institute of Mechanics, Moscow State University

Corresponding Author:[email protected]

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Ivan S Utkin
Institute of Mechanics, Moscow State University
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Ilya N. Bindeman
University of Oregon
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Rapid progress in investigation of zircon records for U-Th-Pb ages and O and Hf isotopes in igneous rocks require understanding how magma bodies are formed and evolve in the crust. We here present a 2D model of magma bodies formation in granitic crust by injection of rhyolitic or andesitic dikes and sills. We combine this model with our zircon crystallization/dissolution software and compute zircon survival histories in individual batches of magma and country rocks.
Simulations reproduces incremental accumulation of intruded magma into magma chambers generating eruptible and interconnected magma batches with melt fraction >50 vol% that form in clusters. The rate of melt production is highly variable in space and time. The volume of eruptible melt strongly depends on the input rates of magma Q and the width W of the dike injection region. For example, dikes injection with Q=0.25 m3/s with W=500 m during 4 ka generate 20 km3 of melt while no significant melt forms if W=4 km. Injection of andesitic dikes produces only slightly more melt than rhyolite to granite injection despite of much larger thermal input.
Due to rock melting most of zircons loose significant portion of their old cores and, thus, average age. Magmatic zircons in the periphery of the intrusion form very quickly while in its central part crystals contain old cores and young rims and can grow during several hundreds of ka. We observe diverse proportions of crustal melt/newly intruded magma, which translates into diverse O and Hf isotope distribution in zircons.