Magma Chamber Formation by Dike Accretion and Crustal Melting: 2D Thermo-compositional Model with Emphasis on Eruptions and Implication for Zircon Records

Formation of large magmatic eruptible bodies beneath active volcanoes or plutons in the Earth crust involves complicated thermo-mechanical processes related to magma injection, host rock melting, solid-melt separation, etc. Revealing enigmas requires joint efforts of geologist, geophysics and geomechanisists. We present a 2D model of magma body formation in granitic crust by injection of rhyolitic or basaltic dikes and sills. An elastic analytical solution enables computation of rock displacement in response to magma intrusion. Phase diagrams for magma and host rocks predict melting/crystallization. We combine this model with our zircon crystallization/dissolution software and compute zircon survival histories within individual batches of magma and country rocks. Incremental accumulation of intruded magma generates interconnected magma batches of eruptible melt with melt fractions >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 injection region of dikes and on eruptions. For example, dikes injection with Q=0.125 m3/s with W=5 km during 100 ka generates ~50 km3 of eruptible melt while no significant melt forms if W=10 km. Injection of basaltic dikes produces more melt for the same flux rate. Frequent and small eruptions led to smaller magma bodies that are located deeper in the system, while systems with rare but voluminous eruption forms large melt. Due to partial melting, most host rock zircons loose significant portion of their old cores and, thus, their average age is reduced. Magmatic zircons in the periphery of the intrusion form very quickly due to rapid dikes cooling while in its central part crystals contain old cores and young rims and can grow during several hundreds of ka. The code is available for user specific situations for anyone with minimal programming skills by changing magma flux rates, magmatism duration, compositions, depths to match measured zircon record that can be constrained based on geophysical and petrogeochronologcal observations. Figure. Stages of growth of the magma body for 100 ka at different magma flux rates by injection of dikes and sills without eruptions. White contours show regions with eruptible magma (melt fraction > 50 vol%)