From crucible through subduction to batholiths

Abstract

There are many lines of evidence to be evaluated in considering the identity and origin of primary magmas; conclusions reached cannot be valid unless they satisfy the constraints imposed by phase equilibrium experiments. Interpretation of laboratory phase equilibrium experiments is not always unambiguous. Experiments in small crucibles in the presence of water under pressure satisfied many petrologists that batholithic granites involved partial fusion of crustal rocks during the culmination of regional metamorphjsm. Then, the concept of subducted oceanic lithosphere provided a relatively high-silica source material for magma generation at mantle depths, and dehydration of the sinking slab provided water to lower the melting temperature of overlying mantle peridotite. Batholiths may include material derived from magmas whose genesis was initiated in all three environments. Phase equilibrium data are available to explore the melting products of subducted micaceous sediments and calcareous oozes trapped within basalt. Limestones or siliceous limestones could escape complete dissociation or melting to considerable depths, possibly for long-term storage in the mantle. Phase relationships in the system basalt-andesite-rhyolite-H_2O through the pressure interval from depths where metamorphosed basaltic ocean crust melts, to the near-surface levels where batholiths are emplaced and andesites are erupted, is fundamental for understanding this magmatic system. On balance, the phase equilibrium data do not favor the concept of primary granite or tonalite from mantle or subducted crust. Primary water-undersaturated granite magma is a normal product of partial fusion of the crust, but temperatures of normal regional metamorphism are too low to generate tonalite liquids. The water content of large batholithic bodies is probably less than 1.5%. Uprise and crystallization produces water-saturated liquids in the upper regions and margins of magma chambers, for satellite intrusions or eruption. Gravity drives magma and energy upward from subducted oceanic crust, and the final, uppermost expression of the process is represented by the batholiths and attendant volcanoes.