Hypothesis for the origin of convergent margin granitoids and Earth’s continental crust by thermal migration zone refining

摘要

The production of Earth’s granitoids is generally attributed to magma intrusion, fractional crystallization and assimilation but the details of how granitoid plutons form remains widely debated. In light of recent experimental results which indicate that partially molten wet andesite in a temperature gradient evolves into a granitic bulk composition at the cooler end of the gradient (in a process called thermal migration), I present a model for at least some of Earth’s granitoids forming by a top-down thermal migration zone refining process. According to this model, convergent margin igneous activity builds a thick volcanic pile which becomes a barrier to further ascent of magma, leading to magma underplating by injection of sills at the base of the pile. When magmas arrive at the location of underplating, they react and release heat and water to the overlying materials (previously intruded sills), resulting in a downward moving zone having a near-steady-state temperature gradient. This leads to compositional differentiation by wet thermal migration taking place over million year time scales; this in situ differentiation process occurs in the middle of the underplated region but not on the more rapidly cooled edges of the sills. Modeling using the IRIDIUM program shows this process can produce sequences of granitoid that are kilometer or greater in thickness; regardless of granitoid thickness, the bottom of the system maintains a near constant thickness of hornblende gabbros. The model provides a logical connection between andesitic stratovolcanoes and underlying, more silicic intrusive series plutons—both reflect ascent of andesitic composition magmas, with the implication that convergent margin magmatic systems evolve temporally from stratovolcanoes to plutons once magma ascent is inhibited and underplating begins. The model provides an alternative to the standard view that granitoids result from cooling of large bodies of magma and could help to resolve long-standing questions concerning: geophysical observations of magma chambers; the compositions of minerals in granitoids; and the development of preferred mineral orientations in granitoids. It provides a consistent model in that it explains the systematic normal compositional zoning of plutons within the context of an incremental growth process dictated by geochronology. Most importantly, the model is predictive, emphasizing the importance of examining granitoids in the vertical dimension. The hypothesis that thermal migration plays a role in granitoid formation can be tested by analysis of non-traditional stable isotope systems such as Fe, Mg and Si that should show a signature of thermal diffusion. The modelpredicts that the tops of overlying granitoids will have relatively heavy isotopic compositions whereas underlying hornblende gabbros will have relatively light isotopic compositions. Examination of existing iron isotope data and new silicon isotope data are consistent with the hypothesis and point to the need for more thorough testing.