THE SOURCE-TO-PLUTON CONNECTION IN OROGENS: INSIGHTS FROM INTRUSION SIZE AND SPATIAL DISTRIBUTION

Granitic intrusions are important tectonic elements of the mid- to upper crust of all orogens, and their emplacement represents a major heat and material transport process. The size, shape and spatial distribution of plutons in orogens likely reflects the degree and distribution of partial melting in the lower crust and upper mantle, which in turn is ultimately controlled by factors such as the fertility of the source rocks and the thermal structure and history of the orogen. The location and size of granitic intrusions in the mid- to upper crust is governed by the self-organization of melt and heat transport within and away from the source region. Superimposed on this intrinsic or background control are local magma-channeling and space-making processes due to regional deformation (i.e., dilation due to fault/shear zone displacements, folding, etc.). Constraints on the degree of melting in the lower crust of ancient orogens can be estimated knowing the volumes of exposed plutons and their aerial extent. Calculations of pluton volumes are possible because most intrusions are tabular to wedge shaped and their horizontal width (or area) scales with their vertical thickness according to an empirical power law. To a first approximation the total volume of granite in an area can be equated to the amount of melt extracted from the source region and the rates of melt withdrawal, transport and emplacement must be balanced at the crustal scale. If the material in a given intrusion is derived from a source region whose horizontal area is equal to the corresponding pluton then the required total degree of partial melting must be high (>25%) and/or the source must be much thicker than the pluton (e.g., up to 11 times thicker for 10% melting). This situation corresponds to that in magmatic arcs, where closely spaced plutons emplaced in narrow belts form by high degrees of localized lower crustal melting induced by mantle derived underplating. If melt is scavenged from a much wider region, then lower degrees (<10%) of partial melting and thinner sources (<2 times pluton thickness) are required. This corresponds to plutonism in collisional orogens and Archean granite-greenstone belts, in which low degrees of partial melting over wide areas of the lower crust produced intrusions with a characteristic spacing (0.5 to 3 times the average pluton radius).