The 'space problem' in the emplacement of granite

Abstract

The space problem in granite emplacement has received an increasing amount of attention in recent years, particularly with the application of structural geology techniques and measurements of magnetic fabrics in plutons. Of the classical emplacement mechanisms, cauldron subsidence and stoping have received little challenge. Diapiric emplacement and ballooning (in-situ inflation at the emplacement site) may generate similar structural effects and there is potential for confusion. Diapirism as a mechanism of transport and ascent of granitic magmas has been seriously questioned and alternative hypothesies, involving dyking ascent, are well developed. Pluton emplacement models involving dilation along major faults and shear zones are now being widely applied particularly in transcurrent and extensional settings. Emplacement in active contractional structures has been documented in a small number of cases. In terms of general models there is an increasing belief that granitic magma can be emplaced in a great variety of ways, and that this reflects a set of complexly interlinked, and as yet not fully understood, controls. A single, simple underlying theme may be that granite is fundamentally syntectonic and occurs mainly in convergent orogenic settings. As a fluid it will contribute to, and become part of, the regional (effective) stress in the orogen. This suggests that in the weak lower crust much of the strain of emplacement is, like the fluid pressure itself, shared regionally, i.e. the space problem is regionally solved. In the mid crust rheological and structural heterogeneities may strongly modify, but will not fundamentally alter, this situation.