EXTENSIONAL PLUTON-EMPLACEMENT MODELS - DO THEY WORK FOR LARGE PLUTONIC COMPLEXES

Shallow-crustal to mid-crustal magma-emplacement models commonly rely on local or regional extension to solve the ''room'' problem. We argue that existing extensional models explain the emplacement of sheeted-d ike complexes or internally layered plutons but are not as easily reco nciled with the emplacement of elliptically shaped plutons or batholit hs for the following reasons: (1) rock strengths and crustal stresses prevent large voids from existing at depth and therefore require wall- rock extension to be balanced by magma influx; (2) rates of extension are controlled by fault slip rates of millimetres to centimetres per y ear and thus require numerous small injections of magma; and (3) becau se of their sheetlike shapes and small thicknesses, these small magma injections will cool rapidly and form sheeted plutons or dike swarms. Extension may, however, facilitate emplacement of elliptically shaped plutons in concert with other mechanisms if certain kinematic constrai nts are satisfied.