BURSTING THE BUBBLE OF BALLOONING PLUTONS - A RETURN TO NESTED DIAPIRS EMPLACED BY MULTIPLE PROCESSES

A popular model for the emplacement of roughly spherical plutons is th at of ''ballooning'' or in situ inflation of a magma chamber, In a com mon version of this model magma ascends until loss of heat or buoyancy causes the outermost magma to crystallize and cease ascent, while the hotter ''tail'' of magma continues to rise and expand the already cry stallized outer margin. This expansion forms a concentric, gneissic to mylonitic foliation and flattening-type strain in the outer margin of the pluton by means of subsolidus deformation and pushes aside the su rrounding country rock to form a dynamothermal aureole that postdates regional structures. Our reexamination of three supposedly ballooned p lutons (Ardara, Ireland; Cannibal Creek, Australia; Papoose Flat, Cali fornia) and evaluation of published descriptions of many others indica te that this model is largely incorrect. Deflections of country-rock s tructures, strains, and porphyroblast-matrix relationships indicate th at only minor to moderate expansion (usually 30% or less) occurred dur ing emplacement, that other emplacement mechanisms must have occurred, and that regional deformation continued during and after emplacement. Internal structures indicate that when magma chamber expansion did oc cur, it did so by flow of magma, that magmatic foliations and lineatio ns formed late in the magma chamber evolution, that enclave shapes are neither good strain markers nor indicators of the magnitude of expans ion, and that only minor internal subsolidus deformation results from emplacement. This study indicates that many plutons previously interpr eted as post-tectonic ballooning plutons are better viewed as syntecto nic, nested diapirs emplaced by a variety of country-rock material-tra nsfer processes. This nested diapir model implies that magma ascent ma y occur by rise of large magma batches (instead of transport in dikes followed by ballooning), that magma chamber dynamics differ from that in the ballooning model, and that normally zoned plutons may form by i ntrusion of several pulses of magma rather than by in situ crystal fra ctionation from a single parent melt.