A COMPOSITE VIEW OF MAGMA ORGANIZATION AND EMPLACEMENT MECHANISMS FROM THE UPPER CRUST OF ARGENTINA AND THE MIDDLE CRUST OF NORWAY

In thickened orogens, do crustally-derived batholiths originate in aerially extensive migmatite terranes? How do magma emplacement mechanisms change with depth? These questions can be addressed where exposures provide direct spatial and temporal links between large batholiths and regional migmatite terranes and 3-D constraints on structural geometry can be made. We present two examples of magma chamber assembly from the upper crust of the Argentine Precordillera and the middle to lower crust of the Norwegian Caledonides. The 470 Ma Vega pluton (Norway) is a 25x15 km S-type granodiorite that intrudes folded metasedimentary host rocks. The pluton contains volumetrically significant amounts of metasedimentary xenoliths and mafic magmatic enclaves. The pluton solidified during a regional migmatization event at 480-470 Ma. Evidence for melting within the pluton is seen in large composite sheets of intermingling diatexite and granodiorite. It is hypothesized that diatexite within adjacent metasedimentary host rocks was generated by heat from the Vega pluton. Evidence for local host rock melting and the large amount of intact metasedimentary xenoliths, lithologically similar to the host rocks, suggest that the Vega pluton has not migrated significantly from its source. The Chita granite pluton (Argentina) was emplaced at <2 km depth into previously folded, weakly metamorphosed mudstones and sandstones of the Agua Negra Formation. Nearly 2 km of vertical relief and complete wall to roof rock exposures display a severely discordant intrusive contact. No evidence of significant emplacement-related ductile strains is evident in the host rocks. Xenoliths occur but are not volumetrically significant. We infer that these two systems display a composite view of the crustal melt generation/ascent/emplacement process in orogens that have undergone significant crustal thickening. The Chita pluton represents the most fractionated portions of the magma column and was emplaced via mechanisms such as stoping, diking, and/or cauldron subsidence. The Vega pluton reflects the early organization of crustally-derived magmas that entrained xenoliths during the melting/transport event. We speculate that the Vega pluton may provide a mid-crustal analog for the physical processes that generated shallow crustal plutons such as the Chita granite.