ASSEMBLY OF A DIKE-FED MAGMA CHAMBER - THE JACKASS LAKES PLUTON, CENTRAL SIERRA-NEVADA, CALIFORNIA

The mechanisms of ascent, assembly, and emplacement of granitic magma in the crust are critical to understanding the dynamics of continental margin growth, get these mechanisms remain controversial and poorly u nderstood. Detailed study of structural and petrologic fabrics in the middle Cretaceous Jackass Lakes pluton-wall-rock system, central Sierr a Nevada, California, coupled with U-Pb geochronology, indicates that the pluton formed via sheet-like assembly of a dike-fed magma chamber, Final emplacement of the pluton was facilitated by multiple brittle a nd ductile mechanisms that were active at different times and places w ithin the system; this supports hybrid viscoelastic emplacement models as realistic alternatives to end-member models (i.e., dike versus dia pir), Fracture propagation, which initiated approximate to 40% of the space required for emplacement, may have been facilitated by a small c omponent of are-parallel dextral shear that produced north-northwest-s triking tension gashes, A combination of ductile wall-rock shortening during lateral expansion of sheets, and return flow of elongate, stron gly deformed wall-rock septa, produced an additional approximate to 25 % of the space required. Other mechanisms, including coeval formation of the overlying Minarets caldera and stoping in the subvolcanic part of the magma chamber, must account for the remaining approximate to 35 % space, implying that vertical transfer of material is an important e mplacement mechanism at shallow crustal levels.