THREE-STEP EMPLACEMENT MODEL FOR THE JACKASS LAKES PLUTON, SIERRA NEVADA, CA: THE CONSTRUCTION OF A PLUTONIC "FOOTPRINT" AND SUBSEQUENT INCREMENTAL GROWTH

The Jackass Lakes pluton (JLP) is a 98-97 Ma, 3 Kb, 175 km2 granodiorite pluton in the central Sierra Nevada batholith, consisting of diorite, granodiorite and leucogranite (porphyry) units intruding local plutonic metavolcanic/sedimentary host rock and pendants. Overlapping LA-ICP-MS zircon ages, petrology and initial geochemistry are compatible with a genetic link between plutonic and volcanic units. Strains associated with coupled magmatic and host rock fabrics average 50% W-E shortening and 72-123% vertical extension. New 1:10,000 scale mapping of both plutonic and host units was combined with m-scale analyses, 3D cross section construction, fabric and strain measurements, petrography, U-Pb zircon dating, and XRF whole rock and isotopic geochemistry to help constrain emplacement of the JLP. The intrusive growth involved three phases: (1) growth of a dioritic to granodioritic “footprint” with limited leucogranite separation, and ductile host rock shortening, and extensive stoping and roof collapse forming thousands of stoped blocks and several large roof pendants; (2) intrusion of smaller granodioritic units into the footprint by magma wedging, widespread recycling of older into younger units, more limited stoping, and the collection, potentially by crystal-melt separation during regional shortening, of increasingly large leucogranitic sheets and batches; and (3) continued engulfment of xenoliths and mushy cognate inclusions and formation of leucogranites. Thus, the JLP records a complex emplacement history (during the downward displacement of the overlying volcanic pile) involving multiple material transfer processes that shifted from displacement of host rock (to form the plutonic footprint) to internal magmatic processes, which displaced or recycled the footprint.