THE EMPLACEMENT THE JACKASS LAKES PLUTON, SIERRA NEVADA, CA: A THREE-STEP INCREMENTAL GROWTH MODEL INTO ITS OWN VOLCANIC EJECTA

The Jackass Lakes pluton (JLP) is a 98-97 Ma, c. 3 Kb, 175 km2 granodiorite pluton in the central Sierra Nevada batholith. The JLP consists of diorite, granodiorite and leucogranite (porphyry) units intruding local plutonic and widespread metavolcanic/sedimentary host rock and pendants with the largest portion of leucogranites ponding beneath metavolcanic roof pendants. Overlapping LA-ICP-MS zircon ages, petrologyand initial geochemistry are compatible with a genetic link between plutonic and volcanic units. Strains associated with coupled magmatic and host rock fabrics average c. 50% W-E shortening and c. 70-120% vertical extension. Thus, the JLP provides an excellent field laboratory to explore the incremental growth of an intrusive complex into its own volcanic ejectaduring strong regional shortening and crustal thickening. 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 and evolution of the JLP. The intrusive growth involved three phases: (1) growth of a dioritic to granodioritic “footprint” with leucogranite separation,ductilehost rock shortening, and extensive stoping and roof collapse forming 1000’s 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, stoping, and the collection, potentially by crystal-melt separation during regional shortening, of increasingly largeleucogranitic 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.