CAUSES OF MAGMA COMPOSITIONS AND EXTENT OF MAGMA-MAGMA INTERACTION IN THE JACKASS LAKES PLUTON, CENTRAL SIERRA NEVADA, CA

The 98-97 Ma Jackass Lakes Pluton (JLP) is a 13 km by 17 km rectangular pluton that offers a record of the growth and magma evolution of the upper magma plumbing of the Sierra Nevada batholith. It formed 2 myrs before the 95-85 Ma, 1,100 km² Tuolumne intrusive complex (TIC) to the north, which grew during a time and in a region undergoing magma focusing and produced large magma mush bodies after a short, initial time of magmatic sheeting. It is crucial to test whether the initial TIC sheeting went further back in time in the TIC periphery to understand the length scales and timing of arc focusing and maturation: While McNulty et al. (1996) proposed extensive diking for the growth of the JLP, Pignotta et al. (2010) described an early diking stage followed by the assembly of magmas into irregular bodies via magma mixing/mingling. To test these hypotheses and define the extent of magma-magma interaction and thus magma body sizes, the granodiorites of the west central part of the JLP were mapped at 1:10,000 scale and petrologically examined.

The main phase of the JLP (Kj) is a K-feldspar porphyritic biotite granodiorite. It is intruded by the Anne Lake hornblende biotite granodiorite (Kja) via sharp contacts. Two hornblende-bearing biotite granodiorite units have gradational contacts with each other and the Kja. The finer grained, more mafic Kja grades into the coarser grained, less mafic Rutherford phase (Kjr) to the west. Further west, the Kjr grades into the Fernandez Pass phase (Kjf), which contains coarser grained, more euhedral hornblende and biotite. The compositions and gradational contacts between these units indicate that they may represent different magma batches injected rapidly to form an interconnected magma chamber ~ ≥4 km east to west. The Kj, which has enclave swarms and stoped blocks, is older than Kja/Kjr/Kjf based on crosscutting relationships, including stoped blocks of Kj found in the latter and the younging direction of crosscutting schlieren layers. LA-ICPMS U-Pb zircon geochronology in progress will determine age relations between Kj and Kja/Kjr/Kjf. Petrography, whole rock XRF major oxide and LA-ICPMS trace and rare earth element analyses in progress will test for the importance and extent of fractionation and mixing/mingling processes that were responsible for the petrologic differences between all JLP phases.