INVESTIGATING THE PETROLOGIC LINKS OF THE JACKASS LAKES VOLCANIC-PORPHYRY-PLUTONIC COMPLEX, SIERRA NEVADA BATHOLITH

The 98-97 Ma Jackass Lakes pluton (JLP) in the central Sierra Nevada batholith is a well-exposed 175 km² resurgent pluton of largely granodiorite and minor diorite that intruded into contemporaneous dacitic and rhyolitic volcanic ejecta (Km) and the Post Peak subvolcanic porphyries (PPP) of the ca. 101-97 Ma Merced Peak Caldera sequence (McNulty et al., 1996; Pignotta et al., 2010). These ages show that all three units are coeval, allowing for a petrologic study and comparison of the volcanic-porphyry-plutonic units. More LA-ICPMS U-Pb zircon geochronology of all units is underway, including the not yet dated PPP.

We are testing the petrologic relationship of the JLP volcanic-porphyry-plutonic complex with the following hypotheses: 1) the JLP granodiorite is compositionally complementary to the more felsic PPP leucogranite and meta-rhyolites that formed from melt-extraction from the magma reservoir leaving behind crystal cumulates. Or, 2) all three units may be compositionally the same (equivalent).

Mapping of ~25 km² of the north- and west-central JLP at 1:10,000 scale included the main porphyritic hornblende-bearing biotite granodiorite (Kj), the equigranular hornblende biotite granodiorite (Kja, Anne Lake phase), the PPP biotite leucogranite, and the dacitic to rhyolitic, lithic-bearing crystal tuff (Km). The mapping redrew some contact lines mapped by Peck (1980) and revealed new granodiorite phases of the JLP: Kja2 (type of Anne Lake phase), Kjr (Rutherford Lake granodiorite), and Kjf (Fernandez Pass granodiorite). The characteristic differences include variations in volume % mafic minerals, grain size, and texture. Kj stoped blocks are found in the minor phases. Mafic enclave swarms are found exclusively in the Kj, along with stoped blocks of Km and PPP. The nature of contacts between JLP phases varies. While Kj-Kja, Kj-Kja2, and Kj-Kjr contacts are sharp, Kja-Kja2, Kja-Kjr, and Kjr-Kjf are gradational over ~30 ft. Contacts between JLP, PPP, and Km are sharp. Magmatic foliations in the JLP strike N-NW (McNulty et al., 1996; Pignotta et al., 2010).

Petrography and XRF bulk rock element analyses will further test the two hypotheses by examining the thin sections for hints of crystal accumulation and determining if the PPP and Km units are fractionates from the JLP, or the same, or a combination of the two models.