MAPPING OF THE JACKASS LAKES VOLCANIC-PORPHYRY-PLUTONIC COMPLEX IN THE CENTRAL SIERRA NEVADA

The 99-97 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 and the subvolcanic porphyries of the ca. 101-97 Ma Merced Peak Caldera sequence (Peck, 1980; McNulty et al., 1996; Pignotta et al., 2010). The JLP was originally mapped entirely by Peck (1980) at 1:62,500 scale and locally by McNulty et al. (1996), Pignotta et al. (2010), and Krueger and Yoshinobu (2016). No petrologic study has yet been done, which is the purpose of this mapping.

The original units of the JLP include the main, medium to coarse-grained porphyritic hornblende-bearing biotite granodiorite (Kj), the medium-grained equigranular hornblende biotite granodiorite (Kja, Anne Lake phase), which is 10-15% more mafic than the Kj, the fine-grained biotite leucogranite of the Post Peak porphyry (Kpp), and the dacitic to rhyolitic, lithic-bearing crystal tuff (Km) from the associated metavolcanic pendants.

In summer 2022, ~25 km² of these units in the north- and west-central JLP were mapped at 1:10,000 scale as part of an USGS EDMAP funded grant. The mapping revealed new JLP granodiorite phases and redrew some contact lines mapped by Peck (1980). The new granodiorite phases include Anne Lake II (Kja2), Rutherford Lake (Kjr), and Fernandez Pass (Kjf). The characteristic differences include variations in volume % mafic minerals, grain size, and texture. XRF whole-rock elements and petrography underway will examine the petrologic connection between these units.

The nature of contacts between JLP phases varies. While Kj-Kja, Kj-Kja2, and Kj-Kjr contacts are sharp, Kja-Kja2 and Kja-Kjr are gradational over ~30 ft. Kjr appears to grade into Kjf, but the contact is not preserved. Contacts between JLP, Post Peak porphyry, and metavolcanics are sharp and non-gradational. Magmatic foliations in the JLP strike N-NW, in line with previous studies by McNulty et al. (1996) and Pignotta et al. (2010).

The field maps are currently getting digitized in ArcGIS using the USGS GeMS format to make them compatible with digitized FEDMAP and STATEMAP maps. In addition, the new findings from our detailed mapping aid in researching pluton growth and evolution and the volcanic-plutonic connection.