THE JACK MAIN INTRUSIVE COMPLEX: AN OUT-OF-SEQUENCE MIGRATING PLUTON FORMED DURING LATE CRETACEOUS MAGMATIC FOCUSING, CENTRAL SIERRA NEVADA, CA

The central Sierra Nevada (CSN) underwent eastward migration (140 to 85 Ma) and local spatial magmatic focusing (ca. 105 to 85 Ma) centered on the NE Tuolumne Intrusive Complex (TIC). The Jack Main Intrusive Complex (JMIC), located west of the TIC in the focusing region, is composed of four units, younging from SE to NW: Quartz Diorite of Mount Gibson (Kgi), Granodiorite of Bearup Lake (Kbu), Granodiorite of Lake Vernon (Klv), and Granodiorite of Boundary Lake (Kbl). We are exploring compositions (petrography, whole rock XRF), structures and contacts (field mapping), and age (LA-ICPMS U-Pb zircon geochronology) of each unit.

The Kgi is a gray, medium grained, equigranular quartz diorite containing sparse hornblende phenocrysts with relict pyroxene cores. The Kgi forms a sharp contact with the Kbu, a light, medium grained, equigranular granodiorite containing subhedral hornblende and biotite. The Kbu is in gradational contact with Klv over 10s of meters, seen in increasing hornblende size and decreasing mafic mineral abundance in the Klv. The Klv is a light, medium grained, locally K-feldspar porphyritic (<5 cm) granodiorite with characteristic euhedral hornblende and biotite giving it a spotty appearance. Laser ablation ICP-MS U-Pb zircon geochronology yielded a crystallization age of 96.73±0.96 Ma. The Kbl is a light, medium grained, locally porphyritic biotite granodiorite to granite. Kbu and Klb display mingling with fine-grained quartz diorite magmas. Enclaves range from globular to thin and are parallel to concurrent N-S and E-W magmatic foliations. Our initial fieldwork, overall map pattern, and the relative ages of units suggest that the JMIC migrated during emplacement towards WNW (309°). This migration direction is opposite to west-east arc migration (071°) and to the CSN inward focusing (055°) during the late Cretaceous. However, similar migration patterns are also seen in the large John Muir and Whitney intrusive suites to the south of the TIC with migration directions of 323° and 130°, respectively. The unusual orientation and lower-order scale of the JMIC migration leads us to consider driving mechanisms localized within the crustal column.