THE JACK MAIN INTRUSIVE COMPLEX IN THE LATE CRETACEOUS CENTRAL SIERRA NEVADA, CA: A MIGRATING PLUTON WITHIN A MAGMA FOCUSING REGION

Between ca. 105 to 85 Ma, the central Sierra Nevada underwent regional spatiotemporal magmatic focusing centered on the 95-85 Ma Tuolumne Intrusive Complex (TIC). The Jack Main Intrusive Complex (JMIC) was emplaced 5 km W of the TIC 2 myr earlier along a migration direction opposing both regional arc migration and internal spatiotemporal focusing patterns. We used field mapping (nature of contacts, magmatic fabrics), petrography, LA-ICPMS U-Pb zircon ages, and whole-rock element and Nd and Sr isotope compositions to characterize the petrology and structure of the JMIC to help constrain the nature of a migrating, normally zoned intrusive complex within a regional focusing system.

The JMIC is an overlapping concentrically zoned, NW migrating intrusive complex composed of four plutons younging and increasing in SiO₂ 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). The Kgi (96.81±0.33 Ma) is a quartz diorite (~59 wt. % SiO₂) bearing hornblende on relict pyroxenes. The Kbu is a hornblende-biotite granodiorite (~65 wt. % SiO₂). The Klv (96.22±0.55 Ma) is a locally K-spar porphyritic (<5 cm), hornblende-biotite granodiorite (~68 wt. % SiO₂). The Kbl (93.52±0.37 Ma) is a K-spar porphyritic, biotite-granite (~71 wt. % SiO₂). Whole rock isotope analyses yielded εNd(0) values of -5.5 (Kgi), -6.7 (Kbu, Klv), and -6.9 (Kbl) and Sr(i) values of 0.706549 (Kgi), 0.707048 (Kbu), 0.706724 (Klv), and 0.706813 (Kbl). Thus, the four JMIC units are isotopically related and gradually increase in crustal melt component with time. This implies that co-linear whole-rock major oxide and trace element trends are likely related to two-component (juvenile and evolved) magma mixing, possibly modified by fractional crystallization.

The shifts to more silicic elemental and evolved crustal isotopic values along the JMIC’s migration track are similar to the trends in the normally zoned, nested TIC that grew inward at the center of the focusing system. Like the TIC, the JMIC underwent magma recycling of older units into younger, but in a NW migration direction opposing regional focusing and arc migration. This suggests the chemical and isotopic patterns of these systems are not linked to the map patterns of the plutons or regional arc migration.