GROWTH AND INTERNAL EVOLUTION OF THE BOX SPRINGS PLUTONIC COMPLEX, PENINSULAR RANGES BATHOLITH, RIVERSIDE – CALIFORNIA

The Box Springs Plutonic Complex (BSPC), in the Western Transition Zone of the Peninsular Ranges Batholith, features a core of massive high-silica biotite tonalite surrounded by foliated tonalites and granodiorites. We present an updated geological map, petrology, geochronology, and structural analysis, to elucidate the formation and internal development of the BSPC.

The BSPC has an elongated elliptical shape trending NW to SE. Four main units have been identified: the central zone comprises biotite tonalite (Kbt) and heterogeneous granite (Kbfg), both with similar SiO₂ from 70 to 76 wt% and low K₂O from 1 to 2 wt%. The edges of the pluton consist of heterogeneous biotite tonalite (Kbht) with lower SiO₂ values of 65 to 76 wt% and higher K₂O from 3 to 4.5 wt% as well as porphyritic granodiorite and granite (Kbgg) with higher SiO₂ from 72 to 78 wt% and K₂O from 3 to 5 wt%. Additionally, biotite-hornblende tonalite (Kbft), and heterogeneous porphyritic granodiorite and tonalite (Kbhg) represent hybrid zones. The internal contacts are mostly gradational with evidence of magma mingling, schlieren, and compositional layering.

The host rock (Kprb) is a Cretaceous tonalite exposed in only two localities, one in the south and in the east. Kprb is characterized by low SiO₂ values of 52-72 wt% and enrichment in CaO, MgO, Fe₂O₃, Al₂O₃, compared to the BSPC.

Zircon U-Pb ages indicate that all the units were emplaced simultaneously within error. Kbgg from 100 to 103 Ma, Kbht at 100 Ma, Kbfg at 99 Ma, and Kbt from 98 to 99 Ma.

All the units, except for the central one (Kbt), are arcuate in plan-view and have an overprinted inward-dipping magmatic and locally subsolidus foliation, a feature distinct from the regional foliation pattern, which is NW-SE striking. This suggests that the units are discordant intrusions in the form of cone sheets.

We propose that the BSPC resulted from the emplacement of at least three intrusions: The Kbgg intruded first, followed by the simultaneous intrusion of Kbht and Kbfg. Finally, the growing and overpressured magma chamber created conditions required to intrude the upper cone sheets and Kbt in the center, which may explain its lack of the magmatic foliation compared to the other units. Older units experienced this overpressure and developed a similar overprinting foliation pattern as suggested by Karlstrom (2009).