INTRUSIVE HISTORY OF THE ASH MOUNTAIN COMPLEX, SEQUOIA NATIONAL PARK

The Ash Mountain Complex (AMC), in southwestern Sequoia National Park, is a series of granitoid rocks, ranging from gabbro to peraluminous granite, that mutually cross cut and have been an enigma of Park geology since first described by Ross (1958). The compositional and intrusive diversity in the Ash Mountain Complex bears on the relationship of high- and low-SiO₂ magmas that are commonly associated in the Sierra Nevada batholith.

New fieldwork identifies two sequences of intrusion in the AMC. Cross cutting relationships indicate the order of intrusion within the first sequence is gabbro, black diorite, white diorite and early granite (Fry’s Point granite). This earlier series is cross-cut by a fine-grained granodiorite porphyry and late granitic pegmatite dikes. The contact between the porphyry and the Fry’s Point granite is sharp and the porphyry contains angular inclusions of diorite and granite, which suggests that the first sequence was mostly solid at the time of intrusion of the second. The contact between the porphyry and pegmatites, often gradational, is consistent with late fluid exsolution from the porphyry.

New SHRIMP-RG U/Pb analyses yield ages of ca. 105 Ma for the gabbro, black diorite, and Fry’s Point granite and an age of ca. 103 Ma for the porphyry. The overlap in ages at 105 Ma indicates simultaneous intrusion of the first AMC sequence. Simple concentric zonation in cathodoluminescence (CL) images of the zircons and the tight clustering of the U/Pb ages suggest that the zircon populations of the gabbro and diorite are relatively homogeneous and lack inherited cores. The zircon populations of the porphyry and Fry’s Point granite commonly contain inherited cores. Titanium and hafnium trace element analysis (which indicate temperature and degree of fractionation of the magma) indicate that both of the diorites cooled quickly experiencing little fractionation of Hf, perhaps quenched by contact with the cooler Fry’s Point granite. The porphyry crystallized at an even lower temperature, consistent with field observations.

Overall, the results show sources produced diverse magma types that intruded simultaneously into mid-crustal levels, but that thermal contrast and, potentially, intrusion rate did not allow significant magma mixing.