William Munson
William Munson

Correlation of volcanic and plutonic activity in convergent margin arcs: Insight from the age and geochemistry of igneous clasts from the Cretaceous forearc basin Rosario and Valle Groups of the Peninsular Ranges magmatic arc of southern and Baja California

William Munson
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. David Kimbrough

May 8th, 2013, 2:30pm
CSL 422, 11:20am
watch Will’s defense here

Continental arc magmatism is a fundamental mechanism of continental growth. Understanding the igneous plumbing system of arcs requires an integrated view of volcanic and plutonic evolution. Studies of convergent margin arcs however typically focus on either modern volcanic arcs or the deeply eroded batholith underpinnings. There are few settings on earth that provide extensive well-preserved supracrustal volcanic-sedimentary sequences together with their contemporaneous plutonic underpinnings.
This project is aimed at refining our understanding of volcanic-plutonic connections in magmatic arcs by linking the in situ history of the Cretaceous Peninsular Ranges batholith to the forearc basin sedimentary record. Age, petrology and denudation history of PRB basement rocks are well-established. The batholith is divided into two distinctly different belts. An older western belt dominated by gabbro, tonalite and monzogranite with zircon U-Pb ages of ~140-100 Ma; and a younger eastern zone dominated by La Posta-type high Sr/Y tonalite-trondhjemite-granodiorite intrusives with zircon U-Pb zircon ages of ~100-90 Ma. The older western belt preserves an extensive partially eroded volcanic-sedimentary cover along its western edge referred to as the Santiago Peak Volcanics (SPV) in the north and the Alisitos Group in the south. The younger eastern belt is much more deeply eroded so that the volcanic-sedimentary cover of the batholith has been completely stripped away.
The focus of this study is volcanic clasts from forearc basin conglomerates that are thought to represent the coeval-cogenetic cover of the eroded batholith. New whole rock major- and trace-element geochemistry (n = 208) and laser ablation ICPMS and TIMS zircon U-Pb ages (n = 24) are reported here for clasts of Cretaceous Rosario and Valle Groups strategically sampled over a 700 km stretch from the northern Santa Ana Mountains, California, USA to the Vizcaino Peninsula in Baja California, Mexico. While a primary emphasis was placed on sampling volcanic clasts, a smaller population of plutonic clasts were also collected and analyzed.
Zircon U-Pb ages of forearc basin plutonic (n=6) and volcanic clasts (n=18) range from 137.7 ± 2.0 to 95.1 ± 1.2 Ma closely mirroring the intrusive history of the batholith and confirming a coeval-cogenetic relationship. Zircon U-Pb ages from the 18 Late Cretaceous Rosario Group volcanic clasts range more narrowly from 123.5 ± 2.0 to 95.1 ± 1.2 Ma, and 15 of the volcanic clast ages are 107.7 ± 2.0 to 95.1 ± 1.2 Ma. In situ SPV samples from this segment of the batholith are older and range in age from ~130-110 Ma. The young clast ages suggests that a large proportion of the Rosario Group volcanic clasts were derived from more easterly parts of the batholith. The distinctive high Sr/Y signature of the eastern La Posta-type batholith was not detected in any of the samples. Collectively however, the volcanic clasts are slightly more alkaline compared to the SPV.
In detail, clast suites from different stratigraphic horizons and locations in the Cabrillo Formation in San Diego and the Baker Canyon conglomerate in the Santa Ana Mountains are distinguishable in terms of whole rock composition. This may reflect progressive erosion in catchment areas of individual drainages feeding submarine fans and/or migration or progressive expansion of catchment areas over time.