ABSTRACT This guide begins with an overview of the internal structure and petrology of the Catalina Schist terrane as exposed on Santa Catalina Island, California, followed by a discussion of the tectonic setting and exhumational history of the terrane, and the Cenozoic tectonic and geological evolution of the Inner Borderland, within which it lies. The guide then presents an itinerary for a three-day field trip from 9–11 May 2020. Next, we present a tectonic model for the formation of the Catalina Schist, followed by a discussion of its relationship to the Pelona, Orocopia, Rand, and related schists in southern California. This field trip generally follows the GSA guide published in GSA Field Guide 59 (available at https://pubs.geoscienceworld.org/gsa ): Platt, J.P., Grove, M., Kimbrough, D.L., and Jacobson, C.E., 2020, Structure, metamorphism, and geodynamic significance of the Catalina Schist terrane, in Heermance, R.V., and Schwartz, J.J., eds., From the Islands to the Mountains: A 2020 View of Geologic Excursions in Southern California: Geological Society of America Field Guide 59, p. 165–195, https://doi.org/10.1130/2020.0059(05) .

ABSTRACT This guide begins with an overview of the internal structure and petrology of the Catalina Schist terrane as exposed on Santa Catalina Island, California, followed by a discussion of the tectonic setting and exhumational history of the terrane, and the Cenozoic tectonic and geological evolution of the Inner Borderland, within which it lies. The guide then presents an itinerary for a three-day field trip from 9–11 May 2020. Next, we present a tectonic model for the formation of the Catalina Schist, followed by a discussion of its relationship to the Pelona, Orocopia, Rand, and related schists in southern California.

The Santiago Peak volcanics, in the northern Santa Ana Mountains, are the northernmost exposures of the Santiago Peak–Alisitos magmatic arc present along the western edge of the Peninsular Ranges batholith. Remnants of deeply eroded volcanic sequences in the Santa Ana Mountains consist of subaerial basaltic-andesite to rhyolite lavas, rare basalt, welded tuff, and pyroclastic rocks that were emplaced across deformed Middle Jurassic turbidites. Subalkaline lavas have mixed calc-alkaline and tholeiitic affinities. Relatively primitive ε Sr (−18 to +5) and ε Nd (+7.5 to +0.1) values for the lavas plot along the mantle array. Silicic lavas have higher ε Sr and ε Nd values in comparison to mafic lavas. Parental magmas were derived from hydrous melts of relatively depleted mantle wedge, followed by fractionation and the assimilation of up to 10% crustal materials. The whole-rock compositions, isotopic data, and U/Pb and 40 Ar/ 39 Ar ages (128–110 Ma) of the Early Cretaceous Santiago Peak volcanics and related Estelle Mountain volcanics overlap with emplacement ages of plutons of the western zone of the Peninsular Ranges batholith. The volcanic rocks are interpreted as the volcanic component of the arc plumbing system of the batholith. Arc rocks are in turn unconformably overlain by a forearc sequence of Upper Cretaceous through Tertiary strata that indicate deep erosion of the Santiago Peak volcanics by 95 Ma. Volcanic clasts of Turonian age within the forearc sequence yield U/Pb ages of 108–106 Ma. Age data and whole-rock geochemistry of the volcanic clasts indicate that they were eroded from supracrustal volcanic rocks located farther east within the Elsinore block.

Magma mixing was an important process in the genesis of plutonic suites of the Peninsular Ranges batholith, San Diego County transect. Contrary to expectations, minimum Hf arc mantle model ages (Hf TAM ) calculated from Lu-Hf spot analyses of zircon from 15 granite samples and one gabbro sample indicate a Neoproterozoic component in granites from the western zone of the batholith and even older crustal components, including a Paleoproterozoic component, in those from the eastern zone. The delineation between western and eastern zones in the San Diego County transect of the batholith corresponds closely with a rapidly formed suture zone marked by the western limit of Jurassic S- and transitional I-S-type granites, magnetic and gravity anomalies, and the δ 1 8 O gradient. Zircon U-Pb ages, many reported herein for the first time, indicate that Early Cretaceous I-type plutons were emplaced into the western zone of the batholith and stitched across both the suture zone and the central belt of deformed Jurassic S-type and I-S-type granites. I-type plutons that intruded east of the suture zone are mainly Late Cretaceous in age. Zircon U-Pb ages, measured as much as possible from the same grains used for 176 Hf/ 177 Hf analyses, not only provide a record of crystallization ages but also of the degree of zircon inheritance—of which there is little for Cretaceous western-zone I-type granites. The variation in 176 Hf/ 177 Hf (εHf (t) ) values for the population of zircon grains from each plutonic sample is therefore interpreted to reflect the degree of magma mixing between crustal- and mantle-derived components between the time of melt generation and final pluton construction, a process that can only be reconciled with open-system chemical behavior. We consider the process of formation of the short-lived suture zone and the S-type granites of the Peninsular Ranges to be examples analogous to the short lived Bundarra Supersuite of the New England batholith (Jeon et al., 2012). The new Hf data of this study are compared to published Nd-Sm model age data for the Peninsular Ranges batholith and to new zircon Hf data for the Tuolumne intrusive suite of the Sierra Nevada batholith.

Improved depositional age constraints and stratigraphic description of rocks in San Diego require designation of a new Upper Jurassic formation, herein named the Peñasquitos Formation after its exposures in Los Peñasquitos Canyon Preserve of the city of San Diego. The strata are dark-gray mudstone with interbedded first-cycle volcanogenic sandstone and conglomerate-breccia and contain the Tithonian marine pelecypod Buchia piochii. Laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon 206* Pb/ 238 U ages of 147.9 ± 3.2 Ma, 145.6 ± 5.3 Ma, and 144.5 ± 3.0 Ma measured on volcaniclastic samples from Los Peñasquitos and Rancho Valencia Canyons are interpreted as magmatic crystallization ages and are consistent with the Tithonian depositional age indicated by fossils. Whole-rock geochemistry is consistent with an island-arc volcanic source for most of the rocks. The strata of the Peñasquitos Formation have been assigned to the Santiago Peak volcanics by many workers, but there are major differences. The Peñasquitos Formation is marine; older (150–141 Ma); deformed everywhere and overturned in places; and locally is altered to pyrophyllite. In contrast, the Santiago Peak volcanics are nonmarine and contain paleosols in places; younger (128–110 Ma); undeformed and nearly flat lying in many places; and not altered to pyrophyllite. The Peñasquitos Formation rocks have also been assigned to the Bedford Canyon Formation by previous workers, but the Bedford Canyon is distinctly less volcanogenic and contains chert, pebbly mudstones, and limestone olistoliths(?) with Bajocian- to Callovian-age fossils. Here, we interpret the Peñasquitos Formation as deep-water marine forearc basin sedimentary and volcanic strata deposited outboard of the Peninsular Ranges magmatic arc. The Upper Jurassic Mariposa Formation of the western Sierra Nevada Foothills is a good analog. Results of detrital zircon U/Pb dating from an exposure of continentally derived sandstone at Lusardi Creek are consistent with a mixed volcanic-continental provenance for the Peñasquitos Formation. A weighted mean U/Pb age of 144.9 ± 2.8 Ma from the youngest cluster of detrital grain ages is interpreted as the likely depositional age. Pre-Cordilleran arc zircon age distributions (>285 Ma) are similar to Jurassic deposits from the Colorado Plateau, with dominant Appalachian-derived Paleozoic (300–480 Ma), Pan African (531–641 Ma), and Grenville (950–1335 Ma) grains, consistent with derivation either directly, or through sediment recycling, from the Colorado Plateau Mesozoic basins and related fluvial transport systems. Appalachian- and Ouachita-like detrital zircon age distributions are characteristic of Jurassic Cordilleran forearc basins from northeast Oregon to west-central Baja California, indicating deposition within the same continent-fringing west-facing arc system.

The most striking feature of the eastern Peninsular Ranges batholith is the large volume of relatively homogeneous tonalite and low-K granodiorite distributed in a series of large zoned Late Cretaceous intrusive centers referred to as La Posta–type plutons. The Sierra San Pedro Mártir pluton in northern Baja California is an outstanding example, and this study was undertaken to test models for the origin of these large composite arc plutons as well as to investigate along-strike variability within this Late Cretaceous belt. The Sierra San Pedro Mártir pluton consists of a nested series of granitoids divided into hornblende, biotite, and muscovite zones that become progressively more felsic and younger inward to a slightly more mafic and lower-K muscovite core zone. Zircon and monazite U/Pb ages from each of the zones indicate composite assembly of the pluton over an ~7 m.y. time span (97–90 Ma), consistent with field evidence and internal compositional variability of the pluton. The Sierra San Pedro Mártir pluton consists of high-Na, high-Al calcic granitoids that contrast with high-K calc-alkaline granitoid intrusive suites typical of Sierra Nevada Late Cretaceous intrusive centers. Whole-rock major-element, trace-element, and rare earth element (REE) data from an ~20-km-long traverse from the margin to the core of the Sierra San Pedro Mártir pluton document compositions that closely match chemical characteristics of Archean high-Al tonalite-trondhjemite-granodiorite (TTG). REEs are in general strongly fractionated, with high (La/Yb) N ratios typical of high-Al TTG. However, large variations in heavy (H) REE abundances and light (L) REE/HREE abundance ratios within the Sierra San Pedro Mártir pluton are comparable to the total range of REE variability within the Peninsular Ranges attributed by previous workers to regional west to east variations across the batholith. High Sr contents and lack of strong Eu anomalies indicate a general lack of plagioclase in the source residue. Compositions are consistent with deep crustal or slab melting from a basaltic source region with residual garnet and amphibole. Hornblende-plagioclase thermobarometry indicates emplacement depths around 17 km and crystallization temperatures ranging from 650 °C to 700 °C. Unlike La Posta–type bodies to the north, which are exclusively ilmenite-series granitoids, the Sierra San Pedro Mártir pluton is partly magnetite-series rocks in the outer hornblende zone. Ilmenite-series rocks preferentially sequester Fe in biotite via Tschermak exchange. Relatively low 87 Sr/ 86 Sr initial isotopic compositions of 0.7038–0.7050 and δ 18 O whole-rock values of 8.5‰ in the Sierra San Pedro Mártir pluton are more typical of the western zone of the Peninsular Ranges batholith. The along-strike variation of La Posta–type centers may be correlated to progressive distancing from Proterozoic North American cratonal basement and/or diminishing contributions of subducted sediment and associated basement into the Cretaceous melt source region of the magmas.

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