The La Posta TTG Flareup
The defining characteristic of the Peninsular Ranges of southern and Baja California is a petrologically distinctive, high Sr/Y plutonic suite of 95 & 3 Ma tonalite, trondhjemite, and granodiorite that is referred to as the La Posta TTG suite. These rocks are chemically similar to high-Al tonalite-trondhjemite-granodiorite gneiss terrains of Archean crust produced by high pressure melting of mafic source regions. La Posta belt magmatism records an eastward relocation of the Cretaceous arc accompanied by abrupt chemical change to TTG compositions. This shift was preceded by tectonic shortening and thickening of the crust along the locus of pluton emplacement. Intrusions occur as a series of large internallyzoned intrusive centers with outcrop areas that range up to at least 1400 km2. Field and thermobarometric data indicate emplacement at -3-6 kbar depths followed by rapid uplift and denudation at rates of -1-2 mmlyr. Zircon U-Pb ages (n=43) from throughout the -1300 km length of the batholith indicates the bulk of the La Posta rocks were emplaced in a narrow time frame from -98-92 Ma. Mainphase biotite k hornblende tonalite from individual zoned intrusive centers were emplaced over -3-4 m.y. time intervals followed by small volume peraluminous core zone intrusions. A series of distinctly earlier (c. 105-11 0 Ma) and smaller "precursor" intrusions parallel the western edge of the main locus of intrusion. The La Posta TTG belt is more deeply exhumed in the north (up to 25 km) relatively to south (-10 km) providing an oblique cross-section through arc crust. Zircon oxygen isotopic compositions indicate that high 6180 predominate in the deeper northern part of the La Posta TTG belt (7.6-12.2) while lower values characterize the shallower southern part (6.3-7.2). This pattern indicates the possibility of significant and previously unrecognized depth gradients in a180 in the PRB consistent with large input of supracrustal contaminants into deeper parts of the batholith relative to shallower parts. The implication is that vertical gradients in 6180 within the eastern zone La Posta belt may be as great as the wellestablished west to east surface gradients across the northern part of the batholith. Because the volume La Posta-type crust in the Peninsular Ranges may easily exceed a million cubic kilometers, simple mass balance considerations require supracrustal recycling on a massive scale. The fact that high a180 La Posta-type zircon are from rocks with Sri values mostly ~0.706co nstrains the nature of the supracrustal contaminant. Assimilation of high Sri Julian Schist-type metasedimentary wallrock to account for La Posta zircon oxygen isotope compositions is unrealistic on several counts. We speculate that large-scale sediment underplating of isotopically primitive accretionary prism material may have played an important role leading up to La Posta-type melt generation. This view is supported by a sediment deficit in adjacent forearc basin & accretionary prism belts of the California borderland.

Was deeply accreted Catalina Schist an important component of the La Posta source region?
Studies at oceanic convergent margins show unambiguously that large volumes of continental material are recycled into the mantle by sediment subduction and forearc subduction erosion. The fate of this subducted continental material is poorly known; mass balance consideration suggest that at least 95 percent of this material is recycled to deep mantle circulation. The Late Cretaceous La Posta-type TTG suite of the eastern Peninsular Ranges batholith however displays evidence that accretionary prism sediments were subducted and magmatically recycled into the Late Cretaceous the on a very short ( 4 0 Ma) timescale. The La Posta suite represents a tremendous flare-up of deep (garnet-involved) partial melting preceded by an episode of crustal shortening and presumed crustal thickening from ca. 98-92 Ma. Evidence from the Catalina Schist subduction complex terrane directly outboard of the PRB demonstrates that these rocks were being underthrust and metamorphosed synchronously with La Posta magma generation. Elevated 6180 values from La Posta plutons indicate substantial contributions of isotopically unevolved (initial Sr < 0.706) intermediate to mafic composition supracrustal materials into the melt souce regions consistent with contributions from the Catalina Schist accretionary complex. Further, inherited zircon in the La Posta rocks are dominated by a 120-100 Ma component that closely matches the age of the western PRB as well as the dominant detrital zircon component in Catalina Schist. These results are consistent with "conveyor belt-type" recycling of western PRB material into the forearc and then downward via underthrusting into the magma source region of La Posta melts.

INVESTIGATORS
David Kimbrough and Marty Grove