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Description
The Julian Schist, a belt of amphibolite-grade metamorphic rocks, lies along the axis of the Peninsular Ranges. Inclusions of the schist are found in plutons as old as 234 ± 39 Ma. Geochemical data from the Peninsular Ranges batholith indicate that western plutons developed in oceanic lithosphere whereas eastern plutons developed in transitional to continental lithosphere. The Julian Schist is found on either side of the now cryptic join between these two lithospheric types and has been interpreted to represent an overlap sequence. The Julian Schist has undergone two phases of deformation. The first phase, D1, produced a prominent schistosity, S1, that is NW striking and steeply dipping. S1 foliation was produced during NE-SW contraction in the Early Cretaceous, ~115 Ma. Locally, tight F2 folds fold S1. The age and tectonic significance of F2 folds are unknown. The Julian Schist is divided into the following two belts. (1) A west-southwest belt defined by the presence of K-feldspar and cordierite, and (2) a northeast belt defined by the local occurrence of staurolite and the absence of K-feldspar and cordierite. Andalusite and sillimanite are present in both belts. Pressure-temperature (PT) conditions in the northeast belt ranged from 2.7-6.5 kb and 500-650 °C. PT conditions in the west-southwest belt ranged from 1.0-3.2 kb and 575-700 °C. Statistical analysis utilizing the Student t-test to determine differences in means in geochemical data from the northeast and west-southwest belts shows that 38 out of 42 elements (90.5%) cross a zero difference in means value with four elements (MgO, Ni, Zn, and Ba) being statistically dissimilar (9.5%). However, U, Th, Ti, Al, and REE have uniform abundances throughout the Julian Schist outcrop belt. The chemistry of pelitic rock samples of the Julian Schist are compositionally similar to shales that have not been subject to significant metasomatism and implies that the whole rock composition of the Julian Schist was not affected by diagenesis and metamorphism. The ratios of Al2O3/TiO2, Th/U, the Eu anomaly, and REE patterns for samples of the Julian Schist indicate a provenance like old upper continental crust. Such an interpretation along with an inferred Early Triassic age suggest that the Julian Schist may have formed along the truncated edge of North America during the Late Permian-Early Triassic. This truncation event may have extended into northwestern Mexico as far south as Hermosillo where uplifted Upper Precambrian through Permian miogeoclinal sediments acted as the source of detritus of mudstones in the Julian Schist.
