The Provenance of Modern Sands from Baja California Rivers (Mexico): Petrographic Constraints from Light and Heavy Minerals

Modern sand composition of three rivers, reflects the nature of the source region, which lies in the central part of the Alisitos arc (Peninsular Ranges, Baja California, Mexico). The sand detrital modes correspond well with the main structural units drained by El Rosario, San Fernando, and San Vicente rivers: (1) the Early Cretaceous oceanic arc of the Alisitos Group, (2) the Paleozoic to Mesozoic continental margin metasedimentary rocks, (3) the Cretaceous plutons, (4) the Late Cretaceous to Tertiary sedimentary rocks, and (5) the Tertiary volcanics. San Vicente, San Fernando, and El Rosario are chiefly fed from erosion of a magmatic arc in various stages of dissection and mostly consist of minor feldspatho-lithic (Fl) to quartzo-litho-feldspathic (qFL) sand and dominant quartzo-feldspatho-lithic (qLF) and litho-feldspatho-quartzose (lQF) sand. Increasing degree of dissection is indicated by progressive increase in quartz, K-feldspar, sedimentary and metamorphic, and decreasing of volcanic lithic fragments. Sand, within the Lv field, microlitic (Lvmi), contains felsitic (Lvf), and lathwork (Lvl) types, and trace amounts of vitric grains (Lvv), such as pumice particles. Andesitic volcanic province of the Alisitos arc shed quartz-poor sand containing mainly microlitic lithic fragments and plagioclase, sand derived from more felsic rhyolites and rhyodacitic and trachyandesitic products contains largely felsitic volcanic lithics whereas minor lathwork lithics are mainly derived from subordinate basalts. The abundance of igneous rock fragments and volcanic and sedimentary lithics of the three studied rivers sand faithfully represents the relative abundance of a heterogeneous [volcanic+plutonic+sedimentary] bedrock exposure in each drainage basin. In the medium to fine sand of the drainage systems, the transparent heavy minerals suite includes mainly hornblende, pyroxene, epidote, titanite and zircon, and minor quantities of staurolite, rutile and garnet. Other heavy detrital species identified in the San Fernando and San Vincente river sand are tourmaline, sillimanite, kyanite and andalusite. Unstable heavy minerals (Amphibole, Pyroxene and Apatite) predominate in El Rosario River compared to San Fernando and San Vicente rivers sand that are enriched in the moderately stable species such as epidote, kyanite and sillimanite. Hornblende and pyroxene grains show mainly corroded to etched morphologies due to dissolution processes whereas epidote remains unaltered. Pyroxene grains are more corroded than amphiboles. Sedimentary recycling produced subrounded to rounded in zircon grains in the San Fernando river sand. Also, sand composition typified by the heavy minerals reflect provenance from multiple and differently weathered sources. Specifically, San Fernando and San Vincente river abundance and weathering textures of the heavy minerals match predominant sedimentary and metasedimentary bedrock lithologies and El Rosario volcanic source rocks.