GEOCHEMISTRY AND GEOCHRONOLOGY OF THE CLASTIC ROCKS OF THE MORITA FORMATION, NORTHERN SONORA, MEXICO

During Triassic to Early Cretaceous time, the north-western part of Mexico, southern Arizona and southern California underwent extensive arc magmatism that is recorded in Mesozoic basins-fills of Sonora. Here we present results from petrography, whole-rock geochemistry (major, trace and rare earth elements) and detrital zircon geochronological studies on the clastic rocks of the Lower Cretaceous Morita Formation exposed in the Sierra Anibacachi and Rancho Bufalo areas of northeastern Sonora. Based on petrography, sandstones of this formation are classified as feldspathic litharenite, litharenite, sublitharenite,and lithic arkose. The sandstone, siltstone and shale samples from the Morita Formation show considerable variations in major and trace elements concentrations. The chondrite normalized rare earth elements (REE) patterns of these rocks are fractionated with light REE enrichment and flat heavy REE patterns with negative Eu anomaly. The chemical index of alteration (CIA) and plagioclase index of alteration (PIA) values as well as A-CN-K diagrams show the low to intense weathering in the source region. ΣREE concentrations in zircons vary significant from sample to sample (101 ‒ 4384 ppm). Zircons from both Sierra Anibacachi and Rancho Bufalo sections show wide variations in Th/U ratios (0.12 ‒ 2.1; 0.12 ‒ 1.91; respectively) and these ratios are well within the range of igneous origin. REE pattern, elemental ratios, bivariate and ternary plots suggest that the clastic rocks of the Morita Formation were mainly derived from felsic and intermediate source rocks.

Detrital zircon geochronology of the Morita Formation indicates Proterozoic and Triassic as dominant age groups, whereas Paleozoic and Jurassic zircons occur in subordinate proportions. Detrital zircon grains with ages between ca. 1000 and ca. 2114 Ma were derived from Precambrian basements of southwestern North America, and/or were recycled from overlying Neoproterozoic and Paleozoic strata. The early Mesozoic continental volcanic arcs in northwestern Mexico, southern Arizona and southern California served as source for the Triassic-Jurassic detritus of the Morita Formation, however, Early Cretaceous zircons from the contemporaneous Alisitos arc are absent.