Geochemistry of the sedimentary rocks from the Antimonio and Río Asunción formations, Sonora, Mexico: Implications for weathering, provenance and chemostratigraphy

Geochemical (major oxides, trace and rare earth elements) studies were carried out on the siliciclastic rocks of the Antimonio and Río Asunción formations to interpret the paleoweathering conditions and provenance. The weathering indices like the Chemical Index of Alteration (CIA) and Plagioclase Index of Alterations (PIA) indicate a low to moderate intensity of chemical weathering. The chondrite normalized REE patterns of the Antimonio and Río Asunción formations have LREE enriched, flat HREE patterns with a negative Eu anomaly. However, a few samples from these formations show a positive or an absence of the Eu anomaly, suggesting that the sediments were dominantly derived from felsic source rocks with a minor contribution from mafic source rocks. The vertical distribution of trace elements and their ratios indicate that sequence I of the Antimonio Formation presents a signature for felsic source rocks, however, in sequence II the sediment source changed, i.e. mafic rocks were recorded (SAAF06 and SAAF07). Sequences III to VI indicate a felsic source rock. The basal and upper parts of sequence VII (samples SARAF02 and SARAF10) of the Río Asunción Formation indicate mafic source rocks, whereas the remaining samples were largely contributed by felsic source rocks. The lower values of ferromagnesian trace elements and their ratios in sequences VIII and IX reveal a felsic source for the upper part of the Río Asunción Formation. The Antimonio and Río Asunción formations received sediments from two different sources: Permo-Triassic granitoids of northwestern Sonora and Precambrian granitoids from Caborca and North American blocks that contributed felsic rocks. Whereas mafic elements were derived from Precambrian amphibolites and Cambrian basalts located north and south of the study area.