PROVENANCE OF THE CLASTIC ROCKS OF THE MORITA FORMATION, NORTHERN SONORA, MEXICO: EVIDENCES FROM GEOCHEMISTRY AND DETRITAL ZIRCON U-PB AGES

The Morita Formation of the Bisbee Group, Sonora is represented by a thick sedimentary sequence formed as the filling of the Bisbee Basin, deposited during the early Cretaceous. Geochemistry and detrital zircon geochronological studies were carried out on the clastic rocks of the Morita Formation that crop out in the Northern Sonora, Mexico. The purposes of the present study is to deduce the potential stratigraphic variation in provenance signatures.

The framework grains identified in the Morita Formation are monocrystalline quartz, polycrystalline quartz, orthoclase, plagioclase and lithic fragements. The lower part of the section is dominated by lithic arkose and feldsparthic litharenite whereas upper part is dominated by subarkose. Geochemically, the Morita Formation is classified into subarkose, litharenite, sublitharenite, wacke and shale. Large variations in ΣREE content are observed between sandstone, siltstone and shale samples of the Morita Formation (61 ± 26; 114 ±28; 152 ± 24; respectively). In chondrite normalized REE plot, the Morita Formation show LREE enriched and relatively flat HREE patterns with negative Eu anomaly.

Detrital zircon U-Pb geochronology of the Morita Formation of northern Sonora indicates that it contains five distinctive zircon populations: (1) Proterozoic; (2) Palaeozoic; (3) Triassic; (4) Jurassic; and (5) Early Cretaceous. Detrital zircon data from the Morita Formation provide useful information to locate source terranes for these grains. Detrital zircon grains with ages between ca. 1000 and ca. 2114 Ma were mainly derived from Precambrian rocks of the Caborca block. Furthermore, plutonic rocks of Mazatzal Province in the southwestern U.S may also contribute significant amount of zircon grains to the Morita Formation. Paleozoic detrital grains were likely derived from the Caborca block. Triassic zircon grains derived from Triassic arc magmatism in the Mesozoic Cordilleran arc exposed in Mesozoic strata on the Colorado Plateau. The Jurassic zircons were derived from Jurassic continental arc in the northwestern Mexico and southern Arizona. The Early Cretaceous detrital zircons with ages between 120 and 137 Ma were derived from the Alisitos volcanic arc presently exposed in the southern and Baja California.