ADVANCES IN THE GEOCHRONOLOGY AND GEOCHEMISTRY OF THE NORTHERN SONOBARI COMPLEX, NORTHWESTERN MEXICO

The Sonobari Complex is an igneous-metamorphic complex that includes vestiges of the Pangea amalgamation, the building of a continental magmatic arc, and the collision of an intraoceanic arc. The northern part of this complex, in southern Sonora, consists of a volcano-sedimentary sequence, which underwent greenschist facies orogenic metamorphism before being unconformably covered by the Upper Triassic-Lower Jurassic Barranca Group. The metamorphic rocks and its coverture are intruded by Late Cretaceous tonalitic to granodioritic plutons. The protoliths of volcanic rocks intercalated in the metamorphosed sequence display distinctive geochemical behavior. Mafic rocks have similar trends as enriched mid-ocean ridge basalts (E-MORB), while intermediate to felsic rocks have trends similar to calc-alkaline magmas related to subduction zones.

Detrital zircon studies in metasedimentary rocks yield main peaks at 1014, 1191, 1206, 1423, 1611, 1684 and 1773 Ma. One of the studied samples also yields peaks at 338, 432, 561 and 620 Ma. On the basis of the geological and geochronological data, the metamorphic complex must be metamorphosed between the Late Mississippian and Middle Triassic, during an orogenic event probably related with collision between Gondwana and Laurentia. The tectonic setting interpreted from the geochemical and geochronolgical data implies a late Paleozoic marginal volcanic arc contributing with calc-alkaine rocks to the sedimentary basin bordering southern Laurentia, which evolved to a back-arc basin with E-MORB basalts, maybe related to a change of the subduction angle.