A chemical and isotopic study of the Laramide granitic belt of northwestern Mexico: Identification of the southern edge of the North American Precambrian basement

Along the Laramide belt of northwestern Mexico, granitic rocks of similar b ulk composition show isotopic and trace element signatures that help to del ineate the position of the southern edge of the North American Precambrian basement. In the northern part, the Laramide plutons (the "northern granite s") intruded Proterozoic crystalline rocks and a thick Late Proterozoic thr ough Paleozoic miogeoclinal cover of North American affinity. In the centra l part, the granitic bodies (the "central granites") were emplaced into a s equence of Paleozoic eugeoclinal rocks overlain by Late Triassic elastic un its. The southern part of the belt (the "southern granites") intruded a les s-known crust characterized by middle to late Mesozoic island-are-related v olcanic and sedimentary rocks of the Guerrero terrane. Data from a suite of metaluminous to slightly peraluminous calc-alkalic granitic rocks along th e belt display north-to-south geochemical and isotopic variations, which co uld correlate with the type of intruded basement. The northern and central granites are characterized by strongly fractionated, light rare earth eleme nt (REE)-enriched patterns, which display generally pronounced negative eur opium anomalies, whereas the southern granites have lower total REE enrichm ents and much flatter chondrite-normalized slopes displaying almost no euro pium anomalies. Isotopic results also suggest regional variations, as shown by the following initial Sr and epsilon (Nd) ranges: 0.7070 to 0.7089 and -4.2 to -5.4, respectively, for the northern granites; 0.7060 to 0.7079 and -3.4 to -5.1 for the central granites; and 0.7026 to 0.7062 and -0.9 to +4 .2 for the southern granites. On the basis of their isotopic similarities, the Proterozoic mafic to intermediate lower crust revealed by xenoliths fro m young volcanic flows in southern Arizona and northern Mexico is interpret ed as a reasonable parental source for the northern and central granites; h owever, mantle-derived melts are not excluded. The more primitive southern granites are interpreted to come from a source that lacked Proterozoic base ment. Instead, they were probably derived by mixing of juvenile mantle melt s with partial melts of the lower parts of the Guerrero terrane. In general , the north-to-south compositional variations of the Laramide granitic rock s of northwestern Mexico reflect the crustal structure underneath the batho liths. The Sr and Nd data indicate that the edge of the North American Prec ambrian basement extends approximately southeastward from the coastal batho fith of central Sonora; then, about 200 km south of Hermosillo in southern Sonora, the edge bends eastward and continues to the east beneath the Sierr a Madre Occidental volcanic province.