IMPLICATIONS FOR THE TRANSITION FROM OLIGO-MIOCENE SUBDUCTION TO LATE MIOCENE TRANSTENSION IN SOUTHERN SONORA, MEXICO FROM IGNEOUS GEOCHEMISTRY

Miocene volcanic rocks in the coastal province of Sonora, Mexico were erupted before and after the ~15-12.5 Ma transition from subduction to transtension/rifting in the proto-Gulf of California. We present major and trace element geochemical data from well-dated volcanic sections in this province in order to gain a better understanding of the plate tectonic transition and how it influenced the geochemistry of erupted magmas. Syn-subduction volcanic rocks (24 – 15.2 Ma) range from calc-alkalic basalt to rhyolite and have typical arc affinities, including a depletion in the high field strength elements (HFSE) with the characteristic relative depletions in Nb-Ta. Post-subduction volcanic rocks (12.5 – 8.25 Ma) range from calc-alkalic basalt to rhyolite (including abundant 12.5-11 Ma dacite and rhyolite) and the mafic end-members have an intraplate signature with residual arc depletion characteristics in the HFSE. Our study documents a progressive change in trace element compositions that begins prior to the end of subduction and continues until volcanism shuts off in coastal Sonora several million years after subduction ends, with little or no evidence for post-subduction tholeiitic basalts. A more pronounced period of geochemical change between 15 and 11 Ma in Sonora is likely the result of the changing plate boundary configuration. However, the 15-11 Ma transition period is also gradual and only distinguishes itself from the longer lived (27 to 8.25 Ma) geochemical changes in Sonora when trace element concentrations and ratios are analyzed statistically. Subduction of an increasingly young and buoyant oceanic slab in the early Miocene may provide an explanation for the gradual changes in Sonoran geochemistry observed long before subduction ceased. The youthful properties of the subducting slab, which caused the subsequent locking of the subduction zone ca. 15-12.5 Ma, may also provide an explanation for the triggering and existence of post-subduction magmatism in Sonora via thermal equilibration of the slab with the asthenosphere. These observations from southern Sonora provide additional constraints for plate tectonic reconstructions of the west coast of the North America during the Oligo-Miocene and estimates of the amount of cumulative strike-slip motion outboard and inboard of Baja California.