PETROLOGY OF PLIO-PLEISTOCENE VOLCANIC ROCKS IN THE NORTHERN GULF OF CALIFORNIA RIFT SYSTEM: INSIGHTS INTO CRUSTAL COMPOSITION

 Thinned continental crust and/or “transitional” crust are believed to occur in the northern Gulf of California. Here the rift system changes and the transition between oceanic to continental transform regimes takes place. Based on seismic reflection and bathymetric data we have located exposed and buried volcanoes in the Delfin basins and along the coast of Baja California.  We are interpreting the petrologic and isotopic data from volcanic rocks from the axial zone of the lower Delfin Basin (LDB), and the southwestern of the Upper Delfin Basin (UDB), including Roca Consag (RC) and Volcan Prieto (~2.5 Ma), to understand the source of magmas and the nature of the crust in this transitional zone.

Chemical compositions of volcanic glass in the LDB vary from sub-alkaline rhyolite to andesite (>56 % SiO₂). Samples from a seamounts in the UDB and the Tiburon Fracture Zone are rhyolite and dacite pumice, respectively. All samples are highly vesiculated and have <3% microphenocrysts (plag and two px). Roca Consag in the Wagner Basin is a low-K, lithoidal, microlithic dacite of unknown age.

The Nd and Sr isotopic compositions of andesite from Volcan Prieto and dacite from RC are similar and indicate a common mantle source, one that is depleted in LIL relative to CHUR/UR (e_Nd=+4.5 and +4.1, ⁸⁷Sr/⁸⁶Sr=0.7038 and 0.7037, respectively). Rhyolite pumice from a seamount in the UDB has e_Nd=+2.4 and Pb that is more radiogenic than Pb from Volcán Prieto and RC. In spite of their similar isotopic compositions RC differs from Volcán Prieto in trace element compositions. Roca Consag has similar trace element composition to rhyolite-andesite from LDB and UDB where plagioclase fractionation is suggested by Eu_n depletion in rhyolites. Andesite magmas in the northern Gulf may derive from: (1) a MORB-type source with variable assimilation of continental crust, or (2) a more primitive and less depleted mantle with respect to typical MORB. Andesite and rhyolite melts within the active basins are interpreted to be co-genetic with variable assimilation of continental crust and crystal fractionation. Pb isotopic compositions of these magmas are consistent with both of these two possible sources for intermediate lavas. Differences in trace elements with similar isotopic composition appears to result from melting at varying depths in a common mantle.