SAN-QUINTIN VOLCANIC FIELD, BAJA-CALIFORNIA-NORTE, MEXICO - GEOLOGY, PETROLOGY, AND GEOCHEMISTRY

The San Quintin Volcanic Field (SQVF) is unique for the Baja Californi a peninsula as the only known location of intraplate-type mafic alkali c volcanism and the only known source of peridotitic and granulitic xe noliths. It consists of 10 distinct Quaternary volcanic complexes. The oldest cones mainly erupted primitive magmas (Mg # >64)(Mg # = 100 x Mg/(Mg + (0.85 x Fe-Total))), which carried occasional small xenoliths . As the SQVF evolved with time, differentiated magmas (Mg # <64) beca me increasingly common, but primitive magmas, virtually devoid of xeno liths and unusually rich in olivine phenocrysts, dominanted at the you ngest cones. Abundances of incompatible elements declined during evolu tion of the SQVF, implying a temporal increase in the extent of partia l melting in the mantle, or progressive exhaustion of these elements i n the source. Samples from two cones, Maze and Ceniza, show relatively low Ce/Pb, epsilon(Nd), and Pb-206/Pb-204 and high Sr-87/Sr-86, which we interpret as evidence for crustal contamination of these magmas. S mall isotopic variations for the other cones are collectively interpre ted to reflect involvement of at least three mantle components beneath the SQVF. Ranges in isotopic composition overlap for primitive and di fferentiated rocks, supporting fractional crystallization as the mecha nism for deriving the latter from the former. Most differentiated rock s can be successfully modeled by fractional crystallization of olivine , plagioclase, clinopyroxene, and spinel from primitive parents. The l argest and most abundant xenoliths were carried by differentiated magm as, indicating that fractional crystallization took place within the m antle, below the level of peridotite entrainment, and reflecting the i mportance of fractionation-elevated volatile contents for driving thes e differentiated magmas rapidly to the surface. Primitive rocks of the SQVF are unusual compared to other reported intraplate-type mafic alk alic suites from around the world in having relatively high Al2O3 and Yb, as well as low La/Yb and CaO/Al2O3. These characteristics and tren ds of rising Al2O3 and falling CaO with decreasing incompatible elemen t abundances are all consistent with origins for the SQVF primitive ma gmas by progressive partial melting of spinel Iherzolite at unusually shallow levels in the mantle.