MIDDLE JURASSIC-EARLY CRETACEOUS (166-140 MA) ISLAND ARC MAGMATISM OF NW MEXICO: EVIDENCE FROM THE META-IGNEOUS ROCKS OF THE SIERRA EL ARCO AND COEVAL MAGMATIC ROCKS IN BAJA CALIFORNIA

The Jurassic-Cretaceous geologic record of subduction-related magmatism is widespread along southern California and northwestern Mexico. However, the record is fragmented and displays wide variations in lithology, composition, and age, making it challenging the complete and coherent understanding of the geologic evolution along the arc. Here, we focus on the petrogenesis of subduction-related igneous rocks and minor metamorphic rocks in the southernmost exposures of the Alisitos terrane at the Sierra El Arco, by whole-rock geochemistry, Sr-Nd-Hf isotopes, and U-Pb zircon geochronology. Our results were compared with the coeval Jurassic rocks of the Vizcaíno and Guerrero terranes in northwestern México. The lithology in the Sierra El Arco consists of greenschist-facies meta-igneous rocks and further northeast, in the Calmallí area, amphibolite and paragneiss are dominant. The meta-igneous rocks of the Sierra El Arco range in composition from peridotite to granodiorite, with associated basaltic to dacitic volcanic rocks. These rocks display negative εSr_t (average -8.4, n=23) and positive εNd_t (average +5.8, n=23) and εHf_t values (+14, n=2), suggesting an origin from mantle-derived melts with small amounts of contamination. Geochemical and isotopic data of the amphibolite suggests a protolith similar to the meta-igneous rocks. Paragneiss samples display εSr_t (+166), εNd_t (-7) and, εHf_t (-3 and -9) values, suggesting the incorporation of continentally derived detritus in their protoliths, similar to Triassic-Middle Jurassic metasedimentary rocks reported from northwestern Mexico. These metasedimentary rocks deposited probably in a marginal basin between the arc axis and the continental margin. Middle Jurassic magmatism in the Cedros Island (Vizcaíno terrane) and Sierra El Arco (Alisitos terrane) define an εSr_t vs. εNd_t evolutionary trend within the mantle array, suggesting a common origin within an island arc setting. Jurassic magmatism of continental affinity from regions of the Alisitos and Guerrero terrenes is defined by positive εSr_t (average +39, n=15) and negative εNd_t values (average -4.2, n=15). Based on Sr-Nd two end-member mixing curves, we suggest that coeval continental affinity magmatism could be explained by the assimilation of Late Triassic metasedimentary rocks, or a lower crustal, which differs from the Precambrian crystalline basement of northern Mexico.