Document Type

Honors Project On-Campus Access Only


Magmatic arcs exert tremendous control over the growth and evolution of continental crust, functioning as factories for new crust and distilleries where it is refined to an overall intermediate composition. The deepest levels of arcs are their least understood domain, especially with regard to the processes controlling the interaction of mantle and crustal components, due to the rarity of deep arc materials available for direct study. In broad terms, the evolution of the lithosphere beneath North America is studied by looking at mafic volcanic rocks and their mafic to ultramafic xenoliths associated with such arcs and quantifying their ages and geological origins. Recent zircon U-Pb ages and Hf isotopic values indicate that xenoliths from the Quaternary San Quintin Volcanic Field (SQVF) represent the only known material from the base of the Peninsular Ranges Batholith (PRB) that underlies much of the Baja California Peninsula. Xenoliths from SQVF are broadly divided into crustal (plagioclase-bearing, clinopyroxene and/or orthopyroxene granulite with minor olivine) and mantle (spinel lherzolite) categories. 92% of zircons from crustal granulite nodules are Cretaceous, dating to ca. 145 to 90 Ma. This range overlaps the most voluminous phase of PRB construction, an intriguing result given its strike-perpendicular zonation. Mafic volcanic and volcaniclastic rocks of the Alisitos arc, constructed offshore from ca. 140 to 105 Ma and accreted to the continental margin by ca. 105 Ma, underlie the western PRB and host the SQVF, while felsic intrusives of the ca. 100 to 90 Ma La Posta suite mark the eastern domain. This age span suggests that studied xenoliths initially formed as deep-level intrusive equivalents to exposed extrusive and shallow intrusive products of the Alisitos arc. Furthermore, ca. 100 to 90 Ma ages plus microstructural evidence for magmatic infiltration of earlier formed material suggest that the deep PRB continued to develop as the La Posta suite invaded the crust and may point to the presence of unrecognized volumes of La Posta-age additions beneath the western PRB. Zircon Hf isotopic values for Cretaceous grains span from +2.1 to +8.4, consistent with geochemical data from the Alisitos arc indicating a significant depleted mantle component for these materials. Subequal proportions of remaining grains are Jurassic, Triassic, Proterozoic, and Archean in age, and yield negative Hf isotopic values. The presence of pre-batholithic grains at deep levels of the western PRB points to incorporation of a small amount of continental material during emplacement and strongly supports a fringing, non-exotic origin of the Alisitos arc.



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