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The Peninsular Ranges Batholith of southern and Baja California is the largest segment of a Cretaceous magmatic arc that was once continuous from northern California to southern Baja California. In this batholith, the emplacement of igneous rocks took place during a single sequence of magmatic activity, unlike many of the other components of the Cordilleran batholiths which formed during successive separate magmatic episodes. Detailed radiometric dating has shown that it is a composite of two batholiths. A western batholith, which was more heterogeneous in composition, formed as a static magmatic arc between 140 and 105 Ma and was intrusive in part into related volcanic rocks. The eastern batholith formed as a laterally transgressing arc which moved away from those older rocks between 105 and 80 Ma, intruding metasedimentary rocks. Rocks of the batholith range from undersaturated gabbros through to felsic granites, but tonalite is the most abundant rock throughout. Perhaps better than elsewhere in the Cordillera, the batholith shows beautifully developed asymmetries in chemical and isotopic properties. The main gradients in chemical composition from W to E are found among the trace elements, with Ba, Sr, Nb and the light rare earth elements increasing by more than a factor of two, and P, Rb, Pb, Th, Zn and Ga showing smaller increases. Mg and the transition metals decrease strongly towards the E, with Sc, V and Cu falling to less than half of their value in the most westerly rocks. Oxygen becomes very systematically more enriched in18O from W to E and the Sr, Nd and Pb isotopic systems change progressively from mantle values in the W to a more evolved character on the eastern side of the batholith. In detail the petrogenesis of the Peninsular Ranges Batholith is not completely understood, but many general aspects of the origin are clear. The exposed rocks, particularly in the western batholith, closely resemble those of present day island arcs, although the most typical and average tonalitic composition is distinctly more felsic than the mean quartz diorite or mafic andesite composition of arcs. Chemical and isotopic properties of the western part of the batholith indicate that it formed as the root of a primitive island arc on oceanic lithosphere at a convergent plate margin. Further E, the plutonic rocks appear to have been derived by partial melting from deeper sources of broadly basaltic composition at subcrustal levels. The compositional systematics of the batholith do not reflect a simple mixing of various end-members but are a reflection of the differing character of the source regions laterally and vertically away from the pre-Cretaceous continental margin.

Rare earth element (REE) patterns of plutonic rocks across the Cretaceous Peninsular Ranges batholith vary systematically west to east, transverse to its long axis and structural trends and generally parallel to asymmetries in petrologic, geochronologic and isotopic properties. The batholith can be divided into three distinct parallel longitudinal regions, each defined by distinct REE pattern types. An abrupt transition occurs between rocks with slightly fractionated REE patterns in the western (coastal) region and rocks with middle to heavy REE fractionated and depleted patterns in the central region. Further to the east a second transition to strongly light REE enriched rocks occurs. The slopes of the REE patterns within each of these regions are largely independent of rock type. The first REE transition is closely coupled to regional discontinuities in other parameters: elimination of negative Eu anomalies, an increase in Sr content, and a marked restriction in petrologic diversity. This transition occurs over a range of initial 87 Sr/ 86 Sr ratios and Ci 18 0 values, but approximately correlates to a major shift in the emplacement style of the batholith from a stationary arc to a rapidly eastward-migrating (cratonward) arc.The sense of the regionally consistent REE trends cannot be explained by crystallization, assimilation, combined crystallization-assimilation, or mixing processes. The consequences of assimilation and high-level differentiation are not observed generally, despite the sensitivity of the REE to these processes. Geochemical and petrological features argue that the partial melting of mafic source rocks in which plagioclase-rich (gabbroic) residual assemblages abruptly gave way laterally and downward to garnet-bearing (eclogitic) residual assemblages produced all the changes associated with the first REE transition. The change in residual assemblages from gabbroic to eclogitic was superimposed on source regions already zoned in light REE abundances, 87 Sr/ 86 Sr and 18 0. Temperature and pressure constraints on the source regions place them in a subcrustallocation. The calcic nature of the batholith and the dominance of tonalite and low-K 2 0 granodiorite in all its regions argue that the source materials are broadly basaltic in composition. Experimental studies are consistent with the generation of the abundant tonalitic magmas by the partial melting of basalt under both low and high pressure conditions. Arc basalts such as those commonly erupted in modern island arcs and continental margins are inferred to have provided much of the source material and the heat. Additional high-18 0 components are needed in the more easterly source regions. These materials must be distributed so as to contribute equally to the range of magmas that occur in a given local region, and must preserve the calcic nature of batholithic sources. Altered basalts of ancient oceanic crust and possibly their associated metasediments, previously incorporated into the lithosphere beneath the continental margin during earlier ...

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