PETROLOGY OF THE PLUTONIC ROCKS OF THE CALMALLI-SAN FRANCISQUITO REGION, SOUTHERN PENINSULAR RANGES BATHOLITH, BAJA CALIFORNIA, MEXICO

The Peninsular Ranges batholith (PRB) shows a compositional zonation between a western zone dominated by mafic plutons and an eastern zone of mostly felsic plutons. Although this zonation is well-developed along the length of the PRB, the southernmost part of the PRB remains poorly known. In this region, the mafic Calmallí pluton intruded into Jurassic island arc rocks composed of metamorphosed igneous and sedimentary rocks of local volcanic and continentally derived provenance. Outcrops of felsic plutonic suites are predominant east of the Calmallí pluton towards the San Francisquito region. There is no evidence of the host rocks in the eastern zone except for igneous and meta-igneous xenoliths in the felsic rocks. We studied the petrogenesis of the plutonic rocks and xenoliths to infer the nature of the basement rocks of the eastern side of the southern PRB. The Calmallí pluton (105-100 Ma) consists of gabbro, diorite, and tonalite yielding εSr_t (average -3.6, n= 6), εNd_t (average +3.3, n=6), and εHf_t (average +6, n=3) values that suggest an origin from mantle-derived melts with a low degree of contamination. The almost flat rare earth elements (REE) patterns and Eu anomalies in these rocks suggest a fractionation under the plagioclase's stability field. The eastern plutonic rocks are granodiorite and granite (100-92 Ma) with εSr_t (average -4.7, n=9), εNd_t (average +0.8, n=9), and εHf_t (average +2.6, n=3) values that suggest a mantle-derived melt genesis with increased contamination. Most of these acidic rocks show a characteristic depleted-HREE pattern suggesting differentiation under the influence of garnet in deep cortical reservoirs. The xenoliths are basic to intermediate in composition and yield Sr, Nd, and Hf isotopic signatures and REE patterns similar to the Calmallí pluton; thus, we suspect a similar origin. Despite differences in composition, the basic, intermediate, and acidic plutonic rocks of the Calmallí-San Francisquito region display an εSr_t vs. εNd_t correlation trend that indicates progressive contamination of mantle-derived magmas. Our results of the Sr-Nd and Nd-Hf isotopic mixing curves suggest a contaminant with the isotopic composition of Triassic paragneisses of northwestern Mexico.