GEOLOGIC ANALYSIS OF THE SAN JER"NIMO PLUTON IN THE SOUTHERN PART OF THE PENINSULAR RANGES BATHOLITH, BAJA CALIFORNIA, MEXICO

The San Jerónimo Pluton (SJP) is located in the southern part of the Peninsular Ranges Batholith. From modal classification, the rocks of the pluton vary from gabbro to granite. Diorite and tonalite showing discrete cummulitic textures volumetrically dominate the complex. The U-Pb ages of zircons from tonalites vary from 115.0 ± 2.3 Ma to 104.3 ± 6.1 Ma. According to the mafic/felsic rock ratio, the SJP can be divided in two zones: a) A mafic northern zone dominated by gabbroic and dioritic rocks containing a high content of disseminated magnetite; in this zone, the tonalite is intruded by Fe-rich dikes. b) A southern zone where mafic and felsic rocks roughly crops out in the same proportion, but their magnetite content decreases considerably and the presence of Bt in dioritic rocks is showed. The REE patterns shown by the rocks in northern zone suggest a relationship with an evolved melt, in the southern zone the tonalite and diorite rocks have very similar REE patterns with light rare earth elements (LREE) enrichment with respect to heavy rare earth elements (HREE). A genetic relationship is suggested from the analyses of the evolutionary trends of Harker-type diagrams and the Q-Di-Ol-Pl tetrahedron. According with geochemical discrimination diagrams, the plutonic rocks are metaluminous, show I type affinities and calc-alkaline and calcic trends, all this trends are typical island arc characters. The geometry of the intrusive rocks is related to the style of emplacement. It also exist a close relationship between the NE-SW orientations of the vertical longitudinal fractures with the magmatic foliation which were possibly developed during the emplacement. It is inferred that Fe-rich dikes were formed by liquid immiscibility in the mafic magmas. Forced emplacement of the dikes of magnetite is interpreted from the abundant slickenside textures. The evolution of the complex in the northern zone is synthesized in three main events: a) an initial state of rapid emplacement of low viscosity gabbroic to dioritic magma at temperatures near 1,200^oC; b) a cooling phase in the mafic magma in which crystalline aggregates and liquid segregation by immiscibility between iron-rich and silicate melts occurred; and c) a late stage where the partially melted gabbro-diorite body was intruded in its southern and eastern flanks by tonalitic magma.