Description
Rock magnetic, paleomagnetic and magnetic mineralogic analysis of plutonic rocks and xenoliths from 29 K-Ar dated sites within the northern Peninsular Ranges batholith indicate that the magnetic stability of these rocks is mineralogically controlled. The relative stability of magnetic remanence during AF and thermal demagnetization is governed by the relative intensity contributed by stable magnetite and unstable hematite within a rock, rather than their coercivity or magnetic domain structure. Deuteric oxidation of magnetite into hematite below 550° c is a possible mechanism that produces the unstable remanence of the hematite. Compositions of magnetite, hematite and ilmentite in all rocks measured are close to the theoretical end-members of their respective solid solution series. The western samples contain magnetite, hematite and ilmenite while magnetite is absent in the east. The distribution of these phases define a western magnetite series belt and an eastern ilmenite-series belt. The presence of these paired belts and their association with geochemical, geochronological, lithological, structural and isotopic relationships across the batholith suggest two fundamentally different source regions for rocks on either side. The western magnetite-series rocks within the gabbroic belt represent rocks which originated by partial fusion of subducted oceanic lithosphere and the overlying mantle wedge, whereas the eastern ilmenite series rocks within the tonalite belt were produced through partial fusion of the lower continental crust. Paleomagnetic analysis of 12 western plutons ranging in age from about 115 to 90 mybp suggest an 11° northern translation and a 42° clockwise rotation relative to stable North America. This regional batholith direction is in agreement with previous studies. Reconstruction of the Baja Peninsula to its original site based on the paleomagnetic data presents certain problems, particularly with the overlapping of the Peninsula with Mexico and the rates of northward translation. The average northward component is calculated to be about 2.5 cm/yr, which is equal to the average northward component of the Farallon plate relative to the North American plate.
