PETROLOGIC FEATURES AND TECTONIC CONTEXT OF DISCRETE PLUTONS IN THE SOUTHWESTERN MARGIN OF THE PENINSULAR RANGES BATHOLITH, MEXICO

The results of a survey in a ~1,500 km² area of the PRB southwestern margin are presented. From aeromagnetic analysis, and the petrologic and structural features of 14 discrete plutons and their enclosing rocks, we identified the typical western magnetite-rich and eastern ilmenite-rich zones separated by a wide zone showing gradual changes. Main lithologic associations of the western Zone I are gabbro-diorite-tonalite with magnetite>titanomagnetite (with exsolved ilmenite). The main units of the central Zone II are diorite, quartzdiorite and tonalite with magnetite and titanite. The Zone III contains quartzdiorite-tonalite with ilmenite, titanite and rutile. Enclosing rocks of Zone I are Late Jurassic-Early Cretaceous (JK) intermediate volcanic and plutonic rocks; in Zone II, volcanic, pyroclastic and volcanoclastic rocks with psammitic units and pelitic horizons are dominant. Zone III contains granitic gneisses and muscovite-rich psammitic xenoliths. The enclosing rocks of the zones I and II show geochemical island-arc affinities while xenoliths from the Zone III contain Jurassic to Middle Proterozoic zircons. Zone II plutons intrude intra and back-arc sequences of the JK arc, which were isoclinally deformed when the emplacement of the K plutons took place. The K-plutons and the JK sequences of zones I and II are characterized by right-lateral transpressional deformation. According with the eastward decreasing Ca/Na ratio of plagioclases, K plutons vary from gabbro to tonalite. The mafic plutons are smaller than the eastern ones which can reach batholithic dimensions. Magnetoteluric and aeromagnetic data of discrete mafic plutons suggest that the mafic bodies show deeper cylinderlike shapes compared with the eastern plutons. K plutons are metaluminous, show I type affinities and calc-alkaline and calcic trends. Genetic relationships are suggested from geochemical data. The enclosing J volcanic rocks are richer in Cr, MgO y CaO than similar units of the K Alisitos arc. The plutons of the Zone III are similar to the ilmenite-rich intrusives described in the literature. Changes in the orientation between the zones can be attributed to left-lateral regional deformation. It is proposed a J-K continuous process of magmatic accretion. The J arc and back-arc were deformed and later intruded by the K plutons.