NEPHELINE.RICH FOIDOLITES AND RARE.EARTH MINERALIZATION IN THE EL PICACHO TERTIARV INTRUSIVE COMPLEX, SIERRA DE TAMAULIPAS, NORTHEASTERN MEXICO

The El Picacho Tertiary intrusive complex is located in the west-central part of the Sierra de Tamaulipas, in the alkaline province of northeast Mexico. The complex cuts Cretaceous limestone, and it consists of gabbro, kaersutite diorite, syenite, nepheline-rich foidolites (ijolite, urtite, juvite), and trachytic and phonolitic dikes. The nephelinerich foidolites are affected by alkali metasomatism and contain apatite-rich veins enriched in the rare-earth elements. The nepheline-rich foidolites consist of nepheline (55-91 9o), pyroxene (4-35V0) (Di36Hd3eAe25 to Dil8Hd37Ae45) and orthoclase (0-22V0), They are silica-undersaturated rocks (44-50 wt.olo SiOt with a strongly alkaline character (11-15 wt.9o Na2O + K2O) and low contents of Mg (l-4 w.q0 MgO), corresponding to fractionated products of a silica-undersaturated parental magma. The alkaline metasomatism is represented by a slightly peralkaline aegirine fenite, which consists of albite, orthoclase, cancrinite pseudomorphs after nepheline, and Na-pyroxene (DilaHd27Ae5e to Die1Hd27Ae72), Mafic-mineral-free fenite also is found in the complex and consists of orthoclase and albite, with minor amounts of interstitial quartz. The apatite-rich veins are composed of radial aggregates of apatite, spherulitic chalcedony, quartz, siderite, calcite, barite, and britholite, which is the main R.EE-mineral in the veins. The total REE content in the veins varies from 14600 to 296@ ppm, with strong enrichment in ZREE (La,/Lu ratios range from 308 to2258). A silicate-carbonate liquid immiscibility model is postulated and discussed in order to explain coherently the petrogenetic relationship of the nephelinerich foidolites with fenite metasomatism and apatite-rich veins enriched in LREE.