La Cueva Intrusive Complex and Dome, Northern Coahuila, Mexico
La Cueva intrusive complex shows a remarkable sequence of sodic igneous rocks. The major rocks, in order of intrusion, are: (1) quartz monzonite, (2) sodic syenite and gabbro of the differentiated series, and (3) sodic microsyenite. Most of the other igneous rocks exposed are part of a younger series of hypabyssal rhyolitic intrusions. La Cueva intrusive complex is within La Cueva dome. Striking changes in dips along the flanks of the dome result largely from multiple intrusion and variation in the shape of individual intrusive masses. Field relationships indicate forceful emplacement of the main mass of the complex in three stages: (1) early laccolithic intrusion of quartz monzonite, (2) intrusion of the differentiated series which tilted and broke through the laccolith, thrusting aside the country rock, and (3) intrusion of a pluglike body of microsyenite through the center of the differentiated series. Field associations suggest that the rocks may have been derived from the same regional magma, but the proportion of felsic to mafic rocks is unusually high. The large volume of rhyolitic rock may be the result of dilution of rising gabbroic magma by assimilation of constituents of the basement rocks that melt at low temperatures. The silica-deficient rocks may have been isolated in a subsidiary magma chamber, where they underwent a divergent trend of differentiation.