THE LA-BRENA-EL-JAGUEY-MAAR COMPLEX, DURANGO, MEXICO .1. GEOLOGICAL EVOLUTION

The La Brena - El Jaguey Maar Complex, of probable Holocene age, is one of the youngest eruptive centers in the Durango Volcanic Field (DVF), a Quaternary lava plain that covers 2100 km2 and includes about 100 cinder and lava cones. The volcanic complex consists of two intersecting maars - La Brena and El Jaguey - at least two pre-maar scoria cones and associated lavas, and a series of nested post-maar lava and scoria cones that erupted within La Brena Maar and flooded its floor with lava to form one or more lava lakes. We believe that El Jaguey Maar formed first, but pyroclastic deposits associated with its formation are exposed at only a few places in the lower maar walls. A perennial lake in the bottom of El Jaguey marks the top of an aquifer about 60 m below the lava plain. Interaction of the rising basanitic magmas with this aquifer was probably responsible for the hydromagmatic eruptions at the maar complex. In the southeastern quadrant of La Brena and in most parts of El Jaguey, the upper maar walls expose a thick pyroclastic sequence of tuffs, tuff breccias, and breccias that is dominated by thinly layered sandwave and plane-parallel surge beds and contains minor interlayered scoria-fall horizons. We conclude that these deposits in the upper walls of both maars erupted during the formation of La Brena, based on: (1) thickness variations in a prominent scoria-fall marker bed interlayered with the surge deposits; (2) inferred transport directions for ballistic clasts, channels, and dune-like bedforms; and (3) lateral facies changes in the surge deposits. Some of the surge clouds from La Brena apparently travelled down the inner southwestern wall of El Jaguey, fanned out across its floor, and climbed up the opposite walls before emerging onto the surrounding lava plain. These clouds deposited steep, inward-dipping surge deposits along the lower walls of El Jaguey. Following this hydromagmatic phase, which was responsible for the formation of the maars, a series of strombolian eruptions took place from vents within La Brena. At many places along the maar rims these eruptions completely buried the surge beds under a thick sequence of post-maar scoriae and ashes. The outer flanks of the maar complex and the surrounding lava plain are also blanketed by post-maar ashes. The final phase of activity involved effusive eruptions of post-maar lavas from vents on the floor of La Brena. The evolutionary sequence from hydromagmatic eruptions during formation of the maars, through strombolian eruptions of the post-maar scoriae and ashes, and finally to the post-maar lavas appears to reflect the declining influence of magma-groundwater interactions with time. Basanitic magmas from all eruptive stages carried spinel-lherzolite and feldspathic-granulite xenoliths to the surface. The La Brena - El Jaguey Maar Complex contains the only known hydromagmatic vents in the DVF and the largest spinel-lherzolite xenoliths, which range up to 30 cm diameter. These two observations indicate an unusually rapid ascent rate for these basanitic magmas compared to those from other DVF vents.