Petrology and Petrogenesis of Basaltic Rocks and Their Inclusions: Studies from the Rio Grande Rift, the Roman Comagmatic Province, and Oceanus Procellarum

Citation

Baldridge, Warren Scott (1979) Petrology and Petrogenesis of Basaltic Rocks and Their Inclusions: Studies from the Rio Grande Rift, the Roman Comagmatic Province, and Oceanus Procellarum. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/zw9m-1m71. https://resolver.caltech.edu/CaltechTHESIS:08012024-013913576

Abstract

Mafic volcanism associated with the central Rio Grande rift is mainly of Pliocene and Pleistocene age. Volcanic rocks occur as isolated small shield volcanoes and cinder cone fields at or near the top of the graben fill. Structurally, the central Rio Grande rift consists of the Espanola and Albuquerque-Belen basins (grabens), offset from each other 50-60 km along the transverse Jemez lineament. The largest and most compositionally complex volcanic field (Cerros del Rio) occurs in this offset zone. On the basis of petrography and whole-rock chemistry, mafic lavas of this field can be divided into four groups: high-alkali olivine tholeiite, basaltic andesite, alkali olivine basalt, and basanite. All were erupted approximately contemporaneously. Quantitative modeling based on detailed phase chemistry shows that none of these lavas can be related to each other by any simple processes involving observed phenocryst and xenocryst phases.

Latite-andesite, also present in this field, is derived from basaltic andesite by subtraction of about 35 percent of olivine, pyroxene, and calcic plagioclase, and by addition of 17 percent of xenocrystic quartz and sodic plagioclase. Quartz and plagioclase occur as single crystals or as single phase aggregates; quartz rarely has a euhedral habit. These features suggest that quartz and plagioclase may be relict high- pressure phenocrysts. Thus, basaltic andesite magma bodies, residing in independent crustal reservoirs, may evolve by gravitative differentiation. The lighter, relict high-pressure quartz and sodic plagioclase phenocrysts are concentrated at the tops of these reservoirs by flotation simultaneously as the heavier olivine, pyroxene, and calcic plagioclase sink.

Lavas from the northern Albuquerque-Belen basin consist of lower alkali olivine tholeiite, similar to basalt of the northern rift. Those from the central Albuquerque-Belen basin consist of olivine tholeiite, basaltic andesite, and alkali olivine basalt of relatively restricted compositional range.

Pre-Pliocene volcanism occurred mainly in the upper Oligocene/lower Miocene and in the late Miocene. The earlier phase of volcanism erupted lavas ranging in composition from olivine nephelinite to quartz tholeiite, while the later phase erupted both alkali olivine and tholeiitic basalts. Hence no well-defined change in magma composition occurred with time.

Experimental studies suggest that the olivine tholeiite was derived from >18% partial melting of spinel pyrolite at about 35 km and the alkali olivine basalt and basanite from approximately 10% partial melting at 50-70 km. Thermodynamic calculations emphasize the hazards of inferring pressure (depth) and temperature of origin from basaltic lavas which are not primitive. The distribution of parental magmas suggests a model in which a transverse shear zone taps a sub-crustal diapir and bleeds magmas from various depths between 35 and 70 km. In contrast, the "normal" genesis of basaltic magmas along the rift occurs at the top of the diapir immediately beneath the base of the crust.

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