Date of Award

1999

Degree Name

Doctor of Philosophy

Department

Geological Sciences

College

Science

Advisor(s)

Elizabeth Y. Anthony

Abstract

The Pleistocene Potrillo volcanic field (PVF) resides within the southern axis of the Rio Grande Rift, New Mexico, U.S.A., near the eastern extent of the Basin and Range Province. Its alkalic mafic volcanism has resulted in several hundred cones, flows and maars distributed over approximately 4,600 km2. Alignments are segregated into two halves relative to the East Robledo fault system that dissects this field. Three of the five maars have brought peridotitic and lower to upper crustal xenoliths to the surface; several older, non maar-related flows from the west half of the field host ultramafic clots. Stratigraphic relationships, 3He surface exposure dating and 40Ar/39Ar methods confirm this area has been active since 1 Ma ago to as recently as 20 ka ago. The Kilbourne Hole maar was dated at 28 ka by the acquisition of exhumation ages of deposits using age differences between syn-maar and proximal non-maar deposits. Elemental and isotopic signatures indicate source heterogeneity coupled with varying degrees of partial melting and polybaric crystal fractionation events. Melts underwent early clinopyroxene fractionation within the mantle, but then the easternmost volcanic complexes experienced a second, shallow-level olivine fractionation history. Fundamental differences existing between the east and west half of the field are further established by 87Sr/86Sr (0.703087 - 0.703917), 143Nd/144Nd (4.4 - 6.7 ƐNd), 206Pb /204Pb (18.363 - 19.081), 207Pb /204Pb (15.520 - 15.584), 208Pb /204Pb (38.192 - 38.699), and magmatic 3He/4He ( 5 - 1 5 R/Ra) isotopic data for ten lavas. Two isotopic groups are observed: Group I is characterized by higher ƐNd and Pb isotopes and lower 87Sr/86Sr.Group II melts display the opposite trends. At least three mantle reservoirs may have contributed to the melts: (1) PREMA, (2) HIMU, and (3) either EM1 or Lower Crust for Group II. The primitive (Mg # < 64) lavas have relatively high Al203 and Yb associated with low La/Yb and CaO/ Al203. similar to the San Quintin volcanic field in Baja California Norte, Mexico (Luhr et al., 1995). These observations are consistent with progressive partial melting of spinel Iherzolite at unusually shallow mantle levels.

A complex magmatic history for the Potrillo volcanic field is now elucidated from combining improved Quaternary dating methods with detailed geochemical studies and structural information. The magma dynamics model integrates temporal, spatial and chemical evidence in light of magma emplacement and neotectonic parameters. Five phases of volcanism are recognized. The punctuated volcanic activity, presence of both monogenetic and polygenetic centers, and evidence for shifting eruption foci across 30 km lateral distances during approximately 106 year time frames are explained in terms of a crack coalescence model by Takada. Changes in magma input rates and/or differential stress fields are invoked. These findings for the Potrillo volcanic field are all causes for concern with respect to prediction of future activity trends within small mafic fields in intraplate extensional terranes.

Language

en

Provenance

Received from ProQuest

File Format

p. 212

Rights Holder

Wendi Joan Whitehead Williams

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