Genesis of the Costilla Reservoir sill, New Mexico
- Johns Hopkins Univ., Baltimore, MD (United States). Dept. of Earth and Planetary Sciences
The Costilla Reservoir sill is a 220 m-thick, tabular hypabyssal intrusion in the Latir volcanic field of north central New Mexico. This late Oligocene body consists of olivine-bearing basaltic andesite ranging from 53 to 57% SiO[sub 2]. The chemical variations are not due to in-situ crystal settling, for modal and whole rock chemical profiles reveal no such redistribution. Settling calculations predict a cumulate pile in such a large igneous body, so the absence of settling suggests that a single, thick molten sheet never existed. Hence the authors conclude emplacement was episodic, extending over a period commensurate with solidification time (100+ years). The lack of internal chill zones precludes injection over a significantly longer period. Since no crystals were redistributed post-emplacement, the chemical differences within the body result from the injection of initially heterogeneous magma. The first pulse of magma, which is represented by the 10--20 m nearest each contact, differs from the bulk of the sill in FeO[sub total], Sc, and average An content of plagioclase. The subsequent injections are not chemically uniform, but their variations are gradational. The chemical disparities within the Costilla magma cannot be fully explained by preplacement fractionation of the major phases (plag, olivine, cpx, opx). The heterogeneity may have existed from the time and place of magma genesis, which appears to be in the lower crust. The majority of pyroxene and feldspar phenocrysts exhibit features (extensive fracturing, disequilibrium textures) suggesting they did not crystallize directly from this magma. Rather, they may be restitic grains inherited from a partially melted source area. Additionally, some of these crystals have been spawned by the abundant supply of xenoliths, some of which were disaggregating in the magma.
- OSTI ID:
- 6135812
- Report Number(s):
- CONF-921058-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 24:7; Conference: 1992 annual meeting of the Geological Society of America (GSA), Cincinnati, OH (United States), 26-29 Oct 1992; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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