Pre-caldera forming pyroclastic deposits: Implications for the origin of the large volume Rainier Mesa ash-flow sheet
- Michigan State Univ., East Lansing, MI (United States). Dept. of Geological Sciences
Small ash-flow layers directly underlying the voluminous Rainier Mesa ash-flow sheet are chemically equivalent to the overlying ash-flow. However, chemical variability of these precaldera-collapse pyroclastic deposits indicates they could not have erupted from the large volume Rainier Mesa Member (RMM) magma body. The RMM is a voluminous chemically and mineralogically layered and zoned ash-flow sheet. Directly underlying the RMM is a sequence of tephra-fall deposits interbedded with thin ash-flow layers, traditionally interpreted as pre-caldera-collapse deposits from the large magma body. Chemical comparison of 14 thin flows with the overlying RMM was made in order to decipher the relationship between earlier erupted ash-flow layers and the large ash-flow sheet. Within each thin ash-flow layer, there is substantial variation in major, trace, and REE concentrations over a thickness of only 0.5 to 5 m. The extremely small volume of pre-Rainier Mesa ash-flow layers would represent a negligible volume of the magma body. The observed chemical zoning would not occur if these small volume deposits erupted from the RMM magma body. Major and trace element variation among the small ash-flow layers spans that of the entire high-silica portion of the RMM ash-flow sheet, which makes up 90% of the total volume of the ash-flow. Moreover, REE concentrations among the thin ash-flow layers are as variable as the REE concentrations of the entire overlying ash-flow sheet. Because there is significant chemical variation both within and among small ash-flow layers underlying the RMM, the authors reject the possibility of a larger differentiating magma body as the source of the small pre-Rainier ash-flow layers.
- OSTI ID:
- 5770822
- 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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