Fluorine geochemistry of quaternary volcanic rocks from Southwestern British Columbia: Some petrogenetic implications |
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| Authors: | Nathan L Green |
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| Institution: | (1) Department of Geology, University of Alabama, 35486 University, Alabama, USA |
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| Abstract: | Fluorine contents in 38 Quaternary volcanic rocks, representing calc-alkaline andesite eruptive groups from the Garibaldi Lake area, were determined by a selective ion-electrode method. A close relationship is evident between F abundance and the type of ferromagnesian phenocrysts present in the andesitic rocks. Hypersthene andesites have the lowest F contents (142–212 ppm), whereas hornblende-biotite andesites exhibit the highest F values (279–368 ppm); hornblende andesites have intermediate F contents (238–292 ppm). The hornblende-free Desolation Valley basaltic andesite has a lower F content than the hornblende-bearing Sphinx Moraine basaltic andesite (122 ppm versus 317–333 ppm).Different eruptive suites can be grouped on the basis of F differentiation patterns into (1) a hornblende-free lava series in which the F content of basaltic andesite is less than that of andesite, and (2) a hornblende-bearing lava series in which F contents remain constant or decrease slightly from basaltic andesite through dacite. Fluorine variation in the former series was controlled largely by fractionation of anhydrous minerals, whereas that in the latter was influenced by crystallization of amphibole, biotite and apatite.The distinctive F variation patterns of the two lava series appear to represent real differences in the volatile contents of Garibaldi Lake magmas. These different volatile concentrations may reflect varying degrees of magma-wallrock interaction, differences in the initial volatile contents of the primary magmas, or some combination of these factors. |
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