Composition of primary postcumulus amphibole and phlogopite within an olivine cumulate in the Stillwater Complex, Montana

Abstract

Al Introduction Al Acknowledgments A2 Distribution, texture, and microscopic relations of amphibole and phlogopite A2 Mineralogy and petrology of the olivine cumulate A2 Variation in amount of amphibole and phlogopite in the olivine cumulate A2 Microscopic textures and associations A2 Techniques A6 Composition and substitution mechanisms of amphibole A6 Composition and substitution mechanisms of phlogopite A15 Assemblage-controlled compositional variations in brown amphibole A18 Compositional variation between assemblage groups within the same sample A18 Compositional variation among the assemblage groups within the same hornblende crystal A20 Variation in hornblende composition with stratigraphic position A25 Variation in phlogopite composition with stratigraphic position A28 Comparison of trace elements information A28 Interpretations and conclusions A29 References cited A34 FIGURES 1. Graphs showing combined modal volume of amphibole and phlogopite correlated with stratigraphic position in cyclic unit 2 A3 2. Photomicrographs showing textures of primary brown amphibole and phlogopite in olivine cumulates from cyclic unit 2 of the Peridotite zone of the Ultramafic series, Nye Basin area A4 3-8. Graphs showing: 3. Classification and nomenclature of amphiboles A14 4. Al versus (Na, K)A for the hornblendes in table 2 A17 5. Al IV -(Na, K)A versus A1 VI + 2Ti + Cr for hornblendes in table 2 A17 6. NaB versus A1 VI + 2Ti + Cr for hornblendes in table 2 A17 7. Al versus Ti, Cr, and A1 VI for hornblendes in table 2 A18 8. NaB versus A1 VI + 2Ti + Cr + (Na, K)A for hornblendes in table 2 A18 9. Triangular diagram showing phlogopite compositions from olivine cumulate of cyclic unit 2 cast as phlogopite, annite, and siderophyllite end members A20 10. Triangular diagram showing phlogopite compositions in terms of Ti,6-OSO (octahedral site occupancy), and 6-(Si + A1 VI) in structural formula A20 11. Plots of phlogopite compositions All 12. Sketches showing compositional variation in zoned hornblendes A26 13-16. Graphs showing: 13. Formula units of Cr and Ti in zoned hornblende and associated chromite A29 14. Variation in selected compositional properties of hornblende with stratigraphic position A30 15. Variation in selected compositional properties of phlogopite with stratigraphic position A32 16. Trace element variation in olivine cumulates with stratigraphic position A33