GEOLOGICAL SURVEY OF CANADA BULLETIN 486 DISTRIBUTION, NATURE, AND ORIGINS OF THE 24OO BP ERUPTION PRODUCTS OF MOUNT MEAGER, BRITISH COLUMBIA: LINKAGES BETWEEN MAGMA CHEMISTRY AND ERUPTION BEHAVIOUR M.V. Stasiuk, J.K. Russell, and C.J. Hickson **"urces Ressources naturerres I *l 331{fl o Her Majesty the Queen in Right of Canada, 1996 Cataloguc No. M42-486E ISBN 0-660-16450-7 Available in Canada from Ceological Survey of Canada offices: 601 Booth Street Ottawa, Ontario KIA 0E8 3303-33rd Street N.W., Calgary, Alberta T2L 2A7 100 West Pender Street Vancouver, B.C. V6B lR8 or from CanadaCommunicationGroup Publishing Ottawa, Onta o KIA 0S9 A deposit copy of this publication is also available for reference in selected public libraries across Canada P ce subject to change without notice Cover illustration View westward across Lillooet River to the base of Plinth Peak. Bridge River tephra (BRvto forms a 60 m thick deposit at the base of the peak, overlying rock avalanche unit AvbxDl. The precipitous slopes are prone to rock avalanches and lead to the thiik accumulations of tephra at the base of the stope. GSC 1996-095 Authors' addresses M.V, Stasiuk laboratoire de physique de maSmatique Uniwrsiti de Paris J.K. Russell D epartment of C eolo gica I Sciences University of British Columbia 6339 Srores Road Vancouter, B r itis h C olum bia v6T 124 CJ. Hicksol Ceolo gical Sur,,ey of Canada 100 West Pender Street Vancouver, B,C, v6B lR8 Manuscript submitted: 1994-06 Approvedfor publication: 1995- l0 CONTENTS I Abstract/R6sumd 2 Summary/Sommaire 5 Introduction 5 Overview and purpose 6 Geological background 1 Field observations 7 Plinth Peak avalanche deposit (unit Aybxpr) 1 Airfall pumice (unit BRvtf.l l2 Pyroclastic flow (unit BRvbxpn) t2 Welded block and ash breccia (unit BRvbxr) 12 Pyroclastic 11ow (unit BRYbxpn) t2 Debris flow (unit BRvs) t2 Welded block and ash breccia (unit BRvbx2) t3 Outburst flood deposit (unit BRyla) l4 Indurated block and ash breccia (unit BRybx3) t4 Lava flow (unit BRvd) l5 Avalanche of Plinth Assemblage (unit Avbxpz) 15 Petrographic characteristics 15 Plinth Assemblage (unit PLA) 15 Bridge River volcanic assemblage 15 Mineralogy t8 Crystallization history l9 Banded pumice rocks 20 Geochemistry 20 Analytical conditions 20 Rock chemical compositions 22 Discussion 23 Eruption trigger 23 Progression of activity 23 Transition in eruption behaviour 24 Acknowledgmcnts 24 Referenccs Appendix l. Major and trace element compositions ofMount Meager volcanic rocks Tables I . Locations and dimensions of airfall pumice sections used for thickness and maximum pumice and lithic measurements l8 2. Outcrop localities, descriptions and petrographic features of representative samples of the Bridge River Volcanic Assemblage 21 3. Whole rock major and trace element chemical compositions of select Bridge River Asscmblage volcanic rocks with estimated analytical uncenainties 2t 4. Whole rock major and trace element chemical compositions of representative Plinth Assemblage lavas.