GEOLOGY AND GEOCHEMISTRY OF THE ACTIVE AZUFRE-PLANCHON-PETEROA VOLCANIC CENTER (35015 ' S, SOUTHERN ANDES): IMPLICATIONS FOR CORDILLERAN ARC MAGMATISM by Daniel Richard Tormey B.S. Civil Engineering and Geology, Stanford University (1983) Submitted to the Department of Earth, Atmospheric, and Planetary Sciences In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy at the Massachusetts Institute of Technology February, 1989 ©Daniel Richard Tormey, 1989. All rights reserved The author hereby grants to M.I.T. permission to reproduce and distribute copies of this thesis document in whole or in part. Signature of Author: Department of Earth, Atmospheric, & Plane6fry Sciences, MIT 6 January 1989 Certified by: I . rederick A.Frey Thesis Supervisor Accepted by: S,, Dr. Thomas H. Jordan Chairman, Departmental Committee on Graduate Students ARCH?," AP 111989 Geology and Geochemistry of the Active Azufre-Planchon-Peteroa Volcanic Center (35015'S, Southern Andes): Implications for Cordilleran Arc Magmatism by Daniel Richard Tormey Submitted to the Department of Earth, Atmospheric, and Planetary Sciences on 6 January 1989 in partial fulfillment of the requirements for the degree of Doctor of Philosophy ABSTRACT Magmatism in convergent margin settings is the dominant mechanism of mass transfer from the mantle to the crust and of crustal growth during the Phanerozoic. The results of a detailed study of the Azufre-Planchon-Peteroa Volcanic Center and of regional comparison of volcanoes in the Southern Volcanic Zone of the Andes between 330S and 420S constrain the sources, processes, and rates operative during magmatism of the southern Andean volcanic front. The Azufre-Planchon-Peteroa Volcanic Center is located at 350 15'S, in a transition zone of crustal thickness. Continental crust of the volcanic front is 35 km thick south of 370S and thickens to 55 km at 34.50S (Hildreth and Moorbath 1988). Azufre, the oldest (<0.55 m.y.) volcano of the center, is a multi-cyclic, bimodal basaltic andesite-dacite stratovolcano. Planchon is the northernmost basaltic volcano of the volcanic front, and grew relatively rapidly. Peteroa is the most recent and active center; it erupts calc-alkaline mixed magmas of andesitic and dacitic composition. Basalts and basaltic andesites of Azufre and Planchon are related by a plagioclase-poor, anhydrous fractionating mineral assemblage; high alumina basalt is produced from tholeiitic precursors by high pressure crystallization. During high pressure crystallization, small degree partial melts of the lower crust (probably an arc - derived garnet granulite protolith) rich in Rb, Cs, Th, and La/Yb contaminate evolving magmas. Azufre dacites form at low pressures by fractionation of a plagioclase rich mineral assemblage. Dacites are contaminated in the upper crust by limestone and by partial melts of igneous country rock, increasing 87Sr/86Sr, a0 18, Rb, Cs, and Th in evolving magmas. Two depths (upper and lower crust) of contamination are evident in the Planchon-Azufre system, with 10-15% crust incorporated in each environment. This style of polybaric, polysource compositional variation is prevalent among volcanoes between the segment boundaries at 34.50S and 370S, where crustal thickness changes rapidly. In the thin crust region south of 370S, lavas evolve dominantly by low pressure fractional crystallization, with minimal amounts of crustal -2- contamination. The 370S segment boundary is an important discontinuity in the predominant depths of fractional crystallization and amounts of crustal contamination. The 370S segment boundary is also an important discontinuity in the amount of fluxing of the mantle wedge by fluids derived from subducting oceanic crust, and the corresponding degrees of partial melting of garnet lherzolite mantle in response to the fluxing. Relative to lavas north of 370S, lavas south of 370S have low Rb/Cs (high amount of slab fluxing of the overlying mantle wedge) and low La/Yb (high degrees of melting of a garnet Iherzolite source). U-Th disequiblibrium data and 238U enrichment correlate with Rb/Cs and 10Be, indicating U-Th fractionation was via a low Th/U slab-derived fluid. To allow sampling of lavas in 238U-230Th disequilibrium, the combined rate for melt coalescence, ascent, and eruption must be 1.33-0.44 m/year. Thesis Supervisor: Frederick A. Frey Professor of Geochemistry -3- Table of Contents Title page........................................................................................................................1 Abstract ........................................................................................................................... 2 Table of Contents................................................... ................................................. 4 List of Figures....................................................................... .................................... 7 List of Tables................................................................................................................... 10 Acknowledgements ..................................................................................................... 11 CHAPTER ONE GEOLOGIC HISTORY OF THE ACTIVE AZUFRE-PLANCHON- PETEROA VOLCANIC CENTER (35015' S, ANDEAN SOUTHERN VOLCANIC ZONE), WITH IMPLICATIONS FOR THE DEVELOPMENT OF COMPOSITIONAL GAPS....................................... 12 Abstract ............................................................................................................. 13 Introduction .................................................................................................................... 13 Basement Geology .......................................................... ....................................... 16 Volcan Azufre ................................................................................................................ 19 Volcan Planchon...........................................................................................................25 Rio Teno Debris Avalanche................................................................. ................. 27 Volcan Planchon II........................................................................................29 Volcan Peteroa.............................................................................................................29 Hydrothermal Activity .......................................................................... ................... 30 Hazard Assessment ................................................................................................ 31 Implications For Compositional Gaps ..................................... .......... 32 Summary ........................................................................................................................ 39 CHAPTER TWO GEOCHEMISTRY OF THE ACTIVE AZUFRE-PLANCHON- PETEROA VOLCANIC CENTER (35015' S, ANDEAN SOUTHERN VOLCANIC ZONE), EVIDENCE FOR MULTIPLE SOURCES AND PROCESSES CONTRIBUTING TO THE DEVELOPMENT OF A CORDILLERAN ARC MAGMATIC SYSTEM ................................................ 42 Abstract...........................................................................................................................43 Introduction ................................................................................................... ..... 43 Stratigraphy and Geologic History .................................... ............ 48 Mineralogy and Petrography ............................................................................ .... 57 Planchon I and II............................................................................................ 57 Olivine ............................................................................................................. 61 Clinopyroxene..................................................................................................... 65 Oxides ...................................................................... 68 Plagioclase ............................................................................................ 72 Melt Inclusions...............................................................................................75 Azufre .................................................................................................................. 75 Peteroa ................................................................................................................ 79 Do W hole Rock Analyses Represent Liquids?................. ...................... .. ... 83 Summary of Mineralogy and Petrography.................................. 84 Compositional Variation .................................................... .................................... 88 Volcan Planchon................................................................................................ 88 Volcan Azufre .......................................................... 04 Volcan Peteroa........................................................................................................ 111 -4- Summary of Compositional Variation ......................................................... 16 Discussio n...................................................................................................................... 11 8 Models of Major Element Variation ..................................... 118 Planchon ................................................... 118 A zufre ........................................................... 30 Peteroa .........................................................