LATE CRETACEOUS PLUTONIC AND METAMORPHIC OVERPRINT OF PROTEROZOIC METASEDIMENTS OF ONTARIO RIDGE, EASTERN SAN GABRIEL MOUNTAINS, CALIFORNIA A Thesis Presented to the Faculty of California State Polytechnic University, Pomona In Partial Fulfillment Of the Requirements for the Degree Master of Science In Geological Sciences By Scott B. Zylstra 2017 ACKNOWLEDGEMENTS Many thanks to Drs. Jonathan Nourse and Nicholas Van Buer for countless hours in research, assistance and editing on this project; to Zhan Peng, Anthony LeBeau and Karissa Vermillion for laboratory assistance; to the MENTORES program and the Cal Poly Geology Department for funding; to Joshua Schwartz and Carl Jacobson for valuable insight; and to Jennifer, Gwendolyn and Lydia Jean Zylstra, without whom this work would not be possible (and with whom sometimes has been scarcely possible). iii ABSTRACT The rifting of Rodinia, particularly in western Laurentia, has been a longstanding geologic problem, as Proterozoic rocks in the western U.S. are few and far between. The eastern San Gabriel Mountains contain a large metasedimentary package that has never been dated or thoroughly mapped. We present U-Pb detrital zircon geochronology results of 439 grains dated at CSUN’s LA-ICPMS; also 24 plutonic grains and 19 metamorphic rims of detrital grains analyzed on Stanford’s SHRIMP-RG, along with a detailed map, cross section and stratigraphic column for these Ontario Ridge metasediments. Probability plots of 206Pb/207Pb ages for five quartzite samples show three distinct peaks at ~1200, 1380-1470 and 1740-1780 Ma. Maximum depositional age is constrained by 3 grains between 906 to 934 Ma, which are 11-28% discordant. The ~1200 Ma peak distinguishes the Ontario Ridge quartzites, and is rarely seen in the western U.S. There are a few minor possible western sources, or, due in part to similarities with the Big Bear Group (Barth and Wooden, 2009), the rocks may represent sediment shed from Rodinia’s conjugate rift pair with western Laurentia. (Barth and Wooden, 2009). The detrital age signature also appears to match preliminary data from the Potato Mountain block (Premo et al., 2007), displaced ~8 km from the main metasediment body by the left-lateral San Antonio Canyon fault. The metasediments are intruded by pre-metamorphic granodiorite of Icehouse Canyon (85.9±0.6 Ma) and post-metamorphic quartz diorite of Ontario Peak (75.8±0.9 Ma). Two quartzites, metamorphosed to upper-amphibolite facies, contain zircons with iv metamorphic rims (75.7±1.2 Ma: 12 grains and 76.7±2.4 Ma: 7 grains), consistent with the last documented plutonic and metamorphic event in the San Gabriel Mountains. These new data provide important keys for future research into the Proterozoic and Mesozoic history of southwestern North America, including the breakup of Rodinia. v TABLE OF CONTENTS SIGNATURE PAGE ......................................................................................................... ii ACKNOWLEDGEMENTS ............................................................................................... iii ABSTRACT .................................................................................................................... iv LIST OF FIGURES ....................................................................................................... viii INTRODUCTION ............................................................................................................ 1 Location and Access: .................................................................................................. 1 Previous Work ............................................................................................................ 3 Purpose and Objectives .............................................................................................. 4 Research Questions and Hypotheses ..................................................................... 5 METHODS ..................................................................................................................... 8 Geologic Mapping ....................................................................................................... 8 Sample Collection ....................................................................................................... 9 Zircon Separation........................................................................................................ 9 U/Pb Analyses at California State University: Northridge ...........................................13 U/Pb Analyses at Stanford .........................................................................................15 RESULTS ......................................................................................................................17 Rock Units of the Eastern San Gabriel Mountains ......................................................17 Young Quaternary ..................................................................................................17 Older Quaternary ....................................................................................................19 Tertiary Volcanics ...................................................................................................20 Tertiary/Cretaceous Mylonites ................................................................................20 Cretaceous Granitoids ............................................................................................27 Triassic Plutonics ...................................................................................................34 Ontario Ridge Metasediments ................................................................................34 Cucamonga Granulite .............................................................................................57 Precambrian Basement ..........................................................................................58 Stratigraphy of the Ontario Ridge Metasediments ......................................................59 Member 10 .............................................................................................................60 Member 9 ...............................................................................................................61 Member 8 ...............................................................................................................61 vi Member 7 ...............................................................................................................62 Member 6 ...............................................................................................................63 Member 5 ...............................................................................................................64 Member 4 ...............................................................................................................65 Member 3 ...............................................................................................................66 Member 2 ...............................................................................................................66 Member 1 ...............................................................................................................67 Geochronology of the Ontario Ridge Study Area........................................................68 DISCUSSION ................................................................................................................78 Precambrian Basement..............................................................................................78 Ontario Ridge Metasediments ....................................................................................79 General Distribution and Thickness ........................................................................79 Structure .................................................................................................................82 Original Stratigraphy ...............................................................................................87 Age .........................................................................................................................91 Cretaceous Plutonics and Metamorphism ................................................................112 CONCLUSIONS ..........................................................................................................114 Recommendations ...................................................................................................116 REFERENCES ............................................................................................................118 APPENDICES .........................................................................................................122 21 Summary of U-Pb Zircon Analyses: Stanford University SHRIMP lab Summary of U-Pb-Th Detrital Zircon Analyses: CSUN ICP-MS lab Plate 1: Geologic Map of Ontario Ridge Metasediments Plate 2: Stratigraphic Section of Ontario Ridge Metasediments Plate 3: Structural Cross Section Through Ontario Ridge Metasediments vii LIST OF FIGURES Figure 1: Overview .............................................................................................................. 2 Figure 2: Snow and pines .................................................................................................... 3 Figure 3: Glass mount ....................................................................................................... 13 Figure 4: Metamorphic zircon ..........................................................................................