Rock and age relationships within the Talkeetna forearc subduction complex in the Nelchina area, Southern Alaska Item Type Thesis Authors Barefoot, John D. Download date 07/10/2021 06:12:08 Link to Item http://hdl.handle.net/11122/10269 ROCK AND AGE RELATIONSHIPS WITHIN THE TALKEETNA FOREARC SUBDUCTION COMPLEX IN THE NELCHINA AREA, SOUTHERN ALASKA By John D. Barefoot, B.S. A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science in Geology University of Alaska Fairbanks December 2018 APPROVED: Elisabeth Nadin, Committee Chair Rainer Newberry, Committee Member Mary Keskinen, Committee Member Paul McCarthy, Chair Department of Geosciences Leah Berman, Dean College of Natural Science and Mathematics Michael Castellini, Dean UAF Graduate School Abstract Subduction-zone processes are challenging to study because of the rarity of good exposures and the complexity of rock relationships within accretionary prisms. In south-central Alaska, a remarkably well-preserved exposure of subduction-related outcrops is located at the foot of Nelchina Glacier. Here, the crystalline basement of the Talkeetna volcanic arc is in contact with the melange of its associated accretionary complex along the Border Ranges fault. A new zircon U-Pb age of an amphibolite from the Talkeetna arc mid-crustal basement just north of the fault is 188.9 ± 2.2 Ma, coincident with previously published dates from the mafic section of the arc. A new amphibole 40Ar/39Ar age from the same outcrop yields a plateau age of 182.6 ± 1.3 Ma, reflecting cooling/exhumation of this part of the arc. The melange south of the arc and the Border Ranges fault, known as the McHugh Complex, comprises sheared metasedimentary rocks, metavolcanic rocks, and chert, and in the Nelchina area it includes a roughly 100-m- diameter block of pillow lavas that are undeformed but altered. Detailed compositional data show that the pillow lava block formed in an intraplate setting. New whole-rock 40Ar/39Ar analyses of two pillow-lava samples yielded irregular plateaus with an approximate age of 60 Ma, which we interpret to be largely reset due to reheating. Hypabyssal dikes crosscut the melange, as well as younger accretionary prism deposits in the area, and provide a new zircon U- Pb age of 53.0 ± 0.9 Ma, which coincides with ages of near-trench plutonism across southern Alaska. This plutonism has been ascribed to subduction of a spreading ridge that migrated eastward along the southern Alaska margin. These new ages constrain the McHugh Complex formation and subsequent hydrothermal alteration to pre-55 Ma. We suggest that the pillow lava was originally part of a Triassic (or earlier) seamount that was decapitated and incorporated into the melange as the oceanic plate entered the subduction zone. The pillow lava subsequently i underwent extensive hydrothermal alteration that almost completely reset its age during the ridge subduction event. We further posit that the Talkeetna volcanic arc and its associated accretionary prism sediments were in their current configuration during the ca. 55 Ma plutonism that was common throughout southern Alaska. ii Table of Contents Page Abstract ............................................................................................................................................ i Table of Contents .......................................................................................................................... iii List of Figures ..................................................................................................................................v List of Tables ................................................................................................................................ vi Chapter 1: Introduction ....................................................................................................................1 Melange..................................................................................................................................... 3 Geologic Background ................................................................................................................5 Southern Alaska terranes .................................................................................................... 5 The McHugh Complex ........................................................................................................6 Southern Alaska Near-Trench Magmatism ........................................................................ 7 References..................................................................................................................................9 Chapter 2: Rock and age relationships of the Talkeetna forearc subduction complex in the Nelchina area, southern Alaska................................................................................................17 Abstract ....................................................................................................................................18 I. Introduction ..........................................................................................................................19 II. Geologic Background..........................................................................................................21 III. Methods..............................................................................................................................23 Mapping ............................................................................................................................ 23 Mineralogy ........................................................................................................................ 24 Geochemistry .................................................................................................................... 24 U-Pb Geochronology ........................................................................................................ 25 40Ar/39Ar Geochronology.................................................................................................. 25 IV. Results................................................................................................................................26 Map Units.......................................................................................................................... 26 Gabbro, undifferentiated (Jgu).................................................................................... 26 Metasedimentary rocks (Mzms) ..................................................................................28 Metavolcanic rocks (Mzmv) ........................................................................................29 iii Mesomelange (Mzm).................................................................................................. 31 Deformed melange (Mzdm)........................................................................................ 32 Hypabyssal (“felsite”) dikes (Pgi) ...............................................................................32 Chickaloon Formation (Pgcg)......................................................................................33 V. Discussion ...........................................................................................................................33 Cooling and uplift of the Talkeetna arc .............................................................................33 McHugh Complex deposition ............................................................................................34 Post-McHugh Complex deposition and deformation ........................................................37 VI. Conclusions........................................................................................................................39 VII. Acknowledgements ..........................................................................................................39 VIII. References ...................................................................................................................... 41 Appendix (40Ar/39Ar methodology) ........................................................................................ 67 References for Appendix .................................................................................................. 68 Chapter 3: Conclusion....................................................................................................................69 Future Work ............................................................................................................................ 71 References................................................................................................................................72 Appendix........................................................................................................................................74 iv List of Figures Page Figure 1.1: Regional map showing the general terrane configuration of southern Alaska............14 Figure 1.2: Detailed map of the Chugach-Prince William composite terrane .............................. 15 Figure 1.3: Schematic plate configuration map of the west coast of North America ....................16 Figure 1: Map showing the general terrane configuration of southern Alaska..............................51 Figure 2: Map of Nelchina area melange showing recently exposed bedrock............................. 52 Figure 3: Photomicrograph of amphibolite sample 16Ba21 ..........................................................53