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Discipline: Ecology Environmental and Earth Sciences
Subcategory: Geosciences and Earth Sciences
David Quezada - California State University of Northridge
Co-Author(s): Joshua Schwartz and Mike Cho, California State University of Northridge, Northridge, CA Harold Stowell, University of Alabama, Tuscaloosa, AL Keith Klepeis, University of Vermont, Burlington, VT
High-grade metasedimentary rocks located south of the Alpine Fault in western Fiordland, New Zealand represent the host rocks into which a Mesozoic Cordilleran arc was constructed. These rocks consist of variably metamorphosed and deformed quartzofeldspathic paragneiss, marble, and calc-silicate. They comprise nearly 40% of the Cretaceous lower crust; however, little is known about their depositional age and sedimentary history. Detrital zircon cores were analyzed by LA-SF-ICPMS from paragneisses in northern Fiordland to 1) evaluate sources of sediment, and 2) determine whether samples from various mapped units share similar or contrasting depositional histories. Samples consist of two paragneisses from George Sound and two paragneisses from Bligh Sound, all of which were intruded by the Western Fiordland Orthogneiss and were metamorphosed at granulite-facies conditions at 116 to 120 Ma (Gebauer, 2016). Results from all four samples reveal that zircon cores are dominantly Cretaceous, and give error weighted average ages of 116.3 ± 2.1 Ma (n=25) and 119.9 ± 1.5 Ma (n=42) for the George Sound samples, and 120.9 ± 1.4 Ma (n=61) and 120.9 ± 1.6 Ma (n=25) for the Bligh Sound samples. These results are identical to metamorphic rim dates determined by SHRIMP-RG from the same samples that give Ti-in-zircon temperatures ranging from 700-830ºC (Gebauer, 2016). We therefore interpret these young dates as recording metamorphic growth during amphibolite to granulite facies metamorphism. Both George and Bligh Sound samples also yield zircons with dates of ca. 143 Ma. These grains have Th/U >0.3 suggesting that they may be detrital and indicate an Early Cretaceous/Late Jurassic depositional age. Pre-Cretaceous grains in both suites give Paleozoic to Mesozoic dates, with peak populations of 173, 193, 202, 220, 231, 254, 272, 292, 318, and 367 Ma. Few Precambrian zircons were observed. Collectively, our data display strong similarities to detrital zircon age patterns in tectono-stratigraphic terranes of the Eastern Province of New Zealand which are dominated by Triassic to Permian grains, and very few Devonian to Silurian populations. These data highlight the utility of detrital zircon analysis in reconstructing the paleogeography of the paleo-Pacific margin of Gondwana prior to Cretaceous high flux magmatism.
Funder Acknowledgement(s): National Science Foundation
Faculty Advisor: Joshua J. Schwartz, joshua.j.schwartz@gmail.com
Role: Once I decided on a hypothesis, I studied detrital zircons gathered from samples of the George Sound Paragneiss (GSP) from Fiordland, New Zealand, using the LA-SF-ICPMS at California State University of Northridge, and methods of 238U/206Pb radioactive dating. By using these methods I gathered and compiled ages of the GSP and from then on I compared it to other authors that had studied the GSP unit and nearby terranes, to find what comparisons or contrasts I could identify by these data sets.
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