T19 Origin and Evolution of the Continental Crust: A Celebration of the Career of Paul Mueller

Sunday, 22 October 2017
8:00 AM - 12:00 PM
Skagit 2

PALEOPROTEROZOIC CRUSTAL GROWTH IN THE CENTRAL US: HF AND OXYGEN ISOTOPE EVIDENCE FROM MAGMATIC ZIRCON IN THE YAVAPAI PROVINCE OF NORTH-CENTRAL COLORADO

Paper number: T19-13
Program: Technical Sessions
Time: 11:00 AM – 11:15 AM

Authors

Abstract

The nature of continental crust growth in deep time is strongly debated. Different approaches, many using radiogenic isotopes and commonly on whole rock samples (Sr, Nd, Hf), have been used to characterize crustal growth through time. In the last decade, combined in situ measurements of d18O, Hf, U-Pb on zircon evolved as the most robust and reliable way to assess juvenile vs. recycled nature of the crust.

These methods were applied to Paleoproterozoic orthogneisses to assess two competing models, both based on Nd and U-Pb dating, for crustal evolution of the Yavapai terrane in north-central Colorado (CO). One involves continuous southward accretion of arc crust, the other rifting in central CO with juvenile input from the mantle.

Nearly 300 δ18O analyses were carried out on zircons from 22 samples from northern, central and southern regions in the Yavapai terrane, sampling the oldest magmatic rocks in each domain as previously determined by ion microprobe U-Pb zircon dating. d18O averages were calculated for each sample after post-analysis vetting by CL imaging and Raman spectroscopy. Three felsic samples contain several distinct groups, but most samples showed limited scatter in δ18O (4.8-6.6) and mainly within uncertainty for zircon in equilibrium with the mantle, and typical for older, juvenile crustal provinces. Overall, felsic samples show a wider range of δ18O (4.3-6.6) than intermediate-felsic ones (5.2-6,2), but averages for both groups are within the mantle array. Samples with unusually low δ18O (3.1-4.4) may be a result of magma interaction with hydrothermal fluids.

Some U-Pb magmatic ages in the southern region are 20 to 30 Myrs younger than the typical 1760-1785 Ma range to the north, but there is no age vs. δ18O correlation, and thus no trend towards more crustal input into the parent magma source over time. There are no distinct differences between the N and S regions or a particular mantle input in the central domain, postulated to be produced by rift magmatism in one of the models. In whole rock Nd vs. δ18O space most samples plot near mantle values for both systems, except for 4 samples from the central and 2 from the southern region with a weak trend towards an older crustal component. This will be explored using Hf isotope analyses obtained on zircon domains matched with the oxygen results.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-306422

© Copyright 2017 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.