Collaborative Research: The Building of a Continent: The Archean to Proterozoic Growth and Modification of Western Laurentia

Washington State University, Pullman WA

Investigators

Abstract

The formation, growth, and modification of the continents is a fundamental topic in the geosciences. The key to understanding these processes lies in the rocks that form the "basement" of the continents. These rocks formed earlier in the history of the Earth and provide the foundation on which the continents were built. The age of these basement continental rocks, how they formed, and how they have been amalgamated and modified to form the foundation of the present day continents, however, is far from clear. The main reason for this is that these basement rocks are often buried under younger rocks or sediments and consequently are not exposed at the surface of the Earth. Where basement exposures do exist--at the cores of many continents and in isolated blocks that have been tectonically brought to the surface--these exposures provide critical pieces of the puzzle of the growth, evolution, and modification of the continents. One such area of basement rocks is the Clearwater complex, a nearly continuous block of crystalline basement exposed across a 115-km-long region of north central Idaho. Recent U-Pb zircon dating of rocks in the Clearwater complex by the PIs and their students have identified discrete periods of crust formation at 2.66 and 1.86 Ga and garnet Lu-Hf and monazite U-Th-Pb geochronology have demonstrated a complex metamorphic history from ~ 1.3 to 1.0 Ga. These new data and the fact that this area represents the largest belt of Neoarchean and Paleoproterozoic crust exposed in the Northwest U.S. outside of the Wyoming Province, that has been largely unexplored, make it a compelling area for further research. The primary goal of this proposed research is to use the Clearwater complex to examine three important transitions (2.7-2.6 Ga, 1.9-1.7 Ga, and 1.3-1.1 Ga) in Earth?s history from the Neoarchean to the Mesoproterozoic. This work will be accomplished through two principal tasks. First, we will examine the magmatic record preserved in orthogneisses to document the change in the nature of crust formation in this region from 2.6 to 1.8 Ga, which is preserved in the zircon U-Pb and Hf, and whole-rock Hf-Nd isotopic records. Second, we will examine the metamorphic record preserved in spatially associated metasedimentary rocks in order to document the timing and crustal conditions during the subsequent modification of this region during the Mesoproterozoic. The tools for this part of our work will be P-T phase equilibria studies that are integrated with monazite U-Th-Pb and garnet Lu-Hf and Sm-Nd geochronology. The detailed chronology obtained on these rocks will be accomplished by analysis of monazite inclusions in situ within garnet and subsequent dating of the same garnet by Lu-Hf and Sm-Nd methods. This integrated approach will allow insight into the nature of the coupling of these chronometers during regional metamorphism that will be of broad use to the metamorphic community.