Petrologic Characterization and Age of Eclogite Bodies in the Brooks Range, Northern Alaska
Introduction
The Brooks Range is a 700-mile-long mountain range that stretches across remote northern Alaska and has a tectonic history that is poorly understood. The most deeply buried metamorphic rocks in the Brooks Range are mostly blueschist-/greenschist-facies rocks that occur as a continuous belt referred to as the Schist belt (SB) and lies adjacent to the lower pressure greenschist facies Central belt (CB; Till, 2008; Moore et al., 2004). The SB contains only two eclogite bodies (e.g., Gottschalk, 1998; Till et al., 2008), which occur as localized lenses surrounded by blueschist-/greenschist-facies rocks. The mineralogy, peak burial depth, and P-T history of these rock has not been studied. We have samples from both eclogite bodies and initial observations show that they range from perfectly preserved eclogites that are primarily garnet+omphacite to some eclogites that have undergone significant retrograde metamorphic reactions and contain hornblende+plagioclase. The age of the metamorphism is also poorly known with current models ranging from ~170 to 130 Ma (Till, 2016; Lemonnier, 2016).
In this study, I will be analyzing the eclogite samples through various methods to determine the maximum pressures of burial, the P-T history, and the age of high-pressure metamorphism. These will aid in understanding the tectonic and metamorphic evolution of the Brooks Range.
Research Methods
(1) Optical petrographic analysis of eclogite thin sections to identify mineral assemblages, reaction textures, and zoning patterns.
(2) Elemental compositional maps using the Scanning Electron Microscope (SEM) will be used to aid in mineral identification, mineral inclusion identification, textural analysis, and compositional zoning patterns. Elemental maps will also be used to detect and locate zircons that will be used for isotopic dating.
(3) In-situ U/Pb dating of zircons will be used to date the high-pressure metamorphism and potentially exhumation. I will date zircons identified in thin sections from the SEM maps. I will date zircons found as inclusions in garnet and omphacite that will determine the age of high-pressure metamorphism and other zircons not associated with the high-pressure minerals that may give the age at which the high-pressure rocks were exhumed. This work will be done using Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS) in the lab at UF.
Goals
The results of this work will address important questions regarding the tectonic history of the Brooks Range. Many models have been proposed for the timing and evolution of the Brooks Range (e.g., Moore et al., 1997; Till, 2016), but these are hindered by the lack of understanding of the P-T history of the SB and the unknown age of the high-pressure metamorphism. My data, along with data collected by current UF graduate students will help test models for Brooks Range tectonics and for the processes of exhumation of high-pressure rocks. Establishing the age of high-pressure metamorphism from my U/Pb zircon dating will also provide an independent check of ages determined by direct dating of garnet using Lu/Hf and Sm/Nd dating systems by current UF graduate student Minh Pham.
Photo from the Field in the Brooks Range
University of Florida, Department of Geological Sciences
Dr. James Vogl
Involved in research since December 2023
