Diachronous Redistribution of Hf and Nd Isotopes at the Crystal Scale—Consequences for the Isotopic Evolution of a Poly-Metamorphic Crustal Terrane

Adrien Vezinet,Yan Luo,Chiranjeeb Sarkar,D Graham Pearson,Emilie Thomassot

doi:10.3390/geosciences12010036

Abstract

In metamorphic rocks, mineral species react over a range of pressure–temperature conditions that do not necessarily overlap. Mineral equilibration can occur at varied points along the metamorphic pressure–temperature (PT) path, and thus at different times. The sole or dominant use of zircon isotopic compositions to constrain the evolution of metamorphic rocks might then inadvertently skew geological interpretations towards one aspect or one moment of a rock’s history. Here, we present in-situ U–Pb/Sm–Nd isotope analyses of the apatite crystals extracted from two meta-igneous rocks exposed in the Saglek Block (North Atlantic craton, Canada), an Archean metamorphic terrane, with the aim of examining the various signatures and events that they record. The data are combined with published U–Pb/Hf/O isotope compositions of zircon extracted from the same hand-specimens. We found an offset of nearly ca. 1.5 Gyr between U-Pb ages derived from the oldest zircon cores and apatite U–Pb/Sm–Nd isotopic ages, and an offset of ca. 200 Ma between the youngest zircon metamorphic overgrowths and apatite. These differences in metamorphic ages recorded by zircon and apatite mean that the redistribution of Hf isotopes (largely hosted in zircon) and Nd isotopes (largely hosted in apatite within these rocks), were not synchronous at the hand-specimen scale (≤~0.001 m3). We propose that the diachronous redistribution of Hf and Nd isotopes and their parent isotopes was caused by the different PT conditions of growth equilibration between zircon and apatite during metamorphism. These findings document the latest metamorphic evolution of the Saglek Block, highlighting the role played by intra-crustal reworking during the late-Archean regional metamorphic event.

Highlights

Combining the various isotope systematics accessible in zircon (ZrSiO4), such as U-Pb and Lu-Hf isotopes, has become a classical means for studying the crustal remnants exposed in early-Archean terranes (e.g., [1,2,3,4,5])
Complementary information can be obtained from the study of rare earth element (REE)-rich minerals, such as apatite, titanite and, monazite [9] which, in addition to hosting trace elements that can be used to discriminate their origin [10], can host appreciable amounts of radioactive U, Th and Sm, which result in measurable amounts of radiogenic Pb and Nd with laser ablation protocols (e.g., [11,12,13,14])
Sample LA12-08 was selected as representing a felsic igneous rock in the area that was least modified by metamorphism since magma crystallization

Summary

Introduction

Combining the various isotope systematics accessible in zircon (ZrSiO4), such as U-Pb and Lu-Hf isotopes, has become a classical means for studying the crustal remnants exposed in early-Archean terranes (e.g., [1,2,3,4,5]). Zircon chemical and isotopic compositions reflect only one aspect of a broader, and in some instances protracted, story Using solely these compositions might introduce bias in models for the Hadean/Archean evolution. Complementary information can be obtained from the study of rare earth element (REE)-rich minerals, such as apatite, titanite and, monazite [9] which, in addition to hosting trace elements that can be used to discriminate their origin [10], can host appreciable amounts of radioactive U, Th and Sm, which result in measurable amounts of radiogenic Pb and Nd with laser ablation protocols (e.g., [11,12,13,14]) These properties, and the relatively high reactivity of REE-rich minerals during metamorphism, make these minerals wellsuited to providing more definite constraints on the timing and conditions of metamorphic processes [15,16,17,18,19,20]

Methods

Results

Conclusion