Petrogenesis of the Nashwaak Granite, West-Central New Brunswick, Canada: Evidence from Trace Elements, O and Hf Isotopes of Zircon, and O Isotopes of Quartz

Wei Zhang,David R Lentz,Kathleen G Thorne

doi:10.3390/min10070614

Wei Zhang, David R Lentz + Show 1 more

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https://doi.org/10.3390/min10070614

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Abstract

The petrogenesis of the Pridoli to Early Lochkovian granites in the Miramichi Highlands of New Brunswick, Canada, is controversial. This study focuses on the Pridoli Nashwaak Granite (biotite granite and two-mica granite). In situ trace elements and O and Hf isotopes in zircon, coupled with O isotopes in quartz, are used to reveal its magmatic sources and evolution processes. In the biotite granite, inherited zircon cores have broadly homogenous δ18OZrc ranging from +6.7‰ to 7.4‰, whereas magmatic zircon rims have δ18OZrc of +6.3‰ to 7.2‰ and εHf(t) of −0.39 to −5.10. The Hf and Yb/Gd increase with decreasing Th/U. Quartz is isotopically equilibrated with magmatic zircon rims. The biotite granite is interpreted to be solely derived by partial melting of old basement rocks of Ganderia and fractionally crystallized at the fO2 of 10−21 to 10−10 bars. The two-mica granite has heterogeneous inherited zircon cores (δ18OZrc of +5.2‰ to 9.9‰) and rims (δ18OZrc of +6.2‰ to 8.7‰), and εHf(t) of −11.7 to −1.01. The two-mica granite was derived from the same basement, but with supracrustal contamination. This open-system process is also recorded by Yb/Gd and Th/U ratios in zircon and isotopic disequilibrium between magmatic zircon rims and quartz (+10.3 ± 0.2‰).

Highlights

The Canadian Appalachians formed by accretion of several Gondwanan microcontinents to Laurentia as a result of Early to Late Paleozoic closure of the Iapetus and Rheic oceans [1]
New Brunswick is mostly underlain by Neoproterozoic Ganderian basement that is overlain by Cambro–Ordovician, quartz-rich, passive margin sedimentary rocks
Both the biotite granite and two-mica granite in the Nashwaak Granite have similar U and Th content (Table S3), but zircon rims in two-mica granite in the Nashwaak Granite have similar U and Th content (Table S3), but zircon rims the two-mica granite have a wider range of Th/U and Hf content

Summary

Introduction

The Canadian Appalachians formed by accretion of several Gondwanan microcontinents to Laurentia as a result of Early to Late Paleozoic closure of the Iapetus and Rheic oceans [1]. Closure of the Tetagouche–Exploits backarc basin along the Bamford Brook Fault in Ganderia resulted from the Salinic orogeny (450–423 Ma). This was followed by the Acadian orogeny due to the northwest flat-slab subduction of Avalonia beneath Ganderia along the Caledonia Fault (421–400 Ma) [2]. Ganderia is bordered to the north by arc volcanic rocks of the peri-Laurentian Notre Dame zone, and by the Avalonian microcontinent along the Caledonian Fault to the south [3]. Both Ganderia and Avalonia are Minerals 2020, 10, 614; doi:10.3390/min10070614 www.mdpi.com/journal/minerals

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