Inception and early evolution of the Ordovician Macquarie Arc of Eastern Gondwana margin: Zircon U-Pb-Hf evidence from the Molong Volcanic Belt, Lachlan Orogen
The Ordovician Macquarie Arc is faulted against and surrounded by coeval Gondwana-derived quartz turbidites, in the Lachlan Orogen of southeastern Australia. How these juvenile, island arc rocks were emplaced and structurally interleaved with continental margin sequences of eastern Gondwana is an on...
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ftanucanberra:oai:openresearch-repository.anu.edu.au:1885/202494 2023-05-15T17:09:59+02:00 Inception and early evolution of the Ordovician Macquarie Arc of Eastern Gondwana margin: Zircon U-Pb-Hf evidence from the Molong Volcanic Belt, Lachlan Orogen Zhang, Qing Buckman, S. Bennett, Vickie Nutman, Allen P. 2020-03-30T00:30:26Z application/pdf http://hdl.handle.net/1885/202494 https://doi.org/10.1016/j.lithos.2019.01.008 en_AU eng Elsevier 0024-4937 http://hdl.handle.net/1885/202494 doi:10.1016/j.lithos.2019.01.008 © 2019 Elsevier B.V Lithos Journal article 2020 ftanucanberra https://doi.org/10.1016/j.lithos.2019.01.008 2020-03-30T22:14:58Z The Ordovician Macquarie Arc is faulted against and surrounded by coeval Gondwana-derived quartz turbidites, in the Lachlan Orogen of southeastern Australia. How these juvenile, island arc rocks were emplaced and structurally interleaved with continental margin sequences of eastern Gondwana is an ongoing debate. Understanding the inception of this arc is critical in building an accurate Early Paleozoic reconstruction for eastern Gondwana. However, the arc's inception and early evolution are poorly-constrained due to the lack of reliable geochronological data. Integrated field observations, zircon U-Pb-Hf, mineral and whole rock geochemistry are presented here for the oldest units of the Molong Volcanic belt, which is a central strand of the dissected Macquarie arc. Geochemistry indicates that these are calc-alkaline rocks with high-K (locally shoshonitic) to medium-K affinities. The stratigraphically-lowest mafic volcanic and volcano-sedimentary rocks of the Mitchell Formation have a unimodal earliest Ordovician zircon UPb age of 479.8 ± 3.8 Ma, with uniform depleted mantle like initial zircon εHf values of +12 to +13. The lack of pre-Ordovician zircons and the uniform positive initial εHf value indicate that the Macquarie arc was initiated in an intra-oceanic setting, far from the influence of eastern Gondwana. The stratigraphically overlying Fairbridge Volcanics includes some more evolved volcano-sedimentary components and has a polymodal, but mostly Ordovician, zircon UPb age population, with a youngest component at 444.3 ± 2.4 Ma. Overall, its Ordovician grains show a greater spread in initial εHf values from +14 down to +8 than the Mitchell Formation, which suggests some continental influence, at least 35 million years after the arc was initiated. The lack of any significant Gondwanan inheritance in Early Ordovician volcaniclastic rocks of the Macquarie Arc along with geochemical comparisons with modern island arcs, suggest that the arc evolved outboard of Gondwana probably as a result of steep, easterly-directed subduction. As the Macquarie arc approached eastern Gondwana there would have been increasing quantities of continent derived sediment subducted beneath the arc which may account for the decreasing and more variable zircon initial εHf values and the appearance of sparse Precambrian continental detritus at the arc's moribund stage. The collision of the Macquarie island arc with Gondwana initiated the Benambran Orogeny and reflects an important mechanism of episodic continental growth involving the addition of juvenile oceanic crust to a continental margin, which contrasts and alternates with periods of purely accretionary growth. This project was supported by the GeoQuest Research Centre, University of Wollongong, Australia, and China Scholarship Council (No. 201506400054), China. We give our thanks to Yang Song for the funding support on EMPA and LA-ICPMS experiments (National Science Foundation of China for Youth, Grant No. 41402178), Yang Song and useful discussion on comparing with the Tibet geology Article in Journal/Newspaper Macquarie Island Australian National University: ANU Digital Collections Lithos 326-327 513 528 |
institution |
Open Polar |
collection |
Australian National University: ANU Digital Collections |
op_collection_id |
ftanucanberra |
language |
English |
description |
The Ordovician Macquarie Arc is faulted against and surrounded by coeval Gondwana-derived quartz turbidites, in the Lachlan Orogen of southeastern Australia. How these juvenile, island arc rocks were emplaced and structurally interleaved with continental margin sequences of eastern Gondwana is an ongoing debate. Understanding the inception of this arc is critical in building an accurate Early Paleozoic reconstruction for eastern Gondwana. However, the arc's inception and early evolution are poorly-constrained due to the lack of reliable geochronological data. Integrated field observations, zircon U-Pb-Hf, mineral and whole rock geochemistry are presented here for the oldest units of the Molong Volcanic belt, which is a central strand of the dissected Macquarie arc. Geochemistry indicates that these are calc-alkaline rocks with high-K (locally shoshonitic) to medium-K affinities. The stratigraphically-lowest mafic volcanic and volcano-sedimentary rocks of the Mitchell Formation have a unimodal earliest Ordovician zircon UPb age of 479.8 ± 3.8 Ma, with uniform depleted mantle like initial zircon εHf values of +12 to +13. The lack of pre-Ordovician zircons and the uniform positive initial εHf value indicate that the Macquarie arc was initiated in an intra-oceanic setting, far from the influence of eastern Gondwana. The stratigraphically overlying Fairbridge Volcanics includes some more evolved volcano-sedimentary components and has a polymodal, but mostly Ordovician, zircon UPb age population, with a youngest component at 444.3 ± 2.4 Ma. Overall, its Ordovician grains show a greater spread in initial εHf values from +14 down to +8 than the Mitchell Formation, which suggests some continental influence, at least 35 million years after the arc was initiated. The lack of any significant Gondwanan inheritance in Early Ordovician volcaniclastic rocks of the Macquarie Arc along with geochemical comparisons with modern island arcs, suggest that the arc evolved outboard of Gondwana probably as a result of steep, easterly-directed subduction. As the Macquarie arc approached eastern Gondwana there would have been increasing quantities of continent derived sediment subducted beneath the arc which may account for the decreasing and more variable zircon initial εHf values and the appearance of sparse Precambrian continental detritus at the arc's moribund stage. The collision of the Macquarie island arc with Gondwana initiated the Benambran Orogeny and reflects an important mechanism of episodic continental growth involving the addition of juvenile oceanic crust to a continental margin, which contrasts and alternates with periods of purely accretionary growth. This project was supported by the GeoQuest Research Centre, University of Wollongong, Australia, and China Scholarship Council (No. 201506400054), China. We give our thanks to Yang Song for the funding support on EMPA and LA-ICPMS experiments (National Science Foundation of China for Youth, Grant No. 41402178), Yang Song and useful discussion on comparing with the Tibet geology |
format |
Article in Journal/Newspaper |
author |
Zhang, Qing Buckman, S. Bennett, Vickie Nutman, Allen P. |
spellingShingle |
Zhang, Qing Buckman, S. Bennett, Vickie Nutman, Allen P. Inception and early evolution of the Ordovician Macquarie Arc of Eastern Gondwana margin: Zircon U-Pb-Hf evidence from the Molong Volcanic Belt, Lachlan Orogen |
author_facet |
Zhang, Qing Buckman, S. Bennett, Vickie Nutman, Allen P. |
author_sort |
Zhang, Qing |
title |
Inception and early evolution of the Ordovician Macquarie Arc of Eastern Gondwana margin: Zircon U-Pb-Hf evidence from the Molong Volcanic Belt, Lachlan Orogen |
title_short |
Inception and early evolution of the Ordovician Macquarie Arc of Eastern Gondwana margin: Zircon U-Pb-Hf evidence from the Molong Volcanic Belt, Lachlan Orogen |
title_full |
Inception and early evolution of the Ordovician Macquarie Arc of Eastern Gondwana margin: Zircon U-Pb-Hf evidence from the Molong Volcanic Belt, Lachlan Orogen |
title_fullStr |
Inception and early evolution of the Ordovician Macquarie Arc of Eastern Gondwana margin: Zircon U-Pb-Hf evidence from the Molong Volcanic Belt, Lachlan Orogen |
title_full_unstemmed |
Inception and early evolution of the Ordovician Macquarie Arc of Eastern Gondwana margin: Zircon U-Pb-Hf evidence from the Molong Volcanic Belt, Lachlan Orogen |
title_sort |
inception and early evolution of the ordovician macquarie arc of eastern gondwana margin: zircon u-pb-hf evidence from the molong volcanic belt, lachlan orogen |
publisher |
Elsevier |
publishDate |
2020 |
url |
http://hdl.handle.net/1885/202494 https://doi.org/10.1016/j.lithos.2019.01.008 |
genre |
Macquarie Island |
genre_facet |
Macquarie Island |
op_source |
Lithos |
op_relation |
0024-4937 http://hdl.handle.net/1885/202494 doi:10.1016/j.lithos.2019.01.008 |
op_rights |
© 2019 Elsevier B.V |
op_doi |
https://doi.org/10.1016/j.lithos.2019.01.008 |
container_title |
Lithos |
container_volume |
326-327 |
container_start_page |
513 |
op_container_end_page |
528 |
_version_ |
1766066384310108160 |