The Cadomian (500-600 Ma) Convergent Margin of Gondwana: Is there Evidence for it in North Africa?

By Prof. Robert Stern

 The University of Texas at Dallas

Date: 27 November 2020

14:00 (GMT)

15:00 Nigeria and Morocco time, 16:00 Egypt and South Africa time

Online Seminar (Google Meet)

https://meet.google.com/cfd-huum-gup

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Bob is Professor of Geosciences and Director of the Global and Magmatic Research Laboratory at the University of Texas at Dallas. He has more than 40 years of geoscientific research experience, studying active convergent margin processes and products in the Mariana arc system Izu-Bonin-Mariana Arc in the Western Pacific as well as ancient (900-550 million year old) crust exposed in the Arabian-Nubian Shield of Egypt, Sudan, Ethiopia, Saudi Arabia, Jordan and Israel. Stern is an expert on the Geology of the Dallas–Fort Worth Metroplex and the geology of Iran, and has made important contributions to the geology of the Caribbean and the Gulf of Mexico. These studies involve research at sea and on land. Geodynamic contributions include ideas about how new subduction zones form and the evolution of plate tectonics. He and his students and co-authors have published more than 200 peer-reviewed scientific papers.

Stern is also interested in generating educational animations and videos of geoscientific processes. He is head of UTD Geoscience Studios. He shares supervision of the UTD Geosciences Micro-imaging lab. He is also co-director of the Permian Basin Research Laboratory. Stern is a Fellow of the Geological Society of America and of the American Geophysical Union and is Editor-in-Chief of International Geology Review. In 2019 he was awarded the International Prize of the Geological Society of Japan.

The Cadomian (500-600 Ma) Convergent Margin of Gondwana: Is there Evidence for it in North Africa?

The Pan-African event in Africa includes two very different crust-forming events that overlapped in time and space but reflected different plate tectonic processes.  The first episode reflected an ~E-W oriented Wilson Cycle that encompassed most of Neoproterozoic time (~300 m.y.) and affected most of Gondwana whereas the second episode encompassed less time (~100 m.y.) and was focused in northern Gond Intense Neoproterozoic magmatism generated much juvenile crust, especially in the Arabian-Nubian Shield (ANS) (Dixon and Golombek, 1988; Reymer and Schubert, 1984) and also reworked older crust to the west, in the Saharan Metacraton (Abdelsalam et al., 2002). This episode is revealed by Tonian and Cryogenian arc magmatism, followed by Ediacaran collisional and post-collisional magmatism, ultimately welding the Greater Gondwana supercontinent together. As this supercontinent cycle ended ~600 Ma, a new subduction system formed along the northern margin of the supercontinent and a new magmatic arc was established in Ediacaran-Early Cambrian time (620-500 Ma). Igneous rocks and sediments generated during this episode, referred to as the Cadomian (Crowley et al., 2000), make up much of the crust of W Europe; some western segments are called Avalonian. Cadomian and Avalonian crust now underlies large tracts of eastern N America, southern Europe, Iran and Turkey. Cadomian rocks dominate the basement of Europe, SW of the Baltic craton, including Iberia, Armorica and Bohemia (Albert et al., 2015; Linnemann et al., 2011; Tilhac et al., 2017b). Cadomian crust can be traced eastwards into SE Europe, Anatolia and Iran, and perhaps further into Central Asia (von Raumer et al., 2002).

beneath northern Gondwana that started about 620 million years ago and ended about 500 million years ago during what is known as the Cadomian crust-forming event.  Most igneous rocks formed between about 570 and 525 million years ago.  Cadomian crust is well-known from western and southern Europe and from eastern North America but is much less well-known from Turkey and Iran.  We used published age and compositional data and contributed new data in order to better understand this ancient magmatic system. Cadomian magmatism included arc-like igneous rocks in the main arc and alkalic igneous rocks that formed in a back-arc setting; these igneous rocks are associated with sedimentary rocks. Geochemical and isotopic modeling reveal that basaltic magmas were the main input, that these formed by partial melting in the upper mantle, and that basaltic magmas evolved further in deep crustal hot zones to form granitic magmas through a combination of assimilating older contiental crust and fractional crystalization of basaltic magmas.

Cadomian crust offers a good opportunity to study the record of Late Neoproterozoic-Early Cambrian crust formation and reworking associated with arc magmatism in northern Gondwana (Moghadam et al., 2017c). This was the second episode of intense arc magmatism that affected Gondwana in Neoproterozoic and Early Cambrian time.

The Cadomian-Avalonian magmatic arc formed during the subduction of Prototethys beneath northern Gondwana, as evidenced by the presence of Ediacaran high-P rocks, ophiolites and accreted arc complexes (e.g., (De Araujo et al., 2014; Kounov et al., 2012a; Murphy and Nance, 1989; Triantafyllou et al., 2020)) (Fig. 1A). Magmatism began ~620 Ma and ended ~500 Ma, although most activity occurred between 530 and 570 Ma. This single continental magmatic arc broke into fragments that rifted away during the Paleozoic and Mesozoic to open the Rheic and Paleotethys Oceans (Nance et al., 2002), and eventually accreted to southern Laurasia. Recent studies of U-Pb ages and Hf-isotopes in zircon have addressed the age and geochemical evolution of Cadomian magmatic rocks in western and southern Europe as well as the provenance of Cadomian sedimentary rocks (e.g., (Abbo et al., 2015; Avigad et al., 2015; Zlatkin et al., 2014)). Although Cadomian rocks in Europe are well characterized, Cadomian magmatism in Iran-Anatolia is less well known. This paper reports new and compiled bulk rock Sr-Nd and zircon U-Pb and Lu-Hf isotopic data for Cadomian magmatic rocks from Iran and Anatolia. This is an enormous tract of continental crust, almost 3000 km long and hundreds of km wide. We use these data to estimate rates of magmatic addition and to constrain how the source associated with the Cadomian magmatism of Iran and Anatolia evolved. We also discuss tectonic and magmatic mechanisms and their roles in controlling magmatic activity during the evolution of this part of the great Cadomian-Avalonian magmatic arc.