Elsevier

Precambrian Research

Volume 359, 1 July 2021, 106211
Precambrian Research

Two-stage crustal growth in the Arabian-Nubian shield: Initial arc accretion followed by plume-induced crustal reworking

https://doi.org/10.1016/j.precamres.2021.106211Get rights and content

Highlights

  • The El-Shadli volcanic province in Egypt’s Eastern Desert yields a U-Pb zircon age of ~700 Ma.

  • The magmatism was produced by remelting of accreted oceanic lithosphere.

  • The remelting was likely driven by a mantle plume during the break-up of Rodinia.

  • The continental crust of the Arabian-Nubian Shield stabilized by plume-induced crustal reworking.

Abstract

Island-arc accretion during the assembly of Gondwana has been widely regarded as the main mechanism for Neoproterozoic crustal growth in the Arabian-Nubian Shield (ANS). However, processes involved to transform the newly accreted juvenile terranes into a typical continental crust remain unclear. Here, we present geochemical, isotopic, and U-Pb geochronological data from the El-Shadli volcanic province (80 km × 35 km and > 10 km thick) in the Eastern Desert of Egypt, which lies in the north-western part of the ANS, and overlies strongly deformed, previously accreted arc terranes. The El-Shadli volcanic province consists mainly of a mafic-felsic bimodal suite and subordinate intermediate rocks that intrude the mafic rocks of the suite. The bimodal suite rocks are tholeiitic, whereas the intermediate rocks have a calc-alkaline affinity. The bimodal suite and the intermediate rocks both yield U-Pb zircon ages of ~ 700 Ma implying they are coeval. High zircon εHf(t) values for the bimodal suite (average εHf(t) = +11.46) as well as the intermediate rocks (average εHf(t) = +9.76) indicate they were either magmatic extractions derived directly from a depleted mantle source, or the products of remelting of juvenile crust. Oxygen isotope data for zircon yield similar δ18O values for both the bimodal suite (average δ18O = 4.94‰) and intermediate rocks (average δ18O = 4.79‰). These are lower than typical mantle values indicating the parental magma in both cases interacted with hydrothermal fluids. Based on the petrological, geochemical, and isotopic data, we suggest that the El-Shadli bimodal suite and the intermediate rocks were produced by reworking of MORB-like and arc-like oceanic lithosphere, respectively, most likely driven by a mantle plume during the break-up of Rodinia. The recognition of the El-Shadli volcanic province as a likely mantle plume-induced post-kinematic magmatism provides a mechanism for the transformation of newly accreted juvenile crustal terranes into a chemically stratified normal continental crust. In addition, such plume events may result in new mantle extractions that are converted into new continental crust.

Introduction

There is a strong view that at least 60–70% of Earth’s continental crust was mostly formed by the end of the Archean (2.5 Ga) (Arndt, 2013, Belousova et al., 2010, Dhuime et al., 2018, Dhuime et al., 2012, Hawkesworth et al., 2020, Taylor and McLennan, 1995). However, the mechanism(s) responsible for the generation of such continental crust and the processes related to crustal growth (including the geodynamic settings) are widely debated (Arndt and Davaille, 2013; Arndt, 2013; Hawkesworth et al., 2013, Hawkesworth et al., 2020, Kemp and Hawkesworth, 2014, Taylor and McLennan, 1995, Wang et al., 2020). In particular, few studies have considered how accreted juvenile crustal terranes were transformed into the chemically stratified continental crust.

Outcrops of Archean rocks are generally restricted to high-grade metamorphic terranes, and in most cases, deformation and metamorphism have substantially modified their original compositions and primary structures. These complications have led to the search for younger analogues of juvenile crustal generation/growth to investigate the mechanism(s) responsible for the crustal growth process. The Arabian-Nubian Shield (ANS) forms one of the largest exposures of Neoproterozoic juvenile continental crust on Earth (Fig. 1a: Pease and Johnson, 2013), and its crustal generation and cratonization/stabilization represent one of the largest events of continental crustal growth since the Archean (Bentor, 1985, Stern, 1994). The ANS crust is generally well exposed, and the low degrees of metamorphism make it one of the best areas to study continental crust formation.

Island-arc accretion during the assembly of Gondwana is generally considered to be the main mechanism for Neoproterozoic crustal growth in the ANS (Fritz et al., 2013, Johnson et al., 2011, Johnson and Woldehaimanot, 2003, Kröner et al., 1991; Stern, 1994, 2002). However, the size of the ANS and its Neoproterozoic growth rate significantly exceeds that of Cenozoic examples of crustal generation via the addition of juvenile mantle materials to arcs along subduction margins (Reymer and Schubert, 1986, Reymer and Schubert, 1984). Although mantle plume-related magmatism has been proposed as an alternative mechanism for Neoproterozoic crustal generation in the ANS (Stein, 2003, Stein and Goldstein, 1996), no direct record of Neoproterozoic plume events has been reported.

Bimodal volcanism produces intercalations of mafic and felsic rocks that are commonly found in magmatic rift systems above mantle plumes, such as the Cenozoic magmatism of the East-African rift (Rooney, 2020, Rooney, 2017) and Neoproterozoic bimodal rift-related volcanism (Cheng et al., 2020, Kjøll et al., 2019, Li et al., 2010, Li et al., 1999; X.-H. Li et al., 2008a; Z. X. Li et al., 2003b, Lyu et al., 2017). In their paleogeographic reconstruction of the Neoproterozoic supercontinent Rodinia, Li et al., 2008a, Li et al., 2008b, Li et al., 2013 suggest that the ANS terranes were likely located between India, Australia-East Antarctica, and the Sahara craton, along the fringes of the Rodinia superplume at ~ 825–680 Ma. This time interval coincides with the rifting and fragmentation of the supercontinent Rodinia (Li et al., 2013; Z. X. Li et al., 2008b, Li and Zhong, 2009), and is characterized by widespread plume-related bimodal volcanic and plutonic complexes, as well as mafic and ultramafic dikes, such as those reported in South China (Li et al., 1999; Z. X. Li et al., 2008b, Li et al., 2003b, Wang et al., 2007), Tarim (Zhang et al., 2006), Australia (Wingate et al., 1998), Southern Africa (Frimmel et al., 2002) and Laurentia (Ernst et al., 2010, Ernst and Buchan, 2001, Heaman et al., 1992, Park et al., 1995).

Here, we describe a voluminous magmatic event in the El-Shadli volcanic province (80 km × 35 km and > 10 km thick) in the Eastern Desert of Egypt, which lies in the north-western part of the ANS (Figs. 1b and 2). Using whole-rock geochemical and Sr-Nd isotope data, and integrated U-Pb-Hf-O isotopic and trace element data from zircons, we investigate the origin and tectonic setting of this voluminous bimodal volcanism and examine the potential role of mantle plume-induced magmatism in the crustal growth of the Arabian-Nubian Shield.

Section snippets

Geological background

The East African Orogen (EAO) represents the largest orogen involved during the assembly of Gondwana, with orogenic activity peaking at ~ 650 Ma (Fritz et al., 2013, Merdith et al., 2017, Stern, 1994). The EAO consists of multiple zones of accretion and collision between East and West Gondwana, with the Neoproterozoic ANS located along its northern margin (Hamdy et al., 2017, Johnson et al., 2011, Johnson and Woldehaimanot, 2003, Pease and Johnson, 2013, Stern, 1994) (Fig. 1a). The ANS is

Analytical methods

Analytical techniques are described in detail in the supplementary information (Supplementary Tables S1 to S3). Whole-rock major elements were determined for 18 samples (11 mafic, 5 felsic, and 2 intermediate rock samples) using X-ray fluorescence at the Bureau Veritas Lab, Perth (Supplementary Table S1). Whole-rock trace elements and Sr-Nd isotope data were determined using inductively coupled plasma mass spectrometry (ICP-MS) and thermal ionization mass spectrometry (TIMS), respectively, at

Whole-rock geochemical data

Samples of the El-Shadli volcanic rocks are fresh with minor alteration/weathering, as indicated by low loss on ignition (LOI) with values below 1 wt% for felsic rocks and ranging between 1 and 3 wt% for the mafic and intermediate rocks (Supplementary Table S1). As rock composition can be modified by alteration/weathering (Polat and Hofmann, 2003), the effect of alteration on the mobility of the major and trace elements was evaluated before discussing the geochemical and petrological

Tectonic setting of the EI-Shadli bimodal volcanic province

The tectonic affinity of the El-Shadli bimodal volcanic province is controversial, with some arguing that the province was formed in an island arc setting (Abdel-Karim et al., 2019, Faisal et al., 2020, Khudeir et al., 1988, Shukri and Mansour, 1980) and others suggesting a continental rift-related origin (Stern et al., 1991). The latter interpretation was supported by relatively few geochemical data from a small sampling area at W. Abu-Hamamid (yellow star in Fig. 2), within the El-Shadli

CRediT authorship contribution statement

Hamed Gamal El Dien: Conceptualization, Formal analysis, Investigation, Methodology, Writing - original draft. Zheng-Xiang Li: Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Writing - review & editing. Mohamed Abu Anbar: Investigation, Project administration, Writing - review & editing. Luc-Serge Doucet: Formal analysis, Investigation, Methodology, Writing - review & editing. J. Brendan Murphy: Conceptualization, Writing - review & editing. Noreen.J. Evans:

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We would like to thank Dr. Allen Kennedy and Mr. Hao Gao for helping in operating the SHRIMP and data reduction, Ms. Elaine Miller for assisting with CL imaging, and Mr. Brad McDonald for assistance with LA-ICP-MS analyses of zircon. We are greatly indebted to Dr. Josh Beardmore for proofreading. We thank the associate editor Dr. Kamal Ali and two anonymous reviewers for their constructive comments. This work was supported by the Australian Research Council Laureate Fellowship grant to Z.-X.L.

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