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Late Ediacaran post-collisional A-type syenites with shoshonitic affinities, northern Arabian-Nubian Shield: a possible mantle-derived A-type magma

Gahlan, Hisham and Asimow, Paul and Al-Kahtany, Khaled (2016) Late Ediacaran post-collisional A-type syenites with shoshonitic affinities, northern Arabian-Nubian Shield: a possible mantle-derived A-type magma. Arabian Journal of Geosciences, 9 (12). Art. No. 603. ISSN 1866-7511.

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The Abu Rumeil syenitic rocks represent the inner ring dyke of the Katherina Ring complex, southern Sinai, Egypt. They are divided petrologically into two types, alkali feldspar syenite and quartz syenite. The mineralogy and geochemistry of the syenites indicate an alkaline nature with a shoshonitic affinity. Although rare mafic xenocrysts overgrown by primary K-feldspars and overlapping rare earth element (REE) patterns indicate some role for crustal contamination, the trace element chemistry shows a dominant mantle contribution. The geochronology and field relations imply that the Abu Rumeil syenites were emplaced in a post-collisional, within-plate tectonic setting, yet they express the enrichments in large-ion lithophile elements relative to high field strength elements generally characteristic of subduction influence. We suggest that this signature is inherited from partial melting of a lithospheric mantle source previously affected by subduction during assembly of the Arabian-Nubian Shield. Little evidence of the early evolution of the suite is preserved; there are no associated mafic rocks. We therefore restrict our attention to a petrogenetic model that can explain the relations among the observed felsic composition. The REE patterns of all samples are enriched in light REE and fractionated, but it is notable that there are small positive Eu anomalies in the alkali-feldspar syenites contrasting with small negative Eu anomalies in the quartz syenites. Positive Eu anomalies suggest a cumulate nature for the alkali-feldspar syenites; there are also breaks in the slopes of most variation trends between the alkali-feldspar syenites and the quartz syenites. The general trends in all major oxides and trace elements within the suite can be modeled by fractional crystallization of feldspars—with smaller roles for pyroxene, biotite, apatite, and Fe-Ti oxides—from an intermediate liquid to form the quartz syenites and by assimilation of the near-liquidus phases into the same starting liquid to form the alkali feldspar syenites. The geothermobarometry of pyroxenes and amphiboles suggests shallow emplacement (<10 km depth) and crystallization temperatures ranging from 1100 °C down to 800 °C.

Item Type:Article
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Asimow, Paul0000-0001-6025-8925
Additional Information:© 2016 Saudi Society for Geosciences. First Online: 19 August 2016.
Subject Keywords:Sinai; Syenite; Post-collision; Shoshonite; Within-plate
Issue or Number:12
Record Number:CaltechAUTHORS:20161014-090824859
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Official Citation:Gahlan, H., Azer, M., Asimow, P. et al. Arab J Geosci (2016) 9: 603. doi:10.1007/s12517-016-2629-x
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:71087
Deposited By: Ruth Sustaita
Deposited On:14 Oct 2016 16:56
Last Modified:03 Oct 2019 16:03

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