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Geochemical and Sr–Nd isotopic constraints on the petrogenesis and geodynamic significance of the Jebilet magmatism (Variscan Belt, Morocco)

Published online by Cambridge University Press:  20 September 2013

ABDERRAHIM ESSAIFI ,
SCOTT SAMSON  and
KATHRYN GOODENOUGH
ABDERRAHIM ESSAIFI*
Affiliation:
Geology Department, Cadi Ayyad University, B.P. 2390, Marrakech 40000, Morocco
SCOTT SAMSON
Affiliation:
Department of Earth Sciences, Syracuse University, Syracuse, NY 13244, USA
KATHRYN GOODENOUGH
Affiliation:
British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK
*
†Author for correspondence: [email protected]
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Abstract

In the Variscan fold belt of Morocco, the Jebilet massif is characterized by Palaeozoic metasedimentary rocks intruded by syntectonic magmatism that includes an ultramafic–granitoid bimodal association and peraluminous granodiorites emplaced c. 330 Ma, intruded by younger leucogranites c. 300 Ma. The mafic–ultramafic rocks belong to a tholeiitic series, and display chemical and isotopic signatures consistent with mixing between mantle-derived and crust-derived magmas or assimilation and fractional crystallization. The granites within the bimodal association are mainly metaluminous to weakly peraluminous microgranites that show characteristics of A2-type granites. The peraluminous, calc-alkaline series consists mainly of cordierite-bearing granodiorites enclosing magmatic microgranular enclaves and pelitic xenoliths. Detailed element and isotope data suggest that the alkaline and the peraluminous granitoids were formed in the shallow crust (<30 km) by partial melting of tonalitic sources at high temperatures (up to 900°C) and by partial melting of metasedimentary protoliths at relatively low temperatures (c. 750°C), respectively. Mixing between the coeval mantle-derived and crust-derived magmas contributed to the large variation of initial εNd values and initial Sr isotopic ratios observed in the granitoids. Further contamination occurred by wall-rock assimilation during ascent of the granodioritic plutons to the upper crust. The ultramafic–granitoid association has been intruded by leucogranites that have high initial Sr isotopic ratios and low initial εNd values, indicating a purely crustal origin. The heating events that caused emplacement of the Jebilet magmatism are related to cessation of continental subduction and convective erosion/thinning of the lithospheric mantle during plate convergence.

Keywords

bimodal magmatismperaluminous graniteSr–Nd isotopesVariscan orogenyMorocco
Type
Original Articles
Information
Geological Magazine , Volume 151 , Issue 4 , July 2014 , pp. 666 - 691
Copyright
Copyright © Cambridge University Press 2013 

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