The late cretaceous turabah ring dyke, central Arabian shield, kingdom of Saudi Arabia : geology, geochemistry and tectonic significance

الملخص EN

The Turabah ring dyke, in the central Arabian Shield of Saudi Arabia, is a Late Cretaceous alkaline A-type granite.

It was emplaced at shallow crustal levels in a Pan-African assemblage of calcalkaline metavolcano-sedimentary association, syn-tectonic metagabbro- diorite, granodiorite-granite gneiss, tonalite gneiss and syenite.

The metavolcanics and the metagabbro-diorite exhibit petrological and geochemical characteristics of mantle-derived island-arc rocks, which were formed by partial melting of a mantle wedge above subduction zone.

The granodiorite-granite gneisses are calc-alkaline I-type granites.

They and the metagabbro-diorite show continuous major and trace element trends, suggesting that the two suites are genetically related through fractional crystallization process.

The tonalite gneisses possess low contents of REE (51-74 ppm), Zr (43-72 ppm), Rb (2-17 ppm), K2O (0.35-1.14 wt.

%), Nb (3-5 ppm) and have trace element patterns not modified greatly by fractionation.

These chemical aspects may suggest their derivation via partial melting of lower crust.The rocks of the Turabah ring dyke consist of subsolvus monzo-and syenogranites.

They are geochemically evolved (SiO2 = 68-77%), metaluminous to mildly peralkaline and display a within plate A-type geochemical signature with enrichment in Fe2O3 (1-4 wt %), Y (7-35 ppm), Nb (3-19 ppm), Rb (45-101 ppm), Zr (101-446 ppm) and depletion in CaO (0.5-1.9 wt %), MgO (0.02-0.6 wt %), Ba (14-820 ppm) and Sr (5-213 ppm).

A model involving partial melting of a mafic lower continental crust in an extensional environment, to produce a granodioritic melt, can explain the origin of these rocks.

Direct contamination of that granodioritic melt with an old continental crust followed by fractional crystallization played an important role in the evolution and chemical characterization of the Turabah A-type granites.

The complex evolution history of the area may be reflected in the formation quasi-oval fractures, along which the fractionated melt was intruded, giving rise to the ring structure in the area.