Abstract

A petrological assessment of 11 volcano-sedimentary belts in the Central Arabian Shield (CAS) using 118 new analyses is presented. The same samples have been used in a Rb-Sr dating programme also reported in this volume. The oldest volcano-sedimentary belts (‘sequence’ C; >900 Ma) are chemically immature bimodal suites of low-K tholeiites and sodic dacite/rhyolite depleted in lithophile elements. These lavas have chemical characteristics similar to immature island arcs such as the Tonga–Kermadee and Lesser Antilles arcs. Younger lavas of ‘sequence’ B (900–700 Ma) are slightly more mature in composition, being predominantly calc-alkaline and low-K arc tholeiite series with higher lithophile element contents. They are comparable with Pacific island are averages. The youngest voluminous lavas (‘sequence’ A; 700–570 Ma) are generally calc-alkaline or high-K, calc-alkaline series lavas with moderately high lithophile element abundances. They are comparable to volcanic arcs such as Central America and Indonesia, which are transitional between island arcs and continental margin volcanic arcs.

The cratonization process in the CAS thus involved the superimposition of three volcano-sedimentary sequences, formed in progressively maturing volcanic arcs. Early-formed, immature island arcs were succeeded by chemically more mature island arc deposits and then by deposits formed in volcanic arc(s) transitional between island arcs and continental margins. The evolution ceased at this stage and probably did not advance to the equivalent of an Andean continental margin.

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