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.

Five examples of disseminated rare element mineralization with sufficient resource potential to warrant exploration and detailed study occur in association with late Precambrian, posttectonic alkali granites in the northwestern Arabian shield. All are characterized by enrichment in a suite of granitophile elements, which typically includes Nb, Ta, Sn, rare earth elements, Y, Th, U, and Zr, but they differ in size, geometry, mode of occurrence, and lithology.Two types of mineralization can be distinguished. The Jabal Sa'id type, represented by the Jabal Sa'id aplite-pegmatite, the Jabal Hamra' silexite, and minor pegmatites occurring near at least two other alkali granites, is characterized by high total iron to total alkali ratios and high K•O/Na•O ratios. The Ghurayyah type, represented by the essentially microgranitic Ghurayyah, Jabal Tawlah, and Umm al Birak intrusions and minor dikes at a fourth locality, is characterized by major oxide abundances approximating those of typical alkali granites. Both types evolved from alkali granite magmas formed by high-temperature, dry melting of late Proterozoic island-arc crust. They probably represent crystallization under deuteric (Ghurayyah type) and hydrothermal (Jabal Sa'id type) conditions of residual fractions enriched in granitophile elements and volatiles. Rare earth element distribution patterns probably reflect the composition of the volatile phases.Systematic surface sampling has been undertaken over all five occurrences, and pilot drilling programs have been completed at Ghurayyah, Jabal Tawlah, and Jabal Sa'id. Indicated resources range from 6 million metric tons (Jabal Tawlah and Umm al Birak) through ñ20 million metric tons (Jabal Sa'id and Jabal Hamra') to 440 million metric tons (Ghurayyah). The highest average grades for Nb (3,400 ppm), Sn (380 ppm), Y (5,200 ppm), and Zr (37,000 ppm) occur at Jabal Tawlah; the highest Ta grades (200 ppm) at Ghurayyah; and the highest U (130 ppm) and Th (830 ppm) grades at Jabal Sa'id. Enhancement in light rare earth elements occurs at Jabal Hamra' (La, 2,600 ppm and Ce, 3,400 ppm) and sporadically at Jabal Sa'id (La, 600 ppm and Ce, 1,300 ppm). Extreme enrichment in heavy rare earth elements and Y occurs at Jabal Tawlah, Ghurayyah, and Jabal Sa'id.Mineralogical studies indicate that Nb may be present in columbite-tantalite or pyrochlore; rare earth elements are concentrated in monazite, bastnaesite, and synchesite-Y; Th is present in thorRe; and zircon is invariably abundant.

No abstract

No abstract

No abstract