Late Proterozoic lavas of the Central Arabian Shield—evolution of an ancient volcanic arc system

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.

A late Precambrian (∼ 710 Ma) high volcanicity rift in the southern Eastern Desert of Egypt

“…This approach has been useful in past studies of the Arabian-Nubian Shield, where it has demonstrated the geochemical similarities between the metavolcanic rocks of the Arabian-Nubian basement and those of modern arcs and back-arc basins (e.g., BAKOR et al, 1976;STERN, 1981;ROOBOL et al, 1983;HEIKAL & AHMED, 1984). We follow this general philosophy here to examine one of the largest exposures of basement metavolcanic sequences in Egypt, the Shadli Metavolcanics of the South Eastern Desert.…”

Rb-Sr isotope study of latest Proterozoic volcano-sedimentary belts in the Central Arabian Shield

“…The depositional environment of both these facies was shallow marine, fluviatile or subaerial. Roobol et al (1983) showed the lavas to be of calc-alkaline or high-K calcalkaline series, with major oxide and trace element characteristics similar to Cenozoic lavas erupted in mature volcanic arcs, such as the Indonesian islands or the Central American Cordillera.…”

“…The study utilized the recently developed methods of automated mass spectrometry which make possible the analysis of relatively large numbers of samples, allowing a more reliable assessment to be made of linear arrays (isochrons) of data. The analytical work has been carried out by DPFD; sampling and assessment of the field relationships were the responsibility of NJJ, CRR, and MJR; and the authors are jointly responsible for the interpretations (see also Roobol et al 1983). …”

Pan-African volcanism: petrology and geochemistry of the Dokhan Volcanic Suite in the northern Nubian shield

“…In Saudi Arabia, similar volcanic suites including the Hummah suite (573 ±23 Ma; Darbyshire et al 1983), Shammar suite (572-557 Ma; Hadley, 1973) and Murdama suite (633±15Ma;Aldriche?a/. 1978; Roobol et al 1983) are calc-alkaline, orogenic rocks. Schmidt & Brown (1982) and Stuckless et al (1983) showed that the 620-560 Ma magmatism in northwestern Arabia predominantly involved calc-alkaline I-type melts.…”