Donald G. Hadley

Geochronology of the Arabian Shield, western Saudi Arabia: K-Ar results

An orogenic event, correlated with the Pan-African event in eastern Africa, affected the Arabian Peninsula between 510 and 610 m.y. ago and is well-recorded geochronologically. The event probably included two thermal pulses or maxima, the first occurring between 560 and 610 m.y. ago and the second between 510 and 540 m.y. ago. The earlier pulse, the more severe one, included the majority of the igneous activity and metamorphism. During the last part of the 510- to 610-m.y. period, left-lateral strike-slip faulting occurred along a set of northwest-trending en echelon fracture zones, whose composite displacement may be as large as 240 km. At least one and probably more orogenic events affected the Arabian Peninsula before the Pan-African event, but only minimum ages can be assigned to these, because thermal effects of the 510- to 610-m.y. event have reset K-Ar ages. Major diorite-granite batholiths, however, formed before 760 m.y. ago.

LATE PROTEROZOIC CRUSTAL HISTORY OF THE ARABIAN SHIELD, SOUTHERN NAJD PROVINCE, KINGDOM OF SAUDI ARABIA

The Arabian Shield was formed by successive accretions of newly formed crust between 1,000 and 600 Ma ago. Successively younger island arcs formed to the East as West-dipping subduction zones shifted eastward. West of Bishah, the volcanic-plutonic crust had consolidated against Africa by about 780 Ma ago when westward-directed subduction ceased at the Nabitah suture.

THE MIOCENE TIHAMA ASIR OPHIOLITE AND ITS BEARING ON THE OPENING OF THE RED SEA

In early Miocene time (ca. 2. Ma ago), dike swarms, layered gabbros, granophyres, and basalts of the Tihama Asir ophiolite in S.W. Saudi Arabia were emplaced during the initial stages of Red Sea rifting and separation of the African and Arabian plates. As rifting began, dikes invaded Paleozoic and Mesozoic sedimentary and Precambrian metamorphic rocks of the Arabian plate along a northwesterly trend paralleling the axial trough of the Red Sea. With increased separation along the rift edge, new crust was accreted to the continental margin. This crust consisted of a 4 km wide zone of subparallel dikes having chilled margins against one another and lacking screens of older continental rock. This dike complex is analogous to the sheeted dike swarms described from the Cyprus, Oman, and Newfoundland ophiolites. Layered gabbros and granophyric intrusions within the dike swarm are products of magmatic differentiation developed during the initial stages of rifting. The differentiation trends indicate that the parent magma was tholeiitic and evolved by crystal fractionation in much the same manner as lavas at Thingmuli Volcano, Iceland. Late Miocene tilting of the sedimentary rocks and dike swarm toward the Red Sea axis and later erosion have exposed the continental-oceanic crust boundary along a narrow zone. The continental Baid Formation (19 Ma) rests unconformably on the Tihama Asir ophiolite.

SEDIMENTARY ROCKS AND BASINS OF THE ARABIAN SHIELD AND THEIR EVOLUTION

Proterozoic sedimentary rocks of the Arabian Shield consist of three depositional phases. Phase I (the oldest) includes the Baish, Bahah, and Jiddah Groups, which consist of an immature assemblage dominated by fine-grained meta-graywacke, graphitic schist, chert, marble, and subordinate polymictic conglomerate and meta-siltstone. Clastic rocks of Phase I are composed solely of volcanic material and do not contain plutonic or sialic detritus. Meta-sandstone, polymictic conglomerate, coarse-grained meta-graywacke, and abundant marble (stromatolitic in places) characterize Phase II, the Ablah, Halaban, and Murdama Groups. Plutonic and volcanic components are abundant in the clastic suite; K-feldspar detrital fragments are common in the Halaban and Murdama. The Shammar and Jubaylah Groups form Phase III. Sedimentary rocks of this phase are fine- to coarse-grained terrigenous clastic rocks, boulder conglomerate, and stromatolitic limestone and dolomite. All phases include abundant eruptive volcanic rocks.