Precambrian ophiolites of Arabia; a summary of geologic settings, U-Pb geochronology, lead isotope characteristics, and implications for microplate accretion, Kingdom of Saudi Arabia

Abstract

Disrupted ophiolites occur in linear belts as much as 900 km long between micro plates that collided during the late Proterozoic to form the Arabian Shield. The U-Pb zircon ages and lead-isotope data from these ophiolitic rocks help constrain the history of accretion of the Arabian Shield and thereby contribute to the definition of its microplates and terranes. Microplates of the central and western Arabian Shield are generally thought to represent intra-oceanic island arcs that range in age from about 900 Ma to 640 Ma; however, a region of the eastern Arabian Shield contains rocks of early Proterozoic age and may represent an exotic continental fragment entrained between the arc complexes. The Ophiolites of the Yanbu suture (northwestern Arabian Shield), dated by U-Pb (zircon) and Sm-Nd (mineral isochron) methods, yield model ages of 740-780 Ma. These are the among the oldest well-dated rocks in the northwestern Arabian Shield. Ages from the Jabal al Wask complex overlap with ages of adjacent arc rocks. This overlap in age supports geologic and geochemical evidence that the complex represents a fragment of back-arc oceanic lithosphere formed during arc magmatism. Older dates of about 780 Ma for gabbro from the Jabal Ess ophiolite suggest that these gabbros are either fore-arc oceanic crust or fragments of oceanic crust on which an arc was built. Gabbro samples from ophiolites of the Bir Umq-Port Sudan suture (westcentral Arabian Shield) yield zircons with ages of 820-870 Ma and ≥1,250 Ma. The 820-870 Ma dates overlap with ages of the oldest nearby arc rocks; this favors an intra-arc or near-arc setting for the ophiolites. The older zircons suggest that middle or early Proterozoic crustal material, possibly derived from the Mozambique belt of Africa, was present during back- or intra-arc magmatism. Plagiogranite from the Bir Tuluhah ophiolitic complex at the northern end of the 900-km-long Nabitah mobile belt was dated by the zircon U-Pb method at approximately 830 Ma. This date is within the range of the oldest dated arc rocks along the northern and central parts of the Nabitah suture zone, but is approximately 100 million years older than the oldest arc plutons (tonalites) associated with the southern part of the belt. These age relations suggest that the northern part of the Nabitah belt contains an extension of the Bir Umq-Port Sudan suture that was rotated parallel to the Nabitah trend during collision of the arc terranes of the northwest Arabian Shield with the Afif plate to the east. Feldspar lead-isotope data are of three types: 1) lead from the ophiolitic rocks and arc tonalites of the northwestern Arabian Shield and ophiolitic rocks of the Nabitah suture zone is similar to lead in present midocean ridge basalt, 2) anomalous radiogenic data from the Thurwah ophiolite are from rocks that contain zircons from pre-late Proterozoic continental crust, and 3) feldspar from the Urd ophiolite shows retarded uranogenic lead growth and is related either to an anomalous oceanic mantle source, or in an unknown manner to ancient continental mantle or lower crust of the eastern Arabian Shield.