Zircon morphology and U-Pb geochronology of seven metaluminous and peralkaline post-orogenic granite complexes of the Arabian Shield, Kingdom of Saudi Arabia

Abstract

The U-Pb zircon method was used to determine the ages of seven metaluminous-to-per-alkaline post-orogenic granites located throughout the Late Proterozoic Arabian Shield of Saudi Arabia. Zircons from the metaluminous rocks are prismatic, with length-to-width ratios of about 2-4:1 and small pyramidal terminations. In contrast, zircons from three of the four peralkaline complexes either lack well developed prismatic faces (are pseudo-octahedral) or are anhedral. Some of the zircons from the peralkaline granites contain inherited radiogenic lead. This complicates interpretation of the isotopic data and. in many cases, may make the U-Pb method unsuitable for determining the age of a peralkaline granite. Zircons in the metaluminous granites do not contain inheritance and thus, best-fit chords calculated through the data have upper concordia intercepts that indicate the age of intrusion, and lower intercepts that indicate simple episodic lead loss. The results show that these granites were emplaced during multiple intrusive episodes from 670 to 510 Ma (Late Proterozoic to Cambrian). INTRODUCTION Stoeser and Camp (1985) have divided the Arabian Shield into five terranes that are separated by four suture zones (fig. 1). The three western terranes, the Asir, Hijaz, and Midyan appear to be underlain by rocks of primitive-to-mature island-arc composition (Greenwood and others, 1976; Camp, 1984; Jackson and others, 1984). The three arc terranes are separated by narrow suture zones that contain ophiolitic complexes (Bakor and others, 1976; Claesson and others, 1984; Camp, 1984; Nasseef and others, 1984). The rocks within the western arc terranes were formed approximately 950 to 700 Ma (Fleck and others, 1980; Calvez and others, 1983). The two eastern terranes, the Afif and Ar Rayn, appear to be more evolved and include mature island-arc rocks and older continental crust (Stacey and others, 1980; Calvez and others, 1983; Stacey and Stoeser, 1983; Agar, 1985). The oldest known rocks of the Arabian Shield (1800-1600 Ma) are felsic orthogneisses from the southern Afif terrane (Stacey and Hedge, 1984; Stacey and Agar, 1985). Most of the Afif terrane, however, appears to contain rocks younger than 820 Ma. The Afif terrane is separated from the western arc terranes by the Nabitah erogenic belt. This belt may be interpreted to be the result of continental collision among these terranes during 680-640 Ma (Schmidt and others, 1979; Stoeser and others, 1984). The Afif and Ar Rayn terranes are separated by the Al Amar suture zone that formed between 630-620 Ma (Calvez and others, 1983; Stacey and others, 1984). All of the terranes of the Arabian Shield appear