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The REM (relative element magnitude) program is designed as an aid in the characterization of geochemical anomalies. The program ranks the magnitudes of anomalies of individual elements within a multielement geochemical anomaly. The REM (relative element magnitude) program is intended to aid in the determination of the type of mineralization. It is designed to characterize a geochemical anomaly on the basis of elemental association as well as the relative magnitude of each individual element anomaly within the overall association.

Trace-element contents for 46 postorogenic granitoids vary by as much as two orders of magnitude; most samples are strongly enriched in incompatible elements (such as the heavy rare earths, yttrium, niobium, and uranium) and depleted in the compatible elements (such as barium, strontium, scandium, europium, and cobalt). These trace-element characteristics are typical of A-type granites found in other areas of the world. In spite of the wide range in trace-element contents, no samples contained economically significant concentrations of a single element. Samples range from peralkaline to strongly peraluminous; most are weakly peraluminous or metaluminous. All three subgroups (peralkaline, metaluminous, and peraluminous) are enriched in uranium and hafnium and are depleted in compatible elements. The peralkaline granites are generally enriched in yttrium and niobium-the strongly peraluminous granites in rubidium and tantalum; both groups are also somewhat enriched in the heavy-rare-earth elements. Several of the peraluminous granites are known to be tin-bearing and, like tin-bearing granites found elsewhere in the world, their chondrite-normalized, rare-earth-element patterns are flat and have large negative europium anomalies. Samples from several plutons have very anomalous ratios for geochemically similar elements such as yttrium and holmium, niobium and tantalum, and zirconium and hafnium. These ratios may reflect differential complexing with a halogen-rich aqueous phase that may be characteristic of plutons that have been subjected to an ore-forming process. A program to test these hypotheses, through study of known mineralized areas and laboratory experiments, should be implemented. The regional trends for the chemical data and geochemical correlations over the large area sampled, suggest that the postorogenic granites were derived from a single protolith that formed by the mixing of oceanic sediments from the west*continental sediments from the east.

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Concentrations determined for all of the trace elements included in this study of postorogenic granites from the northeastern Arabian Shield are best described by log-normal distributions. The trace elements are divided into two groups: (1) compatible lithophile and siderophile elements (strontium, cobalt, scandium, manganese, europium, and titanium) and ( 2) incompatible lithophile elements (uranium, thorium, tantalum, rubidium, and rare-earth elements, except europium). The compatible elements exhibit greatest concentrations in the metaluminous postorogenic granites, and concentrations decrease with increasing degree of magma evolution. Economic potential for these elements and other geochemically similar elements is considered to be low. The concentrations of the incompatible elements increase with increasing degree of magma evolution and are greatest in the peralkaline and peraluminous granites. There is some geologic evidence that pegmatite and vein-forming processes were operative toward the end stage of postorogenic magmatism in the northeastern Arabian Shield, and therefore there is some probability for economic potential for these elements. It is suggested that such potential is greatest where highly evolved postorogenic granites intruded volatile (generally water)-rich country rocks.

Geochemical data for samples from 55 postorogenic plutons of the southwestern Arabian Shield show that these granitoids are generally less evolved than similar granitoids of the eastern Arabian Shield. Radioelement contents are low to normal relative to typical granitic rocks and uranium contents are distinctly low relative to rocks of the eastern Arabian Shield. The data indicate low favorability for ore deposits of elements that are typically enriched in granites at the end stages of magmatism such as tin, tungsten, niobium, tantalum, molybdenum, rare-earth elements, and radioactive elements. Regional geochemical patterns are not consistent with the north-south trending microplate boundary that has been proposed by other workers. Correlation coefficients for trace-element data obtained during the current study also argue against sharp compositional breaks in the protolith for the postorogenic granites. Consideration of the data presented here and similar data for granitoids of the eastern Arabian Shield, suggest the existence of a compositionally gradational protolith of continental affinity to the east and oceanic affinity to the west. \J All tables arc at the end of the report. See page 24. Jabal Kor, Jabal Taweel, and Jabal Amoudah) contain peralkaline rocks. In addition, Schmidt (1980a) notes that the central part of Jabal Khashmadheeb and small plug about 20 km southeast of Al Mu'taridah (fig. 1) are also peralkaline, but these areas were not sampled as part of the current study. Only seven samples are truly peralkaline, and thus they were not treated statistically as a separate subset of the data. All of the known peralkaline rocks within the study area, except Jabal An, lie along a north-south trend of peralkaline plutonism which was originally described by Stoeser and Elliott (1980). Samples from all of the postorogenic plutons along this trend are hypersolvus alkaji feldspar granites. Within the eastern part of the study area, Schmidt (1980a and'Vfsuggcsts that all of the alkali feldspar granites are the youngest plutonic rocks. The pluton at Jabal An is spatially separated from the main trend of peralkaline rocks within the Arabian Shield, and its trachytic texture is also unique. Gonzalez (1973) reports a whole-rock K-Ar age for Jabal An of 21.6+3.5 Ma, but the sample locality is not documented, and the technique and sample medium provide data of questionable validity given the proximity to the Tertiary basalt field. Experimental and theoretical studies (Watson and Harrison, 1983) have shown that the saturation limit of zirconium is much higher in peralkaline rocks than in rocks that are saturated or over-saturated with respect to alumina. Peralkaline rocks from the study area show contents of zirconium that are high relative to most of the non-peralkaline samples (table 2), but in contrast to peralkaline rocks outside of the study area (Stuckless and others, 198$ 1985) the peralkaline samples from the study area (except for Jabal An) are not noticeably enriched in uranium, thorium, niobium, or rare-...

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