Geochemical Evolution of Rare Metal-Bearing A-Type Granites from the Aja Batholith, Hail Terrain, Saudi Arabia

The Aja Batholith northeastern Arabian Shield, consists of five granite varieties namely hornblende alkali feldspar granite, granophyre, Arfvedsonite-Aegirine alkali fieldspar granite, porphyritic alkali granite and albite granite Zircon, sphene, thinite, Samraskite, Ta-selenide pyrochlore, fluorite, magnetic and hematite are magmatic rare metalbearing accessory minerals. In some rock varieties,hematite is Post-Magmatic where local metasomatic effects are visible in varying degrees and played a significant role in the genesis of Silica-Hematite-Rich rocks (hematosilconite). The granite varieties in the batholith are highly siliceou(Si0₂=68-75%)with high K₂O(3 72-5 52%) and Na₂O(2 52- 6 25%) and low AI₂0₃(8 35-13 5%), MgO (0 07-0 4%), CaO(0 17-1 5%), Ba(20-409 ppm) and Sr(6-134 ppm). The alumina saturation index (A/CNK) for most samples is < I while the agpaitic index is > I confirming their alkaline to peralkaline character. The granites have high and variabie contents of Rb (81-495 ppm) Ga (20-52 ppm), Nb (10- 414 ppm), Zr (290-31915 ppm), Y (22-552 ppm), Sn (3-101 ppm), and total rare earth elements (362-3518 ppm). These data indicate that the Aja batholith represents Post-Orogenic A-type granites. The generation of their parental magma probably involved partia1 melting of lower crustal rocks as a result of the decomposition of F-rich biotite Comparison of the geochemical data with many spectalized rare Metal-Bearing granites show that the Aja granites are enriched in Nb, Ta, Sn, Zr and REE, especially the Arfvedsonite-Aegirine alkali granite the albite granite and the hematosilconite. Although the geochemical effects of metasomatism is locally visible, nonetheless, most of the chemical variations in the content of these elements appeal to be the result of in situ fractional ciystallization(feldspars, ferromagnesian minerals and rare metal bearing accessory phases) and fluorine complexing. The strong positive correlation of alkalis (Rb and K) that was mobile during metasomatism with elements that were not (Nb and Ta), argues strongly against metasomatic alteration. In the granites. However, the hematosilconite consistently rich in Fe, St, Ca Pb, Zn,Sn and poor in Na, K, Rb relative to the glanite suggesting the local effect of metasomatism.