Rb-Sr isotopic studies of postorogenic granites from the eastern Arabian Shield, Kingdom of Saudi Arabia

Abstract

Thirteen Rb-Sr whole-rock isochrons for postorogenic granites of the eastern Arabian Shield yield ages that range from 567 to 617 Ma, a similar range to that previously determined ages for leucocratic, evolved granites in that region. The dated plutons range widely in terms of degree of petrologic evolution from primitive (at Najran) to highly evolved (at Jabal al Caharra) and include several plutons that are anomalously enriched in tin and tungsten (Jabal Tarban, Jabal al Gaharra, Jabal Khinzir, and Jabal Minya). Although no uniform relationship was established between age or initial 87 Sr/ 86 Sr and degree of petrologic evolution, the more evolved plutons tend to be younger and to have slightly higher initial 87 Sr/ 86 Sr values. There is a tendency for metalliferous plutons to have elevated initial 87 Sr/ 86 Sr values, but not all plutons with elevated 87 Sr/ 86 Sr values are known to be metalliferous. Several of the dated plutons are situated within the Najd fault zone, and thus, some of the ages are useful in determining times of strike-slip fault activity. The youngest pluton cut by Najd faulting is at Jabal Tukhfah (573±13 Ma). Displacement at this locality is about 2 km, and if normal rates of strike-slip movement are assumed, this displacement suggests that movement for one strand of the Najd faults ceased by about 560 Ma. Available data indicate that postorogenic granites tend to be older in the southern part of the Arabian Shield. This suggests that plutonism started in the south and progressed to the north. Initial 87 Sr/ 86 Sr values also form a regional pattern. These ratios tend to be higher in the eastern part of the Arabian Shield, and suggest one source of continental affinity to the east and one of oceanic affinity to the west. The distribution of initial strontium isotope ratios does not clearly discriminate between the various models for Shield evolution; however, a sedimentary source region of mixed end members seems more compatible with the data pattern than models based on discrete boundaries between unrelated accreted blocks.