Geochemical and isotopic constraints on island arc, synorogenic, post-orogenic and anorogenic granitoids in the Arabian Shield, Saudi Arabia

Abstract

The Arabian Shield preserves a protracted magmatic record of repeated amalgamation of juvenile subduction terranes that host granite intrusions ranging in age from the early Neoproterozoic to the Cambrian, which were emplaced into convergent and within-plate settings. Geochronology and whole-rock geochemistry of sampled Saudi Arabian granitoids define and distinguish four discrete age groups: 1) ~845–700Ma island arc and synorogenic granitoids (IA+Syn), 2) ~640–610Ma granitoids from the Nabitah and Halaban Suture (NHSG), 3) ~610–600Ma post-orogenic perthitic (hypersolvus) granitoids (POPG), and 4) <600Ma anorogenic aegirine-bearing perthitic (hypersolvus) granitoids (AAPG). Groups 1, 2 and 3 include suites ranging from I–S- to A-type granites that have REE signatures typical of volcanic arc settings and show intra-suite variation that could be controlled by a combination of crustal assimilation and fractional crystallisation. Their mafic parental magmas have N-MORB-, or arc-tholeiite-like geochemistry. By contrast, group 4 A-type granites are more enriched in HREE and in incompatible elements such as Nb, Rb, Ga, Nd, Zr and Y and have lower Ce/Yb and higher Y/Nb ratios. These granitoids are interpreted to have been emplaced into within-plate and back-arc settings. Granitoid data also provide evidence that there may be two distinct mantle sources to the mafic parents of the granite suites. These are distinguished as contaminated and enriched mantle using Nb and Y and Nd isotopes. All granitoid suites are isotopically juvenile (ɛNd +3 to +6) and fall between the upper field crustal values of the Paleoproterozoic Khida terrane (ɛNd +1) and contemporary depleted mantle. However, Nd isotopes distinguish contamination in group 1–3 mafic end-members beneath sutures which are interpreted to be derived from the contemporary MORB-type mantle wedge with subsequent crustal assimilation and fractionation to I- and A-type granitoids. The youngest (after 600Ma) A-types (group 4) emplaced into extensional within-plate and back-arc settings require a new enriched mantle source that this study interprets to be associated with delamination.