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The younger granitoids of the Shalatin-Halaib district in the south Eastern Desert of Egypt are of syenogranite compositions. Based on petrological and geochemical studies, the syenogranites have been differentiated into two distinct types, namely biotite-hornblende subsolvus syenogranite (SGR I), showing 1-type affinity, and biotite-bearing hypersolvus syenogranite (SGR II) with A-type affinity. On the basis of rare earth element (REE) patterns, SGR II is further subdivided into SGR Ha and SGR IIb. SGR I is characterised by low Rb/Sr ratios and high field strength element (HFSE) concentrations (e.g. Nb, Ta, Y), fractionated LREE with flat HREE, high LREE/HREE ratios, presence of a negative Nb-Ta anomalies and lack of negative Ba and Eu anomalies. On the other hand, SGR II is characterised by high Rb/Sr ratios and HFSE concentrations, slightly fractionated (SGR IIa) to unfractionated (SGR IIb) LREE with a positive slope for the HREE, absence of negative Nb-Ta anomalies and significant negative Bar Ce, Zr and Eu anomalies.

According to field and geochemical characteristics, the late tectonic SGR I may be produced by dehydrated partial melting of an amphibolitic source, followed by high-pressure fractionation processes. The post-tectonic SGR II is probably formed directly from the residual magma by crystallisation of biotite, plagioclase, K-feldspar and accessory phases such as monazite, allanite and titanite in a shallow-level magma chamber occurring along strike-slip faults at convergent margins. Generally, the syenogranites were generated during a period of rapid tectonic transition from crustal thickening during subduction (SGR I) to crustal thinning during extension (SGR II), which represent the final magmatic stage of an extensive arc system. (C) 2002 Elsevier Science Limited. All rights reserved.