Geochemistry, chronology and petrogenesis of the Wadi Yebigue pluton, Central Tibisti Massif, Libya

Abstract

The Wadi Yebigue pluton is a high-level, post-orogenic, composite granitoid body intruded into the Central Tibisti Massif of southern Libya. It consists mainly of three phases: a fine-grained, two-mica granite core intruded into a porphyritic, coarse-grained biotite granite which grades marginally into a texturally similar hornblende-biotite granite. The porphyritic granites have an age of 558 m.y. with an intial 87Sr/ 86Sr ratio of 0.70640, and the two-mica granite has an age of 548 m.y. with an initial 87Sr/ 86Sr ratio of 0.70693. The virtual identity of ages and initial ratios, combined with the close field relationships, indicate that the porphyritic and two-mica granite phases are cogenetic. Compositional variations within the suite of porphyritic indicate that the biotite was derived by fractionation of mafic minerals and plagioclase from the hornblende-biotite granite. Removal of K feldspar crystals from this magma reduced Ba contents and maintained constant K and Rb contents throughout the series. The high A1 content of the two-mica granite, however, makes it impossible to explain the two-mica magma by crystal fractionation of the porphyritic granites. The suites were apparently formed by two-stage melting of a garnet granulite source with low Rb/Sr at the base of the crust. The proposed model requires: (1) early melting and subsequent fractionation after emplacement to yield the porphyritic granites; and (2) partial melting at a slightly younger time with garnet retention in the source to produce the two-mica granites.