The Pan-African intrusive complex of Ghorabat area, southern Egypt: Geochemical and mineralogical constraints on arc-related and anorogenic magmatism

The Pan-African granitoids of the Ghorabat area in southern Egypt are compo sed of two distinct calc-alkaline and alkaline suites. The calc-alkaline su ite has a wide compositional range (e.g. silica ranges from 46-77 wt%). The suite is made up of gabbro, diorite, tonalite, granodiorite and porphyriti c high-K granite. The suite exhibits geochemical characteristics of are-rel ated magmatism such as enrichment in LILE (Ba & Sr) coupled with depletion in HFSE (Nb, Y & Zr). Major and trace element variations within the suite r eflect a dominant role of crystal-liquid fractionation process in its evolu tion. The early stage of crystallization was controlled by amphibole and pl agioclase fractionation, whereas in the late stage, K-feldspar was the majo r fractionated phase. The alkaline suite includes metaluminous subsolvus syenogranite and dominan t alkali feldspar granite with ubiquitous enclaves. The A-type geochemical affinity of the alkaline granites is manifested by high abundances of some HFSE (Nb and Y) compensated by very low values of LILE (Ba and Sri. The enc laves are chemically alkaline and range in composition from monzonite to gr anodiorite. The alkaline suite is distinct from the calc-alkaline suite by a higher Fe2O3*/(Fe2O3* + MgO) ranging from 0.85 to 0.97. The enclaves and their hosted alkaline granites do not follow the trend of calc-alkaline sui te on AFM diagram, but constitute a distinct different trend parallel to th e AF side. Mineralogical study reveals two distinct trends of mafic mineral associatio ns: i) Mg-rich trend (magnesio-hornblendes and phlogopite) for the calc-alk aline suite and ii) iron-rich trend (edenitic-hastingstic hornblendes and a nnitic biotites) for the alkaline suite. The two suites could be derived from melting of amphibolitic lower crust un derplated by basaltic magmas with variable water fugacity. Tonalitic melt w as generated at an early stage of subduction under high f(H2O). Fractional crystallization of the tonalitic melt produced the lithologic diversity and chemical variation among the calc-alkaline suite. Dehydration melting at t he end stage of subduction resulted in mozonitic melt which further evolved to yield the post-orogenic alkaline granites.