Post-collision, A-type granites of Homrit Waggat Complex, Egypt: petrological and geochemical constraints on its origin

Abstract

The Homrit Waggat Complex (HWC) is a ring complex which belongs to the younger granite province of the Nubian Shield. It comprises a central biotite granite encircled by an intermediate ring of mylonitized pink granite. both of which have been emplaced by cauldron subsidence in a granodiorite body. The mylonitized pink granite is locally cut across by hypersolvus coarse-grained red granite. The mylonitized pink granite is characterized by fully-developed mylonitic fabrics and suffered inflation and flattening due to a ‘ballooning effect’ which suggests that it has experienced stronger ductile deformation than the granodiorite and biotite granite. The HW granites are mildly peraluminous, highly fractionated and enriched in Rb, Nb. Y, REE (except Eu) and F with corresponding depletion in Mg, Fe, Ti, Ca, Sr and Ba. REE fractionation patterns and Eu Eu∗ values decrease from granodiorite ( La Yb n = 6.24−6.69 ; Eu Eu∗ = 0.95−1.56 ) to biotite granite ( La Yb n = 2.48−4.68 ; Eu Eu∗ = 0.25−0.75 ). The mylonitized pink and red granites display flat patterns and large negative Eu anomalies ( La Yb n = 0.33−1.09 ; Eu Eu∗ = 0.01−0.07 , respectively). Although accessory minerals (e.g. apatite, allanite and fluorite) exerted some control on REE concentrations, volatile transfer largely controls REE abundances, particularly HREE (heavy rare-earth element). Geochemical data indicate that the granites of HWC are post-collision, A-types. The less fractionated granodiorite has an I-type and volcanic arc geochemical signature. Least-squares analyses of major elements and modelling of REEs indicate that the HW A-type granites were formed through two stages of differentiation. A first stage, crystalmelt fractionation, was dominated by the removal of plagioclase, biotite, quartz and apatite from the granodiorite magma to yield the biotite granite. Further crystal fractionation of the biotite granite liquid with K-feldspar as a dominant separating phase in addition to plagioclase, biotite and quartz gave rise to the mylonitized pink granite (second stage). Volatile phase transfer occurred during the second stage of fractionation in the magma chamber. The red granite represents the last crystallizing granite type from a residual, volatile-rich fluids which accumulated and emplaced as ring dykes in the solidified part of the mylonitized pink granite.