Petrological and geochemical constraints on the evolution of late Pan-African Bakriya post-orogenic ring complex, Central Eastern Desert, Egypt

摘要

The post-orogenic Bakriya ring complex (BRC) consists of older diorite rocks in the middle part of the complex, outer syenogranite and a central subsolvus alkali feldspar granite. The latter constitutes a major part of the complex and was emplaced during cauldron subsidence. The mineralogical composition of the rock types of the BRC indicate their crystallization under variable water pressures suggesting their emplacement at different levels within the Earth's crust. Field relationships and element patterns of the rocks of the Bakriya ring complex reveal an evolution in two suites, namely: a diorite-syenogranite suite and alkali feldspar granite suite, both with several pulses of emplacement. The diorite rocks of the BRC are metaluminous and exhibit geochemical features of calc-alkaline magmatism. The syenogranite and alkali feldspar granite exhibit alkaline and peralkaline affinity, respectively. The syenogranite and alkali feldspar granite have geochemical characteristics of highly fractionated A-type post-orogenic magmatism such as high total alkalis, Nb, Zr andY contents with concomitant low values of Mg, Ca and Sr. The fractionated REF patterns of the diorite-syenogranite suite have LREE-enrichment patterns {(La/Sm)_n = 3.1 and 4.2, on average, respectively I with slightly flat HREE segment {(Gd/Lu)_n = 0.69 and 0.77, on average, respectively}. The HREE patterns of both diorite and syenogranite are characterised by slight negative Eu anomalies (Eu/Eu= 0.8 and 0.78, on average, respectively). Fractional crystallization has played an important role in the evolution of the diorite-syenogranite suite. The chemical variation of the diorite-syenogranite suite is likely to be attributed to plagioclase, K-feldspar, biotite, hornblende, apatite and titano - magnetite fractionation. The REE patterns of the alkali feldspar granite suite, on the other hand, are characterised by enrichment in HREE {(Gd/Lu)_n = 0.04-0.29} and strong negative Eu anomalies (Eu/Eu = 0.13-0.32) indicating a significant role of plagioclase fractional crystallization. The geochemical data suggest that the alkali feldspar granite has evolved by means of fractional crystallization through removal of K-feldspar, biotite, plagioclase, together with some accessories such as apatite and Fe-Ti oxide phases. From the HREE-enrichment in some samples of alkali feldspar granite it may be suggested that fluorine-complexes played some role during its evolution.