Abstract

Granites constitute the main rock components of the Earth’s continental crust, which suggested to be formed in variable geodynamics environments. The different types of granitic rocks, their compositional characteristics, tectonic settings and magma sources are outlined. Mineralogical classification of granites includes four rock types: tonalites, granodiorites, granite (monzogranite and syenogranites) and alkali-feldspar granites. Alphabetical classification subdivided granites into: I-type, S-type, A-type and M-type granites. Moreover, formation of granitic magmas requires distinctive geodynamic settings such as: volcanic arc granite (Cordilleran); collision-related granites (leucogranites); intra-plate and ocean ridge granites. The Eastern Desert of Egypt (ED) forms the northern part of Nubian Shield. Both older and younger granites are widely exposed in the ED. Old granites (OG) comprise tonalites and granodiorites of syn- to late-orogenic granitoid assemblages. They are calcalkaline, I-type, metaluminous and display island arc tectonic setting. Younger granites (YG) on the other hand, include granites, alkali-feldspar granites and minor granodiorites. They are of I- and A-type granites and of post-orogenic to anorogenic tectonic settings. The majority of the YG are alkaline, A-type granite and of within-plate tectonic setting (WPG). The A-type granites are subdivided into: A2-type postorogenic granites and A1-type anorogenic granites. Granite magma genesis involves: (a) fractional crystallization of mafic mantle-derived magmas; (b) anatexis or assimilation of old, upper crustal rocks (c) re - melting of juvenile mafic mantle – derived rocks underplating the continental crust. Generally, older I-type granitoids were interpreted to result from melting of mafic crust and dated at approximately 760–650 Ma, whereas younger granites suggested to be formed as a result of partial melting of a juvenile Neoproterozoic mantle source. Moreover, they formed from anatectic melts of various crustal sources that emplaced between 600 and 475 Ma.

Highlights

  • Granitoid rocks are the most abundant plutonic rocks in the continental crust, which are diverse and their magma sources have long been debated

  • Using the QAP diagram (Figure 1) granites are classified into four granite domains: tonalite, granodiorite, granite and alkali feldspar granites according to IUGS [2]

  • Genetic alphabetical classification includes: I-type granites (I = igneous); S-type granites (S = sedimentary); M-type granites products of mantle melts (M = mantle) and A-type (A = anorogenic) [3]: A = type granites were referred as alkaline or anorogenic granites [4]

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Summary

Introduction

Granitoid rocks are the most abundant plutonic rocks in the continental crust, which are diverse and their magma sources have long been debated. Island arc granitoids (I-type granites) are produced above subduction zones, which characterized by large masses of batholiths made of diorites, quartz diorites, tonalites, granodiorites and minor granites They are calc alkaline with small to high K contents and their Sr. initial ratio (Sri) are in the range 0.704–0.705 [16]. Eby [18] divided A-type granites into two categories according to their Y/ Nb ratios: a group have low Y/Nb ratios and generally low initial Sr./ Sr. ratios that formed by differentiation of basaltic magmas of OIB – like (plume) mantle sources; the second group of A-type granitoids characterized by higher Y/Nb ratios (1.2–7) and highly variable initial Sr./86 Sr. ratios This group shows a complex petrogenetic history as having a significant mantle component or may be totally of crustal origin. This could be helpful in understanding their compositional variation, tectonic environments and magma evolution

General outlines
Origin and source of magma
Age dating of granitoid rocks
Findings
Concluding remarks

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