The role of mantle and the ancient continental crust in the generation of post-collisional high-K calc-alkaline and alkaline granites, with main reference to the Arabian-Nubian Shield

Abstract

Continental-scaled magmatic activity occurs after the end of collision-orogenic stages of the Wilson cycle. This post-collisional magmatism is widely exhibited, from 635 to 540 Ma, in a series of large Neoproterozoic (Pan-African) orogenic belts throughout North Africa. Special reference is given to the Arabian-Nubian Shield (ANS) and the LATEA metacraton (Tuareg shield). For comparison, two Late Paleozoic-Early Mesozoic transcontinental belts, the Variscan in Western Europe and the east Central Asian Orogenic Belt, are briefly discussed. Two main types of consecutive magmatic suites characterize the Late Neoproterozoic (Ediacaran) and Late Paleozoic-Early Triassic (Variscan) post-collisional magmatic events despite their different formation age and geographic location: (a) Early post-collisional suites composed of dominant HKCA monzogranite, with secondary quartz monzonite, granodiorite, and related high-K calc-alkaline gabbro, diorite; (b) Late post-collisional to intra-continental suites of Alk syenogranite, alkali feldspar granite and quartz syenite, in places with Na-rich pyroxene and amphibole; volcanic activity and numerous dike swarms are abundant. The mafic rocks related to Alk granite suites are monzogabbro and monzonite (shoshonitic series). Granitoids of the same type, HKCA or Alk, are very similar in mineral and chemical composition throughout the belts and in different belts. This similarity is observed in regions with very distinct types of crust, e.g. the dominant juvenile oceanic island arc crust in the ANS, and Archean-Paleoproterozoic continental crust in the LATEA metacraton. The isotope U-Sr-Nd-Hf-O signatures suggest that HKCA granite magmas in these regions were produced from different crustal sources with varying contribution of mantle-derived basic materials to the granitoid magma generation. For HKCA granites, a model of crustal anatexis, accompanied with a limited influx of H2O, K, LILE, HFSE from underplated basalt magmas (convective diffusion in the contact of silicic and mafic liquids) is suggested. The most likely model of the Alk silicic magma generation is partial melting of related mantle-derived mafic rocks (monzogabbro, monzonite in the lower crust. Data on the enhanced temperature of A-type silicic magmas, more than 1000 °C, suggest that magma-generation could occur even at the depth of uppermost lithospheric mantle.