High magnesian granitoids in the Precambrian continental crust: implication for the continuum between ferro-potassic and magnesio-potassic rock suites

Abstract

Ferro-potassic (Fe–K) to magnesio-potassic (Mg–K) hornblende–biotite granitoids (HBGs) with a wide range of SiO₂ from 52 to 77 wt% are typical of Precambrian continental crust, and are generally related to late- and post-collisional orogenic events. Despite their broadly uniform mineralogy and tectonic environment, the chemical compositions of these granites suggest different sources from lower crust for the Fe-K suites to enriched mantle for the Mg–K counterparts. Here we propose a classification of the magnesian rocks into moderate-Mg, high-Mg and ultra-Mg, with corresponding examples from various parts of the world such as the Closepet-type, sanukitoids, and their high-Mg counterparts of the Elan-type. The Elan-type granitoids are characterized by the highest MgO, Cr, Ni, εNd(T) and lowest TiO₂, FeOt/(FeOt + MgO) contents among all the Precambrian HBGs, and are hence close to a true end-member. The Precambrian HBGs show common features such as a wide range of SiO₂ contents, enrichment in LILE (K, Ba, Sr), LREE, Nb-Ti negative primitive-mantle normalized anomalies and weak negative or no Eu-anomalies in intermediate suites. The Mg# of magmatic and cumulus/ relict amphiboles displays a compositional array of ultra-Mg to Fe granites and shows positive correlation with the whole-rock Mg#. Continuum between the compositions of Mg–K and Fe–K granitoids is reflected in a gradual decrease in Mg, Cr, Ni of the rocks and Mg of the mafic silicates, increase in Ti, P, Zr, Y, Zn, U, total REE, and decrease of the Nd initial isotopic composition, together with a change in the paragenesis of Fe–Ti-oxides. We interpret these features to result from mixing of metasomatized mantle and crustal magmas and their homogenization at the lower crust level. The lower-crustal source component increases from the Mg–K to Fe–K series. During their passage to higher crustal levels, the magmas underwent fractional crystallization and crustal contamination leading to the wide range in silica content.