The origin of small-scale geochemical and mineralogic variations in a granite intrusion

Abstract

A post-tectonic unzoned granite intrusion in the Meatiq Dome, a Late Proterozoic metamorphic complex in the Central Eastern Desert of Egypt, shows significant chemical and mineralogic heterogeneity on the scale of sampling (∼5 kg). Whole rock analyses of 21 samples indicate small variations in SiO₂, Al₂O₃, Na₂O and K₂O, and larger systematic variations in the less abundant major elements such as FeO, TiO₂, CaO, MnO and MgO, and trace elements such as Sc, Cr, Co, Rb, Sr, Zr, La, Ce, Nd, Sm, Tb, Yb, Lu, Ta, Th and U. These variations cannot be accounted for by processes such as marginal accretion, assimilation, alteration by late stage fluids, or multiple intrusion. Instead, we proposed a model involving 35 percent solidification of the granite magma followed by partial solid-liquid segregation during emplacement, resulting in rocks containing 7–71 volume percent early-formed solids. Such randomly distributed local segregation could have been caused by filter pressing, flow differentiation, and possibly gravity segregation, either singly or in combination. Thus, each sample is interpreted as a mixture of two end-members with nearly constant compositions: an early-formed solid assemblage of crystals and a complementary residual liquid. Early formed solids are enriched in TiO₂, FeO, CaO, P₂O₅, Sc, Co, Cr, Sr, La, Ce and depleted in SiO₂, Rb, Yb, Lu, Ta, Th, and U, while the residual liquid has complementary enrichments and depletions. This simple mixing model is consistent with field and petrographic observations, experimental studies pertaining to the crystallization sequence in the system Ab-Or-Qtz-H₂O at 1 Kb, and physical properties of silicic magmas. Furthermore, it is quantitatively supported by trace-element data for minerals, computed endmember compositions at 35% crystallization using mineral analyses and reasonable Kd values, and internal consistency in the percent solid for each sample computed from each of 17 elements in the inferred end-members. We suggest that this model might also apply to other small epizonal granite intrusions that show small-scale chemical heterogeneities.