Mineralogical and mineral-chemical changes in a fractionation-dominated diorite-monzodiorite-monzonite sequence: evidence from cathodoluminescence
Wenzel, Thomas; Ramseyer, Karl
European Journal of Mineralogy Volume 4 Number 6 (1992), p. 1391 - 1400
published: Dec 15, 1992
manuscript accepted: Mar 25, 1992
manuscript received: Apr 22, 1991
ArtNo. ESP147050406003, Price: 29.00 €
Abstract The plutonic complex of the Meissen massif, E-Germany, consists mainly of granodioritic to monzogranitic intrusions surrounded by rocks lower in silica. Cathodoluminescence investigations (CL) on a diorite-monzodiorite-monzonite sequence from Freital and on isolated monzonitic outcrops revealed complex growth patterns in apatite, plagioclase and K-feldspar. Under CL, apatite crystals from diorite and monzodiorite show growth and corrosion zonations with 5% of the crystals containing well-rounded, probably relictic cores. In apatites from the monzonites, additional yellow luminescing rims are developed. The violet and yellow luminescing zones are caused by different abundances of Sm3+, Dy3+, Eu2+, Ce3+ and Mn2+, respectively. Apatite compositions in diorite and monzodiorite suggest alkaline affinities of their parental melt. The dioritic plagioclases luminesce with a homogeneous bluish-green colour, which is characterized by the Mn2+ and Fe3+ peaks at 560 and 750nm, respectively. Increase of the 750nm peak in monzonitic plagioclases generates a brownish CL, indicating higher amounts of Fe3+. This agrees well with the occurrence of hematite in the monzonite. The CL spectra of K-feldspars show peaks at 710 and 440nm, caused by the incorporation of Fe3+ and ?Ti4+/Al-O centers, respectively. The successive enrichment of mainly blue luminescing K-feldspar in the Freital sequence may result from fractionational crystallization of the basic magma. This assumption is favoured by whole-rock geochemical data. In addition, apatites and feldspars of isolated monzonites gave identical results, indicating that these rocks underwent "dioritic and monzodioritic stages" comparable to the Freital sequence. An increase of the Fe3+ induced red luminescence in late K-feldspar may suggest that metasomatic influences play an important role during the latest stage of crystallization. This is in agreement with the occurrence of chlorite pseudomorphs after biotite.