Trace Element Composition of Zircons from Rapakivi Granites of the Gubanov Intrusion, the Wiborg Massif, as a Reflection of the Fluid Saturation of the Melt

Abstract

SEM-EDS and SIMS in situ methods were used to study the trace element composition of zircon from rapakivi granites of the Wiborg massif: wiborgites of the second phase, trachytoid granites of the third phase, as well as aplitic granites from their contact zone. All three rock varieties are available for study in the building stone quarry of the Vozrozhdenie deposit (Karelian Isthmus), where the granites of the Gubanov intrusion are mined. Zircon composition from all rock types shows traces of active fluid impact. This impact is manifested both at the level of zircon internal structure (dark zones and areas on BSE-image) and in the contents of trace and rare-earth elements, which significantly increase in the altered zones that differ in the BSE color. The total REE content in the studied zircon reaches 9400 ppm. Zircon from granites of the third phase show an opposite slope of LREE and HREE distribution pattern, i.e., “bird wings” profile (Sm_N/La_N < 1). In the discrimination diagrams, the majority of the analyzed spots falls into the field of hydrothermal zircon. It is possible to assume that a source of fluid that affected zircon in all types of granites was fluid-saturated melts that produced trachytoid granites of the third phase.