Trace Elements of Pegmatitic Quartz and Their Regional Distribution in two Pegmatite Fields of Southern Norway

Abstract

In this study, we analysed the trace elements Li, Ge, Al and Ti in quartz of 155 granitic pegmatites from the Froland and Evje–Iveland pegmatite fields by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). These elements are the most common trace elements in quartz and their concentrations can be utilized to determine the degree of melt fractionation (Al, Li and Ge) and the crystallization temperature of pegmatites by applying the Ti-in-quartz geothermometer. Both pegmatite fields are part of the south Norwegian pegmatite province that was formed during the Sveconorwegian orogenesis at the western margin of the Fennoscandian shield, when the Bamble Complex was thrust over the Telemark Block along the Porsgrunn–Kristiansand Fault Zone (PKFZ). Quartz of the Froland pegmatites has relatively homogeneous compositions with 11.8 ± 6.7 ppm Li, 1.5 ± 0.8 ppm Ge, 44.7 ± 22.7 ppm Al and 7.7 ± 3.3 ppm Ti. The low Ti corresponds to crystallization temperatures of 535 ± 31°C. The temperatures are highest along the north-western margin of the pegmatite field (>550°C), close to the PKFZ. The most differentiated pegmatites occur along the north-eastern margin of the field, away from the PKFZ. The regional pattern of the quartz chemistry implies that the formation and emplacement of Froland pegmatite melts occurred in conjunction with the Sveconorwegian overthrusting. Quartz of the Evje–Iveland pegmatites has more variable compositions with 7.3 ± 4.5 ppm Li, 3.0 ± 2.9 ppm Ge, 81.7 ± 58 ppm Al and 21.4 ± 11.4 ppm Ti. The high Ti content indicates crystallization temperatures of 582 ± 48°C. The highest crystallization temperatures were detected along the northern margin and the central northern part of the field. Low crystallization temperatures were detected in the central eastern and south-western part of the Evje–Iveland field. The irregular regional zoning of the Evje–Iveland field contradicts the hypothesis that a parent granite intrusion is underlying the pegmatite field. The lack of coeval granite intrusions in both pegmatite fields suggests that the source of the pegmatite melts is to be found at deeper crustal levels and remains enigmatic at the current stage of knowledge.