Trace element evolution of quartz in the charnockitic Kleivan granite, SW-Norway: The Ge/Ti ratio of quartz as an index of igneous differentiation

Abstract

Trace element analyses of quartz from the layered charnockitic Kleivan granite in the Rogaland Igneous Province (Rogaland Municipality, SW-Norway) reveal that the trace element composition of quartz can track igneous evolution during differentiation of the magma. Combined SEM-CL imaging and LA-ICP-MS analyses allow identification and analysis of primary igneous quartz. Al, Ti, Li, and Ge are the most abundant trace elements in the Kleivan quartz and each element shows a characteristic evolution during differentiation of the granitic magma. The Ge/Ti ratio of quartz is a reliable index of the magmatic evolution of granitic igneous systems, since Ti and Ge show pronounced compatible and incompatible characteristics, respectively. The temperature of the quartz crystallisation varies from 850 °C during charnockite formation to 645 °C in the most evolved aplitic–granite and granitic pegmatite. Therefore, the regular evolution of Ti and Ge in quartz during cooling of the melt may be used as an igneous geothermometer. The Li evolution pattern of quartz is mostly controlled by the Li content of the melt hence by the absence or presence of Li-bearing phases forming during solidification of the melt. Al in quartz is controlled by the aluminium saturation index of the melt. Accordingly, Al in quartz is constant during crystallisation of a metaluminous magma and increases during peraluminous conditions.