Trace Element Evolution of Quartz During Igneous Differentiation of Granitic Melts: is it Erratic or Systematic?

To document the trace element evolution of granitic quartz we sampled 120 granitic pegmatite localities in South Norway. The granitic pegmatites comprise two major pegmatite fields that were thoroughly studied throughout the past decades in terms of their major and accessory mineralogy and stable and radiogenic isotopes. Therefore, we have a solid geological framework to study and interpretate the petrogenesis of granitic quartz. The two pegmatite fields share many similarities and are nearly indistinguishable in terms of their mineralogy, radiogenic isotope signature and, partially, in their accessory mineral assemblages. Accordingly, both pegmatite fields comprises REE-Nb-Ta type pegmatites with Rb/Sr-ages close to 850 Ma and have relatively low 87Sr/86Sr isotope ratios of only 0.7062-0.7064. The trace and major element distribution in feldspars is indistinguishable and follows overlapping trends during igneous evolution of the granitic melts from typical K/Rb ratios in K-feldspar of 360 in the most primitive pegmatites to 100 at the most evolved localities. However, a recent study of the REE distribution in K-feldspar (Larsen, 2002, The Canadian Mineralogist, 40, 137-151) unequivocally demonstrate that the parent melts forming the two pegmatite fields were derived from different source regions and that the REE mineral assemblage that buffered the REE-distribution in K-feldspar and the melts show subtle but distinctive differences. Contrary to feldspar, the distributions of structurally bound trace elements in quartz (i.e. either sustituting for Si or acting as charge compensators that are confined to lattice vacancies or open strutural channels) confirm the incompatibility of the two pegmatite fields. Laser Ablation ICP-MS analysis of quartz show that the concentration of Li, Be, Al, P, Ti and Ge change systematically during igneous differentiation towards progressively more evolved pegmatites. Ti has a pronounced compatible geochemical behavior whereas Ge and Li are incompatible and obtain highest concentrations in the most evolved pegmatites. Particularly the Ge/Ti and the Ti/Li ratios of quartz efficiently trace the igneous evolution of the granitic melts. Moreover, when plotting these ratios against other trace elements in quartz the two pegmatite fields follows widely different evolutionary paths during igneous differentiation of the granitic melts. Sudden abrupt deviations from the igneous path are recorded at some localities and, although not fully resolved, may record the onset of H2O saturation and nucleation of an aqueous phase. In conclusion, our studies demonstrate that structural bound trace elements in igneous quartz in deed follow systematic and predictable trends during AFC-processes in granitic magma chambers. Moreover, when compared to other common phases in the granitic mineral assemblage, quartz may be superior in distinguishing the origin and evolution of granitic rocks in regions that have experienced long and complex igneous histories.