GEOL 5903 Seminar: Shae Nickerson
7 December 2020
GEOL 5903 Seminar: Shae Nickerson
Title: Pegmatite formation by crystal fractionation of a parental magma or partial melting of crustal rocks?
Time: 2:30 p.m. - 3:30 p.m.
Presentation via TEAMS.
Abstract:
Granitic pegmatites are very coarse-grained igneous rocks that can contain minerals rich in trace elements and rare-earth elements (REE). These elements are essential to the technology that we use every day, making them economically important. For successful pegmatite exploration and mining, an understanding of how and where pegmatites form is important. Since the mid-1800s, the origin of granitic pegmatite has been controversial, and various models have been developed for how the melts that form them are generated. Currently, two main competing hypotheses are proposed: (1) fractional crystallization of a parental granitic magma, and (2) low-degree partial melting of metasedimentary or amphibolitic crustal rocks. The Pilbara pegmatite province in Australia is an example of pegmatites with clear parental pluton(s), based on similar ages and geochemical characteristics, as well as spatial association and evidence from regional zonation, where chemical fractionation of the pegmatites increases with distance from the source pluton. Examples of pegmatites that formed from partial melting of crustal rocks occur in the Sveconorwegian province in Norway and Sweden. Many of these pegmatites are not spatially associated with granitic plutons and if they are, they are either too old or too young to have formed from the residual melt of the pluton. However, the Tørdal pegmatites in that area are an exception as they are spatially associated with the Tørdal granite and are the same age, although they do not show a pattern of increasing degree of fractionation with distance from the pluton. The Tørdal pegmatites are hosted by amphibolite which could be the source, although mica chemistry from the pegmatites compared to the host rock is not consistent with an origin by partial melting of the amphibolite. At the present time, a single model that can be applied to the origin of all economically important granitic pegmatites has not been formulated.