Rutile occurrence and trace element behavior in medium-grade metasedimentary rocks: example from the Erzgebirge, Germany

Abstract

Metamorphic textures in medium-grade (~500-550°C) metasedimentary rocks from the Erzgebirge give evidence of prograde rutile crystallization from ilmenite. Newly-crystallized grains occur as rutile-rich polycrystalline aggregates that pseudomorph the shape of the ilmenites. In-situ trace element data (EMP and SIMS) show that rutiles from the higher-grade samples record large scatter in Nb content and have Nb/Ti ratios higher than coexisting ilmenite. This behavior can be predicted using prograde rutile crystallization from ilmenite and indicates that rutiles are reequilibrating their chemistry with remaining ilmenites. On the contrary, rutiles from the lowest grade samples (~480°C) have Nb/Ti ratios that are similar to the ones in ilmenite. Hence, rutiles from these samples did not equilibrate their chemistry with remaining ilmenites. Our data suggest that temperature may be one of the main factors determining whether or not the elements are able to diffuse between the phases and, therefore, reequilibrate. Newly-crystallized rutiles yield temperatures (from~500 to 630°C, Zr-in-rutile thermometry) that are in agreement with the metamorphic conditions previously determined for the studied rocks. In quartzites from the medium-grade domain (~530°C), inherited detrital rutile grains are detected. They are identified by their distinct chemical composition (high Zr and Nb contents) and textures (single grains surrounded by fine grained ilmenites). Preliminary calculation, based on grain size distribution of rutile in medium-grade metapelites and quartzites that occur in the studied area, show that rutiles derived from quartzites can be anticipated to dominate the detrital rutile population, even if quartzites are a minor component of the exposure.

Metal mobility during hydrothermal breakdown of Fe-Ti oxides: Insights from Sb-Au mineralizing event (Variscan Armorican Massif, France)

“…On the other hand, the breakdown of detrital rutile to titanite during prograde metamorphism (Otago Province, New Zealand) has been shown to release significant amounts of W (Cave et al, 2015), later incorporated in scheelite disseminated in Otago Schist and in orogenic gold veins (Cave et al, 2016). More generally, breakdown of Fe-Ti oxides is common during diagenetic, metamorphic and hydrothermal reactions (Morad and Aldahan, 1986;Luvizotto et al, 2009;Rabbia et al, 2009), a process that can redistribute metals at a regional scale.…”

The Cedrolina Chromitite, Goiás State, Brazil: A Metamorphic Puzzle

“…It has also been demonstrated that hydrothermal solutions and metamorphic fluids are capable of introducing incompatible elements in the alteration system of a chromitite, giving rise to the formation of exotic accessory minerals, such as monazite, galena, bismuthinite, and antimony that normally do not occur in unaltered chromitites [13,14]. In a number of contributions [15][16][17][18] it has been shown that certain trace elements can be successfully used to evaluate the crystallization temperature of rutile, as well as the behavior of the mineral under different metamorphic reactions. Likewise, trace element concentrations in zircon can be used to identify its geochemical signature and, therefore, probable source rock.…”

Benefits of a Multiproxy Approach to Detrital Mineral Provenance Analysis: An Example from the Merrimack River, New England, USA

“…The divider here is taken from Triebold et al () and indicates that the vast majority of rutile were derived from metapelites. Published rutile data from Zack et al () and Zack, Moraes, & Kronz (2004a) and Luvizotto et al () are shown for comparison; metamafic rutile from the Catalina schist in California has unusually high Nb and low Cr, but all other rutile plots in the correct zones. (b) Rutile that was derived from the metamafic rocks is characterized by Nb less than 600 ppm.…”