A Temperature-Composition Framework for Crystallization of Fractionated Interstitial Melt in the Bushveld Complex from Trace Element Systematics of Zircon and Rutile

Document Type

Article

Publication Date

7-1-2018

Abstract

The near-solidus crystallization history of the Paleoproterozoic Bushveld Complex, the world’s largest layered intrusion, has been investigated using the in situ trace element geochemistry (LA-ICP-MS) of accessory minerals that crystallized from late, highly fractionated pockets of interstitial melt in layered cumulates and from granitic magmas in felsic roof rocks. Zircon with simple to complex sector zoning occurs in mafic–ultramafic rocks in interstitial pockets that contain quartz–biotite–plagioclase and local granophyric intergrowths. Chondrite-normalized rare earth element patterns are typical of igneous zircon and Ti is negatively correlated with Hf in most samples. Ti-in-zircon thermometry of the cumulates (T= 950–730°C) records the onset of zircon saturation through to the solidus, with notably cooler temperatures determined for Upper Zone and roof rock zircon (T= 875–690°C). Forward modelling of proposed Bushveld parental magmas using rhyolite-MELTS consistently yields similar temperatures for zircon saturation (800–740°C) from highly fractionated melts (~5–20% remaining melt) with late-stage, near-solidus mineral assemblages similar to those observed in the rocks. Anomalously high and variable Th/U (2–77) in zircon from orthopyroxenites in the Critical Zone, including those associated with the PGE-rich UG2 chromitite and Merensky Reef in the Upper Critical Zone, can be related to U loss from the fractionated interstitial melt during exsolution of late, oxidized Cl-rich fluids. In addition to zircon, rutile occurs throughout the Critical Zone of the Bushveld Complex in two different textural settings, as interstitial grains with quartz and zircon and with chromite, each with distinctive chemistry. Euhedral rutile needles found in interstitial melt pockets have relatively high HFSE concentrations (Nb= 1000–20 000 ppm; Ta= 100–1760 ppm), high Zr-in-rutile temperatures (1000–800°C), and are magmatic in origin. Rutile associated with chromite, either as rims or inclusions, is strongly depleted in HFSE (Nb <1000 ppm; Ta <100 ppm) and in Cr and Sc relative to magmatic rutile, and represents a sub-solidus exsolution product of Ti from chromite. Exploring the near-solidus evolution of mafic layered intrusions such as the Bushveld Complex using the trace element chemistry of accessory minerals provides a novel approach to constraining the late stages of crystallization from highly fractionated interstitial melts in these petrologically significant intrusions.

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