Trace element systematics of zircon from granophyres of the Archean Stillwater Complex, Montana, USA. Ver Hoeve, Thomas James
The ca. 2.7 Ga Stillwater Complex, an 8 km-thick mafic-ultramafic layered intrusion in southwestern Montana, contains a suite of granophyres hosted within plagioclase-rich cumulates of the Banded Series that are interpreted as late-stage magmatic differentiates. The granophyres occur as discordant bodies ranging from cm-thick veins to bodies tens to hundreds of meters in thickness. The incompatible element-enriched granophyres contain accessory U-Th-Pb-bearing minerals (zircon, titanite, rutile) that yield similar ages to the mafic cumulates for low-U minerals that have remained closed since crystallization. High-U zircon (>1000 ppm) in the granophyres is variably metamict due to self-irradiation and the metamict zones have been altered during subsequent hydrothermal activity resulting in open-system U-Pb systematics. In this study, the consequences of elemental redistribution during alteration of zircon from four granophyres from the Stillwater Complex have been investigated using scanning electron microscopy (SEM) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). SEM imaging reveals strongly contrasting zonation and internal structure of zircon. A granitic pegmatite core from the Lower Banded Series contains pristine igneous low-U zircon (37-154 ppm) with no evidence for metamictization or alteration (e.g., no measurable Ca). The other three samples from the Middle Banded Series and Upper Banded Series contain high-U zircon (358-12,440 ppm) that is partially to completely metamict. Non-stoichiometric Ca (up to 37,800 ppm) characterizes the metamict zones or grains, which also show significant gains in Ti, Nb, rare earth elements, and nearly complete loss of Li in the most altered zircon. Ti-in-zircon thermometry from the pristine zircon of the low-U granophyre yields temperatures of 701 to 799°C (mean = 752°C) that are interpreted as the temperatures of crystallization of these late, fractionated melts. Thermometry from altered zircon produces highly variable results (584-1065°C) reflecting the addition of Ti during alteration. The long-term stability of zircon in the geological record makes it a powerful geochronological, isotopic (e.g., O, Hf, Nd, Li), and geochemical tool, however, the results of this study demonstrate the potential for significant open-system behavior and elemental mobility within zircon depending on the relative state of metamictization and extent of hydrothermal alteration in these damaged zones.
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