Zircon trace element geochemistry of the neoarchaean late-granite suites along the southern margin of the Zimbabwe craton, Zimbabwe

In-situ laser ablation quadrupole inductively coupled plasma mass spectrometry (LA-Q-ICP-MS) analyses of zircon grains from the ca. 2635–2620 Ma Chilimanzi and Razi granite suites along the southern margin of the Zimbabwe Craton in Zimbabwe reveal that a majority of zircons experienced post-magmatic chemical alteration. A minority (5 %) of the analyses show unaltered (Type-1) patterns. Type-1 zircons show low abundances of light rare earth elements (LREE) relative to heavy rare earth elements (HREE) and they preserve igneous zoning textures. The majority of the grains analysed exhibit altered (Type-2) patterns indicated by overabundance in LREE concentrations and such grains are partially or completely metamict (radiation damaged). Type-2 grains experienced radiation damage due to high initial contents of radioactive elements including U and Th incorporated at the time of crystallization. The structurally defective zircons experienced pervasive chemical alteration from Type-1 to Type-2 compositions via susceptible regions such as fractures under fluid-mediated conditions. Lack of analyses with Th/U ≤ 0.08, which is a diagnostic feature of zircons precipitated from hydrothermal-melts supports the view that Type-2 compositions derived from alteration of Type-1 zircons. LREE overabundance in the zircon suites is also explained by inadvertent analysis of mineral inclusions and/or contaminants such as Fe, Ti, and Mn oxides along fractures.
Our study established a LREE index, LREE-I = 32 as a discriminant for altered and unaltered zircon analyses. LREE-I < 32 (altered compositions) correlates with known alteration features such as cathodoluminescence dark core domains, partial or complete overprint of igneous textures in fluid-mediated conditions, overabundance in LREE, and high contents of non-structural elements in zircon such as Ti, Li, Th and U. The Ti-in-zircon thermometry on unaltered analyses of the Chilimanzi Suite's Great Zimbabwe granite constrains the crystallization temperature at 698–738 °C, consistent with temperatures at zircon saturation for felsic magmas. Our study emphasizes that zircon chemistry is a pre-requisite data layer that should be incorporated in analytical protocols to complement conventional, discordance-based data filter for effective screening of U-Pb analyses for age determinations.