Can Spinel-Group Mineral Chemistry Reliably Distinguish Altered Ultramafic Rocks from Different Tectonic Settings? Evidence from Komatiites, Mantle Residues, and Layered Intrusions

Abstract

Orogenic belts are commonly littered with dismembered and altered ultramafic-mafic bodies that could plausibly represent a variety of origins (e.g. ophiolite, exhumed mantle, continental intrusion). Because these bodies are often preserved with limited geological context, their origins may be difficult to resolve, resulting in competing tectonic models. One tool to interpret the tectonic setting in which such ultramafic-mafic bodies formed is the major-element composition of spinel-group minerals. The potential utility of this method is a consequence of the spinel-group minerals’ major element sensitivity to petrological variables. However, numerous studies have documented the susceptibility of these minerals to post-crystallization alteration, obscuring primary chemistry and reducing the reliability of spinel-group minerals as a tectonic discriminator. Despite this, they remain a widely utilised tool in a range of mafic–ultramafic settings of diverse age. In this study, we evaluate whether spinel-group mineral chemistry can reliably distinguish between the primary tectonic environments of ultramafic rocks that have experienced significant metamorphism and/or hydrothermal alteration. We focus on three petrologically-distinct tectonic environments: (1) komatiites, which represent the crystallization of high-Mg lavas derived from high degrees (>30%) of partial melting; (2) the mantle portion of ophiolites, which reflect multiple phases of melt extraction; and (3) layered intrusions, which form during the intrusive crystallization of mafic magma. Using a new dataset comprising 2457 spot analyses from 48 ultramafic rocks, back-scattered electron imaging, and elemental maps, we characterize the effects of secondary alteration for all localities studied, before applying multivariate statistical analysis to the filtered (secondary analyses excluded) and un-filtered (all analyses) datasets. Our filtered dataset demonstrates that almost all elements and chemical proxies, including those traditionally used to infer tectonic significance from spinel-group mineral chemistry (e.g. Al2O3, Cr-number), cannot distinguish melts from residual mantle rocks and are thus not useful tectonic setting discriminators. TiO2 is the only element considered to represent a coarse tectonic discriminator: spinel-group minerals with > 0.2 wt.% TiO2 likely crystalized from melts (komatiites and layered intrusions), while those within TiO2 contents < 0.1 wt.% likely formed in residual mantle rocks. Compositions between 0.1 and 0.2 wt.% TiO2 fall into a zone of uncertainty where other lines of evidence are required to determine the tectonic setting of ultramafic rocks. This proxy is applicable only to ultramafic rocks and should not be used to distinguish mafic volcanic rocks from different tectonic settings.