Evaluating the utility of spinel-group mineral chemistry to distinguish the origin of altered ultramafic rocks in orogenic belts

Session: Evolution of Orogenic Belts Through Time: Insights from Sedimentation, Deformation, Magmatism, and Metamorphism, Part I

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Abstract:

Orogenic belts are littered with dismembered and altered ultramafic-mafic bodies that could represent a variety of origins (e.g., ophiolite, exhumed mantle, continental intrusion), with these interpretations placing disparate constraints on the tectonic evolution of orogens. These potential origins are routinely assessed using the major-element composition of spinel-group minerals, as the principal elements that constitute these minerals are highly sensitive to petrologic variables (e.g., magma source and composition, crystallization temperature, oxygen fugacity). Despite this utility, 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. In this study, we evaluate the utility of spinel-group mineral chemistry to distinguish between altered ultramafic rocks formed in three petrologically-distinct tectonic environments: (1) komatiites, which represent the crystallization of high-Mg lavas derived from extremely 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 crystallization of mafic magma in a magma chamber. Using a new dataset comprising 2457 spot analyses from 49 ultramafic rocks, back-scattered electron imaging and elemental maps, we characterize the effects of secondary alteration for all localities studied, before applying principal component analysis (PCA) to the filtered (secondary analyses excluded) and un-filtered (all analyses) datasets. When considering the filtered data, our PCA demonstrates that TiO₂ is the only measured element capable of distinguishing melts (komatiites and layered intrusions; TiO₂ >0.175 wt.%) from mantle residues (TiO₂ <0.15w wt.%), with these tectonic environments indistinguishable using all other elements and chemical proxies (e.g., Cr#, Fe²⁺#, Fe³⁺#).