TRACE ELEMENT SYSTEMATICS AND ZONING IN RUTILE: IMPLICATIONS FOR THERMOMETRY & PROTOLITH APPLICATIONS IN HIGH-PRESSURE/LOW-TEMPERATURE (HP/LT) METAMORPHIC ROCKS

Rutile is a common mineral in high-pressure/low-temperature (HP/LT) metamorphic rocks where it serves as an important host for transition metals and high-field strength elements. The extent to which these elements substitute in for Ti⁴⁺ in the rutile crystal structure is controlled by variables such as temperature, pressure, the composition of the rock in which the rutile grew, as well as interactions with fluids. Rutile composition can thus be used to track the metamorphic conditions, protoliths and geochemical evolution of HP/LT metamorphic rocks. However, previous attempts to use rutile as a geothermometer or protolith indicator in paleosubduction zones have yielded inconsistent results, indicating that the trace element (TE) behavior and systematics of rutile in subduction zones is not well-understood. This study aims to investigate the TE systematics of rutile in HP/LT metamorphic rocks to understand how differences in thermal gradients, protoliths, and histories of fluid-rock interaction can influence the TE composition and zoning patterns of rutile. To evaluate the effects of these different variables, a detailed TE composition and zoning study of rutile in HP/LT rocks from six paleosubduction zones that represent different subduction endmembers is being conducted. Preliminary TE studies of rutile from the Sivrihisar Massif (SM), south Motagua Fault Zone (SMFZ), and the Rio San Juan (RSJC) and Samana Metamorphic Complexes (SMC) show that, regardless of textural position, all rutile from the SMFZ, RSJC and SMC have Nb/Cr that indicate a metamafic origin, consistent with bulk-rock data suggesting a MORB protolith. However, rutile from metamafic rocks in the SM have Nb/Cr that indicate both metamafic and metapelitic origins. This suggests that some SM rutile records interactions with (meta-)sedimentary material, which is consistent with the more enriched bulk-rock signatures exhibited by these samples. In some cases, Nb/Cr varies with the textural position of the rutile, indicating that the rutile may be tracking temporal changes in bulk-rock composition during subduction-related metamorphism. Rutile Zr concentrations, which are used to derive metamorphic temperatures, also vary in this sample suite, with rutile in rocks with MORB protoliths exhibiting less scatter in Zr concentrations.