Figure 1. Compositional maps from a garnet-bearing sample from Xanthi, Greece Left: Manganese (red) and magnesium (blue) zoning indicates multiple growth stages within garnet. Right: Composite elemental map shows distribution of minerals differentiated by color.

Investigating the Ultrahigh-Pressure Metamorphic History of the Rhodope Metamorphic Complex

Pacifica (Kitrea) Takata-Glushkoff, Class of 2019

During continental collisions, portions of Earth’s crust can be subducted to depths >100km. At these conditions, the minerals in the subducted crust metamorphose to ultrahigh-pressure (UHP) mineral assemblages and produce distinct microstructural patterns. As these metamorphic rocks are later exhumed back to Earth’s surface, some of the petrological evidence of UHP metamorphism is lost. The Rhodope Metamorphic Complex (RMC), in northeastern Greece and southern Bulgaria, is one of about 25 locales worldwide, which contains documented evidence of UHP metamorphism. Within the RMC, four locations preserve evidence of UHP metamorphism, but this evidence has been overprinted to varying degrees. Because of this variation, the RMC provides an excellent field site to investigate UHP metamorphism and the factors that favor its preservation.

Our research team traveled to Greece this summer to collect and analyze 65 rock samples from multiple locations within the RMC. We have analyzed these samples using scanning electron microscopy (SEM) and electron probe micro-analysis to address the following overarching questions:

1) What pressure-temperature-time path(s) did these rocks follow during subduction and exhumation? 2) Which of these rocks preserve indicators of UHP metamorphism? 3) Do the indicators of UHP metamorphism vary according to bulk rock composition or the pressure-temperature-

time path?

My laboratory research this summer focused on one sample that I collected from outcrops exposed along the Xanthi River in northern Greece. I collected the sample from a folded sequence of marble and metamorphosed mudstones. This specimen is particularly important for addressing our central questions because it contains garnet with concentric rings of different colors that suggest multiple stages of growth. Back in the lab, I asked the following questions:

1) Do these garnet crystals preserve compositional evidence of multiple stages of growth? 2) Do the mineral inclusion types and orientations display patterns associated with multiple stages of garnet growth?

Compositional analysis on the SEM (Fig. 1, Left) revealed concentric Mn domains (red) that show depletion and enrichment patterns consistent with the hypothesis that there were two successive growth events. The garnet crystals also increase in Mg (blue) from the cores out to the rims. This pattern can either be the original Mg zoning distribution or the result of later modifications to the original zoning. I also mapped the distribution of mineral inclusions and their orientations in garnet (Fig. 1, Right). Some patterns correspond with the successive garnet growths. The cores are characterized by aligned quartz inclusions that are at an angle to the dominant microstructural fabric of the rock and non-linear quartz-rich domains align with the inner edge of the second growth on both garnets.

Moving forward, we will further investigate these patterns to answer questions about the pressures, temperatures, and timing of the multiple stages of garnet growth. Such investigations provide insight about how this rock traveled to and from its maximum depth. We will then place this rock’s history into the larger plate tectonic context of the region.

Faculty Mentor: Emily Peterman Funded by the National Science Foundation