Mafic inclusions in migmatite domes: exceptional pressure-temperature-time records of crustal flow

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

Most of the materials that make up continents have been heated and deformed during tectonic events, such as when continents collide to create mountains, and most of the transformations that occur take place deep underground. The classical view of how hot, formerly-deep rocks become exposed at the Earth’s surface invokes processes unrelated to their high-temperature transformation (metamorphism), such as erosion that may occur over many millions of years, long after metamorphism. More recently, geoscientists have discovered some places where formerly-deep, hot rocks were rapidly transported toward the Earth’s surface, not as magma (one mechanism for rapid transport), but as mostly-solid, flowing crust. The central hypothesis of this research is that rapid ascent of deep rocks to the Earth’s surface occurs on a large-scale, not just in unusual, local sites. Most rocks do not record their earlier, deep history, but some do, and these can be investigated to understand the conditions, mechanisms, and timing of transport of deep, hot rocks toward the Earth’s surface, with implications for understanding the thermal and structural evolution of continents, including the distribution of mineral resources. Societal benefits of this project involve the training of graduate and undergraduate students in field and analytical skills in an important science, technology, engineering, and science (STEM) discipline. The project also represents a collaboration between researchers at the University of Minnesota and the University of Montpellier (France), including interaction and creation of educational materials for the general public for natural history museum. The principal investigators disseminate their research findings in part through the organization of an international field-forum on crustal flow systems. Domal structures comprised of high-grade metamorphic rocks (gneiss domes) are accessible manifestations of deep-crustal flow systems in mountain systems and represent powerful and efficient mechanisms of mass and heat transfer in continental crust. The material in domes may be deeply sourced, but most gneiss domes record emplacement at relatively shallow crustal levels, as the dominant quartzofeldspathic rocks in domes are poor recorders of pressure-temperature paths. This research takes advantage of the more complete record of pressure-temperature-time history preserved in mafic rock inclusions in quartzofeldspathic gneiss, with a specific focus on orogenic eclogite and amphibolite in five gneiss domes of the French Massif Central. Based on previous work showing that eclogite and host rocks in one dome in this region record the same age, and that this age represents the timing of high-pressure metamorphism, this research tests the idea that domes are the ‘tip of the iceberg’ of regional deep-crustal flow systems. According to this hypothesis, dome-rocks can be used to investigate the conditions, timing, and consequences of lateral and vertical flow via field and analytical studies that include calculation of pressure-temperature-time histories of high-pressure mafic rocks (eclogite) and associated amphibolite in a system of spatially-related domes. Key data in this research include the composition and age of zircon, rutile, and titanite as determined by integrated thermobarometry and Laser ablation split stream - inductively coupled plasma mass spectrometry (LASS-ICPMS) Uranium-Lead (U-Pb) geochronology and trace element analysis, as well as in situ oxygen isotope and microstructural analysis of zircon to evaluate the role of fluids in recrystallization during high-pressure - high-temperature metamorphism in the deep orogenic crust. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.