New Petrogenetic Approaches to the Study of Ultrahigh-temperature and Ultrahigh-pressure Rocks
Date of Award
Doctor of Philosophy (PhD)
Geology and Geophysics
The degree to which rocks are heated and pressurized during mountain building controls fundamental characteristics of Earth’s lithosphere such as density and chemical composition, and influences cycling of life-essential elements (e.g. H, C, N, P). “Extreme” metamorphic rocks that reached exceptionally high pressures and/or temperatures compared to common examples are key to plumbing the limits of metamorphic processes. Extreme metamorphism encompasses high-pressure granulites (HPGs) formed during orogenic crustal overthickening, ultrahigh-temperature (UHT) rocks formed during intense heat flow, and deeply subducted ultrahigh-pressure (UHP) rocks returned to the surface. Critically, methods for recognizing these rocks are debated. This dissertation presents a series of studies that identify extreme metamorphism, scrutinize classical indicators of extreme metamorphism, and test promising diagnostic approaches. After the introductory material in Chapter 1, Chapter 2 documents HPG metamorphism in silica-undersaturated rocks of the Acadian/Neoacadian Brimfield Schist of the Appalachian mountains in Connecticut, U.S.A. Pseudosection modelling and ternary feldspar thermometry show metamorphism at ~18 kbar and ~1,040 °C. This result places a key new benchmark on metamorphic grade and crustal thickness during orogenesis. Findings offer promising opportunities to compare the scale of metamorphism during microcontinent-continent convergence of Appalachian orogenesis with that during continent-continent convergence of Caledonian orogenesis in Europe characterized by extreme metamorphism. Syenite-like leucosomes in the rocks give insight into syenite genesis. Chapter 3 tests the hypothesis that oriented needles, plates, and blebs (lamellae) of oxides and phosphates in garnet are solid-state precipitates formed by exsolution processes. Lamellae textures are associated with extreme metamorphism and could be a valuable tool for identifying extreme metamorphic rocks and understanding their formation. Large crystallographic datasets show that the studied lamellae species develop sets of crystallographic orientation relationships (COR) that align crystallographic planes or directions (chains of atoms) to those of the host garnet, consistent with an exsolution origin. Observations of correlation between crystallographic plane spacing and COR formation improve methods for predicting and understanding COR formation. These findings, together with textural and logical evaluation of lamellae textures, provide strong evidence for a lamellae exsolution origin and justify the use of these textures to interpret garnet chemistry in extreme metamorphic rocks. Chapter 4 builds upon Chapter 2 and applies principles from Chapter 3 to evaluate the origin of silicate (+oxide and phosphate) lamellae textures in garnet from the Brimfield Schist. Exsolved silicate lamellae can signify UHP precursor garnet formed at pressures ≥5 GPa; the Brimfield Schist textures serve as a key signpost to metamorphic conditions during Appalachian orogenesis. Lamellae COR and the chemical trends of garnet and lamellae indicate exsolution from garnet and reveal a UHP stage for metasedimentary gneiss from the Brimfield Schist. This finding demonstrates deep subduction of sedimentary material during Appalachian microcontinent-continent convergence and carries implications for processes recycling subducted sediments to orogenic roots. The studied lamellae assemblages are remarkably diverse, making results applicable to many localities of extreme metamorphism. Chapter 5 approaches the problem of non-standardized COR analysis and develops a novel framework for classifying and interpreting lamellae COR that may be applied to all species in garnet. This work extends the edge-to-edge matching system used to understand exsolution in alloys to garnet-lamellae systems for the first time and shows that the low-strain configurations produced by edge-to-edge matching alignments at the host-lamellae interface are successful predictors of lamellae COR in garnet. Edge-to-edge matches are shown to be correlated with needle-shaped precipitates. Several possible uses of edge-to-edge matching COR for discerning the state of the host rock during metamorphism are presented; these methods could answer the open question of why COR vary in different rocks. Chapter 6 tests a possible confounding factor of COR comparative analysis by examining correlations between small ruptured inclusions in garnet and lamellae COR. Findings show that the studied textures are not correlated with COR trends, confirming that samples with and without these ruptured inclusions may be directly compared. This strengthens the uses of lamellae COR for interpreting rocks from different settings. Nano-scale imaging of contacts between garnet and quartz lamellae shows features consistent with an exsolution origin. Chapter 7 explores the ways that the classical UHP indicators coesite and diamond are recognized in petrographic thin sections. The far-reaching effects that UHP rocks have on the interpretation of orogenic belts justify great scrutiny of UHP indicators. Examples of preparation artifacts or other mineral phases that mimic Raman spectra or visual characteristics of diamond and coesite are highlighted. These cases are presented to draw attention to updates for best practices that would help standardize ways that UHP rocks are identified and provide stronger confidence in UHP reports.
Keller, Duncan Sutherland, "New Petrogenetic Approaches to the Study of Ultrahigh-temperature and Ultrahigh-pressure Rocks" (2021). Yale Graduate School of Arts and Sciences Dissertations. 357.