Rare earth elements in metamorphic environments [electronic resource] : the Karrat rare earth element deposit, Greenland and the ultra-high pressure North Qaidam terrane, China
Keywords: Geological and Environmental Sciences.

Research presented in this dissertation is focused on rare earth elements (REE) in the Paleoproterozoic metamorphic environment of the Karrat Isfjord and the Paleozoic ultra-high pressure (UHP) North Qaidam metamorphic terrane. REE mineralization has been discovered in the Karrat Isfjord region of west central, Greenland (~72°N) at Niaqornakavsak (up to 2.6 wt. % Y2O3 + REE2O3) and Umiamako Nuna (up to 2.3 wt. % Y2O3 + REE2O3). Mineralization is found within the amphibolite capping unit of the metasedimentary Qeqertarssuaq Formation. Replacement textures of host amphibolite minerals by REE mineralization minerals are found throughout the deposit suggesting that the amphibolite is the protolith of the REE mineralization. REE mineralization has been divided into four primary lithologies at Niaqornakavsak: (1) Calcium Carbonate Amphibolite (CCA) is characterized by a carbonate (ankerite and calcite) dominated matrix with significant fluorite, Fe-Mg amphiboles, ferrokinoshitalite, and chondrodite with trace bastnasite, monazite, allanite, fergusonite, thorium silicate, magnetite, ilmenite, sphalerite, apatite, and unknown Ba-rich minerals; (2) Dark Calcium Carbonate Amphibolite (DCCA) has a primary mineralogy of green amphibole + biotite + carbonate (ankerite and calcite) with trace bastnasite, monazite, allanite, fergusonite, magnetite, ilmenite, fluorite, Fe-Mg amphiboles, plagioclase, and apatite; (3) Biotite Layered Carbonate (BLC) has a biotite dominated matrix (> 50% modal) with significant calcite and trace magnetite, ilmenite, allanite, monazite, apatite, and fergusonite; and (4) Light Biotite Layered Carbonate (LBLC) has the same assemblage as BLC with the addition of a significant plagioclase component. Samples similar to CCA have been discovered at the surface at Umiamako Nuna, but were not found in drill hole exploration of the subsurface. Umiamako Nuna mineralization within the drill holes is similar to that at the base of the Niaqornakavsak mineralization with (1) a green amphibole + biotite + carbonate assemblage that appears as a more biotite rich DCCA and (2) calcite veins that cut across a plagioclase + biotite dominated matrix with trace green amphiboles. A distinctly separate occurrence of REE mineralization can be found at depth at Umiamako Nuna with an assemblage of carbonate (ankerite and calcite) + fluorite + talc + Fe-Mg amphiboles in the zone of highest Y2O3 + REE2O3 concentration (~1.0 wt. %). Bastnasite and monazite are the primary REE minerals in this zone. Unlike the mineralization at Niaqornakavsak (up to ~40% wt. % Fe2O3) the Umiamako Nuna mineralization at depth is Fe-depleted (< 10 wt. %). The F-component in micas (up to 2.0apfu) and amphiboles (up to 1.2apfu) formed in the REE mineralization at Niaqornakavsak and Umiamako Nuna suggest that in addition to REE and CO2 the fluid responsible for metasomatism was also enriched in F. Carbonate isotope compositions at Niaqornakavsak overlap with the compositional field expected for primary igneous carbonatites (δ13C = -3.7 to -7.6‰ and δ18O = 8.3 to 12.8‰). Carbonate samples at Niaqornakavsak exhibit a linear trend with increasing 13C and 18O that we attribute to mixing with carbonate in the protolith amphibolite (δ13C = -2.7‰ and δ18O = 15.6‰) in addition to Rayleigh fractionation. Carbonates from Umiamako Nuna are characterized by a carbon isotope signature within the expected range for primary igneous carbonatites (δ13C = -5.5 to -7.5‰). Monazite isotopic ages from Niaqornakavsak, Umiamako Nuna (surface), and Umiamako Nuna (depth) vary between 1.8 -- 1.9 Ga, but the age ranges exhibited by individual analyses show significant overlap. 207Pb/206Pb, 206Pb/238U, and 208Pb/232Th isotope ratios were all recorded in some samples with the most consistent agreement between the ratios in a single sample resulting in a range of 1825 -- 1863 Ma. Zircon (1915 ± 39 Ma) and sphene (1894 ± 42 Ma (207Pb/206Pb) and 1873 ± 49 Ma (208Pb/232Th)) ages were also calculated for the terrane and attributed to the timing of metamorphism. A F-REE-CO2 rich fluid likely derived from a ferrocarbonatite source is responsible for the formation of the REE mineralization at Niaqornakavsak during the Himalayan-scale continental collision of the North Atlantic and Rae cratons during the Paleoproterozoic. Multiple zircon age populations were discovered in the eclogite samples from the North Dulan Belt within the UHP North Qaidam terrane, China. In-situ, in-section analytical techniques retained the textural relationships between individual zircon and garnet allowing for REE compositional comparison between the included zircons and their garnet hosts. Garnets in both samples contain a Fe-Mn-HREE rich core, a Mg-rich mantle, and a Ca-rich, HREE-depleted rim. Zircon age populations of 420.7 ± 7.7 Ma in sample D5A and 427 ± 10 Ma in sample D5B exhibit a flattened, HREE-depleted pattern that would be expected in a metamorphic environment with significant garnet growth sequestering HREE. Zircons with this flattened pattern occur as inclusions within the HREE-depleted garnet rims with resulting flat partitioning patterns and HREEKD Zrcn/Grt ≤ 1. These zircons formed cogenetically with the garnet rims at calculated peak P-T conditions of 780°C at 29.8kbar in sample D5A and 716°C at 23.6kbar in sample D5B. Younger zircons with ages of 406.6 ± 4.5 Ma in D5A and 404.1 ± 8.9 Ma in D5B have a Ti-in zircon crystallization temperature of ~775°C and likely formed during retrograde symplectite growth. Using conditions calculated for peak and retrograde metamorphism an exhumation rate of 4.0mm/a has been calculated for sample D5A. Both samples exhibited isothermal decompression prior to symplectite growth during exhumation.