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This proposal seeks funds to study the metamorphic evolution of blueschists and eclogites from the islands of Syros and Sifnos in the Cyclades Blueschist belt, Greece. The Cycladic islands contain high-pressure metamorphic rocks that are believed to be the dismembered roots of the mountain belt formed during Eurasia-Africa subduction and fragmented during the continental extension that followed collapse in the Aegean area. The islands of Syros and Sifnos contain excellent exposures of fresh blueschists and eclogites that have been tectonically juxtaposed with greenschist facies rocks. Fundamental questions remain about the timing and duration of subduction metamorphism, the timing and conditions of exhumation of the high pressure rocks into the middle crust, and the relationship between different tectonic units: these issues will be addressed in this study. The major goal of this project is to determine the fine structure of the P-T-t history in blueschists, eclogites, and greenschists of different tectonic units in order to constrain the burial and uplift history of the blueschist/eclogite belt.
A compelling reason to conduct this study is the recent development of three new geothermometers and two new geochronology techniques that can be applied to the study of blueschists. Recent work has demonstrated that the solubility of Ti in quartz and zircon, and the solubility of Zr in rutile are highly temperature sensitive. Preliminary work indicates that these elements can be analyzed with a precision that results in a temperature uncertainty of less than 5 degrees C, providing an unprecedented glimpse into the thermal history of a rock. In addition, exploratory work has demonstrated that SIMS dating of the common blueschist accessory allanite has the potential for constraining the time of allanite formation to within a few million years. Finally, depth profiling of metamorphic rims on zircon using SIMS, coupled with Ti solubility measurements, will provide precise T-t points for each zircon.
These new methods will be used in concert to construct high resolution P-T-t histories of each tectonic block and enable major inferences to be drawn about the tectonic assembly of this terrain that have been hereto for impossible. This will lead to resolution of some of the major outstanding questions about the evolution of the Cycladic blueschist belt. It will also provide new insights into the thermal structure of subduction zones and the processes responsible for the subduction and exhumation of blueschists.
The Cyclades form the geologic link between mainland Greece and Turkey, and detailed understanding of the tectonic evolution of this terrane will greatly assist tectonic reconstructions of the eastern Mediterranean. Furthermore, the thermometric and chronologic techniques we will develop through their use will have great application to rocks from other terrains. Finally, knowledge gained about the workings of subduction and exhumation will provide new insights into the subduction process and the interaction of plates that has shaped the Earths surface.
|Photograph of folded blueschist from near the type locality of glaucophane on Syros, Greece|
This project involves quantifying the paragenesis of accessory minerals in suites of metamorphic rocks from a range of metamorphic terranes including documentation of the textural setting of accessory phases and their chemical variability (e.g. zoning patterns). Thermodynamic data will be estimated by integrating experimental values with observed natural paragenesis so that predictive models of reaction relationships between major and accessory phases can be developed. Thermometers involving the distribution of Y between garnet, monazite, and xenotime will be extended to higher metamorphic grades using more sensitive analytical tools than the electron microprobe (LA-ICP-MS or SIMS). These projects will be undertaken on suites of rocks from areas chosen for their metamorphic diversity and compositional variability. Paragneisses from the Adirondacks and the Valhalla Complex, and calc-pelites from Vermont and the Cordillera Darwin, Chile have been identified as suites to examine initially.
It is now known that trace elements preserve parts of a rocks history that are not preserved by major elements, so a major potential of the proposed research is the discovery of segments of Earth history that were previously unknown. Tools developed in this research effort will be directly applicable by other researchers to other areas. Additionally, new insights into the specific histories of the areas studied will be forthcoming as a natural consequence of this work.
|X-ray maps (top) and SE images (bottom) showing Y zoning in monazite from migamites from the Valhalla Complex, B.C. 5 generations of monazite are apparent. Numbers are SIMS ages.|
This research is to develop and to test application of a powerful analytical tool that will be used to determine the temperature of quartz crystallization in a wide range of geologic settings. The proposed geothermometer, which will be based on the Ti content of quartz that crystallized in equilibrium with a pure-TiO2 phase (rutile), promises to provide valuable information about the thermal history of rocks equilibrated at P & T conditions for which no suitable geothermometers now exist, and to provide additional and probably more precise constraints on T estimates for other rocks that are based on existing thermometers.
Calibration of the new thermometer will be accomplished using established experimental techniques and using standard equipment (both piston-cylinder and cold-seal). Crystals of quartz and rutile will be equilibrated with fluid (either supercritical H2O, or at higher temperatures, a hydrous silicate melt) at a range of temperatures (400 1100°C) and pressures (~0.1, 1, and 2 GPa). At the same time, techniques will be developed to optimize the analysis of newly equilibrated quartz using the electron microprobe for relatively high concentrations (>~10 ppm) and the ion microprobe for low concentrations (<~50 ppm) of Ti. Application of the thermometer will then be tested by using it to characterize the thermal history of selected igneous and metamorphic rocks.
Outstanding features of this Ti-in-Quartz thermometer include its broad applicability, its anticipated high precision, and its intrinsic simplicity. In accord with basic thermodynamic principles, the temperature of quartz crystallization will vary inversely with the log of its Ti concentration, assuming equilibrium with the mineral rutile. Determination of Ti concentration is very straightforward: it can be analyzed using the electron microprobe, with anticipated errors leading to uncertainties in temperature of about 2° near 800°C, and about 20° near 650°C where roughly 10 -20 ppm Ti (corresponding to the lower limit of detection based on existing data) is expected. Still lower Ti contents can be precisely measured using an ion microprobe, dramatically reducing the uncertainty (to ±1°C at ~500°C) and allowing determination of lower equilibration temperatures possibly down to 400°C and below.
The proposed thermometer is expected to find wide use among Earth scientists working on a variety of geologic problems. Because quartz is one of the most abundant minerals in Earths crust, and because it tends to be relatively mobile yet stable over a broad P & T range, multiple generations of quartz each recording its own segment of thermal history can be found in rocks from a variety of geologic environments. These include, but are not limited to, samples recrystallized during regional metamorphic events (including ultra-high pressure rocks for which accurate T estimates are now lacking), samples crystallized from partially molten systems (e.g., migmatites and granite plutons, as well as silicic volcanic rocks), ore deposits, and possibly even the silica cements that bind clastic grains in relatively low-T sedimentary environments. The impact of the thermometer is broadened even more when it is recognized that even in the absence of rutile, it provides an accurate estimate of the minimum T of equilibration, and even then may be accurate to within a few degrees C in the many systems for which TiO2 activity is close to that required for rutile precipitation.
CL image of a quartz inclusion within a plagioclase phenocryst in a migmatite from New Hampshire. Mosaic structure shows where quartz was melted during anatexsis