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PRESSURETEMPERATURE OF METAMORPHISM AND SOURCE OF SILICA IN ECLOGITE FROM THE SANBAGAWA METAMORPHIC BELT, SHIKOKU, JAPAN Abstract Sample 15MGHA06 was excavated from the Shirataki Unit of the Sanbagawa Belt in central Shikoku, Japan. Optical mineralogical examination using petrographic microscopes identified the species from the occurrence of several index minerals including kyanite, rutile, omphacite (Na-rich pyroxene), and quartz. The sample, a rutilequartz-bearing nebulitic eclogite with SiO2 measured at 66 wt% is an uncommon occurrence. Application of thermometry on the sample of eclogite constrained the peak temperature and pressure of metamorphism. Results indicate a temperature of 749-823℃ and a pressure of 3.5-7.1 GPa at a calculated depth of 105-213 km. The unusually high quartz content is hypothesized to be caused by mechanical mixing of pelitic sediment in the accretionary mélange during the subduction of a MORB protolith. A secondary species of quartz crystals is sourced from the dehydration process of magnesium-chlorite and muscovite recrystallizing to form phengite, aluminosilicate (kyanite), quartz, and water. Additional water released lowered the solidus of the eclogite and allowed for partial melting and further metamorphism.

Devin Keating, J. Moloney, E. Wisehart, T. Volante, J. Joerger, and W. Kilpack

INTRODUCTION Eclogite represents the highest temperature and pressure metamorphic grade that forms in nature. It is formed from mafic igneous rocks and can be metamorphosed in subduction zones down to depths at the base of the crust. Sample 15MGHA06 was exhumed from the Iratsu Eclogite Mass in the Shirataki Unit of the Sanbagawa High-Pressure Metamorphic Belt in the Besshi region, central Shikoku Island, Japan by Dr. Christy Till and Meghan Guild. The high pressure, low temperature Sanbagawa belt is paired with the low pressure, high temperature Ryoke-Abukuma belt to the west, running northeast to southwest across the island (Winter, 2010). The Sanbagawa belt contains an eclogite lens unusually rich in quartz and encased in dunite. These belts are formed through tectonic deformation from the convergent boundary of the Philippine Sea Plate subducting underneath the Amur Plate at an average rate of 43 mm/yr (See Fig.1) (Eurasia, 2016). While the events that formed the Sanbagawa belt began roughly 117 Ma, the eclogite mass formed over a period spanning 91-76 Ma (Aoya, Endo, Mizukami, and Wallis, 2013), when lenses of mid-ocean ridge basalt (MORB) material were entrained in the accretionary mélange from the subducting Philippine plate as it dove beneath the Amur Plate. Determination of the protolith, the source of high silica content, and the temperature and pressure of metamorphism is the premise of the following sections. The tectonic environment of the Nankai Trough, the name given to this area of the subduction zone, is unique. It is a low angle subduction with a very large accretionary prism (See Fig. 2). The accretionary prism is about four kilometers thick and may go to depths greater than ten kilometers (Futagami, Morono, Terada, Kaksonen, Inagaki, 2013). The trough also contains a megasplay fault zone overlaying a décollement zone filled with Shikoku Basin

sediment all of which overlies the subducting oceanic plate (Futagami et al., 2013). The faults in this location have produced megathrust earthquakes in the past and may continue to produce large magnitude quakes in the future (Eurasia, 2016). METHODS Preliminary data collection was done by observation of thin sections using optical mineralogy techniques using a petrographic microscope to resolve relative mineral abundances. Using the assemblage of minerals and a guide from Dave Hirsch on naming metamorphic rocks, the rock type was determined to be eclogite (2006). Major elemental and mineral compositions of the eclogite were examined by Electron Microprobe (EM) and X-ray Fluorescence analysis, in data provided by Dr. Christy Till. Trace element composition was determined by Inductively Coupled Plasma Mass Spectrometry (ICPMS), also from data Dr. Till provided. The data was normalized for mineral and whole rock composition with weight percentage and Mg# also calculated. The normalized weight percents of the minerals indicated that the eclogite contains ~66 wt % SiO2 (See Fig. 3). Mineral compositions for garnet, epidote, amphibole, ‘mica 1’, ‘mica 2’, and spinel were listed as part of the bulk composition. In order to identify the micas, the weight percentage of each of these minerals was entered into an online mineralogy database, where ‘mica 1’ is determined to be similar to biotite, and ‘mica 2’ is similar to phengite (Barthelmy, 2014). After identifying the micas, the ratio weight percentage of each was compared to garnet by using an excel sheet thermometry tool, created by Meghan Guild, that calculated an estimated temperature range based on the comparison between biotite-garnet and phengite-garnet. Thermometry for biotite-garnet provided a temperature of 823℃ and the temperature calculated for phengite-garnet used three different Mg# to provide three temperatures for different types of

systems and protoliths. Index minerals and geographical context were used to determine that the protolith was a mid-ocean ridge basalt, thus allowing for the possible temperatures to be narrowed down to one of the three estimates -- that of a basaltic protolith (Winter, 2010). This temperature estimate, which used an Mg# of 67, was selected as the temperature for all further calculations. The basaltic protolith temperature range is 749-823℃, which is within the eclogite formation temperature range. The pressure range was determined to be ~3.5-7.1 GPa corresponding to a depth of ~105-213 kilometers, calculated by using the established temperature range and a schematic petrogenetic graph for a basaltic composition from Poli and Schmidt (2002) (See Fig. 4). OBSERVATIONS Sample 15MGHA06 is an eclogite that displays a characteristic porphyroblastic appearance; that is, containing a matrix of green minerals with red almandine garnets. The matrix minerals are fine grained phaneritic (< 1 mm), while the garnets are approximately 0.51.5 millimeters (average size 0.9 mm) and are crushed and cracked with a poikiloblastic surface texture. The hand sample also displays several dark uneven continuous bands. This can be seen in the thin sections as a migmatite nebulitic texture, or a mixing of igneous and metamorphic rock that is a sign of partial melting. The thin sections (15MGHA06-2 through 15MGHA06-04) display a slight migmatitic texture with randomly oriented inequant grains. The slides contain the index minerals omphacite, rutile and quartz in addition to garnet. The garnet crystals, recognizable by their high relief and isotropic appearance under cross polarized light, are prekinematic, idioblastic and subidioblastic and display a regular, tensile fracture set, oriented parallel to the loading axis of the stress they experienced. The fractures are filled with SiO2 (See Fig. 5) and contain inclusions of rutile,

quartz, and a “mystery mineral” with screw dislocations that resemble synthetic quartz (See Fig. 6). High resolution photopetroscopic images obtained using a Nikon NIS-Elements system reveal the macrostructure of the sample (See Fig. 7). The index minerals are clearly visible in the thin sections. Quartz is evident in all of the slides not only as inclusions in the garnet but as a major component of the fabric. Many of the omphacites display exsolution lamellae and occasionally contain poikiloblasts. Rutile appears exclusively as inclusions within the garnets, since they would have broken down into titanite without the protection of the more resilient host mineral during any exhumation process. In addition, the high alumina-silicate content of the sample is manifested as kyanite, a high pressure polymorph. It is evident in all of the thin sections as it is pale blue under plane-polarized light, and first-order gunmetal gray under crosspolarized light. DISCUSSION The protolith of the sample was determined to be a MORB based on the regional geologic context and rutile content as well as referring to index minerals of a metabasite. Mechanical mixing with pelitic material in the accretionary mélange during the beginning of subduction was synkinematic with low grade metamorphism. Relict textures remaining from this mixing process were overprinted by metamorphic textures later in the evolution of the sample as it metamorphosed into an eclogite at greater pressures and temperatures. At this point, the sample continued to its maximum subducted depth of greater than 150 kilometers, exceeding the chlorite out boundary (Poli and Schmidt, 2002) generating water and silica from the dehydration process shown by equation one.

(Eq. 1) 3𝑀𝑔5 𝐴𝑙2 𝑆𝑖3 𝑂10 (𝑂𝐻)8 + 5𝐾𝐴𝑙3 𝑆𝑖3 𝑂10 (𝑂𝐻)2 → 5𝐾𝑀𝑔3 𝐴𝑙𝑆𝑖3 𝑂10 (𝑂𝐻)2 + 8𝐴𝑙2 𝑆𝑖𝑂5 + 𝑆𝑖𝑂2 + 12𝐻2𝑂

The additional water released from this reaction lowered the solidu enough to produce a nebulitic migmatite texture at hand sample scale. The leucosomes are 5-7cm in length and 2-3 cm in width and the melanosome is 3-4 cm in length and 2-3 cm in width (See Fig. 8). At this depth, the eclogite then experienced compressional or tensional stresses resulting in parallel fractures in the garnets that are in-filled with quartz. This was the final stage in the prograde metamorphism and marks the beginning of the exhumation path. The sample was embedded in a dunite block during exhumation where it was exposed on the surface (See Fig. 9). An unknown mineral found overlying one of the garnets in thin section, this along with the small mineral size impeded us from finding accurate interference figures and colors that could be used to help identify it. Interestingly, the unknown mineral shows a “fish scale” appearance (Fig. 6) as the stage is rotated it resembles screw locations of quartz, albeit synthetic quartz. CONCLUSION The pressure and temperature of metamorphism of the eclogite was found to be 3.5-7.1 GPa corresponding to 749-823 ºC respectively, determined by using a thermometry tool developed by Meghan Guild, indicating a high pressure and medium temperature deformation conditions. The pressures correlate to a depth of 105-213 km, assuming that one kilobar is equal to thirty kilometers. The pressure range was constrained further by extending the gradient signifying conditions that are not present in our Earth (See Fig.10). With the given temperatures of formation, a max pressure was found at these values to be between 4.42-4.93 GPa. This

creates a range of 3.56-4.42 or 3.56-4.93 GPa, depending on the temperature of formation, which is within the original range calculated (Winter, 2010). The accretionary mélange is formed from scraping of oceanic material and underplating continental material during subduction. The protolith is a MORB with pelitic sediment entrained from an accretionary mélange mixing process as well as dehydration and partial melting (See Fig. 2). The mélange is roughly four kilometers thick, and may be more than ten kilometers in depth. Assuming the temperatures in the mélange have the potential to reach 300℃, which allows for the mobilization of quartz to begin to increase silica content and form low-grade chlorite within the MORB (Winter, 2010). A bimodal distribution of quartz crystals indicate formation at two distinct P-T-D-t conditions. The secondary species of quartz is sourced from the dehydration process of magnesium-chlorite and muscovite recrystallizing to form phengite, aluminosilicate (kyanite), quartz, and water (Eq. 1). From the dehydration reaction, a large amount of water was released (Till, 2012), lowering the solidus of our sample, and allowing for partial melting and further metamorphism to occur. During the study of this sample, other questions were considered but not examined. Such as, were there multiple environments of varying P-T-D as the sample was subducted. Electron backscatter imaging could be used to study the zoning of the garnets to understand their evolution. The garnets enclose other minerals that were present within the matrix during growth and were not consumed or recrystallized during progressive metamorphism. These minerals may contain information about the melt composition, P-T-D conditions, and possibly the time spent in each environment. The minerals encased in the garnets are protected from exterior conditions

affecting the bulk rock. By completing an X-Ray analysis on the rutile inclusions, we may be able to ascertain further data about the burial and exhumation process. Further research using thermobarometry of zoned garnets may find narrower constraints for temperature and pressure of deformation, and describe a more definitive process of metamorphism. During this study, the use of optical mineralogy, thermobarometry, context from relevant literature, and data describing the chemical and mineralogical composition of the rock was used. It was conclusive that the eclogite sample taken from the Sanbagawa Metamorphic Belt on Shikoku, Japan underwent complex metamorphism to contain a high silica content with a MORB protolith that is now encased within a dunite lens and was exhumed through some process to now be on the surface after being subducted to the base of the crust.

FIGURES

Figure 1. Location of eclogite sample is indicated by the purple star. Indicates plate movements as well for the regional area. (Eurasia, 2016)

Figure 2. Cross-section of the Nankai Trough Subduction zone. (Futagami et al., 2013)

Figure 3. A graph created by Jayde Moloney using data provided by Dr. Christy Till showing oxides by wt % of the bulk composition.

Figure 4. Original figure is from Poli and Schmidt (2002) and edited by Devin Keating to highlight the pressure and temperature range of the eclogite sample.

Figure 5. Cracked garnet in XPL showing parallel fractures infilled by quartz and other minerals. scale bar is 1000 μm. Image taken by Tom Volante

Figure 6. Unidentified mineral at 40x magnification. Image taken by Tom Volante

Figure 7. High-Resolution Image of Thin Section 15MGHA06-4 in XPL scale bar is 1000 μm. Image taken by Tom Volante

Figure 8. Sample 15MGHA06 : Picture of sample with uncut face on left and cut face on right. Photos taken by Jolene Joerger

Figure 9. Point (1) initial protolith, Point (2) maximum subducted depth. Point (3) projected possible exhumation path. Poli and Schmidt (2002) and edited by Jayde Moloney

Figure 10. Inferred P/T Path of Sanbagawa Facies Series. Discontinuous graph shows extension of boundary of conditions not present in Earth denoted by dashed line.. Note: Max pressures possible at each T are within the calculated P range. Original Source: http://www.tulane.edu/~sanelson/eens211/contact®ional_metamorph.htm Edited by William Kilpack

References

Aoya, M., Endo, S., Mizukami, T., and Wallis, S.R., 2013, Paleo-mantle wedge preserved in the Sanbagawa high-pressure metamorphic belt and the thickness of forearc continental crust: Geology, v. 41, p. 451–454, doi: 10.1130/g33834.1. Barthelmy, D., 2014, Mineralogy Database: Mineralogy Database, http://www.webmineral.com/. Eurasia Eurasia, http://amurianplate.weebly.com/ (accessed November 2016). Futagami, T., Morono, Y., Terada, T., Kaksonen, A.H., and Inagaki, F., 2013, Distribution of dehalogenation activity in subseafloor sediments of the Nankai Trough subduction zone: Philosophical Transactions of the Royal Society B: Biological Sciences, v. 368, p. 20120249–20120249, doi: 10.1098/rstb.2012.0249. Hirsch, D., 2006, Classifying Metamorphic Rocks Nelson, S.A., 2004, Contact & Regional Metamorphism - Tulane University: Contact & Regional Metamorphism, http://www.tulane.edu/~sanelson/eens211/contact®ional_metamorph.htm (accessed November 2016). Poli, S., and Schmidt, M.W., 2002, Petrology of Subducted Slabs: Annual Review of Earth and Planetary Sciences, v. 30, p. 207–235, doi: 10.1146/annurev.earth.30.091201.140550. Till, C.B., Grove, T.L., and Withers, A.C., 2011, The beginnings of hydrous mantle wedge melting: Contributions to Mineralogy and Petrology, v. 163, p. 669–688, doi: 10.1007/s00410-011-0692-6. Winter, J.D., 2010, Principles of Igneous and Metamorphic Petrology: Upper Saddle River, NJ, Pearson Education.