The 1984 discovery of coesite and coesite pseudomorphs in metamorphic rocks

Thesis Proposal Spring 2012 Megan Regel 6/19/12 Thermobarometry and Geochronology in the Dulan region, North Qaidam Ultrahigh- Pressure metamorphic terrane: Resolving Spatial Variation of Ages, Temperatures and Pressures. Introduction The 1984 discovery of coesite and coesite pseudomorphs in metamorphic rocks introduced the paradigm of ultrahigh-pressure (UHP) metamorphism in continental collision systems (Chopin, 1984). Over the last 25 years, UHP rocks have been found in more than 20 locations worldwide, suggesting that UHP processes are an integral part of continental collisions (Chopin, 2003; Liou et al., 2004). UHP processes, such as exhumation, are problematic: how does continental crust (density 2.8 g/cc 3 ) achieve pressures and temperatures of ~26-35 kbar and ~600-950 C (Liou et al., 2004) and stay there for ~10-20 M.y. without melting, and how do these rocks get back to the Earth s surface? Estimating peak pressure-temperature (P-T) conditions of UHP rocks is essential to understanding exhumation processes (Ernst et al., 2005; Hacker, 2006). There are several large-scale models that explain the exhumation of large UHP and HP terranes. For example, moderate peak temperatures and a long duration of UHP metamorphism suggest a thick, slow subducting slab, the center of which would be insulated from high mantle temperatures (Kylander-Clark et al., 2011). Another possibility is slab detachment from the subducting continental crust to form separate units, which predicts juxtaposed UHP and HP units (Li and Gerya, 2009). The Dulan region of the North Qaidam UHP terrane (Fig. 1) is a useful study area because the UHP rocks underwent a long (~20 My) period of UHP metamorphism (Mattinson et al., 2007; Zhang et al., 2010b), but maintained moderate temperatures of up to ~700 C rather

than heating completely to the melting point. However, high-pressure (HP), high-temperature (HT) granulite facies rocks also lie to the west of UHP eclogites (Figs. 2). To explain the relationship between HP granulites and UHP eclogites in the Dulan region, two hypotheses have been proposed (Song et al., 2003a; Yu et al., 2011). One hypothesis suggests that the granulites represent an overprinting of UHP eclogites during exhumation (Song et al., 2003a). A second hypothesis suggests that HP granulites are a separate tectonic unit that were juxtaposed with UHP eclogites at the end of uplift (Christensen, 2011; Yu et al., 2011). I propose to determine the peak P-T of HP granulites and UHP eclogites with zirconium-in-rutile thermometry to test the two hypotheses. Geological Setting The Dulan UHP region (Figs. 1 and 2) is located at the eastern end of the North Qaidam UHP terrane about 30 km northeast of the town of Dulan (Song et al., 2003a; Song et al., 2003b). The Dulan region is primarily composed of ortho- and para-gneisses, which enclose boudins and lenses of UHP eclogites, and HP granulites (Christensen, 2011; Mattinson et al., 2007; Song et al., 2003a; Yu et al., 2011). The host gneisses contain a tightly to isoclinally folded amphibolite facies foliation with a northwest strike and northeast dip (Mattinson et al., 2007; Yu et al., 2011). Song et al. (2003) separated the Dulan UHP exposure into the North and South sub-belts based on spatial location, mineral assemblages, and chemical composition of the eclogites. However, more recent work suggests that the two sub-belts are petrologically similar, rendering the distinction unnecessary (Zhang et al., 2010b). UHP eclogites (Fig. 2) outcrop in an area about 140 km 2 in both granitic and pelitic gneisses (Song et al., 2003a; Zhang et al., 2010b). UHP eclogite mineral assemblages are garnet + omphacite + rutile ± phengite ± epidote ± quartz. Preserved coesite inclusions, which suggest

UHP conditions above about 25 kbars, are present in paragneiss and eclogite from the Dulan region (Song et al., 2003b; Zhang et al., 2010b; Zhang et al., 2009b). UHP eclogite in the SDB contains rare kyanite (Christensen, 2011; Mattinson et al., 2007; Song et al., 2003a; Yu et al., 2011). Retrograde amphibolite facies assemblages of hornblende + plagioclase + epidote or zoisite ± biotite ± clinopyroxene, along with symplectite along fractures and grain boundaries is also apparent (Mattinson et al., 2007; Song et al., 2003a). Peak P-T of the UHP eclogites is 27 34 kbar and 610 800 C (Song et al., 2003a; Yang et al., 2001; Zhang et al., 2008; Zhang et al., 2009a; Zhang et al., 2010b). Estimated retrograde P-T conditions from the Dulan region (Fig. 3) and petrographic textures suggest that the UHP eclogite decompressed directly to amphibolite facies without pausing in the granulite facies (Zhang et al., 2009a; Zhang et al., 2010b). U/Pb, and Gt-Omp Whole Rock Sm/Nd geochronology of the eclogites spans an age range of 422 459 Ma (Christensen, 2011; Mattinson et al., 2007; Mattinson et al., 2006; Song et al., 2003a; Song et al., 2003b; Zhang et al., 2009a; Zhang et al., 2010b) which suggests that eclogite facies metamorphism (HP + UHP) lasted for approximately 37 Myrs. Coesite bearing samples from the Dulan region give ages ranging from 423 to 446 Ma, which suggests that UHP conditions lasted for at least 20 Myrs (Song, 2006; Zhang et al., 2009a; Zhang et al., 2010b). HP granulites outcrop in the western end of the SDB (Fig. 2; Christensen, 2011; Song et al., 2003a; Yu et al., 2011). The granulites are defined by the mineral assemblage garnet + clinopyroxene + plagioclase ± zoisite/clinozoisite ± rutile ± kyanite in both mafic and intermediate compositions; P-T conditions of 14.5 18.5 kbar and 800 950 C, with no P-T estimated in eclogite facies (Christensen, 2011; Song et al., 2003a; Yu et al., 2011). An amphibolite facies retrogression assemblage (P-T conditions of 12 14 kbar and 735 800 C) is apparent in symplectite intergrowths of plagioclase and amphibole (Song et al., 2003a; Yu et

al., 2011). Based on the inferred replacement of omphacite by plagioclase, the low jadeite component in clinopyroxene and higher almandine content of garnet, Song et al. (2003a) suggested that the granulite facies rocks in the west represent eclogites overprinted during exhumation. However, more recent textural analysis of mineral relationships within granulite facies suggests that granulite facies assemblages do not represent overprinting of UHP eclogites, and thus are interpreted to be a separate tectonically emplaced unit (Christensen, 2011; Yu et al., 2011). Zircon U/Pb geochronology of the HP granulites suggests ages ranging from 418 449 Ma (Christensen, 2011; Yu et al., 2011). These ages overlap with the UHP eclogites, suggesting that metamorphism of the two units was concurrent. The textural relationships and P-T estimations of the UHP eclogites and HP granulite facies rocks described above led to the proposal of two models of exhumation. Hypothesis 1 predicts that a continuous temperature gradient of ~610 900 C (Song et al., 2003a) exists between the east and west, which is based on the interpretation of the granulites as an overprinting of eclogites during slow exhumation. Hypothesis 2 predicts that the granulites do not represent overprinting during exhumation; a temperature discontinuity (Fig 4B) between HP granulites and UHP eclogites is caused by an inferred fault separating the two units. This separate tectonic unit was juxtaposed just prior to amphibolite facies retrogression, which affected the entire Dulan region. Both HP and UHP rocks experienced amphibolite facies retrogression (Fig. 5; Christensen, 2011). Thermometry Differentiating between the two proposed hypotheses that explain the presence of HP granulites and UHP eclogites in the Dulan region requires precise temperature measurements. The Fe 2+ -Mg exchange thermometer is the most commonly used. At increased temperatures and

pressures, Fe 2+ exchanges with Mg in the minerals garnet and omphacite (Ravna and Paquin, 2003). Fe 3+ does not take part in the exchange with Mg between garnet and omphacite, which means that the concentration of Fe 3+ relative to Fe 2+ needs to be determined. Unfortunately, the electron microprobe (EMP) is unable to distinguish between Fe 3+ and Fe 2+, and the ratio of Fe 3+ to Fe 2+ cannot be ascertained. Furthermore, the ratio of Fe 3+ and Fe 2+ cannot be accurately calculated by charge balance, which increases the error range of the estimated temperatures to ±100-300 C (Hacker, 2006; Ravna and Paquin, 2003). In order to distinguish between the two models, I will use the zirconium-in-rutile thermometer. This thermometer does not rely on an accurate calculation or measurement of the Fe 3+ /Fe 2+ ratio, which contributes to the low estimated uncertainty of ±30 C (Tomkins et al., 2007). Rutile (Rt) in equilibrium with zircon and quartz or coesite incorporates zirconium (Zr) in trace amounts; Zr concentration in Rt increases with increasing temperature, but decreases with increasing pressure (Zack et al., 2004; Tomkins et al., 2007). Tomkins et al. (2007) calibrated the effect of pressure on the incorporation of Zr into Rt. Because the high temperatures achieved during UHP metamorphism are able to re-equilibrate matrix Rt, both inclusion and matrix Rt crystals will be analyzed (Zack et al., 2004). Rt inclusions within garnet likely retain peak metamorphic P-T information and can be compared with matrix Rt crystals in order to determine the extent of re-equilibration. For the Dulan region, Zr concentrations in rutile are expected to be ~ 150 ppm for the UHP eclogites and ~2000 ppm for the granulites. Although low, these Zr concentrations can be accurately measured by EMP. This method has been used with success in blueschists from Sifnos, Greece (Spear et al., 2006) and recently in the Dulan region (Zhang et al., 2010a) providing confidence in its ability to determine precise temperature estimates. Zr-in- Rt thermometry is capable of distinguishing relative temperatures between individual samples,

even when the temperatures vary by only 35 C and will therefore be useful to determine the presence or absence of a temperature gradient in the Dulan region (Zack and Luvizottow, 2006). Methods To test the hypotheses, I will select 6 samples, 5 from the SDB and 1 from the NDB (Fig. 2), and perform petrographic analyses to understand the textures and to choose rutile crystals for temperature analysis. One sample will be granulite and five samples will be eclogite. Then, I will undertake EMP analyses of the Zr concentrations in Rt crystals to document metamorphic temperatures. These temperatures will allow me to calculate a more accurate P-T estimation that will test the two hypotheses. I propose to use UHP eclogites for this work because they are spatially distributed across the UHP field area and have the correct mineral assemblage (rutile, zircon, and quartz) in equilibrium to accurately estimate peak temperature using the Zr-in-Rt thermometer (Zack et al., 2004). Initial petrographic work will focus on picking the freshest samples, documenting the mineral assemblage, and documenting the textural context of rutile- whether as an inclusion in garnet or along grain boundaries in the matrix. Rutile inclusions in garnet can be protected from retrogression and are more likely to provide peak temperatures (Zack et al., 2004). Then, following the procedure of Zack et al. (2004), zirconium concentrations from the chosen rutile crystals will be analyzed on an electron microprobe (EMP). I plan to analyze 20-50 spots per sample; on large rutile crystals, multiple spot analyses are possible. Spot locations must be 10 µm from the grain boundary to prevent phase boundary contamination (Zack et al., 2004); I will choose crystals that satisfy this requirement. It is likely that high temperatures will have reset some inclusions; 20-50 rutiles will be analyzed to assure reset crystals are known and matrix data is accurate. The temperatures will be calculated using the Tomkins et al. (2007)

calibration with current, reliable phengite barometry data (Fig. 3; Zhang et al., 2010). Not all samples currently have phengite barometry data; I will calculate phengite pressures for those samples lacking data following the method of Ravna and Paquin (2003). To evaluate the time at which these temperatures were recorded, I will use existing ages, and collect zircon U-Pb ages where needed using the method of Mattinson et al. (2006). The combination of geochronology, trace element analyses, and pressure-temperature estimates will allow me to determine the timing of UHP peak temperatures and pressures in the Dulan region. Once the temperatures are calculated, I will plot the results (as in Fig. 4a and 4b) to evaluate the two hypotheses discussed above. A continuous temperature gradient with higher temperatures to the west but lower to the east would support the overprinted eclogite model. However, a lack of continuous temperature gradient would support the hypothesis that the granulite facies rocks were faulted into position prior to the exhumation of UHP eclogites. Timeline: Winter-Spring 2012: Sample selection and initial petrological work completed. June-July 2012: Sample crushing and preparation. Complete Zirconium-in-Rutile Thermometry and trace element analyses at Washington State University GeoAnalytical Laboratory. August 2012: U/Pb SHRIMP Geochronology at Stanford University. Write and submit AGU abstract. Fall 2012: Data Analysis and plotting of spatial relations to determine presence and significance of patterns. Winter 2013: Begin writing thesis Spring 2013: Defend Thesis Summer 2013: Finish revisions

Budget: Analytical Work: WSU GeoAnalytical Lab SHRIMP at Stanford University 5 days @ $550/day 2 days @ $1700/day Round-trip Travel to Analytical work: WSU- 360 miles @ $.55/mile Stanford University- round-trip airfare $200 (carpool- 1 vehicle) $250 x 2 flights Per Diem and Lodging WSU Stanford 4 nights @ $60/night 5 days @ $40/day 5 nights @ $70/night 5 days @ $40/day TOTAL $7570 References: Chopin, C., 1984, Coesite and pure pyrope in high- grade blueschists of the Western Alps: a first record and some consequences: Contributions to Mineralogy and Petrology, v. 86, p. 107-118. -, 2003, Ultrahigh- pressure metamorphism: tracing continental crust into the mantle: Earth and Planetary Science Letters, v. 212, no. 1-2, p. 1-14. Christensen, B., 2011, Geochemistry, geothermobarometry, and geochronology of high- pressure granulites and implications for the exhumation history of ultrahigh- pressure terranes: Dulan, western China [Master's: Central Washington University, 156 p.] Ernst, W., Hacker, B., and Liou, J., 2005, Petrotectonics- geochronology of ultrahigh- pressure (UHP) crustal and upper mantle rocks- implications for Phanerozoic orogeny: Geochimica et Cosmochimica Acta Supplement, v. 69, p. 291. Hacker, B., 2006, Pressures and temperatures of ultrahigh- pressure metamorphism: implications for UHP tectonics and H2O in subducting slabs: International Geology Review, v. 48, no. 12, p. 1053-1066. Kylander- Clark, A. R. C., Hacker, B. R., and Mattinson, C. G., 2011, Size and exhumation rate of ultrahigh- pressure terranes linked to orogenic stage, University of California, Santa Barbara, p. 33. Li, Z., and Gerya, T. V., 2009, Polyphase formation and exhumation of high- to ultrahigh- pressure rocks in continental subduction zone: Numerical modeling and application to the Sulu ultrahigh- pressure terrane in eastern China: Journal of Geophysical Research, v. 114, no. B9, p. B09406.

Liou, J. G., Tsujimori, T., Zhang, R. Y., Katayama, I., and Maruyama, S., 2004, Global UHP Metamorphism and Continental Subduction/Collision: The Himalaya Model: International Geology Review, v. 46, p. 1-27. Mattinson, C. G., Menold, C. A., Zhang, J. X., and Bird, D. K., 2007, High- and Ultrahigh- Pressure Metamorphism in the North Qaidam and South Altyn Terranes, Western China: International Geology Review, v. 49, p. 969-995. Mattinson, C. G., Wooden, J. L., Liou, J. G., Bird, D. K., and Wu, C. L., 2006, Age and duration of eclogite- facies metamorphism, North Qaidam HP/UHP terrane, Western China: American Journal of Science, v. 306, no. 9, p. 683-711. Ravna, E. J. K., and Paquin, J., 2003, Thermobarometric methodologies applicable to eclogites and garnet ultrabasites: Notes in Mineralogy, v. 5, p. 229-259. Song, S., 2006, Evolution from Oceanic Subduction to Continental Collision: a Case Study from the Northern Tibetan Plateau Based on Geochemical and Geochronological Data: Journal of Petrology, v. 47, no. 3, p. 435-455. Song, S., Yang, J., Liou, J. G., Wu, C., Shi, R., and Xu, Z., 2003a, Petrology, geochemistry and isotopic ages of eclogites from the Dulan UHPM Terrane, the North Qaidam, NW China: Lithos, v. 70, no. 3-4, p. 195-211. Song, S. G., J.S., Y., Xu, Z. Q., J.G., L., and R.D., S., 2003b, Metamorphic evoloution of the coesite- bearing ultrahigh- pressure terrane in the North Qaidam, Northern Tibet, NW China: Journal of Metamorphic Geology, v. 21, p. 631-644. Spear, F. S., Wark, D. A., Cheney, J. T., Schumacher, J. C., and Watson, E. B., 2006, Zr- in- rutile thermometry in blueschists from Sifnos, Greece: Contributions to Mineralogy and Petrology, v. 152, no. 3, p. 375-385. Tomkins, H. S., Powell, R., and Ellis, D. J., 2007, The pressure dependence of the zirconium- in- rutile thermometer: Journal of Metamorphic Geology, v. 25, no. 6, p. 703-713. Yang, J., Xu, Z., Zhang, J., Chu, C. Y., Zhang, R., and Liou, J. G., 2001, Tectonic significance of early Paleozoic high- pressure rocks in Altun- Qaidam- Qilian Mountains, northwest China: MEMOIRS- GEOLOGICAL SOCIETY OF AMERICA, p. 151-170. Yu, S., Zhang, J., and Real, P. G. D., 2011, Petrology and P T path of high- pressure granulite from the Dulan area, North Qaidam Mountains, northwestern China: Journal of Asian Earth Sciences, v. 42, no. 4, p. 641-660. Zack, T., and Luvizottow, G., 2006, Application of rutile thermometry to eclogites: Mineralogy and Petrology, v. 88, no. 1, p. 69-85. Zack, T., Moraes, R., and Kronz, A., 2004, Temperature dependence of Zr in rutile: empirical calibration of a rutile thermometer: Contributions to Mineralogy and Petrology, v. 148, no. 4, p. 471-488. Zhang, G., Ellis, D. J., Christy, A. G., Zhang, L., and Song, S., 2010a, Zr- in- rutile thermometry in HP/UHP eclogites from Western China: Contributions to Mineralogy and Petrology, v. 160, no. 3, p. 427-439. Zhang, G., Song, S., Zhang, L., and Niu, Y., 2008, The subducted oceanic crust within continental- type UHP metamorphic belt in the North Qaidam, NW China: Evidence from petrology, geochemistry and geochronology: Lithos, v. 104, no. 1-4, p. 99-118. Zhang, G., Zhang, L., Song, S., and Niu, Y., 2009a, UHP metamorphic evolution and SHRIMP geochronology of a coesite- bearing meta- ophiolitic gabbro in the North Qaidam, NW China: Journal of Asian Earth Sciences, v. 35, no. 3-4, p. 310-322.

Zhang, J. X., Mattinson, C. G., Yu, S. Y., Li, J. P., and Meng, F. C., 2010b, U- Pb zircon geochronology of coesite- bearing eclogites from the southern Dulan area of the North Qaidam UHP terrane, northwestern China: spatially and temporally extensive UHP metamorphism during continental subduction: Journal of Metamorphic Geology, v. 28, no. 9, p. 955-978. Zhang, J. X., Meng, F. C., Li, J. P., and Mattinson, C., 2009b, Coesite in eclogite from the North Qaidam Mountains and its implications: Chinese Science Bulletin, v. 54, no. 6, p. 1105-1110.

Fig. 1. Location map of North Qaidam UHP region. Yellow box represents Dulan study area. Mattinson et al. (2006, 2007).

Fig. 2.Geologic map of the Dulan region showing the distribution of high- pressure granulites at the far western end (labeled with g) and ultrahigh- pressure eclogites towards the east (labeled with e). The gray unit represents ultrahigh- pressure eclogite and granulite host gneiss. North and South Sub- belts are labeled. Line A- A marks the temperature gradient suggested by the two models. Modified after Christiansen (2011).

Fig. 3. Summary of previous P- T calculations and data for Dulan UHP host gneisses and eclogites and HP granulites. The points represent averages of temperatures reported by the listed authors. The range spans 550 915 C and 8 30 kbars. Each box is labeled with its metamorphic facies. Modified after Christiansen (2011)

Figures 4A and 4B shows schematic diagrams of temperature predictions for the 2 hypotheses. Figure 4A shows a continuous temperature gradient of approximately 200 C from the western granulites (A) to the eastern UHP eclogites (A ). This continuous temperature gradient is expected if the granulite facies rocks represent overprinting of UHP eclogites during exhumation. Figure 4B shows the expected temperature discontinuity if the granulites were emplaced tectonically later. The break in the temperature gradient is explained by a fault that has not yet been mapped.

Figure 5 shows exhumation paths predicted by the two hypotheses that explain the juxtaposition of HP granulites and UHP eclogites in the Dulan region. Song et al. (2003a,b; green line) predict that the UHP eclogites experienced HP granulite facies overprinting during exhumation. Yu et al. (2011) and Christensen (2011) predict that UHP eclogites and HP granulites are separate tectonic units with separate P- T histories (red and blue lines). All hypotheses predict that UHP and HP rocks experienced amphibolite facies metamorphism at the end of exhumation.