John C. Ayers

Recycling lower continental crust in the North China craton

Foundering of mafic lower continental crust into underlying convecting mantle has been proposed as one means to explain the unusually evolved chemical composition of Earths continental crust, yet direct evidence of this process has been scarce. Here we report that Late Jurassic high-magnesium andesites, dacites and adakites (siliceous lavas with high strontium and low heavy-rare-earth element and yttrium contents) from the North China craton have chemical and petrographic features consistent with their origin as partial melts of eclogite that subsequently interacted with mantle peridotite. Similar features observed in adakites and some Archaean sodium-rich granitoids of the tonalite-trondhjemite-granodiorite series have been interpreted to result from interaction of slab melts with the mantle wedge. Unlike their arc-related counterparts, however, the Chinese magmas carry inherited Archaean zircons and have neodymium and strontium isotopic compositions overlapping those of eclogite xenoliths derived from the lower crust of the North China craton. Such features cannot be produced by crustal assimilation of slab melts, given the high Mg#, nickel and chromium contents of the lavas. We infer that the Chinese lavas derive from ancient mafic lower crust that foundered into the convecting mantle and subsequently melted and interacted with peridotite. We suggest that lower crustal foundering occurred within the North China craton during the Late Jurassic, and thus provides constraints on the timing of lithosphere removal beneath the North China craton.

Constraints on timing of peak and retrograde metamorphism in the Dabie Shan Ultrahigh-Pressure Metamorphic Belt, east-central China, using U–Th–Pb dating of zircon and monazite

Abstract The Dabie Shan Ultrahigh-Pressure Metamorphic (UHPM) Belt occupies the suture between the Yangtze and Sino-Korean blocks in east-central China. The timing of UHPM in the Dabie belt is controversial, and most recent data come from dating of zircons. Monazite has recently been recognized as useful for dating of multiple tectonic events due to its preservation of multiple growth zones, and monazite growth has been documented to occur during prograde, peak, and retrograde metamorphism. Zircons and monazites from UHP mafic rocks from Maowu and a UHP jadeite quartzite from Shuanghe were imaged in thin sections and in mineral separates and dated using a high-resolution ion microprobe. Maowu mafic rocks are unique in that they contain high light rare earth element concentrations and therefore contain abundant monazite. Maowu eclogites and garnet pyroxenites contain zircons with mean 206 Pb/ 238 U age of ∼230±4 Ma, and monazites from a clinopyroxenite have 208 Pb/ 232 Th ages of ∼209±4 Ma. Multiple lines of evidence suggest that the measured ages represent the timing of new growth and recrystallization and are not cooling ages (i.e., they do not correspond to the cessation of diffusional Pb loss as rocks cooled below the closure temperature during exhumation). Shuanghe jadeite quartzites contain zircons with cores that define a discordia with an upper intercept age 1921±22 Ma and lower intercept age of 236±32 Ma, interpreted to represent the ages of the source rocks and of peak metamorphism, respectively. Zircon rims contain jadeite inclusions that restrict growth to pressures greater than 1.5 GPa. Their pooled age of 238±3 Ma agrees well with the lower intercept defined by cores, suggesting that Pb loss from cores and growth of new rims occurred during UHPM at ∼235–240 Ma. Monazite records multiple events during retrograde metamorphism. Maowu clinopyroxenite and Shuanghe jadeite quartzite experienced monazite growth at ∼209 Ma, interpreted to reflect regional amphibolite facies overprinting resulting from pervasive retrograde fluid infiltration. Only the jadeite quartzite records growth at 223±1 Ma. Estimated exhumation rates for Maowu are 7–8 km/Ma.

Solubility of apatite, monazite, zircon, and rutile in supercritical aqueous fluids with implications for subduction zone geochemistry

Solubilities of accessory minerals (apatite, monazite, zircon and rutile) in supercritical aqueous fluids have been measured to evaluate the role of these fluids in the mobilization of accessory mineral-hosted trace elements. We have characterized the effects on solubility of pH, XH2O (addition of CO2), pressure (P = 1.0-3.0 GPa), temperature (T =800-1200 °C), and dissolved silicate and NaCl concentration. Fluorapatite solubility in pure H2O is low, not more than 0.4 wt% at all conditions studied, but increases strongly with decreasing pH. Changes in P, T, XH2O MNaCl (the molality of NaCl), and dissolved silicate concentration have comparatively little effect on apatite solubility. Monazite is even less soluble in H2O (not more than 0.2 wt% ). Limited data suggests that monazite solubility increases with increasing P and T and with decreasing pH, but is insensitive to MNaCl. Zircon reacts with H2O to form baddeleyite (ZrO2) + silica-rich fluid. ZrO2 solubility in H2O and 1 m HCl is less than 0.2 wt% . Zircon, and therefore ZrO2, solubility in quartz-saturated fluids± HCl ±NaCl and in H2O -CO2 fluids is also very low. Rutile is more soluble than the other minerals examined, in the wt% range, and its solubility increases with increasing P and T. Results indicate that high P-T aqueous fluids can dissolve significant amounts of Ti but very little Zr, and little phosphate unless the fluids are acidic. In most cases, apatite, monazite and zircon will remain present during episodes of aqueous fluid metasomatism and therefore will exert control, as ‘residual phases’, over element distribution. The higher solubility of rutile relative to other accessory minerals at high pressure may result in the depletion of high field strength elements relative to large ion lithophile elements observed in subduction zone volcanics.

Low temperature replacement of monazite in the Ireteba granite, Southern Nevada: geochronological implications

The Ireteba pluton is a relatively homogeneous, ∼64 Ma (zircon ion probe age) two-mica granite that was intruded by two 16 Ma Miocene plutons at depths ranging from 5 to 13 km. Deeper levels of the Ireteba and Miocene plutons were ductilely deformed at 15–16 Ma. At shallow levels remote from the Miocene plutons, the Ireteba granite appears to have experienced little Miocene heating and deformation. Monazites from different portions of the pluton reflect the different histories experienced by the host rock. Irregularly shaped (patchy) zones with high huttonite component (ThSiO4) are widespread in monazite at deep levels adjacent to Miocene plutons but less common in shallow-level rock; monazite grains with extensive replacement generally have irregular, embayed surfaces. In undeformed rocks distant from the Miocene plutons, monazites are less modified and more nearly euhedral, though fine networks of replacement veins are common and irregular rims are evident in some grains. Secondary monazite from these samples is poorer in huttonite. Ion probe Th–Pb dating yields 60–65 Ma ages for magmatic and some replacement zones in monazite from the shallow samples, and veins yield apparent ages as young as mid-Tertiary. Monazites from deep samples yield a few 55–65 Ma ages for remnant magmatic zones and abundant Miocene ages for replacement zones (∼14–18 Ma). These data demonstrate extensive Miocene replacement of magmatic monazite, especially at deep levels near Miocene plutons, and they suggest an early replacement episode as well. Both events were probably related to influxes of fluid; the first may have been associated with initial solidification of the Ireteba pluton and the second with the Miocene plutons and/or extensional deformation. Ambient temperatures at the time of replacement indicate that secondary monazite growth occurred at T as low as 400°C or less.

Rutile solubility and mobility in supercritical aqueous fluids

AbstractExperimental and thermodynamic data and the apparent immobility of Ti under metamorphic conditions suggest that rutile is very insoluble in aqueous fluids at upper crustal conditions. New solubility measurements at 1.0–2.93 GPa and 800–1200°C show, however, that under certain pressure and temperature conditions rutile is quite soluble in H2O. Solubilities were estimated from the measured weight loss of a single crystal equilibrated with a known mass of fluid in a piston cylinder apparatus. Measured solubilities in H2O range from 0.15 wt% (wt loss crystal/wt fluid) at 2.93 GPa and 1000°C to 1.9% at 1.0 GPa and 1100°C. Solubility increases with increasing temperature and with decreasing pressure in a manner given by the following fit to the experimental data:

TEXTURAL DEVELOPMENT OF MONAZITE DURING HIGH-GRADE METAMORPHISM : HYDROTHERMAL GROWTH KINETICS, WITH IMPLICATIONS FOR U, TH-PB GEOCHRONOLOGY

\log _{10} m_{Ti} = - 7049/T - (0.589* P)/T + 5.14

Experimental study of zircon coarsening in quartzite ±H2O at 1.0 GPa and 1000 °C, with implications for geochronological studies of high-grade metamorphism

wheremTi is the molality of Ti in the fluid,T is in degress Kelvin andP is in MPa. The effect of fluid composition on rutile solubility was also examined at 1.0 GPa and 1000°C for H2O-CO2, 1m NaCl, and 1m HF fluids. Kesults suggest that solubility depends on the mole fraction of H2O in the fluid but is independent of ionic strength and fluid pH. This behavior implies that Ti dissolves as the neutrally-charged hydrolysis product Ti(OH)4. The free energy of this species was calculated for each set of experimental conditions. TheP-T dependence of rutile solubility suggests that aqueous fluids derived from subducted oceanic lithosphere would dissolve rutile or other Ti-rich minerals from the deepest portion of the mantle wedge and precipitate them at higher levels. Subsequent melting of the base of the mantle wedge would form HFSE-depleted IAB.

In situ oxygen isotope analysis of monazite as a monitor of fluid infiltration during contact metamorphism: Birch Creek Pluton aureole, White Mountains, eastern California

The Northern Dabie Complex in east central China lies between the Sino‐Korean plate to the north and the Yangtze plate to the south. The Northern Dabie Complex has been variously proposed to represent a Paleozoic magmatic arc on the Sino‐Korean plate, an exhumed piece of subducted Yangtze plate crust, or crust produced almost entirely by Cretaceous extension-related magmatism. Trace element compositions of Northern Dabie Complex orthogneisses and granites show arc signatures similar to those of ultra-high-pressure rocks in the central Dabie, but no mineralogical evidence of ultra-high-pressure metamorphism is present in the samples investigated here. Field relationships, textures, major and trace element compositions, and ion microprobe U-Pb zircon protolith crystallization ages reveal three distinct types of gneiss: diorite gneiss xenoliths (770 6 26 Ma, 95% confidence limit), those within first-genation highly deformed migmatitic grey gneisses (747 6 14 Ma), and those cross-cut by secondgeneration Cretaceous weakly foliated felsic gneisses (127 6 4 Ma). Unfoliated Cretaceous granites (117 6 11 Ma, monazite Th-Pb age 5 117 6 1 Ma) intrude secondgeneration gneisses. Cretaceous second† Corresponding author e-mail: john.c.ayers@ vanderbilt.edu. generation gneisses and granites yield zircon inheritance ages of ca. 2 Ga, 700‐800 Ma, and (rarely) 227‐271 Ma, indicating that the Northern Dabie Complex is not simply a Cretaceous extensional terrane. The 700‐800 Ma zircon ages are similar to those of granitic gneisses from the central ultra-highpressure zone (698 6 47 Ma) and are characteristic of the Yangtze craton. «Nd values suggest that Cretaceous rocks in the Northern Dabie Complex formed by partial melting of basement with very low «Nd and not by melting of first-generation or diorite gneisses. Nd-depleted mantle model ages are consistent with the time of formation of the Yangtze craton at 1.4‐2.5 Ga. The Northern Dabie Complex is interpreted to be an extension of the Yangtze craton that was unaffected by ultra-high-pressure metamorphism. The Sino‐Korean/Yangtze suture must lie to the north of the Northern Dabie Complex.