Isotopic constraints on age and duration of fluid‐assisted high‐pressure eclogite‐facies recrystallization during exhumation of deeply subducted continental crust in the Sulu orogen

Abstract

Fluid availability during high-grade metamorphism is a critical factor in dictating petrological, geochemical and isotopic reequilibration between metamorphic minerals, with fluid-absent metamorphism commonly resulting in neither zircon growth/recrystallization for U-Pb dating nor Sm-Nd isotopic resetting for isochron dating. While peak ultra-high pressure (UHP) metamorphism is characterized by fluid immobility, high-pressure (HP) eclogite-facies recrystallization during exhumation is expected to take place in the presence of fluid. A multichronological study of UHP eclogite from the Sulu orogen of China indicates zircon growth at 216 ± 3 Ma as well as mineral Sm-Nd and Rb-Sr reequilibration at 216 ± 5 Ma, which are uniformly younger than UHP metamorphic ages of 231 ± 4 to 227 ± 2 Ma as dated by the SHRIMP U-Pb method for coesite-bearing domains of zircon. O isotope reequilibration was achieved between the Sm-Nd and Rb-Sr isochron minerals, but Hf isotopes were not homogenized between different grains of zircon. The HP eclogite-facies recrystallization is also evident from petrography. Thus this process occurred during exhumation with fluid availability from decompression dehydration of hydrous minerals and the exsolution of hydroxyl from nominally anhydrous minerals. This provides significant amounts of internally derived fluid for extensive retrogression within the UHP metamorphosed slabs. Based on available experimental diffusion data, the consistent reequilibration of U-Pb, Sm-Nd, Rb-Sr and O isotope systems in the eclogite minerals demonstrates that time-scale for the HP eclogite-facies recrystallization is c. 1.9-9.3 Myr or less. This provides a maximum estimate for duration of the fluid-facilitated process in the HP eclogite-facies regime during the exhumation of deeply subducted continental crust.

Empirical calibration of the clinopyroxene–garnet magnesium isotope geothermometer and implications

“…This method follows the idea that the state of intermineral O isotope equilibrium may be a good indicator of with slope of unity that best fit the data for each locality represent the same degree of Mg isotope fractionation between clinopyroxene and garnet (i.e., the same Δ 26 Mg clinopyroxene-garnet value) at the same temperature. Error bars indicate the external precision of ±0.07 ‰ (2SD) on δ 26 Mg. Data are from Table 2 the state of Nd isotope equilibrium between clinopyroxene and garnet in eclogites (Zheng et al 2002(Zheng et al , 2003Zhao et al 2006). This is because the attainment of isotopic exchange equilibrium between mineral pairs and its preservation during later geological processes (e.g., retrograde metamorphism) is controlled by diffusion.…”

Phanerozoic tectonics of the South China Block: Key observations and controversies

“…Along the Dabie-Sulu belt, the UHP rocks were rapidly exhumed to crustal levels along the Dabie-Sulu orogenic belt (e.g., Zhao et al, 2006;Liu et al, 2008a;Zheng et al, 2009). In the meantime, the SCB finally collided with the NCB along the Qinling orogen (e.g., Jiang et al, 2010).…”

Isotopic disequilibrium in ultrahigh-pressure and retrograde metamorphism of eclogite and gneiss from the Chinese Continental Scientific Drilling in the Sulu orogen, China: evidence from mineral Nd–Sr–O isotopic composition

“…Geochemical studies have demonstrated that supracrustal material was subducted to mantle depth of more than 120 km, recrystallized within a short time span under UHP metamorphic condition and then was rapidly exhumed (e.g., Zheng et al 2003c). The time period for the UHP/HP metamorphism has been constrained between 215 and 245 Ma by different radiometric methods (Li et al 1993(Li et al , 2000Ames et al 1996;Hacker et al 1998;Chavagnac et al 2001;Ayers et al 2002;Liu et al 2004a, b;Liu et al 2006a, b;Wu et al 2006;Zhao et al 2006). The Sulu UHP-HP belt is located between the Wulian-Qingdao-Yantai fault in the northwest and the Jiashan-Xiangshui fault in the south (Fig.…”

“…On the basis of radiometric dating and O isotope analysis on UHP metamorphic rocks in the DabieSulu orogenic belt, Zheng et al (2002) proposed that the O isotopic composition can be used as an indicator to test the conditions of Nd and Sr isotopic equilibrium of metamorphic minerals. Subsequently, abundant studies on eclogite, gneiss, and granite in the Dabie-Sulu orogenic belt have shown that d 18 O variation between cogenetic minerals can provide a reasonable test to the validity of mineral Sm-Nd and Rb-Sr chronometers (Zheng et al 2002(Zheng et al , 2003aLi et al 2004;Xie et al 2004Xie et al , 2006Zhao et al 2006). Most of these studies have shown that the Sm-Nd isotopic system preserves the information of UHP eclogite-facies metamorphism and Sm-Nd mineral isochrons of eclogite and gneiss give UHP metamorphic ages.…”

“…Sm-Nd and Rb-Sr isochron ages acquired from two or more metamorphic mineral phases were commonly used for dating of HP and UHP metamorphic rocks in the last decades (e.g., Li et al 1993Li et al , 2000Li et al , 2004Chavagnac and Jahn 1996;Thöni 2002;Xie et al 2004;Glodny et al 2005;Zhao et al 2006). Nevertheless, if the isotopic equilibrium between metamorphic minerals failed to be achieved during the metamorphic recrystallization or if earlier equilibrium phases were disturbed by later geological overprints, isochron obtained from the metamorphic minerals may give meaningless ages.…”