SHRIMP U‐Pb zircon dating from Sulu‐Dabie dolomitic marble, eastern China: constraints on prograde, ultrahigh‐pressure and retrograde metamorphic ages

Abstract

Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircon from Sulu-Dabie dolomitic marbles is characterized by distinctive domains of inherited (detrital), prograde, ultrahighpressure (UHP) and retrograde metamorphic growths. The inherited zircon domains are darkluminescent in CL images and contain mineral inclusions of Qtz + Cal + Ap. The prograde metamorphic domains are white-luminescent in CL images and preserve a quartz eclogite facies assemblage of Qtz + Dol + Grt + Omp + Phe + Ap, formed at 542-693°C and 1.8-2.1 GPa. In contrast, the UHP metamorphic domains are grey-luminescent in CL images, retain the UHP assemblage of Coe + Grt + Omp + Arg + Mgs + Ap, and record UHP conditions of 739-866°C and >5.5 GPa. The outermost retrograde rims have dark-luminescent CL images, and contain low-P minerals such as calcite, related to the regional amphibolite facies retrogression. Laser ablation ICP-MS trace-element data show striking difference between the inherited cores of mostly magmatic origin and zircon domains grown in response to prograde, UHP and retrograde metamorphism. SHRIMP U-Pb dating on these zoned zircon identified four discrete 206 Pb/ 238 U age groups: 1823-503 Ma is recorded in the inherited (detrital) zircon derived from various Proterozoic protoliths, the prograde domains record the quartz eclogite facies metamorphism at 254-239 Ma, the UHP growth domains occurred at 238-230 Ma, and the late amphibolite facies retrogressive overprint in the outermost rims was restricted to 218-206 Ma. Thus, Proterozoic continental materials of the Yangtze craton were subducted to 55-60 km depth during the Early Triassic and recrystallized at quartz eclogite facies conditions. Then these metamorphic rocks were further subducted to depths of 165-175 km in the Middle Triassic and experienced UHP metamorphism, and finally these UHP metamorphic rocks were exhumed to midcrustal levels (about 30 km) in the Late Triassic and overprinted by regional amphibolite facies metamorphism. The subduction and exhumation rates deduced from the SHRIMP data and metamorphic P-T conditions are 9-10 km Myr )1 and 6.4 km Myr )1 , respectively, and these rapid subduction-exhumation rates may explain the obtained P-T-t path. Such a fast exhumation suggests that Sulu-Dabie UHP rocks that returned towards crustal depths were driven by buoyant forces, caused as a consequence of slab breakoff at mantle depth.

Not all folds and thrusts in the Yangtze foreland thrust belt are related to the Dabie Orogen: Insights from Mesozoic deformation south of the Yangtze River

“…The process produced the blueschistfacies unit, the HP eclogite-facies unit, the UHP eclogitefacies unit, the low-amphibolite-and greenschist-facies unit in the Dabie Orogen Zheng et al, 2003;Xu et al, 2009;Li et al, 2009aLi et al, , 2010a. A considerable amount of geochronological data has confirmed that the deformation of the Qinling-Dabie Orogen happened in the Triassic-Early Jurassic times (244-180 Ma;Webb et al, 1999;Hacker et al, 1998Hacker et al, , 2000Hacker et al, , 2006Zhu et al, 2005;Liu et al, 2006aLiu et al, , 2006bZheng, 2008;Li et al, in press). The two sutures are obviously offset by the Tan-Lu Fault which is characterized by two-stages of sinistral strike slip movement (128-100 Ma and 161 Ma; Figures 1 and 2; Zhu et al, 2001Zhu et al, , 2005Wang, 2006).…”

Structural anatomy of the exhumation of high‐pressure rocks: constraints from the Tongbai Collisional Orogen and surrounding units

“…255 Ma as a result of continental subduction (Liu et al, 2008a), slightly earlier than the timing of peak UHP metamorphism in the Dabie-Sulu Orogen at ca. 240 Ma (Li et al, 2004;Liu et al, 2004bLiu et al, , 2006aLiu et al, , 2006bWan et al, 2005;Hacker et al, 2006;Wu et al, 2008). The 40 Ar/ 39 Ar age of ca.…”

Diachroneity of continental subduction and exhumation: Constraints from the Permian-Triassic HP metamorphic terrane in the Tongbai orogen, central China

“…Two groups of zircon U-Pb ages of 245-240 Ma and 225-220 Ma are common for UHP metamorphic rocks from the central and southern Dabie orogen [46][47][48][49]. The former was generally interpreted as the age of metamorphic dehydration event occurring in the transition from HP to UHP conditions during continental subduction, and the latter as the age of metamorphic dehydration event in the transition from UHP to HP conditions during the exhumation of deeply subducted continental crust [42,50].…”