Geochronology and isotopic character of ultrahigh‐pressure metamorphism with implications for collision of the Sino‐Korean and Yangtze cratons, central China

Abstract

The collision zone between the Sino‐Korean and Yangtze cratons in central China is marked by subduction‐related metamorphic rocks composed of crustal protoliths. These range in metamorphic grade from ultrahigh‐pressure eclogite to low‐greenschist facies. The paragenesis of the ultrahigh‐pressure rocks (coesite and diamond‐bearing) indicates subduction to depths greater than ∼120 km as a result of continental collision. Despite extreme metamorphic conditions, zircons have not undergone Pb loss but rather illustrate two periods of growth: crystallization between 700 and 800 Ma and ultrahigh‐pressure metamorphism at 218 ± 2.5 Ma. Previous studies have predicted a time‐transgressive collision, but age data presented in this study indicate that the collision was approximately coeval along the length of the suture. U‐Pb zircon ages of these ultrahigh‐pressure metamorphic rocks are 218.4 ± 2.5 Ma and 217.1 ± 8.7 Ma for the Dabie Mountains and Shandong peninsula, respectively. The similar ages imply that the Tan‐Lu fault, which offsets the ultrahigh‐pressure metamorphic rocks, is a secondary feature not related to collision or subduction. Whole rock isotopic data show that both Rb‐Sr and U‐Pb were open systems during metamorphism but that Sm‐Nd probably remained closed. Nd initial isotopes display crustal signatures, indicating that the crust subducted as a coherent slab rather than becoming intercalated with mantle material at depth. Sm‐Nd data in conjunction with U‐Pb zircon dating indicate that the ultrahigh‐pressure rocks are not typical Yangtze craton basement (2.9 Ga) but that they originally crystallized in a rift environment between ∼700 and 800 Ma. Discrepancies between 40Ar/39Ar plateaus, Sm‐Nd isochrons, and U‐Pb zircon ages render calculation of a P‐T‐t path unrealistic.

Provenance and tectonic-paleogeographic evolution: Constraints from detrital zircon U–Pb ages of Late Triassic-Early Jurassic deposits in the northern Sichuan basin, central China

“…For example, the accretion of the SCB to NCB was suggested to occur along the Shangdan suture zone in Silurian-Devonian following the fact that the South Qinling belt (SQB) began to receive sediments from the North Qinling in middle Paleozoic (Gao et al, 1995;Kröener et al, 1993;Zhang et al, 1997). In contrast, paleomagnetic data (Enkin et al, 1992;Yang et al, 1992), together with ultrahigh-pressure metamorphism at 230 Ma in the easternmost Qinling-Dabie belt (Ames et al, 1996;Hacker et al, 1998;Okay et al, 1993) blocks. The discovery of the Mianlue suture zone between the South Qinling and SCB was invoked to reconcile the above inconsistence and draw a conclusion that it is the Late Triassic collision between the SCB and South Qinling that finally resulted in the accretion of the SCB to NCB Zhang, 1999, 2000;Ratschbacher et al, 2003).…”

Sedimentary provenance of the Hengyang and Mayang basins, SE China, and implications for the Mesozoic topographic change in South China Craton: Evidence from detrital zircon geochronology

“…1A), each with distinctive crustal ages and tectonic histories (HNGBMR, 1988;Ren, 1991;JXGBMR, 1988;Duan et al, 2011;Li et al, 2011;Wong et al, 2011). The basement of the Yangtze Block consists of Archean rocks (Ames et al, 1996;Qiu et al, 2000) while that of the Cathaysian Block dominantly Paleo-Mesoproterozoicsome Late Achaean components ($2.5 Ga;JXGBMR, 1988;Chen and Jahn, 1998). The collision of the two blocks resulted in the 0.9-1.3 Ga Sibao Orogeny, which coincides with the worldwide Grenville event (Li et al, 2002b;Rino et al, 2008;Santosh et al, 2009).…”

Multichronometric Evidence for an In Situ Origin of the Ultrahigh‐Pressure Metamorphic Terrane of Dabieshan, China

“…However, the destabilization of rutile into ilmenite is indicative of an earlier, higher PT condition. The Triassic UHP metamorphic age of the Bixiling Complex was established on the basis of seven Sm-Nd garnet-omphacite-WR isochrons between and Ma (Chavagnac and Jahn 210 ‫ע‬ 9 218 ‫ע‬ 4 1996), which are close to the 220-230 Ma obtained on U-Pb zircon ages (Ames et al 1993;Ames 1995;Maruyama et al 1995;Rowley et al 1997;Hacker et al 1998). The primary intrusive age of the Bixiling Complex was constrained by a Sm-Nd garnetonly isochron (7 points, ; Chavagnac MSWD p 0.7 and Jahn 1996).…”