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

Abstract

High-pressure (HP) metamorphic terrane in the Tongbai orogen comprises two HP slices (I and II) and a tectonic mélange zone in the northeast and a blueschist-greenschist zone in the southwest. HP slice I is represented by the northern and southern eclogite zones on the two sides of the Tongbaishan antiform. HP slice II is represented by retrograded eclogite-bearing metamorphic enclaves in Cretaceous gneissic granites in the Tongbai Complex. U-Pb, Lu-Hf, Rb-Sr and 40 Ar/ 39 Ar multichronometric data indicate that the peak metamorphism of HP slice I took place at ~255 Ma, whereas the metamorphic ages of HP slice II are as young as 232-220 Ma. By contrast, the tectonic mélange zone near the suture was metamorphosed at ~256 Ma. Such a diachroneity of different slices across the direction of the orogen in the Hong'an-Dabie-Sulu HP/UHP terrane is ubiquitous, and it can be interpreted by a syn-subduction detachment/exhumation model. Furthermore, the metamorphic age of HP slice I in the Tongbai orogen is older than that of the equivalent HP slice in the Hong'an orogen by ~15 Ma, suggesting that the diachroneity may have also existed along the direction of the orogen. A seesaw-type subduction/exhumation model is proposed to explain this age disparity and the subduction of the South China Block becomimg shallower towards the west. The processes and mechanism of exhumation of ultrahighpressure (UHP) metamorphic rocks to the Earth's surface from mantle depths exceeding ~100 km remain as one of the most difficult problems in earth science. It appears that a simple mechanism involving buoyancy and erosion is unable to explain the rapid exhumation required for the preservation of coesite. Much attention should be paid to the deepseated tectonic processes of continental slab. Based on the nature of structural contacts and existence of metamorphic gaps between different HP/UHP metamorphic units in the Dora Maira Massif of the western Alps, Chopin [1] andSchreyer [2] inferred that the end of the downgoing slab might have successively broken to several slices during its subduction along very low geothermal gradients (~7C/km). The earlier subducted slices could be uplifted as the overlying hanging-walls during the subduction of the later underlying slices. Liu and Hu [3] have formerly noticed the similarity of HP/UHP terrane from the Dabie Mountains and the Dora Maira Massif. Lately Liu et al. [4,5] distinguished UHP, HP and low-temperature (LT) HP slices and a tectonic mélange zone with the time difference of exhumation of ~30 Ma in the Hong'an orogen on the basis of thermobaric structure and geochronological data, and proposed a diachronous subduction/exhumation model for the formation of

Tectonic evolution of the Tongbai-Hong’an orogen in central China: From oceanic subduction/accretion to continent-continent collision

“…Two groups of age record were obtained for rocks in both belts. In the Wuguan-Guishan complex, the 40 Ar/ 39 Ar ages of hornblendes and some muscovites were determined to be 310-300 Ma, whereas the 40 Ar/ 39 Ar ages of most muscovite are concentrated to 260-250 Ma, in agreement with those obtained from the Liuling Group and Nanwan flysch (Zhai et al, 1998;Liu et al, 2013b;Chen et al, 2014b;our unpublished data). Similarly, in the Xiongdian eclogite belt, although the metamorphic ages obtained for metamorphic zircon domains are clustered at ~310 Ma, garnet-whole-rock Lu-Hf and Sm-Nd isochron ages are gathered to 270-250 Ma (Cheng et al, 2009(Cheng et al, , 2010b(Cheng et al, , 2013Cheng and Vervoort, 2015;Brouwer et al, 2011).…”

“…(2) As mentioned previously, although the metamorphism of the oceanic-type eclogites from Xiongdian eclogite belt in the Hong'an orogen was dated at ~310 Ma using zircon U-Pb method, the garnet-whole-rock Lu-Hf and Sm-Nd isochron or garnet-omphacite tie-line ages obtained for the same eclogites are concentrated to 270-250 Ma Cheng et al, 2009Cheng et al, , 2010bCheng et al, , 2013Cheng and Vervoort, 2015;Brouwer et al, 2011), suggesting that these oceanictype eclogites may have been reworked by later continental subduction. (3) Muscovite 40 Ar/ 39 Ar ages obtained from the Liuling Group, Nanwan flysch, Foziling Group and some Wuguan-Guishan complex are clustered at ~270-250 Ma (Niu et al, 1994;Faure et al, 2003;Ratschbacher et al, 2006;Liu et al, 2013b;our unpublished data), while these Devonian flysch sequences may represent the hangingwall accretionary complex of the subduction zone . Therefore, the continuous oceanic subduction may have lasted for ~60 Myr, i.e.…”

“…Zhang and Liou, 1994;Hacker et al, 2000;Zhang et al, 2004;Ernst et al, 2007). However, research on the Tongbai Complex shows that the Tongbai Complex resembles the Northern Dabie Complex in antiform (dome) structure with NWW-SEE subhorizontal extension, rock association of deformed granites and metamorphic rocks dominated by TTG gneisses, eclogite/ amphibolite Figure 5 Schematic cartoons demonstrating the tectonic evolution from oceanic subduction/accretion to continent-continent collision between the North China Block (NCB) and South China Block (SCB) during the Late Paleozoic to Mesozoic (modified after Liu et al, 2004bLiu et al, , 2013aLiu et al, , 2013b. (a) The Tongbai orogen; (b) the Hong'an orogen.…”

Detrital zircon trace elements from the Mesozoic Jiyuan Basin, central China and its implication on tectonic transition of the Qinling Orogenic Belt

“…Many other geologic records have also been listed to demonstrate the Late Paleozoic oceanic subduction in the Qinling Orogen, such as subduction-related deformation [90], zircon trace elements and types of mineral inclusions [91,92], Permian-Triassic HP metamorphic terrane in the Tongbai Orogen [93], muscovite 40 Ar/ 39 Ar ages obtained from the Nanwan flysch (262 ± 2 Ma) and the Foziling Group (271 -261 Ma) [62,94], and the island-arc volcanics from the Permian to Early Triassic along the south margin of the South Qinling Belt [45,95,96]. In addition, the Late Paleozoic-Early Triassic trough in the Qinling Orogen might be the response to the extension [96,97].…”

A newly found Paleozoic Japan-type subduction–accretion system

“…250 Ma [4,5], although some authors consider that the final collision occurred as late as 230-220 Ma [6]. Such long-evolved orogenic belts have been found elsewhere in China indicated by accurate geochronologic research, such as the Tongbai orogen [7] and the Trans-North China orogen [8,9].…”