Combined geochemistry and geochronology constrains coupled subduction of oceanic and continental crust in the Huwan shear zone, central China

Abstract

Subduction of rocks into the mantle results in high-pressure metamorphism and the formation of eclogites from basaltic precursor rocks. In general, many kilometers of oceanic lithosphere are ultimately consumed prior to the subsequent continental slab subduction and collision. The exposure of the eclogites derived from oceanic subduction and continental subduction at the surface of Earth today record provide different P-T-t records of the subduction process. The Huwan shear zone in the Hong'an orogenic belt, marking a former ocean-continent transition zone, has been the focus of many studies on subduction-related high-pressure metamorphism. In this study, Lu-Hf garnet, U-Pb zircon, and ArAr mica ages are combined with geochemical data to understand the origin of two coexisting eclogite bodies exposed along the Xuehe River in the Huwan Shear zone. In total, the results indicate that the two eclogites have different protoliths but experienced a similar metamorphic history. This observation requires new tectonic model for the coupled subduction of oceanic and continental crust in subduction zones. Combined geochemistry and zircon U-Pb geochronology suggest distinct oceanic and continental affinities for the eclogite protoliths. The Lu-Hf dates of 261.5 ± 2.4 Ma of the continental-type eclogite and 262.7 ± 1.7 Ma of the oceanic-type eclogite reflect garnet growth and are interpreted to closely approximate the age of eclogite-facies metamorphism. Therefore, both the geochemically oceanic-and continental-type eclogites underwent the same episode of Permian eclogite-facies metamorphism. The Permian Lu-Hf ages of ca. 262 Ma and the obtained Triassic Ar-Ar ages (~240 Ma) of the oceanic-type and continental-type eclogites imply coupled subduction and exhumation of oceanic and continental crustal materials in the Hong'an orogenic belt during the Permian and the Triassic. Though limited, the geochemical and geochronological results of this study, together with the discrepant Carboniferous dates for the nearby eclogites of previous studies, apparently suggest that the Huwan shear zone was not always a single coherent unit but instead comprises different tectonic slices that were metamorphosed at different times before final assembly. Some slices of the oceanic and continental crust underwent two subduction cycles during the Carboniferous and the Permian, whereas some eclogites registered only a single subduction-exhumation loop during the convergence between the South China Block and the North China Block in the Huwan shear zone. The consistent ages of the oceanic-and continental-type eclogites disfavor the traditional mélange model that high-pressure rocks are dismembered fragments that have been assembled and intercalated with rocks devoid of any high-pressure history at shallow crustal levels, forming a tectonic mélange.

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

“…(1) Zircon U-Pb and mineral-whole-rock Rb-Sr isochron ages of continental-type eclogites in the Tongbai orogen are determined to be ~255 Ma , which has been confirmed by garnet-whole-rock Lu-Hf dating (Cheng 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 .…”

“…Trace element characteristics of zircon and mineral inclusion types in metamorphic zircon domains provide convincing evidence for this conclusion (Sun et al, 2002;Wu et al, 2009;Zhou et al, 2015). Moreover, some continental materials may have also involved in the Carboniferous subduction (Liu X C et al, , 2011cCheng and Vervoort, 2015).…”

“…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). At present, it is unclear whether or not two-stage metamorphism and two age groups correspond one-to-one.…”

“…Zircon U-Pb dating indicates that their protoliths formed at ~425 Ma, whereas the HP metamorphism took place at ~310 Ma. However, garnet Sm-Nd and Lu-Hf dating gave the isochron ages as young as ~270-250 Ma (Jian et al, 1997(Jian et al, , 2000Li et al, 2001;Sun et al, 2002;Cheng et al, 2009Cheng et al, , 2013Cheng and Vervoort, 2015;Liu X C et al, 2009Liu X C et al, , 2011cPeters et al, 2013;Zhou et al, 2015). Peak P-T conditions of metamorphism were estimated to be 590-640C and 2.0-2.4 GPa (Cheng et al, 2009;Zhou et al, 2014).…”

Paleo‐ to Mesoproterozoic Tectono‐Magmatic Events Recorded in the Huwan Complex from the Dabie Orogen, Central China: Evidence from Petrology and U‐Pb Geochronology

“…The Huwan complex (Ratschbacher et al, 2006) or the Huwan shear zone (Cheng and Vervoort, 2015) is located at the northern margin of the Dabie belt and is ca. 10 km in length (Fig.…”