The metamorphic evolution and tectonic significance of the Sumdo HP–UHP metamorphic terrane, central-south Lhasa Block, Tibet

Abstract

The Lhasa terrane, forming one of the main tectonic components of the Himalayan–Tibetan orogen, has received a lot of attention as it records multiple episodes of plate spreading, subduction and collision within the realm of the Paleo-Tethys Ocean. A review of the mineralogical, petrological, geochemical and geochronological data of eclogites, associated blueschists and garnet-bearing mica schists from the Sumdo high- and ultrahigh-pressure metamorphic (HP/UHP) complex in the central/southern part of the Lhasa terrane, is present here so that the origin and tectono-metamorphic evolution of this important suture can be deduced. By re-evaluating the original published P–T conditions for the metamorphic rocks of the Sumdo Complex, we consider that the Sumdo Complex has experienced low temperature HP/UHP metamorphic conditions, characteristic of fast subduction (and exhumation) in a typical oceanic subduction zone setting. The original wide spread in the maximal peak P–T conditions could be reduced in size due to thus far unknown inconsistencies in the usage of applied geothermobarometric techniques. The remaining spread in the maximal P–T conditions (c. 200°C/10 kbar) of the HP/UHP regions can be explained by a mechanism that the rocks from individual tectonic slices were subducted to different depths and followed by juxtaposition on their way back to the surface. A re-consideration of the isotopic ages of eclogites from the Sumdo Complex demonstrates that the opening of the Paleo-Tethys Ocean, located in between the two major components of the Lhasa terrane, was initiated prior to c. 280 Ma and the eclogite facies metamorphism is likely to be of late Permian (c. 260 Ma) to early Triassic age (245–225 Ma), recording different ages of subduction from individual slices of the oceanic crust. The closure of the Paleo-Tethys Ocean resulted, no earlier than 210 Ma, in the final collision between the northern and southern Lhasa blocks. This final collision event may have been triggered by the initial subduction of the Bangong–Nujiang Tethys Ocean in the north.

Metamorphic evolution and age constraints of the garnet-bearing mica schist from the Xindaduo area of the Sumdo (U)HP metamorphic belt, Tibet

“…8; Cheng et al 2012;Yang et al 2014;Huang et al 2015;Li et al 2017). The c. 100°C temperature difference between the calculated metamorphic P-T conditions obtained by these two methods may be caused either by a different choice of equilibrium minerals or the complicated tectonic evolution of the Sumdo Complex (Zhang et al 2018).…”

“…Geochemical and isotopic data show that the eclogites were derived from either a mid-ocean ridge basalt (MORB) or an oceanic island basalt (OIB) protolith and experienced eclogitefacies metamorphism during Late Permian to Early Triassic times (Yang et al 2009;Cheng et al 2012Cheng et al , 2015Cao et al 2017;Zhang et al 2018). P-T conditions of the eclogite from the Sumdo Complex have been reported by several researchers, ranging from 450-800°C and 2.5-3.9 GPa (Yang et al 2009(Yang et al , 2014Zhang, Zhang & Zhao, 2011;Cheng et al 2012Cheng et al , 2015Zhang et al 2018). These P-T conditions are located within the lawsonite eclogite (LE), epidote eclogite (EE) or the dry eclogite stability fields formed by low-temperature (LT) to medium-temperature (MT) metamorphism.…”

Geochronologic Constraints on the Timing of Metamorphism and Exhumation of the Tillotson Peak Complex in Northern Vermont

“…Therefore, 40 Ar/ 39 Ar analyses of amphiboles from within the TPC may be difficult (see Veblan, 1989, 1991). Actinolite is not targeted as frequently as other K-bearing minerals, however it can be dated successfully to examine the timing of greenschist-facies metamorphism (e.g., Borojević Šoštarić et al, 2014;Zhang et al, 2018, and references therein). Glaucophane that has been separated from blueschist and dated often yields younger 40 Ar/ 39 Ar ages than white mica from the same sample (McDougall and Harrison, 1999).…”

HP–UHP metamorphism and tectonic evolution of orogenic belts: introduction

“…The second paper concerns the Sumdo eclogite terrane, which is a newly recognized HP-UHP metamorphic belt in Tibet (Yang et al 2009). C. Zhang et al (2018a) summarize progress in mineralogical, petrological, geochemical and geochronological studies of eclogites and related blueschist and garnet-bearing mica schists of the Sumdo terrane. On the basis of these studies, C. Zhang et al (2018a) propose a new tectonic model involving the northward subduction of Paleo-Tethyan oceanic crust before 280 Ma and the collision between the South Lhasa and North Lhasa terrains during 266-225 Ma.…”

“…C. Zhang et al (2018a) summarize progress in mineralogical, petrological, geochemical and geochronological studies of eclogites and related blueschist and garnet-bearing mica schists of the Sumdo terrane. On the basis of these studies, C. Zhang et al (2018a) propose a new tectonic model involving the northward subduction of Paleo-Tethyan oceanic crust before 280 Ma and the collision between the South Lhasa and North Lhasa terrains during 266-225 Ma. These new studies are of vital importance since they represent important pieces of the puzzle that contribute to understanding of the formation and tectonic evolution of Tibet plateau.…”