Construction of P–T–t paths for eclogite in the Tongbai orogen by combining phase equilibria modelling with zircon inclusion composition

Abstract

The interpretation of metamorphic ages for zircon domains in eclogite remains challenging because of the uncertainties in interpreting their rare earth element (REE) patterns and mineral inclusions. This has seriously hindered the construction of robust pressure (P)–temperature (T)–time (t) paths for eclogites from collisional orogens. In this study, a composite approach is used to obtain a reliable P–T path for eclogite from the Tongbai orogen and quantitatively attribute zircon U–Pb ages to specific P–T intervals. An integrated study using isochemical phase diagram section (pseudosection) modelling, multi‐equilibrium thermobarometry, and the Zr‐in‐rutile thermometry with petrographic observations indicates that the target eclogite experienced multistage metamorphic evolution. The evolution is characterized by a clockwise P–T path with compressional heating for peak eclogite facies metamorphism at 588 ± 20°C and 26.7 ± 2.8 kbar and two stages of isothermal decompression from an early stage of amphibole eclogite facies metamorphism at 15–10 kbar to a late stage of amphibolite facies metamorphism at pressures of <10 kbar. Metamorphic zircon domains in eclogite and quartz vein show relatively flat patterns of heavy REE without notable Eu anomalies but different cathodoluminescence responses and middle REE (MREE) contents. This indicates their growth in different stages of eclogite facies metamorphism, which are categorized into two groups. The first group is composed of bright rims or homogenous grains that are characterized by low MREE contents, suggesting the existence of abundant amphibole during the metamorphism. The compositional isopleths of amphibole inclusions indicate that this group of zircon domains would form by hydration during crustal exhumation to amphibole eclogite facies at 240 ± 2 Ma. The second group consists of dark sector mantles and shows high MREE contents, indicating the prograde to peak metamorphism with significant decomposition of amphibole or lawsonite during crustal subduction to eclogite facies at 245 ± 2 Ma. Therefore, the combination of the isopleth thermobarometry of mineral inclusions with the Zr‐in‐rutile thermometry and the zircon REE partitioning is an effective means to link the metamorphic P–T conditions to the ages of metamorphic zircons in the different stages of collisional orogeny.

Significance of selective crystal entrainment and differential crystal‐melt separation in petrogenesis of granites from the Tongbai orogen

“…The southern belt represents the subducted northern margin of the SCB (Figure 1b) and is composed of the blueschist-greenschist zone, the southern high-pressure (HP) eclogite zone, the Tongbai Complex, the northern HP eclogite zone and the Balifan tectonic mélange from south to north, which mainly exposes Triassic metamorphic rocks and Cretaceous granite with sporadic Precambrian basement (Cheng et al, 2011;Hu et al, 2012;Liu et al, 2008Liu et al, , 2010Zhang, Gao, Chen, et al, 2020;Zhang et al, 2021;Zhou et al, 2020). The northern belt consists of the Kuangping Group, the Erlangping Group, the Qinling Group, the Guishan Complex and the Nanwan flysch (Figure 1b; Liu et al, 2011Liu et al, , 2013, which represents metamorphosed accretionary materials on the southern margin of the NCB, arc or back arc-related rocks, a microcontinent, a mixture of the Qinling microcontinent and forearc flysch, and a forearc flysch formation, respectively (Hacker et al, 2004;Liu et al, 2011Liu et al, , 2013Liu et al, , 2015Meng & Zhang, 2000;Okay et al, 1993;Ouyang & Zhang, 1996;Ratschbacher et al, 2003Ratschbacher et al, , 2006Wu & Zheng, 2013;Xue et al, 1996;Zhai et al, 1998).…”

Thermal Structure of the Paleo‐Continental Subduction Zone: Insights From Quantitatively Constrained Prograde P–T Paths of Exhumed LT/UHP Eclogites in the Dabie Orogen

“…The lower P – T conditions constrained by the stability of the mineral assemblage included in garnet cores than by isopleths of garnet cores are commonly found in natural eclogites (e.g., López‐Carmona et al., 2014; R. Y. Zhang et al., 2012; Q.‐Q. Zhang et al., 2021), suggesting that either disequilibrium growth or compositional resetting of garnet grains.…”