Pseudosection modelling and garnet Lu–Hf geochronology of HP amphibole schists constrain the closure of an ocean basin between the northern and southern Lhasa blocks, central Tibet

Abstract

The P-T-t path of high-P metamorphic rocks in subduction zones may reveal valuable information regarding the tectonic processes along convergent plate boundaries. Herein, we present a detailed petrological, pseudosection modelling and radiometric dating study of several amphibole schists of oceanic affinity from the Lhasa Block, Tibet. The amphibole schists experienced an overall clockwise P-T path that was marked by post-P max heating-decompression and subsequent isothermal decompression following the attainment of peak high-P and low-T conditions (~490°C and 1.6 GPa). Pseudosection modelling shows that the amphibole schists underwent water-unsaturated conditions during prograde metamorphism, and the stability field of the assemblage extends to lower temperatures and higher pressures within the water-unsaturated condition relative to water-saturated model along the prograde path. The high-P amphibole schists were highly reduced during retrograde metamorphism. Precise evaluation of the ferric iron conditions determined from the different compositions of epidote inclusions in garnet and matrix epidote is crucial for a true P-T estimate by garnet isopleth thermobarometry. Lu-Hf isotope analyses on garnet size separates from a garnet-bearing amphibole schist yield four two-point garnet-whole-rock isochron ages from 228.2 AE 1.2 Ma to 224.3 AE 1.2 Ma. These Lu-Hf dates are interpreted to constrain the period of garnet growth and approximate the timing of prograde metamorphism because of the low peak metamorphic temperature of the rock and the well-preserved Mn/Lu growth zoning in garnet. The majority of zircon U-Pb dates provide no constraints on the timing of metamorphism; however, two concordant U-Pb dates of 222.4 AE 3.9 Ma and 223.3 AE 4.2 Ma were obtained from narrow and uncommon metamorphic rims. Coexistence of zircon and sphene in the samples implies that the metamorphic zircon growth was likely assisted by retrogression of rutile to sphene during exhumation. The near coincident radiometric dates of zircon U-Pb and garnet Lu-Hf indicate rapid burial and exhumation of the amphibole schists, suggesting a closure time of c. 224-223 Ma for the fossil ocean basin between the northern and southern Lhasa blocks.

Petrogenesis and metallogenic implications of Cretaceous magmatism in Central Lhasa, Tibetan Plateau: A case study from the Lunggar Fe skarn deposit and perspective review

“…The discontinuous Jurassic ophiolite along the SNMZ is regarded to have formed in a mature back‐arc basin related to the southward subduction of the Bangong–Nujiang oceanic plate beneath Northern Lhasa (M. J. Xu et al, ; Zhong et al, ). The Sumdo eclogites and amphibole schists along the eastern LMF represent the Paleo‐Tethyan oceanic subduction and continental collision between Central and Southern Lhasa during Late Permian to Middle Triassic (~270–228 Ma; D. D. Cao, Cheng, & Zhang, ; Cheng, Liu, Vervoort, & Lu, ; Xu, Dilek, et al, ; K. J. Zhang, Xia, et al, ). Central Lhasa was once a separate continent with Precambrian crystalline basement (Dong et al, ; D. G. Hu et al, ) and covered by a Permian–Carboniferous metasedimentary sequence (Leier, Kapp, Gehrels, & DeCelles, ) and Late Jurassic–Early Cretaceous volcanic‐sedimentary sequence (i.e., Zenong Group; Y. Chen et al, ; L. Y. Wang, Zheng, Yang, et al, ; T. S. Yang et al, ).…”

Post-peak metamorphic evolution of the Sumdo eclogite from the Lhasa terrane of southeast Tibet

“…= -0.135,  fit = 2.22)conditions are obtained using compositions of the garnet outer rim and the adjacent inclusions. Despite the high uncertainties and the large  fit values, the P-T conditions are consistent with the mineral stability field and the X Grs and X Pyr isopleth sections.Amphibole isopleths can be, at times, a powerful tool for inferring the P-T metamorphic path of rock(Gazley et al, 2011;Palin et al, 2014;Cao et al, 2017). The high modal proportion of amphibole (~50 %) and the sharp chemical zoning make it feasible to deduce a certain segment of the overall P-T loop through amphibole isopleths.…”