Formation of the Xigaze Metamorphic Sole under the Tibetan continental margin reveals generic characteristics of the subduction initiation process

Abstract

Metamorphic soles found under allochthonous oceanic lithosphere, or ophiolites, are derived from the earliest lower plate oceanic crust accreted to upper plate mantle and are considered diagnostic of intraoceanic subduction initiation. Their metamorphic evolution is inferred to reflect the thermal structure at the site of subduction nucleation, with granulite-bearing soles linked to initiation at hot spreading centers. Here we present garnet Lu-Hf geochronology for the granulite-bearing sole of the Xigaze ophiolite in South Tibet, whose oceanic crust formed around 130 Ma through continental forearc extension. Our study shows that sole metamorphism was ongoing by 144 Ma, 14 million years before oceanic forearc spreading. The granulite-bearing sole thus formed following subduction initiation under a continental margin and not a spreading center. Our results imply that the metamorphic characteristics of soles are independent of the specific tectonic setting at the subduction nucleation site and rather provide generic constraints on the subduction initiation process.

Retentiveness of rare earth elements in garnet with implications for garnet Lu‐Hf chronology

“…With no clear exception, and regardless of rock composition, mineral assemblage or metamorphic grade, Lu‐Hf garnet ages are similar to, or older than, the accessory mineral U–Pb ages that are interpreted to date peak metamorphism (Figure 1b). The data thus do not reflect down‐skewing of Lu‐Hf ages, as predicted for garnet‐clinopyroxene assemblages (Bloch et al, 2015), and in fact can be readily interpreted to reflect the nature of garnet as a prograde mineral that typically forms earlier in the petrogenetic history of metamorphic rocks than zircon or monazite (e.g., Anczkiewicz et al, 2007; Godet et al, 2021; Guilmette et al, 2018; Guilmette et al, 2023; Smit, Hacker, & Lee, 2014) and may even retain age information on metamorphic events that accessory minerals appear to have missed (e.g., Lihter et al, 2022; Thiessen et al, 2019). Although these data generally indicate the robustness of Lu‐Hf ages, they do not exclude that Lu diffusion affects such ages in individual cases.…”

“…Observations made from natural garnet generally indicate that REE diffusion in natural garnet is relatively sluggish. Garnet that has undergone long‐lived thermal overprinting during high and ultrahigh‐temperature ( HT ; UHT ) metamorphism commonly preserves strong REE zoning (e.g., Carlson, 2012; Cutts & Smit, 2018; Guilmette et al, 2018; Guilmette et al, 2023; Pownall et al, 2019; Pyle & Spear, 2000; Rubatto et al, 2020; Smit, Ratschbacher, et al, 2014). Estimates of REE diffusivities constrained from diffusion zoning in natural garnet are lower than those determined in most experiments (Carlson, 2012) and are approximated only by REE diffusivities determined in a high‐pressure diffusion experiment (van Orman et al, 2002).…”

Detachment Fault‐Hosted Subduction Re‐Initiation of the (Ultra)Slow‐Spreading Western Neo‐Tethys in the Jurassic

“…Since transform fault‐related SI proposed by the SIR creates intraoceanic subduction and suture zones perpendicular to the ridges, it contradicts the kinematics and plate configurations of the Jurassic western Neo‐Tethys Ocean (Maffione, Thieulot, et al., 2015). On the other hand, multiple lines of evidence have also suggested that some Early Cretaceous ophiolites in the eastern Neo‐Tethys (such as the Yarlung Tsangpo ophiolites in southern Tibet) were generated in a tectonic location quite close to the continental margin (e.g., Guilmette et al., 2023; Huang et al., 2015), which disagrees with the spontaneous intraoceanic SI at transform faults or fracture zones in the SIR model. Ridge‐ and/or continental margin‐parallel SI should therefore be considered for the generation of the abovementioned Neo‐Tethyan ophiolites in the geological history of both the Jurassic (Maffione, Thieulot, et al., 2015; van Hinsbergen et al., 2015) and Early Cretaceous (Guilmette et al., 2023; C. Zhang et al., 2019).…”