Reconciling Orogenic Drivers for the Evolution of the Bangong‐Nujiang Tethys During Middle‐Late Jurassic

Abstract

Identifying arc-trench systems along with spatial and temporal variations in their record of tectono-magmatic events is crucial for determining the orogenic divers and evolution of orogenic systems. New geochronological and geochemical data of Jurassic igneous rocks, as well as detrital zircon data from contemporaneous sedimentary units, within the eastern Bangong-Nujiang suture in central Tibet indicate the existence of an approximately 1,200-km Middle-Late Jurassic magmatic arc system. This arc system can be divided into two distinct along-strike segments, which are characterized by magmatic activity extending from 166 to 160 Ma in the east and 170-148 Ma in the west, followed by magmatic gaps at 160-120 and 148-125 Ma, respectively. An accretionary prism, magmatic arc, and retro-arc sedimentary units are identified from south to north in the eastern segment. The 166-160 Ma arc includes high-K calc-alkaline granitoids, and high-Mg andesites, dacites, and rhyolites, which collectively can be interpreted to originate from partial melting of ancient lower crust and mélange diapirs above a north dipping subduction zone. Our analysis reveals the existence of an overall compressional arc-trench system along strike, which overlaps with a phase of 170-160 Ma ophiolite generation and a rock association of 160-148 Ma slab-derived adakites and oceanic island basalt-type rocks, and is followed by an overall magmatic gap during 148-125 Ma with subsequent 125-105 Ma extensive magmatism. We infer that these records may reflect sequential tectonic events, including subparallel ridge-trench collision (170-160 Ma), slab window formation (160-148 Ma), subsequent subduction termination (148-125 Ma), and final Lhasa-Qiangtang amalgamation (125-105 Ma).

Mesozoic Subduction Accretion History in Central Tibet Constrained From Provenance Analysis of the Mugagangri Subduction Complex in the Bangong‐Nujiang Suture Zone

“…Both detrital composition and detrital zircon spectra of the Group 1 sandstones are similar to those of the Sewa Formation in the southern Qiangtang basin (Figures 7 and 9), suggesting they may have been deposited in the same approximately southward palaeodrainage system from shallow marine (Sewa Formation) to deep marine (Group 1) settings. The Group 1 sandstone may be correlated with the Dongqiao‐Amdo mélange ~180 km along‐strike to the east but is distinct with the Beila‐Naqu mélange to the south of the Dongqiao‐Amdo mélange, which includes local granitoids and volcanic rocks (S. M. Li et al, 2020; Figures 1 and 9).…”

“…Li et al (2014); and Zhu et al (2011); the Mugagangri Complex data set is from Huang et al (2017); C. Li, Wang, et al (2019); S. Li, Ding, et al (2017); S.M. Li et al (2020); and Zeng et al (2016); the upper unit (Tma) data set in Gaize is from S. Li, Ding, et al (2017). The (interpreted) depositional age interval of each sample is shown.…”

“…The Mugagangri Complex was first established in the Nima area (Figure 1b) as the Mugagangri Group and described as Middle Jurassic flysch (Wen, 1979). Recent studies show that the Mugagangri Complex consists of coherent sedimentary sequences of Qiangtang affinity and siliciclastic mélange that can be divided into several units, including the Early Cretaceous Zhaguo Formation, several Triassic to Jurassic subunits, the Middle Jurassic Gamulong Formation, Dongqiao‐Amdo mélange, Daru mélange, and Beila‐Naqu mélange (Fan, Li, Liu, et al, 2015; Huang et al, 2017; S. Li et al, 2017; C. Li, Wang, et al, 2019; S. M. Li et al, 2020; Sun et al, 2019; Zeng et al, 2016). The shallow marine Shamuluo Formation (Figure 2) is structurally juxtaposed against the Mugagangri Complex in the Najiangco area; it is composed of sandstone, shale, and limestone and was constrained to be Oxfordian to Kimmeridgian with recycled orogen‐derived provenance from the Qiangtang interior (Ma et al, 2018).…”

“…The Mugagangri Complex is exposed along a >1,500‐km‐long east‐west belt and consists of mainly siliciclastic‐matrix mélange, strongly dismembered formations, and coherent turbiditic sandstone sequences along the Bangong‐Nujiang suture zone in central Tibet (Figure 1a). Previous studies in the Gaize (Fan, Li, Liu, et al, 2015; Huang et al, 2017; S. Li, Ding, et al, 2017; Zeng et al, 2016), Nima (C. Li, Wang, et al, 2019), and Dongqiao (S. M. Li et al, 2020) areas (Figure 1a) concluded that all sandstones in the Mugagangri Complex were sourced from the Qiangtang terrane (Fan, Li, Liu, et al, 2015; Huang et al, 2017; S. Li, Ding, et al, 2017; Zeng et al, 2016). However, the tectonic setting in which clastic sedimentary rocks of the Mugagangri Complex were deposited is debated.…”

“…However, the tectonic setting in which clastic sedimentary rocks of the Mugagangri Complex were deposited is debated. Some authors interpreted rocks in the Mugagangri Complex mainly as disrupted forearc basin and accretionary prism rocks related to the northward subduction of the Bangong‐Nujiang Tethyan oceanic lithosphere beneath the Qiangtang terrane during the Late Triassic to Jurassic, or even as late as the Early Cretaceous (Fan, Li, Liu, et al, 2015; Huang et al, 2017; S. Li, Ding, et al, 2017; C. Li, Wang, et al, 2019; S. M. Li et al, 2020; Zeng et al, 2016). Additionally, some authors interpreted the Mugagangri Complex to include abyssal plain strata that were deposited along the passive continental margin of the Qiangtang Terrane during the Late Triassic (S. Li, Ding, et al, 2017; Zeng et al, 2016).…”