Provenance and tectonic setting of the Triassic clastic deposits in the Napo basin, South China: evidence from petrography, whole-rock geochemistry and detrital zircon U–Pb geochronology

Abstract

The provenance and tectonic setting of the Lower–Middle Triassic clastic sediments from the Napo basin, South China, have been examined here using detrital modes, whole-rock geochemistry and detrital zircon U–Pb ages. Field investigations indicate that these sediments consist of fan delta, slope and turbidity fan facies with dominantly southward palaeocurrent directions. Detrital modes and geochemical characteristics of the clastic rocks indicate that they were derived from mixed magmatic arc and Palaeozoic successions in a continental island arc setting, with no significant sediment recycling. The U–Pb age spectra of sandstone detrital zircons from different stratigraphic positions are similar, with one major group (300–230 Ma), two subordinate groups (400–320 Ma and 480–420 Ma, respectively) and two scattered groups (1200–800 Ma and 2000–1700 Ma, respectively). Thus, we consider that the north late Permian – Middle Triassic volcanic rocks and the uplifted Palaeozoic sedimentary/volcanic sequences constituted the predominant sources. The detritus derived from the late Permian Emeishan mafic rocks is subordinate and limited. The pre-Devonian zircons are likely sedimentary-recycled or magmatic-captured instead of directly derived from the early Palaeozoic orogen (e.g. Yunkai massif) and Neoproterozoic Jiangnan orogen because of the topographic barrier of a magmatic arc and carbonate platform. Considering the spatial and temporal distribution characteristics of the volcanic arc and ophiolite, we suggest that the Triassic Napo basin was a fore-arc basin within a continental island arc setting, which developed in response to the northward subduction of the Babu–Cao Bang branch ocean beneath the South China Block.

Triassic detrital records in the western Dian‐Qiong suture, South China: Implications for the eastern Paleo‐Tethys evolution

“…In addition, samples from the northern portion (Figure 4b,c) show similar zircon age spectra to the Early–Middle Triassic successions in the southeastern Song Hien (Figure 4f, Halpin et al, 2016; Wang et al, 2021), Napo (Figure 4g, Xia et al, 2021), and northern‐central Nanpanjiang (Figure 4h, Lehrmann et al, 2015; Yang et al, 2012) basins, implying the predominant inputs from the magmatic arc. But samples from the southern portion (Figure 4a,d–e) display the decreasing or absence of G1 and extreme enrichment in G2 or G3, indicating a recycled orogenic provenance.…”

“…Probability density plots for detrital zircon U–Pb ages from the Middle Triassic turbidite sandstones in the Malipo basin (a–e), compared with those from the early Middle Triassic turbidites in the Song Hien basin, western Dian‐Qiong suture (f, Halpin et al, 2016; Wang et al, 2021), the early Middle Triassic turbidites in the Napo basin to the south of the volcanic arc (g, Xia et al, 2021) and the Middle Triassic turbidites in the central‐northern Nanpanjiang basin to the north of the Napo‐Jingxi‐Pingxiang volcanic arc (h, Lehrmann et al, 2015; Yang et al, 2012). …”

“…After the intense and short tectonothermal activity induced by the Emeishan plume ( c . 260 Ma, Fan et al, 2008; Zhou et al, 2006), this ocean started and continued to subduct northward beneath the SCB in the Late Permian–Middle Triassic, resulting in the development of the island volcanic arc and associated fore‐arc basins in Napo and Pingxiang areas (Song et al, 2013; Xia et al, 2021; Figure 7a,b). As the southward subduction and closure of Ailaoshan‐Song Ma‐Song Chay ocean (Faure et al, 2014), the NVT and Eastern Indochina block amalgamated along the Song Chay suture zone in the Early–earliest Middle Triassic ( c .…”