Triassic trachytic volcanism in the Bangong–Nujiang Ocean: geochemical and geochronological constraints on a continental rifting event

The Bangong–Nujiang suture zone (BNSZ), which separates the Gondwana-derived Qiangtang and Lhasa terranes, preserves limited geological records of the Bangong–Nujiang Ocean (BNO). The timing of opening of this ocean has been hotly debated due to the rare and complicated rock records in the suture zones, which span over 100 Ma from Carboniferous–Permian to Early Jurassic time, based on geological, palaeontological and palaeomagnetic data. A combination of geochemical, geochronological and isotopic data are reported for the Riasairi trachytes, central BNSZ, northern Tibet, to constrain its petrogenesis and tectonic settings. Zircon U–Pb dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yields mean ages of 236 Ma. Geochemically, these rocks are high-K calc-alkaline with moderate SiO2 (59.1–67.5 wt%) and high K2O + Na2O (8.1–11.6 wt%) contents. They are enriched in light rare earth elements with negative Eu anomalies, and show enrichments in high-field-strength elements with positive ‘Nb, Ta’ anomalies, similar to the intra-continental rift setting-related felsic lavas from the African Rift System. The high positive zircon ϵHf(t) and bulk ϵNd(t) values, as well as high initial Pb isotopes, imply a heterogeneous source involving both asthenospheric and subcontinental lithospheric mantle. The field and geochemical data jointly suggest that the Riasairi trachytes within the Mugagangri Group were formed in a continental rift setting. We interpret that the continental-rift-related Riaisairi trachytic lavas as derived from the southern margin of the Qiangtang terrane, implying that the BNO would have opened by Middle Triassic time, well after the commonly interpreted break-up of the Qiangtang terrane from Gondwana.

Geology and depth structures of the main Karatau strike-slip fault, Southern Kazakhstan

The Main Karatau fault is a classical crustal strike-slip zone. It originated as a continental rift structure in the Late Proterozoic and had been developed incessantly for almost 1 billion years as inherited structure. The fault was subjected to polyphase deformations associated with both dextral and sinistral shifts. The Main Karatau fault crosses the Earth’s crust, including the structures of granite-metamorphic layer and granulite-basitic layer and fades without crossing the Moho discontunious. The amplitude of displacement of the Syr-Daria and Chu-Sarysu blocks relative to each other along the Main Karatau fault is estimated at approximately 200 km.

Late Permian ultrapotassic rhyolites in SE Thailand: Evidence for a Paleotethyan continental rift basin

Volcanic rocks in the Chanthaburi zone are rarely reported and important for investigating the tectonic evolution of Paleotethyan Ocean in SE Thailand. Four rhyolitic samples from the Ko Chang Island yield zircon ages of 254–258 Ma, confirming the presence of Late Permian volcanic rocks in SE Thailand. These rocks consist of Group 1 rhyolites and Group 2 rhyolitic ignimbrites and have high K2O contents of 4.92–7.10 wt.% and A/CNK values of 1.10–1.69. They are enriched in LREEs, Rb, Th, U, Zr and Y, and show negative anomalies of Ba, Sr, Nb, Ta and Ti with obvious Eu anomalies. Their whole-rock εNd (t) values range from −1.7 to −3.1. Zircon in-situ εHf (t) and δ18O values range from 0.0 to +5.6 and 8.2‰ to 9.6‰, respectively. They belong to peraluminous, ultrapotassic A-type rhyolites, and were derived from partial melting of a mixed source of Mesoproterozoic metasedimentary rocks with a component of juvenile mafic crust. These ultrapotassic rhyolites formed in a continental rift setting in response to the rollback of subducted Paleotethyan oceanic slab beneath the Indochina Block. Combining previous geological observations, we propose that there are some sporadically distributed continental rift basins along the Eastern Paleotethyan domain during the Permian.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5635390

Petrogenesis and tectonic significance of the middle Neoproterozoic highly fractionated A-type granite in the South Qinling block

The South Qinling block, a segment of the Yangtze craton involved in the Qinling–Dabie orogen, is critical for understanding the tectonic evolution of eastern China. However, the tectonic setting of the South Qinling block and the northern margin of the Yangtze block during middle Neoproterozoic time has long been the subject of debate, with two distinctly different models (continental rift or volcanic arc) proposed. Here, a comprehensive study of zircon U–Pb geochronology and geochemistry has been carried out on the Chengwan granitic pluton from the Suizao terrane in the South Qinling block. The granites are monzogranite and syenogranite in lithology, and are mainly composed of potash feldspar, quartz, plagioclase and biotite. This suite has long been regarded as a Palaeozoic magmatic pluton, but zircon U–Pb ages of 809 ± 9 Ma and 816 ± 4 Ma are obtained in this study. The granites are metaluminous to strongly peraluminous with high alkali contents, and exhibit highly fractionated features, including high SiO2, low Zr/Hf ratios, rare earth element tetrad effects and enrichment of K and Rb. They show Hf–Nd isotopic decoupling, which may be genetically related to their petrogenetic process. Based on the geochemical features and the positive εHf(t) values of the zircons, it is indicated that the granites may have been derived from partial melting of juvenile tonalitic rocks by biotite breakdown under fluid-absent conditions. The Chengwan granite geochemically belongs to the A2-subtype granites, suggesting that it might have formed in a post-orogenic tectonic setting. The highly fractionated A-type granite in this study may represent extensional collapse shortly after the collisional events in the South Qinling block, and thus indicate a tectonic regime switch, from compression to extension, as early as middle Neoproterozoic time. Integrating our new data with documented magmatic, metamorphic and sedimentary events during middle Neoproterozoic time in the region may support a continental rift model, and argues against arc models.