Madagascar is one of the world's largest islands, separated from the Africa continent by the Mozambique Channel (Figure 1a). The main water divide of Madagascar follows the longitudinal axis of the island and separates drainages of the Indian Ocean from those of the Mozambique Channel (Figure 1b), although the topography is strongly asymmetric with eastward-flowing rivers shorter (<150 km) than westward flowing rivers (>300 km). Madagascar was at the center of the Gondwana supercontinent and was surrounded by the ancient Africa continent and the continent of the Seychelles-India (Wit, 2003). The rifting between Madagascar and Seychelles-India started between 120 and 92 Ma based on ages of basaltic intrusions and dikes at the eastern margin of Madagascar (Melluso et al., 2005;Torsvik et al., 1998). Final separation is dated to the late Cretaceous from volcanic provinces and the oldest seafloor magnetic anomaly in Indian Ocean, Chron 34 (∼84 Ma) (Eagles & Hoang, 2014), and is limited to being older than the Deccan volcanic province eruption at the western margin of the India peninsula (∼65 Ma) (Collier et al., 2008). Rifting between Madagascar and Seychelles-India has formed the paired mountain ranges along the coast at the conjugate margins of eastern Madagascar and western India (Gunnell & Harbor, 2008).Unlike the west margin where the central high plateau gradually flattens into the coastal plain, the east margin is characterized by an escarpment where the topography abruptly increases from the coastal plain, rapidly rising to the high flat plateau, with the water divide sitting at or near the eastern margin of the highlands. The topography of the conjugate margin of western India, the Western Ghats, is similar and is well-recognized as a great escarpment (Gunnell & Harbor, 2008Mandal et al., 2015), but Madagascar has not received the same attention, even though the topography exhibits the same strong asymmetry.Although there is no major post-rift tectonics on Madagascar, there is evidence for Cenozoic uplift. A major erosional unconformity from Oligocene to Miocene (30-16 Ma) is revealed in offshore wells on the western continental margin (Delaunay, 2018). This unconformity is likely regional given that Oligocene sediment is also

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The Huangsha uranium mining area is located in the Qingzhangshan uranium-bearing complex granite of the Middle Nanling Range, Southeast China. This uranium mining area contains three uranium deposits (Liangsanzhai, Egongtang, and Shangjiao) and multiple uranium occurrences, showing favorable mineralization conditions and prospecting potential for uranium mineral resources. Chloritization is one of the most important alteration types and prospecting indicators in this mining area. This study aims to unravel the formation environment of chlorites and the relationship between chloritization and uranium mineralization, based on detailed field work and petrographic studies of the wallrock and ore samples from the Huangsha uranium mining area. An electron probe microanalyzer (EPMA) was used in this study to analyze the paragenetic association, morphology, and chemical compositions of chlorite, to classify chemical types and to calculate formation temperatures and n(Al)/n(Al + Mg + Fe) values of chlorite. The formation mechanism and the relationship with uranium mineralization of the uranium mining area are presented. Some conclusions from this study are: (1) There are five types of chlorites, including the chlorite formed by the alteration of biotite (type-I), by the metasomatism of feldspar with Fe–Mg hydrothermal fluids (type-II), chlorite vein/veinlet filling in fissures (type-III), chlorite closely associated with uranium minerals (type-IV), and chlorite transformed from clay minerals by adsorbing Mg- and Fe-components (type-V). (2) The chlorite in the Huangsha uranium mining area belongs to iron-rich chlorite and is mainly composed of chamosite, partly clinochlore, which are the products of multiple stages of hydrothermal action. The original rocks are derived from argillite, and their formation temperatures vary from 195.7 °C to 283.0 °C, with an average of 233.2 °C, suggesting they formed under a medium to low temperature conditions. (3) The chlorites were formed under reducing conditions with low oxygen fugacity and relatively high sulfur fugacity through two formation mechanisms: dissolution–precipitation and dissolution–migration–precipitation; (4) The chloritization provided the required environment for uranium mineralization, and promoted the activation, migration, and deposition of uranium.

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Middle Neoproterozoic sedimentary strata are widely distributed on the periphery of the Yangtze Block. In the western Jiangnan Orogen, they are divided into the Lengjiaxi and Banxi groups by the “Wuling unconformity,” and the Banxi Group is further divided into the Madiyi Formation and Wuqiangxi Formation by the “Xihuangshan unconformity.” However, the timing and tectonic significance of the Wuling and Xihuangshan unconformities remain unclear, which hampers our understanding of the Precambrian tectonic evolution of the Yangtze Block. Zircon U–Pb dating and Lu–Hf isotopic analysis were performed on the sedimentary rocks above and below the two unconformity boundaries in the western Jiangnan Orogen. These data were used to trace sedimentary provenance and provide new insights into the basin evolution and tectonic significance of the unconformities. Combined with previous studies, the Wuling unconformity is bracketed to have formed between ~830 and 813 Ma, and the provenance of the sediments above the unconformity remained unchanged. The detrital zircons from the upper parts of the Lengjiaxi Group and lower parts of the Banxi Group show the primary peak ages of 800–1000 Ma, 1.0–1.30 Ga, 1.40–1.90 Ga, and 2.30–2.60 Ga, and the provenance mainly derived from the southwestern margin of the Yangtze Block, Cathaysia Block, and Jiangnan Orogen. The provenance from the Cathaysia Block occurred in the upper part of the Lengjiaxi Group, indicating that the Yangtze Block and Cathaysia Block merged in the western Jiangnan Orogen earlier than the formation time of the Wuling unconformity (~830–813 Ma) and the collisional time in the eastern Jiangnan Orogen (~820–800 Ma). Thus, the collision between the Yangtze and Cathaysia blocks may have undergone a scissor-like closure process from west to east. The formation time of the Xihuangshan unconformity was at ~800–779 Ma. The field contact relationships changed from an angular unconformity to a disconformity and then to conformity, from north to south, indicating that the Xihuangshan unconformity was controlled by tectonic movement in the north. The provenance of the sedimentary strata changed above the Xihuangshan unconformity. The detrital zircon age peaks of the upper Banxi Group are 755–1000 Ma, 1.90–2.10 Ga, and 2.35–2.70 Ga, and the detritus were derived from the northern margin of the Yangtze Block and the Jiangnan Orogen. This unconformity is coeval with that of the ~800–780 Ma collisional orogeny at the northern and northwestern margins of the Yangtze Block. Thus, the Xihuangshan unconformity is likely a response to the collision orogeny in the northern and northwestern margins of the Yangtze Block and induces the transition of sedimentary provenance.