Mud volcanoes and diapirs are geological structures formed due to arch piercing or diapiric intrusion of ductile sedimentary materials into the overlying strata along high permeability channels. A detailed study on the processes controlling the formation of mud volcanoes and diapirs in the northern continental margin of the South China Sea is of vital importance to the exploration of economically viable oil and gas reservoirs and can be helpful to the exploration of natural gas hydrate in a sedimentary basin. The fluid seepage structures that occur in the Mesozoic and Cenozoic sedimentary basins of the northern South China Sea show significant differences in their morphological and tectono-structural characteristics. We used high-resolution seismic profiles and instantaneous frequency profiles to understand the mechanisms that are critical with respect to the differential development of the investigated piercement structures. Differences in stress field do not directly lead to the difference in the scale of mud volcanoes or diapirs. Fractures may play an important role in the formation of mud volcanoes and diapirs. The thickness of the sediment was found to have a strong impact on the formation of fluid leakage structures that thicker sediments are more conducive to the development of mud diapirs and the thinner one is more likely to form mud volcanoes.

The timing of the Lhasa–Qiangtang collision following the closure of the Bangong–Nujiang Tethys Ocean has not been well constrained. An integrated study of whole-rock geochemistry and zircon U–Pb–Hf isotopes was carried out for Early Cretaceous quartz diorite-porphyrites and granites from the Yilashan and Amdo areas, northern Tibet. LA–ICP–MS zircon U–Pb dating reveal that the Yilashan and Amdo granitoids were emplaced at ~121–110 Ma. These granitic rocks display selective enrichment of light rare earth elements, large ion lithophile elements (e.g., Rb, U) and Th, but depletion of Sr and high field strength elements (e.g., Nb, Ta, Ti) compared to its neighboring elements. These new data, combined with regional geological setting, show that these igneous rocks were formed under a geodynamic setting of the Lhasa and Qiangtang (–Amdo) collision with oceanic slab breakoff and asthenospheric upwelling. The BNTO had been closed at ~121–110 Ma in the study area. Yilashan-Amdo granitoids roughly yield high (⁸⁷Sr/⁸⁶Sr)_i ratios and obvious negative ε_Nd(t) and zircon ε_Hf(t) values along with old Nd T_DM and zircon Hf T_DM2 ages. Together with their variable U–Pb ages, these features indicate a Precambrian “hidden” crustal source beneath the northern Lhasa and Amdo terranes. The YLSS S-type granophyres were derived from partial melting of Paleoproterozoic lower crustal metagraywackes, whereas the YLSZ quartz diorite–porphyrites and the Amdo I- and A-type granites were mainly derived from partial melting of Paleo–Mesoproterozoic lower crustal mafic rocks with a certain amount of addition of mantle-derived melts. Minor amounts of the materials originated from the Amdo orthogneisses may also be involved in the formation of the YLSZ quartz diorite–porphyrites and the Amdo I-type granites. In addition, the Yilashan ophiolite was intruded by the ~112–108 Ma granophyric and quartz diorite–porphyritic intrusions before its final emplacement into the surrounding strata.

Post-collisional K-rich magmatic rocks of the Qinghai–Tibet Plateau provide important information for understanding the continental collision and uplift of this plateau during the Cenozoic. However, the source of these K-rich rocks is still debated, and limited investigations have been conducted in the western Kunlun Mountains (WKM) in the northwestern margin of the plateau. In this paper, we present geochemical, geochronological, and Sr–Nd–Hf isotope data for Quanshuigou post-collisional K-rich igneous rocks from the WKM. Geochronological results show that these rocks were emplaced during two stages: (1) ca. 5.3 Ma, as an intrusive suite of granite porphyries; and (2) ca. 4.8 Ma, as a suite of shoshonites. These rocks are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), and depleted in heavy rare earth elements (HREEs) and high-field-strength elements (HFSEs). Compositions of the granite porphyries classify these rocks as high-K calc-alkaline peraluminous A-type granites. Combining the negative zircon ε _Hf(t) values (−6.9 to −1.1) with Hf crustal model ages (T_DM2) of 1.16–1.53 Ga and negative ε _Nd(t) values (−5.31 to −4.96) with two-stage Nd model ages (T_DM2) of 1.22–1.46 Ga, these signatures suggest that the granite porphyries most probably originated from partial melting of thickened lower crust with limited input of mantle-derived magmas. In contrast, the shoshonites have low SiO₂ contents (50.84–53.94 wt.%), relatively high Mg^# values (50–57), and ε_Nd (t) values ranging from −6.18 to −5.34, with single-stage Nd model ages (T_DM1) of 0.97–1.02 Ga. Our analyses show that the parental magma of the shoshonites probably formed by partial melting of EMII-type lithospheric mantle, with the addition of subducted and recycled oceanic sediments. Hence, we conclude that these granite porphyries and shoshonites formed in a post-collisional, extension-related geodynamic setting and that the melting was probably triggered by motion on the Altyn Tagh strike-slip fault system. GRAPHICAL ABSTRACT

Cenozoic alkali-rich porphyries are widely distributed in the junction zone between the Sanjiang Orogenic belt and the Yangtze Plate. They are of great significance for understanding the regional geodynamics, tectonic evolution, and metallogenesis. However, the origin of these porphyries remains controversial. In this study, new petrological, geochemical, and geochronological data are presented for Cenozoic syenite porphyry from the Beiya porphyry Au-polymetallic deposit in western Yunnan. Zircon U-Pb dating results show that the Beiya syenite porphyries formed around 36.3–35.0 Ma, coinciding with the magmatic peak in the Jinshajiang-Red River (JSJ-RR) alkali-rich porphyry belt. Geochemical analyses indicate that the Beiya porphyries have potassic characteristics and an arc-like geochemical affinity, with C-type adakite affinity, suggesting a post-collisional setting. The JSJ-RR fault zone is unlikely to be the primary mechanism responsible for the formation of this alkali-rich porphyry magmatism. Instead, the development of the Beiya alkali-rich porphyries is likely associated with the convective removal of the lower part of the overthickened lithospheric mantle and asthenospheric upwelling during the Eocene–Oligocene. Their magmas probably originated from the partial melting of Paleo–Mesoproterozoic garnet amphibolite facies rocks in the thickened lower continental crust, with the addition of shoshonitic mafic magmas produced by the partial melting of metasomatized lithospheric mantle triggered by asthenospheric upwelling. This study provides additional reliable evidence to further constrain the origin of Cenozoic alkali-rich porphyries in the JSJ-RR belt.