Sedimentary and Source-to-Sink Evolution of Intracontinental Basins: Implications for tectonic and Climate Evolution in the Late Mesozoic (Southern Junggar Basin, NW China)

Sedimentary investigations, petrography, heavy mineral and conglomerate component analyses, and detrital zircon U-Pb geochronology were conducted to reconstruct the sedimentary and source-to-sink evolution of the Southern Junggar Basin, an intracontinental basin in the late Mesozoic. A paludal deltaic environment evolved into a fluvial environment, and abruptly prograded into alluvial fan and aeolian environments in the Late Jurassic, which was replaced by fan deltaic and lacustrine environments in the Early Cretaceous. Three source-to-sink systems were identified, according to different source-to-sink system features. In the northern piedmont of the Tianshan Orogenic Belt, the North Tianshan Orogenic Belt mainly provided sediments in the Late Jurassic. The North Tianshan and Central Tianshan Orogenic Belt both supplied sediments in the Early Cretaceous. In the northern piedmont of the Bogda Orogenic Belt, the Bogda Orogenic Belt was constantly the primary provenance, and the Tianshan Orogenic Belt also provided sediments. Sediment recycling occurred in the basin margin in the Late Jurassic and more metamorphic rocks were denudated in the Early Cretaceous. The source-to-sink system shrank in the Late Jurassic and expanded in the Early Cretaceous. This source-to-sink evolution and the conglomerates in the Kalazha Formation with seismite structures responded to the aridification in the Late Jurassic, the uplift of the Bogda and Tianshan Orogenic Belts in the Late Jurassic, and the exhumation of the Bogda and Tianshan Orogenic Belts in the Early Cretaceous.

The Changshagou gold deposit, Eastern Tianshan, NW China: Orogenic gold mineralization overprinting a Porphyry gold occurrence

The temporal-spatial relationships of porphyry and orogenic gold mineralization in the Eastern Tianshan Orogenic Belt are ambiguous. The newly-discovered Changshagou deposit in this belt contains both porphyry and orogenic gold mineralization, which are characterized by polymetallic-sulfide veinlets and quartz-pyrite veins, respectively. Fluid inclusions in the porphyry mineralization episode were trapped at 290–340 °C with salinities of 3.0–8.0 wt.% NaClequiv. The homogenization temperatures and salinities in the orogenic mineralization episode range from 240 to 300 °C and 1.0–5.0 wt.% NaClequiv. Coexisting V-type and L-type fluid inclusions with similar homogenization temperatures are indicative of fluid immiscibility. The δ18Ow and δDw values range from 7.6 to 9.1 ‰ and −70.9 to −84.0 ‰ in the porphyry mineralization episode, and from 6.4 to 7.1 ‰ and −65.7 to −72.1 ‰ in the orogenic mineralization episode, overlapping magmatic and metamorphic ranges, respectively. The pyrite δ34S values range from 3.5 to 4.9 ‰, falls into the magmatic range. Pyrite in porphyry and orogenic mineralization episodes yield Re-Os isotopes ages of 269.1±2.9 Ma and 257.4±2.4 Ma. The porphyry and orogenic gold mineralizations are genetically associated with the quartz syenite porphyry and Kanggur strike-slip shear activity, respectively.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5635584

Petrogenesis and Tectonic Significance of the ~276 Ma Baixintan Ni-Cu Ore-Bearing Mafic-Ultramafic Intrusion in the Eastern Tianshan Orogenic Belt, NW China

The Baixintan mafic-ultramafic intrusion in the Dananhu-Tousuquan arc of the Eastern Tianshan orogenic belt is composed of lherzolite, olivine gabbro, and gabbro. Olivine gabbros contain zircon grains with a U-Pb age of 276.8 ± 1.1 Ma, similar to the ages of other Early Permian Ni-Cu ore-bearing intrusions in the region. The alkaline-silica diagrams, AFM diagram, together with the Ni/Cu-Pd/Ir diagram, indicate that the parental magmas for the Baixintan intrusion were likely high-Mg tholeiitic basaltic in composition. The Cu/Pd ratios, the relatively depleted PGEs and the correlations between them demonstrate that the parental magmas had already experienced sulfide segregation. The lower CaO content in pyroxenites compared with the Duke Island Alaskan-type intrusion and the composition of spinels imply that Baixintan is not an Alaskan-type intrusion. By comparing the Baixintan intrusion with other specific mafic-ultramafic intrusions, this paper considers that the mantle source of the Baixintan intrusion is metasomatized by subduction slab-derived fluids’ components, which gives rise to the negative anomalies of Nb, Ti, and Ta elements. Nb/Yb-Th/Yb, Nb/Yb-TiO2/Yb, and ThN-NbN plots show that the Baixintan intrusion was emplaced in a back-arc spreading environment and may be related to a mantle plume.

Two late Carboniferous belts of Nb-enriched mafic magmatism in the Eastern Tianshan: Heterogeneous mantle sources and geodynamic implications

Identification of subduction to post-collisional tectonic transitions is critical to the study of orogenic belts. To characterize such a transition in the Tianshan Orogenic Belt, a systematic study was conducted on the late Carboniferous (305–301 Ma) Hongshankou dolerite and Dikan’er basalt of Eastern Tianshan. The Hongshankou dolerites have relatively high Ti and Nb contents, akin to Nb-enriched arc basalts. Based on the Nb/La ratios, these dolerites can be divided into low-Nb/La (0.35–0.40) and high-Nb/La (0.67–1.4) groups, which were likely derived respectively from slab melt-metasomatized mantle wedge and a mixed mantle source involving depleted super-slab and enriched sub-slab asthenospheric components. Like the low-Nb/La dolerites, the Dikan’er basalts possess low Nb/La (0.42–0.46) ratios, suggesting a mantle source previously modified by slab components. In addition, the Dikan’er basalts have variable Nb contents and can be grouped into normal arc basalts and Nb-rich basalts that can be attributed to a common mantle source with different degrees of mantle melting, as demonstrated by the positive correlations of La/Sm with La and Nb. By integrating available data, two late Carboniferous belts of Nb-enriched mafic magmatism are recognized in the Eastern Tianshan, with one in the Yamansu arc (336–301 Ma) and the other in the Bogda Mountains (305–301 Ma). The former is characterized by low Nb/La (<0.6) ratios, reflecting derivation from mantle metasomatized by slab-derived melt during a subduction process; the latter exhibits an abrupt Nb/La increase from 0.6 to 1.4, indicating significant input of sub-slab asthenospheric mantle that was probably induced by slab break off. Accordingly, we propose that the tectonic transition from subduction to post-collision in the Eastern Tianshan occurred in the latest Carboniferous (305–301 Ma) and was marked by the abrupt input of deep and enriched asthenospheric mantle.