Geochemistry, Chronology and Genesis of Marine Basalts in Houhongquan Area, Beishan, Gansu and Inner Mongolia

The marine Permian strata developed in the Beishan area, Gansu and Inner Mongolia, is one of the youngest marine sedimentary strata in the southern margin of the Central Asian orogenic belt. To further explore the timeline of the Permian paleo-Asian Ocean closure and the later collisional orogeny, five representative sections of the Permian marine volcaniclastic-clastic strata in the Houhongquan, Beishan, Gansu and Inner Mongolia were selected for lithology-petrography and distribution characteristics studies, and petrography, elemental geochemistry and whole-rock Sr-Nd isotope studies were conducted on the basalts in the marine strata. In addition, a typical rock profile was selected for LA-ICP-MS zircon U-Pb dating. The ages of the rhyodacite and the sand-bearing bright micrite bioclastic limestone are 273±1.7 Ma and 275.8±1.4 Ma, respectively. Combined with published age data and paleontological data, it is clear that the eruption age of the volcanic rock is between the Artinskian age of Early Permian to the Ward stage of the Middle Permian. It should be determined as the Shuangbaotang Formation. The SiO₂ contents of basalt in Houhongquan area ranges from 47.48% to 54.56%, mainly basalt and basalt andesite, belonging to sodium-tholeiitic basalt. The total content of rare earth elements (ΣREE) is relatively high, ranging 74.90×10^-6-173.61×10^-6, LREE/HREE is between 2.32-6.55, showing slight enrichment of light rare earth elements relative to heavy rare earth elements, with (La/Yb)_N=1.30-6.20. On the whole, chondrite-normalized distribution curve shows slightly enriched LREE with the gentle distribution curve, relatively enriched elements such as La, Ce, Nd, Zr, Hf, and depleted Ta, Nb, P, Ti and other elements to varying degrees, δEu=0.83-1.20, no obvious Eu anomaly. (⁸⁷Sr/⁸⁶Sr)_i is 0.702 024-0.706 432, the ε_Nd(t)values between 1.99 and 6.54 (except -0.59 and -0.83). Therefore, the basalt magma in the Houhongquan area is formed by low-degree melting of the garnet peridotite, which is close to the original mantle composition. During its evolution and diagenesis, it was not strongly contaminated by the crust, but a strong contaminant occurred in the magma source area. It is speculated that the asthenospheric mantle has been modified by terrigenous sedimentary rocks, continental crust or altered oceanic crust, and lithospheric delamination may be an important mechanism for its generation. It shows that the Paleo-Asian Ocean was closed in the early period, and the rift basin was formed by further extension and rifting in the Early-Middle Permian.