New Zircon U-Pb Age of Tuff in the Third Section of the Tuchengzi Formation: Constraints on the JurassicCretaceous Boundary of Continental Strata in Western Liaoning

Objective Western Liaoning has a large area of Mesozoic continental strata,which is unique for the study of Mesozoic continental strata in China and even all over the world.The Jurassic-Cretaceous boundary of Mesozoic continental strata in Western Liaoning has been controversial since 2013 when the Jurassic-Cretaceous boundary at 145 Ma was established in the international stratigraphic table.The Tuchengzi Formation is a key Mesozoic stratum in Western Liaoning,which is widely distributed and has a clear top and bottom.

Triassic Granitic Magmatism at the Northern Margin of the North China Craton: Implications of Geochronology and Geochemistry for the Tectonic Evolution of the Central Asian Orogenic Belt

The early Mesozoic marked an important transition from collisional orogeny to post-orogenic extension at the northern margin of the North China Craton(NCC). In this study, we undertook zircon U-Pb dating and whole-rock majorand trace-element geochemical analyses of early Mesozoic granitic rocks in the Chifeng area to establish their geochronological framework, petrogenesis, and implications for the tectonic evolution of the eastern Central Asia Orogenic Belt(CAOB). Zircon U-Pb dating results show that these rocks were emplaced in three stages during the Triassic:(1) syenogranites during 250-248 Ma,(2) granodiorites during 244-243 Ma, and(3) monzogranites and granodiorites during 232-230 Ma. These Triassic granitoids belong to the high-K calc-alkaline series and are evolved I-type granites. They have high SiO2 and low Mg O contents with enrichments in light rare-earth elements, Zr, Hf, Rb, Th, and U, and depletions in Ba, Nb, Ta, Sr, and Eu. These geochemical data indicate that the granitoids were derived from partial melting of a lower-crustal source under relatively low-pressure conditions and subsequently underwent extensive fractional crystallization. Considering both the geochemical data and regional geological information, we propose that the 250-248 Ma syenogranites were emplaced in an extensional environment linked to slab break-off after closure of the Paleo-Asian Ocean(PAO) along the Solonker-Xra Moron-Changchun suture zone. The 244-243 Ma granodiorites were formed in a compressional orogenic setting during collision between the Erguna-Xing’an-Songliao composite block and the NCC. The 232-230 Ma granodiorites and monzogranites were emplaced during the transition from compressional orogeny to post-orogenic extension. Overall, the early Mesozoic tectonic evolution of the Chifeng area can be divided into three main stages:(1) closure of the Paleo-Asian Ocean and extension related to slab break-off during the Early Triassic;(2) continuous collisional compression during the Middle Triassic after closure of the PAO;and(3) post-orogenic extension during the Late Triassic, most probably due to lithospheric delamination after amalgamation of the Erguna-Xing’an-Songliao composite block and the NCC.

Uprising period and elevation of the Wenyu granitic pluton in the Xiaoqinling District, Central China

Xiaoqinling District is an important gold-producing area in China.It ranks second to Jiaodong with regard to gold deposits.The uprising period of the Wenyu granitic pluton and the wall-rocks of the deposit,as well as the mineralizing depth and reserved place of gold ore bodies,are significant to ore exploration.Fission-track(FT)analysis of zircons and apatites of granitic rocks from the Wenyu granitic pluton shows that apatite FT(AFT)data modeling indicates a rapid cooling rate of 20°C/Ma from 138 to120 Ma after emplacement at 138 Ma.Thermal evolution and inversion curves suggest a secondary phase of fast cooling and uprising from 45 to 35 Ma,and 35 Ma,respectively,with a cooling rate of 6.7°C/Ma and a denudation quantity of^4.3 km.The last cooling phase took place from<4 Ma,with an average cooling rate of^11.3°C/Ma and a denudation amount of 1.3 km.Total exhumation quantity of 5.6 km and uprising elevation of 7.3 km are similar to the estimated results of fluid inclusions from the Dongtongyu and Wenyu gold deposits.The39Ar/40Ar dating of sericite from the fault planes of the Xunmadao-Xiaohe and Taiyao faults demonstrate two uprising activities of the ore-host metamorphic complex.The Huashan and Wenyu granitic plutons intensively occurred during 77 and 45 Ma,respectively.These data sets are valuable for understanding the uplifting process and for preserving gold ore bodies in the Xiaoqinling area,as well as for further studies on tectonic evolutions of the Taihua Complex and the Qinling-Dabie Orogen.