Characteristics And Genesis Of The Tianzishan Pluton In The Liziyuan-Taiyangsi Area,Gansu Province

The Tianzishan pluton,located in the eastern part of the western Qinling orogenic belt,is a representative acid intrusion in this belt.A detailed study on the Tianzishan pluton would give a clue to better understand the genesis and tectonic setting of the Indosinian magmatic rocks in the western Qinling orogenic belt.A detailed petrology,chronology and geochemistry study has been carried out on the Tianzishan pluton,with focus on an analysis of petrogenesis and tectonic setting.LA-ICP-MS zircon U-Pb dating yields ages of 241±1.7 Ma for the Tianzishan medium-to fine-grained monzogranites,indicating the Tianzishan pluton is the product of early Indosinian magmatism in the western Qinling orogenic belt.Rocks of the Tianzishan pluton are mainly composed of medium-to fine-grained monzogranites and minor porphyritic monzogranites.The medium-to fine-grained monzogranites consist of quartz?20%–25%?,plagioclase?30%–35%?,K-feldspar?20%–25%?,biotite?10%–15%?,and locally hornblende.The porphyritic monzogranites are composed of quartz?20–25%?,plagioclase?30%–35%?,K-feldspar?20%–25%?,hornblende?10%–15%?,and locally biotite.The Tianzishan monzogranites show relatively low Ce(30.8×10^-652.1×10^-6)and Zr(60.1×10^-6245×10^-6)content,and Ga/Al ratio,indicative of I-type granitic characteristics.The monzogranites also show high Sr content,but low Y(4.37×10^-65.97×10^-6)and Yb(0.52×10^-60.66×10^-6)contents,indicative of apparent adakitic characteristics.Meanwhile,the rocks exhibit high SiO₂?68.35%⁷4.63%?,Na₂O+K₂O?7.73%¹1.05%?,and K₂O?3.74%⁶.64%?contents,and are enriched large-ion lithophile elements?e.g.Rb,K,Ba,and Sr?,and depleted in high-field-strength elements?e.g.Nb,Ta?.They are strongly enriched in LREEs and relatively depleted in HREEs?LREE/HREE=16.68²4.64;?La/Yb?_N=23.19⁴1.88?,with negligible to slightly positive Eu anomalies??Eu=0.74².04,in an average of 1.33?.These features indicate that the residual phases include garnet but no plagioclase.Detailed petrologic and geochemical data indicate that the Tianzishan monzogranites were formed from partial melting of the thick lower crust rocks under the post-collision extension setting.