The Dafosi granites from the Zoulang Transition Zone are weakly peraluminous (A/CNK =1.03 ～ 1.06),with high SiO2 (76.7％ ～ 78.9％),Na2O + K2O (7.7％ ～ 8.3％),Rb (303 × 10-6 ～ 383 × 10-6),Nb (32 × 10-6 ～ 42 × 10-6) and heavy rare earth element (Yb ～8 × 10-6) contents,low MgO (～0.1％) and CaO (0.5％ ～0.6％) contents,high FeOT/MgO (6.3 ～7.6),Ga/A1 (＞3 × 10-4) and Rb/Ba (3.0 ～6.2) ratios,as well as large Ba,Sr,Eu and Ti depletions.The Dafosi pluton belongs to A-type granite,and was likely derived from a pelitic source with magma temperature above ～820℃.Zircon δ18O,εHf(t) and two-stage depleted mantle Hf model ages of the Dafosi granites are 7.8‰ ～8.6‰ (ave.8.24 ± 0.13‰),-4.8 ～-2.0,and 1540 ～ 1717Ma,respectively.The Jinfosi granites from the northern margin of the North Qilian Orogen are also peraluminous with A/CNK about 1.0 ～ 1.1.They have large variations of SiO2 (65.5％ ～75.0％),MgO (0.6％ ～2.2％),Fe2O3 (1.9％ ～5.2％) and TiO2(0.3％ ～ 0.8％) abundances.They resemble the Chaidanuo granites from the North Qilian Orogen in major and trace elements,and were likely derived by partial melting of greywacke and amphibolite with magma temperature about 800 ～900℃.Zircon δ18O,εHf (t) and two-stage depleted mantle Hf model ages of the Jinfosi granites are 7.4‰ ～ 9.7‰ (ave.8.03 ± 0.36‰),-0.5 ～ + 1.9,and 1289 ～ 1439Ma,respectively.Zircon U-Pb SIMS dating of the Dafosi granite pluton from the Zoulang Transition Zone and the Jinfosi granite pluton from the northern margin of the North Qilian Orogen yielded ages of 426.1 ± 2.8Ma and 424.0 ± 1.6Ma,respectively.The occurrence of coeval magmatic rocks including A-type granite in the adjacent tectonic units indicates that by ～425 Ma the Noah Qilian Ocean was already closed and the region was under an extensional regime of post-collisional stage.%走廊过渡带大佛寺花岗岩为弱过铝质(A/CNK=1.03～ 1.06),SiO2 (76.7％ ～78.9％)、全碱(Na2O+ K2O=7.7％～ 8.3％) 、Rb(303 × 10-6～ 383 × 10-6)、Nb(32×10-6 ～ 42×10-6)、重稀土(Yb ～8×10-6)含量以及和FeOT/MgO(6.3～7.6) 、Ga/Al(＞3×10-4) 、Rb/Ba(3.0 ～6.2)比值较高,MgO(～0.1％)、CaO(0.5％～0.6％)含量较低,Ba、Sr、Eu、Ti强烈亏损,属A型花岗岩,其源岩可能为泥质岩.大佛寺花岗岩中锆石δ18O和εHf(t)值分别为7.8‰～8.6‰(平均8.24±0.13％‰)和-4.8～-2.0,Hf同位素两阶段亏损地幔模式年龄1540 ～ 1717Ma,岩浆温度达到～820℃以上.北祁连造山带北缘金佛寺花岗岩为过铝质(A/CNK=1.0～1.1),SiO2 (65.5％～75.0％)、MgO(0.6％～2.2％)、Fe2O3(1.9％ ～ 5.2％)、TiO2 (0.3％ ～0.8％)含量变化较大,其主量和微量元素特征与北祁连造山带的柴达诺花岗岩相似,源岩可能包括杂砂岩和角闪岩.金佛寺花岗岩的锆石618O为7.4％～9.7‰(平均8.03±0.36‰),εHf(t)在-0.5～ +1.9之间,Hf同位素两阶段亏损地幔模式年龄为1289 ～ 1439Ma,岩浆温度达到800～ 900℃.走廊过渡带大佛寺花岗岩、北祁连造山带北缘金佛寺花岗岩的锆石U-Pb SIMS年龄分别为426.1±2.8Ma、424.0±1.6Ma,不同构造单元发育同时期岩浆活动以及A型花岗岩的出现,表明在～425Ma北祁连洋盆已经闭合,北祁连造山带及邻区进入到后碰撞拉伸阶段.
展开▼
