再论巴楚麻扎山正长岩体岩石成因和构造意义

巴楚麻扎山正长岩体形成于286~277 Ma，属于晚二叠世，与南天山造山带晚二叠世花岗岩(300～260 Ma)相一致。正长岩样品具有高硅高碱的地球化学特征，SiO₂和全碱质量分数分别为62.98%～67.29%和10.08%～11.22%，且Na₂O>K₂O。大部分样品具正Eu异常,Sr具轻微负异常，P和Ti表现出明显的负异常，Ga/Al值(2.80～3.26) ×10^-4和(Zr+Nb+Ce+Y)值(405.11~606.49)×10^-6，与A₁型花岗岩的地球化学特征相似，形成于板内裂谷环境。样品具有较区内同时代基性岩脉更为亏损的Sr-Nd同位素组成(ε_Nd(t)=+3.4～+4.3、I_Sr=0.703 03～0.703 44)和更为年轻的Nd模式年龄(T_DM=0.6～0.7 Ga)。地球化学数据表明，正长岩体是受软流圈组分改造过的底侵基性下地壳重熔后经结晶分异的产物。结合区域地质资料发现，塔里木盆地和南天山造山带二叠纪岩浆活动的差异与二者所处的构造位置密切相关，塔里木盆地二叠纪岩浆活动与软流圈上涌导致的主动裂谷有关，而与南天山造山带后碰撞伸展阶段岩浆活动受控于已存在的构造活动带(缝合带) 或者是因伸展导致的拆离明显不同。

A Further Discussion of the Petrogenesis and Tectonic Implication of the Mazhashan Syenites in the Bachu Area

The Mazhashan syenites in the Bachu area were formed in the Late Permian with the zircon SHRIMP age ranging from 286 to 277Ma, and these ages are consistent with the ages (300-260 Ma) of the post-collisional granites in the southern Tianshan orogen. These syenites are high in silica and alkaline with SiO₂=62.98%-67.29 % and K₂O+Na₂O=10.08%-11.22% , and often show Na₂O>K₂O. Most of the samples have positive Eu anomalies and slightly negative Sr and significantly negative P and Ti anomalies. They formed in an intra plate rifting setting as they have geochemical affinity with A1 type granites with Ga/Al =（2.84-3.28）×10^－4 and Zr+Nb+Ce+Y =（405.11-606.49）×10^－６. The syenites have more depleted Sr-Nd isotopic compositions (ε_Nd(t) =+3.4-+4.3, I_Sr=0.703 03-0.703 44) and younger Nd model ages (T_DM=0.6-0.7 Ga) than the coeval mafic dikes. Geochemical data indicate that the syenites were originated from the remelting of the underplated mafic lower crust which composition had been changed by the asthenospheric upwelling. Combining with regional geological data, the authors found that the magmatic activity differences between the Tarim Basin and the South Tianshan Orogen in the Permian stage were probably related to their different tectonic localities. The Permian magmatism in the Tarim Basin was related to the active rifting triggered by the asthenospheric upwelling, this is different from the magmatism during the post collisional extension in the South Tianshan Orogen, which was controlled by the pre existent mobile zone (suture) or by the extension resulted detachment zones.