内蒙古林西三叠纪花岗岩中黑云母地球化学特征及成岩意义

关于内蒙古林西地区早中生代岩浆岩产出的构造背景尚有争议。作者选择林西地区早三叠世新林镇花岗闪长岩和晚三叠世四力本黑云母花岗岩为研究对象,在岩相学研究的基础上,利用电子探针对花岗岩中黑云母的化学成分进行分析,探讨岩石成因和成岩大地构造背景。研究表明:新林镇花岗闪长岩中的黑云母为镁质黑云母,四力本黑云母花岗岩中的黑云母为铁叶黑云母,总体均显示富铁(TFeO质量分数为22.81%~36.65%)、贫镁(MgO质量分数为2.44%~10.56%)的特征。新林镇花岗闪长岩成岩温度为785~805℃,压力为5.39~8.88MPa,lgfO_2为-13.2~-13.8,属造山带钙碱性花岗岩;岩浆物质以壳源为主,兼具幔源特征,形成于早三叠世碰撞造山阶段。四力本黑云母花岗岩成岩温度为690~710℃,lgfO_2为-16.3~-17.1,为非造山带碱性花岗岩;花岗岩的物源以壳源为主,形成于晚三叠世造山期后的演化阶段。早三叠世林西地区处于强烈的接触碰撞阶段,在晚三叠世则进入造山期后的伸展演化阶段。

Geochemical signatures and petrogenesis significance of biotite from the Triassic granite in Linxi area,Inner Mongolia,China

The Linxi area in the Xingmeng orogenic belt (XMOB) is characterized by widespread Mesozoic granitoids and complex tectonic evolution and its tectonic setting still a controversial topic among geologists.Based on the petrography research of the Xinlinzhen granodiorite (early Triassic in age) and Siliben biotite granite (late Triassic in age),the mineralogical characteristics of biotite are studied by electron microprobe analysis so as to investigate the petrogenesis and tectonic setting of the granitoids.It shows that the biotite are iron-rich (TFeO=22.81％ ～36.65％) and magnesium-poor (MgO=2.44％10.56％).Biotite in Xinlinzhen granodiorite belongs to magnesia biotite;while the biotite from Siliben biotite granite belongs to siderophyllite.The formation conditions of the Xinlinzhen granodiorite is t=785～ 805℃,p =5.39 ～8.88 MPa and lgfo2 =-13.2 ～-13.8.It reveals that the Xinlinzhen granodiorite belongs to the orogenic calc-alkaline granite,derived mainly from crustal material,with characteristics of mantle-derived magma,and formed in early Mesozoic collision orogenic stage.The formation condition of the Siliben biotite granite is t=690～710℃ and lgfo2=-16.3 ～-17.1,belonging to the anorogenic calc-alkaline granite,derived from crustal material,and formed in late Mesozoic post orogenic evolution stages.It concludes that the Linxi area is under collision orogenic stage during early Mesozoic,followed by postorogenic evolution stages during late Triassic.