北祁连山硬柱石蓝片岩p-T条件相平衡计算及其岩石学意义

北祁连硬柱石蓝片岩主要分布在甘肃省肃南县九个泉一带,是目前中国唯一报道的、确切地含有硬柱石的蓝片岩。文中在详细的岩石学和矿物学研究基础上,根据矿物共生组合的不同,将北祁连低温蓝片岩进一步划分为绿纤石蓝片岩、硬柱石蓝片岩和绿帘石蓝片岩。绿纤石蓝片岩的特征变质矿物组合为蓝闪石(>40%)+绿纤石(30%)+绿泥石(10%)+钠长石(8%)+石英(5%)+硬柱石(<3%)±方解石/文石(<1%)。硬柱石蓝片岩的矿物组合为蓝闪石(35%~40%)+硬柱石(35%~40%)+绿泥石(10%)+钠长石(10%)+石榴石(1%~2%)+黝帘石/斜黝帘石(<2%)+石英(<1%),副矿物有磷灰石和榍石,总含量小于2%。绿帘石蓝片岩的矿物组合为蓝闪石(30%~35%)+黝帘石/斜黝帘石/绿帘石(~30%)+绿泥石(15%)+钠长石(15%)+石榴石(2%)+石英(<2%),副矿物有金红石、磷灰石和磁铁矿,总含量小于2%。利用矿物内部一致性热力学数据和Domino/Theriak软件计算了这三种类型的蓝片岩形成的峰期温压条件,它们分别是绿纤石蓝片岩为320~350℃,0.75~0.85GPa;硬柱石蓝片岩为335~355℃,0.8~0.95GPa;绿帘石蓝片岩为345~375℃;0.75~0.85GPa。北祁连低温蓝片岩带由硬柱石蓝片岩相到绿帘石蓝片岩相的转化代表了俯冲变质过程中的递进变质过程。

p-T condition and phase equilibrium of lawsonite-blueschists in northern Qilian Mountains and its petrologic significance

Lawsonite-bearing blueschist in northern Qilian Mountains is the only lawsonite blueschist occurrence so far reported in China, which is exposed in Nine Spring of Gannan County, Gansu Province. Based on the petrologic and mineralogic studies, we divide the low temperature blueschists of northern Qilian Mountains into three groups: pumpellyite blueschist, lawsonite blueschist and epidote blueschist. The characteristic mineral assemblage of pumpellyite blueschist is glaucophane (>40%) + pumpellyite (30%) + lawsonite (3%) + chlorite (10%) + albite (8%) + quartz (5%) + calcite (1%). Lawsonite blueschist is composed of glaucophane(35%-40%)+ lawsonite(35%-40%) + chlorite(10%) + albite(10%) + garnet(1%-2%) + zoisite(<2%) + quarte(<1%). Epidote blueschist consists of glaucophane(30%-35%)+zoisite/clinozoisite/ epidote (~30%)+ chlorite(15%)+ albite (15%)+ garnet (2%)+ quartz(<2%)and rutile (1%) and magnetite (1%). Using internally consistent mineral thermodynamic database of Holland and Powell (1998), and Domino/Theriak calculation software by de Capitani and Brown (1987), we calculate the p-T pseudosections showing the metamorphic p-T conditions for the three types of blueschists. The metamorphic p-T conditions we obtained are 320-350 ℃ and 0.75-0.85 GPa for pumpellyite blueschist, 335-355 ℃ and 0.8-0.95 GPa for lawsonite blueschist and 345-375 ℃ and 0.75-0.85 GPa for epidote blueschist. The transition from lawsonite blueschist facies to epidote blueschist facies in northern Qilian Mountains occurred during the prograde subduction metamorphism.