北大别主簿源花岗岩和片麻岩矿物的氧同位素平衡及其Rb-Sr年代学效应

The mineral O isotopic equilibrium of Zhubuyuan granite and gneiss in the North Dabie Mountains and the Rb- Sr geochronologic affection

对大别造山带北部主簿源中生代花岗岩侵入体及其围岩片麻岩进行了矿物氧同位素分析，同时对同一样品进行了矿物 Rb- Sr内部等时线定年。结果表明，花岗岩和片麻岩矿物的氧同位素温度大小顺序为：角闪石 >磁铁矿 >榍石 >石英 >黑云母 >长石，遵循缓慢冷却条件下扩散控制的氧同位素交换封闭顺序，指示这些岩石没有受到后期热液蚀变的扰动。根据黑云母-长石-磷灰石-全岩内部 Rb- Sr等时线测定，花岗岩的年龄为 (118± 3) Ma，代表了岩浆侵位冷却年龄；片麻岩的年龄为 (122± 1) Ma，代表了片麻岩受大面积燕山期岩浆侵位热烘烤达到高温同位素平衡后的冷却年龄。因此，矿物之间的氧同位素平衡与否 ,能够对矿物 Rb- Sr体系封闭后平衡状态的保存性以及矿物内部等时线定年的有效性予直接制约。

The oxygen isotope compositions of minerals were measured for a Mesozoic granite and its host gneiss at Zhubuyuan in North Dabie. The same samples were also dated by mineral Rb- Sr isochron method. The results show that oxygen isotope temperatures of both granitic and gneissic minerals are in the sequence of hornblende > magnetite > titanite > quartz > biotite > feldspar, being in concordant with the sequence of closure temperatures for oxygen diffusion under slowly cooling conditions. The intermineral Rb- Sr isochron composed of biotite, feldspar, apatite and whole- rock yields an age of (118± 3) Ma for granite which points to the time for cooling of the granitic magma, and an age of (122± 1) Ma for the gneiss which is interpreted to represent the cooling age of the gneiss subsequent to high- temperature isotope reequilibration by baking due to the voluminous emplacement of granitic magmas in this region. In this regard, oxygen isotope equilibrium between the minerals provides a direct constraint on the Rb- Sr isotope equilibrium and thus the validity of the mineral isochron.