北秦岭中生代沙河湾岩体环斑结构特征及有关问题的讨论

Characteristics of Rapakivi Texture in Mesozoic Shahewan Granite of North Qinling Mountains and Some Related Problems

该岩体的环斑结构主要发育于边部含巨斑状黑云角闪石英二长岩中 ,环斑长石粒径一般 2cm× 4cm。形态多呈自形、半自形 ,有些为卵形。环斑长石由核部钾长石和多层或单层斜长石外壳两部分构成。钾长石内核呈肉红色 ,一般是由单颗粒组成 ,具卡氏双晶 ,普遍发育规则的条纹结构 ;中心部分钾长石分子含量Or为 95 ,边部为 84。外壳斜长石牌号一般为An2 0± ,为奥长石。内核和外壳中均发育石英、斜长石、黑云母、角闪石等矿物的包裹体 ;包裹体在其边部较多 ,中部较少 ,钾长石斑晶中的石英包裹体呈不规则的凹面状和水滴状。岩石中主要矿物具有 2个世代。这些特征显示 ,沙河湾岩体中的环斑结构与典型的环斑结构是相同的。亦表明典型的环斑结构可以出现于不同的时代和构造环境。由于其形成时代和产出背景不同于典型环斑花岗岩 。

Rapakivi texture of the Shahewan pluton mainly occurs in megacryst bearing biotite horblende quartz monzonite. Rapakivi feldspars are mostly about 2 cm×4 cm in size and enhedral in shape, with some assuming ovoid form. A rapakivi feldspar grain consists of alkali feldspar core and plagioclase mantle. The core is generally composed of a single alkali feldspar grain characterized by Or 95～84, Karlsbad twin and perthitic texture. The plagioclase mantle is oligoclase with An=20±. Both the core and the mantle have inclusions of biotite, hornblende, quartz and plagioclase. The quartz inclusion in alkali feldspar megacrysts is concave and drop like in shape. The major minerals of the Shahewan pluton have two generations. Their textures are comparable with those of the typical rapakivi granites. This suggests that the typical rapakivi texture can be formed in various epochs and different tectonic settings. As the age and tectonic setting of the Shahewan pluton obviously differ from those of typical rapakivi granites, whether the granite belongs to rapakivi granite or a new type of granites needs further investigation.