太古代太华群基性变质岩角闪石及其成因意义

Amphiboles From Basic Metamorphic Rocks in the Archeozoic Taihua Group and its Genetic Significance

太华群位于秦岭北段,是该区最老的变质基底,经 Nd—Sr 同位素研究,测得模式年龄为2.6Ga。太华群原岩为一套火山—沉积建造,火山岩主要为富铁拉斑玄武岩,经变质作用形成角闪岩、斜长角闪岩以及紫苏辉石角闪麻粒岩等角闪质岩石。变质岩系受到了混合岩化作用。角闪石的矿物化学和结构特点为 Mg/(Fe^(2+)+Mg)>0.5,(Ca+Na)_B>1.34,N_(aB)<0.67,C_(aB)>1.34,Si 以6.5～7.25为主,根据国际分类,应属于钙质角闪石亚族,主要为镁普通角闪石,其中一个样 Si>7.50,为阳起石。同时取得了角闪石的光性、X 光衍射及红外光谱资料。角闪质岩石从低级角闪岩相到角闪麻粒岩相,属于中压相系,并以角闪岩相为主。形成的温压条件为:压力>5 kb,温度550℃～600℃。太华群变质岩系从年龄、建造、岩类、岩石化学以及含矿性等各方面特点与世界上广泛发育的绿岩带颇为相似,亦应属于绿岩带。只是变质程度较高,以角闪岩相为主,而不是绿片岩相。太古代基性岩有自己的特点,类似于大洋中脊拉斑玄武岩,但也不尽相同,其地壳构造、岩石圈厚度和运动机制与当代均有差异。

The Taihua Group is distributed over the northern slopes of Qinling Regions.It forms the oldest metamorphic basement of the district.The Nd-Sr modal age is 2.6 Ga.The original rocks belong to a set of volceno sedimentary formation.The volcanic rocks are composed mainly of Fe rich tholeiite.The basic metamorphic rocks in the group include amphi- bolite,plagio-amphibolite and hyperthene amphibole granulite.The metamorphic rock series have undergone chorismitization. The chemical and structural characteristics of amphibole crystal are Mg/(Fe^(2+)+Mg)>0.5,(Ca+Na)_B>1.34,Na_<0.67,Ca_B>1.34, Si 6.50—7.25.So the amphiboles can be grouped into the calcic am- phibole subfamily according to the international classfication.And most of the minerals belong to magnesiohornblende.Only one sample (Si>7.59) is actinolite.Their optical character,X-diffraction and infra-red of the amphibole have been measured. The amphibolides in the group vary from lower amphibolite ficies to amphibole granulite ficies,the greagter part belonging to the amphi- bolite facies,all belonging to mid-pressure facies series.The formation condition:P>5kb,T=550～600℃. The Taihua Group is quite similar in age,formation,petrography、 petrochemistry and minerogenetic property to greenstone zones distribu- ted over the world.Thus,the metamorphic rock series should belong to a greenstone zoon.The distinction between them is that the former has a higher metamorphic degree,most being amphibolite facies rather than greenschist facies. The Archean basic volcanic rocks have their own features which are similar to those of mid-ocean ridge tholeiite.The princinple differential from the latter depends on the tectonics of the crust,the thickness and the movement mechanism.