东秦岭太华群变质岩系及其形成条件

METAMORPHIC ROCK SERIES OF TAIHUA GROUP AND CONDITIONS FOR ITS FORMATION IN EASTERN QINLING

太华群变质岩系是秦岭地区最古老的变质基底,构成华北板块的南缘。测得两个Nd-Sr 法模式年龄皆为2.6Ga。形成于嵩阳运动之前,属晚太古代。太华群变质岩系形成于p=7Kbar,t=550—600℃条件下,以中压相系角闪岩相为主。原岩为一套海相火山-沉积建造。火山岩主要为富铁拉班玄武岩,它与世界上广泛分布的太古代绿岩带玄武岩有相似的地球化学性质。太华群基本具有缘岩带的特征。

Metamorphic rock series of Taihun Group is an oldest metamorphic basement in Qinling region which forms the southern border of North China plate. Two Nd-Sr model ages are 2.6 Ga. The metamorphic rock series was formed before the Songyang movement, belongs to the late Archaean.The metamorphic rock series of Taihua Group is composed of five kinds of rocks: 1. amphibolite, plagioclase amphibolite and hyperthene amphibole-granulite 2.biotite-plagioclase gneiss; 3. quarzite and magnetite quarzite; 4. muscovite-graphite schist and muscovite-quartz schist; and 5. marble. Based on the mineral association and mineral thermobarometric data, the metamorphic rock series is from epidote-amphibole facies to amphibole granulite facies, and belongs to a mid-pressure facies series formed at conditions: p = 7×105Pa and t = 550-600℃.The metamorphic rock series has undergone migmatization.Original rocks of the metamorphic series are reconstructed: the amphibolite was iron-rich tholeiite (Fe-TH), the others were pelite, sandstone and marlite, which formed a marine volcanic-sedimentary formation of great thickness and multiple cycles.The structure and crystal chemistry of amphibole fron the first kind of rocks are Mg/ (Fe2++Mg)>0.5, (Ca+Na)B>1.34, NaB<0.67, CaB>34, Si = 6.50-7.25, so they be long to calc-amphibole subfamily, containing mainly magnesiohornblende.The age, sedimentary formation, rock association, petrography, petrochemistry and metallogeny of the metamorphic rock series of Taihua Group are similar with those of greenstone zones which are widely distributed over the world, so it also belongs to greenstone zone, wich only higher matamorphic grade (it represents mainly amphibolite facies) in the Taihua Guoup than that in other regions of the world (greenschist facies). Therefore the term greenstone is extended as we use here.Some petrochemical properties of TME in Archaean Fe-TH are similar with those in MORB, but they are apparently defferent. Tholeiite of Arhaean greenstone has its specific nature, because crustal structure, lithospheric thickness and movement mechanism in Archaean are greatly different from those in recent time.