Formation And Evolution Of The Iron-bearing Rock Series Of Gongchangling Area, Liaoning Province

The Gongchangling iron deposit is Located in the Anshan-Benxi area,Liaoning province, being famous for its Anshan-type iron ore(banded iron ore),and specially for its high-grade iron ore. The iron-bearing rock series,occurring in the "granite sea" in the from of Large residual inclusions,shows a general structural style of anticlinorium and has a marginalmigmatization contact relationship with the granite of c. 2.5Ga. Thesynthetic section of the rock series. with a total thickness of 500-800m.gives the following stratigraphic sequence from old to young(in assendingorder):leucoleptynitic rock formation, hornblendic rock formation, and iron-bearing rock formation in the lower part, key bed consisting of biotiteleptynite and garnet-mica-quartz schist in the middle, iron-bearing, rockformation, and leucoleptynitic rock formation in the upper.There are a series of residual sedimentary structures in the biotitelephtynite of the key bed, including the thin interbedding of quartzmagnetite poor are and the biotite leptynite. So it is clear that theoriginal rocks of the leptynite were formed by sedimentation. The biotiteleptynite and garnet-mica-quartz schist occur in the same layer, butgeochemical and zircon researches indicate that their original rocks camefrom different sources, the former from intermediate-acid volcanic rockswithout strongly weathering, while the latter from continental crust materialwith strongly weathering.There are two kinds of plagioclase in the iron-bearing rockseries, one of them is the lower plagioclase amphibolite occurring in thelow hornblendic rock formation, which has generally layered, lamellar andbanded structures, with somewhat large change in their mineral kinds andproportion. Some detrital zircons and microsopherules are discovered inartificial heavy concentrates. Hence it can be concluded that the originalrocks of the lower plagioclase amphibolite are mainly basaltic tuffs,taffite and tuffaceous sedimentary rocks. Geochemically, the lower plagioclaseamphibolite is relatively high in SiO₂ and low in FeO, and exhibits REE patterns of slight enrichment in LREE and negative Eu anomaly, but withoutthe trend of linear distribution between inactive incompatible elements. Thechemical characteristics are on the whole similar to those of the calc-alkalic basalt, The other is the upper plagioclase amphibolite existing inthe upper iron-bearing rock formation, which has ordinarily massivestructure showing the homogeneous distribution of minerals in the rocks. Theoriginal rocks of the amphibolite are predominatly pure basaltic tuffs orlavas. The upper plagioclase amphibolite is geochemically different from thelower one mentioned above, being relatively high in TFeO and low in SiO₂,having the LREE depleted or flat REE patterns without obvious Eu anomely andshowing good linear relationships between inactive incompatible elements. Thecomposition features are typical of oceanic tholeiite, implying that thebasaltic magma was formed in a relatively low pressure extension setting. Thedifferences of the two kinds of rooks in chemical composition, apart fromthe reason that the original rocks of the lower plagioclase amphibolite wereinfluenced by exogenic action in the sedimentary processes, resulted mainlyfrom the contamination of the basaltic magma forming the lower plagioclaseamphibolite by continental materials. The change in the compositions of theamphibolitic rocks is consistent with the extenison processes of thecontinental basement of the Anshan-Benxi basin.The REE analyses of 5 quartz magnetite poor ores suggest that the REEpatterns are very similar to those of the same ores of the iron-bearing rockserise of Archaean(Fryer, 1983), with TREE being very low, LREE slightly rich,and positive Eu anomaly obvious,Some other kinds of rocks occurring in the area have also been studiedpetrologically and geochemically.By using sensitive high mass-resolution ion microprobe and zircon grainstepwise evaporation technique, a lot of zircons from different kinds ofrocks in the iron-bearing rock series are determined geochronologically. Theage of magmatic zircons is 2.99 Ga for the Gongchangling gneissic granite,which is roughly lcoated in the core of the Gongchangling anticlinorium, andon which probablly was directly laid down the iron-bearing rock serise. Thedetrital zircons, derived from the continental crust basement, have theages of 3.25-3.36 Ga(migmatized leucoleptynite), 3.08-3.53 Ga(quartzite) and3.03-3.44 Ga(fuchsite quartzite) respectively. The detrital zircons from thebiotite leptynite are determinated as 2.7-2.5 Ga. It is considered that theage of 2.7 Ga represents approximately the time of intermediate-acidvolcanism and relevant sedimentation, i.e., the time of the formation of theiron-bearing rock series. The conclusion is also supported by other evidences. The Gongchangling iron-bearing rock series has undergone three majorstages of metamorphism and deformation, happening respectively before,during and after the intrusion of c. 2.5 Ga granite. The highestmetamorphism degree is up to low amphibolite facies.The study above indicates that the materials constituting the iron-bearing rock series were supplied by two different sources. The protolithformation corresponds to the volcanic-sedimentary formotion being made up ofbasic(tholeiitic and calc-alkalic)-intermediate-acid(dacitic)volcano-sedimentary rocks, Fe-Si chemical sedimentary rocks(being probably relatedto basic volcanism) and continental detrital materials with differentdegrees of maturity. According to the basement properties, scale of thesedimentary basin, protolith assemblages, formation processes and conditions,it can be concluded hat the iron-bearing rock series was formed in anenvironment similar to the back-are basin on continental margin. And it isalso considered that the basin formation, the sedimentation of the iron-bearing rock series, the main period of metamorphism and deformation, and thegranite intrusion of the late Archaean, all happening in 2.75-2.50Ga,reflect the different development stages of a same geological evolutionalprocess.Combined with the regional geological background, the geological de-velopment in Anshan-Benxi area can be divided into three main stages: a, theformation of the basement on which the iron-bearing rock series wasdeposited unconformably, and which has continental crust properties, and long-term complex evolutionary history (3.8-2.75 Ga): b, the formation of theiron-bearing rock series, late Archaean metamorphism and defomation, andgranite intrusion (2.75-2.5 Ga) and c, its late evolution since proterozoic(after 2.5 Ga).