遼寧弓長嶺含鐵巖系的形成與演化

1、遼寧弓長嶺含鐵巖系的形成與演化The Gongchangling iron deposit is Located in the Anshan-Benxiarea,Liaoningprovince,being famous foritsAnshan-type ironore （ bandediron ore） ,and specially for its high-grade iron ore. The iron-bearingrock series,occurringin the granitesea in the from of Large residualinclusions,shows

2、a general structural style of anticlinorium and has amarginalmigmatizationcontactrelationshipwith the graniteof c. 2.5Ga.Thesynthetic section of the rock series. with a total thickness of500-800m.gives the following stratigraphic sequence from old to young（ in assendingorder ）:leucoleptynitic rock f

3、ormation, hornblendic rock formation, and iron-bearing rock formation in the lower part, key bed consisting of biotiteleptynite and garnet-mica-quartz schist in themiddle,iron-bearing,rockformation,and leucoleptyniticrock formationin theupper.There are a series ofresidualsedimentarystructuresin theb

4、iotitelephtynite of the key bed, including the thin interbedding ofquartzmagnetite poor are and the biotite leptynite. So it is clear thattheoriginal rocks of the leptynite were formed by sedimentation. Thebiotiteleptynite and garnet-mica-quartz schist occur in the same layer,butgeochemical and zirc

5、on researches indicate that their original rockscamefrom different sources, the former from intermediate-acid volcanicrockswithout stronglyweathering,while thelatter from continentalcrustmaterialwith strongly weathering.There are two kinds of plagioclase inthe iron-bearing rockseries, one of them is

6、 the lower plagioclaseamphibolite occurring in thelow hornblendic rock formation, which hasgenerally layered, lamellar andbanded structures, with somewhat largechange in their mineral kinds andproportion. Some detrital zircons andmicrosopherulesare discoveredinartificialheavy concentrates.Hence itca

7、n be concluded that the originalrocks of the lower plagioclaseamphiboliteare mainly basaltictuffs,taffiteand tuffaceoussedimentaryrocks. Geochemically, the lower plagioclaseamphibolite is relativelyhigh in SiO2 and low in FeO, and exhibitsREEpatternsofslightenrichmentinLREEand negativeEu anomaly, bu

8、t withoutthetrendoflineardistribution between inactive incompatible elements. Thechemicalcharacteristics are on the whole similar to those of the calc-alkalicbasalt, The other is the upper plagioclase amphibolite existing intheupper iron-bearingrock formation,which has ordinarilymassivestructureshow

9、ing the homogeneous distribution of minerals in the rocks.Theoriginalrocks of the amphiboliteare predominatly pure basaltictuffsorlavas. The upper plagioclase amphibolite is geochemically differentfrom thelowerone mentioned above,beingrelativelyhigh inTFeOand lowin SiO2,havingtheLREEdepleted or flat

10、REEpatternswithoutobvious Eu anomely andshowing good linearrelationshipsbetween inactiveincompatible elements. Thecomposition features are typical of oceanictholeiite,implying thatthebasalticmagmawas formed ina relativelylowpressure extension setting. Thedifferences of the two kinds of rooks inchemi

11、calcomposition, apartfromthereason thatthe originalrocks ofthelower plagioclase amphibolite wereinfluenced by exogenic action in thesedimentary processes, resulted mainlyfrom the contamination of thebasaltic magma forming the lower plagioclaseamphibolite by continentalmaterials. The change in the co

12、mpositions of theamphibolitic rocks isconsistentwiththeextenisonprocesses ofthecontinentalbasement oftheAnshan-Benxi basin.The REE analyses of 5 quartz magnetite poor oressuggest that the REEpatterns are very similar to those of the same oresof the iron-bearingrockseriseof Archaean（Fryer,1983）,with

13、TREEbeingvery low, LREE slightly rich,and positive Eu anomaly obvious,Some otherkinds of rocks occurring inthe area have also been studiedpetrologicallyand geochemically.Byusingsensitivehighmass-resolutionion microprobeand zircon grainstepwise evaporation technique, a lot of zircons fromdifferent ki

14、nds ofrocks in the iron-bearing rock series are determinedgeochronologically. Theage of magmatic zircons is 2.99 Ga for theGongchangling gneissic granite,which is roughly lcoated in the core ofthe Gongchangling anticlinorium,andon which probabllywas directlylaiddown the iron-bearingrockserise.Thedet

15、ritalzircons,derived fromthecontinental crust basement, have theages of 3.25-3.36 Ga（migmatizedleucoleptynite）, 3.08-3.53 Ga（quartzite） and3.03-3.44 Ga（fuchsitequartzite） respectively.The detritalzirconsfrom thebiotiteleptyniteare determinated as 2.7-2.5 Ga. It is considered that theage of 2.7 Garep

16、resents approximately the time of intermediate-acidvolcanism andrelevant sedimentation, i.e., the time of the formation oftheiron-bearing rock series. The conclusion is also supported by otherevidences.The Gongchangling iron-bearingrock serieshas undergone threemajorstages of metamorphism and deform

17、ation, happening respectivelybefore,during and after the intrusion of c. 2.5 Ga granite. Thehighestmetamorphism degree is up to low amphibolite facies.The studyabove indicates that the materials constituting the iron-bearing rockseries were supplied by two different sources. The protolithformationco

18、rresponds to the volcanic-sedimentary formotion being made up ofbasic（ tholeiitic and calc-alkalic） -intermediate-acid（ dacitic）volcano-sedimentary rocks, Fe-Si chemical sedimentary rocks（beingprobablyrelatedtobasicvolcanism ） and continentaldetritalmaterialswith differentdegreesofmaturity. Accordin

19、gto thebasement properties,scale of thesedimentary basin, protolith assemblages, formationprocesses and conditions,it can be concluded hat the iron-bearing rockseries was formed in anenvironment similar to the back-are basin oncontinental margin. And it isalso considered that the basin formation,the sedimentation of the iron-bearing rock series, the main period ofmetamorphism and deformation, and thegranite intru