Composition, Geochemical And Chronological Characteristics Of The Pre-khondalite Series In Jining, Inner Mongolia

Research area is located in Jining area in the northern margin of eastern part ofkhondalite belt in western north China craton. This area mainly develops the khondaliteseries and S-type granites, while a small amount of pre-Khondalitee series metamorphicbasement rocks is found which is weak in previous researches. Pre-Khondalitee seriesmetamorphic basement rock is composed in this area of metamorphic supracrustal rockand metamorphic plutonic rock:Ⅰ. Geochemistry and tectonic environment of pre-Khondalitee seriesmetamorphic supracrustal rockIn this area, pre-Khondalitee series metamorphic supracrustal rock is composed ofbasic granulite rock association and biotite hornblende plagioclase gneiss rockassociation.1. On the basis of sampling locations and rare earth distribution characteristics,basic granulite rock association is divided into2types:1) Sanchakou basic granulite (REE\|smooth) belongs to tholeiite series and is a setof transitional mid ocean ridge basalt (T-MORB). Its (La/Yb) N value is between1.87-2.78and Tectonic distinguishing casting point falls in MORB or close to P-MORBin N-MORB.2) Tanyaocun basic granulite (LREE\|rich) also belongs to tholeiite series and is aset of oceanic island tholeiite (OIT). Its (La/Yb)N value is between7.33-7.60andTectonic distinguishing casting point falls within plate basalt area.2. On the basis of SiO2contents in the rocks, biotite hornblende plagioclase gneissrock association is divided into basic biotite hornblende plagioclase gneiss rock association and intermediate-acid biotite hornblende plagioclase gneiss rock association:1) The basic rock types in basic biotite hornblende plagioclase gneiss rockassociation include pyroxene amphibolite and amphibolite. The main, minor rare earthand Tectonic distinguishing diagrams explain that it has the same primary rock formationwith Sanchakou basic granulite and is a set of transitional mid ocean ridge basalt（T-MORB） which is formed under Sanchakou basic granulite retrogrademetamorphism.2) Dabeigou rock association in intermediate-acid biotite hornblende plagioclasegneiss rock association has rock types of biotite plagioclase gneiss and biotite monzoniticgneiss and is a set of continental arc volcanic formation of shoshonite series. Sanchakourock association, hornblende monzonitic gneiss lithology, is a set of mid ocean ridgehigh-K calc-alkaline volcanic formation.Ⅱ. Geochemistry and tectonic environment of pre-Khondalitee seriesmetamorphic plutonic rocksmetamorphic plutonic rock is composed of hypersthene granitic gneiss, light pinkishred middle-fine grained granite (alkali) and grayish biotite monzonitic granitic gneiss.Hypersthene granitic gneiss is a set of high-K calc-alkaline-shoshonite series; Lightpinkish red middle-fine grained granite is a set of Syn-collision granite of shoshoniteseries; Grayish biotite monzonitic granitic gneiss is a set of high-K calc-alkalinecontinental arc granitoid.Ⅲ. Chronology characteristics of pre-Khondalitee series metamorphicbasementChronology characteristics indicate that pre-Khondalitee series metamorphic basement rocks in this area were formed in Neoarehean（~2.5Ga）and mainly subject to3metamorphism transformations, which happened during~2.45Ga,~1.95Ga and~1.85Ga. The former two periods (~2.45Ga and~1.95Ga) respectivelystand for the2collision time between Yinshan block of western north China craton land mass and Erdos block:~2.45Ga metamorphism event forms basic granulite rock association in the zone, and~1.95Ga metamorphism event forms Khondalite belt across the western continental blocks, and these2events create extensiv e development of Nearly horizontal shear deformation structure in old metamorphic basement in the zone. And the third period (~1.85Ga) stands for the formationof mesozone of north China craton, and brings about NEE extrusion deformationstructure in old metamorphic basement in the zone.Based on the comprehensive analysis on Geochemistry and Chronology characteristics of pre-Khondalitee metamorphic basement rocks in Jining, the conclusioncan be reached that: pre-Khondalitee metamorphic basement in Jining is composed of transitional mid ocean ridge basalt, oceanic island tholeiite, continental arc granitoid and Syn-collision granite,etc. A fairly complete rock type combination stands for Geological Evolution of oceanic lithospheric subduction to subsequent continental subduction and collision, and the collision time is~2.45Ga.。...