Origin And Geological Significance Of The Haishenmiao Neoarchean Granitic Pluton In The Songshan Area

Songshan is the typical area where the Neoarchean Era granitic rock are well exposed. The Haishenmiao granitic pluton is a Metamorphic deformation granitic pluton which disintegrated from Dengfeng complex rock. We have obtained the following geological results through studying the petrologic feature,The petro-graphic features,geochemical characteristics,geochronology,origin and tectonic environment:(1) Field geological surveied:Haishenmiao granitic pluton with the features of grayish-white colour,medium-grained structure,Fine grained granite structure,gneissic structure trondhjemite,main mineral plagioclase(60%-65%),quartz(30%-35%),minor mineral biotite(5% ±)and few muscovite distributes like rock column from north to south and is about 7.3 km long,5km wide,exposed area of about 17km2.(2) We obtain two handed point age: 2519 Ma ± 14 Ma and 2551Ma±38Ma by U-Pb zircon chronology study. So The crystallization age of The Haishenmiao granitic pluton is 2600Ma~2500 Ma, and it is the production of magma movement in Songyang period and its intrusion age is Neoarchean Era.(3) The rocks which have features of high silicon,low aluminum,rich alkali and poor calcium magnesium shows calcium alkaline characteristics with δ(1.14~2.13).With the content of SiO2 increase, the content of TiO2,Al2O3,FeOt,CaO,MgO,P2O5 decrease,and it shows that they are from the same origin, and mainly dominated by crystallization differentiation and have a good correlation.(4) The pluton has low ΣREE, with differentiated LREE/HREE patterns :LREE/HREE=9.95~21.61,(La/Yb)N=(21.07~56.67),(La/Sm)N=4.21~7.74,(Gd/Yb)N=2.01~4.97. HREE strongly loss and δEu weakly positive anomalies and negative anomaly possibly related with the differentiation of plagioclase. Th,K,La,Pb,Nd,Sm shows consistent positive anomaly evidently but U,Ta,Nb,P and Ti shows negative anomaly, and high field strength element Zr-Hf shows negative anomaly weakly or no anmalies. It has the similar geochemical characteristics with the high Si Cenozoic adakite with high content of Sr, high ratio of Sr/Y, low content of Cr,Ni,Yb,Y.(5) The mineral assemblages and geochemical characteristics of the rocks show that Haishenmiao granitic pluton has the similar features with basaltic rocks from island arc or continental margin arc. geochemical characteristics shows that it formed by the partial melting of subducted oceanic crust which is aqueous basaltic under the high temperature and high pressure.