Petrogenesis And Prospecting Significance Of The Laoniushan Granites, In The South Margin Of North China Craton

Laoniushan granite complex,in the northwest of Jinduicheng, is located in the south margin of North China Craton . Based on the study of field geology,mineralogy,geochemistry, Sr-Nd isotope, and the ziron U-Pb ages of Laoniushan granites, this study made the following preliminary awareness.According the geological characteristics of the field rock, the rock composition,texture and structure and their relationship, combined with U-Pb zircon dating, Laoniushan complex can be divided into four intrusive stages.The firist stage is adamellite and granodiorite. The second is coarse -prophyry biotite monzogranite. The two stages were formed in early Mesozoic .The third stage is mid-coarse-prophyry and mid-prophyry biotite monzogranite which is the major rock type of the complex and the fouth fine-prophyry biotite monzogranite.Magmatic crystallization age of rock are 223Ma、214Ma、151Ma、146Ma, formed in the Late Triassic and Late Jurassic. Therefore Laoniushan granite complex are formed by Triassic and Mesozoic granites.The Laoniushan granites are belong to high K aluminous, peraluminous shoshonitic-cal-alkaline I -type granite. SiO₂ content of these granites from low to high, but MgO,CaO and Na₂O from high to low from early to late stage. The trace element spider diagrams and Chondrite-normalized REE patterns are similar with each other, and show that they are the evolution of the homologous magma. The crystallization temperatures of Laoniushan granites are 735⁸06℃,and the pressure are relatively high,>1.2Gpa.MgO is 9.60%¹1.77% in Biotite ,Mg# in hornblende between 0.57 to 0.72, t and unobviousδEu abnormity, all of these show that Laoniushan rock belongs to crust-mantle-typegranite. I_Sr=0.70638⁰.70877,ε_Nd（t）=-11.3¹6.83,ε_Hf（t） =-24.6^- 17.2, T_DM=1.7¹.9Ga, T_DM2=1869²242Ma, all these suggest that Magma sources are ancient lower crust., However, the development of mgamtic mafic inclaves suggest that mantle materials involved in the formation of the granites. Mineral assembleges of Laoniushan complex and Jinduicheng granitic porphyry are different, the former are K-feldspar and plagioclase,, the latter is K-feldspar, different content of dark minerals biotite, the former high; different types of minerals, the former sphene, magnetite, apatite, zircon, etc. the latter is magnetite, apatite, zircon, etc. Rock structure are different, the former porphyritic structure, showing the deep intrusion, the latter porphyritic texture, showing shallow emplacement. From the geochemical point of view,the two granite is similar but not identical,the former is low-silicon, aluminum, alkali and magnesium-richs,Jinduicheng granitic porphyry has a relatively lowΣREE content, the Laoniushan rock belong to high-Sr and low-Yb, Jinduicheng granitic porphyry is low-Sr and low-Yb,which shows that granite porphy formed shallow, Jinduicheng porphyryε_Hf（t） changes in range, showed a largerεHf （t） values, and there is more young mantle components join in.These geochemical differences may be responsible for different metallogenic ability.