Geochemical And Chronology Characteirstics Of The Intrusive Rocks And Its Significance In Yangjiazhangzi Area, The Western Liaoning Province

The Western Liaoning Province(Liaoxi) is located in the east segment of Yanshan fold beltwhich is the north rim of North China Craton. This area is characterized by immense ofMesozoic intrusive rocks and large deposits of the Yangjiazhangzi porphyry skarn type Modeposit, Lanjiagouskarn type Mo deposit, Bajiazi Pb-Zn-Ag deposit and other polymetallic (Au,Cu, Mo, Pb, Zn) deposit. Due to the lack of research on the types, attitudes and geochemicalcharacteristics of the intrusive rocks in Yangjiazhangzi area, it leaded to a hundred schools ofthought contend of the Mesozoic tectonic environment in Liaoxi and restricted the cognition ofthe mineral resources in Yangjiangzhangzi area. For the above factors, we carry outsystematic studies of petrology, lithogeochemistry and isotope chronology of the intrusive rocksin the this area, provide the petrological evidence for the crustal evolution during Mesozoic inLiaoxi, and guide further ore-prospecting work in this area.According to our field geological survey, we found there are mainly four types of intrusiverocks in Yangjiazhangzi area. They are medium-coarse grain syenogranite, porphyriticsyenogranite, porphyaceous syenogranite and fine-grained hornblende monzonite. The mainrocks are medium-coarse grain syenogranite, porphyritic syenogranite and porphyaceoussyenogranite, which present as the big batholith of NE distribution. Zircon U-Pb isotope dating ismainly181~188Ma which shows they emplaced in the early Jurassic. Fine-grained hornblendemonzonite present as stock or dyke of NS distribution. Zircon U-Pb geochronology is227Mawhich shows they emplaced in the late Triassic. The geochemistry analyses of the three types of rocks are as follows. The porphyriticsyenogranite and porphyaceous syenogranite contain higher SiO2(68.89~77.38),Al2O3(12.79~15.34), Na2O(2.64~4.91), K2O(4.16~5.53), A/CNK(0.87~1.16), A/NK(0.97~1.25),high Differentiation Index (86.61~96.67),Alkalinity Ratio (1.9~2.45),SolidificationIndex(0.21~5.96). The diorites and monzonite contain low SiO2(51.4~61.73), higher Al2O3(14.84~16.58), Na2O(4.06~4.91), K2O (2.68~4.17), A/CNK(0.42~0.68), A/NK(1.55~1.69),Differentiation Index (50.75~36.72), Alkalinity Ratio (3.2~6.1),SolidificationIndex(23.83~36.72). Al2O3, MgO, FeO, CaO, Na2O, TiO2and P2O5abundances in theporphyritic syenogranite and porphyaceous syenogranite exhibit a negative correlation with SiO2,whereas AR, DI and K2O exhibit a positive correlation with SiO2. P2O5increasing trend showeda negative correlation with the SiO2, higher DI, which indicating the porphyritic syenograniteand porphyaceous syenogranite are high differentiation I-type granites. The fine-grainedhornblende monzonite belong to metaluminous high-K calc-alkaline rock series, but theporphyritic syenogranite and porphyaceous syenogranite are weak peraluminous high-Kcalc-alkaline rock series. The porphyritic syenogranite and porphyaceous syenogranite areenriched of high field strength elements Th, La, Nd, Hf and Gd, and depleted of high fieldstrength elements Ti, Ho and large ion lithophile elements Ba, Sr. The fine-grained hornblendemonzonite are enriched of high field strength elements Gd, Er and large ion lithophile elementsBa, Sr, and depleted of high field strength elements Nb, Hf, Ti, Pr, Y, Yb. Thechondrite-normalized REE patterns of the three-type rocks decline slightly to right, and obviousfractionation of LREE with unobvious fractionation HREE. Considering the zircon saturationtemperatures, we obtain the formation temperatures of the granitic rocks range from745°C to783°C, and the average temperature around765°C. The granitic rocks are low temperaturegranites.The fine-grained diorites and fine-grained monzonite in Yangjiazhangzi area may formed inthe intercontinental collision between North China Craton and Mongolian massif, due tothe crustextrusion, shortening, superposition, uplift. The porphyritic syenogranite and porphyaceoussyenogranite are the post-orogenic high differentiation I-type granites, relating toIzanagi Plate subduction to North China Craton in NW direction, and they may formed in the edge of the northChina plate under the post-orogenic setting.