Geodynamic Setting Of Mesozoic Gold Metallogcny In The Western Shandong Province

The scale and characteristics of gold metallogeny in the Luxi area along the southeasternmargin of the North China Craton is in marked contrast with those in the Jiaodong area alongthe eastern Tan-Lu fault. This thesis summarizes the petrologic features，major and traceelement geochemistry, compositions of main minerals including feldspar, biotite andhornblende, zircon U-Pb geochronology, fission track chronology and Lu-Hf isotopes ofore-bearing magmatic rocks and combines these results with salient regional geophysical data,S, Pb, H-O, C-O, He-Ar isotopes, and fluid inclusion data on ores in Luxi and Jiaodong todiscuss the geodynamic setting of the gold mineralization in western Shandong Province.Luxi and Jiaodong area have different lithospheric structures. In a detailed synthesis,this work combined the magnetotelluric data, gravity data, heat flow and other geophysicaldata of the profiles crossing Tan-Lu fault which reveal that the lithospheric thickness andcrustal thickness of Luxi area are64-85Km and35-38km respectively. The former isobviously larger than the Jiaodong region with thickness of63-72km, and the latter has nomarked difference with Jiaodong area.The combined Sr-Nd-Pb and Lu-Hf geochemical features of intermediate-maficmagmatic units and their xenoliths in Shandong Province show that the mantle beneath theLuxi area is mainly of EM1type, and the mantle in the eastern part, close to the Tan-Lu faultshows mixed EM1and EM2features, whereas the mantle beneath the Jiaodong area is mainlyof EM2type. The EM1type is related to recycling ancient lithosphere mantle, whereas theEM2type is associated with the Yangtze craton subduction.The zircon U Pb data on the basement rocks in the Luxi area show that the protoliths ofthe TTG (tonalite-trondhjemite-granodiorite) gneisses, granitoids and amphibolites formed at2572.2±8.8Ma,2531±12Ma, and2572±32Ma respectively. The Mesozoic Tongshi complexin Guilaizhuang gold deposit formed at178.42.1175.61.7Ma; the diorite porphyry ofTongjing complex related to Yinan gold deposit was emplaced at128.0±5.4Ma. During theMesozoic, Luxi area had two tectono-magmatism-metallogeny events: around180Ma and130Ma.The Tongshi complex belongs to high-K alkaline and aluminum-peraluminous series,with the geochemical data displaying weakly negative europium anomalies, enrichment oflarge ion lithophile elements, and depletion in high field strength elements, suggestingfractional crystallization of magmas derived from enriched mantle together with youngercrustal components. The Tongjing complex belongs to high-K calc-alkaline and aluminum series, and lacks negative europium anomalies. The rock display high Sr, low Y content, andMg#value between52.19and67.13suggesting high Mg diorite composition derived fromenriched mantle. The Jinchang complex of Jinchang gold deposit belongs to I-type granitederived from the partial melting of lower crust and mixed with the mafic magma fromenriched mantle.The average zircon crystallization temperature of Tongshi complex is672.53℃, and theoxygen fugacity is-14.8to-11.9. The average zircon crystallization temperature of Tongjingcomplex is743.68℃, and the oxygen fugacity is-12.2to-11.7. The EPMA data of pargasite inthe Tongshi complex, shows that the rim and core compositions are identical, the magmachamber depth of Tongshi complex is estimated to be between9and31.6km with mean valueof21km. but the temperature, pressure, oxygen fugacity and H2O content in melt of Tongjingcomplex shows a reduction from core to rim. The magma chamber depth of Tongjing complexis4.8to21.5km with mean value of15km, and its emplacement depth is between1.9and16.4Km, with a mean of8.4km.From Tongshi complex cooling stage to133-140Ma, the uplift rate is estimated as0.34-0.39km/Ma; from130Ma to91Ma, its uplift rate is1.13m/Ma; then from90Ma tocurrent level, its uplift rate is45.5-47.7m/Ma and the mean denudation depth is4.2km. FromTongjing complex cooling stage to87-100Ma, the uplift rate is estimated as0.33-0.51Km/Ma;then from87-100Ma to current level, its uplift rate is63.6-73.1m/Ma and the meandenudation depth is6.4km. In addition, the uplift rate and depth of Luxi area is higher thanthat of Jiaodong.Comparison of S, Pb, C-O, H-O, He-Ar isotope and fluid inclusion characters of oresfrom Luxi and Jiaodong areas, lead to the inference that ore-forming fluid of Luxi area has acrust-mantle mixed magmatic hydrothermal signature with dominant input from mantlesources. The ore-forming materials were derived from complexes during the same period.However, the ore-forming materials in the Jiaodong area were derived from multiple sources,including the110-130Ma granites and intermediate-mafic dykes.In summary, the collision between the Yangtze Craton and the North China Cratonprobably marks the prelude for lithosphere modification of Luxi and Jiaodong, but thedifferent extent of modified lithospheric mantle induced by the Jurassic-Cretaceous subductionof the Pacific plate and subsequent asthenosphere upwelling is the main factor of metallogenicdifferences in these two areas.