Magmatic Evolution Process Of Dikes In Guocheng Fault Zone, Jiaodong Peninsula: Implications For North China Craton Lithosphere Thinning And Gold Ore Formation

Deep-source mafic dikes, controlled by regional lithosphere extension, play an important role inunderstanding the continental dynamics. Their temporal-spatial relationship to gold deposits hasbeen a significant research field for many geologists. In this study, we focus on the EarlyCretaceous dikes intruded intensively in the Guocheng fault zone secondary to the Tan-Lu faultsystem (deep to upper mantle), Jiaodong Peninsula within the southeast margin of the North Chinacraton (NCC). After extensive field work and petrographic study, the results of zircon U-Pbchronology, whole-rock geochemistry, isotopic geochemistry and microscopic geochemistry of theGuocheng dikes, and the morphology and microscopic analyses of sulfide melt inclusions inphenocrysts from these rocks are presented. These results are used in an attempt to investigate thesource and petrogenesis of these dikes, constrain the Mesozoic lithosphere thinning of the easternNCC, and understand the gold enrichment process in the magmatic chamber and the transfer of oremetals from the magma to the hydrothermal fluid during gold ore formation. The main results andconclusions are summarized as the follows:(1) These dikes in the Guocheng fault zone dominantly intruded in NE direction extensionalfaults, resulted from the extensional strike-slip movement of the Tan-Lu fault system in the EarlyCretaceous. 95 % volume of the dikes emplaced immediately after gold ore formation, and thecumulative thickness is locally far more than their wall rocks revealed by laneways crosscut theserocks vertically, which is perhaps the best example and accordingly an ideal place to test the dikepetrogenesis and their implications for the evolution of the eastern NCC. Petrographic study showsthat these dikes are dominated by diorite porphyry with lesser amounts of lamprophyre andmonzonite porphyry.(2) Zircon cathodoluminescence (CL) images show well-developed magmatic oscillatory zonesand the acquired LA-ICP-MS zircon U-Pb weighted mean ²⁰⁶pb/²³⁸U ages are 114±2 Ma (MSDW=1.5) for monzonite porphyry and 116±1 Ma (MSDW=0.8) for diorite porphyry,respectively. Earlier magmatic events in the northwest Jiaodong Peninsula represented by someinherited or captured zircons also occur in these dikes. Our results, combined with preciouslypublished data on gold deposits (Rb-Sr dating of auriferous pyrite and Ar-Ar dating of muscoviteand sericite), further confirm a significant magmatic action after gold ore formation in theJiaodong Peninsula.(3) Bulk-rock geochemical data show that the Guocheng dikes range in composition from quartzmonzonite to monzogabbro with an SiO₂ content varying from 44.75% to 71.38%. These dikes arecomposed of high-K calc-alkaline to shoshonitic or ultrapotassic rocks, which are characterized byhigh and wide MgO (Mg^#= 30-70), Cr (5.4-571.4 ppm) and Ni (3.3-235.5 ppm) contents. Theyare enriched in large ion lithophile elements (LILE, e.g., Sr, Rb, Ba) and light rare earth elements(LREE; (La/Yb)_N=18-66), depleted in high field strength elements (HFSE, e.g., Nb, Ta, Zr, Hf, Ti)and heavy rare earth elements (HREE), and possess uniform isotopic compositions having highinitial ⁸⁷Sr/⁸⁶Sr (0.70775-0.71164) and lowε_Nd (t) (-20.8-13.5) and ²⁰⁶pb/²⁰⁴pb (17.128-17.557).Since crustal contamination during the magma ascent is insignificant, the Guocheng dikes couldreflect the nature of their magma source. Their geochemical features can not been interpreted bypartial melting of an enriched mantle or melts derived from foundered lower crust, withsubsequent interaction with mantle peridotite. Therefore, we propose that these rocks originatedfrom the mixing between mantle-derived mafic and crust-derived felsic magmas, resulting in highcontents of Mg, Cr and Ni and enrichment of Sr, K, Rb, Ba and Pb simultaneously, which isconsistent with linear relationship of their isotopic compositions. A simple modeling of Sr-Ndisotope show that mixing the depleted mantle with granitic melt remelted from crust can accountfor the present Sr-Nd isotope features and element compositions, in which the proportion of crustalmaterials participated in monzonite porphyry (＞60%) is higher than those in diorite porphyry andlamprophyre (5%-40%). Quantitative model indicates the behaviors of incompatible elements andREE are also influenced by fractional crystallization of pyroxene, hornblende, plagioclase,K-feldspar, biotite, apatite and zircon. MFC (mixing and fractional crystallization) dominantlycontrol the magma evolution of the Guocheng dikes, confirming the existence of crust-mantleinteraction in the eastern NCC during the Late Mesozoic.(4) Electron probe microanalyses (EPMA) show that the amphibole and clinopyroxenephenocryst's mantle in diorite porphyry and lamprophyre respectively has sharply higher MgO(Mg^#) and Cr₂O₃ contents in contrast to their cores. The plagioclase phenocryst in monzoniteporphyry has reverse zoning. These compositional variations of phenocrysts indicate that the dikesoriginated from a mixing magma source, which coincides with the petrography exhibiting biotiteenclaves in amphibole phenocrysts and the presence of acicular apatites. This crust-mantleinteraction is related to the lithosphere extension, which results in asthenosphere upwelling leadingto decompressional melting of crustal materials to form felsic magma reservoir. Along with hightemperature asthenosphere magma injecting into the crust-derived magma chamber, material andenergy exchange occurred, as different temperature, viscosity and chemical compositions of themixing members, to form the inhomogeneous mixing magma source of these dikes. Mineral chemistry show that the pressure and temperature of the magma source is about 0.79-1.33 GPa(26-44 km; middle-lower crust level) and 924-1278℃respectively. Our findings make sure adeep dynamical process of asthenosphere upwelling in the eastern NCC during the Late Mesozoic.(5) Crust-mantle magma mixing genesis (with asthenosphere magma injecting into crust-derivedmagma reservoir) of the Guocheng dikes provides direct evidence for asthenospheric underplatingthinning process of the eastern NCC during the Late Mesozoic. It indicates that asthenosphereupwelling and its chemical erosion to lithospheric mantle and lower crust may be the dominantthinning mechanism in the Cretaceous. Dynamical background is probably due to the secondextensional sinistral strike-slip movement of the Tan-Lu fault system triggered by the obliquesubduction of the Izanagi plate. Combined with tectonic evolution of the NCC and stagedmagmatisms during the Mesozoic, and the existence of the Early Mesozoic foundering lower crust(e.g., researches on eclogite xenoliths entrained by adakitic intrusions from the Xuzhou-Huaibeiregion, Xinglonggou high Mg volcanic rocks in western Liaoning), we propose that the NCC hadexperienced staged thinning process, i.e., delamination thinning in the Early Mesozoic andunderplating thinning in the Late Mesozoic respectively, and firstly point out the Tan-Lu faultzone may play a significant role in the lithosphere thinning, i.e., stress release.(6) Based on petrographic study in detail, we discovered primary melt inclusions (PMIs) inequilibrium with silicate melt inclusions in phenocrysts (including hornblende, plagioclase andbiotite) from diorite porphyry. The globular shape and necking of the sulfides and differentsulfide/silicate melt ratios in single inclusions indicate that these sulfides were trapped in amolten stage, documenting the fact that the magma had exsolved sulfide melt upon crust-mantlemagma mixing in some parts of the magma chamber. EPMA microscopic analyses show that Auwould be preferentially enriched in the sulfide liquid. Relicts of the magmatic sulfides coexistingwith Fe-oxides in phenocrysts from the diorite porphyry indicate that the sulfide melt can beinstantaneously destabilized during later stage of the magma evolution and release Au to anore-forming hydrothermal fluid. The mean Au/Cu and Ag/Au ratios of sulfide melt inclusions,within an average factor of about 2.5 and 2.0 respectively, is identical to those of ore sulfides,and the dikes have Pb isotopic compositions similar to ore sulfides, suggest that the ore-forminghydrothermal fluid inherited the element and isotope features of the sulfide melt in the magmasource. This discovery in the Mesozoic dikes in the NCC is the first, providing a critical linkbetween mafic dikes and gold deposits and showing new insights into the formation of othermagmatic hydrothermal deposits.