Chronology And Geochemical Characteristcs Of Chibosong Mafic-ultramafic Rocks From Tonghua And Its Constraints On Ore-forming Process

A lot of mafic-ultramafic intrusives occur in Chibosong area in Tonghuaregion, Jilin province. The Chibosong-Ⅰintrusive is the most important one inthem due to occurrence of the copper-nickel deposits. Based on the former'sstudying, chronology and geochemistry of Chibosong mafic-ultramafic rocks arestudied in this paper. The major achievements are as followings.1. Chronology of Chibosong mafic-ultramafic rocks SHRIMP zircon U-Pb dating from Chibosong gabbro in Chobosong Ⅰintrusive indicates that euhedrel and irregular zircons yield several group ofconcordia ages, i.e. 2497±13Ma, 787±35Ma, 321±10Ma (six spots), 217±11Ma (two spots), and 134±7Ma (four spots). On the basis of analyses of zirconCL images and regional tectonics, the age of 134±7Ma is considered to be theemplacing age of Chibosong mafic-ultramafic rocks.2. Rock series and origin of Chibosong intrusiveThe major element data of Chibosong mafic-ultramafic rocks indicate thatthey belong to sub-alkali series and low-potassium tholeiitic series, and displaytholeiitic evolutionary series.Petrography and geochemistry of Chibosong mafic-ultramafic rocks suggestthat they should be resulted from the accumulation of mafic magma.3 Magma resource and its nature for Chibosong mafic-multramafic rocksThe Mg# values for Chibosong peridotite and gabbro range from 61 to 78.Sc , Co, and Ni contents for them are from 21.85 to 34.22, 66.22 to 207.57, and194.90 to 4699.67, respectively. The evidence mentioned above implies thatChibosong mafic-multramafic rocks should be of characteristics of primarymagma and could be derived from partial melting of peridorite in enrichedmantle. Sr-Nd isotopic compositions of the mafic-ultramafic rocks, such as Isrvalues ranging from 0.7063 to 0.7088 and the negative εNd(t) values, alsodemonstrate above-mentioned consideration. In addition, Sr-Nd isotopiccompositions for the mafic-ultramafic rocks, together with Archean inheritedzircons in them, shows that intensive crustal hybridization could took placeduring the formation and evolution of the primary magma, which is also provedby the depletion of high-field strenghth elements in them.4. Tectonic background of formation of Chibosong mafic-ultrafic rocksBoth major and minor elements in the Chibosong intrusive indicate thatthey could formed in the active continental margin. On the other hand, theoccurrence of contemporary syenite porhyry dykes in this region, together withmafic-ultramafic dykes, constitute the bimodal magmatism in the region,implying an intensive extensional tectonic environment. This tectonicenvironment could be consistent with the peak stage of lithospheric thinning inMesozoic period. Many Mesozoic basins and large-scale metal mineralizationalso formed during this period. Mesozoic large-scaletectonic-magmatism-mineralization should be associated with the subductionof the paleo-Pacific plate.5. Relationship between Chibosong mafic-ultramafic magmatism andmetallogenetic processThe deep differentiation of mafic-ultrafic primary magma should beresponsible for the formation of ore-forming hydrothermal fluid. The pulseintrusion of differentiated magma results in distribution of intrusive phases. Thelatter furthur controls the occurrence of disseminated copper-nickelmineralization. However, the structural fractures formed in the late stage ofmagmatism restrict the Cu-Ni vein mineralization.