Late Hercynian-indosinian Mafic-ultramafic Rocks Of The Central-Eastern Part Of Jilin Province And Cu-Ni Metallization

This thesis presents the geochronology, geochemistry and mineralization featuresof the Late Hercynian-Indosinian mafic-ultramafic intrusions in central-eastern part ofJilin province, NE China. The characteristics of the magma source, evolution, thetectonic environment and dynamics mechanism are discussed. The tectonic evolutionof the study area during Late Hercynian to Indosinian is studied. The relationshipbetween the tectonic evolution process and the Ni-Cu mineralization is summarized.The mafic-ultramafic intrusions distribute along the subsidiary faults of twomajor fractures (Huifahe and Fuerhe-Gudonghe fractures) in central-eastern part ofJilin province. The SHRIMP zircon U–Pb and Ar/Ar ages of these intrusions concludethat two stages of mafic–ultramafic magmatism occurred in this area during LateHercynian to Indosinian.Mafic-ultramafic intrusions formed in Late Hercynian located in No.2belt ofHongqiling area are mainly consisisted of amphibole pyroxenite, gabbro andlherzolite. They are characterized by enrichment in large ion lithophile elements (Ba,Rb, Sr, Pb) relative to high field strength elements (Ta, Nb, Zr), indicating that theywere derived from a metasomatized lithospheric mantle. On the other hand, they showE-MORB-like geochemical compositions such as Nb/Yb and Th/Yb ratios, suggestingthat an asthenospheric mantle was involved. Crustal assimilation seems notsignificantly modify the geochemical compositions of the mantle-derived magmasduring ascent.Mafic-ultramafic intrusions formed in Indosinian mainly distribute inSandaogang, Hongqiling, Chajian, Erdaogou, Piaohechuan and Changren area. Therock types of these intrusions are relatively complex. Some of them consist of muti-types of rocks with a characterisitic of strong differentiation (such as theHongqiling No.1intursion). Others are intrusions with simple rock type. Samples ofIndosinian mafic-ultramafic intrusions are enriched in LREE, Th, U, Sr and Pb,depleted in Ta, Nb and Ti, which can be explained by subduction-relatedmetasomatism, and suggest that the magmas have been contaminated by continentalcrust during ascent. These samples also shows E-MORB-like εNd(t), Nb/Zr and Nb/Lavalues, suggesting that an asthenospheric mantle was involved.The two stages of mafic-ultramafic magmatism indicate that the study areaexperienced three stages of tectonic evolution events from Late Hercynian toIndosinian. Middle–Late Permian (272~258Ma), mafic–ultramafic intrusions arecorrelated with the first stage magmatism to a slab break–off regime following theinitial collision between the North China Block and the Songnen–ZhangguangcaiRange Massif (SZRM). The Late Hercynian intrusions were probably formed byinteraction of asthenospheric and metasomatized lithospheric mantle. In this scenario,upwelling asthenospheric mantle would have underwent decompression melting andinduced partial melting of overlying metasomatized lithospheric mantle. During LatePermian to Early Triassic (258~240Ma), the study area was under the compressionalenvironment related to the final closure of Paleo–Asian Ocean, which made the LateHercynian intrusions show same NW direction foliation as Hulan Group.Middle–Late Triassic (240~212Ma), mafic–ultramafic intrusions emplaced under thepost–orogenic extensional environment associated with the final amalgamation of theNCB and the Songnen–Zhangguangcai Range Massif. Consequently, the Indosinianmafic–ultramafic intrusions were derived from the partial melting of lithosphericmantle heated by upwelling of asthenospheric mantle in a post–orogenic extensionalsetting in response to delamination of the lithospheric mantle.The Indosinian mafic–ultramafic intrusions are ore-bearing intrusions ofconsiderable economic value, however there is no economic mineralization related tothe Late Hercynian intrusions. According to the tectonic evolution of the study area, the reason might be: first of all, the Indosinian mafic–ultramafic intrusions formed ina post–orogenic extensional setting after the final amalgamation of the NCB and theSongnen–Zhangguangcai Range Massif, which is relatively stable. A stable tectonicenvironment may be conducive to the differentiation and mineralization. Secondly,the Indosinian mafic-ultramafic magmatism has a larger scale relative to the LateHercynian, is a favorable factor of mineralization. And the composition of the LateHercynian intrusions are close to magmas derived from metasomatized lithosphericmantle, sulfide is hard to extract from that. Finally, the Indosinian mafic–ultramaficintrusions experenced crustal contamination, promotes the segregation of sulfide.