Geochemical Characteristics And Tectonic Setting Of Neoproterozoic Mafic-ultramafic Rocks In Western Margin Of The Yangtze Craton--Exampled By The Gaojiacun Complex And Lengshuiqing No.101 Complex

Recently, several important progresses have been made in the geodynamics concerning the evolution of mafic and ultramafic rocks. Furthermore, numerous, economically important, large to giant magmatic Cu, Ni, Cr, Co, and platinum group elements (PGEs) sulphide deposits and Fe-V-Ti deposits are associated with mafic and ultramafic layered intrusions.Recent studies indicate that Neoproterozoic magmatic rocks on the western margin of the Yangtze craton correspond to those on the southern and southeastern margins. To understand their origin is potentially important to the study on the evolution of the supercontinent Rodinia. However, previous studies were focused on Neoproterozoic acid magmatic rocks on the western margin of the Yangtze craton, with little attention being paid to basic magmatic rocks. The Gaojiacun intrusive complex is a typical and the biggest Neoproterozoic mafic-ultramafic intrusion on the western margin of the Yangtze craton. The Lengshuiqing intrusive complex hosts magmatic Cu-Ni sulphide deposit. Thus, the Gaojiacun and Lengshuiqing mafic-ultramafic intrusive complexes are chosen for the studying objects in this dissertation. Petrology and mineralogy, major, trace element, Sr-Nd-Pb isotopes, and precise geochronology are presented to provide an insight into geochemistry and tectonic setting of the Gaojiacun and Lengshuiqing complexes. The purposes of this study are as follow: (1) to determine the forming age and geochemical characteristics of the Neoproterozoic mafic-ultramafic rocks on the western margin of the Yangtze craton; (2) to discuss the genetic relationship between basic magma evolution and Cu-Ni - (PGE) deposit, and to provide constraints on assessing the ore-forming potential of the mafic-ultramafic intrusive complexes in the Yanbian area, Sichuan province; (3) to characterize the nature of their mantle sources and processes, and to understand the relation between tectonic setting and the assembly and breakup of the supercontinent Rodinia. Studies on the geochemistry and tectonic setting of the mafic-ultramafic intrusive complexes on the western margin of the Yangtze craton play an important role in understanding the processes of Neoproterozoic tectonicevolution and clarifying the debates on the assembly and breakup of the supercontinent Rodinia in South China. The present study leads to the following conclusions:(1) The Gaojiacun intrusion and the Lengshuiqing intrusion hosting magmatic Cu-Ni sulphide ore deposit are both well-differentiated mafic-ultramafic intrusive complexes and intrude into the meta-volcanic rocks and schists of the Mesoproterozoic Yanbian Group, although their sizes are different. Both intrusions are composed of mafic-ultramafic accumulating rocks. The former is a layered intrusion characterized by two accumulating cycles, corresponding to two pulses of parental magma. In contrast, the latter is a concentric ring body formed by fractional crystallization. The Cu-Ni sulphide ores, representing a typical magmatic sulphide deposit, occur at the bottom and in the margin of the Lengshuiqing No. 101 complex and consist mainly of pyrrhotite, pentlandite and chalcopyrite.(2) The single zircon U-Pb age of (840 + 5) Ma for intrusion of the hornblende gabbro in the major rock phase of the Gaojiacun complex indicates that there occurred extremely intense Neoproterzoic basic magmatism on the western margin of the Yangtze craton. The 40Ar/39Ar dating of hornblende from the upper hornblende gabbro of the Lengshuiqing No. 101 complex records an age of (821 ± 1) Ma. It is suggested that the formation of the Gaojiacun intrusion is almost contemporaneous with that of the Lengshuiqing intrusion.(3) The chondrite-normalized rare earth element (REE) patterns and the primitive mantle-normalized trace element "spider diagram" of the Gaojiacun complex are similar to that of the Lengshuiqing complex. These data indicate that they are likely derived from the same mantle source. The samples from the two intrusions are characterized by positive ε Nd (t...