Studies Of Neoproterozoic Mafic And Ultramafic Intrusions In Western-central China And Their Constraints On Breakup Of Rodinia Supercontinent

The various igneous rocks of the middle Neoproterozoic (~ 800 Ma) that are well developed in the South China Platform have been received a great deal of attention to many researchers over the world. The nature of magmatic activities in this period, whether in mantle plume or in island arc of continental margin, directly constrains the re-construction and breakup of Rodinia supercontinent, as well as the location of the South China platform in the re-constructed model of Rodinia supercontinent. On the basis of field investigations, three Neoproterozoic mafic and ultramafic intrusions in Qinling-Qilian-Kunlun orogenic super-belt and adjacent areas were selected for comprehensive studies on petrology, geochemistry, mineral structures, melt and fluid inclusions and SHRIMP zircon U-Pb dating in this paper. Rock-forming processes and environments of the three intrusions have been analyzed. These studies play important roles in comparing analysis and exploring the regional tectonic settings in the Neoproterozoic era and in tracing information for tectono-magmatism associated with breakup of the Rodinia Supercontinent. Some achieves are as following:1. Jinchuan Cu-Ni-ore-bearing ultramafic intrusion occurs as lensoid or dyke-like bodies in NW direction in the Longshushan Rise, the south margin of Alax block. It consists of dunite, lherzolite, plagioclase lherzolite and olivine-pyroxenite. Zircon U-Pb SHRIMP analyses yield precise magmatic crystallization ages of 827? Ma for the ultramafic body and 828? Ma for the accompanied dykes. Petrological and geochemical studies reveal that the parental magma of Jinchuan Cu-Ni ultramafic body is high-Mg picritic tholeiite, and mafic dykes within or outside the body are also the products of tholeiitic intrusion from the same parental magma. Nd isotope, REE and trace elements reveal that this body and mafic dykes were sourced from a long-term enriched mantle (eNd(t) < -3). The results suggest that Jinchuan ultramafic body and the accompanied dykes are deep-seated intrusions related to within-plate mantle plume magmatic activity.2. Based on detailed tectono-petrographic investigations, this paper demonstrates that Hannan Complex is a large layered cumulate hybrid body that consists of ultramafic zone, gabbro zone and gabbro-diorite zone from the bottom to the top. Troctolite layers that steadily occur in the basal part of the complex reflect that theparental magma generates in a "dry" environment rather than the "wet" island arc, which is characterized by crystallization of plagioclase earlier than that of pyroxenes and shows a cumulate sequence of dunite-troctolite-olivine gabbro-gabbro from bottom to top. The negative anomaly of Zr and Hf in some rocks can be readily explained as excessive cumulation of plagioclase in troctolite and light-colored gabbro in the low part of the cumulate sequence, because Kd of Zr and Hf between plagioclase and melt is much lower than that between pyroxene and melt. Discovery of ilmenite exsolution lamellae in plagioclase suggests that minerals crystallized from a high-T and Ti-rich parental magma. Combining all aspects of fact, for instance, the LREE-enriched patterns of all rocks, the E-MORB-affinity trace element spidergram shown by most cumulate gabbro and gabbroic diorite, and occurrence of the large ore deposit of V-bearing titanic magnetite, we can conclude that Hannan complex crystallizes from a high-temperature magma at an environment of mantle plume.3. Songshugou dunite body and accompanied garnet-pyroxenite and amphibolite and Fushui complex body in north Qinling Orogenic Belt have received a great deal of attention. Using CL and ion microprobe (SHRIMP) techniques, this study obtains a high-pressure metamorphic age at 501+ 10 Ma for garnet pyroxenite (Sm-Nd isochron age is 1030+46Ma for the protolith) near Songshugou dunite body and a magmatic age at 490+10 Ma for Fushui island-arc gabbro-diorite complex. The two concordant ages constructed the Early Paleozoic tectonic pattern of plate subduction and collision in the North Qin...