Dikes And It's Tectonic Setting In Barenzhelimu Area, Inner Mongolia Province

The Mesozoic Volcanic rocks and dykes are grow widely in Barenzhelimu area, inner Mongolia province. it may provide the basis formations for differentiating regional tectonic evolution in Mesozoic about the study of the rock geochemistry and the spatial distribution. The essay takes the dykes in Barenzhelimu area as the study objects and uses the ways of systemic Petrology, geochemistry and Zircon U-Pb chronology to define the formation age of the dykes. Also we use the method of the isotope "Hf" chronology to probe the Magma sources and its' nature. And at least we clarify the forming tectonic setting for the dykes in this area.The dykes' Temporal and spatial distribution feature in Barenzhelimu area.The dykes distributed widely in this area. In space, the dykes accompanied with the Mesozoic Volcanic rocks emerged banded and its' direction is north-east. In some portion the dykes are concentrated relatively. And they make up the N-E Intrusive rock belt. In time, the dykes in this area may divided into two period. The early development time is about 137 ma and the late time is about 113 ma. In this area, the Magma intrusion started from Late Jurassic and ended in early Cretaceous.The mainly types of the dykes and their geochemistry featureIn the study area, the mainly types of the dykes are Granite porphyry, Monzonite porphyry, Diorite and Diorite porphyry etc. their geochemistry feature are mainly similar. They all belongs to High-K calc-alkaline rocks. And their Trace Elements feature are::LREE enrichment, HREE loss, Negative Eu anomaly and enrichment of large ion lithophile elements. The dykes' geochemistry features (major elements, trace elements and REE) indicated that they are similar with the feature of the component of the bottom of the crust. According to the tectonic history, we analyzed that the dykes in this area are suspected to be the outcome in the Period of crustal uplift after Plate collision..The dykes' Zircon U-Pb chronology in Barenzhelimu areaThe essay uses the way of LA-ICP-MS to define the age on zircon. The result demonstrates that the Crystallization age of emplacement of dykes in this area is between 113±61 Ma ---137.2±1.5 Ma. The emplacement time of Diorite which is 137.2±1.5 Ma,(i.e. late Jurassic) is earlier than other types of dykes, while Granite porphyry and Diorite porphyry's emplacement time are 121.6±1.7 Ma and 113±61 Ma respectively(i.e. early Cretaceous.). And they are later than Diorite' on time. Combined with geological data in this area, we analysis that the diorite intrusive into the strata of this area in late Jurassic firstly. Accompany with the beginning of the Volcanic activity in late Jurassic, magmatic activity intensified. Then the Diorite which intrusive earlier was destroyed by the Granite porphyry and Diorite porphyry who intrusive late in early Cretaceous.The dykes' Zircon isotope Hf chronology in Barenzhelimu areaThe essay uses the way of LA-ICP-MS to determination of isotopic Hf's composition. And we take the granite porphyry and diorite as the test samples. The result show that the zircon particles of the samples are coarse, and develop the oscillatory zoning. Where they come from is upper mantle or lower crust. And they are outcomes of partial melting from upper mantle or lower crust. The reason of their conformation related with the role of lithospheric extension after collisional orogeny.The dykes' cause and its' tectonic settingBy the study of the geochemistry, Zircon U-Pb chronology and Zircon isotope Hf chronology of dykes, the author think that the initial magma of the dykes in this area was from upper mantle and lower crust. It is the outcome who had been formed because something melt partially from upper mantle and lower crust. And the dykes in this area took shapes in a extensional environment after the collisional.During the late Jurassic and the early Cretaceous, the effect of the orogeny of the Mongolia-Okhotsk strengthened.and all the eastern China was in a dextral extensional environment. In this stage the crust' feature is early brittle extension and sudden upwelling of hot in deep. At the same stage, the compression had been decreasing in north-west and trending fault zone in north-east depressurized relatively as north Daxing'anling areas of the Pacific plate motion change of direction. In the context of this regional structure, the study area had been undergoing the effect of the extension, thinning and asthenospheric upwelling of the lithospheric. Then it prompt the things from the upper mantle and lower crust to melt. The forming magma had experienced the course of the fractional crystallization and the continental crust contamination. After this, the magma intrusive increased along the north-east direction. It is the reason why the mid-acid dykes grow widely in Barenzhelimu area.