The Study On The Late Paleozoic Basic-intermediate Dyke Swarms In Baogutu-Urho Area,Western Junggar

Late Paleozoic basic-intermediate dyke swarms are wildly distributed in Western Junggar, especially in Baogutu-Urho Area. The dyke swarms can be divided into three groups based on their distribution and formation age: Urho basic-intermediate dyke swarms, Xiaerpu basic-intermediate dyke swarms and Baogutu basic-intermediate dyke swarms. These dyke swarms which recorded the vital information on magmatic and tectonic evolution of Western Junggar in Late Paleozoic are of great value of probes and indicators. This dissertation based on previous research results takes these dyke swarms as research objects and studies systematically on their field geology, petrology, chronology, geochemistry, petrogenesis and structural environment. It can provide new evidences for the further understanding of the Late Paleozoic magmatic and tectonic evolution in Western Junggar.Urho dyke swarms, Xiaerpu dyke swarms and Baogutu dyke swarms are densely distributed in Late Palaeozoic strata and intrusions. The extension direction of the NE-SW-trending dyke swarms are basically the same with Darbut Fault. Urho and Baogutu dyke swarms are parallel to Darbut Fault and Xiaerpu dyke swarms intersect it by minor angels. The rocks are mainly composed of diorite,diabase, diorite-porphyrite and diabase porphyrite.Detailed geochronological study shows that the formation age of Xiaerpu and Baogutu dyke swarms are similar. They were both generated in the early stage of Late Carboniferous(313Ma). Urho dyke swarms were generated in late Early Permian(283~286Ma), which were posterior to Xiaerpu and Baogutu dyke swarms.Urho, Xiaerpu and Baogutu dyke swarms have a high concordance in geochemical characteristics. They all have features of low K calc-alkalic rocks with high level of A12O3 and Mg#. The dyke swarms all have low REE and low level of fractionations between light and heavy REE. The anomalies of δEu are not obvious and the REE distribution patterns are all of the right-oblique type.The dyke swarms are all characterized by the relative enrichments of large ion lithophile elements and depletion of high field strength elements, showing the geochemical characteristics of magmas from the subduction zone. The content of Cr, Ni which shows a great difference in different samples and high average values on the whole between the three has a positive correlativity with the content of MgO. Positive εNd(t)values which are essentially consistent and relatively low initial Sr isotopic ratios indicate the depleted mantle-related source. Moreover, Xiaerpu and Baogutu dyke swarms show geochemical characteristics of adakitic High-Mg andesite and sanukitic High-Mg andesite.Petrogenetic study shows that Urho, Xiaerpu and Baogutu dyke swarms were all generated by partial melting of the mantle wedge. Urho dyke swarms were metasomatized by slab released fluid. In addition to slab released fluid, Xiaerpu and Baogutu dyke swarms which show geochemical characteristics of adakitic High-Mg andesite and sanukitic High-Mg andesite were also metasomatized by slab released sediment melts.Xiaerpu and Baogutu dyke swarms were generated in the residual sea evolutionary stage in the early stage of Late Carboniferous, at which time Junggar oceanic basin was almost closed and the rotation and deformation further developed on the edge of Kazakh orocline. Partial melting occurred when the mantle wedge was metasomatized by slab released fluid and sediment melts. Upwelling basaltic magmas intruded into Carboniferous strata and Baogutu dyke swarms took shape. Xiaerpu dyke swarms took shape after the process that magma mixing occurred to some extent between basaltic magmas and granitic magmas generated by partial melting of the lower crust.Urho dyke swarms were generated in the lagged arc evolutionary stage in late Early Permian, at which time Junggar oceanic basin had already closed. The subduction between plates still went on after the extinction of trench-arc-basin systems and the occurrence of the continental basin. Partial melting occurred when the mantle wedge was metasomatized by slab released fluid. Upwelling lagged arc magmas intruded into Early Permian strata and Urho dyke swarms took shape.