| Qinling Shang-Dan suture zone is the boundary between the south and north Qinling, which is also the main suture zone of Qinling orogenic belt. The research of the Shang-Dan suture zone has the vital significance for Qinling tectonic evolution. In this paper we study Sanshilipu gabbro complex rock which exposed along the Shang-Dan suture zone, it is located at the southern margin of the north Qinling, the shape is lenticular and covers an area of about 15 square kilometers. On the basis of field geological investigation, Through the petrology, geochemistry, high-precision LA-ICP-MS zircon U-Pb isotopic dating research to discusses the genesis, tectonic environment and chronology information, further serving constraints for the restoration of the tectonic evolution of the Qinling orogenic belt. Paper’s main understanding is as follows:The rock composition is complex, the west is mainly gabbro-nerite gabbro, central lithology is uniform with the grain of gabbro, the east is gabbro-diorite. The main research is gabbro which in central. The main rock-forming minerals as follows: plagioclase, olivine, pyroxene, hornblende, contain a little of biotite, quartz and potassium feldspar, the light color mineral content is higher, accounting for about 20%.In Sanshilipu gabbro complex rock SiO2 content between 48.57%- 51.25%,Na content is high and K content is low, it shows the calc-alkaline in AMF illustration, riched part LILE(Ba, Sr, Pb) and loosed of part HESF(Nb, Ta, Zr, Hf, Ti), low TiO2 content and the large range Nb/Ta ratios(3.73 to 11.8), REE distribution patterns were nearly flattened-LREE REE enrichment, and Zr/Y=1.05 ~ 3.62, less than 4, above geochemical characteristics indicates that Sanshilipu gabbro complex rock formed in an island-arc environment. Magma source area mainly carry deep sea sediments subduction of oceanic crust and fluid, at the same time have a mantle material to join. Weak positive Eu anomaly, may be the result of melted.The zircon’s color is transparent, euhedral- subhedral shaped, length 90μm- 150μm. CL image shows a significant wildly magmatic shock belts, The content of Th and U are higher, Th/U= 0.51 ~ 0.91, are greater than 0.4. Zircon chondrite-normalized rare earth element is leftist, HREE is high. These features are shown magmatic zircons, dating results may represent diagenetic age of the Sanshilipu gabbro complex rock. The above studies show Sanshilipu gabbro complex rock formed in 414± 2.8Ma. The presence of the island-arc gabbro complex rock indicates that Shang-dan Ocean under thrust to the North Qinling at 415 Ma. Yangtze plate and North China plate has not happened collision at this area, Shang-dan ocean had not disappeared, the overall in extrusion environment.Field geological observations Sanshilipu gabbro complex rock intruded Danfeng ophiolite. The average content of La elements in rock is about 141 times higher than chondrite, the ophiolite is 0.6 to 2 times, a larger difference between them; Ba and Sr element average contentment are about 870 ppm in Shilipu gabbro complex rock, but they show lower levels in Danfeng ophiolite. Two different formation age, geochemical data and analysis of the above chronology shows that Sanshilipu gabbro complex rock is not the part of Danfeng ophiolite. |
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