Discovery of the Plagiogranites in the Diyanmiao Ophiolite,Southeastern Central Asian Orogenic Belt,Inner Mongolia,China and Its Tectonic Significance

摘要

In this study, plagiogranites in the Diyanmiao ophiolite of the southeastern Central Asian Orogenic Belt(Altaids) were investigated for the first time. The plagiogranites are composed predominantly of albite and quartz, and occur as irregular intrusive veins in pillow basalts. The plagiogranites have high SiO_2(74.37–76.68 wt%) and low Al_2O_3(11.99–13.30 wt%), and intensively high Na_2O(4.52–5.49 wt%) and low K_2O(0.03–0.40 wt%) resulting in high Na_2O/K_2O ratios(11.3–183). These rocks are classified as part of the low-K tholeiitic series. The plagiogranites have low total rare earth element contents(∑REE)(23.62–39.77 ppm), small negative Eu anomalies(δEu=0.44–0.62), and flat to slightly LREE-depleted chondrite-normalized REE patterns((La/Yb)N=0.68–0.76), similar to N-MORB. The plagiogranites are also characterized by Th, U, Zr, and Hf enrichment, and Nb, P, and Ti depletion, have overall flat primitivemantle-normalized trace element patterns. Field and petrological observations and geochemical data suggest that the plagiogranites in the Diyanmiao ophiolite are similar to fractionation-type plagiogranites. Furthermore, the REE patterns of the plagiogranites are similar to those of the gabbros and pillow basalts in the ophiolite. In plots of SREE–SiO_2, La–SiO_2, and Yb–SiO_2, the plagiogranites, pillow basalts, and gabbros show trends typical of crystal fractionation. As such, the plagiogranites are oceanic in origin, formed by crystal fractionation from basaltic magmas derived from depleted mantle, and are part of the Diyanmiao ophiolite. LA–ICP–MS U–Pb dating of zircons from the plagiogranites yielded ages of 328.6±2.1 and 327.1±2.1 Ma, indicating an early Carboniferous age for the Diyanmiao ophiolite. These results provide petrological and geochronological evidence for the identification of the Erenhot–Hegenshan oceanic basin and Hegenshan suture of the Paleo-Asian Ocean.