Lithogeochemistry and isotopic compositions of Soklu alkaline volcanic rocks, Dongnai

Abstract

The alkaline volcanic rocks were distributed in the SokLu mountains, in the territory of Quang Trung commune, Thong Nhat district, Dong Nai province, and considered Soklu Formation, exposed as a truncated cone-shaped dome with an area of more than 12.65km2, located close to the National Highway 20 from Dau Giay to Da Lat, 3km north of Dau Giay junction, with an altitude of 408.3m, prominent in basalt cover, the formation consists mainly of trachyandesite nearly homogeneous in the whole massif. The rock is ashen-grey, light grey colored, porphyritic texture with the microcrystal groundmass of needle-shaped plagioclase arranged parallel to each other (trachytic texture), directive structure (flow structure). The phenocrysts mineral composition accounts for about 10÷15%, scattered, sometimes concentrated in clusters to form a phenocrysts clump texture, including plagioclase (40÷60%), pyroxene (20÷25%), olivine (15÷20%), the groundmass mineral composition accounts for about 85÷90%, including main glass (45-55%) and the rest are needle-shaped plagioclase microcrystals arranged directive in clusters (15÷20%), pyroxene (10÷15%), ore (>5%). The results of research on lithogeochemical characteristics and isotopic compositions of Sr, Nb, Pb show that SokLu alkaline volcanic is characterized by high SiO2 content (56.08-57.58wt%), a high total of alkaline (Na2O+K2O) vary from 9.90-10.87wt%, low Na2O/K2O ratio (1.09-1.36), it is typical of the alkaline sodium and total iron Fe2O3t accounts for 5.74-3.37wt%, low iron content F=FeO/MgO (1.62-2.55) and K2O/MgO ratio of 1.33-2.00. Soklu alkaline volcanic has high Sr content (1244.30-1298.33ppm), the total content of light rare earth elements is mainly ∑LREE (321.57-339.13ppm), but high rare earth elements occupy the secondary ∑HREE (21.04-21.47ppm), Rb/Sr ratio of 0.08-0.09. The primitive isotope 87Sr/86Sr ratio of 0.704792-0.704807, 143Nd/144Nd ratio of 0.512716-0.512722, 206Pb/204Pb ratio of 18.2282-18.24, 207Pb/204Pb ratio of 15.5839-15.59, 208Pb/204Pb ratio of 38.4560-38.50, reflecting the mantle origin. The analysis result of Ar-Ar isotopic age of Soklu alkaline volcanic for 0.33±0.17Ma, corresponding to Middle Pleistocene.