The rocks of Lascar belong to the calc-alkaline series. SiO2 concentrations range from 55.5 to 67.8% by weight, and the rocks have medium to large concentrations of potassium. The magmas are contaminated by the local crust, but not to the extent found in the Galan or Purico complex eruption products. The chemistry of Lascar's rocks is fairly similar to those of neighbouring Tumisa volcano.