Magmatic rocks with δ¹⁸O values significantly lower than normal mantle-derived magmas can be generated by a series of special geological processes during the formation and evolution of crustal magmas. While there is negligible effect of magma differentiation on the oxygen isotope composition of magmas, theoretical calculation suggests that high-T water-rock interaction is a significant process to decrease the oxygen isotope ratios of silicate rocks.  The low δ¹⁸O magmatism usually takes place in rifting tectonic zones, with development of caldera collapse and subsequent remelting of low δ¹⁸O rocks. The granites at Seychelles in Indian Ocean and Nianzhishan in Northeast China represent two examples of low δ¹⁸O granites crystallized from low δ¹⁸O magmas.