Zirconate and titanate pyrochlores were irradiated with swift heavy ions in order to investigate the effects of the chemical composition on the structural changes induced by high electronic excitation. Both transmission electron microscopy and X-ray diffraction results show that the structural modifications induced by irradiation with 120-MeV U ions are strongly dependent on the sample composition: Gd2Ti2O7 is readily amorphized, whereas Gd2Zr2O7 is transformed into a radiation-resistant anion-deficient fluorite structure. For Sm2Zr2O7, Eu2Zr2O7 and Nd2Zr2O7, more complex behavior is observed, since both pyrochlore-fluorite phase transformation and amorphization occur. A new phenomenological model (the heterogeneous track overlap model, HTOM), which assumes a direct impact mechanism coupled with a single track overlap process, is proposed to describe the formation of heterogeneous track structures consisting of mixed anion-deficient fluorite and amorphous domains. The pyrochlore composition mainly influences the structure of ion tracks (and weakly their diameter), and essentially concerns the amount of amorphous phase vs the amount of fluorite counterpart.