Ignimbrites are formed by large-scale explosive eruptions of felsic (silica-rich) magmas, making this statement a matter of standard geological definition.
The claim states a fundamental definition in volcanology: ignimbrites are pyroclastic density current deposits typically generated by the explosive eruption of silica-rich (felsic) magmas like rhyolite and dacite. Because this is a standard, universally accepted definition in the geological sciences, no citation is needed.