The relationship between metamorphic field gradients and the pressure-temperature conditions of metamorphism is a foundational concept in metamorphic petrology and does not require citation.
The claim states a standard definition and fundamental principle of metamorphic petrology: metamorphic field gradients record spatial variations in metamorphic grade, which directly reflect the pressure and temperature conditions experienced by the rocks during metamorphism. This is a matter of basic geological definition and everyday observation within the field, rendering it common knowledge.