Differentiation mechanisms of felsic magmas: Evidences from an Appalachian crustal section

A fundamental petrological paradox exists regarding the formation of the continental crust: the partial melting of the mantle produces basaltic magmas, whereas the continental crust is predominantly composed of andesitic to felsic rocks. Previous studies indicate that basaltic magmas primarily generate andesitic magmas via high-pressure differentiation within the lower crust (>700 MPa). However, the differentiation mechanisms responsible for evolving these andesitic magmas into felsic magmas in the middle-to-upper crust, after their extraction from deep source regions, are not well-constrained by direct geological evidence. The primary reason for this knowledge gap is the scarcity of natural crustal sections that continuously recorded magmatic evolutionary paths. A recent study identified an about 24 km thick (4~28 km depth) continuous section of the middle-to-upper crust in the Appalachian orogeny of New England, USA. This section exhibits continuous compositional stratification from the bottom (700 MPa) to the top (100 MPa). This feature demonstrates that polybaric fractional crystallization could be a key mechanism driving the further evolution of the andesitic magma into felsic magma within the middle-to-upper crust. This study provides critical geological evidence for the trans-crustal magmatic system model.