Crustal section in the New England Appalachians records polybaric fractional crystallization-driven magma differentiation

Ultramafic-mafic cumulates in the lower crust suggest that fractional crystallization (FC) drives the differentiation of primitive basaltic arc melts to intermediate compositions. Polybaric FC in the middle to upper crust may further evolve melts to silicic compositions, predicting compositional stratification with depth. However, such stratification is not observed in exposed arc sections. We investigate the Kinsman suite in the New England Appalachians, which exhibits depth-dependent compositional stratification, offering a critical test of the polybaric FC model. The Kinsman suite could originate from (1) polybaric FC of intermediate melts derived primarily from primitive arc melts with a minor contribution from crustal metasedimentary rocks, or (2) metasedimentary anatexis with selective entrainment or unmixing of peritectic minerals. Focusing on the Cardigan pluton (emplaced at ∼1 to 7 kbar, ∼406–403 Ma), the largest magma body within the Kinsman suite, we observe systematic increases in SiO₂ and Rb and decreases in MgO, FeO_total, Al₂O₃, CaO, Sr, Ba, Sc, and Yb with decreasing emplacement depth. This compositional stratification, coupled with field evidence of cumulates, a continuous evolution of mineral geochemistry, and thermodynamic modeling, support continuous polybaric FC in the middle to upper crust of the Acadian arc. Zircon Hf isotopes indicate a common magma source for the Kinsman suite and contemporaneous ultramafic-mafic cumulates in northeastern Connecticut, with ∼65–95% contribution from primitive arc melts and ∼5–35% from crustal metasedimentary rocks. These findings highlight the Kinsman suite as a compelling example of polybaric FC in a continental arc, providing key insights into the magma differentiation and granite formation in the middle to upper crust.