Dehydration-melting and fluid recycling during metamorphism: Rangeley formation, New Hampshire, USA

Abstract

Muscovite and biotite dehydration-melting reactions near the peak of metamorphism played a significant role in the reaction and fluid history of the Rangeley Formation in southwestern New Hampshire, USA. Evidence for in situ melting includes: (1) the consistency among theoretical phase equilibria, observed reaction textures, and the inferred P-T conditions; (2) disseminated, centimeter-scale, leucocratic quartz+plagioclase+muscovite pods; (3) diffusion and growth zoning of major and trace elements in garnet that are best explained as the result of high-T muscovite and biotite breakdown; and (4) oxygen isotope evidence that high-T back-reaction of K-feldspar to muscovite near peak metamorphic conditions did not involve an isotopically disequilibrium (externally derived) fluid. Isotopically equilibrated fluids were apparently stored in melt pockets and then reused as the melts crystallized, thereby driving retrogression. Prograde moscovite dehydration-melting reactions further imply P≥4 kbar at T≤∼65°C, so that loading occurred before the peak of metamorphism at T∼725°C. Oxygen isotope compositions of retrograde garnet that grew during cooling between T∼650°C and T∼550°C are consistent with closed-system models, indicating that previous back-reaction of K-feldspar to muscovite did not disturb the isotope compositions of the rocks. Late-stage growth of additional retrograde garnet, staurolite, and chlorite at T∼475°C requires infiltration of externally derived H2O, but this retrograde infiltration did not affect garnet and staurolite isotope compositions, as expected for differing rates of infiltration-driven hydration vs isotope alteration. Late-stage infiltration continued after garnet and staurolite growth ceased, as evidenced by systematic differences in isotope trends near the base of the nappe for minerals with fast oxygen isotope diffusion rates (quartz, muscovite, and biotite) vs minerals with slow diffusion rates (garnet, staurolite, and sillimanite). This infiltration may reflect the dewatering of structurally lower levels after nappe emplacement. If so, then nappe emplacement occurred at T∼475°C.