P-T paths of contact metamorphism constraints on emplacement mechanism of magmas of the zoned Fangshan granitic Plutons, Zhoukoudian, Western Hills of Beijing, China

Abstract

P-T path of a contact aureole can provide important insights into the magma emplacement mechanism for nearly concentrically zoned granitic plutons. The Mesoproterozoic Xiamaling formation immediately adjacent to the southern Fangshan granitic Plutons in the Western Hills of Beijing, China, developed three contact metamorphic prograde staurolite zone, sillimanite zone and K-feldspar zone. These zones preserved corroded pink-andalusite wrapped by pale-andalusite, corroded garnet inclusion inside the K-feldspar porphyroblasts and staurolite, which add to well-established sequences of mineral growth. These sequences allow reconstruction of the complete P-T path for each prograde zone on the pseudosection diagram obtained by full system composition-based phase equilibrium modeling The resultant prograde P-T paths are composed of two segments, with the first one isobaric heating and the second distinct compression and heating. The staurolite zone shows the P-T path initiating from ~2.0 kbar/350 °C to ~1.8 kbar/510 °C and ending at ~3.3 kbar/580 °C. The sillimanite zone path began from ~2.0 kbar/350 °C to ~1.5 kbar/500 °C then ended at ~3.8 kbar/580 °C. The K-feldspar zone path started from ~2.0 kbar/350 °C to ~1.7 kbar/510 °C, then terminated at ~3.5 kbar/670 °C, approaching the initial partial melting temperature condition, then documented a retrograde path cooling down to ~630 °C. The two prograde P-T path segments of the three zones suggest a in situ heating followed by a ductile return flow from ~7 km down to ~11 km depth of the metapelites from the Xiamaling formation, responsable to a diapiric emplacement of the magmas for the zoned composite Fangshan Plutons.