Thermodynamic constraints on the composition of orogenically thickened lower crust

Abstract

Orogenically thickened lower crust is the key site of crustal differentiation, crustal deformation, and Moho modification. However, the composition of thickened lower crust is still highly debated. Here, we calculate a set of pseudosections with mafic lower crust compositions in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O₂ (NCKFMASHTO) system. Our modelling results show that the maximum thickness of the mafic lower crust increases with the Moho temperature (T_Moho). In addition, the lithologies of stable mafic crust are characterized by medium-pressure (MP) to high-pressure (HP) granulites at 40–50 km, HP granulites and garnet-omphacite granulites at 50–60 km, and garnet-omphacite granulites at 60–70 km. Under the Pamir geothermal conditions, mafic rocks with high SiO₂ (>50.2 wt%), X_Mg (>0.70), X_Ca (>0.49), or low X_Al (<0.11) could be stable at 70 km; however, only ~10% of global mafic granulite xenoliths lie within this compositional range. Further modelling indicates that if T_Moho reaches 900–1000°C, neither the lower crust nor the upper mantle has significant strength relative to the upper crust and that only ~5–37% of mafic materials are gravitationally stable at 70 km. This implies that the base of doubly thickened (70 km) crust is dominated by intermediate-felsic rocks, consistent with the low V_p and V_p/V_s values seismically observed in young orogenic crustal roots. Thus, most mafic materials at >70 km could delaminate into the deep mantle. Our results provide insights on the formation of extremely thick crust with a predominantly intermediate-felsic base and the crustal thickness variation in continental collision zones.