Fluid Regime of the Amphibolite-Facies Metamorphism in the Dzhugdzhur–Stanovoy Fold Area (Far East)

摘要

Chromatographic and electrochemical measurements, combined with computer simulation of thenatural mineral parageneses and estimation of the stability field of muscovite-bearing assemblages, yielded aconsistent model of the fluid regime for the amphibolite-facies metamorphism of the Dzhugdzhur–Stanovoyfold area (DSFA). The model allows the fluid differentiation into "internal" and "external" fluids. The "internal" fluid is formedby the volatiles of the rock, while the "external" fluid arrived from an outer source: the mantle or other reservoir.It is established that the chromatographic and electrochemical measurements refer to the "external" fluid,whereas the redox state estimated from the mineral equilibria is related to the "internal" fluid, whose composi-tion is buffered by the equilibrium mineral assemblage. The "external" fluid trapped by rocks preserves its ownredox state only at the regtrograde stage, when the solid-phase reactions slacken and the buffer role of the min-eral assemblages is minimized. This aspect explains the contradiction between the wide variations in the oxi-dation state of the mineral equilibria (log fO_2 from –15 to –20), on the one hand, and the persistent oxidation state of the external fluid established by the chromatographic and electrochemical methods, on the other hand.The main reason for the wide development of hornblende-bearing assemblages in the amphibolite-facies meta-morphic rocks of the Dzhugdzhur–Stanovoy fold system is the high H_2Opressure in the "external" fluid. According to the obtained data, the composition of the "external" fluid is determined by the conditions P_(H_2O)0.7 P_S andP_(CO_2)/P_(H_2O)=0.01-0.3. The oxidation potential of the "external" fluid is close to that of the H_2O–Csystem under carbon-saturated vapor conditions.