Pressure Estimated from Syngenetic Melt and Fluid Inclusions: Probable Reasons for Discrepancies

摘要

Estimation of fluid pressure during crystallization of natural water-saturated magmas is important to resolve a number of problems related to petrogenesis and ore formation. Therefore, the Gudzhir granite porphyry pluton, with which Mo and W deposits of the Dzhida ore field in Transbaikalia are related, has been studied. This intrusion was formed as a result of successive emplacement of three magma portions from the same deep-seated source and in situ crystallized vein aplite that is a product of crystallization differentiation. Melt inclusions (Mis) in quartz phenocrysts and fine-grained mesostasis contain both crystalline aggregate and gas bubbles and an insignificant aqueous solution; the gas bubbles and the aqueous solution homogenize into liquid at 230-270℃. Melt inclusions less than 3 μm in size, on being heated higher this temperature, collapse. Therefore, the temperature of their total homogeniza-tion was determined accurate to ±15℃ under restricting pressure of argon up to 5 kbar. The content of H_2O　in MI (C_(H_2O)) was calculated within +15% (lσ) from　thermal and volumetric data [1 ] and taking into account the water content in daughter muscovite and the volume of quartz, which turned to melt from inclusion walls at the moment of homogenization of Mis [2, 3].