Pressure estimated from syngenetic melt and fluid inclusions: Probable reasons for discrepancies

Abstract

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°e . Melt inclusions less than 3 µ m in size, on being heated higher this temperature, collapse. Therefore, the temperature of their total homogenization was determined accurate to ± 15°e under restricting pressure of argon up to 5 kbar. The content of c 2 e in MI ( ) was calculated within ± 15% (1 σ ) 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]. Most likely, MIs in phenocrysts characterize the early stage of magma crystallization during the preintrusion period, whereas MIs in quartz of mesostasis of granite porphyry and vein aplite fit the late stage of crystallization. The former differs from the latter in the higher (790–760°e against 750–740°e ) and wider range of (7.3‐4.1 wt % against 4.5 ± 0.5 wt %). Along with MI, syngenetic primary inclusions of magmatic fluid (IMF) homogenized at 300–290°e , less