METASOMATISM IN THE GENERATION OF GRANULITE VEINS: MASS BALANCE, MASS TRANSFER, AND REFERENCE FRAMES

Abstract

In the Seguin subdomain, Grenville Province, dark hornblende–garnet-rich quartz-absent metagabbro is transected by anastomosing light-coloured veins rich in orthopyroxene, clinopyroxene, plagioclase, and, in some cases, quartz. Three types of veins form a smoothly transitional series from fine-grained diffuse veins and patches to a more defined intermediate vein type, and finally to coarsegrained tonalitic veins with abrupt contacts to the metagabbro host. The textures and contact relations of diffuse and intermediate veins are suggestive of channelized subsolidus dehydration of the metagabbro. Mass-balance calculations show that dehydration was accompanied by metasomatism; all reasonable solutions require the addition of SiO2, Na2O, and large ion lithophile elements (LILE) to the host rock to produce the veins. Reference frames for the evaluation of mass transfers can be represented by two cases: (1) constant volume, and (2) constant FeO, MgO, V, and Zr requiring volume increases by factors of 1?3—3?3. The simplest model (minimizing the number of mobile constituents and giving progressively larger volume increases for more leucocratic veins) is based on constant FeO, MgO, V, and Zr. In this case, model mineral reactions and mass transfers for two types of veins, distinguished on the outcrop as (1) diffuse and (2) intermediate, are: <f>$$\begin{array}{c}\left(1\right)\hbox{ }1.1\hbox{ Hbd }+0.3\hbox{ Grt }+1.5{\hbox{ Pl }}_{1}\hbox{ }+\hbox{ }12.2{\hbox{ SiO }}_{2}\hbox{ }+\hbox{ }0.9{\hbox{ Na }}_{2}\hbox{ O }+\hbox{ }1.0{\hbox{ Al }}_{2}{\hbox{ O }}_{3}+\hbox{ }1.0\hbox{ CaO }\\ \hbox{ }\to 1.8\hbox{ Opx }+\hbox{ }2.1\hbox{ Cpx }+\hbox{ }3.2{\hbox{ Pl }}_{2}\hbox{ }+\hbox{ }2.6\hbox{ Qtz }+\hbox{ }0.3\hbox{ Ilm }+1.1{\hbox{ H }}_{2}\hbox{ O }+\hbox{ }0.2{\hbox{ K }}_{2}\hbox{ O }\\ \left(2\right)\hbox{ }1.0\hbox{ Hbd }+\hbox{ }0.7\hbox{ Grt }+\hbox{ }1.5{\hbox{ Pl }}_{1}\hbox{ }+\hbox{ }16.7{\hbox{ SiO }}_{2}\hbox{ }+\hbox{ }1.4{\hbox{ Na }}_{2}\hbox{ O }+\hbox{ }0.3{\hbox{ K }}_{2}\hbox{ O }+\hbox{ }0.9{\hbox{ A1 }}_{2}{\hbox{ O }}_{3}\\ \hbox{ }\to \hbox{ }2.4\hbox{ Opx }+1.4\hbox{ Cpx }+4.1{\hbox{ Pl }}_{2}+0.3\hbox{ Ilm }+\hbox{ }2.9\hbox{ Qtz }+1.0{\hbox{ H }}_{2}\hbox{ O }\end{array}$$</f> These mass balances show that SiO2, Na2O, and Al2O3 were added, and H2O lost, in producing the vein assemblages. Minimum fluid/rock volume ratios of 3–20 imply flow-through. The fluid did not induce melting of hornblende+plagioclase, and hence it must have had relatively low aH2O. The open-system transformation of hornblende–garnet-rich metagabbro to pyroxene-rich veins was driven primarily by the introduction of silica and feldspar components in a low-aH2O supercritical fluid at granulite-facies P–T conditions. The application of quantitative mass balance, taking into account propagated errors and a critical appraisal of the possible reference frames, enhances our ability to resolve open-system processes.