Eclogite-facies vein systems in the Marun-Keu complex (Polar Urals, Russia): textural, chemical and thermal constraints for patterns of fluid flow in the lower crust

Abstract

Metasomatic amphibole-eclogite sequences grew in selvages of quartz veins from the Marun-Keu complex (Polar Urals, Russia) during high-pressure metamorphism. Relicts of a pre-metasomatic eclogite-facies assemblage are present in the wallrock layers as irregular patches. Wallrock interstitial quartz trails lying at a high angle to reaction fronts provide evidence for grain-scale pore channelisation which may be produced by intergranular-fluid compositional gradients parallel to the quartz trails. Disequilibrium at vein-wallrock scale is inferred from wallrock mineral heterogeneity and from variable initial Sr isotope ratios in mineral separates. Mass-balance calculations between relicts and wallrock assemblages reveal chemical imbalances caused by open system-behaviour with two way mass-transfer. The vein-wallrock system registers a prograde history from 408–434 °C (relicts) to 526–668 °C (vein precipitates). Vein and metasomatic assemblages formed during a single fluid-rock interaction process, implying high heating rates (≥100 °C/Ma), which could result from heat advection by large-scale fluid circulation.

Appendix

The Gresens-Grant equations

According to Grant (1986), the initial and final concentrations of the i ^th component, respectively, $C^{o}_{i}$ and $C^{F}_{i}$ are related by the following expression:

$$C^{F}_{i} = \frac{{m^{o} }} {{m^{F} }} \cdot {\left( {C^{o}_{i} + \Delta C_{i} } \right)}$$

(A1)

where m^o and m^F are the equivalent masses prior to and after the transformation and ΔC _i is the gain or loss of mass of the i ^th component, Δm_i, divided by m^o:

$$\Delta C_{i} = \frac{{\Delta m_{i} }} {{m^{o} }}$$

(A2)

Since ΔC _i is null for those components which remain immobile during the metasomatic process, from expression (A1) the following relation is derived for immobile components:

$$\frac{{m^{o} }} {{m^{F} }} = \frac{{C^{F}_{i} }} {{C^{o}_{i} }}$$

(A3)

Relation (A3) implies that $C^{F}_{i}$ and $C^{o}_{i}$ values for immobile components must define a straight line through the origin giving by the equation:

$$C^{F} = \frac{{m^{o} }} {{m^{F} }} \cdot C^{o} $$

(A4)

which was named isocon by Grant (1986).

According to relation (A3), the isocon equation becomes:

$$C^{F} = \frac{{C^{F}_{n} }} {{C^{o}_{n} }} \cdot C^{o} $$

for a process conserving the n^th component;

$$C^{F} = C^{o} $$

for constant mass; and

$$C^{F} = \frac{{\rho ^{o} }} {{\rho ^{F} }} \cdot C^{o} $$

for constant volume (where ρ is the rock density).

Dividing relation (A1) by $C^{o}_{i}$ and re-arranging terms, we obtain:

$$C^{F}_{i} = C^{o}_{i} \cdot \frac{{m^{o} }} {{m^{F} }} \cdot {\left( {1 + \frac{{\Delta C_{i} }} {{C^{o}_{i} }}} \right)}$$

(A5)

which indicates that all components experiencing the same relative variation must lie on a line through the origin. It is important to note that ${\Delta C} \mathord{\left/ {\vphantom {{\Delta C} {C^{o}_{i} }}} \right. \kern-\nulldelimiterspace} {C^{o}_{i} }$ is the relative mass variation of the i ^th component referred to the masses of that component prior to the metasomatic transformation:

$$\frac{{\Delta C_{i} }} {{C^{o}_{i} }} = \frac{{\Delta m_{i} }} {{m^{o}_{i} }}$$

(A6)