Phase relationships

The metacarbonate and related rocks of the study area are very complex metamorphic rocks in which various chemical reactions have taken place to produce the minerals assemblages that are presently found within them. The above information on the association of minerals and their chemical composition will now be used in combination with experimental data, where available, to determine as far as possible the nature of the chemical reactions that have taken place and the conditions of pressure and temperature of metamorphism. The mineralogy of the metasomatic rocks of the Silgará Formation can be examined in the CaO-Al₂O₃-F_eO-M_gO-K₂O-Na₂O-M_nOF_e22O₃- TiO2-H₂O-CO2 chemical system, in the presence of SiO2 (quartz) and a H₂O-CO2 binary fluid. Lopez & Soto (1999) have deduced from an ACF diagram (Figure 12a) that the formation of the mineral assemblages in calc-silicate rocks is due to an increase in the chemical potential (μ) of CaO from pelitic rocks to marble, which is balanced by diffusion of Al₂O₃ (and probably SiO2) in the opposite direction. These authors consider that the occurrence of quartz in all the reported zones indicates that μSiO2 remained constant during this process. The mineralogy and the compositional variation of minerals from a representative calc-silicate rock of the Silgará Formation, containing the defined reaction zones, are depicted in an ACF projection (Figure 12b). T-XCO2 diagrams are used to predict the sequence of metamorphic mineral assemblages that would be ob-served at constant fluid composition as shown in Figure 13. However, this metamorphism, although easy to understand, is probably not representative of natural processes. The reactions encountered in progressive metamorphism generate H₂O and CO2 that will tend to change the fluid composition, and representing a problem. A process that serves very well to drive devolatization reactions is infiltration by fluids (Spear, 1993). Changing fluid composition at constant temperature and pressure is just as effective for driving reactions as changing the temperature or pressure at constant fluid composition. Numerous mineral reactions should take place in the investigated rocks during metamorphism. The calcite-dolomite reaction, which controls the Mg content of calcite, indicates retrograde metamorphic temperatures of 470-550oC. The Mg content of calcite in equilibrium with dolomite was shown by Harker& Tuttle (1955), Goldsmith & Heard (1961) and Goldsmith & Newton (1969) to increase with rising temperature. However, some obstacles that are normally encountered in carrying out an interpretation of mineral assemblages in these petrochemical type of rocks are: (1) generally it is not yet possible to identify using textural features of the rock, the reactants that produced a mineral presently found; (2) the primary or secondary nature of dolomite and quartz in some of rocks is unclear; (3) in rocks undergoing metamorphism, a gaseous phase may not be present and molecules of H₂O and CO2, as well as other relatively volatile constituents, may be concentrated in a grain-boundary phase (Ramberg, 1952; Thompson, 1955; Barth, 1962); (4) normally, in dealing with natural mineral assemblages, the minerals are not "pure" and one or more compositional variables must be considered in addition to variation in the concentration or activity of H₂O and CO2.

rutile + calcite + quartz = titanite + CO2,

which was investigated by Hunt & Kerrick (1977), who located the reaction at 500°C for a total pressure of 2 kbar and an intermediate CO2:H₂O ratio. The examined metasomatic rocks evidently reached higher temperatures, titanite should be present in place of rutile and quartz, as found. Titanite is particularly common and abundant in these rocks, and it was probably formed by metasomatic introduction of titanium. The rare presence of grossularite is of interest in relation to the experiments and calculations of Gordon & Greenwood (1971), who showed that at 600-700°C and 2 kbar this mineral is stable only in the presence of H₂O-rich fluids, with CO2/(H₂O + CO2) < 0-2. Under these conditions of pressure and fluid composition and at a temperature less than about 600 °C, grossularite may crystallize according to the reaction:

calcite + anorthite + quartz = grossularite + CO2.

No information is available concerning the reaction by which scapolite crystallized in these rocks. Scapolite is apparently the product of metasomatism, which implies an introduction of various elements, including Cl and S, possibly as Cl2 and SO2 (Kretz, 1980). Most of the retrograde reactions, including the exsolution of calcite, are considered to have taken place during a cooling process, following the thermal climax, and no attempt will be made to estimate the temperature of these reactions. All reactions, particularly those that require H₂O can take place below the equilibrium temperature, so that a condition of near-equilibrium between reactants and products was never attained. The distribution of minor elements between associated calcite and dolomite may be examined in relation to chemical exchange reactions and the concept of exchange equilibrium. These reactions, although they do not produce new mineral phases, represent one of the several different kinds of chemical changes that take place in metamorphic rocks, all of which are expected to proceed toward equilibrium and a minimum of free energy.

The application of the phase rule to these chemical open systems and the determination of the stability conditions of the mineral assemblages assume local equilibrium (Thompson, 1959). Fluid flow plays a very important role in heat and mass transfer in the crust and is strongly controlled by the layered permeability structure (Cui et al., 2001). Calc-silicate rocks represent only a small fraction of the crust, but they are valuable monitors of fluid flow and fluid-rock interaction and can reveal significant information about the composition of the metamorphic fluid phase and P-T conditions (Ague, 2003). The regional fluid flow transporting volatiles including H₂O and CO2 is an integral part of prograde regional metamorphism. According to Ague (2003), metacarbonate rocks are commonly out of chemical equilibrium, at least in part, with surrounding lithologies such as metaclastic rocks, and consequently, fluids that infiltrate metacarbonate layers can drive significant chemical reaction involving volatiles and other elements, particularly at lithologic contacts (e.g., Ague, 2000) and in alteration zones or "selvages" around syn-metamorphic veins (e.g., Ague & Rye, 1999; Ague, 2002). It is probably that mass transfer of "non-volatile" rock-forming elements has been significant during regional metamorphism of metacarbonate rocks in the Silgará Formation at the CSM, taking into account that evidence of this has been reported in similar geological contexts around the world (e.g., Thompson, 1975; Brady, 1977; Tanner & Miller, 1980; Tracy et al., 1983; Leger &Ferry, 1993; Ferry, 1994; Ague &van Haren, 1996; Widmer & Thompson, 2001; Shmulovich et al., 2001; Carlson, 2002). We con-sider that H₂O required to drive prograde CO2 loss probably came from regional dehydration of surrounding metapelitic schists, although the development of a H₂O-rich diopside reaction zone in the presence or absence of scapolite probably also required an external fluid contribution derived from syn-metamorphic intrusions (orthogneiss masses) emplaced during the final stage of metamorphism of the Silgará Formation.

Discussion (origin of the reaction zones)

Devolatilization of metasedimentary sequences is a process occurs during regionalmetamorphism and play a very important role in the cycling of volatiles through the that Earth's, andmetacarbonate rocks are the source ofmost of the CO2 evolved by regionalmetamorphism, although considerable uncertainly remains regarding the process of fluid release and transport (Ague, 2002). Devolatilization presumably resulted from heating and fluid flow during dynamic regional metamorphism of the Silgará Formation, and that the calc-silicate reaction zones occurring in metacarbonate and related rocks of thismetamorphic unit containmineral assemblages representing the greatest amount of evolatilization reaction progress, which has been documented by Hewitt (1973) at the edges of the metacabonate layers in contact with surrounding pelitic schists and in reaction selvages adjacent to syn-metamorphic quartz veins. Many of the studied calc-silicate rocks, especially in the case of the diopside-bearing type rocks, appear as clearly defined reaction zones with a mineral composition that evolves from pelitic schists to marbles or carbonate- silicate rocks. This sequence of zones is similar to those described in other calc-silicate reaction zones (e.g., Thompson, 1975; Kerrick, 1977; Ague, 2002, 2003).

Ague (2003) estimated fluid compositions in metacarbonate rocks, giving particular attention to the reaction diopside-rich zones, which show the higher H₂O-rich compositions, as required by the topology of the diopside-producing reactions. Hewitt (1973) inferred a large gradient in fluid composition across a lithologic contact based on a sharp in anorthite content of plagioclase from metacarbonate to metapelitic layers, which necessarily not indicate change in XCO2, taking into account that the mineral assemblages in the two layers are very different. On the other hand, Tracy et al. (1983) inferred large changes in XCO2 across a reaction zone adjacent to quartz veins.

According to Ague (2002), the mineral assemblages of the diopside-rich zones are stable at high T (> c.575oC) and relatively low XCO2. This author also consider that the low XCO2 fluid of the diopside-rich zones was present at peak P-T conditions that can not be differentiated from those of the immediately adjacent zones, implying that pressure and temperature were not the main variables controlling their formation. It is probable that prograde reactions and CO2 loss have occurred in the metacarbonate layers within the Silgará metamorphic sequence in response to (1) infiltration of H₂O from dehydration of surrounding pelitic schists (e.g., Hewitt, 1973; Ague&Rye, 1999; Ague, 2000) or (2) advection-driven infiltration of a H₂O-rich fluid external to the metasedimentary sequence (e.g., Tracy et al., 1983; Ague & Rye, 1999; Ague, 2002).

The presence of clinozoisite in the calc-silicate rocks suggests an extremely low XCO2 as been pro-posed in other studies (e.g., Thompson, 1971; Storre & Nitsch, 1972).

These reaction zones are characterized by sharp contacts and a low number of phases that are stable in each zone and their formation could be controlled by diffusion processes as been proposed in other studies (e.g., Thopmson, 1975; Joesten, 1977, 1991; Ague& Rye, 1999; Ague, 2000, 2002, 2003). Another possible origin for the reaction zones, suggested for other calc-silicate rocks, could be an infiltration process. For the calc-silicate rocks examined in this work, some arguments suggest a limited influence of this factor: (1) metamorphic rocks have low porosity (Brady, 1977), fluid circulation would be restricted, producing small-scale reaction bands; (2) infiltration processes generate an asymmetric distribution of calc-silicate bands from metapelites to marbles or carbonate-silicate rocks on both sides of the metapelite intercalations (Lopez & Soto, 1999). A strong evidence for a limited infiltration of aqueous fluids is observed in the diopside-rich zones with the consumption of graphite and the production of scapolite (Nabelek, 2002). Therefore, an infiltration process should be consider in the development of these calc-silicate rocks, which can be reviewed to estimate if it would produce a larger variation in the chemical composition of the minerals compared with the data presented here.

The investigated calc-silicate rocks can be developed as a result of an isochemical metamorphism of pure carbonate rocks (e.g., marbles) or by metasomatic introduction of silica into them or by a combination of these processes. On the other hand, marble layers recognized in the metamorphic sequence of the Silgará Formation at CSM have not enough silica to produce the calc-silicate rocks, in spite of metacarbonate and related rocks with some silica content and quartzites also occurs. According to Campos (1999), the occurrence of veins mainly composed of quartz with minor calcite in the metasomatic zones indicates that a remobilization of SiO2 and CaO can be due to the action of late magmatic fluids associated to the emplacement of the Orthogneiss protolith, which circulated along tectonic discontinuities or schistosity surfaces. We adopt the term "barren calc-silicate rocks" to metacarbonate and related rocks occurring at this region, taking into account that they are characterized by the presence of diopside and scapolite, although lacking wollastonite. We can differentiate these rocks from wollastonite-bearing calc-silicate rocks which are produced by metasomatism of almost pure carbonate rocks. Circulating fluids would react with the carbonates and convert them to calc-silicates, and when all carbonate was used up there would be no more reactions, and the silica would be transported elsewhere, as it is soluble in hydrothermal fluids. Therefore, the absence of wollastonite-bearing calc-silicate rocks at the CSM can be explained by the fact that these rocks require either a protolith richer in calcite than the barren calc-silicate rocks or loss of Mg during metasomatism. However, if the calc-silicates were produced by metasomatism, additional chemical effects of this process should be explained.

The fluid flow plays a very important role in heat and mass transport in the crust and is strongly controlled by the layered permeability structure, which causes focused subhorizontal fluid flow within high permeability lithologic units Cui et al., 2001). Although a component of fluid flow was subhorizontal along bedding, a larger component must have been subvertical through fractures that cut across the calcareous layers (e.g., Cui et al., 2001; Nabelek, 2002). The effects of transient permeability changes associated with mineral reactions on heat and fluid flow, which are very important for layered metasedimentary sequences where metamorphic devolatilization reactions are likely to be more extensive in calc-silicate layers than in either limestone or shale layers, are not well understood (e.g., Hover-Granath et al., 1983). These authors determined that metamorphic reactions in marble layers occur in equilibrium with a CO2-rich fluid phase at all grades, while in calcareous argillite layers they occur in the presence of progressively more H₂O-rich fluids as metamorphic grade increases.

According to Ague (2003), the increases in metasomatic mass transfer toward contacts and veins demonstrate the importance of hydrodynamic dispersion along gradients in fluid composition at high angles to layering and in alteration selvages, consistent with the results of reaction-transport modeling. The regular sequences of mineral assemblages observed at lithologic contacts and in vein selvages, which have not yet established at the CSM, are classic indicators of hydrodynamic-dispersion mass transfer (e.g., Vidale, 1969; Hewitt, 1973; Vidale & Hewitt, 1973; Thompson, 1975; Brady 1977; Ashworth& Sheplev, 1997; Abart et al., 2001, Ague, 2003).

The reaction-transport modeling indicates limited cross-layer advection (Ague, 2003), so most of the regional fluid flow presumably occurred parallel to layering or through fractures (Figure 14), which is consistent with previous studies carried out in other metamorphic belts (e.g., Hewitt, 1973; Rye et al., 1976; Tracy et al., 1983; Bickle & Baker, 1990; Ague, 1994; Ferry, 1994; Ague, 2000, 2003). A considerable component of the layer-parallel flow was concentrated along metapelitic-metacarbonate contacts and internal contacts separating reaction zones with differing properties (such as grain size and mineralogy) within individual metacarbonate layers, which is supported by local source-sink relationships and fractures (Ague, 2003). Metacarbonate and metapelitic rocks have different physical properties, so contacts between them are favorable areas of mechanical weakness, in some cases related to elevated fluid pressures during reaction (Walther, 1996).

Fracturing at lithologic contacts increased porosity and permeability (Ague, 1995); so highly veined contacts were probably sites of elevated fluid flux relative to more "pervasive" flow through non-fractured rock matrix (Breeding et al., 2003). In addition, reactions driven by fluids infiltrating along contacts may have increased rock porosity and permeability at the grain scale and focused layer-parallel flow into the contact areas (Balashov and Yardley, 1998). Most veins that cross-cut metacarbonate layers are surrounded by reaction selvages whose mineral assemblage zonation is similar to that found at lithologic contacts. Advective fluid flow transported the chemical "signature" of the surrounding rock types into the metacarbonate layers along the cracks and in the immediately adjacent wall rock. Hydrodynamic dispersion at high angles to the crack walls operated to transport mass to and from fractures and form the altered selvages. Next to some veins, quartz, calcite, feldspar, titanite, and/or rutile were removed from the wall rock and deposited in the adjacent vein. This local transport or "segregation" is inferred to have occurred by processes dominated by diffusion (e.g., Ague, 2003), and did not necessarily require large fluid fluxes.

A large volume of studies exists on the metamorphism of calc-silicate rocks. However, one of the main reasons for studying these rocks is due to the mineral assemblages are a function of the composition of the fluid, studying the distribution of the mineral assemblages provide considerable information about the behavior of the fluid phase during metamorphism. During the development of the calc-silicate reaction zones between marble and pelite layers, a volume of these lithologies should have disappeared. Garnet-bearing rocks display a sequence of textural and compositional zones with different high-variance mineral assemblages, which can be interpreted as a consequence of diffusion processes (mainly transfer of CaO) between adjacent impure calcite-bearing marble and aluminum- rich metapelites.

The mineral assemblages up to the appearance of scapolite in the diopside-rich zone give little evidence about the compositions of fluids that existed in the rocks during metamorphism, except that if there was very little or an absence of fluid infiltration, the reactions would have driven X(CO2)fluid to higher values (Nabelek, 2002). The first direct evidence for infiltration of aqueous fluids occurs in the diopside-rich zone with the consumption of graphite and the production of scapolite.

There is not doubt that marble has decomposed releasing CO2 from the chemical system and CaO to the pelite. Although compositional gradients in the CO2-H₂O-fluid produced by decarbonation of marble and dehydration of pelite should be considered, it is probably that the amount of circulation of fluids may not have been important. However, it is difficult to establish if the cation diffusion process dominates respect to the fluid flow as the mechanism of mass transfer as been proposed by Thompson (1975), taking into account that in the Silgará Formation metamorphic rocks the circulation of fluids through fractures and lithologic contacts has played a very important role in the development of hydrothermal veins, some of which are observed within calc-silicate reaction bands. Therefore, future research should consider in detail the geometry of the calc-silicate reaction zones to distinguish between diffusion and flow fluid processes, which require more information on the properties of aqueous fluids, mineral phase stability and chemical compositions of coexisting phases in the mineral assemblages in order to develop a model for a metasomatic phenomenon.

These metamorphic conditions agree well with those estimated by Garcia et al. (2005) for the high-grade metapelites of the Silgará Formation at the CSM in which are intercalated the metacarbonate and related rocks and also with the P-T conditions deduced for metapelites. This research has shown that phase-relationship analysis of reaction zones developed along metapelite-marble contacts represent a powerful tool contributing to understanding the metamorphic evolution of the Santander Massif in the geological context of the Colombian Andes.

We propose the name of a non economic mineralization "reaction calcic exoskarn" (except by the exploitation of marble) for the metacarbonate and related rocks that form part of the metamorphic sequence of the Silgará Formation at the CSM, taking into account the composition and texture of the resulting skarn, the available terminology for these rocks, and the following aspects suggested by Meinert (1992): (1) skarns can form during regional or contact metamorphism and from a variety of metasomatic processes involving fluids of different origin (magmatic, metamorphic, meteoric, and/or marine); (2) they can be found adjacent to plutons, along faults and major shear zones, in shallow geothermal systems, on the bottom of the seafloor, and at lower crustal depths in deeply buried metamorphic terrains; (3) what links these diverse environments, and what defines a rock as skarn, is the mineralogy, which includes a wide variety of calc-silicate and associated minerals but usually is dominated by the presence of garnet and diopside. The "reaction calcic exoskarn" that crops out at the CSM probably has developed by isochemical metamorphism of thin interlayered shale and carbonate levels where metasomatic transfer of components between adjacent lithologies (e.g., Zarayskiy et al., 1987) may occur on a millimeter to centimeter scale. It is known that the composition and texture of the protolith tend to control the composition and texture of the resulting skarn. On the other hand, most economically important skarn deposits (which are not our case) result from large-scale metasomatic transfer, where fluid composition controls the resulting skarn and ore mineralogy.

Acknowledgements

The present work forms part of the undergraduate Thesis carried out by S. Gómez and G. Avila at Universidad Industrial de Santander. We are very grateful to the Universidad Industrial de Santander, entity that supported fieldwork. We are indebted to doctors Alan Boyle from the Department of Earth& Ocean Sciences at University of Liverpool (England) and Akira Takasu from the Department of Geosciences at Shimane University (Japan) for contributing us with the acquisition of electron backscatter diffraction and electron microprobe analyses. The authors also acknowledge to the anonymous referees for their critical and insightful reading of the manuscript. We are most grateful to the above-named people and institutions for support.

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