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Metamorphic Petrology

Figure 1: Plane polarized light digital image showing leucosomes of quartz and plagioclase and selvages of intergrown biotite and sillimanite (sample ARR96-106).

 

POSSIBLE MELTING REACTIONS AND P-T CONDITIONS

Peak metamorphic pressures and temperatures are constrained by the mineral assemblages, reaction textures, chemical zoning analyses, and the application of a petrogenic grid (Spear, 1993; Spear et al., 1999). The metamorphic history of these migmatites is best characterized by melting reactions outlined by Spear et al. (1999).

Possible Melting Reactions:
Many melting reactions are possible for these rocks. If a water-rich fluid was present, then the first melting reaction encountered in the heating of these rocks would be

muscovite + albite (plagioclase) + H2O vapor = Al2SiO5 (sillimanite) + melt (1)

(Spear et al., 1999). However, given that these rocks were heated at elevated pressures, it is most likely that porosity was very low and little fluid was available to drive this reaction. Most likely, reaction (1) was of minor importance in the partial melting of these rocks.
The next possible melting reaction that these rocks would encounter is:

muscovite + albite (plagioclase) = K-feldspar + Al2SiO5 (sillimanite) + melt (2)

(Spear et al., 1999). This is interpreted as the main melting reaction. Evidence for this reaction is seen in the large amounts of sillimanite in the selvage domains of these migmatites. The “melt” in this reaction is represented by the leucosomes of quartz and plagioclase (Fig. 1). This reaction is interpreted to represent the minimum P-T conditions for these migmatites.
A continuous reaction responsible for garnet growth would also be encountered upon further heating:

biotite + Al2SiO5 (sillimanite) = garnet + K-feldspar + melt (3)

(Spear et al., 1999). However, once cooling or isothermal decompression begins, this reaction will reverse. The fourth melting reaction to be encountered by these rocks would be a biotite melting reaction:

biotite + Al2SiO5 (sillimanite) = cordierite + garnet + melt (4)

(Spear et al., 1999) This reaction is interpreted to be an upper constraint on metamorphic P-T conditions, as there is no evidence of its occurrence (i.e. no cordierite is present in these rocks). A fifth melting reaction would have occurred if these rocks had experienced higher pressures:

biotite + garnet = orthopyroxene + Al2SiO5 (sillimanite) + melt (5)

(Spear et al., 1999). The absence of orthopyroxene in these rocks suggests that this reaction did not occur.

The Absence of K-feldspar:
The main melting reaction that is believed to have taken place (reaction 2) produces K-feldspar, which has not been recognized in these rocks. The lack of K-feldspar may reflect that melting reaction (1) is more important than originally thought. Alternately, the absence of K-feldspar can be attributed to the growth of retrograde muscovite by the reaction:

Al2SiO5 (sillimanite) + K-feldspar + H2O = muscovite + quartz (6)

(Spear et al., 1999). This reaction uses water vapor exsolved from the crystallizing melt to produce muscovite from K-feldspar. Retrograde muscovite is prevalent in these rocks and it appears to cross cut the foliation. It also appears within the leucosomes, as well as the selvages.

Figure 2: Crossed polarized light digital image showing garnet porphyroblast with chlorite inclusions rimmed by sillimanite (sample ARR96-117).

 

Other Metamorphic Reactions:
Chlorite inclusions in garnet are interpreted to represent synchronous early prograde garnet (Fig. 2) and chlorite growth by the reaction:

chloritoid + biotite = garnet + chlorite +H2O (7)

(Spear, 1993). This reaction is the lower boundary on the garnet + chlorite stability field. Its position in P-T space shifts depending on the manganese concentration in garnet. Additional garnet growth is likely to have occurred through the continuous reaction:

chlorite + muscovite + quartz + anorthite = garnet + biotite + albite + H2O (8)

(Spear, 1993). Growth of staurolite, dissolution of garnet, and continued dissolution of chlorite probably occurred by the reaction:

garnet + chlorite + muscovite = staurolite + biotite +H2O (9)

(Spear, 1993).

Figure 3: Plane polarized light digital image showing extensively resorbed Staurolite rimmed by sillimanite and biotite (from sample ARR96-117).

 

The staurolite-out reaction occurs near the muscovite melting reaction (2):

staurolite = garnet + biotite + Al2SiO5 (sillimanite) (10)

(Spear, 1993). This reaction would explain the highly resorbed staurolite in ARR96-117 (Fig. 3). The occurrence of sillimanite and biotite rimming garnet suggest the dissolution of garnet by continuous reaction 3 in reverse:

garnet + K-feldspar + melt = biotite + Al2SiO5 (sillimanite) (3a)

(Spear et al., 1999; Fig. 2). This reaction also helps to explain the absence of K-feldspar. From these constraints, it is proposed that these migmatitic schists reached peak P-T conditions of 675 – 850 degrees C and 4-8 kbars (Fig. 5).

 

Figure 4: Garnet chemical zoning maps showing minor core to rim increases in Fe/(Fe+Mg) and calcium. Plagioclase zoning map shows a core to rim increase in percent anorthite.

 

The generally flat major element profiles suggest that prograde growth zoning is not preserved due to diffusion related to elevated temperatures during partial melting. Steps in zoning near the rims are likely due to garnet growth during partial melting (Spear et al., 1999) or possibly staurolite dissolution. Calcium zoning in garnet shows a slight decrease moving from core to near-rim. This corresponds with a decrease in inclusion density. The calcium step up at the rim (Fig. 4) is interpreted to reflect garnet growth during partial melting.

The plagioclase core-rim step (Fig. 4) in percent anorthite may represent two generations of plagioclase. The more albite rich core may be plagioclase that crystallized from the melt. The higher anorthite rims are interpreted as plagioclase growth during retrograde garnet dissolution.

 

 

Petrogenic Grid

Figure 5: Petrogenic grid showing reactions involved in the metamorphism and partial melting of migmatites ARR96-106 and ARR96-117 (after Spear, 1993 and Spear et al., 1999). Gray region shows peak metamorphic conditions for these migmatites. Mn/(Mn+Fe+Mg) ratios for garnet + chlorite stability field are inferred from chemical zoning analyses (Fig. 4).