Appendix 2: Petrology of the analyzed samples
Analytical details concerning the electron microprobe analyses and X-Ray maps are summarized in Appendix 1.
Sample KG8
The mineral assemblage of this sample is complex and consists of garnet, omphacite, amphibole, plagioclase, epidote, rutile, ilmenite, titanite, haematite, chlorite and apatite. All these minerals show complex relations with several growth-dissolution events and associated varied textures and compositions. Representative mineral compositions are given in Table 3.
Garnet
Garnet is present as porhyroblasts or porphyroblastic aggregates of up to 1 mm in (integrated) length. The distribution of zoning suggests that at least some of these aggregates resulted from fragmentation of original garnet porphyroblasts (Figs. S1a-d). Individual grains and aggregates of grains show cores richer in Xsps and Xgrs and rims richer in Xprp, indicating prograde growth (Figs. S1a-d). However, all types of grains show complex growth-dissolution features denoted by irregular growth zoning patterns with sharp cross-cutting relations among successive growth layers. Discontinuities are best shown by the distribution of Xgrs contents that either show a continuous decrease from core-to-rim (with subtle large-scale oscillations) or sharp decreases in rims overgrowing irregular dissolved cores (Figs. S1d-f). This feature is mirrored by the distribution of Xprp and Mg# (= Mg/(Mg+Fe2+)), which either increases smoothly toward the rims or abruptly increases in the low-Xgrs overgrowths (Figs. S1c, e, and f). These relations indicate two main phases of prograde garnet growth, separated by a phase of (retrograde?) garnet resorption. The two garnet growth phases are clearly identified by the statistical maxima of garnet composition in the Xgrs-Xprp-Xalm+Xsps diagram of Fig. S1g.
The composition of garnet is Xsps-poor and Xalm-rich, typical of type-C (i.e., low-T) eclogites of Coleman et al. (1965; Figs. S1g-h). However, the low-Xgrs overgrowths have compositions approaching those of garnets of type-B (i.e., medium-T) eclogites (Fig S1g). At these conditions, diffusion in garnet allowed local diffusional-zoning to occur, involving an increase in Mg, mostly along fractures cutting across the lower-T core regions of fragmented porphyroblasts (e.g., the relatively high Xprp and Mg# region located close to the core of the large garnet grain shown in Fig. S1c; see profile #1 in Fig, S1e). These relations indicate deformation (fragmentation) during the higher-T event associated with growth of Xprp-rich rims and overgrowths.
Garnet underwent a second phase of dissolution after the growth of Xprp-rich rims and overgrowths. During this phase, it was replaced by fine-grained coronas made of low-Na omphacite, calcic amphibole, calcic-to-sodic plagioclase and, locally, epidote (see below).
Omphacite
Two main textural settings and compositional types of omphacite were identified (Figs. S2a-d). The largest grains, up to 0.5 mm in length, are dispersed in the matrix of the rock and show a composition distinctly rich in Al and poor in Ca, indicating relatively high Xjd contents (ca. 0.5; Fig. S2e). Smaller grains (ca. 0.1 mm) appear intergrown with amphibole, plagioclase and, locally, epidote in coronas around garnet. The composition of these grains is poorer in Al and richer in Ca (Figs. S2a and b), but they are still classified as omphacite (Xjd ca. 25) in the classification scheme of Morimoto et al. (1988). The compositional range of this corona omphacite is fairly large, approaching the composition of calcic ("augitic") clinopyroxene and aegirine augite. In general, the composition of the matrix grains is slightly richer in Mg and Mg# than that of the corona omphacite (Figs. S2c-d and f). The main compositional changes involving Al, Ca and Na indicate a decrease in pressure from the matrix-forming to the corona-forming events.
Amphibole
Four textural types of amphibole and several chemical varieties of the calcic and sodic-calcic series (following the classification scheme of Leake et al. 1997) were identified in sample KG8 (Figs. S3a-d). The earliest grains are present as inclusions in garnet with calcic compositions ranging from ferrohornblende to hastingsite (Fig. S3e). Matrix grains (up to 0.2 mm) have relatively homogeneous cores with sodic-calcic composition ranging from magnesiokatophorite to magnesiotaramite (Fig. S3e). These grains display thin rims of calcic composition, similar to that of amphibole grown in the coronas around garnet (Figs. S3a-d) and, consequently, are interpreted as overgrowths related to the corona-formation phase. In these coronas, the composition of amphibole is richer in Al and poorer in Mg and Mg# when closer to garnet and, as a whole, amphibole grown during this phase ranges from ferropargasite, pargasite to magnesiohastingsite. Finally, grown grains dispersed in the matrix and commonly associated with chlorite, albite and titanite (Fig. S3a-d) are of magnesiohornblende composition (Fig. S3e).
The complex growth history of amphibole indicates a P-T path with important inflections in slope during much of the metamorphic history of the sample. This is best illustrated using chemical species that are generally a function of P or T, such as those depicted in Figs. S3f-i, developed by Laird and Albee (1981). In these diagrams, it is seen that inclusions in garnet formed along a prograde path at relatively low pressure (low Na(B) and increasing Sum(A), [iv]Al, [vi]Al and Fe3+. The sodic-calcic compositions of matrix grains (rich in Na(B) and intermediate in Sum(A), [iv]Al, [vi]Al and Fe3+ denote a substantial increase in pressure and perhaps a slight decrease in temperature. This phase is followed by a significant decrease in pressure and increase in temperature, as indicated by the low Na(B) and the highest Sum(A), [iv]Al, [vi]Al and Fe3+ contents of the pargasitic compositions of the corona amphibole. Finally, decreasing pressure and temperature is indicated by the magnesiohornblende compositions of the retrograde grains. ´
Plagioclase
Plagioclase is present in two textural sites, forming part of the coronas around garnet and associated with late hornblende (Fig. S4a). The first type displays the largest compositional range, from andesine to albite composition, whereas the second is essentially of albite composition. As a whole, anorthite contents range from 0.43 to 0.07 (Fig. S4c) with the higher values present in plagioclase adjacent to garnet and the lowest in retrograde grains associated with hornblende. This trend is consistent with cooling onwards from the corona-forming event.
Other phases
Epidote is found associated with the coronas, where it shows the highest Al contents and as late retrograde replacements, mostly after garnet (Figs. S5a and f). Chlorite is rare but is present in two textural types: included in garnet and replacing garnet, amphibole and omphacite (Fig. S5b). Inclusions in garnet are richer in Al, Mg, and Mg# (Figs. S5g and h).
The sample also contains ilmenite, rutile, hematite and titanite (Figs. S5c-e). The earliest phase (probably magmatic relicts) is ilmenite, which is present as grains dispersed in the matrix and included within garnet. They show variable compositions, Mn ranging from 0.05 to 0.15 cations per 6 O and 4 total cations (Fig. S5i). Ilmenite within garnet and in the matrix is replaced by rutile, indicating increasing pressure, though rutile, in turn, is also replaced by thin films of Mn-rich ilmenite (Figs. S5c-e), probably during the ensuing decompression-induced garnet dissolution and corona formation. Titanite is associated with retrograde magnesiohornblende (Fig. S5c), and hematite is distributed in fractures and grain contacts among garnet grains (Fig. S5d).
Sample KG79a
The mineral assemblage identified in this amphibolite sample is relatively simple and is mostly made up of garnet, calcic amphibole and quartz; chlorite, biotite, apatite, ilmenite, rutile and titanite are present in minor amounts. Plagioclase and omphacite are lacking. Representative mineral compositions are given in Table 3.
Garnet
Garnet is present as porphyroblasts and small grains up to a few hundred µm in size (Figs. S6a-c). They display normal growth zoning with cores rich in Xsps and rims rich in Xprp and Mg#, whereas the content of Xgrs slightly increases from cores to rims of the grains (Figs. S6b-f). The composition of garnet is relatively poor in pyrope, being similar to that of type C (i.e., low-T) eclogite of Coleman et al. (1965; Fig. S6e).
Except when garnet rims are adjacent to quartz (black in Figs. S6a-c), the rims of garnet appear dissolved and replaced by Al-rich amphibole (green in the online version in Fig. S6a). As seen in Fig. S6a, the contours of the amphibole replacements depict the original idiomorphic habit of garnet. Hence, most rims do not represent the original composition of peak garnet rims, except for those adjacent to quartz.
Amphibole
The textural and compositional characteristics of amphibole are variable (Fig. S7). Four phases of amphibole growth, from earliest to latest, are identified: cores of matrix grains, rims of matrix grains, replacements 1 after garnet, and replacements 2 after garnet (Fig. S7a). The cores and rims of the matrix grains show a prograde trend with increasing Al and decreasing Ca and Mg# (Figs. S7b-d) from actinolite to magnesiohornblende. This trend, however, includes a significant increase in Nab towards the rims (Fig. S7f and i), denoting a substantial increase in pressure during prograde growth (Laird and Albee, 1981). Subsequently, an important phase of amphibole growth occurred related to garnet replacement, (after Grt1 in Fig. S6), as indicated by the different atomic ratios depicted in Figs. S7b-d. At this stage not only amphibole grew in the place of garnet but also overgrew previous rims of matrix grains. The mantles of this type of amphibole replacing garnet clearly show the former idioblastic habit of garnet (Figs. S6a and S7a-d). Its composition ranges from magnesiohornblende to pargasite (Fig. S7e). Finally, another phase of amphibole growth occurred after garnet replacement (after Grt2 in Fig. S7). This type of amphibole is located towards garnet in the mantles, in close contact with the corroded rims of garnet (Figs. S6a-d and S7a-d). Its composition is ferropargasite, bearing the highest Al and lowest Mg# and Si contents.
Relative to the matrix grains, the trend of amphibole in these garnet replacement mantles and matrix amphibole overgrowths involves a decrease in pressure and increase in temperature, as indicated by decreasing Na(B) and increasing Sum(A), [iv]Al, [vi]Al and Fe3+ contents towards the ferropargasitic compositions (Figs S7f-i). It should be noted that the rims of the matrix grains were clearly in contact with former idioblastic garnet, indicating that the peak garnet composition (now recorded in the garnet rims in contact with quartz; Figs. S6a-d) was in equilibrium with the magnesiohornblende rims of matrix amphibole, in spite of the fact that the latter do not record the highest temperature attained.
Other phases
Epidote is found as discrete, relatively homogeneous grains rich in Fe3+, apparently in equilibrium with garnet and the rims of matrix grains of amphibole (Figs. S8a and S5f). Chlorite is present as replacements of garnet, amphibole and biotite (Figs. S8b and c). It is relatively poor in Al and does not show large variations in Mg#. Biotite (Al[vi] = 0.64-0.67 apfu; Ti = 0.14 apfu; Mg# = 0.46-0.47) is rare and present in the mantles replacing garnet associated with amphibole (Fig. S8d). Ilmenite and rutile form discrete grains dispersed in the matrix and do not show reaction relationships between them (Fig. S8e). Ilmenite is rich in Mn (0.35 apfu; Fig. S5i). Titanite (Al = 0.36 apfu) is rare and present in the mantles replacing garnet.
Sample KG79
The mineral assemblage is typical of amphibolite facies and consists of garnet, plagioclase, quartz, calcic amphibole, epidote, biotite, chlorite, paragonite, muscovite, calcite, apatite, ilmenite, hematite and titanite. The amount of amphibole is low, whereas quartz and plagioclase are abundant. Representative mineral compositions are given in Table 3.
Garnet
Garnet forms porphyrobasts up to 1 mm in width showing normal growth zoning with cores rich in Xsps and rims richer in Xprp and Mg#, whereas the content of Xgrs and Fe Xalm is irregularly distributed (Figs. S9a-d). The composition is comparable to sample KG79a, poor in pyrope and similar to those of type C (i.e., low-T) eclogites of Coleman et al. (1965; Fig. S9e). The grains appear moderately to strongly corroded/dissolved at their rims and along fractures that locally break up larger grains (Figs. S9a-c). In spite of dissolution, however, the rims of garnet do not show diffusive readjustment in terms of XMn and Mg#, though some readjustment in terms of Xalm and Xgrs appears in the cores crosscut by fractures (Fig. S9a). Corrosion/dissolution of the rims is not observed when the grains are adjacent to plagioclase (mostly, albite; see below). This observation, and the lack of relict omphacite, indicate epidote-amphibolite-facies and argue for equilibrium between garnet and plagioclase at peak metamorphic conditions (i.e., the sample is not a retrogressed eclogite).
Amphibole
Amphibole is calcic, fine grained (<0.5 mm in length) and shows concentric to patchy zoning (Fig. S10a-d). The compositions of concentrically zoned grains range from actinolite in the cores to magnesiohornblende in the rims (Fig. S10e). Inclusions within garnet are of actinolite composition, slightly poorer in Mg# than the cores of the matrix grains, whereas grains included in plagioclase are of magnesiohornblende composition. This prograde trend is characterized by increasing Na(B) up to 0.47 atoms pfu (Fig. S10f), almost reaching sodic-calcic composition and indicating a substantial increase in pressure during growth and formation of the peak pressure Grt-Amp-Pl assemblage. Further growth of amphibole occurred after a decrease in pressure and/or increase in temperature, as indicated by magnesiohornblende to tschermakite compositions (Si <6.7 atoms pfu; Fig. S10e) having lower Nab contents (<0.2 atoms pfu; Fig. S10f). Though some of these grains are not texturally related to garnet, these compositions formed after garnet dissolution as in amphibolitic sample KG79a.
Plagioclase
Plagioclase is present in two textural sites, forming part of the matrix and replacing garnet at the rims and along fractures (Fig. S4b). The first type of grains is of albite composition with a very narrow compositional range (Xan = 0.00-0.02), though they show some patchy zoning (Figs. S4b and c). These albite grains occur in sharp contact with garnet along the idioblastic rims of the latter, indicating equilibration with garnet and Nab-rich magnesiohornblende at peak pressure. However, when in contact with xenoblastic garnet, these grains show overgrowths with higher Ca-contents (Xan up to 0.23), indicating plagioclase growth upon garnet dissolution (Figs. S4b and c). This type of composition is similar to that of grains found in fractures and is interpreted to have formed along with Nab-poor magnesiohornblende-tschermakite during the decompression and/or heating event ensuing the peak pressure stage.