Melt loss and the preservation of granulite facies mineral assemblages

Transcription

1 J. metamorphic Geol., 2002, 20, Melt loss and the preservation of granulite facies mineral assemblages R. W. WHITE AND R. POWELL School of Earth Sciences, University of Melbourne, Parkville, Victoria 3052, Australia ABSTRACT The loss of a metamorphic fluid via the partitioning of H 2 O into silicate melt at higher metamorphic grade implies that, in the absence of open system behaviour of melt, the amount of H 2 O contained within rocks remains constant at temperatures above the solidus. Thus, granulite facies rocks, composed of predominantly anhydrous minerals and a hydrous silicate melt should undergo considerable retrogression to hydrous upper amphibolite facies assemblages on cooling as the melt crystallizes and releases its H 2 O. The common occurrence of weakly retrogressed granulite facies assemblages is consistent with substantial melt loss from the majority of granulite facies rocks. Phase diagram modelling of the effects of melt loss in hypothetical aluminous and subaluminous metapelitic compositions shows that the amount of melt that has to be removed from a rock to preserve a granulite facies assemblage varies markedly with rock composition, the number of partial melt loss events and the P T conditions at which melt loss occurs. In an aluminous metapelite, the removal of nearly all of the melt at temperatures above the breakdown of biotite is required for the preservation of the peak mineral assemblage. In contrast, the proportion of melt loss required to preserve peak assemblages in a subaluminous metapelite is close to half that required for the aluminous metapelite. Thus, if a given proportion of melt is removed from a sequence of metapelitic granulites of varying composition, the degree of preservation of the peak metamorphic assemblage may vary widely. Key words: granulites; melt loss; melting; metapelite. Mineral abbreviations: quartz, q; garnet, g; sillimanite, sill; cordierite, cd; K-feldspar, ksp; plagioclase, pl; biotite, bi; muscovite, mu; silicate melt, liq. INTRODUCTION Crustal granulite facies rocks differ from those of lower metamorphic grades in that they contain predominantly anhydrous minerals, lack a free H 2 O-rich fluid phase during metamorphism and involve partial melting. These main features of granulite facies rocks are a consequence of dehydration melting in which H 2 O is strongly partitioned into silicate melt. This process is in contrast to progressive dehydration under subsolidus conditions in which the H 2 O evolved in dehydration reactions is inferred to escape. Instead, the partitioning of H 2 O into silicate melt and the absence of a H 2 O-rich fluid at higher grade conditions, means that the amount of H 2 O in a rock will remain constant unless melt is lost. Although the idea that melt is lost from partially melted rocks is not a new one (e.g. Fyfe, 1973; Powell, 1983; Powell & Downes, 1990), the amount of melt loss that occurred from many terranes is poorly constrained. Estimates of the degree of melt loss from several terranes have been undertaken on the basis of geochemical and or petrographic constraints (e.g. Sawyer, 1987; Wickham, 1987; Barbey et al., 1996; Sawyer et al., 1999). If no melt is lost from granulite facies rocks, high-grade anhydrous assemblages will be retrogressed on cooling via the reversal of the partial melting reactions to hydrous assemblages typical of the upper amphibolite facies (Powell, 1983; Powell & Downes, 1990; Spear et al., 1999; White et al., 2001). Although some degree of hydrous retrogression is common in granulite facies terranes, the effects of such retrogression are generally limited and the high-grade anhydrous assemblages are preserved. Thus, understanding the preservation of granulite facies mineral assemblages with little or no retrogression is important in the context of mid- to lower-crustal processes. The origin and behaviour of melt-bearing metamorphic rocks (migmatites) has been studied widely through field observation (e.g. Tracy, 1978; Brown, 1994; Vernon et al., 1990), geochemical studies (e.g. Sawyer, 1991; Watt & Harley, 1993; Watt et al., 1996; Greenfield et al., 1996) and experimental studies (e.g. Thompson, 1982; Vielzeuf & Holloway, 1988; Carrington & Harley, 1995; Gardien et al., 1995). However, field and geochemical studies are hampered by the fact that only the inferred products of partial melting can be observed and not the melt itself. Furthermore, most experimental studies give little Ó Blackwell Science Inc., /02/$ Journal of Metamorphic Geology, Volume 20, Number 7, 2002

2 622 R. W. WHITE & R. POWELL information on the processes of melt migration and segregation. In this paper we model the effects of melt loss from granulite facies metapelitic rocks, investigating the effects of varying proportions of melt loss at different temperatures. The effects of melt loss on the development and preservation of granulite facies mineral assemblages are illustrated using phase diagrams calculated for two model metapelitic compositions. PHASE EQUILIBRIA CALCULATIONS IN NCKFMASH In the following sections, the effects of melt loss are considered using quantitative P T and T X pseudosections calculated for an aluminous metapelite and subaluminous metapelite in the Na 2 O-CaO-K 2 O-FeO- MgO-Al 2 O 3 -SiO 2 -H 2 O (NCKFMASH) model system. The phase equilibria calculations were undertaken using THERMOCALC 3.0 (Powell & Holland, 1988) and the Holland & Powell (1998; in the form of the Sept, 1999 upgrade) internally consistent thermodynamic data set. The calculations involve the thermodynamic model for silicate melt in NCKFMASH presented by White et al. (2001) based on the haplogranite model of Holland & Powell (2001). The thermodynamics of the other phases are also those used in White et al. (2001). Although the model system does not constitute a complete chemical description of pelitic rocks, it does cover most of the spectrum of pelitic compositions. In particular, Ti and Fe 3+ are not included in this model, and the biotite model assumes that the hydroxyl site is full. All the calculations involve quartz in excess (i.e. are projected from quartz), thus SiO 2 is not included in the bulk rock compositions. The bulk rock compositions given in the figure captions are in oxide molar percent. The compositions in weight percent, also excluding SiO 2, are given in Table 1. The T X pseudosections are used to evaluate the effects of melt loss from particular starting compositions. Such diagrams are a powerful tool in constraining the metamorphic evolution of rocks in the context of melt loss via calculated contours of mineral and melt modes. Melt production in aluminous pelite Before the effects of melt loss on the preservation of granulite facies rock are considered it is important to discuss the role of melt production in producing predominantly anhydrous granulite facies assemblages. This will be done using a P T pseudosection (Fig. 1) and a T X pseudosection (Fig. 2), both constructed for an aluminous bulk rock composition. Figure 1 is a P T pseudosection that shows the stable mineral assemblages and modal melt proportions for an aluminous metapelite that has been heated from subsolidus conditions to temperatures appropriate to granulite facies metamorphism. The rock is considered to be water-saturated immediately below the solidus at 5 kbar, with the H 2 O content of the bulk composition chosen accordingly. Thus, this diagram (and Figs 6 & 7) are only appropriate for a prograde heating path that occurs at pressures above where the water-saturated solidus intersects the fields representing the breakdown of muscovite and quartz. The effect of heating on the mineral assemblage development and melt formation in a rock can be evaluated by looking at a given heating path in Fig. 1. If a rock is isobarically heated at 5 kbar, partial melting will begin at the water-saturated solidus (thick dashed line in Fig. 1). However, the amount of melt produced here will be small. With further heating, major melt-producing steps will occur via muscovite breakdown melting reactions and biotite breakdown melting reactions. At temperatures above the breakdown of biotite, the melt will become progressively drier by dilution as progressively more of the anhydrous solid minerals are consumed to produce melt with increasing temperature. At temperatures above about 800 C the molar melt proportion exceeds 40%. It is unlikely that a rock containing 40 modal % melt will be able to maintain coherence as this amount of melt is likely to exceed the rheological critical melt percentage of Arzi (1978). However, if melt is lost from a rock during prograde metamorphism, melt percentages may stay well below this amount and subsequent melt production in that rock will be reduced. A wide range of P T-melt production histories can be shown Table 1. Bulk rock compositions used in the calculations in weight %. Na 2 O CaO K 2 O FeO MgO Al 2 O 3 H 2 O Total Fig Fig. 2 x ¼ x ¼ Fig. 3 x ¼ x ¼ Fig. 4 x ¼ x ¼ Fig Fig Fig. 8 x ¼ x ¼

3 MELT LOSS AND THE PRESERVATION OF GRANULITES 623 Fig. 1. Calculated P T pseudosection for an aluminous metapelite heated from subsolidus water saturated conditions. The bulk rock composition in terms of Na 2 O:CaO:K 2 O: FeO:MgO:Al 2 O 3 :H 2 O is 2.07:1.03:10.45: 10.95:5.75:48.34: The dashed lines give the molar proportion of melt in the rock on a quartz-absent basis, since the diagram is drawn for quartz in excess. For increasing molar quartz, the contours progressively overestimate the melt modes. The thick solid and dashed lines are P T paths discussed in the text. The P T position of melt loss events are labelled ML1 and ML2. The solid lines along path A and indicates where this figure can be used if melt is lost at ML1 and ML2. in Fig. 1, such as those represented by the P T paths labelled A and B. Melt loss The features of melt loss that are likely to influence the development and preservation of granulite facies assemblages are the bulk rock composition, P T conditions of melt loss, the number of melt loss events and the proportion of melt lost at each event. In this section we will consider melt loss in terms of a hypothetical isobaric (5 kbar) heating path where the effects of differing amounts of melt loss can be assessed, and consider the development and preservation of granulite facies assemblages along given P T paths in terms of fixed degrees of melt loss. When modelling the effects of melt loss using phase diagrams, it is important to consider the scale on which melt loss has occurred. In the following discussion ÔÔmelt loss simply means the migration of melt to outside the volume of equilibration in which it was produced such that it is no longer in chemical communication with it. However, volumes of equilibration during granulite facies metamorphism are commonly unknown, and will change with changing P T conditions, deformation and melt content (e.g. Stu we, 1997). Of key importance to understanding the scales at which given phase diagrams are appropriate is the distinction between melt segregation on a centimetre scale and melt loss on a metre to kilometre scale. In the case of melt segregation, as in the formation of leucosomes, melt migration is on a cm scale. If this scale is larger than that of chemical communication between the melt and any part of the rock in which it is produced, those parts can be considered to have lost melt. If the melt then remains within the segregation it will be in chemical communication with at least part of the source rock. The implications of heterogeneous melt distribution in granulite facies rocks is discussed in more detail below. The effects of the loss of a proportion of the melt from a rock can be shown using T X melt loss diagrams (Figs 2, 3 & 4). The ÔÔX axis in these figures is the proportion of melt removed from the original rock at the specified temperature: X ¼ 0 represents the bulk rock composition assuming no melt loss and X ¼ 1 represents the residual bulk rock composition assuming all melt has been removed. Because the composition of melt within a rock changes with temperature, the diagrams have to be drawn for melt loss at a given temperature. Figure 2 shows the effect of a single melt loss event at 805 C at which point modal melt is 40% for a rock that has been heated from initially fluidsaturated subsolidus conditions. Closed system cooling paths on such diagrams are vertical lines. The solidus undergoes a series of step-like jumps up temperature with increasing melt loss through the muscovite and biotite breakdown melting steps. As an example four melt-loss cooling histories, paths (A-D) on Fig. 2, are used to illustrate the retrograde evolution of metamorphic assemblages from a common granulite facies peak assemblage of garnet, sillimanite, cordierite, K-feldspar and silicate melt. Path A involves 40% of the total melt being lost prior to cooling. In this case the peak assemblage will be progressively retrogressed to a biotite-bearing, cordierite- and garnet-absent assemblage and then a muscovite-bearing assemblage. The conversion of the granulite facies assemblage to an upper amphibolite facies assemblage occurs at just below 750 C, while the rock still contains

4 624 R. W. WHITE & R. POWELL Fig. 2. Calculated T X melt loss pseudosection where X melt loss is the proportion of melt removed from an aluminous metapelite at 805 C and 40% modal melt at X ¼ 0. The bulk rock composition in terms of Na 2 O:CaO:K 2 O:FeO:MgO:Al 2 O 3 :H 2 Ois 2.07: 1.03:10.45:10.95:5.75:48.34:21.41 at X ¼ 0 and 0.50:0.54:7.05:16.76:9.24:62.39: 3.52 at X ¼ 1. The grey arrows labelled A to D show four possible melt loss cooling paths to produce different mineral assemblages at the solidus. Stippled fields indicate the partial preservation of granulite facies assemblages at the solidus. The point labelled x represents the minimum amount of melt loss required to preserve part of the peak assemblage. Fig. 3. Calculated T X melt loss pseudosection showing the effect of two partial melt loss events (66% at 720 C & 40% at 740 C) prior to a final melt loss event at 805 C. The bulk rock composition in terms of Na 2 O:CaO:K 2 O:FeO:MgO:Al 2 O 3 :H 2 OatX¼ 0 is the same as at X ¼ 0 in Fig. 2. The composition at X ¼ 1 is 0.62:0.77:9.35:16.00:8.57:61.57:3.12. The grey arrows show a prograde heating melt loss path and a series (arrows A to C) of possible melt loss cooling paths for the peak and retrograde history. Stippled fields indicate the partial preservation of granulite facies assemblages at the solidus. The point labelled x represents the minimum amount of melt loss required to preserve part of the peak assemblage.

5 MELT LOSS AND THE PRESERVATION OF GRANULITES 625 Fig. 4. Calculated T X melt loss pseudosection showing the effect of melt removal from a subaluminous metapelite at 805 C and ( 34% modal melt (X ¼ 0). The bulk rock composition in terms of Na 2 O:CaO:K 2 O: FeO:MgO:Al 2 O 3 :H 2 O is 1.94:0.97:9.83: 23.74:12.47:30.66:20.39 at x ¼ 0 and 0.44:0.99:6.40:35.47:18.98:34.32:3.40 at x ¼ 1. The grey arrows labelled A to D show the same four possible melt loss cooling paths as those in Fig. 2 in terms of the proportion of available melt lost. Stippled fields involve the partial preservation of granulite facies assemblages at the solidus. The point labelled x represents the minimum amount of melt loss required to preserve part of the peak assemblage. approximately 15% melt and is fluid absent. Given the elevated temperatures and the presence of substantial amounts of melt, it is likely that the degree of retrogression of the granulite facies assemblage is advanced if not complete, given that under these conditions reaction rates will be fast. However, if melt is heterogeneously distributed, different parts of a rock may show different degrees of retrogression. In path B, Fig. 2, 75% of the melt is lost prior to cooling. In this case the solidus is at about 730 C, just below where cordierite would become stable. Although the assemblage (g sill ksp pl bi) should be developed, the small amounts of melt remaining in the rock may be more likely to produce a more patchy and less complete retrogression than for path A if melt was not evenly distributed. If so, relict patches of relatively unretrogressed granulite may be preserved within a biotite-rich but muscovite-absent matrix. For paths C and D, representing 88% and 95% melt loss, respectively, the solidus is crossed while cordierite is still stable. In path C, there would be a small amount of biotite formed at temperatures just above the solidus. In path D, the assemblage stable just below the solidus is biotite-poor. The point labelled (x ) in each T X melt loss diagram represents the minimum amount of melt loss required for a granulite assemblage to be preserved at the solidus, and provides a useful point of comparison between the different T X diagrams. Thus, if a single, discrete, melt loss event is inferred for this bulk rock composition the proportion of melt lost has to be large if unretrogressed or weakly retrogressed granulite facies assemblages are to be preserved. However, given the complex deformation histories of most granulite facies terranes and the likelihood that rocks with large melt proportions could lose coherence, it is more likely that multiple partial melt loss events occur during orogenesis, and this is discussed next. Multiple melt loss events It is currently impossible to draw a generic diagram in which multiple partial melt loss histories can be displayed. However, it is possible to draw diagrams for a specific melt loss history, where the temperature and proportion of melt loss is assumed. For example, Fig. 3 is a T X melt loss diagram involving three melt loss events. The proportion of melt lost in the two lower temperature events is fixed prior to the final melt loss event at 805 C. For this diagram the rock is heated from subsolidus water-saturated conditions to a temperature of about 720 C where it contains 24% melt. Two thirds of the melt is removed and then the rock is heated further to 740 C at which point it contains 15% melt. The second melt loss event involves the removal of 40% of the melt present, prior to heating to 805 C (14% melt) where the effects of different proportions of further melt loss can be assessed. Three paths labelled A to C, involving different amounts of melt loss followed by cooling are shown in Fig. 3 The higher water contents of the lower temperature melts

6 626 R. W. WHITE & R. POWELL (below the breakdown of biotite) means that the partial removal of these melts has a more profound effect on the bulk rock water contents than does removing the same amount of melt at higher temperature. The removal of melt at temperatures below the breakdown of biotite has a strong effect in reducing the higher temperature melt production as the water content of the rock has been reduced. Thus, the total modal amount of melt produced up to 805 C in Fig. 3 is 32% compared with 40% at 805 C in Fig. 2. Furthermore, a heating path involving a number of melt loss events, such as in Fig. 3, allows the preservation of a granulite facies assemblage with much smaller melt loss proportions in the final melt loss event and reduced overall melt production. The melt production history and the effect of melt loss in different rock compositions may vary substantially as shown in numerous partial melting experiments on metapelitic compositions (e.g. Gardien et al., 1995). The aluminous metapelite (Fig. 2) and a subaluminous metapelite (Fig. 4) have different melt production histories that reflect the different stable metamorphic assemblages and mineral modal proportions that develop during metamorphism. The most notable difference between these two rock types is the melt productivity related to muscovite and biotite breakdown. Compared with Fig. 2, most of the melting in Fig. 4 is at higher temperature, reflecting the different muscovite:biotite ratios in these examples. However, by 805 C total melt contents differ by only a few percent. The similarity in total melt production between the two bulk rock compositions reflects the similar total mica content of the two examples. Despite the mode of melt produced in the two examples given above being similar at peak temperatures, the different muscovite:biotite ratios result in markedly different melt production and presumably melt loss histories. That is, muscovite breakdown melting in the more aluminous pelite is a major melt-producing step producing a water-rich low-temperature melt over a small temperature range. Whereas, in the muscovite-poor, subaluminous pelite, the major melting step involving biotite melting occurs over a wider range in temperature and produces a drier melt. The four melt losscooling paths (A-D) shown in Fig. 4 represent the same proportions of melt loss as those in Fig. 2 and allow a useful comparison. Significantly less (x 40%) melt needs to be removed from the subaluminous composition to preserve a granulite facies assemblage than for the aluminous composition (x 75%). Thus, in Fig. 4 path A would have the same effect as path B in Fig. 2 in terms of the degree of preservation of the peak metamorphic assemblage. For larger amounts of melt loss (paths B-D, Fig. 4) the degree of preservation of the peak assemblage increases, as does the temperature at which the solidus is crossed during cooling. Melt distribution Thus far the discussion of melt loss has not addressed the distribution of melt within rocks and the scales at which melt loss occurs have not been explicitly considered. In most granulite facies terranes the presence of discrete leucosomes have been used to infer the segregation of melt within rocks on a centimetre to decimetre scale. Furthermore, high-grade rocks preserve a myriad of inferred melt segregation structures that reflect a wide range of possible structural, magmatic and metamorphic processes operating on various timescales (Brown, 1994; Brown et al., 1995; Brown & Solar, 1998; Hand & Dirks, 1992; Sawyer, 1994, 1996, 2001). The behaviour of melt within the segregations is of importance in understanding the scale of melt loss in granulite facies terranes (Brown, 1994; Sawyer, 1994, 1996, 2001). If the melt within leucocratic segregations remains there and crystallizes during cooling it may result in retrogression of parts of the rock adjacent to leucosomes, but preservation of high-grade assemblages more distal to leucosomes, as in Fig. 5 (a). Alternatively, if large amounts of melt are lost from leucosomes, little to no retrogression is likely, as in Fig. 5b. In this case melt loss can be Fig. 5. (a b). Sketches of two melt segregation melt loss scenarios. The boxes labelled A D in both sketches show different amounts of melt rock interaction (see text for explanation). (a) Sketch showing the possible outcome of melt segregation but no melt loss. In this scenario melt can interact with part of the source rock (b) Sketch showing the preservation of mineral assemblages adjacent to the melt segregation (leucosome). The preservation of the peak metamorphic mineral assemblages adjacent to the melt segregation in this example reflects melt loss from the leucosome.

7 MELT LOSS AND THE PRESERVATION OF GRANULITES 627 considered to have occurred on a metre to kilometre scale. However, given the potentially complex melting melt loss melt crystallization histories that granulite facies may undergo a wide range of possibilities between the two simple scenarios presented in Fig. 5 are possible. The T X melt loss diagrams can be used to model the behaviour of segregated melt. The boxes (A D) shown in Fig. 5 represent parts of a rock with a heterogeneous distribution of melt. Figure 5(a) a represents the situation where melt has segregated but has not left the rock as a whole. Different parts of the rock have different degrees of chemical communication with the melt. Area A in Fig. 5(a) is too distant from the melt segregation to interact with it and can be considered to have lost melt, equivalent to path D in Figs 2 and 4. The boxes labelled C and D in Fig. 5 are able to interact with melt during cooling and would undergo partial to complete retrogression approximately equivalent to cooling paths down the left hand side of Figs 2 and 4. In the areas either side of the leucosome, the interaction between melt and source rock may be highly variable with a range of retrogression possible. The box labelled B represents the melt itself, with no interaction with solid residue minerals and would simply crystallize during cooling. In Fig. 5(b) the peak metamorphic assemblage is preserved adjacent to the leucosome and peak metamorphic minerals occurring within the leucosome are also preserved. Again, box A in Fig. 5(b) is equivalent to path D in Figs 2 and 4. The boxes labelled C and D in Fig. 5 are also approximately equivalent to cooling path D and show little to no retrogression. Unretrogressed high-grade ferromagnesian minerals such as orthopyroxene and garnet within and adjacent to leucosomes is common in many granulite facies metamorphic terranes (e.g. Powell & Downes, 1990; Carson et al., 1997; Sawyer et al., 1999 & White et al., 2001, 2002) is consistent with melt loss having occurred on a large scale, albeit via a process of segregation into leucosomes and transfer from them. Melt loss along P T paths Isobaric melt loss diagrams are limited in their direct applicability to rocks that undergo more complex P T trajectories (e.g. Brown, 1993, 2002). The reactions by which retrogression occurs in rocks following a given P T loop will differ from those predicted in T X melt loss diagrams. However, they will still undergo substantial retrogression during cooling unless melt loss has occurred from them. Figures 6 and 7 are P T pseudosections that show the effect of removing all the melt from Fig. 1 at different temperatures. This series of pseudosections is sequential in that Fig. 7 shows the effect of melt removal subsequent to the melt loss inferred for Fig. 6. Thus, for the construction of Fig. 7, melt was removed at the P T equivalent to the melt loss point in Fig. 6 (labelled ÔÔML1 ) before the final melt loss event (labelled ÔÔML2 ). Thus, each diagram is only appropriate for part of the P T history. Such diagrams allow the effects of melt removal along P T trajectories to be assessed. Furthermore, these diagrams provide a useful guide to inferring the effects of subsequent metamorphic events on rocks that had lost melt during earlier granulite metamorphism. Although Fig. 6. Calculated P T pseudosection for an aluminous metapelite (shown in Fig. 7) having lost melt at the point labelled ML1. The bulk rock composition in terms of Na 2 O:CaO:K 2 O:FeO:MgO:Al 2 O 3 :H 2 Ois 1.19:1.08:10.70:13.62:7.17:55.97: The P T position of melt loss events are labelled ML1 and ML2. Solid lines along path A and path B indicate where this figure can be used if melt is lost at ML1 and ML2.

8 628 R. W. WHITE & R. POWELL Fig. 7. Calculated P T pseudosection for an aluminous metapelite (shown in Fig. 7) having lost melt at the points labelled ML1 and ML2. The bulk rock composition in terms of Na 2 O:CaO:K 2 O:FeO:MgO:Al 2 O 3 : H 2 O is 0.54:0.67:8.97:16.20:8.78:62.09:2.75. The P T position of melt loss events are labelled ML1 and ML2. Paths A, B and C are three hypothetical P T paths for a polymetamorphic terrane. Solid lines along paths A, B and C indicate where this figure can be used if melt is lost at ML1 and ML2. removing all melt at specified P T points is a somewhat crude approach to modelling melt loss processes in natural examples, it does represent an approximate expression of the processes discussed below. As each melt loss event requires the construction of a new pseudosection, more continuous melt loss scenarios involving a large number of melt loss events would require the construction of a large number of diagrams. A notable feature of melt loss is the significant shift in the position and shape of the solidus at pressures above about 3.5 kbar. This change in the position and shape of the solidus is accompanied by a substantial change in the mineral equilibria relationships to lower temperatures, and a less profound change in the up-temperature mineral equilibria relationships. The effect of the second melt loss event is most profound with the solidus and the melt mode contours forming a large embayment at about 4 kbar. This embayment reflects the distribution of water between melt and cordierite at lower pressures. As pressure decreases cordierite mode increases at the expense of melt, and in particular the water contained in that melt. The effect of melt loss on the mineral assemblage evolution of the aluminous metapelite will be assessed in relation to three distinct high-grade metamorphic events (Paths A, B & C; Figs 1, 6 & 7). Path A is a clockwise P T path starting in subsolidus conditions (Fig. 1). Initial melting at the water saturated solidus is followed by muscovite breakdown melting during which the first melt loss event occurs (ML1). Thus at temperatures above this point, Fig. 1 is no longer appropriate and the next segment of Path A is shown as a solid line on Fig. 6. This segment involves a steady increase in melt proportions accompanied by a steady decrease in modal biotite. Further melt loss occurs at point ML2 where the melt mode has reached c. 15 molar %. Thus the rest of the P T path is read from Fig. 7. The second melt loss event has a profound effect on the mineral equilibria relationships along path A which crosses the solidus during decompression cooling at close to 850 C and 5 kbar. At this point the rock is melt absent and lacks free water and further re-equilibration would most likely be slow. Thus the assemblage garnet, sillimanite, cordierite, K-feldspar and quartz would be preserved. If the rock is subsequently metamorphosed along P T path B (Fig. 7), melting does not occur until temperatures close to 800 C. In this path cordierite is consumed with increasing pressure close to peak conditions. With cooling biotite becomes stable and the solidus is reached at a temperature of about 780 C. Thus the peak metamorphic assemblages are similar for both events but they experience different retrograde histories. If a subsequent metamorphic event involves heating to temperatures below the post melt loss solidus, the ability of the rock to attain

9 MELT LOSS AND THE PRESERVATION OF GRANULITES 629 equilibrium could be hindered by the absence of a hydrous fluid or silicate melt. The ability of a rock to achieve an equilibrium assemblage under these conditions would be highly dependent on the rates of heating and cooling, time spent at high temperatures and the intensity of deformation that accompanied metamorphism. The resultant change in metamorphic assemblages that accompany such an overprint would most likely be incomplete. Differentiating unrelated partial metamorphic overprints from post peak reaction textures developed during cooling or decompression from peak conditions in a single metamorphic event can be difficult and has been the source of disagreement in several terranes (e.g. Hand et al., 1992; Collins & Vernon, 1991, 1993; Vernon, 1996). The melt loss history along with P T paths and deformation are likely to have profound influence on the mineralogical response of rocks to different metamorphic events in polymetamorphic terranes. This problem can be highlighted by looking at two sequential P T paths (paths B & C, Fig. 7). The P T paths B and C occur subsequent to path A in which melt loss occurred. Thus we have a theoretical polymetamorphic history involving three discrete metamorphic events in the order A, B & C. Peak metamorphism on path C involves conditions of approximately 4.5 kbar and 760 C. Despite the high temperatures, these conditions are below the solidus for this rock composition (Fig. 7). If equilibrium prevailed the assemblage garnet, sillimanite, cordierite, K-feldspar and quartz would form. However, in the absence of melt or an intergranular fluid reaction rates may be too slow to fully recrystallize the earlier assemblage and partial recrystallization of the assemblage preserved on path B may occur. This recrystallization would involve the breakdown of some of the garnet, sillimanite and biotite to produce predominantly cordierite and may result in a texture involving narrow rims of cordierite surrounding garnet, sillimanite and biotite. In the absence of intense deformation these textures are likely to be fine-grained as reaction rates would be slow despite the elevated temperatures. Such textures could easily be used to invoke post peak decompression following peak metamorphism along path B, shown as a dotted line in Fig. 7 rather than two separate metamorphic events. Although the loss of melt in earlier events has helped the preservation of their high-grade assemblages through later high-grade metamorphism, it may also allow the development of potentially misleading reaction textures. Rehydration The discussion of polymetamorphism above considers melt loss as the only open system process operating. However, the effects of rehydration of granulite facies terranes at temperatures below the wet solidus are potentially important in polymetamorphic terranes (e.g. McGregor & Friend, 1997). Figure 8 is a T X H2 O pseudosection that shows the effects of the addition of H 2 O at 650 C to a melt depleted aluminous metapelite. The composition of the metapelite prior to rehydration is equivalent to the composition along path C in both Figs 2 and 8 The horizontal arrow in Fig. 8 shows varying amounts of water added to the rock. Given the heterogeneous nature of retrogression in many granulite facies terranes such as channelled fluid flow in shear zones, a complete spectrum of retrogression could occur in a single rock on as little as a centimetre scale. An upper limit to the amount of water that can be added to a rock, assuming that water has sufficient access to minerals to allow them to retrogress, is provided by the water saturation line (X ¼ 0.83 at 650 C) in Fig. 8. At this point the rock contains a small amount of water as a free fluid phase. Heating from this point (path A) produces melt at the wet solidus with further major melting steps due to muscovite breakdown and biotite breakdown reactions. At a temperature of 805 C the rock contains c. 18 molar % melt. This amount of melt is less than half that produced in the original protolith ( 40%; Fig. 2). The reduction in the fertility of the rehydrated rock compared with the original protolith reflects the change in bulk rock composition due to melt loss and in particular the reduction in bulk K 2 O. This has the effect of restricting the amount of total mica in the rock and hence total water and melt fertility. The reduction in fertility is even more profound in cases with less water influx such as in path B. Furthermore, it is likely that melt loss at higher temperatures than outlined here would result in a greater reduction in bulk K 2 O and a greater reduction in the amount of water needed to saturate an assemblage at subsolidus temperatures. DISCUSSION The preservation of granulite facies mineral assemblages is an important feature in understanding highgrade processes in the middle to lower continental crust. The diagrams presented here show that melt loss at a given temperature has a significant influence on both up-temperature and down-temperature stable mineral assemblages. The preservation of granulite facies mineral assemblages requires that a substantial proportion of the melt produced during prograde and peak metamorphism to be lost from the source rock. Although it has long been recognised that melt can escape from partially melted rocks (Powell & Downes, 1990; Sawyer, 1991; Brown, 1994), constraining the proportion of melt loss that has occurred is more difficult and has relied on detailed geochemical studies (e.g. Barbey et al., 1990; Wickham, 1987; Sawyer, 1991, 1998). Various amounts of melt removal have been inferred for different terranes ranging from none (e.g. Greenfield et al., 1996) through some (e.g. Kriegsman

10 630 R. W. WHITE & R. POWELL Fig. 8. Calculated T X H2O pseudosection showing the effect of subsolidus water addition to a melt depleted aluminous metapelite. The bulk rock composition in terms of Na 2 O:CaO:K 2 O:FeO:MgO:Al 2 O 3 : H 2 O is 0.69:0.60:7.46:16.06:8.82:60.71:5.67 at X ¼ 0 and 0.52:0.51:6.45:14.67:8.07: 54.97:14.82 at X ¼ 1. The bulk rock composition at X ¼ 0 is equivalent to the composition along arrow C in Fig. 2. & Hensen, 1998) to effectively all (e.g. Powell & Downes, 1990). The abundance of only weakly retrogressed granulite facies terranes is consistent with substantial melt loss on a kilometre to tens of kilometre scale. The preservation of high grade minerals within leucocratic segregations (e.g. Powell & Downes, 1990; White et al., 2001), as well as a number of geochemical studies of migmatites (e.g. Sawyer, 1987; Barbey et al., 1990, 1996) in many terranes are inconsistent with leucosomes simply representing crystallized melt. Although leucosomes are undoubtedly related to partial melting, and may be related to melt segregation and melt movement in granulite facies rocks, the amount of crystallized melt in leucosomes is difficult to estimate. As suggested by Powell & Downes (1990), leucosomes, while being the locus of melting and melt segregation, may be dominated by the solid products of melting reactions. Furthermore, fractional crystallization of melt at temperatures above the stability of biotite in a given rock will produce predominantly anhydrous products that may also form an important component in leucosomes (Brown, 2002). Continued melt loss during the early stages of cooling and crystallization will aid the preservation of anhydrous minerals in leucosomes. On cooling from high temperatures, the solidus is likely to represent a marked change in the metamorphic development of a granulite facies rock. Even for small amounts of melt loss prior to cooling, the retrograde history is water undersaturated and the wet solidus will not be seen. In this case the solidus represents the point on a cooling path where only solid phases are present. In the absence of deformation, melt or water, diffusion and reaction rates are likely to be slow, such that the mineral assemblage in the rock may well become effectively ÔÔfrozen at this point. Thus, although lower grade assemblages may become more stable than the assemblage immediately below the solidus, it is likely that the higher grade assemblages will metastably persist. Thus, on cooling, the degree of retrograde re-equilibration is likely to be less than that predicted by the models due to slower reaction rates. In this case the step-like jumps in the temperature of the solidus with different degrees of melt loss are of particular interest as they control the final stable retrograde assemblage that will be formed on cooling after different amounts of melting and melt loss. The cooling melt crystallization histories of rocks where melt segregation has occurred may be more complex. Different degrees of melt residue interaction may lead to the situation where a rock effectively contains more than one solidus. In the extreme case part of a rock that has lost substantial melt to melt segregations may have a solidus at a temperature just below that of peak metamorphism whereas the segregation or leucosome contains melt until the wet solidus is reached. However, because granulite facies mineral

11 MELT LOSS AND THE PRESERVATION OF GRANULITES 631 assemblages tend to be preserved in both leucosomes and mesosomes, it is more likely that melt loss from both leucosomes and mesosomes in most granulite facies terranes is extensive and that any difference in their cooling path solidus temperatures is negligible. Certain exceptional examples are known where no melt has been lost from the source rocks such as Mt Stafford, central Australia (Greenfield et al., 1996). This terrane is quite different from most granulite facies terranes in that it experienced only low strain, shows significant melt crystallization-related retrogression and, where melt contents were high, the rock has mobilised to form a diatexite. In contrast, most granulite terranes are dominated by predominantly coherent lithological units. Brown (1994) outlined numerous mechanisms of deformation-enhanced melt segregation and migration at melt contents as low as five volume percent. Such mechanisms are consistent with melt loss occurring on a large scale in deforming rocks. Thus there is a clear conceptual distinction between rocks that have lost melt while maintaining their rheological coherence (i.e. without disruption to their original compositional layering), and rocks that have accumulated melt to a point where they have melt fractions larger than the critical melt percentage and become essentially a restite-rich magma. Both situations may have occurred in a single granulite facies terrane. The implications of melt loss on the metamorphic evolution of rocks in polymetamorphic terranes are likely to be profound. With the exception of rocks with extreme amounts of melt loss, the narrow major meltforming fields involving the breakdown of muscovite and biotite effectively become the solidus for subsequent metamorphic events. If the loss of melt from an earlier event is large, the melt production in any subsequent events will be small and the solidus for these later events will be at substantially higher temperatures than it was for the first. The preservation of high-grade mineral assemblages from an early event means there may be little mineralogical change involved in forming an equilibrium assemblage in subsequent high-grade events as the temperature of mineral preservation from the first event and the peak metamorphic temperature of the second event may be similar or involve similar stable mineral assemblages. Furthermore, high-grade mineral assemblages and reaction textures that are commonly assumed to have formed in the presence of melt may actually have been melt-absent if extensive melt loss occurred during earlier high-grade events. Thus melt loss during early metamorphic events is likely to aid in the preservation of the earlier granulite facies assemblages and metamorphic textures during subsequent high-grade metamorphic events. Melt loss may also have a profound effect on the response of lower crustal rocks to deformation. The absence of melt at high temperatures will inevitably affect the rheology of the lower crust, presumably making it stronger than melt bearing equivalents. The change in bulk rock composition that accompanies the loss of melt from granulite facies rocks essentially makes granulite facies metamorphism a one way process. In the absence of later rehydration highgrade assemblages will tend to be preserved. Where later subsolidus rehydration has occurred, the retrogressed granulite will have a different composition and possibly mineral assemblage than at the same P T conditions on the prograde path. The loss of melt removes key elements that are needed to produce hydrous minerals and hence the total water content of the rocks is restricted. This is likely to inhibit melt production during subsequent high-grade metamorphism, despite the rock having been rehydrated and even if it had become water-saturated. Overall, the presence of large sequences of coherent, weakly or unretrogressed, granulite facies gneiss in many terranes is consistent with open system behaviour for melt. Since, at temperatures above the breakdown of biotite, metapelitic rocks consist of water-poor solids and a relatively water-rich melt, the only high-grade process that can dehydrate the overall system is for a proportion of the melt to be lost. The dominance of anhydrous assemblages in granulites at the surface today is consistent with the proportion of melt loss from these terranes being in excess of 50 70% of the melt produced. 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