A sequence of partial melting reactions at Mt Stafford, central Australia

Transcription

1 J. metamorphic Geol., 1998, 16, A sequence of partial melting reactions at Mt Stafford, central Australia J. E. GREENFIELD,1 G. L. CLARKE1 AND R. W. WHITE2 1Department of Geology and Geophysics, University of Sydney, NSW 2006, Australia ( geoffc@mail.usyd.edu.au) 2School of Earth Sciences, Macquarie University, Sydney, NSW 2109, Australia ABSTRACT Metasedimentary gneisses show a rapid change in grade in a 10 km wide low-p/high-t regional aureole at Mt Stafford in the Arunta Block, central Australia. Migmatite occurs in all but the lowermost of five metamorphic zones, which grade from greenschist (Zone 1) through amphibolite (Zones 2 3) to granulite facies (Zones 4 5). The sequence of partial melting reactions inferred for metapelitic rocks is dependant upon protolith, temperature and fluid conditions. The metapelite solidus in Zone 2 reflects vapour-present melting at P#3 kbar and T #640 C, melting having initially been controlled by the congruent breakdown of the assemblage Crd Kfs Bt Qtz. At slightly higher temperature, andalusite in leucosome formed via the reaction Kfs+Qtz+Bt+H OAnd+melt; And+melt having been stabilized by the presence of 2 boron. Sillimanite coaxially replaces andalusite in the high-grade portion of Zone 2. In Zone 3, large aluminosilicate aggregates in leucosome are armoured by Spl Crd±Grt symplectites. Garnet partially pseudomorphs biotite, cordierite or spinel in high-grade portions of Zone 3. Zone 4 Grt Crd Opxbearing metapsammite assemblages and garnet-bearing leucosome reflect T #800 C and P= 2.2±0.9 kbar. In the model KFMASH system the principal vapour-absent melting step reflected significant modal changes related to the breakdown of the As Bt tie-line and the establishment of the Spl Crd tie-line; the bulk rock geochemistry of migmatite samples straddle the Spl Crd tie-line. The aluminous bulk-rock composition of the common bedded migmatite restricted its potential to witness garnet-forming and orthopyroxene-forming reactions, minor textural and modal changes in and above Zone 3 reflecting biotite destablization in biotite-poor assemblages. Key words: Arunta Block; KFMASH; low-p/high-t metamorphism; melting; migmatite. Mineral abbreviations are after Kretz (1983), with the following additions: alm- almandine; ann- annite; as- aluminosilicate; cel- celadonite; crd- cordierite; east- eastonite; en- enstatite; fcrd- cordierite; fs- ferrosilite; mu- muscovite; naph- Na phlogopite; pa- paragonite; pe- pelite; phl- phlogopite; ps- psammite; py- pyrope; sp- spinel end-member. INTRODUCTION (after White et al., 1974; Barton & Brooks Hanson, 1989; Collins & Vernon, 1991). Although low-p/high-t metamorphic Any evidence for prograde reactions that preceded peak belts are common in Proterozoic granulite conditions is obscured in most high-grade rocks by facies terranes, the comparatively low strain experienced progressive recrystallization. However, terranes that by the rocks at Mt Stafford has allowed the processes preserve a sequence of metamorphic isograds provide an involved in their formation to be studied without the opportunity to study the reaction history of the highest complication induced by intense deformation (Greenfield grade rocks through lower grade assemblages of similar et al., 1996). The terrane exposes greenschist to granulite rock types (Chenhall et al., 1988; Pattison & Harte, facies rocks, making it an ideal location to study 1988; Obata et al., 1994). The simplest settings involve migmatite formation and evolution as a function of contact metamorphic aureoles that surround highcrustal temperature. On the basis of field relationships, microgradients level granites, with steep, local geothermal scopic textures and modal data, we infer a series of and areally restricted isograds (Pattison & prograde reactions that are influenced by grade, rock- Tracy, 1991). Unfortunately, most exposed contact type and fluid availability, and interpret their conditions aureoles did not experience supersolidus conditions, so of formation using thermobarometry and modelling in our knowledge of partially melted rocks comes predomi- the K O O MgO Al O SiO H O (KFMASH) nantly from studies of regional metamorphic terranes system. that have more complex geological settings. Migmatites exposed at Mt Stafford, in the Proterozoic Arunta Block of central Australia, are unusual in that temperatures in REGIONAL GEOLOGY excess of 810 C were reached at kbar in a Mt Stafford, in the Anmatjira-Reynolds Range region comparatively simple low-p/high-t regional aureole (Stewart, 1981), forms part of the Proterozoic Arunta Blackwell Science Inc., /98/$14.00 Journal of Metamorphic Geology, Volume 16, Number 3, 1998,

2 364 J. E. GREENFIELD ET AL. Block (Noakes, 1953; Fig. 1), an extensive area of by garnet-bearing migmatite, and garnet orthopyroxene-bearing complexly deformed, low- to high-grade Proterozoic assemblages in metapsammite. Sufficiently metasedimentary rocks and orthogneisses (Shaw et al., high proportions of melt in some Zone 4 rock layers 1984; Stewart et al., 1984). The Mt Stafford area is resulted in the formation of local diatexite. However, the dominated by a metasedimentary sequence called the consistency in whole rock geochemistry of metapelite Lander Rock Beds, inferred to have been deposited at c. layers in migmatites from Zones 2b to 4 indicates that 1870 Ma (Blake & Page, 1988; Compston, 1995). Peak melting proceeded in situ without injection or extraction metamorphism at Mt Stafford occurred during the of melt (Greenfield et al., 1996). Zone 5 contains pre-1820 Ma D1 M1 event (Collins & Williams, 1995). enigmatic biotite cordierite plagioclase diatexite, which The area preserves only limited effects ofad2 M2event has gradational contacts with the northern granite that pervasively recrystallized rocks further east (Clarke (Fig. 1). This granite cuts the metamorphic isograds and et al., 1990). The results of U Pb dating of zircon from was intruded late in the M1 D1 event (Fig. 1; Greenfield syn-tectonic granitoids was used to infer that the D2 M2 et al., 1996). event was short lived and occurred between c Peak metamorphism and melting were contempor Ma (Collins & Williams, 1995). However, more aneous with localized extension in the migmatites, recent U Th Pb dating of zircons and monazites from evidenced by the migration of melt to boudin necks granulite facies metasedimentary rocks recrystallized and narrow (1 cm wide) conjugate shear zones. Drag during the event is consistent with D2 M2 having folding adjacent to high-grade shear zones, which occurred at c. 1600Ma(Vryet al., 1996; Williams et al., locally truncate bedding, is also inferred to have 1996). At Mt Stafford, the D1 M1 event resulted in a occurred at peak conditions on the basis of shear low-p/high-t regional aureole (Collins & Vernon, 1991), zone-leucosome relationships. Whereas Vernon et al. which has been divided into five zones (Greenfield et al., (1990) inferred that these features were the result of 1996), ranging from greenschist (Zone 1) to granulite two compressional deformation events, we infer that facies (Zones 4 & 5) over a distance of 10 km (Fig. 1). they are both part of strain partitioning during one Zone 1 is poorly exposed and contains muscovite extensional D event. It is currently unclear what 1a biotite quartz schist. The lower limit of Zone 2 is defined ultimately caused the localized, exceptionally low- by the subsolidus breakdown of muscovite to andalusite P/high-T metamorphism. The limited mafic rocks in and K-feldspar (Sub-zone 2a). The first recognized the area are themselves metamorphosed, and the leucosome defines the lower limit of Sub-zone 2b. The northern granite intruded after D1a (Greenfield et al., high-temperature section of Zone 2b contains basic sills 1996). However, the intimate relationship shown by metamorphosed to two-pyroxene granofelses, which are the northern granite and Zone 5 rocks is consistent interlayered with andalusite-bearing migmatite. Subzone with it having intruded shortly after, and having been 2c is defined on the basis of sillimanite replacing causally related to, the D1a M1a event (Greenfield andalusite. Zone 3 is characterized by abundant spinel et al., 1996). and a reduction in biotite content. Zone 4 is characterized Kilometre-scale, open to tight, north-west-trending Fig. 1. Geological map of the Mt Stafford area showing bedding trends, metamorphic zoning, the position of the inferred metapelite solidus and the location of samples discussed in the text.

3 PARTIAL MELTING AT MT STAFFORD 365 F1b folds deform the migmatites and D1a extensional zone 2b and the inferred change to supersolidus structures. Recrystallization during this event was not conditions for metapelitic rocks. It is characterized by sufficiently intense to destroy M1a textures. The impersistent, cm-scale lenticular leucocratic segregations eastern granite (Fig. 1) cuts S1b, is surrounded by oriented predominantly normal to bedding in zones of random muscovite-bearing retrogression and metapelite layers (Fig. 2a, Greenfield et al., 1996) that was dated at 1818±15 Ma (Collins & Williams, 1995). reflect extensional structures and the incipient stages Though compositionally similar to, it intruded later of melting (Greenfield et al., 1996). The metapelite than, the northern granite. Cordierite and K-feldspar mineralogy is similar to Zone 2a, and the leucocratic in migmatites near this body are retrogressed to segregations comprise quartz (60%), K-feldspar (30%), muscovite sillimanite or muscovite andalusite assemblages. and biotite (10%). The leucocratic segregations in this Two sets of muscovite quartz dominated zone may also enclose andalusite porphyroblasts, or retrograde shear zones cut all earlier fabrics. The form inter-connected networks. Larger grains of dominant set is east-trending and shows sinistral K-feldspar in the segregations commonly preserve movement, and the subordinate set trends north-east inclusion-rich cores; clear idioblastic rims are rarely and shows dextral movement. No data directly con- mantled by plagioclase. Andalusite occurs impersistently strain the age(s) of these shear zones, but they may in leucosome in the high-grade portion of Zone have been active as late as the Late Devonian to Early 2b, near the Zone 2c boundary. It is prismatic, coarsegrained Carboniferous Alice Springs Orogeny (Collins & (up to 8 cm long) and commonly observed in Teyssier, 1989) when extensive ductile faulting of close association with subidioblastic tourmaline grains. Arunta Block basement rocks occurred. Sub-zone 2c contains sillimanite and more extensive leucosome development. Whereas the mineralogy of METAPELITE PETROGRAPHY the units is similar to that described above for Zone 2a and 2b, subtle variations reflect the increase in Metapelite in the Mt Stafford terrane is interbedded grade. Biotite and quartz are less abundant than in on a decimetre scale with metapsammite, and may Zone 2b, especially in metapelite layers. K-feldspar is represent original turbidite sequences (Stewart et al., abundant and cordierite subordinate in metapelite 1984). The general term bedded migmatite (Greenfield layers, and vice versa in the metapsammite layers. et al., 1996) is used to describe these interbedded units Leucosome fills boudin necks and defines cm-scale (Zone 2b 4) in which the pelitic layer is coarser conjugate shear zones. Andalusite that is rimmed by grained and was more extensively melted than adjacent optically continuous prismatic sillimanite is interpreted psammitic layers. Hybrid diatexite describes migmat- to indicate coaxial replacement of andalusite by ite from Zone 5 that contains a mixture of in situ and sillimanite ( Vernon, 1987; Fig. 2a). Pseudomorphs of injected melt (Greenfield et al., 1996). Migmatite sillimanite commonly retain the prismatic shape and structure within the pelitic layers varies with grade, square [001] faces of the andalusite. Spinel cordierite the terms used below follow the nomenclature of symplectites partially replace aluminosilicate in one Mehnert (1968), Brown (1973), and Pattison & Harte sample (1112 m) from Zone 2c. This would appear to (1988). A more complete description of the rock-types be the incipient stage of the general reaction texture and migmatite structures within the terrane can be described below for Zone 3. found in Vernon et al. (1990) and Greenfield et al. Zone 3 contains the first schlieren migmatite (after (1996). Mehnert, 1968) and migmatite with well-developed Zone 1 metapelite has a schistose fabric defined by melanosome. Where present, the melanosome separates fine-grained idioblastic muscovite, biotite, quartz and psammitic and pelitic portions of the bedded migmatite. chlorite. Muscovite is the dominant mica and chlorite Metapelite layers in bedded migmatite mostly consist is typically subordinate ( 5 10% of the rock). Minor of coarse-grained granoblastic K-feldspar, cordierite minerals include tourmaline, ilmenite, and apatite, and quartz, with or without aluminosilicate (mostly though tourmaline may form up to 20% of the rock. sillimanite), spinel, ilmenite, plagioclase and garnet. Zone 2 is marked by the appearance of K-feldspar Large grains of prismatic sillimanite that partially to and has been divided into three subzones on the basis completely pseudomorph andalusite porphyroblasts of isograds marking the metapelite solidus ( boundary are enclosed by complex corona reaction textures of Zones 2a/2b) and the andalusite sillimanite transition involving symplectites of cordierite and spinel, with or ( boundary of Zones 2b/2c). Sub-zone 2a metapel- without K-feldspar (Fig. 2a & b). Nebulitic leucosome ite contains poikiloblasts of cordierite with inclusions may comprise up to 20% ( by volume) of the metapelite of random fine-grained biotite and rounded quartz, in layers, commonly centred on the aluminosilicate aggregates a fine to medium-grained granoblastic matrix of and forming extensive vein networks (Fig. 2c). K-feldspar, quartz, biotite and ilmenite. Andalusite The striking texture involving high-proportions of occurs as fine to medium-grained ( mm) idioblastic nebulitic leucosome with aluminosilicate, spinel and prisms that are commonly poikiloblastic with cordierite symplectites dominates exposures of Zone 3 inclusions of quartz. bedded migmatite, though the modal content of the Bedded migmatite marks the lower limit of Sub- symplectites is variable. In some rocks, all aluminosilic-

4 366 J. E. GREENFIELD ET AL. Fig. 2. (a) Coaxial replacement of andalusite by sillimanite, and the development of spinel cordierite symplectites surrounding sillimanite (Zone 4). Spinel is mostly separated from cordierite by garnet. Crossed polars, base of photograph 5 mm. (b) Symplectites of spinel cordierite K-feldspar that partially pseudomorph sillimanite (Zone 4). Base of photograph 10 mm. (c) Interconnected network of Zone 3 leucosomes, centred on aluminosilicate spinel cordierite symplectites (e.g. Fig. 2a). (d) Partial replacement of biotite by garnet, in the presence of cordierite and K-feldspar (Zone 3). Base of photograph 3.5 mm. (e) Delicate biotite fingers, optically continuous over large areas, which form skeletal biotite grains that are partially replaced by K-feldspar, quartz and garnet. Zone 3, base of photograph 3.5 mm. (f ) Garnet porphyroblasts in Zone 4 metapelite surrounded by a K-feldspar-rich leucosome. Pen is 13 cm long. (g) Zone 4 garnet porphyroblasts within leucosome. Idioblastic garnet is set against quartz, with partial consumption of biotite and cordierite. Base of photograph 3 mm.

5 PARTIAL MELTING AT MT STAFFORD 367 ate has been consumed by symplectites of spinel and of metasediments and injection of granitic magma cordierite, whereas in other rocks spinel and cordierite sourced from the northern granite (Greenfield et al., only pseudomorph #20% of the square shape of what 1996). Aluminosilicate is less common than in Zone 4, was presumably andalusite. Garnet is present in small but where present it is andalusite. In two specimens proportions in some spinel cordierite symplectites in prismatic andalusite is observed to partially pseudomorph high-grade portions of Zone 3. Where present, garnet sillimanite. is sub-idioblastic and symplectic with spinel, or In migmatites formed as a consequence of regional separates spinel from cordierite in the symplectites metamorphism, it is difficult or impossible to distinguish (Fig. 2a). Such observations are consistent with the between minerals that crystallized from melt and those growth of garnet in the symplectites having occurred formed as inherited or peritectic components of a at the expense of spinel. Metapsammite layers in Zone leucosome, as strain-induced recrystallization will have 3 bedded migmatite commonly consist of coarsegrained commonly destroyed any igneous texture. At Mt granoblastic K-feldspar, cordierite, biotite, and Stafford, the weakly deformed nature of leucosome, and quartz, with or without spinel and garnet. Where well-defined leucosome/mesosome boundaries, provide present, spinel occurs in spinel cordierite symplectites a clear framework to interpret the migmatitic microtextures. in the cores of large cordierite grains. Garnet is finegrained Andalusite (Zone 2c), cordierite (Zones 3 and 4), ( mm), xenoblastic and commonly garnet and orthopyroxene (Zone 4) occur in leucosome, partially pseudomorphs biotite and/or cordierite commonly with leucosome networks spatially related to (Fig. 2d). Biotite is partially pseudomorphed ( Fig. 2e) these minerals (Fig. 2c & f ). Such relationships are by fine-grained K-feldspar and quartz, with or without consistent with them being the peritectic products of garnet, leaving skeletal biotite grains that are inferred melt-producing reactions (e.g. Powell & Downes, 1990). to have been partially consumed by melt-producing Alternatively, poikiloblastic and xenoblastic cordierite reactions. that is entrained in leucosome most probably represents The lower limit of Zone 4 is marked by the restitic material. The cores of large K-feldspar grains in appearance of garnet orthopyroxene cordierite assem- Zones 2c and 3 are poikiloblastic, with randomly blages within metapsammite. Though metapelite layers orientated quartz and biotite inclusions. The inclusion- contain abundant diatexite (after Brown, 1973), metap- rich cores may represent sub-solidus K-feldspar, which sammite layers still preserve relict bedding features provided a nucleus for further inclusion-free K-feldspar ( Vernon et al., 1990; Greenfield et al., 1996) reflecting growth that crystallized with or from melt. Similar low strain and restricted melt migration. Garnet most textures are commonly observed in other peritectic commonly occurs as large poikiloblasts in melanosome minerals, consistent with many having sub-solidus that separates pelitic and psammitic layers, and less nuclei. Within diatexite from Zones 4 and 5, fine-grained commonly in metapelite surrounded by leucocratic patches of quartz, biotite and K-feldspar, with or segregations containing quartz, K-feldspar and rare without garnet, occur in leucosome. They are inferred plagioclase (Fig. 2f & g). Orthopyroxene is medium- to be restitic components on the basis of their fine grain grained (0.5 5 mm), and either prismatic and inclusion size and absence of melt-textures such as idioblastic free or is xenoblastic and poikiloblastic. It occurs in crystal faces, graphic intergrowths, myrmekite, idiomorphic contact with cordierite, K-feldspar and quartz, and zoning in plagioclase, etc. (Tilley, 1924; Ashworth less commonly with garnet and plagioclase. Co-existing & McLellan, 1985; Pattison & Harte, 1988; Vernon & orthopyroxene and garnet occur exclusively in metapsammite Collins, 1988). For example, cordierite in metapsammite layers. In metapelite layers, sillimanite is schollen entrained in Zone 4 diatexite is inclusion-rich, partially to completely pseudomorphed by symplectites xenoblastic and has embayed grain boundaries. In of spinel and cordierite, with or without K-feldspar or comparison, what is inferred to be peritectic cordierite garnet. Grandidierite has also been recognized in in leucosome is inclusion-free, displays sector twinning sillimanite-rich leucosomes of mobilized diatexite. It and has sub-idioblastic or idioblastic grain boundaries. occurs as aligned fibrous, turquoise crystals in contact Random symplectic aggregates of andalusite, biotite with coarse-grained K-feldspar (0.1 4 cm), cordierite and quartz may partially to completely pseudomorph (0.1 1 cm), quartz ( cm) and brown, chromium cordierite in Zones 2 5, most probably as the result of spinel ( mm). Grandidierite also occurs on the water released from the crystallization of leucosome margin of symplectites of spinel and cordierite that ( Vernon et al., 1990). Late fibrous sillimanite and partially replace sillimanite. garnet are also common in Zones 2 and 3. The late Zone 5 is dominated by a heterogeneous unit garnet is commonly symplectic with biotite, and the mapped as hybrid diatexite (Greenfield et al., 1996). fibrous sillimanite occurs in biotite-rich folia that cut This unit is characterized by increased modal proportions the granoblastic texture defined by the prograde of biotite, plagioclase and quartz relative to assemblage described above. Microstructural relation- Zone 4 assemblages (Greenfield et al., 1996), partial to ships are consistent with the fibrous sillimanite having complete loss of bedding structures and the presence formed later than the symplectic aggregates of andalusite, of rotated schollen. The hybrid diatexite is interpreted biotite and quartz ( Vernon et al., 1990), most to contain leucosome sourced from the in situ melting probably as a consequence of reheating of the Mt

6 368 J. E. GREENFIELD ET AL. Stafford terrane during the c Ma D event that end member between 0.96 and 1.00, and magnetite, 2 pervasively recrystallized the Mt Weldon terrane which is pure. Aluminosilicate has negligible Mn O 2 3 further to the south-east. but may contain #0.08 mol p.f.u. of iron, which is concentrated in the cores of large grains. Rare plagioclase has a broad range in An=100 MINERAL CHEMISTRY Ca/(Ca+Na+K) values from An to An for Zones Analyses were carried out on the Cameca SX , but plagioclase in individual samples is uniform electron microprobe housed at the Electron in composition. Large plagioclase grains in Zone 5 Microscopy Unit at the University of New South hybrid diatexite show strong normal zoning from An 98 Wales, operating with an accelerating voltage of 15 kv, to An (core to rim) and have higher anorthite values 57 a beam current of 20 na and PAP data reduction than plagioclase from Zones 2 4. Orthoclase has Or= techniques supplied by the manufacturer. Repre- 100 K/(Ca+Na+K) values between Or to Or with sentative mineral compositions for low- and highgrade no apparent systematic variation. metapelite assemblages are given in Table 1. Grandidierite has a tight X range between 0.59 X =/(+Mg) in almandine-rich garnet ranges and Boron contents are consistently c mol from 0.84 to Ca and Mn are minor components, p.f.u. (nine oxygens; Table 1). Tourmaline grains are with the mole fraction of grossular and spessartine not zoned but may vary in composition between <0.01. Garnet is typically unzoned, with variation of samples: detrital tourmaline has X #0.66, tourmaline X in individual grains being commonly <0.02. in leucosome has X #0.54, and tourmaline within Cordierite has X = Subsolidus cordierite retrograde shear zones has X #0.46. Boron contents has subtle femic zoning involving Mg-rich cores (X = in analysed tourmalines vary from 8.60 to wt.% ) and -rich rims (X = ). Cordierite grains in leucosome are homogenous, and THERMOBAROMETRY there is no discernible difference between cordierite grains in adjacent metapelite and metapsammite layers. Quantitative P T estimates were obtained using The volatile content of cordierite, determined by THERMOCALC (v2.4, Powell & Holland, 1988), with secondary-ion mass spectrometry (Carrington & the expanded internally consistent dataset of Holland Harley, 1996), from Zone 4 in situ migmatite is different & Powell (1990; updated January 1996). The Grt to that of cordierite from Zone 5 hybrid diatexite. Crd Qtz Sill thermometer and barometer of Nichols Zone 4 cordierite contains mol of total et al. (1992), the Grt Bt thermometer of rry & volatiles per formula unit, consisting of c Spear (1978), the Grt Opx thermometer of Harley X ( wt.%) and c X (1.1 wt.%). (1984) and the Opx Cpx thermometer of Fonarev & CO2 H2 O Cordierite from four hybrid diatexite samples contains Graphchikov (1983) were also used to estimate P T mol of total volatiles per formula unit (p.f.u.), conditions. Results are shown in Table 2 for 28 consisting almost entirely of H O (S. L. Harley, representative samples from lowest to highest grade. 2 unpublished data). Generally there is good agreement between the results Biotite defining S or S in the migmatites ranges obtained from THERMOCALC (Table 3; Fig. 3) and 1a 1b from X = , and there may be considerable those obtained from the Grt Crd barometer of Nichols variation (±0.12) within individual samples. There is et al. ( 1992). As the exsolution of magnetite from a general decrease in X in biotite moving from Zone spinel has resulted in uncertainty on the true composi- 2 to Zone 3. Biotite grains in andalusite biotite quartz tion of S1 spinel, spinel-bearing assemblages were not symplectites that pseudomorph cordierite in leucosome used for thermobarometry. Mineral assemblages used have X #0.55. The TiO /(TiO +Al O ) ratio of in the calculations for rocks in Zones 1 2c were biotite varies between 0.05 and Only two of the assumed to be water present. In and above Zone 2c, analysed biotite grains have detectable fluorine; these the mineral assemblages are assumed to have become samples have #0.5 wt.% F. fluid absent, with water partitioned into melt and Orthopyroxene has X = Some grains peritectic cordierite. show subtle zoning with a rimward decrease in X Mg exchange thermometers gave less consistent -1 involving a change of up to Al O contents range results than THERMOCALC, and tended to underestimate temperature conditions for the assemblages 2 3 from wt.%. Hercynitic spinel has X = , and is always observed in Zones 3 5 when these assemblages are more -rich than co-existing garnet. It may contain compared with the results of experimental work and up to 0.14 mol of zinc p.f.u. (4 oxygens) but commonly inferred reactions (e.g. Grant, 1985; Vielzeuf & has negligible Cr O. An exception to this generalization involves the brown-coloured spinel associated was 495±106 C (2 s errors cited) obtained from a Holloway, 1988). The lowest temperature estimate 2 3 with grandidierite, which can contain up to 0.02 mol Zone 1 schist (GC6, Table 2) using the assemblage p.f.u. of chromium (Table 1). Spinel may also contain Bt Mu Qtz, assuming a pressure of 2.0 kbar and up to 0.02 mol p.f.u. of Ti. The opaque minerals are ilmenite, which has ilmenite a =1 (Fig. 3). Zone 3 sample gave an average H2 O P T result of 3.2±1.6 kbar and 756±120 C based on

7 PARTIAL MELTING AT MT STAFFORD 369 Table 1. Representative mineral analyses of the main minerals forming the metapelitic rocks throughout the Mt Stafford area. Zone Sample DC229 DC229 DC229 DC Description Grt core Grt core Crd core Bt Kfs Bt Crd Sil Spl Pl core Grt rim Grt core Opx Bt in Opx Crd Mig type bedded bedded bedded bedded bedded schlieren schlieren schlieren schlieren bedded bedded bedded bedded bedded bedded SiO TiO B O 2 3 Al O Cr O O MnO MgO ZnO CaO Na O K O Total No. Ox Si Tt B Al Cr Mn Mg Zn Ca Na K Total Sample DC176a DC176a DC176a DC176a DC176a DC176 DC176 MB73 MB73 MB73 MB73 Description Sil Bt rim Bt core Gdd core Gdd rim Crd Spl Kfs Crd Bt Pl core Rock type bedded bedded bedded bedded bedded bedded bedded h. diatexite h. diatexite h. diatexite h. diatexite SiO TiO B O Al O Cr O O MnO MgO ZnO CaO Na O K O Total No. Ox Si Ti B Al Cr Mn Mg Zn Ca Na K Total the assemblage Grt Bt Crd Kfs Qtz. The highest lations made using sample 1190f returned P= grade conditions were obtained from the Zone 4 assemblage Grt Crd Opx Bt Kfs: average P T calcu- P=2 kbar (a H2 O =0.25, a =0). On the basis of 2.1±1.1 kbar for T =700 C and T =808±194 C for CO lations made using mineral analyses from sample 9115 cordierite volatile content a 2 was considerable, but CO2 returned T =926±256 CandP=3.2±1.2 kbar (a H2 O = average P T calculations made using garnet cordierite orthopyroxene-bearing equilibria are comparatively 0.25, a =0.25; Table 3), whereas average P T calcu- CO2

8 370 J. E. GREENFIELD ET AL. P (kbar) T ( C) Zone Sample Distance (km) Assemblage (2 s error) Method (2 s error) Method 1 GC6 8.2 Bt Ms 495 (106) Bt Ms 515 (106) Bt Ms 632 (62) Bt Ms 574 (62) Bt Ms 637 (70) Bt Crd Grt 2.7 (3.0) (100) (3.0) (1.6) (116) Bt Crd 638 (60) Bt Crd Grt 2.8 (3.0) (100) (3.0) (1.4) (108) Opx Cpx Pl Grt Crd Bt 3.5 (3.0) (100) (3.0) (22) Grt Crd Bt 2.2 (3.0) (100) (3.0) (1.6) (120) 3 DC Crd Bt Pl 727 (116) Grt Crd Bt Spl 3.3 (3.0) (100) (3.0) (96) (1.0) 3 216A 2.4 Grt Crd Bt Pl 4.3 (3.0) (100) (3.0) (142) 3 4 (2.0) f 2.3 Grt Crd Bt 4.2 (3.0) (100) (3.0) HW Grt Crd Bt 4.4 (3.0) (100) (3.0) Grt Opx Crd Bt 4.7 (3.0) (100) (3.0) (256) (1.2) Grt Crd Bt 4.4 (3.0) (100) (3.0) Grt Crd Bt Pl 5.1 (3.0) (100) (3.0) (1.2) (94) b1 1.9 Grt Bt Grt Crd Bt 4.7 (3.0) (100) (3.0) Grt Opx Crd Bt Pl 4.7 (3.0) (100) (3.0) (88) (1.2) f 1.5 Grt Opx Crd Bt Kf 2.1 (1.1) (200) HW Opx Cpx Pl MB Grt Crd Bt 4.8 (3.0) (100) (3.0) (1.8) (132) Grt Crd Bt 3.5 (3.0) (100) (3.0) MB7 1.2 Bt Crd Grt Pl 4.6 (3.0) (100) (3.0) (126) (1.8) 3 MB Grt Crd Bt 5.2 (3.0) (100) (3.0) (94) (1.2) Grt Crd Pl 4.8 (3.0) (100) (3.0) (94) 3 HW Opx Cpx Pl 785 (100) 6 Table 2. Summary of the thermobarometric results from 28 representative metapelite and metapsammite analyses from across the terrane. 1: Nichols et al. (1992) FAS; 2: Nichols et al. (1992) MAS; 3: Powell & Holland (1988); 4: Nichols et al. (1992); 5: rry & Spear (1978); 6: Fonarev & Graphchikov (1983); 7: Harley (1984). insensitive to variations in a H2 O and a as most of Zone 5 rocks give poor thermobarometric constraints: pressure estimates made from different CO the exchange reactions do not involve fluid 2 (Table 3). Grt Opx thermometry returned temperature estimates methods are less consistent than in the other zones between 759 (9115) and 826 C (1190f ), though some and there are larger errors on the THERMOCALC samples indicate resetting and give T = C. results (Table 2). Pressure estimates made using the Pressure estimates of #3 kbar were obtained using the FAS and MAS systems of the Nichols et al. (1992) method of Nichols et al. (1992) for the mineral barometer vary by up to 1.6 kbar in this zone, assemblages from Zones 2 and 3 (Table 2). consistent with the assemblages used having been

9 Table 3. Representative THERMOCALC (Powell & Holland, 1988) results for assemblages from Zone 4. PARTIAL MELTING AT MT STAFFORD 371 Average P T calculations for sample 9115, Bt Crd Grt Opx Qtz Kfs granofels (Zone 4) phl ann east crd fcrd py alm en fs mgts q ksp activities (a) ss (ln(a)) Average P T independent reactions (for T =800 C and a H2 O =0.25 a =0.25) CO 2 P(T ) s (P) a s (a) b c ln K s (ln K) (1) east+py+3q=phl+crd (2) 2east+2en+3q=2phl+crd (3) 2 mgts+3q=crd (4) 3east+6fs=2phl+ann+3alm (5) east+alm+3q=ann+crd (6) 2ann+3east+3py+9q=5ph+3fcrd T =926 C, s=128, P=3.0 kbar, s=0.6, cor=0.61, sfit=0.86 Effect of varying a H2 O and a a a T s(t ) P s(p) s(fit) CO 2 CO2 H2 O Average P T calculations for sample 1190f, Bt Crd Grt Opx Qtz Kfs granofels (Zone 4) py alm en fs mgts phl ann east ksp crd fcrd q activities (a) a s (ln(a)) Average P independent reactions (for T =700 C and a H2 O =0.3) P(T ) s (P) a s (a) b c ln K s (ln K) (1) en+mgts=py (2) py+east+3q=phl+crd (3) 2en+2east+3q=2phl+crd (4) 3fs+3 mgts=py+2alm (5) py+2alm+3east+9q=3phl+3fcrd (6) alm+east+3q=ann+crd T =700 C, av P=2.1 kbar, s(p)=0.55, s(fit)=1.3 Average T independent reactions (for P=2.5 kbar and a H2 O =0.25) T (P) s (T ) a s (a) b c ln K s (ln K) (1) py+east+3q=phl+crd (2) en+east=mgts+phl (3) 2 mgts+3q=crd (4) 2py+3fs=2alm+3en (5) py+ann=alm+phl (6) 6py+2ann+9q=6en+2phl+3fcrd P av T s (T ) s(fit) Average P T Influence of a H2 O and a a a T s(t ) P s(p) s(fit) CO 2 CO2 H2 O either reset and/or in disequilibrium. Nonetheless, the results are similar, if somewhat less well constrained than those of the Zone 4 assemblages. REACTION SEQUENCE Zone 2 Andalusite is common in Zone 2a, well below the inferred solidus, consistent with it having initially formed via a solid state reaction. The close association of andalusite and K-feldspar in Zone 2a, as well as the disappearance of muscovite and a reduction of modal quartz from Zone 1 is consistent with andalusite forming via the reaction: 1.0 Ms+1.6 Qtz0.5 And +0.5 Kfs+1.0 H O 2 (1) This reaction, and reactions 2 and 3 below were balanced using mineral compositions appropriate to the rocks at Mt Stafford. The onset of partial melting

10 372 J. E. GREENFIELD ET AL. is interpreted to occur at upper amphibolite facies 2 biotite is richer in than Zone 3 and 4 biotite. conditions within Zone 2b, though the water activity Most samples of the bedded migmatite have a attending metamorphism at this grade is not known. remarkably uniform /Mg ratio near X =0.63 The amount of biotite present in the metapelitic rocks, (unconstrained for ferric: ferrous ratio) but show a and water released from the breakdown of muscovite spread in projected alumina content with X = Al (reaction 1) would be consistent with hydrous conditions (Al K)/((Al K)++(Mg)=55 75 (Fig. 4). This having prevailed during peak metamorphism. spread mostly reflects subtle variations in K content The absence of any peritectic ferromagnesium mineral that control the projected position of the bulk rock within the segregations is consistent with the congruent analyses; the spread is similar for samples of bedded melting of the Qtz Kfs Bt Crd assemblage in the host migmatite from throughout the terrane, and there is metapelite. The following reaction is inferred, with the no significant variation in bulk rock geochemistry of leucosome composition comprising crystallized melt metapelite layers with grade (Greenfield et al., 1996). and peritectic minerals represented in brackets on the The Zone 2 three phase field, andalusite biotite basis of point counting of representative leucocratic cordierite is constructed using the composition of segregations: biotite inclusions to andalusite (sample 1117a), as 3.5 Kfs+3.2 Bt+5.8 Crd+4.1 Qtz+1.0 H O 2 biotite grains in the matrix of Zone 2b and 2c samples were most probably significantly affected by retrogression or recrystallization during the c Ma M leucosome (0.61Qtz 0.3Kfs 0.09Bt) (2) event. The projected position of all samples of bedded Patch leucosome with coarse grained idioblastic migmatite is then consistent with the peak assemblage andalusite enclosed by quartz and biotite, with or inferred above. We infer that conditions that accomwithout K-feldspar, is found slightly upgrade from the panied the formation of the Zone 2c assemblages did metapelite solidus (Fig. 1), close to the andalusite not involve free water, as there is a significant reduction sillimanite transition that marks the lower limit of in the proportion of biotite in Zone 2c bedded Zone 2c. The following reaction is inferred for the migmatite relative to that in Zone 2b. Available water formation of peritectic andalusite: would have been increasingly partitioned into melt. 2.7 Kfs+1.1 Qtz+0.5 Bt+1.0 H O0.3 And leucosome ( 0.59Qtz 0.6Bt 0.35Kfs) (3) Zone 3 Moats of leucosome surrounding idioblastic andalus- Metapelitic rocks throughout Zones 3 and 4 commonly ite in Zone 2b are consistent with the focus of the contain large sillimanite grains with moats of symplec- reaction having been centred on the growing andalusite tic cordierite and spinel (Fig. 2a & b). Two samples porphyroblast. This may have been due to the difficulty from Zone 4 preserve large aluminosilicate porphyrob- in nucleating this mineral and/or the poor mobility of lasts involving andalusite cores and sillimanite rims, Al. An analogous situation involving peritectic garnet which are in turn mantled by spinel cordierite symplecis inferred for migmatites at Round Hill, Broken Hill tites (Fig. 2a). As leucosome is centred on these (Powell & Downes, 1990), where garnet grains acted aluminosilicate aggregates, we infer that the increased as the loci for melt formation that resulted in a coarse- temperatures resulted in the newly formed sillimanite grained patch migmatite (see also Loomis, 1979; having broken down in the presence of biotite by the Waters, 1988). At Mt Stafford, we infer that andalusite following reaction: formation was probably initially sub-solidus (i.e. reaction 1), then as temperature rose sufficiently above the Bt+Sil+QtzSpl+Crd+leucosome (Kfs Qtz H O) 2 solidus, melting and continued andalusite growth resulted in patchy, nebulitic, in situ leucosomes centred (4) on the large andalusite grains. With increasing grade, it becomes increasingly inappropriate The variations in mineral assemblage with increasing to use the leucosome compositions (based on metamorphic grade can be represented on AFM point counting) to balance any inferred melt-producing diagrams (after Thompson, 1957), projected from reactions, as there would have been an indeterminate quartz, K-feldspar and melt (after Clarke et al., 1989). proportion of the leucosome generated by meltproducing Figure 4 illustrates the position of major ferromagnesian reactions that would have preceded reactions phases for representative samples from Zone 2, such as reaction 4. The change to conditions appropriate and the position of the bulk rock geochemistry for Zone 3 assemblages thus reflects, in the AFM (recalculated to molecular proportions and projected diagram, the breakdown of the aluminosilicate biotite from K-feldspar and quartz) for representative samples tie-line and the establishment of the spinel cordierite tie- of bedded migmatite. Paired samples represent adjacent line (Fig. 4). This would have induced a significant metapelite and metapsammite portions of bedded change in mineralogy for the bulk rock compositional migmatite, metapelite layers being richer in Al O range of the bedded migmatite, which lay in one Zone (Fig. 4). The variation in X of Zone 2 biotite may three phase field, but was partitioned into two Zone 3 reflect post-peak re-equilibration, but in general Zone fields: aluminosilicate-absent and aluminosilicate-

11 PARTIAL MELTING AT MT STAFFORD 373 Fig. 3. Temperature-distance diagram summarizing the results of thermometry (Table 2) on a profile extending from Zone 1 to the northern granite (Fig. 1). Individual samples were projected along strike onto the profile shown in Fig. 1. THERMOCALC (Powell & Holland, 1988) estimates are represented by columns showing 1 s and 2 s error bars. Garnet cordierite thermometry estimates (Nichols et al., 1992), applied to metapsammitic rocks, are shown as circles with 1 s error bars. Opx Cpx thermometry estimates (Fonarev & Graphchikov, 1983) applied to mafic rocks from the terrane are shown as triangles. The results of garnetorthopyroxene thermometry (Harley, 1984) on Zone 4 rocks are represented by the open ovals. A simple regression line is drawn though the Zone 1 4 estimates, and another through Zone 5 estimates. The dashed line indicates the approximate position of the metapelite solidus. present. In the prograde transition from conditions appropriate for those in Zone 2c to those appropriate for Zone 3 (reaction 4), samples of bedded migmatite with bulk rock geochemistry lying above the spinel cordierite tie-line would have witnessed the destabilization of biotite and a significant reduction in the modal proportion of aluminosilicate, creating the commonplace texture involving moats of spinel cordierite around sillimanite that pseudomorphed andalusite. Alumino- small (Fig. 4). silicate in samples with a bulk rock geochemistry lying below the spinel cordierite tie-line (samples DC150, and the establishment of the garnet cordierite tie-line in the high-grade section of Zone 3, via the reaction: Bt+Spl+QtzGrt+Crd+Kfs+melt ( 5) For the range of bulk rock compositions shown by the bedded migmatite, such a change would not have produced any significant proportion of melt, as the proportion of biotite in most samples would have been DC229, Fig. 4) would have been completely consumed Zone 4 by spinel cordierite symplectites. The AFM Garnet porphyroblasts are rare in Zone 4 metapelite projection does not give any indication of the proportion layers, but common in adjacent metapsammitic layers, of melt generated through any reaction, but field evidence reflecting the comparatively alumina-poor bulk rock is consistent with the reduction in modal biotite and composition of the metapsammite layers (Fig. 4). aluminosilicate via reaction 4 having enabled the gener- However, some metapelite layers in Zone 4 bedded ation of abundant melt. migmatite have compositions close to the alumina The paucity of garnet in bedded migmatite from the content of the common metapsammite horizons (Fig. 4) lower grade portion of Zone 3 is consistent with a and these locally contain leucosome as moats that spinel biotite tie-line having been stable at conditions enclose garnet in a similar style to that of the Zone 2b immediately after the crossing of reaction 4. However, leucosome moats around andalusite (Fig. 2f ). Garnet garnet partially replaces spinel in the spinel cordierite may be idioblastic (Fig. 2 g), or partially pseudomorph symplectites in the high-grade portion of Zone 3. In cordierite and biotite (Fig. 2d). Cordierite in contact addition, garnet partially pseudomorphs biotite, and with garnet is commonly corroded and embayed, consistent skeletal biotite grains preserve evidence for the partial with it having been partially consumed during garnet replacement of biotite by K-feldspar and quartz with growth. Orthopyroxene in Zone 4 migmatite occurs as or without garnet. Such observations are consistent idioblastic grains with rutile exsolution lamellae, consist- with the destabilization of the spinel biotite tie-line ent with it also having been a product of a melt-forming

12 374 J. E. GREENFIELD ET AL. Fig. 4. Al 2 O 3 O MgO diagram, projected from quartz, K-feldspar and melt (after Thompson, 1957) illustrating the composition of representative mineral assemblages (numbered) and bulk rock compositions (open dots; recalculated to modal proportions and projected from K-feldspar and quartz) of bedded migmatite samples for Zones 2 4 at Mt Stafford. Paired bulk rock analyses with a single number represent adjacent pelite and psammite portions of bedded migmatite. Shaded portions of the AFM diagrams represent the three phase fields inferred for the three zones. The dotted line at X #0.62 indicates the /Mg ratio inferred to be representative of the bedded migmatite and used in the construction of Fig. 6. The same data were used in Greenfield et al. (1996). reaction. Rounded inclusions of garnet within orthopyroxene are consistent with a comparatively late timing How consistent is the reaction sequence? of orthopyroxene growth in the reaction sequence, and Although the co-existence of andalusite and melt is the possibility of garnet having acted as a reactant in its consistent with low pressures having prevailed during formation, in the reaction: melting at Mt Stafford, the commonly accepted Grt+Bt+QtzOpx+Crd+Kfs+melt (6) position of the andalusite=sillimanite reaction (Richardson et al., 1969; Brown & Fyfe, 1971; The change in conditions that accompanied the shift Holdaway, 1971) leaves little or no P T space for this to Zone 4 thus resulted in the destabilization of the association. However, the presence of boron or fluorine garnet biotite tie-line and the establishment of the in minor amounts (1 2%) can lower the water- orthopyroxene cordierite tie-line in the AFM projec- saturated solidus by as much as 100 C (Chorlton & tion (Fig. 4). Only a few alumina-poor metapsammite Martin, 1978; Pichavant, 1981; London et al., 1996). layers in bedded migmatite have compositions appro- In Zones 1 and 2, detrital tourmaline is a significant priate to witness this change (Fig. 4), which would minor mineral within metapelite, and grandidierite have resulted in only small proportions of melt. Biotite occurs in extensively melted Zone 4 rocks that are persists in small proportions in most Grt Opx Crdbearing enclosed by comparatively coherent bedded migmatite. Zone 4 samples, most probably due to the The experimental work of Manning & Pichavant influence of TiO and/or O content in stabilizing (1983) and Bénard et al. (1985) indicate that tourmaline biotite in addition to the three minerals predicted by remains insoluble in low-temperature granitic melts. the model KFMASH system (after Clarke et al., 1989). However, Haslam (1980) recorded detrital tourmaline