The Mclntosh Layered Troctolite-Olivine Gabbro Intrusion, East Kimberley, Western Australia

by CHARTER I. MATHISON1 AND PAUL R. HAMLYN2

1 Geology Department, University of Western Australia, Nedlands, Western Australia 6009, Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021 Australia 2Geology Department, University of Melbourne, Parkville, Victoria 3052, Australia (Received 29 November 1985; revised typescript accepted 6 August 1986)

ABSTRACT The well-preserved ?lower Proterozoic Mclntosh intrusion consists of 96 macro-layers with a total stratigraphic thickness of about 6 km. The lowermost rocks in this possible cone-shaped intrusion are hidden, and the roof and the upper layers were removed by erosion. The layered sequence is dominated by 40 bimodal cyclic units of troctolite and olivine gabbro. Minor gabbronorite layers occur throughout the sequence, and are more abundant and more fractionated higher in the sequence. Six imperfect megacyclic units are developed in the upper 2700 m, each unit consisting of several troctolite-olivine gabbro cyclic units followed by a Fe-Ti oxide-bearing gabbronorite. The overall cumulus crystallization order in each megacyclic unit was plagioclase first,closel y followed by olivine, then augite, orthopyroxene, and magnetite successively. Cryptic composition data for troctolites and olivine gabbros show a slight overall decrease of 10 mol per cent An and Fo from the base to the top of the layered sequence (approximate ranges An8O_7O and F°78-68)- Several major fluctuations occur however, and are generally associated with the oxide gabbronorites, which are significantly more fractionated than the adjacent layers (plagioclase AnJ3_60, orthopyroxene Mg32_<59). Each fluctuation comprises a marked progressive discontinuity (rapid normal fractionation) followed by a gradual to rapid regressive discontinuity (or reversal) in the overlying troctolites and olivine gabbros. Apparently, such marked progressive discontinuities have not been described in layered intrusions. A chilled margin and the overall composition of the intrusion suggest an olivine tholeiite parent magma, inferred to have crystallized at P < 6 kb, relatively low pHjO and high fo^ (> NNO buffer). The troctolite-olivine gabbro cyclic units are inferred to have formed by fractional crystallization of periodic additions of new magma. However, the oxide gabbronorites seem too fractionated relative to the underlying layers to have formed by conventional crystal fractionation mechanisms, and they could have resulted from a 'liquid fractionation' process in which fractionated residual magma, instead of rising, periodically became denser and ponded on the temporary floor (a density crossover). Gradual, reversed cryptic trends in the cyclic units above the oxide gabbronorite layers may reflect mixing of this fractionated magma with successive magma additions.

INTRODUCTION The Mclntosh intrusion is 65 km NNE of Halls Creek in the Kimberley district of northeast Western Australia, and is the best preserved example of at least 13 broadly similar intrusions of probable Lower Proterozoic age in the Halls Creek mobile zone (Fig. 1). The intrusion, 6 km E-W and 15 km N-S, produces a well-defined concentric pattern on satellite imagery and aerial photographs due to the internal layering. This study, aimed at determining the overall stratigraphic sequence and the mineral composition patterns is based on 240 samples taken along three main traverses across the intrusion. Microprobe analyses were performed on 50 rocks, and 26 rocks were chemically analysed.

[Journal of Petrology. Vot 28, P«rt I, pp. 211-234, 1987] C Grfdrd Umroiity Prcn 1987 112 CHARTER I. MATHISON AND PAUL R. HAMLYN REGIONAL GEOLOGY The Halls Creek mobile zone (400 km long and 50 km wide) is separated by prominent faults from younger, less deformed, mainly Proterozoic sedimentary rocks (Fig. 1). The general geology has been described by Dow & Gemuts (1969), Gemuts (1971), Thorn (1975, pp. 160-93), Plumb & Gemuts (1976), and Hancock & Rutland (1984). The mobile zone consists mainly of medium to high-grade metamorphic rocks (the Tickalara Metamorphics), intruded by ultramafic, mafic, and granitic bodies. The oldest rocks in the mobile zone have

suffered at least three deformation and metamorphic events, the peak metamorphism Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021 occurring around 1960 m.y. The oldest intrusions belong to the Alice Downs Ultrabasics Pow & Gemuts, 1969), and the best example is the Panton Sill (Hamlyn, 1977,1980) 10 km W of the Mclntosh intrusion.

HALLS CREEK MOBILE ZONE

FIG. 1. Satellite image of the central part of the Halls Creek mobile zone, together with a simplified sketch of the regional geology of the same area showing most of the layered mafic intrusions in the mobile zone. (The satellite imagery is provided through the United States Remote Sensing Satellite System operated by the Department of Commerce, National Oceanic and Atmospheric Administration (DOC/NOAA) in co-operation with the National Aeronautics and Space Administration (NASA) and the EROS Data Centre Facility of the Department of Interior.)

Gabbroic intrusions belonging to the Mclntosh Gabbro suite (Dow & Gemuts, 1969) are probably younger than the Alice Downs Ultrabasics, but not necessarily synchronous with each other (isotopic age data are not available). The following intrusions have been recognized: Salt Lick Creek (Wilkinson et al., 1975), Bow River, Lamboo, Corkwood, Sally Malay (Thornett, 1981), Keller Creek, Violet or Dave Hill (Barrett, 1984), Spring, Mclntosh, Toby, Springvale, Armanda River, and Emull. The intrusions are concentrated in an area centred about 90 km north of Halls Creek, coinciding with a regional gravity high. The intrusions have suffered varying degrees of recrystallization and are generally plagioclase- rich, and are composed of troctolite, olivine gabbro, peridotite, olivine norite, and THE McINTOSH LAYERED GABBRO 213 Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021

1 \ QL1 "TSTRATHMPHI. ' "' * 1 '/,' . . II . :•' //. Fio. 2. Geological map of the Mclntosh intrusion, showing the main oxide gabbronorite horizons (poorly outcropping) in the upper 2500 m of the layered sequence. The positions ofstratigraphic horizons at 1000 m intervals above the arbitrary datum are shown. Strike and dip data apply mainly to small-scale rhythmic layering. Faults, marginal contacts, and trend lines for the strike of layering in the surrounding rocks are inferred from aerial photographs. The grid coordinates are arbitrary and serve to locate points of discussion in the text 314 CHARTER I. MATHISON AND PAUL R. HAMLYN

-- ./- v - - Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021

FIG. 3. Rounded fragments or pillows of chilled gabbro in fine to medium grained gabbroic matrix about 5 m inside the marginal contact at the western margin of the Mclntosh intrusion at 107N 018E (scale is 15 cm long). gabbronorite. They have been explored for Ni-Cu sulfides, chromite, and platinum, and a sub-economic Ni-Cu sulfide body was found in the Sally Malay intrusion (Thornett, 1981).

STRUCTURE The marginal contact (Fig. 2) generally follows the change in topography from the rugged hills of the intrusion (160 m maximum relief) to the flat areas of poorly outcropping gneissic rocks of the Tickalara Metamorphics, and this is confirmed by magnetic data. The surrounding Tickalara Metamorphics are little studied and the detailed structural relationships to the intrusion are poorly known. More extensively recrystallized, weakly layered gabbroic rocks occur beyond the inferred marginal contact around 010E, 030N and 090E, 120N (Fig. 2). These represent older layered intrusions or less probably, disrupted parts of the Mclntosh intrusion. The marginal contact is well exposed at 020E, 107N (Fig. 2), where a contact between chilled gabbro and gneiss is locally strongly sheared. An obvious thermal aureole is lacking. Gabbros become progressively coarser grained for 100 m inwards from this chilled margin, and contain a zone of chilled gabbroic fragments or pillows in a slightly coarser grained gabbroic matrix (Fig. 3). The fragments are rounded rather than angular suggesting continued intrusion before complete solidification, or some other incomplete mixing process. A similar feature was found in the Skaergaard intrusion (Wager & Brown, 1968, fig. 82, p. 116). Prominent internal layering is expressed topographically as concentric strike ridges (mainly troctolites and olivine gabbros) and strike valleys (mainly Fe-Ti oxide gabbronorites in the upper 2500 m of the succession). The gabbroic macrolayers range from 20 to 300 m in thickness, and extend laterally for up to about 15 km along strike. Small-scale structures such as rhythmic layering (layers 1 to 15 cm thick, mainly in some troctolites) and igneous lamination appear to be parallel to the macrolayering. The layering dips inwards from the THE McINTOSH LAYERED GABBRO 215 margins of the intrusion, and dips decrease inwards from about 45° to 50° near the east and west margins to sub-horizontal in the middle, upper part Steep dips ranging from 60° to vertical were recorded in the southern portion of the body from about 1000 to 3500 m above the stratigraphic datum (Fig. 2). The internal structure of the intrusion south of the Alice Downs access road is poorly known because reliable dip measurements are lacking. The intrusion is interpreted to have been a discordant, steep-sided, gently tapering, cone-shaped body. The marginal contacts may have been faulted so that the intrusion was shifted tectonically from its original environment (Hancock & Rutland, 1984). Unknown thicknesses of overlying country rocks and of layered intrusive rocks were removed by Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021 erosion. The conical intrusion narrows downwards into a possibly keel-shaped dyke-like body in which the lowermost layers are probably hidden, and which may represent the feeder. The lower parts of the intrusion were tilted to nearly vertical, but the upper parts remained in their original orientation. In contrast, Dow & Gemuts (1969) suggested that the intrusion (which they named the Mclntosh Sill) was originally a sill-like body and was later folded into a structural basin.

PETROGRAPHY The range of rock types is very restricted (Table 1). Troctolites (plagioclase-olivine cumulates) and olivine gabbros (plagioclase-augite-olivine cumulates) dominate, and minor gabbronorites (essentially plagioclase-augite-orthopyroxene cumulates) are also present. Photomicrographs of common rocks are shown in Fig. 4, and modal compositions are plotted in the stratigraphic column for the whole intrusion in Fig. 5. Plagioclase, augite, olivine, orthopyroxene, and magnetite are the main components, together with minor brown hornblende, ilmenite, suLfides, and rare apatite, and biotite. The rocks are transitional between adcumulates and mesocumulates as they contain less than 5-10 per cent new postcumulus phases (Irvine, 1982), negligible normal zoning in plagioclase, and very low amounts of incompatible components (K2O < 0-1 per cent, P2O 5 < 005 per cent) (Wager & Brown, 1968). Average grain sizes are 1-2 mm, small in relation to the size of the intrusion. Some layers (troctolites particularly) are finer grained (0-3-1 mm) probably due to greater degrees of supercooling.

TABLE 1 Terminology and modal compositions of the main rock types in the Mclntosh intrusion

IUGS and cumulate terms Troctolite (po cumulate) Olivine gabbro (pao cumulate) Gabbronorite (pab cumulate) leucotroctolite (pa/abh)*f leuco olivine gabbro (pao/bh) olivine gabbronorite (pabo/h) melatroctolite (op/abh) mela olivine gabbro (apo/bh) oxide gabbronorite* (pabm/h) Modal compositions plagioclase (vol. per cent) 25-85 (71)J 40-80(67) 55-75 (60)* olivine 12-65(25) 5-15 (7) 0-5 (0) augite 0-5-18 (3) 15-50(24) 0-5 (0) orthopyroxene 0-3-5 (1) 03-5 (2) 7-20(15) magnetite 0-0-5 (tr) 0-4 (tr) 0-10 (8)

* p = plagioclase, o = olivine, a = augite, b = orthopyroxene, h = hornblende, m = magnetite. t Letters indicating cumulus minerals are listed in order of decreasing modal abundance (cf. McCallum et al-, 1980), the diagonal stroke separating cumulus minerals (first) from postcumulus minerals. \ Typical modal compositions are given in brackets. 216 CHARTER I. MATHISON AND PAUL R. HAMLYN Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021

Fio. 4. Photomicrographs of the four main rock types in the Mclntosh intrusion. Each photomicrograph represents a field of view 7 mm x 4-5 mm. A = troctolite or po cumulate, B = olivine gabbro of pao cumulate, C •= gabbronorite or pb(a) cumulate (augite ?post-cumulus), D = oxide gabbronorite or pbam cumulate.

The magmatic mineralogy and textures are generally well preserved, though every rock shows evidence of some postmagmatic overprint (in addition to normal subsolidus cooling effects such as exsolution in pyroxenes). The primary, laminated tabular laths of plagioclase are locally recrystallized to finer grained, randomly oriented, polygonal mosaics of clear plagioclase which is generally 2-4 mol per cent more sodic than primary plagioclase. In contrast, the primary plagioclase laths are cloudy due to exsolved dusty and rod-shaped inclusions (making them dark in hand specimen), and show bent and tapering twin lamellae. Olivine also contains minute exsolved oxide rodlets (1-3 /zm long), and commonly shows deformation lamellae and strain extinction. Thin composite coronas (01-02 mm wide) of intergrown orthopyroxene, amphibole, and spinel are locally developed at some plagioclase- olivine contacts (inner coronas of orthopyroxene 10-40 /an wide and outer coronas of symplectic hornblende-spinel 30-100 /mi wide). The intensity of all these recrystallization effects generally increases towards the margins, and could have been induced by deformation, heat or both during a post-intrusion metamorphic or deformation event. The layered rocks show evidence of only minor alteration, generally less than 2 per cent Augite and orthopyroxene are slightly altered to pale green amphibole, and olivine to minor serpentine and magnetite. Traces of carbonate occur in thin fractures (some analysed rocks contain around 03 wt. per cent CO2).

Troctolites Plagioclase is always cumulus, even in the olivine-rich varieties near the base of the intrusion. Most olivine grains have irregular postcumulus overgrowths extending inter- stitially between plagioclase laths (Fig. 4A), and some olivines (in the same rock) may have a narrow partial reaction rim of orthopyroxene (03 mm wide). Minor augite and ortho- THE McINTOSH LAYERED GABBRO 217 pyroxene have a postcumulus habit. Some troctolites (particularly the finer-grained varieties) have augite oikocrysts (5-20 mm) containing numerous small plagioclase laths.

Olivine gabbros These are distinguished from troctolites by more augite, less olivine, and the change to cumulus augite (Fig. 4B). Olivine has less well developed postcumulus overgrowths than in troctolites. Orthopyroxene occurs as reaction rims on olivine and also as sparse subophitic

patches apparently independent of olivine. Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021

Gabbronorites Three main varieties can be recognized, as follows: (a) olivine-bearing (olivine gabbronorites); (b) olivine-free, oxide-free (gabbronorites, Fig. 4Q; and (c) olivine-free, oxide- bearing (oxide gabbronorites, Fig. 4D) in which the oxide is mainly magnetite. Olivine and cumulus orthopyroxene are generally mutually exclusive so type (a) is rare. Similarly, olivine and abundant magnetite are mutually exclusive. Augite in types (a) and (b) tends to be postcumulus, suggesting the crystallization order plagioclase and olivine first, followed by orthopyroxene and then augite, a reversal of the normal order of augite preceding orthopyroxene in olivine gabbros.

Other rocks The chilled margin consists mainly of plagioclase laths and subophitic to granular augite and orthopyroxene, with minor green hornblende. Fine-grained granular microgabbros and microgabbronorites (0-2-0-5 mm grain size) resembling beerbachites occur as tabular bodies up to 50 m thick and several hundred metres long, generally within 1-2 km of the margins of the intrusion. These rocks are problematical and could represent screens and xenoliths of hornfelsed mafic country rocks (cf. Wilson & Larsen, 1985) or chilled margins engulfed by an advancing magma chamber. They could also represent gabbroic layers or later intrusions that have been largely recrystallized.

STRATIGRAPHIC SEQUENCE The maximum stratigraphic thickness of the layered sequence from the Alice Downs access road in the south to the uppermost layer in the middle of the oval-shaped ridges is 5900 m. A further 600 m of section may be present in the basal or feeder part of the intrusion south of the Alice Downs road. Ninety-six macrolayers have been recognized, but some layers were probably overlooked because of too wide a sample spacing and poor outcrop of some gabbronorites. Troctolite (44 layers totalling 3570 m) makes up 55 per cent of the thickness of the layered sequence, olivine gabbro 32 per cent (38 layers, total 2080 m) and gabbronorite 13 per cent (14 layers, 850 m). The succession is essentially a macrolayered sequence of alternating troctolites and olivine gabbros, with minor gabbronorites (Fig. 5). Most of the succession consists of bimodal cyclic units, each cyclic unit consisting of troctolite and olivine gabbro. At least 40 cyclic units are present (thickness generally 50-200 m). Seventeen of the contacts between the layers were located in outcrop to within about 1 cm, and it is inferred that most of the other contacts are also sharply defined. Layer contacts are defined mainly by the upward change in texture of augite or orthopyroxene from postcumulus to cumulus, or vice-versa, and by changes in the modal proportions of augite, olivine, orthopyroxene, and oxides. Thus, layer contacts are combinations of phase contacts, ratio or modal contacts, and textural contacts (Irvine, 1982). Obvious top or bottom features 211 CHARTER I. MATHISON AND PAUL R. HAMLYN

MODAL COMPOSITION MODAL COMPOSITION 0 SO 100 0 30 100 Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021

oxtdt gobbronorite gobbronorita, oliviM QObbronorit* Olivinc gobbro troctolit* sample locoliont

orthO|>yroK«ni

MOOES

plogioclaM FIG. 5. Stratigraphic column for the layered sequence in the Mclntosh intrusion. Possible megacyclic units are indicated in the upper 3000 m (units I to VT). Modal compositions and specific gravities are also plotted. of the cyclic units that might represent a break in crystallization are lacking. Vertically, most layers seem to be relatively homogeneous or isomodal, although thick troctolite and olivine gabbro layers tend to be stratigraphically variable or graded, becoming more plagioclase- rich upwards. Distinctive marker horizons are scarce because of the repetitive macrolayering, but the lowermost of the three upper oxide gabbronorites (base at 4200 m above datum) is THE McINTOSH LAYERED GABBRO 219 particularly distinctive as it has the most fractionated mineral compositions, and is richer in apatite and brown hornblende than any other layer. Although alternating layers of troctolite and olivine gabbro dominate the layered sequence, several changes occur with increasing height in the intrusion, and are summarized below. 1. The relative proportions of the main rock types (based on layer thicknesses) change with height. Troctolites (83 per cent by thickness) dominate in the lower 2100 m, troctolites (53 per cent) and olivine gabbros (45 per cent) in the middle 1700 m, and gabbronorites (33 per cent) are more abundant in the upper 2700 m. This allows subdivision of the layered Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021 sequence into three zones with somewhat arbitrary boundaries at 1500 m and 3200 m above the datum: a basal melatroctolite zone (2100 m thick), a troctolite-olivine gabbro zone (1700 m), and an oxide gabbronorite-bearing zone (2700 m). 2. The nature of the individual rock types varies according to stratigraphic position. Troctolites and olivine gabbros at higher stratigraphic levels are generally more plagioclase- rich, and gabbronorites tend to be richer in magnetite and olivine-deficienL 3. The thicknesses of the layers change upwards—troctolite layers are thicker nearer the base of the sequence (approx. 300 m, cf. 40 m at the top), and gabbronorite layers are thicker towards the top (up to 240 m, cf. 20 m lower down). 4. The patterns in the sequences of layers change. In the middle and lower parts of the intrusion, the dominant pattern is the alternation of troctolites and olivine gabbros (cyclic layering). In the upper 2700 m, modal and particularly cryptic data suggest the presence of more complicated and imperfect repeat patterns or megacyclic units (Todd et al., 1982). Each megacyclic unit comprises several alternations of troctolite-olivine gabbro, followed by an oxide gabbronorite (a sequence that resembles the overall distribution of rock types in the whole intrusion). Five imperfect repetitions of this sequence in a megacyclic unit can be recognized in the upper 2700 m (Units I to V, Fig. 5). Two troctolite-olivine gabbro repetitions of a possible sixth megacyclic unit occur on one of the hills in the innermost oval 'ring' (056E, 104N, Fig. 2), and presumably an overlying oxide gabbronorite and possibly other megacyclic units were removed by erosion.

MINERALOGY Plagioclase Plagioclase was analysed in 50 rocks with the microprobe (5 determinations per rock), and typical analyses covering a wide stratigraphic interval are given in Table 2. Analysed compositions are plotted against stratigraphic height in Figs. 6 and 7. Plagioclase compositions were also determined optically in 140 rocks using the minimum refractive index of 001 cleavage flakes, and these results agreed closely with the microprobe results (± 2 mol per cent An). The total range in composition is An83 to An5, (An70_80 in most troctolites and olivine gabbros but Ansl_64 in oxide gabbronorites). Plagioclase is commonly zoned through a range of 5-7 mol per cent An, rim compositions typically being more calcic than cores. Reversely zoned plagioclase is not commonly reported in layered intrusions and may result from increasing pHjO during post-cumulus crystallization (Wager & Brown, 1968) or access of less fractionated magma (Young, 1984). Plagioclases in some troctolites and olivine gabbros have small patchy cores separated by sharp contacts from the rest of the crystals. The cores are more sodic by up to 1 mol per cent An, possibly due to supercooling in the early stages of cumulus crystallization (Maaloe, 1976). Total iron is the main minor component in plagioclase (commonly 0-2-0-35 wt. per cent Fe2O3), and is largely present as (? exsolved) submicron-sized 'dust' and minute plates and 220 CHARTER I. MATHISON AND PAUL R. HAMLYN

TABLE 2 Representative microprobe analyses of plagioclase in the Mclntosh intrusion

Height (m) 900 2090 2990 3460 4200 5050 5080 5610 oxide oxide oxide trocto- trocto- olivine gabbro- gabbro- olivine trocto- gabbro- Host rock Ute lite gabbro norite norite gabbro lite norite

SiOj wL per cent 481 46-8 49-2 51-3 53-9 49-8 48-8 53-7 TiO,

A13O3 32-7 331 31-9 29-7 28-6 321 321 28-9 Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021 FejO, (total) 0-23 0-32 023 0-17 0-19 0-26 0-24 0-29 MnO 002 005 001 001 ooo OOO OOO OOO MgO 002 001 002 0O1 000 0O1 0O4 004 CaO 16O 16-6 15-4 13-4 1O9 1*0 15-5 11-9 Na2O 2-24 1 88 2-85 3-97 5-7 2-82 2-77 4-70 K2O (M)l 003 0O9 0-11 0-10 005 0-10 0-16 Total 99-4 98-8 99-8 98-7 99-4 100-1 99-5 99-7 Cations per 32 oxygens Si 8-85 8-69 903 9-45 9-81 9O8 8-96 9-74 Ti 0012 0000 0011 0004 0001 OOOO OOOO OOOO Al 710 7-24 689 6-46 6-13 6-90 6-96 6-18 Fe3 + 0032 0O45 0032 0023 0026 0035 0034 0039 Mn 0003 0008 0002 0002 OOOO 0000 OOOO OOOO Mg 0005 0O03 0005 OO03 0000 0O03 0011 0011 Ca 315 3-31 302 2-63 213 2-92 3O6 2-31 Na 0-800 O678 101 1-42 201 0-996 0-987 1-65 K 0002 0O07 0021 0026 0023 0O12 0O23 0O37 z 19-95 19-98 2002 20O2 20-13 19-95 20O3 19-97 An 79-72 82-86 74-51 64-54 5118 74-36 75-18 57-73 Ab 20-22 16-96 24-97 34-83 48-26 25-34 24-24 41-34 Or 0O6 0-18 0-52 O63 0-56 0-30 0-58 M3

rods (5-50 /an long, < 1 /im thick). Reflectivity and microprobe analysis suggest that the larger inclusions are haematite (microprobe analysis corrected for minor plagioclase contamination: Fe2O3 96-8 wt. per cent, TiO2 1-4 per cent, A12O3 0-3 per cent, MnO 0-2 per cent, MgO 0-1 per cent).

Olivine Typical microprobe analyses are given in Table 3 and micToprobe-derived compositions are plotted against stratigraphic height in Figs. 6 and 7. Although the total composition range is Fo79 to FoS6, olivine is Fo65_76 in most troctolites and olivine gabbros, FoJ6 being recorded in only one rock, a transitional olivine-bearing oxide gabbronorite 3460 m above the datum. Compositions in a given rock agreed within 2-3 mol per cent Fo. The most consistent minor element patterns are shown by MnO and NiO. MnO (range 0-23-0-45 wt. per cent) correlates negatively with Fo, whereas NiO (range 0-03-0-23 wt per cent) correlates positively with Fo but shows more scatter, possibly reflecting variable degrees of Ni depletion due to scavenging by sulfides (Campbell, 1977).

Augite Typical microprobe analyses are given in Table 4, and selected components are plotted in Figs. 6 and 7. The total composition range is Ca4jMg4jFe10 to Ca43Mg36Fe21 (at per n cent). Atomic per cent Mg varies from MgB2 to Mg6A overall, but is generally Mg7j_80 ' troctolites and olivine gabbros, and Mg6A_li in oxide gabbronorites. Cr2O3 varies widely THE MCINTOSH LAYERED GABBRO 221

TABLE 3

Representative microprobe analyses of olivine in the Mclntosh intrusion

Height (m) 900 2090 2500 3460 3950 4840 4940 5880 oxide oxide trocto- trocto- olivine gabbro- gabbro- olivine trocto- olivine Host rock llte Ute gabbro norite norlu gabbro Ute gabbro

SiO2 wt per cent 39-5 38-7 37-9 35-6 37-7 38-9 39-6 37-3 TiO2 OOO 005 OOO 003 OOO 001 003 OOO

A13O, 000 001 002 002 003 OOO OOO OOO Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021 FeO (total) 19-4 22-2 241 36-9 26-2 21-7 204 25-1 MnO O25 031 031 045 030 027 030 040 MgO 411 39O 37-3 270 36-2 39-7 4O2 36-6 CaO 012 000 001 O01 001 002 002 003 NiO 026 025 005 009 002 004 Oil 005 Total 10O6 10O5 99-7 10O1 1005 10O6 10O7 99-5 Cations per 4 oxygens Si 101 100 100 100 099 100 1O1 099 Ti OOOO 0O01 OOOO OO01 OOOO OOOO 0O01 OOOO Al OOOO OOOO 0O01 0001 0001 OOOO OOOO OOOO Fe2 + 0412 0480 O531 0865 O578 0467 O437 O559 Mn 0O05 0007 0007 0011 0007 0O06 OO06 0O09 Mg 1-56 1-50 146 113 1-42 1-52 1-53 145 Ca 0003 OOOO OOOO OOOO OOOO 0O01 0O01 0001 Ni 0005 0005 0001 0002 OOOO O001 OO02 0001 X 3O0 2-99 300 3O1 2-99 300 2-99 301 100MgAMg + Fe + Mn) 78-9 75-5 731 56-2 709 76-2 77-5 71-8

TABLE 4 Representative microprobe analyses ofaugite in the Mclntosh intrusion

Height (m) 900 2090 2500 3460 3950 4200 5050 5610 oxide oxide oxide trocto- trocto- olivine gabbro- olivine gabbro- olivine gabbro- Host rock Hte Ute gabbro norite gabbro norite gabbro norite

SiO2 wt per cent 51-5 5O5 52-3 502 5O4 51O 51-2 51-5 TK)2 040 051 O53 O66 079 039 064 O72 A1,O, 3-44 3-67 2-95 304 2-% 1-70 3-43 2-88 FeO (total) 5-97 7-30 7-78 9O5 7-68 12-8 6-35 104 MnO O12 010 021 017 018 035 018 023 MgO 15-4 14-5 16-3 13-3 15-3 130 15-4 13-8 CaO 21-4 21-7 19-2 21-6 2O1 19-5 211 2O5 Na2O 037 044 047 O45 O48 026 051 047 Cr2O, O47 O06 030 001 O32 OOO O62 002 NiO 000 004 OOO 001 OOO 002 007 006 Total 99-1 98-8 10O0 98-5 98-2 990 9>5 10O6 Cations per 6 oxygens Si 1-912 1-895 1-926 1-909 1-902 1-948 1-900 1-917 Al" 0088 O105 0074 0091 0098 0052 O100 0O83 Al" 0063 0058 0054 0045 0033 0024 0049 0044 Ti 0011 0014 0015 0019 0022 0011 0018 0020 FeJ + 0185 0229 O240 0288 0243 0409 0197 O323 Mn 0004 O003 0007 OO05 0006 0012 0006 0007 Mg 0854 0812 0893 O751 0861 O740 O850 0768 Ca 0850 0874 0757 0879 0813 O797 0840 0817 Na 0027 0032 0034 0033 0035 0019 0037 0O34 Cr 0014 OO02 0009 OOOO OO10 OOOO 0018 O001 Ni OOOO 0001 OOOO OOOO OOOO OOOO 0002 O002 X 4O1 4-02 401 4O2 4O2 4O1 4O2 402 100 Mg/(Mg+Fe + Mn) 81-9 77-8 78-3 71-9 77-6 63-7 8O7 69-9 222 CHARTER I. MATHISON AND PAUL R. HAMLYN from 0-0-62 wt per cent overall, and A12O3 is high (range 2-7-4-3 wL per cent) and structural formulae suggest that Al is mainly in the tetrahedral sites. The overall ranges of other minor components are Na2O 0-3-0-5 wt. per cent, TiO2 0-4-0-9 wt. per cent, and MnO 0-1-0-35 wt. per cent. Cr, Al and possibly Na correlate positively with Mg, while Mn and possibly Ti correlate negatively with Mg. Very minor ?orthopyroxene lamellae (2-4 /an thick) are exsolved parallel to 100 in cumulus augite. Abundant inclusions of opaque and translucent oxide phases are oriented parallel to ?010 and the c-axis of host augite (elongate rods and plates 5-200 /im long, 1-20

/im wide, < 1 /im thick) and parallel to 001. Reflected light examination and microprobe Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021 analysis show that several different oxide phases are present, even in the one plate or rod (cf. Pasteris, 1985). Cr-magnetite (with 4-6 wt. per cent Cr2O3, 2 per cent TiO2, 2-7 per cent A12O3,1 per cent MgO) and minor ilmenite and hercynite were identified. Such inclusions are normally inferred to result from exsolution (e.g., Deer et al, 1978) but subsolidus oxidation of augite (cf. Elsdon, 1971) and of the resulting Ti-magnetite inclusions is possible. Similar oxide inclusions are not reported from augites in some other layered intrusions (Boyd & Brown, 1968) despite similar Fe2O3 and A12O3 contents. The abundant oxide inclusions in the Mclntosh augites may reflect different subsolidus cooling history and conditions (e.g., reheating or high/oj, or perhaps different crystallization conditions.

Orthopyroxene Table 5 lists typical microprobe analyses covering the total composition range, and Mg values are plotted in Figs. 6 and 7. The overall composition range is Ca2Mg79Fe19 to Ca2Mg51Fe47 (Mg80 to Mg52), but is typically Mglo_16 in troctolites and olivine gabbros,

TABLE 5 Representative microprobe analyses of orthopyroxene in the Mclntosh intrusion

Height (m) 900 2090 2500 3460 3950 4200 4940 5610 oxide oxide oxide troclo- trocto- olivine gabbro- olivine gabbro- olivine gabbro- Host rock lite llte gabbro norite gabbro norite gabbro norite

SiO2 wL per cent 54-7 54-4 540 51-8 53-6 5O6 54-2 52-4 TiOj 014 024 0-2O O32 O18 O19 O32 O28 AI2O5 2-06 216 2 53 1-61 2-21 074 2-66 1-62 FeO (total) 127 14-5 15-3 211 16-9 28-3 14-2 21-3 MnO 021 029 030 O40 032 067 O28 O46 MgO 29-2 281 27 5 22-0 26-8 17-8 27-4 21-6 CaO 089 1 16 072 1-69 O69 118 093 114 Cr2O, 014 OOO 010 001 Oil 001 O36 001 NiO 007 002 005 004 007 OOO 015 OOO Total 10O1 10O9 1007 99-0 10O9 99-5 10O5 98-8 Cations per 6 oxygens Si 1-945 1-936 1-932 1-948 1-929 1-959 1-933 1-968 Al" O055 0064 0068 0052 0071 0034 0067 0032 W" 0032 0027 0038 0019 0022 OOOO 0045 OO40 Ti 0004 0006 0005 0009 OO05 0005 0009 0008 Fe1 + 0377 0432 0457 O664 0508 0917 O424 O670 Mn O006 0009 0009 0013 OO10 0022 0008 0015 Mg 1-546 1-492 1-467 1-232 1-440 1030 1-459 1-207 Ca O034 0044 0028 0068 0027 0049 0036 0046 Cr 0004 OOOO 0003 OOOO 0003 OOOO 0010 OOOO Ni O002 OOOl OOOl OOOl 0002 OOOO 0004 OOOO I 4O0 401 4-01 401 4O2 4O2 4O0 3-99 100 Mg/(Mg + Fe + Mn) 8O5 77-2 75-8 64-5 73-5 52-3 771 63-9 THE McINTOSH LAYERED GABBRO 223 and Mg51_10 in oxide gabbronorites. A12O3 (0-8-2-9 wt. per cent), Cr2O3 (0-0-13 wt. per cent), and NiO (0-0-09 wt. per cent) increase, and MnO (0-28-0-67 wt. per cent) decreases, with increasing Mg. Thin lamellae (1-5 pm thick) of Ca-rich pyroxene are exsolved parallel to 100 and are more common in cumulus than in postcumulus orthopyroxenes. Cumulus orthopyroxenes also contain oriented plates and rods of oxide phases parallel to the c-axis. Primary orthopyroxene normally ceases crystallizing at compositions around Mg10 in tholeiitic layered intrusions, and pigeonite crystallizes instead (Deer et al., 1978). However, primary orthopyroxene (without any evidence of inversion from a monoclinic precursor) continues to Mg52 in the oxide gabbronorite in unit III. The miscibility gap between the Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021 Mclntosh Ca-rich and Ca-poor pyroxenes is wider than for most other layered intrusions, and this cannot be entirely due to the small amount of exsolved pyroxene. Pyroxene crystallization trends for some other layered intrusions (Deer et al, 1978, p. 334) suggest that the wider this gap the more delayed the appearance of pigeonite, but the ultimate cause is uncertain.

Magnetite and ilmenite Fe-Ti oxides are generally very minor components of troctolites and olivine gabbros (< 1-2 vol. per cent), but are more abundant in oxide gabbronorites in the upper half of the layered sequence. Magnetite is typically cumulus, whereas ilmenite is postcumulus and commonly in contact with magnetite. Magnetite: primary ilmenite ratios vary widely, but generally increase with height. Magnetite always contains 5-15 vol. per cent secondary ilmenite lamellae 'exsolved' in 111 planes from subsolidus oxidation of Ti-magnetite (Buddington & Lindsley, 1964). Magnetite also contains numerous elongate blebs of pleonaste exsolved parallel to 100, and primary ilmenite contains minor pleonaste lamellae and haematite lamellae, both parallel to 0001. Magnetites were analysed with the microprobe in 24 rocks and typical analyses are listed in Table 6. The microprobe analysis points were sited in homogeneous magnetite, away from exsolved inclusions. The analysed magnetites have low A12O3 (0-2-2-8 wt per cent) and MgO (typically 0-2-0-6 wt per cent), and unexpectedly low TiO2 (0-1-4-9 wt. per cent, generally < 0-6 per cent). Cr2O3 is commonly the main minor component (0-03-6-3 wt. per cent with

TABLE 6 Representative microprobe analyses of magnetite in the Mclntosh intrusion

Height (m) -300 4200 4780 4800 5050 5490 5610 5840 metet- oxide oxide trocto- gabbro- olisine trocto- olivine olivtne gabbro- olicine Host rock liu norite gabbro Ute gabbro gabbro norite gabbro

FeO (total) wL per cent 89-8 910 85-9 84-5 75-9 86-4 91-5 87-1 FeO(calc) 306 30-9 30-5 31-8 31-7 31-8 31O 3O9 Fe2O3 (calc) 65-8 66-8 61-6 58-8 49-4 60-7 66-6 62-6 TiOi 025 0-16 1-07 1-58 1-93 O41 045 047 Cr2O3 0-11 5-64 5-84 13-6 3-43 O20 417 1O9 A12O3 0-69 060 1-78 2-44 106 046 045 042 MgO 006 O29 031 062 O65 008 008 O28 MnO OOO 004 012 025 013 003 004 002 NiO QO1 0-12 O08 014 004 OOO 003 012 Total (calc) 98-8 98-8 99O 99-8 99-7 99-7 98-8 98-7 Ulvcupinel* (mol per cent) 1-4 0-7 05 3-4 6O 5-9 1-2 1-4

Calculated using method of Buddington & Lindsky (1964). 224 CHARTER I. MATHISON AND PAUL R. HAMLYN

TABLE 7 Microprobe analyses of postcumulus brown hornblende from typical rocks in the Mclntosh layered intrusion

Height (m) 2090 4460 2500 4130 210 5*50 Host rock troctolite olivine gabbro gabbronorite

SiO2 wt. per cent 4210 41-96 43-37 41-68 43-22 41-69 TiO2 1-62 2-57 1-67 2-36 2-80 308 Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021 A12O3 1317 13-60 13-49 13-63 11-85 1303 FeO (total) 9-70 9-45 9-57 11-86 10-64 12-79 MnO O07 012 009 0-17 009 Oil MgO 14-84 14-62 14-93 12-95 13-85 12-50 CaO 11-88 11-58 11-80 11-38 11-91 11-77 Na2O 211 2-54 2-23 2-22 1-82 211 K2O O94 080 0-65 115 0-79 089 Cr2O3 000 042 0-39 0-12 0-28 009 NiO 000 006 015 003 0O5 OOO BaO 0O0 000 008 0-10 000 000 Total 96-43 97-72 98-42 97-65 97-30 9806 Cations per 23 oxygens Si 6-214 6125 6-263 6-159 6-347 6154 A11V 1-776 1-875 1-737 1-841 1-653 1-846 AT 0-518 0465 0-559 0-532 0-398 0421 Ti O180 0282 0-181 0-262 0-309 0342 Fe 1199 1154 1156 1-466 1-307 1-579 Mn 0009 0015 0O11 0O21 0O11 0014 Mg 3-270 3182 3-214 2-853 3032 2-751 Ni — 0007 0O17 0O04 0O06 — Cr — 0048 0O45 0O14 0O33 0011 Ca 1-882 1-811 1-826 1-802 1-874 1-862 Na 0-605 0719 0-624 0-636 0-518 0624 K 0-177 0149 0-120 0-217 O148 0168 Ba — — 0O05 0006 — — 100Mg/(Mg + Fe + Mn) 73-0 731 73-4 65-7 69-7 63-3

one exceptional value of 13-6 per cent), and is plotted for the upper part of the layered sequence in Fig. 7. Apart from Cr2O3 contents, the analysed magnetite compositions are of limited use because of substantial subsolidus modification. However, their low TiO2 contents (generally < 1 per cent) and their low volumes of'exsolved' ilmenite (< 15 per cent) suggest that the primary magnetites were very Ti-poor (< 8 wt. per cent TiO2 or < 20 mol per cent ulvospinel, less than normal for gabbroic magnetites) due to relatively oxidizing crystallization and subsolidus conditions (Buddington & Lindsley, 1964). Crystallization/O2 conditions were at least as oxidising as 1 log unit above the NNO buffer at 1000 °C, using the graphical data of Buddington & Lindsley (1964, p. 316) and Spencer & Lindsley (1981, p. 1197).

Brown hornblende Brown hornblende (generally 0-5-2 vol per cent) occurs partly replacing augite at some plagioclase-augite contacts, and as postcumulus reaction rims on orthopyroxene mantling olivine. Microprobe data are given (Table 7) as the hornblende may have crystallized from the last few per cent of trapped magma, and because few analyses are available from layered intrusions. Compositionally, the hornblende is ferroan pargasite to ferroan pargasitic hornblende with moderate TiO2 (terminology of Leake, 1978). Its composition varies THE McINTOSH LAYERED GABBRO 225 slightly with host rock: Mg values range from 73-4 in less fractionated troctolites and olivine gabbros to 63-3 in more fractionated gabbronorites, and Ti contents also increase with fractionation.

Sulfides Sulfides are present in virtually every rock (generally 0-1-0-2 vol. per cent, but reaching a maximum of 0-4 per cent in parts of the uppermost oxide gabbronorite). They commonly occur as subspherical to irregular blebs (01-0-3 mm diam.) at grain boundaries of silicates suggesting separation of an immiscible sulfide liquid by the postcumulus stage. Pyrrhotite Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021 dominates, followed by chalcopyrite and pentlandite in order of decreasing abundance. Pentlandite is more common in troctolites and is virtually absent from oxide gabbronorites. Whole rock analyses commonly have 0-05-0-1 wt. per cent S, suggesting the magma was saturated (or close to saturation) with S during most of its crystallization history (Haughton et al., 1974). However, an effective concentration mechanism was lacking. Analysed S and Cu contents of rocks correlate well (Table 8) suggesting Cu is mainly in the sulfides (chalcopyrite). Ni contents of olivines do not suggest any large-scale depletion of nickel due to scavenging by sulfides (Campbell, 1977).

STRATIGRAPHIC CRYPTIC VARIATION Figure 6 shows the cryptic variation for selected mineral components in the whole intrusion as a function of stratigraphic height. The sample spacing is generally too wide in view of the complex stratigraphy and fluctuating cryptic trends, so three megacyclic units in the upper 1700 m were explored in more detail (Fig. 7). The following parameters are the most sensitive for indicating the degree of fractionation: mol. per cent An in plagioclase (overall range An83_J2); at. per cent Mg in olivine (Mg19_i6), augite (JVf<782_<54) and in orthopyroxene (Mg80_J2); wt. per cent Cr2O3 in magnetite (6-0 per cent) and in augite (0-62-0 per cent), and NiO in olivine (0-25-0 per cent). The original cumulus compositions were probably modified during both the postcumulus and the subsolidus stages, but despite this limitation and the wide sample spacing, distinct cryptic patterns are recognizable. Disregarding the fluctuations for the moment, Fig. 6 shows that the degree of fractionation decreases slightly from the base to about 2000 m above the base, and then increases by about 10 mol per cent Ab in plagioclase and 10 at per cent Fe in olivine and pyroxenes. The cryptic trends are complex in detail, and prominent, irregular fluctuations are associated with relatively fractionated gabbronorites (particularly in the upper 2500 m) at around 600,1000, 3500,4200, 5200, and 5700 m above datum. These fluctuations are combinations of rapidly increased normal fractionation with height (progressive discontinuities, Irvine, 1982) and reversed fractionation (regressive discontinuities or reversals, Irvine, 1982). The progressive discontinuities are associated mainly with oxide gabbronorites, and the regressive discontinuities with the overlying troctolites and olivine gabbros. This particular type of fluctuation is uncommon in layered intrusions. Several smaller fluctuations also occur in troctolites and olivine gabbros throughout the sequence, and two anomalous fluctuations at 600 and 1000 m above datum occur in moderately recrystallized troctolites, and this may explain their relatively fractionated mineral compositions. The fractionation rate with height varies widely. Mineral compositions change slowly with height in some parts (e.g., from 1500 to 3200 m above datum, plagioclase changes progressively from An82 to An72 and olivine from Fo74 to Fo68). However, rapid fractionation is associated with the prominent fluctuations (e.g., at 5500 m, plagioclase changes from An74 to An61 in 20 m and at 5750 m it changes from An60 to An7, in 40 m). 226 CHARTER I. MATHISON AND PAUL R. HAMLYN

o o Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021

80 90 O 0 0 1 0-2 0-3 Atom % Mfl In otfvim and pyroxtnu trt ^t NiO bi ottvrrM (mjaoprot* data) (fntcroproot ooto/ FIG. 6. Cryptic variation in the Mclntosh layered sequence. Selected mineral composition data for mafic minerals and plagioclase show a slight overall fractionation trend from the base to the top, and complex fluctuations associated mainly with oxide gabbronorites. Figure 7 shows the cryptic variation for the megacyclic units in the upper 1700 m in more detail. (Note: Data for troctolites are presented by open circles, olivine gabbros by crosses, olivine gabbronorites by small dots, and oxide gabbronorites by large dots.)

The origin of these marked changes in both the rate and direction of fractionation poses major problems. Microprobe data for the upper three megacyclic units (Fig. 7) show that the basal troctolite in a megacyclic unit is not the least fractionated, and that the regressive discontinuity at the base of each unit is gradational over vertical intervals of THE McINTOSH LAYERED GABBRO 227

top ? 60O60O00 , metret s t 1 H

UNI T ""I * *\ """ r"

^\ x

UNI T x ^> v— /

< Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021 ^^ > 5000- ( J r "V — 13 6 ^. H / / ^ UNI T > <: * • <; r'' < i ^^ 1 n r 65 TO 75 80 60 70 0 5 0 01 02 03 04 (3 01

5 atom % Mg in mafic minerals mot % An n ptagioclase w % Cr2O3 wt% Cr2O3 wt% NiO 4000- magnetite in augite in olivin« FIG. 7. Cryptic variation in megacyclic units in the upper 1700 m of the layered sequence in the Mclntosh intrusion (microprobc data). Locations of samples are shown on the stratigraphic column (troctolite = stippled, olivine gabbro = blank, oxide gabbronorite = black, gabbronorite = vertical lines).

50-650 m. The regressive discontinuities become less gradational from unit IV to unit VI, and successive oxide gabbronorites do not become progressively more fractionated as might be expected.

PETROCHEMISTRY Table 8 lists representative analyses selected from 26 whole rock analyses, together with the composition of the whole intrusion. The CIPW normative mineralogy of the analysed rocks generally agrees closely with the modal mineralogy, and the norms are typically plagioclase-rich, with moderate ol and low hy. Oxide gabbronorites have either very little ol or minor qz in the norm. The analyses show that troctolites and particularly gabbronorites tend to become more fractionated the higher they occur in the sequence. Successively higher gabbronorites generally contain more Fe2+, Fe3+, Ti, Na, and V, but less Cr and Ni. Analyses of the layers in the uppermost complete megacyclic unit (unit V) show httle difference in composition between the troctolite and olivine gabbro, but a major difference in the oxide gabbronorite which contains higher Fe2+, Fe3+, Ti, Na, P, V, Cu, and S. Au, Pd, Ir, and As were determined by radiochemical neutron activation analysis in nine samples covering the range of common rock types. Average values are Au 34 p.p.b., Pd < 1 p.p.b., Ir < 0-1 p.p.b., and As < 0-8 p.p.m. The low Pd and Ir values suggest depletion due to immiscible sulfide segregation during the early fractionation history of the parent magma (cf. Hamlyn et al., 1985), and are consistent with the evidence for sustained sulfur saturation throughout most of the cumulate succession. The intrusion therefore has low potential for Merensky-type platinoid mineralization. TABLE 8. Selected chemical analyses of rocks with other composition data from the Mclntosh intrusion Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021 22 8

Height (m) Chilled margin Whole Intrusion 900 2090 2500 3460 5050 5420 5610 micro- calculated mela- leuco- olivine oxide olivine oxide Rock type gabbronorlte average troctolite troctolite gabbro gabbronorite gabbro troctoltte gabbronorite

SiOj wt. per cent 48-83 47-3 43-83 45-95 46-62 46-52 48-34 46-82 46-90 TiOj O89 0-5 0-13 O06 O18 0-23 0-21 0-13 1-29 AI,O, 15-08 18-9 8-72 2818 2063 15-20 18-66 22-71 1601 Fe,O, 2-21 1-5 1-40 068 118 1O4 O69 058 5-53 FeO 11-90 7-5 1029 2-96 4-73 12-48 4-85 5-83 7-82 MnO 022 0-1 016 004 O09 021 009 008 017 O MgO 8-09 10-4 24-35 5-36 9O4 10O4 9-31 9O0 8-16 I CeO 1046 11-8 8-34 1410 13-95 9-33 14-36 11-47 11-54 NajO 1-84 1-9 064 1-56 1-62 2-05 1-91 2-31 2-48 H K,O 0-29 Ol O02 007 006 009 005 0O9 008 W P2OS 008 00 O01 004 004 007 003 004 007 l» H,O+ 0-59 O87 057 O70 058 063 049 031 CO2 — — O85 031 026 025 021 — — 2 Total 100-48 10O0 99-61 99-88 99-10 99-09 99-34 99-55 10O36 > Cr (p.p.m.) 335 830 105 560 310 515 202 151 IE Ni 178 — 810 140 120 190 54 181 37 Cu 122 175 20 40 130 130 51 121 O S 960 _ 650 125 200 1030 720 120 520 2 + z Mg value (100 Mg/(Mg+Fe at.)) 55-0 71-4 808 76-3 77-3 58-9 77-4 73-5 65-2 > C1PW Norms o or wt per cent 1-67 056 012 042 036 054 030 056 056 ab 15-72 16-27 5-52 13-33 13-97 17-65 16-41 19-58 2O82 > an 31-97 42-61 21-32 7039 49-77 32-58 42-84 51-93 32O3 c di 16-03 13-03 16-68 000 16-42 11-56 23-50 4-28 19-70 r hy 22-94 6-06 7-87 5O6 3-23 14-53 2-42 066 8-68 73 ol 5-98 18-47 46-12 9-59 14O7 19O6 13O4 21-81 7-56 mt 3-25 209 2-08 1O0 1-74 1-53 102 084 807 > im 1-67 O91 026 O12 035 2-38 O40 O24 2-41 3; ap 0-28 - 002 O10 O10 017 007 O10 017 r- Modes Z Plagiodaae (vol. per cent) 53 — 24 86 70 56 68 71 52 Olivine _ — 64 12 8 7 6 24 — Augite 18 — 9 1 21 16 24 3 21 Orthopyroxene 26 — 3 1 1 18 1 1 18 Magnetite 3 - tr tr tr 3 1 1 9 Analysed mineral compositions Plagioclase (An mol per cent) _ — 79-7 830 77-3 64-8 74-6 72-2 58-2 Olivine (Mg at per cent) — — 78-9 75-5 731 56-2 74-5 73-2 — Augite (Mg at per cent) — — 81-9 77-8 78-3 71-9 8O7 77-4 69-9 Orthopyroxene (Mg at per cent) — — 805 77-2 75-8 64-5 76-6 74-7 63-9 THE McINTOSH LAYERED GABBRO 229

PETROGENESIS The Mclntosh intrusion is characterized by repetitive layering and prominent fluctuations in the stratigraphic cryptic trends. Except for the relatively fractionated gabbronorites, surprisingly little progressive fractionation occurs with height for such a thick succession. The main problems posed by the intrusion are the troctolite-olivine gabbro cyclic units, the prominent increases in fractionation represented by oxide gabbronorites, and the gradual reversals in the layers overlying the oxide gabbronorites.

Parent magma and crystallization behaviour Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021 The parent magma is inferred to have been an olivine tholeiite (possibly with moderately high alumina) as indicated by compositions of the chilled margin and of the whole intrusion (Table 8), and the iron enrichment in the oxide gabbronorites (a typical tholeiitic trend). The composition of the chill is broadly similar to that of the whole intrusion except for total iron (higher in the chill, and somewhat high for a tholeiite) and A12O3 (lower in the chill). Application of the data of Roeder & Emslie (1970) and KD = 0-31 for Fe-Mg distribution 2 + between olivine and liquid to the Fe : Mg in the chill indicates that olivine Fo80 should crystallize from the chill and this compares closely with the most magnesian olivines in the Mclntosh cumulates (Fo79). However, A12O3 is lower in the chill than expected in view of the plagioclase-rich cumulates, though some of this discrepancy would be explained if relatively Al-poor (and Fe-rich) gabbros had been eroded from the top of the succession. The overall crystallization order is inferred to have been plagioclase first together with olivine, then augite, orthopyroxene (with olivine disappearing), and magnetite. The normative composition of the chill plots in the olivine field close to the plagioclase-olivine cotectic near the ternary point in the Pl-Ol-Cpx plot of Cox et al. (1979, p. 232). Evidence for crystallization pressures is uncertain and somewhat conflicting, but suggests an upper pressure limit of about 6 kb based on the following lines of evidence. Coronas on some olivine grains suggest the reaction: olivine + plagioclase + H2O = orthopyroxene + amphibole +spinel (cf. Grieve & Gittins, 1975). Deer et al. (1982, p. 267) note that several investigations of similar coronas indicate their formation at high P (> 6 kb) and relatively high T (> 800 °C) during subsolidus cooling in a regional metamorphic environment. Pyroxene compositions (particularly Al contents) have also been used as geobarometers, and the compositions of coexisting augite and orthopyroxene in equilibrium with olivine and calcic plagioclase can be used to infer crystallization pressures using the petrogenetic grid of Herzberg (1978). Using this grid, compositions of typical Mclntosh pyroxenes give P = 3-3-5 kb and T = 1090-1200 °C. The Al contents of pyroxenes are, however, also influenced by other factors such as magma composition, particularly the activity of SiO2 (e.g., Campbell & Borley, 1974). Presence of orthopyroxene reaction rims on Mclntosh olivines may put an upper limit on crystallization pressures, since the reaction relationship in Fe-free, anhydrous experimental systems is reported to disappear at pressures between 1-3 and 5-4 kb, though H2O significantly increases and Fe decreases this pressure (Deer et al, 1982). Other crystallization parameters cannot be well defined either, though the scarcity of hydrous minerals suggests low pHjO and the relatively early appearance of a Ti-poor magnetite suggests high oxygen fugacity (> NNO buffer).

Origin of troctolite-olivine gabbro cyclic units Troctolite is inferred to be the less fractionated, higher temperature cumulate relative to olivine gabbro, though there is no obvious consistent difference in cryptic composition data between adjacent layers. The sharply defined modal and textural contact between troctolite 230 CHARTER I. MATHISON AND PAUL R. HAMLYN and olivine gabbro is inferred to have resulted from fractional crystallization of a small batch of magma, so that the layer sequence in a cyclic unit is a product of the crystallization order (plagioclase and olivine first, then augite). Repetition of this sequence can be explained by periodic addition of new magma. Sharp modal and textural changes at olivine gabbro- troctolite boundaries suggest that the new magma was denser than the resident magma, and spread across the temporary floor (Huppert & Sparks, 1980; Sparks et al, 1984). The contact between troctolite and olivine gabbro could result from normal fractionation, or from a mixing event. Smewing(1981) suggested that peridotite-gabbro contacts in cyclic units in the

Oman ophiolite resulted from mixing. Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021 Formation of cyclic units by processes operating in a closed system such as variable depth convection (Jackson, 1961), periodic supercooling cycles (Wager, 1959) or possibly double diffusive convection (McBirney & Noyes, 1979) would be expected to result in successive cyclic units becoming progressively more fractionated. Slight progressive fractionation does occur in some successive cyclic units but the trend is not continuous, and major reversals in cryptic compositions (10-15 at. per cent Mg and mol per cent An) strongly favour open system mechanisms.

Origin of oxide gabbronorite layers The mineralogy, cryptic compositions and the bulk chemical compositions of oxide gabbronorites show that they are the most fractionated layers. Unexpectedly, they are interlayered with significantly less fractionated troctolites and olivine gabbros, and are not restricted to the very top of the layered sequence. Their cumulus mineral assemblages could result from continuation of the fractionation sequence in a troctolite-olivine gabbro cyclic unit due to the crystallization order: plagioclase + olivine, augite, orthopyroxene (olivine disappearing) + magnetite. Crystallization of Ti-poor magnetite (due to high/Ol) could have resulted in increased SiO2 activity and therefore cessation of olivine and appearance of orthopyroxene. Depleted FeO and enriched SiO2 also favoured sulfur saturation (Haughton et al, 1974), consistent with the larger, more abundant sulfide blebs in most oxide gabbronorites. However, the change from troctolite to oxide gabbronorite typically occurs in only about 100 m vertically. Moreover, oxide gabbronorites are much more fractionated than the olivine gabbros just below them so that a marked progressive discontinuity occurs in the cryptic trends (e.g., in megacyclic unit III, plagioclase decreases from An73 to An32, and orthopyroxene from Mg10 to Mg52 in only about 70 m vertically). This accelerated fractionation suggests that oxide gabbronorites may not be the result of typical crystal fractionation processes. Instead, they may have formed when a dense, fractionated liquid periodically accumulated on the crystallizing front. Thus, the marked increase in fractionation at the bases of the gabbronorites could have resulted from a density crossover (Sparks & Huppert, 1984) in the relationship between the densities of the fractionated residual liquid and the resident magma. Such a density crossover might have involved the following stages: 1. The fractionated residual liquid from the crystallization of troctolite and olivine gabbro was initially less dense than the overlying magma and rose, possibly mixing continuously so that the composition of the overlying magma changed relatively slowly, resulting in gradual, normal fractionation trends in the cumulates. The adcumulate character of the troctolites and olivine gabbros fits the idea of a buoyant residual liquid (Tait et al, 1984). 2. The residual liquid became progressively denser until after the accumulation of olivine gabbro, or several troctolite-olivine gabbro cycles, it became denser than the overlying THE McINTOSH LAYERED GABBRO 231 magma (a density crossover) and it ponded next to the crystallizing front, resulting in accelerated fractionation. The possibility of a density crossover is indicated by calculations of magma density and fractionation densities for the Mclntosh intrusion using the approach of Sparks & Huppert (1984) (fractionation density = ratio of gram formula weight to molar volume of chemical components in the liquid phase being removed by fractional crystallization). The density of the chilled magma (assumed similar to the magma from which troctolites and olivine gabbros crystallized) is calculated to be 2-74 g cm"3 (anhydrous, at 1250 °C) using the method of 3 Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021 Bottinga & Weill (1970), and this decreases to 2-66 g cm" if 1 wt. per cent H2O is present at P = 2 kb. Using the data of Sparks & Huppert (1984) and the modal compositions and mineral compositions for selected Mclntosh rocks, the following fractionation densities are obtained: melatroctolite 2-85 g cm ~ 3, troctolite 2-68 g cm " 3, and olivine gabbro 2-63 g cm ~3. Thus, the residual postcumulus liquid was probably less dense than the overlying resident magma initially, and its density would have increased during accumulation of troctolite and olivine gabbro, and ultimately it may have become denser than the overlying magma. (A similar result is possible even if the assumption about the chill composition is incorrect, and the parent magma had more Al and less Fe than the chill.) The periodically-formed dense residual liquid from which the oxide gabbronorites were derived could have accumulated in place by ponding on the crystallizing front, or it could have been derived from sidewall flow (Chen & Turner, 1980; McBirney et al, 1985) that descended and spread across the temporary floor, possibly as a density flow (Irvine, 1980a). Formation of the oxide gabbronorites by sidewall downflow of denser, more fractionated magma could have also involved mixing with the resident magma, and possible evidence for imperfect magma mixing in such a mechanism is afforded by the pillows of finer-grained gabbroic rocks in oxide gabbronorite matrix (Fig. 3). The different crystallization order in some gabbronorites (orthopyroxene possibly before augite) may also reflect mixing (cf. Todd et al, 1982; Irvine et al., 1983).

Origin of cryptic trends in megacyclic units Cryptic trends for the megacyclic units in the upper part of the layered series show that the marked progressive discontinuities in the oxide gabbronorites are followed by gradual rather than abrupt reversals (Fig. 7). The most gradual reversal occurs above the most fractionated oxide gabbronorite in unit IV, where orthopyroxene changes from Mgi2 to Mg15 and plagioclase from An52 to An77 over 650 m of section, involving seven troctolite-olivine gabbro cyclic units. These reversals are inferred to represent influxes of new magma, but their gradual nature requires explanation. Gradual reversals seem to be more common than abrupt reversals in layered intrusions, and have commonly been interpreted in terms of continued crystallization during addition of new magma and its progressive mixing with the resident magma (e.g., Dunham & Wadsworth, 1978). Mechanical mixing of a single large pulse of new magma is unlikely to occur gradually and continuously over such an extended period as required for the accumulation of 650 m of cumulates in unit IV. A gradual reversal might result if several distinct pulses of magma entered the chamber after the oxide gabbronorite formed, successive pulses mixing progressively less with the fractionated resident magma (cf. Wadsworth et al., 1982). A mechanism involving several pulses is favoured by the existence of several troctolite-olivine gabbro cyclic units. Other mechanisms are capable of producing gradual reversals, such as progressive reduction in the degree of supercooling (Campbell, 1978) or postcumulus infiltration metasomatism (Irvine, 1980b), but the Mclntosh reversals are too great and are spread over too large a stratigraphic interval for 232 CHARTER I. MATHISON AND PAUL R. HAMLYN either mechanism to be the dominant cause. Alternatively, gradual reversals could result from diffusive mixing of new and fractionated magma (Cawthorn, 1983), possibly by compositional stratification due to double diffusive convection (Sparks et al., 1984). Although the cryptic trends are broadly similar for the upper megacyclic units, some differences exist The size of the reversal (measured by the shift of mineral compositions) and the stratigraphic interval over which it occurs vary, the reversals becoming progressively sharper upwards (Fig. 7). Oxide gabbronorites do not become progressively more fractionated in successive megacyclic units, and the thicknesses of the megacyclic units and the number of troctolite-olivine gabbro cyclic units they contain also vary irregularly. The Downloaded from https://academic.oup.com/petrology/article/28/1/211/1494719 by guest on 02 October 2021 reasons for these differences are not clear, but they favour an open system involving periodic additions of magma. Each megacyclic unit is inferred to be a composite cyclic unit in which several magma pulses occurred, and the progressive discontinuities and the gradual reversals are inferred to be related mainly to the periodic formation of the oxide gabbronorite magma. SUMMARY AND CONCLUSIONS The observations fitth e concept of an open system, possibly advancing magma chamber in which there were at least 40 pulses of unfractionated magma, each pulse being denser than the resident magma and resulting in a troctolite-olivine gabbro cyclic unit. Obvious cryptic reversals at the bases of the cyclic units are absent because fractionation did not proceed far before the next pulse. The small grain-size of the layers (0-3-2 mm) suggests a relatively high degree of supercooling, consistent with cooling of new pulses in contact with the resident magma. The layered sequence shows only limited progressive fractionation from the base to the top, except for the sporadic, fractionated gabbronorites. Repeated influxes of new magma coupled with possible losses of fractionated magma would help explain the limited overall fractionation. A slow rate of fractionation (and adcumulate rocks) would also be favoured by a buoyant fractionated postcumulus liquid which therefore continually mixed with the overlying resident magma. However, the density of fractionated postcumulus liquid periodically exceeded that of the overlying resident magma so that rather than mixing with the bulk of the resident magma as previously, the fractionated liquid ponded on the temporary floor, forming the much more fractionated oxide gabbronorite layers and the progressive discontinuities in the cryptic trends. A similar process involving the bottom accumulation of heavy Fe-enriched liquids was suggested by Reynolds (1985) to explain the magnetite-rich layers in the Bushveld Complex. Such processes have been termed 'liquid fractionation' by McBirney et al. (1985) since typical crystal fractionation is not the only mechanism involved. Thus, the Mclntosh magma chamber acted as a density trap for these heavier, fractionated magmas so that they would not be available for eruption (cf. Stolper & Walker, 1980), and the Mclntosh oxide gabbronorites may be an example of this process. Baker & McBirney (1985) suggested that iron-enriched fractionated lavas are relatively scarce in tholeiitic volcanic suites for these reasons.

ACKNOWLEDGEMENTS C. I. Mathison gratefully acknowledges Australian Anglo-American Corporation Limited (Mr C. C. Codner in particular) for air fares, field transport, camp facilities, aerial photographs and other information. A University of Western Australia research grant financed one field trip. F. M. Barrett and D. I. Groves assisted in the field, and Mr J. W. Hillyer of the Electron Microscopy Centre at the University of Western Australia helped with the microprobe analyses. Professor B. E. Leake provided helpful advice on this manuscript. The staff of Pickands Mather & Co. International, in particular Mr R. Halligan, are thanked for their assistance to P. R. Hamlyn. THE McINTOSH LAYERED GABBRO 233

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