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There is textural evidence in the Rhum pluton for crystallization of layered ultrabasic rocks ('harrisite') under conditions of massive olivine supersaturation. Some of these rocks are comb layered. This field, petrographic and experimental investigation of harrisite is aimed at determining (1) the degree of supercooling at which different shapes of olivine crystallized in harrisite, (2) how magma in a plutonic environment became massively supersaturated in olivine and (3) whether the parent magma of harrisite was basaltic or ultrabasic. The results have particular relevance to the conditions under which magmas crystallize close to, or on, their margins and enclosures, sometimes to produce comb layers. Harrisite is here defined as "an ultrabasic, plutonic, igneous rock in layered intrusions which contains >25 vol. percent of olivine. The olivine crystals are characterized by skeletal or dendritic shapes and have larger size than olivines in contiguous layers. Hydrous minerals are essential phases in the rock," Harrisite is a varietal type of peridotite, allivalite or olivine eucirite. Harrisite layers are mainly confined to the Western Layered Series of the Rhum pluton. They range in thickness from 7 cm to 10 m and have a preferred thickness of about 1 m. Individual layers vary laterally in thickness and olivine crystal shape. A few harrisite layers are intrusive into the Layered Series, but the majority crystallized in conformable sequence with cumulate layers. Five percent of the ultrabasic rock of the island is harrisite; sixty percent of the Transition Series is harrisite. The shapes of olivines in igneous rocks are classified into thirteen categories (those found in harrisite are underlined): polyhedral, porphyritic, granular, chain-like, parallel-growth, plate, branching (non-crystallographic, crystallographic, linked parallel-unit), randomly oriented, radiate, dendritic, lattice-work, swallow-tail and irregular olivines. Elongate olivines in harrisite can grow preferentially upwards, downwards, horizontally or in random orientation. There is a gradual, systematic upward change in layers from granular or polyhedral cumulus olivines to skeletal porphyritic olivines, to branching olivines (present only in some layers). This change is accompanied by up to a 10 vol. percent decrease in olivine content and by increases in contents of both chrome magnetite and hydrous minerals; also, olivine crystals increase up to 1000 times in size and plagioclase crystals increase in size up to 10 times. Olivine crystals in harrisite grew as fast as 10-4 cm/s. Olivine in harrisite has a composition range of Fo84- 73. Some crystals are reverse zoned (2 - 2.5 mol. percent Fo). Olivines in cumulate layers are up to 2 mol. percent poorer in Fo than those in contiguous harrisite layers. Augite (e.g., Wo46En46FS9) and rare orthopyroxene crystals (e.g., Wo1.3En79.4 Fs 19.3) indicate crystallization from a transitional magma. Plagioclases range from An87 in the generally broad, unzoned cores of crystals, to approximately An60 at in the extremity of the normally-zoned mantles. Cr-rich kaersutite and phlogopite crystallized by reaction of magma with pyroxene, whereas Cr-deficient varieties of the same minerals crystallized directly from the magma. Chrome magnetite composition is dependent on the enclosing silicate mineral- crystals enclosed in olivine are generally richer in Fe and poorer in Al, Cr and Mg than those enclosed by plagioclase and pyroxene. Variations in olivine, plagioclase and chrome magnetite compositions in one comb layer of harrisite are consistent with upward crystallization of the layer along a thermal gradient in the magma of increasing temperature with height.

The unique Selukwe Subchamber of the Great Dyke is bounded by the Shurugwi greenstone belt (SGB) on the west side for approximately 25 km and granitoids on the east side, as compared to other subchambers of the Great Dyke that are bounded on both sides by granitoids. It is also the narrowest section of the entire Great Dyke. The extensive xenolith suite is found on the western flank and the central zone of the subchamber. This study focuses on the PAR 11 borehole and the surface xenoliths in the Selukwe Subchamber (SSC). The PAR 11 core was drilled into an anomalous sequence of ultramafic rocks situated in the Mafic Succession of the SSC. There are basically two rock types in the PAR 11 borehole: peridotites and pyroxenites. Comparison of the major and trace element geochemistry of the PAR 11 body with the MR 92 data of Coghill (1994) for the SSC reveals that they are similar but less evolved. The mineral assemblages and proportions of phases in the PAR 11 borehole samples are indicative of essentially the same composition as that which formed the layered sequence of the Great Dyke. Therefore, on the basis of the rock types and chemical compositions, the PAR 11 body and the Great Dyke cumulates appear to be petrologically and chemically similar and had the same petrogenesis. There are three rock types in the xenolith suite that have been observed in the mafic succession of the Unki area: peridotites, pyroxenites and gabbros. Major and trace elements show a wide range of compositions that have CaO/Al2O3 ~ 1, which are dissimilar to both PAR 11 and MR 92 borehole data. REE patterns show depletion of LREE, with flat HREEs indicating a different magma to that which gave rise to the Great Dyke. Such flat patterns are typical of a primitive mantle source similar to that of komatiite magma. Stowe, (1974) describes dunite and chromite in the SGB and does not describe pyroxenites and gabbros. Therefore, it is not clear in the first instance that the xenoliths were derived from the SGB. It also does not necessarily mean that these rock types did not occur in the SGB and, if they did, maybe they were derived from an intrusion within the SGB that is at depth and never been seen iv before. The xenoliths do not have mineral compositions that are similar to the Great Dyke and therefore precludes them as having been derived from the Great Dyke Marginal Facies, a possible source of such rocks. Therefore, it is concluded from this study that they were inherited from another source which also does not appear to be the SGB because there is no report of such rock types (other than peridotite) in the SGB. They are also not mantle derived. The metasedimentary rocks that occur as xenoliths are banded iron formation and quartzites and are all clearly derived from the different formations of the SGB. The quartzites are from the Mont d’Or Formation and Wanderer Formation. The BIFs are from the Upper Greenstone and Wanderer Formation. The Shurugwi Greenstones were stripped off from the western flank whereas the Archean granitoids to the eastern flank of the Great Dyke remained. The conclusion from this study is that the Shurugwi greenstones and Archean granitoids of the Selukwe area were intruded by the large volume of new magma that was the parental magma to the Great Dyke. The hot parental magma carried up with it xenoliths from outside the Great Dyke and large blocks from within the Great Dyke to the uppermost rocks of the level of the P1 pyroxenite layer and mafic unit.

Alteration of ultramafic rocks is ubiquitous to their occurrence near the surface. Primary mantle minerals like olivine and pyroxenes are unstable at low pressure and temperatures and undergo hydration (serpentinization), carbonation and weathering reactions forming hydrated and carbonated minerals. Employing a variety of analytical techniques including: electron microprobe, X-Ray diffraction, major and trace element geochemistry, Mg isotopes geochemistry and geochemical modelling this work seeks to contrain conditions ofa lteration and trace changes in composition of peridotite during alteration. In Wadi Fins in the southeast of Oman peridotites outcrop at the bottom of a canyon overlaid by a thick sequence 1.5 km of Cretaceous to Eocene shallow oceanic limestones and dolostones. The peridotites exhibit different types of alteration. While the common view is that serpentinization and carbonation of peridotites is isochemical this is not the case for alteration in Wadi Fins. Peridotites tens of meters below the unconformity are characterized by a striking grid of carbonate and serpentine veins. The calcite veins and relatively low MgO/SiO2 suggest that the peridotites reacted with a hydrous fluid derived from interaction of seawater with the overlying sediments composed of limestones with minor amounts of chert. This is further affirmed by average δ13C, δ18O and 87Sr/86Sr from carbonate veins in the peridotites that are similar to values of the sediments. Clumped isotope thermometry on calcite veins in peridotite establish that they formed at 25–60 ℃. Reaction path modeling of carbonate- quartz derived fluids with peridotite reproduces the observed mineral assemblage composed of carbonate and serpentine with similar Mg and MgO/SiO2 at high water to rock ratios, with carbon, H2O and silica added to the rock by the reacting fluid. Close to the unconformity the altered peridotites are characterized by concentric alteration halos recording variable fO2 and fS2. The partially serpentinized cores preserved primary minerals and record extremely low oxygen fugacities (fO2~10^(-75) bars). Two alteration zones are present evident from the alteration color. These zones exhibit nonisochemical alteration characterized by intergrowths of stevensite/lizardite. The alteration zones record progressively higher (fO2) recorded by Ni-rich sulfides and iron oxides/hydroxides. The alteration zones lost 20-30% of their initial magnesium content, together with mobilization of iron over short distances from inner green zones into outer red zones, where iron was reprecipitated in goethite intermixed with silicates due to higher fO2. The loss of magnesium in this peridotites motivated the final section of work. Mg isotopic compositions of partially serpentinized harzburgites and dunites in Oman are identical to average mantle and bulk silicate Earth (δ26Mg=-0.25‰) while altered periodites from Wadi Fins get heavier with increasing alteration. Analyses of peridotite alteration products including samples from Wadi Fins and carbonates from Wadi Tayin were used to show that isochemical serpentinization at low W/R does not fractionate Mg isotopes. I propose a mechanism that with increasing W/R and co-precipitation of Mg-carbonates and serpentine leads to carbonates with light isotopic compositions (Magnesite δ26Mg =-3.3 and dolomite δ26Mg=-1.91) and serpentine with heavy compositions (up to δ26Mg =-0.96 in serpentine veins). This complementary enrichment-depletion and the finite 14C ages of the carbonates suggest that serpentinization is ongoing along carbonation in Oman at ambient temperatures. Rates of calcite precipitation in travertines inferred from Δ26Mgcal-fl suggest that travertine formation in Oman sequesters a total of 10^6-10^7 kg CO2/yr.

The ultramafic cumulate rocks of the Raglan horizon in the Proterozoic Cape Smith fold belt of northern Quebec have a common parental liquid in equilibrium with olivine of Fo89 composition. Cumulate olivines have experienced a trapped liquid shift to lower forsterite composition proportionally to the fraction of trapped liquid in the cumulate. Anomalously low nickel contents in analyses of both olivine and whole-rock chemistries are observed only in cumulates with the most modal olivine and which are proximal to known Ni-Cu-(PGE) deposits. The calculated compositions of the trapped liquid fraction indicate that most of the Raglan cumulates formed from Fe-rich high-MgO basalts, which are restricted to the base of the Chukotat volcanic stratigraphy. We propose that a lower degree of adiabatic partial melting of a mantle source accounts for the Fe-rich nature of these parental liquids and may provide an explanation for the presence of numerous Ni-Cu-(PGE) deposits in the sills of the Raglan horizon.

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M.Sc. / The ca. 800 km2 Tete Complex of NW Mozambique is located at the eastern end of the 830 ±30 Ma Zambezi Belt, near the transition zone into the Neoproterozoic Mozambique Belt. The Complex is located just south of the Sanangoe Shear Zone where Mesozoic and Late Palaeozoic cover rocks obscure much of the region. Country rocks immediately in contact with the Tete Complex include amphibolitic gneiss, graphite-bearing marble, calcsilicate gneiss, muscovite and biotite schist and quartzite of the Chidue Group. The Tete Complex may have been intrusive into the Chidue Group, although there is evidence inferring tectonic emplacement. Those few contact exposures that exist are equivocal. Some of the rocks within the Tete Complex have been affected by metamorphism up to amphibolite grade, although large proportions of the rocks retain pristine magmatic mineralogy and texture. The Tete Complex contains mafic, ultramafic and anorthositic rocks, dolerite dykes and minor Fe-Ti oxide-rich rocks that occur as rubble. Pyroxenite occurs as thin (<1-2 m), cumulate layers within gabbroic rocks. Most exposed anorthositic rocks occur in the Nyangoma area in the eastern part of the Tete Complex. The anorthosites and leucotroctolites are massive, coarse grained (2-3 cm), and contain plagioclase (An47-An57) megacrysts up to 10 cm in length, interstitial olivine (Fo59-Fobs) and orthopyroxene (En59- En75, mean A1203 = 1.84 wt.%) rimmed by clinopyroxene (mean = Wo 46En38Fs i6), pyrite and Fe-Ti oxides. Secondary biotite, iddingsite, epidote and green spinet are present. The stable coexistence of olivine and plagioclase limits the depth of emplacement to <7-8 kbar, or <20- 25 km; a relatively shallow level of emplacement is favored by the generally fine grain size of the gabbroic and doleritic rocks. Compositions of coexisting plagioclase and mafic silicates (orthopyroxene and olivine) are similar to those of massif-type anorthosites. Previously unmapped meta-anorthosite occurs along the western and northern margin (within the Sanangoe Shear Zone) of the Tete Complex and has been metamorphosed to amphibolite grade. The rock contains plagioclase (An38-An39), with the more Ab-rich compositions related to the formation of garnet (mean = A1m67GrotsPYI6Sp2). Metamorphic orthopyroxene (Enso-En53), clinopyroxene (mean = Wo37En38Fs25), mizzonitic scapolite (Me63), amphibole, biotite and apatite are present. High Cl contents in amphibole, scapolite and biotite (e.g., up to 4.7 wt. % in amphibole), suggest that a Cl-rich metamorphic fluid infiltrated the western margin of the Tete Complex. Olivine melagabbro from the north-central part of the Tete Complex contains plagioclase (An70-An26), olivine (Fo82-Fos4) and clinopyroxene (mean = WanEn1Fs0.2, mean A1203 = 2.56 wt. %), with primitive compositions compared to those in Nyangoma anorthositic rocks and pyroxenites. Pyroxenites are modally dominated by clinopyroxene (mean = Wo46-48En36-39Fsi3-18) with accessory interstitial plagioclases (Ano-An45) and discrete and exsolved orthopyroxenes (En 56-En75). Clinopyroxenes with high A1203 contents up to 9 wt. % are similar to high-Al pyroxene megacrysts. One sample of pyroxenite contains orthopyroxene (En56-En60) and plagioclase (An40-An45) with more evolved compositions compared to those in Nyangoma anorthositic rocks and olivine melagabbro. Normal Fe4- and Na-enrichment trends accompanying fractionation from magmas that may be common to the Nyangoma anorthositic rocks, pyroxenites and olivine melagabbro, are associated with an increase in Al relative to Cr along a line of nearly constant relative Ti content. Gabbro contains olivine and plagioclase crystals that are commonly zoned, thus ranging widely in composition (Fool -Fos°, Anss-Ans2)• Clinopyroxene (mean = Wo36En47Fsi6) constitutes ca. 34 modal % of gabbro. New whole-rock (Nyangoma anorthosite and leucotroctolite) and mineral (plagioclase, clinopyroxene and orthopyroxene) Sm-Nd isotopic data yields ages between 975 ±33 Ma and 1041 ±131 Ma. The igneous crystallization age of the anorthositic rocks is estimated at 1025 ±79 Ma (9-point whole-rock regression). Rb-Sr isotopic compositions for whole-rock samples reveal no meaningful age relationships. Initial Nd isotopic compositions (calculated at 1.0 Ga) correspond to E Nd values between +3.5 and +4.5 (mean = +4.1) with Is, = 0.70276 — 0.70288 (mean = 0.70282), both inferring magmatic derivation from a depleted mantle source, possibly with little or no contamination by Archaean crustal components. TDM model ages range between 1074 and 1280 Ma (mean = 1148 Ma). There is a striking similarity between the Tete Complex anorthosites and those of SW Madagascar in terms of Nd isotopic compositions and the nature of country rocks; in both regions the anorthosites were emplaced either magmatically or tectonically into shelf-type supracrustal metasediments (marbles, quartzites, graphitic schists, etc.). Anorthosites intruded similar country rocks in Draining Maud Land, eastern Antarctica. Although anorthosites from Mozambique and Madagascar share a common depleted mantle signature with little or no contamination by Archaean crustal components, a direct stratigraphic correlation between these two areas (and possibly eastern Antarctica), awaits further geological and geochronological data.