Late Intrusive Phases Research Articles

Critical Metals Mineralization in the Late-Stage Intrusions of Salmi Batholith, Ladoga Karelia, Russia

The paper reports the results of studies on critical metal mineralization genetically related to the late-stage intrusions of Salmi anorthosite-rapakivi granite batholith (SARGB) in the Riphean age. In, Bi, and Be mineralization in skarn-greisen deposits and occurrences at the SARGB endocontact, as well as REE and Nb-Ta mineralization in Li-F granites, understood as the late intrusive phases of the batholith, were studied. It is the first report on columbite-group minerals, as well as REE-Ta-Nb and REE mineralization in SARGB granites. Optical and scanning electron microscopy, EDS and LA ICP MS microanalysis, X-ray fluorescence spectrometry, Raman spectroscopy, and inductively coupled plasma mass spectrometry (ICP-MS) were used. The data obtained show that roquesite formation was mainly triggered by the decay of In-bearing solid sphalerite and chalcopyrite solutions. Zavaritskite, associated with unoxidized sulphides, was derived hypogenically and seldom occurs in ores. A helvine-group mineral association with zinc-enriched spinel (ZnO 22%–25%) seems to have been one of the factors preventing genthelvite formation. The Muzilampi, Hepaoja and Avtodor ore occurrences in Li-F granites display similar REE and Nb-Ta mineralization. They are associated with Y-fluorite and Li-siderophyllite, which contain exceptionally high Nb concentrations (0.25%–0.78%) in Muzilampi granites. Additionally, fluorite-1 is commonly overfilled (to >50%) with micron-sized synchisite and parisite inclusions. Columbite-tantalite-group minerals, present at all the occurrences studied, occur solely as ferricolumbites with a dominant Mn/(Mn + Fe) ratio of <0.2. Biotite and Li-siderophyllite, associated with columbite, have an extremely high iron index Fe/(Fe + Mg) > 0.9 approaching the maximum values (~1.0) in the most differentiated granites.

Ediacaran‐Ordovician Magmatism and REE Mineralization in the Wet Mountains, Colorado, USA: Implications for Failed Continental Rifting

AbstractStructures associated with Ediacaran‐Ordovician alkaline magmatism and the timing of rare earth element (REE) mineralization in the Wet Mountains, CO, were analyzed using field, geophysical, and U‐Th‐Pb isotope methods to interpret their tectonic setting in the context of previously proposed rift models. The Wet Mountains are known for thorium and REE mineralization associated with failed rift‐related, Ediacaran‐Ordovician alkaline intrusions and veins. Structural field data indicate that alkaline dikes and mineralized veins are controlled by a system of northwest‐striking, high‐angle faults and tension fractures formed in a 040°‐directed extensional regime. Magnetic and surface expressions of Democrat Creek and McClure Mountain complexes show tectonic elongation toward ∼045°, consistent with NE‐directed extension. Magnetic data also suggest the existence of a fourth, previously unrecognized mafic‐ultramafic complex of inferred Cambrian age with a similar elongated orientation. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICP‐MS) 208Pb/232Th analysis of low‐uranium zircon from carbonatite dikes and in situ 206Pb/238U LA‐ICP‐MS analysis of monazite in mineralized dikes yielded 465 ± 18 Ma and 489 ± 33 Ma ages, respectively. These ages are consistent with the expected age based on slightly older, cross‐cut syenite dikes and the hypothesized Ordovician end to failed rift‐related magmatism. The Ediacaran‐Ordovician age of alkaline magmatic rocks and the associated northeast‐directed extension direction are similar to those of the along‐strike, Ediacaran‐Cambrian Southern Oklahoma Aulacogen. Therefore, the failed rift system in the Wet Mountains is interpreted to be a northwestern continuation of the Southern Oklahoma Aulacogen with carbonatite magmatism and thorium/REE mineralization representing late intrusive phases.

The Petrological and Geochemical Evolution of Ediacaran Rare‐Metal Bearing A‐type Granites from the Jabal Aja Complex, Northern Arabian Shield, Saudi Arabia

AbstractNew fieldwork, mineralogical and geochemical data and interpretations are presented for the rare‐metal bearing A‐type granites of the Aja intrusive complex (AIC) in the northern segment of the Arabian Shield. This complex is characterized by discontinuous ring‐shaped outcrops cut by later faulting. The A‐type rocks of the AIC are late Neoproterozoic post‐collisional granites, including alkali feldspar granite, alkaline granite and peralkaline granite. They represent the outer zones of the AIC, surrounding a core of older rocks including monzogranite, syenogranite and granophyre granite. The sharp contacts between A‐type granites of the outer zone and the different granitic rocks of the inner zone suggest that the AIC was emplaced as different phases over a time interval, following complete crystallization of earlier batches. The A‐type granites represent the late intrusive phases of the AIC, which were emplaced during tectonic extension, as shown by the emplacement of dykes synchronous with the granite emplacement and the presence of cataclastic features. The A‐type granites consist of K‐feldspars, quartz, albite, amphiboles and sodic pyroxene with a wide variety of accessory minerals, including Fe‐Ti oxides, zircon, allanite, fluorite, monazite, titanite, apatite, columbite, xenotime and epidote. They are highly evolved (71.3–75.8 wt% SiO2) and display the typical geochemical characteristics of post‐collisional, within‐plate granites. They are rare‐metal granites enriched in total alkalis, Nb, Zr, Y, Ga, Ta, REE with low CaO, MgO, Ba, and Sr. Eu‐negative anomalies (Eu/Eu* = 0.17–0.37) of the A‐type granites reflect extreme magmatic fractionation and perhaps the effects of late fluid‐rock interactions. The chemical characteristics indicate that the A‐type granites of the AIC represent products of extreme fractional crystallization involving alkali feldspar, quartz and, to a lesser extent, ferromagnesian minerals. The parent magma was derived from the partial melting of a juvenile crustal protolith with a mantle contribution. Accumulation of residual volatile‐rich melt and exsolved fluids in the late stage of the magma evolution produced pegmatite and quartz veins that cut the peripheries of the AIC. Post‐magmatic alteration related to the final stages of the evolution of the A‐type granitic magma, indicated by alterations of sodic amphibole and sodic pyroxene, hematitization and partial albitization.

Multiple fluid sources/pathways and severe thermal gradients during formation of the Jílové orogenic gold deposit, Bohemian Massif, Czech Republic

The Jílové deposit in the central part of the Bohemian Massif represents a vein to stockwork type of orogenic-type gold deposit. It is hosted by Neoproterozoic rocks of the Jílové Belt and by various magmatic dikes related to the ~355 to ~335Ma Central Bohemian Plutonic Complex. The deposit is situated along the terrane boundary of the Teplá Barrandian and Moldanubian units.The deposit offered an exceptional opportunity to trace O, C, S and Sr stable isotope evolution of parent fluids based on combined mineralogical and geochemical study of carbonate, quartz, scheelite, and sulfide minerals, which represent six stages of mineralization, including the gold-bearing event.Stable isotope data and mineral and isotope thermometry indicate gangue and ore mineral formation between ~350°C and <100°C, which can be divided into 6 stages. Scheelite-bearing assemblages (stages 2–3) precipitated at 292±8°C from a fluid with calculated values: δ18OSMOW=+4.2±0.5‰ and δ13CPDB=−11±1‰. Gold precipitation (stage 5) probably started at about 300°C, but the major event probably occurred at 230±30°C from a fluid with more variable isotope values (δ18OSMOW=+2.5 to +5‰ and δ13CPDB=−9 to −13.5‰). The carbon speciation was characterized by predomination of dissolved CO2 (H2CO3ap.) in the fluids. Some gold, however, undoubtedly precipitated from bicarbonate dominated fluids even at <120°C.Extreme variations in the δ18O values of carbonate minerals, obtained from sampling profiles across individual veins with macroscopic gold, revealed severe thermal gradients during vein formation (~50 to ~100°C difference of crystallization temperatures between the vein margin and core).The sulfur stable isotope composition of sulfide minerals indicates the dominant role of sulfur remobilization from Neoproterozoic rocks and stratiform mineralizations of the Jílové Belt by Variscan hydrothermal fluids. Similarly, the Sr-isotope composition of carbonates indicates both relatively primitive (87Sr/86Sr=0.7055) and more evolved (87Sr/86Sr~0.7090) fluid compositions, probably indicating fluid exchange with the Jílové Belt and the Central Bohemian Plutonic Complex rocks, respectively.Age determination of hydrothermal muscovite (related to stage 2) via 40Ar/39Ar indicated an age of 339.0±1.5Ma for the quartz veins. The mineralization is essentially coeval with the late intrusive phases of the Central Bohemian Plutonic Complex (i.e. the ultrapotassic suite) and with late-orogenic large-scale tectonic movements at the boundary between the two crustal terranes (Teplá-Barrandian and Moldanubian).Based on evaluation of the available age data on the hydrothermal and magmatic activity within the broader area of the Central Bohemian Plutonic Complex, we suggest two intervals of gold mineralization: 347 to 341Ma and 340 to 337Ma. The former interval overlaps with the intrusive activity of the Blatná high-K suite (granodiorite). The associated gold deposits (Mokrsko and Petráčkova hora) exhibit strong affiliation to the intrusion-related-gold-type deposit. The later interval overlaps with the ultrapotassic magmatism and is associated with more or less “classical” orogenic-gold-type deposits (Jílové, Bělčice, Libčice deposits).

Zircon U–Pb and K-feldspar megacryst Rb–Sr isotopic ages and Sr–Hf isotopic composition of the Mesozoic Heyu pluton, eastern Qingling orogen, China

Mesozoic granitoids are ubiquitous in China and have attracted the interests of many geologists not only because of their mineralization potential but also because they carry information on source material, emplacement mechanisms and crustal architecture. Most of the plutons were created by multi-stage magma emplacement and offer particular challenge for unraveling the emplacement and cooling history. A multi-isotope approach combined with single grain dating analyses provides a means to understand magma geneses even in complex magmatic settings. This study presents Rb–Sr isotope data for K-feldspar megacrysts and U–Pb ages and Hf isotopic composition for zircons from the composite Heyu pluton exposed in eastern Qinling orogen, central China. Zircon U–Pb concordia ages of 150 and 140Ma and single K-feldspar Rb–Sr isochron ages between 133 and 121Ma were obtained from early and late magmatic stages, respectively, consistent with slow cooling or isotopic re-equilibration of the Rb–Sr K-feldspar system. The distinct K-feldspar morphology of early and late intrusive phases is controlled largely by the emplacement depth, likely to be associated with a gravity-driven sinking model. Corresponding zircon εHf(t) values are −21.8 to −18.0 and −17.2 to −10.3 for early and late intrusive phases, respectively, indicating different magma sources.

Late Paleozoic rare-metal granitoid magmatism of the Southern Baikal region

The Late Paleozoic intraplate magmatism of the Selenga-Vitim structural zone of the Baikal region (Khamar-Daban Range) produced granitoids of different geochemical types: palingenic calc-alkaline granitoids, subalkaline monzogranites, and rare-metal Li-F granitoids and their subvolcanic analogues. Subalkaline and rare-metal granitoids occur in the periphery of the Late Paleozoic magmatic zone. Rare metal granite magmatism is manifested in this region as nearly N-S trending intrusive-dike belts comprising multiphase intrusions (Kharagul, Urugudei, and Bitu-Dzhida massifs) with an exposed area of ∼10 km2 and an age of formation from 311 to 321 Ma and series of accompanying dikes. The early phases of the intrusions are made up of biotite granites usually with fluorite, which are changed during the late stage by typical topazbearing rare-metal amazonite-albite granites. In the subvolcanic facies, thicker subalkaline dikes of monzonite porphyry, granite porphyry, and elvan are changed by ongonites, topaz rhyolites, and topazites, which occasionally serve as cement in eruptive and fluid-explosive breccias. The development of multiphase intrusions from early biotite granites to late amazonite-albite granites with Li-F mica was accompanied by an increase in SiO2 and, especially, Na2O contents, whereas the level of (FeO + Fe2O3), CaO, and K2O declined. Geochemical evolution includes an increase in the same direction in the contents of F, Li, Rb, Cs, Sn, Be, Ta, and Pb and a decrease in Ba, Sr, Zn, Zr, Th, and U. Similar evolution is also characteristic of the subvolcanic rocks, which emphasizes the genetic relation of the whole intrusive-dike complex of the Khamar-Daban province. Significant differences were detected in the distribution of K, Ba, Sr, and Zr between the calc-alkaline granitoids and rare-metal Li-F granites. The continental crust-normalized patterns of the raremetal granites show positive anomalies for Li, Rb, Nb, and Pb. The rare-metal Li-F granites could not be produced by palingenesis only, and their formation required specific conditions causing extensive accumulation of characteristic trace elements. During the evolution of granite melts, Li, Rb, Ta, Nb, Sn, W, and F are extensively accumulated in late intrusive phases, which indicates an important role of the processes of magmatic and fluid-magmatic differentiation during their formation. The composition and isotope geochemical characteristics of the supposed magma source material correspond to the ancient Precambrian continental crust with a mean model age of more than 1200 Ma.

The Rhyolite-Hosted Volcanogenic Massive Sulfide District of Cuale, Guerrero Terrane, West-Central Mexico: Silver-Rich, Base Metal Mineralization Emplaced in a Shallow Marine Continental Margin Setting

The Cuale mining district, situated in the Cordillera Madre del Sur, ~30 km southeast of Puerto Vallarta, Mexico, comprises a number of small, polymetallic Pb, Zn, Cu, volcanogenic massive sulfide deposits (VMS), and vein and stockwork-hosted deposits with low-sulfidation epithermal characteristics. Massive sulfide mineralization contains high Ag ± Au grades when compared to similar deposits in western Mexico. The massive sulfide bodies are hosted by an interbedded rhyolitic volcanic and volcaniclastic succession, herein termed the Cuale volcanic sequence. Regionally, the Cuale volcanic sequence is part of the Zihuatanejo subterrane belonging to the Guerrero terrane, which is interpreted to be a complex Mesozoic accreted arc terrane hosting the majority of VMS deposits in Mexico. The rocks of the Cuale volcanic sequence are not metamorphosed and are only weakly deformed but exhibit intense chlorite, sericite, and quartz hydrothermal alteration. Three stratigraphic units are identified within the Cuale volcanic sequence: (1) a more than 400-m-thick footwall unit of quartz and plagioclase phyric rhyolite flows, related hyaloclastites, and subordinate heterolithic volcaniclastic breccias; (2) an ore horizon characterized by sedimentary rocks ranging from volcaniclastic conglomerates to black shales, as well as aphyric to quartz and plagioclase phyric rhyolite; and (3) late intrusions composed of quartz and plagioclase phyric rhyolite and subordinate andesitic dikes intruding all units. The sedimentary rocks of the ore horizon are intercalated with rhyolite tuff horizons, some of which exhibit welding or accretionary lapilli that are interpreted as evidence for shallow submarine or subaerial deposition. The rhyolites of the Cuale volcanic sequence are calc-alkaline with only limited compositional variations. However, the trace element signatures reveal a weak trend from the footwall rhyolites, which are characterized by low Zr/Y (1‐1.9) and low chondrite normalized La/Yb ratios (9 of 11 samples between 1.24 and 3.3), to slightly higher values in the ore horizon and late intrusive phase (Zr/Y = 1.6‐7.6 and chondrite normalized La/Yb ratios between 2.25 and 15). This compositional evolution can be explained by slightly decreasing degrees of partial melting related to a transition from amphibole-free assemblages for the footwall to amphibole-bearing assemblages in the residuum for the ore horizon and intrusive rhyolites. This interpretation is supported by the observed volcanic stratigraphy and indicates increasing pressure at the site of melt generation as the volcanic edifice was built. Zircons from five samples of the Cuale volcanic sequence have been dated by the U-Pb method, using isotope dilution thermal ionization mass spectrometry (ID-TIMS). Although some samples show evidence of a minor inherited Pb component, an age range of 157.2 ± 0.5 to 154.0 ± 0.9 Ma has been established. Cuale represents the oldest dated VMS camp in western Mexico as other VMS districts of western Mexico are between 151.3 and 138 Ma. Two styles of mineralization are recognized: volcanogenic massive sulfide mineralization (VMS) and late lowsulfidation epithermal vein and stockwork mineralization characterized by high Au (1.89 ppm) and Zn (2.35%) but low Cu (0.2%) contents. The VMS mineralization is contained in more than 15 small orebodies, which exhibit a range of mineralization styles. Orebodies interpreted as proximal are characterized by stockwork vein networks with high Au and Cu contents, underlying massive pyrite bodies, which grade laterally and vertically into sphalerite- and galena-rich massive sulfide ore. Stockwork mineralization is locally hosted by aphyric rhyolite. More distal orebodies, rich in Pb, Zn, and Ag, are hosted by black shale and locally exhibit laminated or brecciated sedimentary textures in the galena-sphalerite-pyrite ore. The sedimentary textures observed in some massive sulfide orebodies suggest that the sulfide has been transported and deposited in anoxic basins adjacent to the rhyolite domes. Some of the more significant orebodies show evidence of both distal sedimentary ore textures and proximal features indicated by intense quartz-sericite-pyrite alteration in the immediate footwall.

Low geomagnetic field intensity during the Kiaman superchron: Thellier and microwave results from the Great Whin Sill intrusive complex, northern United Kingdom

Coe‐Modified Thellier (CMT) and Microwave (MW) paleointensity experiments were conducted on samples from the multi‐phase late Carboniferous (ca. 301 Ma) to early Permian (ca. 270 Ma) Great Whin Sill complex. Statistically comparable results were obtained with both techniques from high quality data. Forty three samples (25 with CMT and 18 with MW) yielded mean paleointensity values of 12.0 ± 5.8 μT and 16.5 ± 8.2 μT for the early and late intrusive phases respectively, corresponding to VDMs of (1.6 ± 0.8) × 1022 and (2.1 ± 1.1) × 1022 Am2. When combined with previous data, the results suggest a generally low dipole moment during the KRS, reducing from ∼75% of the present value at 301 Ma to 20–27% of the present value later in the superchron. These data imply either a decoupling between processes controlling reversal rate and field strength, or a more complex, intriguing time‐lag relationship. More absolute palaeointensity results are needed to further evaluate these alternatives.

Mineral-chemical studies of metamorphosed hydrothermal alteration in the Kristineberg volcanogenic massive sulfide district, Sweden

The massive sulfide deposits of the Kristineberg area, Sweden, occur within a 2- to 3-km-thick succession of felsic volcaniclastic rocks belonging to the Skellefte Group. The volcanic pile is intruded by a synvolcanic Jorn-type granitoid (Viterliden intrusive complex) and is overlain by a thick sequence of metasedimentary rocks (Vargfors Group). Mineralization occurs at two main stratigraphic levels, at the base of the felsic volcanic succession and at the contact with the metasedimentary rocks of the Vargfors Group. The Kristineberg Cu–Zn mine is the largest deposit (approximately 21 Mt) and occurs at the base of the volcanic pile, close to the contact with the Viterliden intrusive complex. Four smaller deposits (Ravliden, Ravlidmyran, Horntrask and Nyliden) occur along the upper ore horizon. These deposits are thought to be related to a late intrusive phase of the Viterliden complex which cuts the altered volcanic rocks at the Kristineberg deposit. Within an area of about 50 km2 surrounding the Kristineberg deposit, felsic volcanic rocks between the two ore horizons are affected by extensive albite-destructive alteration (sodium depletion) and development of chlorite and muscovite (strong co-enrichment in magnesium and potassium). The Kristineberg deposit is enveloped by a large and partly transposed quartz–chlorite alteration zone, approximately 2 km in diameter, and a distal but coherent pyrite–quartz–muscovite alteration zone extending as far as 4 km from the deposit. Chlorite(±talc) in the mine area is notably magnesium-rich and contains anomalous F, Ba, Zn and Mn. High fluorine is also present in coexisting muscovite and phlogopite. The magnesium-rich chlorite alteration contrasts sharply with the iron enrichment observed in many other felsic, volcanic-hosted Precambrian massive sulfide deposits. This may indicate fixation of iron by large amounts of pyrite in the section or entrainment of large amounts of seawater in the hydrothermal upflow zones. Kyanite is developed locally in the chlorite-rich pipe at Kristineberg in response to regional thermal metamorphism of highly aluminous alteration in the immediate foot-wall rocks. Spectacular, andalusite-bearing quartz–muscovite schists and quartz–biotite–cordierite schists also occur where the altered felsic volcanic rocks are intruded by the late Revsund granite. However, similar metamorphic mineral growth is not observed where the volcanic rocks at the contact are less altered. Deposits near the top of the felsic volcanic succession are characterized by magnesium-rich chlorite alteration in the foot wall and proximal calc-silicate assemblages (dolomite, calcite, tremolite, ±garnet, ±margarite) where the host sedimentary rocks are carbonate-rich. In general, the calc-silicate alteration is restricted to the immediate hanging wall and zones lateral to the deposits and does not represent a regionally extensive exploration target. The two main ore horizons in the Kristineberg area are not linked by any obvious discordant structures or alteration zones. However, mineral-chemical studies highlight several possible fluid flow pathways leading from the Kristineberg deposit to the Ravliden ore horizon, more than 2 km upsection. Overprinting regional metamorphic minerals have inherited the hydrothermal signature of the ore-related alteration. Electronic Supplementary Material is available at http://dx.doi.org/10.1007/s00126-002-0299-y. On that page (frame on the left side), a link takes you directly to the supplementary material.

Zr-Nb-REE Mineralization in Peralkaline Granites from the Amis Complex, Brandberg (Namibia): Evidence for Magmatic Pre-enrichment from Melt Inclusions

The Amis Complex is a late intrusive phase associated with the Brandberg anorogenic granite intrusion. It consists of peralkaline, arfvedsonite-bearing granitic and pegmatitic dikes and sills and is characterized by locally extreme enrichments in REE and rare metals with high charge-ionic radius ratios, such as Zr and Nb. The highest concentrations (e.g., 1.7 wt % Zr, 0.3 wt % Nb, 0.5 wt % total REE) are found in aegirine-albite aplites that formed around arfvedsonite pegmatite cores. Quartz-hosted melt inclusions in coarse-grained arfvedsonite granite and pegmatite give unequivocal evidence that the peralkaline composition and rare metal enrichments are primary magmatic features. Electron microprobe and SIMS analyses of rehomogenized inclusion glasses reveal their peralkaline high Si rhyolite compositions (avg molar [Na + K]/Al = 1.1) and very high contents of H2O (max 10.6 wt %) and F (2.2 wt %). Inclusion glasses also have high concentrations of incompatible trace elements including Nb (min 803 ppm), Zr (min 3,187 ppm), and REE + Y (Σ = min 1,271 ppm). The REE, Zr, and Nb compositions of the aegirine-albite aplites plot along the same linear enrichment trends as the melt inclusion glasses, and Y/Ho ratios mostly display unfractionated near-chondritic values. The chemical and textural features of the aegirine-albite aplites are compatible with an origin by rapid crystallization (compositional quenching) after volatile loss from a residual peralkaline granitic melt of a composition like that of the melt inclusions. Interaction of postmagmatic fluorine-rich aqueous fluids with the arfvedsonite granites caused local remobilization of REE and secondary mineralization of fluorocarbonate phases, such as bastnaesite and fractionation of Y and Ho.

Biotite from rapakivi

This paper deals with the chemical composition of biotites from rapakivi granites. For this purpose, 61 chemical analyses of biotites from composite anorthosite-rapakivi plutons in the Fennoscandian and Ukranian shields were used. Figurative points of the biotite composition were plotted on diagrams designed for biotites byFoster (1960),Winchell (1949) andVelikoslavinsky (1972). Consideration of the diagrams shows that a) the biotites from rapakivi granites have exotic composition not characteristic of other biotites from other igneous rocks, b) that they belong to the annite-siderophyllite series and c) that they crystallized at low pressures and high temperatures. Model crystallization temperatures range from 760° (early intrusive phases) down to less than 435° (late intrusive phase).

Rb-Sr age determinations of rocks from the Okenyenya igneous complex, northwestern Namibia

AbstractThe Okenyenya igneous complex is one of a suite of intrusions which define a prominent northeast-trending linear feature in Damaraland, northwestern Namibia. Precise Rb–Sr internal isochron ages range from 128.6 ± 1 to 123.4 ± 1.4 Ma for the major phases of intrusion identified within the complex. The tholeiitic gabbros forming the outer rings of the complex, and the later alkali gabbros which form the central hills, cannot be distinguished in terms of Rb–Sr ages, although field relations clearly indicate the younger age of the latter. The intrusionsof nepheline-syenite and essexite comprising the mountain of Okenyenya Bergon the northern edge of the complex give ages of 123.4 ± 1.4 and 126.3 ± 1 Ma, respectively, and form the final major phase of intrusion. The ages obtained for early and late intrusive phases define a minimum magmatic ‘life-span’ of approximately 5 Ma for the complex. The determined age of the Okenyenya igneous complex (129–123 Ma), when taken together with the few reliable published ages for other Damaraland complexes (130–134 Ma), suggests that these sub-volcanic complexes were emplaced contemporaneously with the widespread Etendeka volcanics (˜ 130 Ma), and relate to magmatism associated with the breakup of southern Africa and South America with the opening of the South Atlantic Ocean. The linear distributionof intrusions in Damaraland is interpreted to be due to magmatism resultingfrom the upwelling Tristan plume being focused along a structural discontinuity between the Pan-African, Damaran terrain to the south, and Proterozoiccratonic basement to the north.

Compositional variation of minor intrusions and the form of a volcano magma chamber

A number of highly alkaline parasitic bodies and dykes within the south-western basaltic shield volcano on Saint Helena belong to a late intrusive phase. The rocks exhibit systematic compositional variation from phonolite in the central area, outwards through trachyte to trachybasalt. This variation is interpreted as resulting from the tapping of a differentiated magma chamber by a number of independent dykes and parasitic intrusions. The chamber was tapped at varying depths dependent upon the horizontal distance of the individual intrusion or dyke from the apex. The most basic intrusive rocks therefore occur furthest from the centre. The dimensions and volume of the chamber are estimated.