Norman K. Grant

Structural distinction between a metasedimentary cover and an underlying basement in the 600-m.y.-old Pan-African domain of northwestern Nigeria, West Africa

Structural data from an area of gneisses, migmatites, and low-grade metasedimentary rocks in northwestern Nigeria suggest the presence of two contrasting structural styles, one simple and monocyclic and the other complex and polycyclic. The simple structural style is coextensive with one of two distinctive metasedimentary formations, the Birnin Gwari Schist Formation, which is interpreted as a sedimentary cover infolded during the Pan-African event into a basement of complex style consisting of the second metasedimentary formation, the Kushaka Schist Formation, and the gneisses and migmatites. This infolding may have occurred within the limits 730 to 650 m.y. ago, and it was followed by the 550- to 530-m.y.-ago epeirogenic uplift and cooling of the basement assemblage and the later emplacement of the Kusheriki granite 500 ± 4 m.y. ago. The gneissic and migmatitic basement of the Birnin Gwari Schist Formation includes ∼2000-m.y.-old Eburnean rocks, but neither the depositional nor metamorphic ages of the Kushaka Schist Formation are known.

Rare-element–enriched, S-type ash-flow tuffs containing phenocrysts of muscovite, andalusite, and sillimanite, southeastern Peru

Ash-flow tuffs of Neogene age exposed over 2,500 km 2 in the Macusani region of southeastern Peru are the volcanic equivalent of S-type granites. The strongly peraluminous tuffs contain phenocrysts of andalusite, sillimanite, and muscovite and have high 87 Sr/ 86 Sr i (0.7258 and 0.7226) and δ 18 O (+11‰). Elevated concentrations of Li, Cs, Be, Sn, B, and other minor elements compare with those in “tin granites.” Mineral phase relations and composition are indicative of low magmatic temperatures and oxygen fugacities and high a HF/ a H 2 O. The chemical, isotopic, and mineralogical features and regional geologic relations are consistent with models of magma generation involving the incorporation of large amounts of pelitic rock.

40Ar39Ar dating of glauconites: Measured 39Ar recoil loss from well-crystallized specimens

Abstract Nine glauconite samples with relatively high K concentrations and which appear to be well crystallized using normal X-ray diffraction techniques have been studied using the 40 Ar 39 Ar method. The glauconite 40 Ar 39 Ar apparent ages exceed their KAr, RbSr and, in most cases, stratigraphic ages by substantial amounts. 40 Ar 39 Ar release spectra sometimes yield plateaus but these apparent ages have no geological significance. The results indicate that 39Ar is lost by recoil from mineral grains during neutron irradiation, consistent with previously reported observations. The amount of 39Ar loss was measured by isotope dilution for four samples and varied from 29% to 17%. In contrast, radiogenic 40Ar is quantitatively retained during irradiation. The very fine blades which make up glauconite grains yield the mineral susceptible to large amounts of 39Ar loss and unsuitable for 40 Ar 39 Ar dating.

Reequilibration of olivine with trapped liquid in the Duluth complex, Minnesota

A 525-m-long drill core (DDH-221) through the Partridge River intrusion in northeastern Minnesota consists of mesocumulates and/or orthocumulates of troctolite, olivine gabbro, and olivine gabbronorite. The cumulus olivine, whose composition (Fo 31 to Fo 71 ) is a function of the amount of intercumulus liquid, followed an equilibrium crystallization path with this liquid. Estimates of the primary magmatic compositions of the olivine are Fo 57 to Fo 79 . None of the olivine in the drill core samples has retained an original cumulus composition (i.e., one that was in equilibrium with the parental magma). The plagioclase is strongly zoned, and the cumulus cores (An 59 to An 73 ) are thought to record crystallization from the parental magma.

Simulation of primary phase relations and mineral compositions in the Partridge River intrusion, Duluth Complex, Minnesota: implications for the parent magma composition

In order to describe the composition and crystallinity of the initial (parental) magma of the Partridge River intrusion of the Keweenawan Duluth Complex, and thereby understand the mode of emplacement and solidification of the intrusion, we have applied a numerical simulation technique called geochemical thermometry (Frenkel et al. 1988). The parental magma was a low-alumina, high-Ti-P olivine tholeiite similar to typical Keweenawan low-alumina, high-Ti-P basalts associated with the Duluth Complex and from the nearby Portage Lake area of the Lake Superior region. The parental magma was emplaced as a crystal-liquid suspension, followed by chilling of an evolved, leading edge ferrodioritic liquid in the basal zone of the intrusion. The conditions of emplacement at the present crustal location were ∼1,150°C, 2 kbar, and fO2 slightly above the wustite-magnetite (WM) buffer. The main differentiation process after emplacement was the sorting and redistribution of plagioclase and olivine crystals on a local scale accompanied by less efficient convection and minor settling of olivine. Calculated crystallization sequence for the parental magma is olivine+plagioclase (1,240°C)→olivine+plagioclase+magnetite (1,146°C, WM+0.5)→olivine+plagioclase+magnetite+augite (1,140°C, WM+0.5). The calculated compositions of the cumulus olivine and plagioclase in equilibrium with the parent magma at 1,150°C are Fo66.7±1.1 and An64.5±2.5, respectively, and are similar to the estimated average composition of primary olivine (Fo69.1±2.8) and the average composition of plagioclase core (An66.3±2.8) measured in drill core samples through the intrusion (Chalokwu and Grant 1987).

A strontium isotope and trace element profile through the Partridge River Troctolite, Duluth complex, Minnesota

Initial 87Sr/86Sr ratios down a 2,315 foot-long core (DDH-295) through the Partridge River Troctolite are lower in the depth range 710–1,410 feet (0.70443±22) than in the underlying and overlying rocks (0.70500±18). These initial ratio variations define three zones (I, II, and III) thought to be three intrusive units within the troctolite. The best estimate of the Rb-Sr age of the troctolite is 1,050±105 m.y. The olivine compositions and the abundances of the incompatible trace elements Zr, Y, and Rb show identical variations down the core, with downward repetitions of olivine iron-enrichment and incompatible trace element enrichments. Discontinuities in the olivine compositions and incompatible trace element abundances match the initial 87Sr/86Sr ratio discontinuities in the core.The strontium isotope data imply that involatile components of the Proterozoic Virginia Formation and other Archaean rocks were not significantly assimilated by the troctolites, for only three out of the twenty-three initial 87Sr/86Sr ratios depart from normal magmatic values. These three anomalous ratios are not systematically related to the Cu-Ni sulphide mineralization and may be the result of secondary calcite in the troctolite. The strontium isotope systematics of the Virginia Formation xenoliths and underlying hornfelses are highly disturbed as a result of Rb loss.The apparently inverted fractionation sequences in the troctolites are interpreted in terms of downward increases in intercumulus liquid in repeated magma batches and controlled by plagioclase flotation. The amount of intercumulus liquid is estimated to range from 7 to 61 weight per cent.The Cu-Ni sulphides are concentrated in the lower 250 feet of zone I (immediately above the Virginia Formation) and through 400 feet of core across the contact between zones I and II, more than 750 feet above the base. Because the sulphide-bearing troctolites are not systematically related to disturbances in the magmatic initial 87Sr/86Sr ratios, the mineralization probably involved the volatile transfer of sulphur derived from dehydration reactions in the underlying Virginia Formation.

Icelandite and aenigmatite-bearing pantellerite from the McDermitt caldera complex, Nevada-Oregon

Intermediate lavas from the middle Miocene McDermitt caldera complex, Nevada-Oregon, have very high FeO* (= total Fe as FeO; 9.1 to 10.2 wt %) and low MgO (0.4 to 2.1 wt %) contents at SiO 2 contents of 59.5 to 62 wt %. With the exception of significantly higher K 2 O contents (3.1 to 4.7 wt %), these lavas are compositionally very similar to icelandite (“tholeiitic andesite”) from oceanic areas. In addition to the voluminous comendite tuffs of the complex, a thin unit of densely welded crystal-rich pantellerite tuff of probable air-fall origin is exposed on the northern wall of the McDermitt caldera. The pantellerite tuff contains 1.5% aenigmatite phenocrysts. The high FeO*/MgO ratios of the icelandites and the peralkaline nature of the silicic rocks suggest differentiation under conditions of low f O 2 and f H 2 O. The existence at McDermitt of a continuous series of rocks ranging from potassic icelandite to comendite and pantellerite is consistent—as are other geologic, geochemical, and geophysical data—with the derivation of the icelandites and ultimately of the voluminous peralkaline silicic rocks of the complex from mafic magma generated from upwelling mantle material. Labile U released during the crystallization of late subvolcanic bodies of peralkaline silicic magma may have provided an appreciable proportion of the epigenetic U now present in the upper part of the caldera complex. Reducing conditions produced by the very Fe-rich glassy icelandites may have contributed to the fixation of U at the Aurora and Bretz prospects.

RbSr and KAr ages of paleozoic glauconites from Ohio—Indiana and Missouri, U.S.A.

Abstract Glauconite pellets from the Lower Silurian Brassfield Formation on both limbs of the Cincinnati Arch in Ohio and Indiana give a RbSr age of 370 ± 11 Ma, which is substantially younger than the stratigraphic age of the formation. The age is concordantcwith conventional KAr ages of 355 ± 6 and 368 ± 5 Ma for two of the same glauconites. Concordant ages were also obtained from the Viburnum Trend in Missouri, where glauconite pellets from the Davis Formation in an ore-zone collapse structure into the underlying Bonneterre Formation give a RbSr age of 387 ± 21 Ma and conventional KAr ages of 368 ± 5 and 369 ± 5 and 369 ± 5 Ma. A third suite of glauconite from the Bonneterre Formation in the Old Lead Belt ∼ 10 km from the nearest ore body has given a RbSr age of 423 ± 7 Ma and slightly older conventional KAr ages of 434 ± 6, 445 ± 6 and 441 ± 11 Ma. Because these glauconite-bearing rocks have been buried to depths of less than 1 km, thermal resetting of the RbSr and KAr systematics appears unlikely. The initial 87 Sr 86 Sr ratios of the RbSr isochrons are similar to the ratios for vein- and vug-filling dolomite and calcite. This is consistent with resetting of the RbSr and KAr systems during diagenetic changes which included the isotopic equilibration (perhaps by cation exchange) of the Sr in the glauconite with that in the diagenetic and Mississippi Valley-type ore fluids. This interpretation implies that the age of the Mississippi Valley-type mineralization in the Viburnum Trend is Devonian rather than Carboniferous—Permian as has been inferred from paleomagnetic measurements. Cation-exchange experiments with a dilute Sr-bearing solution and an artificial oilfield brine indicate that glauconite adsorbs large amounts of Sr, some of which is sufficiently strongly attached to the glauconite lattice as to resist leaching with ammonium acetate. The introduction of this strongly attached Sr may be the first step in the resetting of the RbSr systematics of glauconite by cation exchange.

The Cumulate Paradigm Affirmed

McBirney and Hunters conclusion that the cumulate paradigm or theory is an impediment to progress is unjustified because they did not demonstrate that the original cumulate terminology cannot be adapted; neither did they question the fundamental cumulate assumption of bottom upward solidification for many large mafic intrusions. Because the decoupling of mineral, modal and whole rock chemical compositions may reflect either the failure of cumulate theory or may be an artifact of sampling of large mafic intrusions on scales smaller than the scale of the heterogeneities produced by the irregular distribution and migration of intercumulus liquids, we conclude that a compelling case to abandon cumulate theory has not been made. We suggest that the determination of whether cumulus and intercumulus phases were in equilibrium with each other, and whether disequilibrium textures might yield a complete record of the crystallization history of the cumulus plagioclase, are likely to be among the approaches that will influence the future development of cumulate theory.

Rb-Sr glauconite systematics and the uplift of the Cincinnati arch

Five well-ordered authigenic glauconite samples from the Belfast Member of the Brassfield Formation in south-central Ohio yield a Rb-Sr isochron age of 337 ± 27 m.y. and an initial 87 Sr/86 Sr ratio of 0.7118 ± 0.0043 (2σ uncertainties). This glauconite age is 20% to 25% younger than the approximate stratigraphic age of the Belfast Member determined by fossil correlation, and it probably represents a geologic event in the postdepositional history of the sedimentary strata. This event is interpreted as an uplift of the pre-Mississippian units composing the Cincinnati arch, the glauconites having isotopically equilibrated their strontium with that of associated fluid phases and carbonates within the Belfast Member at the time of uplift.