Patrick M. Hurley

Ancient age of the Middle Marker Horizon, Onverwacht Group, Swaziland Sequence, South Africa

Abstract Separate laminae of 1–6 mm thickness from a microcrystalline quartz-mica-calcite-bearing sedimentary rock sample from the Middle Marker Horizon of the Onverwacht Group, show concordant Rb-Sr ages within experimental error. A statistical treatment of the data gives an extrapolated value for initial 87 Sr/ 86 Sr of 0.7015 ± 0.0018 (2σ) and an apparent age value of 3355 ± 70 (2σ) my, based on 87 Rbλ = 1.39 × 10 −11 yr −1 . The low initial ratio suggests tha effect of metamorphism has been slight and that this data may be fairly close to the time of deposition of the sediments. The Onverwacht Group is of interest because it is the oldest well-preserved sedimentary-volcanic sequence known, and contains an aromatic-type kerogen and microstructures of unknown origin which morphologically resemble fossil algae.

Rubidium-Strontium Age of the Bosumtwi Crater Area, Ghana, Compared with the Age of the Ivory Coast Tektites

Rocks from the vicinity of Bosumtwi crater, Ghana, and a repre-sentative collection of Ivory Coast tektites have been analyzed mass spectrometrically for rubidium, strontium, and strontium isotopic composition. The data from the rocks of the crater area yield an age of 1.97 x 109 years (λgb = 1.47 χ 10-11 year-1). The data for the Ivory Coast tektites fall on this isochron. This identity of age values for the Ivory Coast tektites and the Birrimian basement rocks of West Africa strongly supports the hypothesis of terrestrial formation for these tektites. The evidence available at present suggests that the Ivory Coast tektites are most probably the fusion products of meteoritic impact at the Bosumtwi crater site.

The isotopic composition of strontium in surface water from the North Atlantic Ocean

Abstract The isotopic composition of strontium in surface water from ten localities in the North Atlantic Ocean has been determined. The results support the conclusion that the Sr 87 Sr 86 ratio of the ten samples is constant within experimental errors. Eight of the ten samples came from the western half of the North Atlantic from the Bahamas to the Northumberland Straits of Nova Scotia. Two samples originated from the vicinity of the West African coast. The mean SrSr 87 Sr 86 ratio is 0.7093 ± 0.0005 (σ), relative to Sr 86 Sr 88 = 0.1194 . This value is in agreement with determinations of most previous analysts. A model is proposed which describes the isotopic composition of strontium in the oceans as a mixture of three isotopic varieties of strontium originating as weathering products of three types of rocks on the continents and in the ocean basins.

KAr age values in pelagic sediments of the North Atlantic

Abstract K-bearing mineral components in pelagic sediments from several locations in the North Atlantic show K-Ar age values generally in the range 200–400 m.y., indicating that most of the K is held in stable allogenic mineral structures. The high content of K in the non-car-bonate fraction indicates that it is part of a major component of the sediment. Different size fractions show a minor change in age value. X-ray analysis and studies of age values in eolian material and the components of marine shales suggest that the K-bearing phase is dominantly illite that is in part transported to the oceanic region by wind from continental source regions of unmetamorphosed ancient (mostly Paleozoic) marine shales. Ocean currents may contribute to the transportation and mixing of the material. Differences in age values at depth in a dated bottom core on the equator suggest that there were changes in direction of the transporting medium, or change of source, during glacial stages in the Pleistocene, which may be a basis of stratigraphic correlation. Differences in age values between different regions may provide a basis for following total current movements, or average long-term wind directions.

Liberian Age Province (about 2,700 m.y.)and Adjacent Provinces in Liberia and Sierra Leone

Whole-rock Rb-Sr dating of rocks from the crystalline basement has disclosed an age province in Liberia and Sierra Leone of about 2,700 m.y. The approximate eastern boundary of this province with the adjoining Eburnean age province of about 2,000 m.y. has been found in eastern Liberia. Much younger rocks of PanAfrican age (about 550 m.y.) bound the ancient province in Sierra Leone and western Liberia in a belt adjacent to the coast. Groups of infolded metasedimentary and metavolcanic rocks have been tentatively identified as associated with two of these ages. The Kambui Schists of Sierra Leone show whole-rock Rb-Sr ages of about 2,700 m.y. The pelitic and iron-bearing metamorphic rocks in the Marampa Formation of Sierra Leone, and similar sections in the Nimba Range, Liberia, appear to be about 2,200 m.y. old, and may fall within the typical Eburnean age range. The age values are scattered, however, and the Nimba rocks may be equivalent t o the Kambui. The Kasila Group of Sierra Leone, and a coastal belt in Liberia believed to be a continuation of the Kasila, yield typical PanAfrican ages of about 550 m.y.

Geology and age of the Parguaza rapakivi granite, Venezuela

The 1,550-m.y.-old Paraguaza granite of the northwestern Guayana Shield in Venezuela represents one of the larger, apparently anorogenic rapakivi intrusive rocks of the world. The massive rapakivi granite intrudes foliated granitic rocks and associated volcanic rocks of trans-Amazonian age in a structural setting transcurrent to the general northeast-southwest trend of the older basement rocks of the Guayana Shield. Age relations and the geochemistry of the rapakivi suggest an anatectic origin from tensional effects developed by internal distortions within a continental mass. The extensive 1,550-m.y.-old Parguaza intrusion in Venezuela is correlated with 1,550-m.y.-old basement rocks underlying the Amazon Basin in Brazil and suggests a widespread “Parguazan” event 1,500 to 1,600 m.y. ago which affected a large part of the northwestern and southern Guayana Shield, extending as far south as the Guapore craton of Brazil. The Parguazan event therefore marks an important Proterozoic episode in the tectonic evolution of the widespread Precambrian Shield area of northern South America.

Radiogenic argon and strontium diffusion parameters in biotite at low temperatures obtained from Alpine Fault uplift in New Zealand

Abstract Rapid uplift along the Alpine Fault zone in New Zealand has exposed ancient mica schists which are estimated to have been at depths up to 9000 ft prior to the fault displacement, on the assumption that the smooth surface represented by the present, flat topped, peak elevations is truly representative of the mature Pliocene surface prior to uplift and glaciation. K-Ar age measurements on the micas show losses of radiogenic argon that vary with depth of burial up to almost complete loss from biotite at a depth of 9000 ft. The mean value of the diffusion parameter ( D a 2 ) calculated is 6 × 10−16 sec−1 at an estimated temperature of 110°C. This value is much higher than expected but is compatible with effectively complete retention of argon at surface temperatures, and with measured diffusion losses at high temperatures, if in the relationship D a 2 = ( D 0 a 2 ) exp (- E RT ) D 0 a 2 = 1 sec −1 and E = 27 Kcals/mole. A single Rb-Sr analysis on one of the low argon biotite samples showed an almost equal loss of radiogenic Sr87.

RbSr isotopic evidence in the origin of potash-rich lavas of Western Italy

Abstract The 87 Sr/ 86 Sr ratio in the potassic lavas of the Roman province has been found to vary from 0.706 to 0.711, with the ratio rather constant for each volcano, but generally increasing from south to north. The increase in 87 Sr/ 86 Sr is found to be correlated roughly with total Rb and Sr, both of which reach unusually high levels of concentration. It is concluded that the continental potassic volcanic association in general (leucite-basalt to potash-trachyte), by analogy with this volcanic province, could not have derived its unusually high Sr from carbonatites or the normal types of basalt, or from the assimilation of limestone. The Sr isotopic evidence suggests that these unusual magmas were derived from the refusion or anatexis of ancient continental sialic rocks. The concentrations of Sr and other trace elements suggest that the sialic rocks were deeply depressed into the high pressure regime where garnet and clinopyroxene are dominant residual phases.

The dependence of K-Ar age on the mineralogy of various particle size ranges in a shale

Abstract The mineralogy and K-Ar age of the whole rock and six size fractions, ranging from Whatever the cause of the low age of the IMd material, it is obvious that the whole rock age is not directly indicative of the age of sedimentation, as it is composed of a mixture of ages in the different size fractions of the shale.

Total-rock UPb and RbSr systematics in the Imataca Series, Guayana Shield, Venezuela

Abstract A combination of data from two isotopic systems may be useful in “seeing through” an intense metamorphic event to the primary age of an ancient terrane. Rb Sr secondary isochrons on thin-slice samples make it possible to select domains within the rock in which Sr isotopes were equilibrated during the high-temperature stage of the metamorphism. A single domain may then be tested for Pb isotopic homogeneity at the same time by use of secondary U Pb isochrons. These monitor the possible effect of recent U loss, and if successful yield values for the common Pb ratios at the time of the thermal event. If the thermal event is both ancient and well-dated by the secondary isochrons, a model age for the protolith may then be established. The method is illustrated here with reference to the Imataca Series, about which we conclude the following: (1)|The primary or protolith age of the Imataca Series is in the range of 3.4–3.7 b.y. (2)|At approximately 2 b.y. ago, a regional metamorphic event up to granulite grade in intensity affected the Imataca. (3)|This metamorphism was accompanied by complete local homogenization of Sr isotopes over distances of centimeters; equilibration of Pb isotopes was apparently less perfect at the same scale. (4)|The granulite event was further characterized by extensive depletion of U relative to Pb. Comparable Rb loss has not taken place, perhaps because of the abundance of K-bearing phases. (5)|Deformation of the rock unit investigated may have taken the form of separate shear surfaces, between which isotopically equilibrated domains on either side of a shear plane are identified by differences in initial Sr isotope ratio.