Ma Isochron Research Articles

Geochemical constraints on the petrogenesis of the Kamiskotia gabbroic complex and related basalts, Western Abitibi Subprovince, Ontario, Canada

The Kamiskotia gabbroic complex (KGC) is a large (170 km 2), late Archean synvolcanic intrusion located in the western Abitibi Subprovince, Canada. It is divided into four stratigraphic units: a Lower Zone (LZ), comprised of adcumulus and mesocumulus peridotites, troctolites and gabbro-norites (Mg number = 73–86); a Middle Zone (MZ) comprised of mesocumulus gabbro-norites (Mg numbemr = 60–74); mesocumulus and orthocumulus gabbro-norites and ferroan gabbro-norites of the Upper Zone (UZ, Mg number=37–64) which are overtain and partly along strike with granophyric rocks of intermediate to felsic composition. The cumulates are characterized by flat REE patterns with La N/Yb N = 0.4–2.6, and range from 0.6–2× chondrites for olivine-bearing LZ adcumulates to 10–25× chondrites for upper UZ meso-to orthocumulates. Europium anomalies are strongly positive for the LZ and and diminish upsection to the upper UZ which has slight negative Eu anomalies. Chilled rocks are found in mixed magma and contact chill outcrops, and have a Mg number of 54–58, La N/Yb N = 0.8–1.2 and slight negative Eu anomalies. Phase relationships and mass balance calculations using the REE indicate that LZ cumulates may be derived from the average chill composition. Similarly upper UZ cumulates may be derived from evolved basalts which overlie the KGC. Assimilation-fractional crystallization modeling indicates that the evolved basalts may be derived from the average chill composition by 70–80% fractionation at low pressures; or lower degrees of fractionation accompanied by recharge of the averagr chill composition into the system. The basalt compositions do not reflect significant assimilation of nearby felsic rocks or average Archean crust during fractionation. Primitive KGC liquids are estimated by adding an average olivine-bearing adcumulate composition to the KGC chill composition in equal proportions to obtain a liquid with an Mg number of 70. The REE content of such a liquid is consistent with a derivation by 34% partial melting of a chemically primitive mantle. Neodymium isotope ratios for KGC cumulates, mineral separates and overlying volcanic rocks plot on a 2710 Ma isochron with ϵ Nd( t) = 2.5 ± 0.8, in close agreement with U-Pb zircon ages for the KGC and Kamiskotia felsic volcanic rocks at 2707 ± 2 Ma and 2702 ± 2 Ma, respectively. The initial value is identical to the isotopically depleted, MORB-like Abitibi mantle signature at 2.7 Ga, and provides supporting evidence for fractionation-dominated supracrustal processes, with no detectable assimilation of an isotopically enriched crustal component. Kamiskotia mafic magmatism is similar to more recent, rift-related systems represented by the Skaergaard Intrusion, East Greenland, and differentiated tholeiitic volcanic fields in Eastern Iceland and along the Galapagos Spreading Center.

Age of a K-feldspar megacrystic granite from the Burgeo intrusive suite, and timing of tungsten mineralization at Grey River, southern Newfoundland

A K-feldspar megacrystic biotite granite phase of the Burgeo intrusive suite, southern Newfoundland, has been dated by the Rb–Sr whole-rock method. The isochron age of 412 ± 5 Ma (initial 87Sr/86Sr ratio = 0.7086 ± 0.0004) is consistent with the younger of two U–Pb zircon ages obtained from other phases of the Burgeo intrusive suite. The southern margin of the granite is intensively deformed in a shear zone that also affects amphibolitic gneisses of the Grey River enclave. Data from deformed granite samples plot below the 412 Ma isochron and may have resulted from reaction and partial reequilibration of the samples with a hydrothermal fluid of low initial 87Sr/86Sr ratio during metasomatic reactions accompanying shear zone deformation. However, alternative hypotheses such as crustal contamination or derivation from two separate and isotopically distinct magmas cannot be discounted. Mica ages between 390 and 370 Ma associated with tungsten mineralization at Grey River confirm that the mineralization was unrelated to the Burgeo intrusive suite. They support previous conclusions, based on geological and geochemical data, that the mineralization was related to posttectonic leucogranite intrusions.

Nd-Sr-Pb systematics and age of the Kings River ophiolite, California: implications for depleted mantle evolution

Sm-Nd whole-rock and mineral data for the Kings River ophiolite define two isochrons of 485±21 Ma and 285±45 Ma age with e_Nd (483)= +10.7±0.5 and e_Nd (285)= +9.9±1.1, respectively. The 483 Ma isochron is defined by samples of the main igneous construct. Samples from crosscutting diabase dikes and flaser gabbro sheets within the peridotite unit yield the 285 Ma isochron. The 483 Ma data provide the first evidence of lower Paleozoic oceanic crust in the Sierran ophiolite belt. New U-Pb analyses of zircons from a plagiogranite lying on the 483 Ma Sm-Nd isochron yield upper and lower intercepts with the concordia of 430^(+200)_(−60) and 183±15 Ma. Published zircon ages have underestimated the primary age of the ophiolite by 200–300 m.y. due to the effects of polymetamorphism. The 483 Ma samples have initial ^(87)Sr/^(86)Sr=0.7023–0.7030, ^(206)Pb/^(204)Pb=17.14–17.82, ^(207)Pb/^(204)Pb=15.37–15.52, ^(208)Pb/^(204)Pb=36.80–37.38. The 285 Ma samples have similar initial 87Sr/86Sr, but more radiogenic Pb. The range in Sr and Pb compositions is probably due to introduction of radiogenic Sr and Pb during multiple post-emplacement metamorphic events. The high e Nd, low ^(87)Sr/^(86)Sr, ^(206)Pb/^(204)Pb, ^(207)Pb/^(204)Pb, ^(208)Pb/^(204)Pb of the least disturbed samples are clearly diagnostic of a midocean ridge origin for the 483 Ma portion of the ophiolite. Igneous activity at 285 Ma is thought to have occurred in an arc or back-arc setting, or perhaps along a leaky transform. The initial e_Nd (483)=+10.7 is indistinguishable from that of the similar age Trinity Peridotite (Jacobsen et al. 1984). This value is the highest yet reported for the Mesozoic or Paleozoic depleted mantle and requires either a mantle source that was depleted ∼ 850 m.y. earlier than average or a source more highly depleted than average. Alternatively, if such values were more typical of the early Paleozoic mantle than is currently thought, then there has been little evolution of the depleted mantle over the last ∼ 500 m.y. This requires that the modern mantle has been refluxed by material with low e_Nd, such as continental crust.

Chemical and isotopic effects of late Archaean high-grade metamorphism and granite injection on early Archaean gneisses, Saglek-Hebron, northern Labrador

Summary Early Archaean (3700–3800 Ma) layered tonalitic to granodioritic Uivak gneisses form approximately 60–70% of the Archaean gneiss complex between Saglek and Hebron fiords, northern Labrador. Effects of late Archaean metamorphic and igneous activity can be recognized on the geochemistry and isotope chemistry of the Uivak gneisses: depletion in rocks affected by 2900 Ma granulite facies metamorphism, with a loss of K, Rb and redistribution of REE and a corresponding enrichment of LIL elements in the amphibolite facies area. LIL enrichment is at least in part by addition from intrusive late Archaean granitoid sheets. The depleted granulites (K/Rb 500–2000) show a scatter of data points on a Rb/Sr- 87 Sr/ 86 Sr diagram to the left of a 3560 Ma reference isochron (ISr 0.700) consistent with marked Rb loss and some homogenization of Sr. Amphibolite facies gneisses from a broad transitional zone into granulite facies have K/Rb values of 200–300 and plot on a c. 3560 Ma isochron with a high ISr (0.7038) consistent with a slight loss of Rb at 2900 Ma but without marked Sr homogenization. Amphibolite facies gneisses away from the transition zone show marked enrichment in LIL elements with unusually low K/Rb ratios (100–200). Rb/Sr data plot close to a 3560 Ma reference isochron, ɛSr 3560 values are strongly negative, interpreted as due to Rb addition at 2900 Ma rather than derivation from an unusually depleted mantle source. Pb-Pb isotope data from the depleted granulite show a cluster of unradiogenic data points. Samples from a monolith of gneisses in a transitional zone between amphibolite and granulite facies yields a 3808 Ma isochron (μ 1 8.5) suggesting there was locally little disturbance of the Pb/U system at 2900 Ma. Pb-Pb data from the Rb-enriched amphibolite facies area show a markedly higher proportion of radiogenic Pb compared to the granulite facies rocks which cannot be explained by U movement alone. Local galena mineralization suggests that Pb as well as U was mobile during the late Archaean and we interpret the results as mixtures of > 3600 crustal Pb with younger material. Sm-Nd data points (Collerson & McCulloch 1982) show a scatter about a 3665 Ma isochron with an average ɛNd 3665 of +2.48. Marked LIL + LREE enrichment is suggested and we interpret the scatter of ɛNd 3665 values as being due to late movement of LREE. The late Archaean granitoid sheets have a chemistry and petrology suggesting derivation largely by partial melting of earlier sial with modifications by fluid redistribution of alkalis. New minerals such as allanite and apatite grow in older rocks adjacent to young veins. Sr, Nd and Pb data all suggest mixing between early Archaean crust and a component with mantle-like isotopic character at c. 2900 Ma. The mixing process was complex and took place by ingestion of older rocks in younger granite, mixing of partial melts of different origin and fluid transport of components from younger intrusions into older gneisses.

RbSr and zircon study of ~2800 Ma Lewisian silicic gneisses from the Torridon Inlier of NW Scotland: Dyke intrusion and an open system

The Torridon Inlier of the Lewisian of Scotland contains ~30 sq. km of granodioritic gneisses. A coordinated study of this inlier, including detailed mapping, geochemical analyses and geochronological work, has indicated that Rb-Sr whole rock analyses yield “ages” that are functions of the local structural setting and of the chemistry and mineralogy of the rock, as well as of the overall geological history of the region. This Archean region has been invaded by a swarm of northwest striking dykes of Inverian (2400 to 2240 Ma) age, accompanied by new foliations and major structures. However, several structurally distinct areas, termed “pips”, 1–2 km 2 in extent, are free of Inverian foliation and largely free of Inverian dykes. The 38 samples analysed for Rb and Sr scatter widely about a 2.66 Ma isochron; but give meaningful results only when subdivided according to their structural setting and lithology. A subset of silicic gneiss samples from the area most strongly overprinted by Inverian structures yields an age of 2240 ± 70 Ma with an initial 87Sr 86Sr of 0.7098 ± 18. A subset of samples from “pips” further north yields an age of2790± 100 Ma (initial 87Sr 86Sr = 0.7020± 10 ), identical to the 2780± 70 Ma U-Pb age obtained from zircon fractions separated from two of the samples. Samples from this northern district, located outside the “pips” in areas showing weak Inverian foliation, or within 50 m of Inverian dykes, or both, have grossly disturbed Rb-Sr systematics. The apparent ages of the felsic gneisses have been increased, while those of their mafic enclaves have been decreased (to as low as 1380 Ma). Field and geochemical evidence relate the increase directly to Rb loss and/or radiogenic Sr gain in the immediate vicinity of major dykes, which themselves have gained Rb and K by comparison with dykes elsewhere in the Lewisian. In terms of Rb-Sr work on complex terranes, these results must caution against direct interpretation of such data, especially in the absence of detailed field knowledge.

The scale of Sr isotopic diffusion during post-metamorphic cooling of gneisses in the Inner Piedmont of Georgia, southern Appalachians

Rb Sr whole-rock isotopic analyses of two suites of contiguous gneiss samples (each ca. 10 cm in width) collected in quarries within the Georgia Inner Piedmont of the southern Appalachians indicate that Sr isotopic diffusion continued on a scale of 10–25 cm for at least 30 Ma after culmination of high-grade regional metamorphism. Variation of Rb/Sr ratios between 14 contiguous slabs of one suite is sinusoidal about an overall mean of 1.5, permitting calculation of a 315 ± 17 Ma isochron. On the basis of previous estimates of the scale of Sr diffusion in feldspar under geological conditions, an approximate closure temperature of 600°C may be calculated. Together with Rb Sr and 40Ar/ 39Ar mineral ages from this quarry, the 315 ± 17 Ma isochron age is interpreted to date cooling through ca. 600°C following culmination of high-grade Devonian regional metamorphism. The average Rb Sr isotopic composition of a contiguous suite from another Inner Piedmont quarry confirms a Paleozoic age for regional metamorphism.

Isotopic dating and structural relationships of granitoids and greenstones in the East Pilbara, Western Australia

A structural RbSr isotopic study has been made on two small areas in the eastern Pilbara block, Western Australia. The sites were chosen because they showed good interrelationships of granitoid and greenstone units. At Warrery Gap, on the western side of the Corunna granitic dome, the acid volcanics of the Duffer Formation (at the top of the lower Talga-Talga Subgroup of the Warrawoona Group) show some updating, but a good 3506 ± 62 Ma isochron, with initial ratio (IR) of 0.7006 ± 0.0011, was recognized, consistent with the 3452 ± 16 Ma zircon measurement of Pidgeon (1978b). Both a penetrative D 1 deformation, and the doming of D 2 preceded intrusion of late tectonic granitoids, dated at 3270 ± 22 Ma (IR = 0.7015 ± 0.0003), into both the Duffer Formation and the overlying pillow lavas of the Salgash Subgroup. Thus, the Salgash Subgroup is much older than suggested by Glikson (1979) and the stratigraphic succession cannot contain a hiatus between the apparently conformable Talga-Talga and Salgash Subgroups of the magnitude he proposed. The granitic domes clearly owe their form to the D 2 deformation rather than to batholitic intrusion, but near horizontal structural lineations suggest that they were not formed by diapiric movements. A granodiorite and pink feldspar granite from just within the Corunna granitic dome are slightly deformed: pooled isochrons indicate an age of 3232 ± 27 Ma but different IRs of 0.7032 and 0.7009, respectively. At Tambourah, in the eastern Shaw granitic dome, local D 2 also deforms an intrusive microadamellite of age 3087 ± 34 Ma and IR = 0.7103 ± 0.0057. There is therefore a real spread in ages of D 2 granitoids and D 2 deformation between about 3300 and 3100 Ma. Layered megacrystic gneiss, at Tambourah, also intruded by the microadamellite, contains a nebulous foliation argued to be local S 1 . Layered and homogeneous megacrystic gneiss produce updated RbSr total rock isochrons of 2995 ± 95 and 2779 ± 38 Ma, respectively. The primary age of these D 1 gneisses is clearly greater than that of the D 2 granitoids and is probably indicated by Pidgeon's (1978c) zircon age of 3417 ± 40 Ma from the Shaw granitic dome. If so, Hickman's (1975) “Migmatite Suite” contains both D 1 gneisses and D 2 granitoids separated in age at Tambourah by ca. 300 Ma, although neither appears to be older than the lower part of the preserved layered sequence. The protocrust on which that sequence was deposited has yet to be directly identified.

Timing of arrival of the Avalon Zone in the northeastern Appalachians: a new look at the Straddling Granite

The "Straddling Granite" was previously thought to straddle the Hermitage Bay – Dover fault system, which marks the boundary between the Avalon and Gander Zones of eastern Newfoundland, and hence to fix the earliest juxtaposition of these two zones before 504 ± 12 Ma. More detailed investigation shows that these rocks are divisible into two distinct geochemical suites, the Indian Point Granite to the west and the Hardy's Cove intrusive suite to the east. Of the samples that provided the 504 Ma isochron, five were obtained from the Hardy's Cove suite and one from within the fault zone; hence the isochron must be taken as representing only a typical date for plutonic rocks within the Avalon Zone. The Indian Point Granite intrudes the Gaultois Granite, dated at 350 ± 18 Ma, and is thus considerably younger than the Hardy's Cove suite. These observations now allow for juxtaposition of the Avalon and Gander Zones to have been as late as Early Carboniferous.

Rb/Sr isotopic and geochemical evolution of a recrystallized shear (mylonite) zone at Broken Hill

Relatively narrow (1 to 1,000 metres) planar zones of intense shear deformation and retrograde metamorphism (retrograde shist zones, RSZ) are common in the Willyama Complex around Broken Hill. A Rb/Sr isotopic study of one of these zones has revealed an unexpected isotopic pattern. All of the analysed points lie to the left of and above the 1665 Ma isochron of the Potosi Gneiss host. This result indicates a different isotopic composition of the zone as a whole rather than a chemical redistribution of Potosi Gneiss within a closed system. The isochron plot scatter is similar to that of ubiquitous metamorphic pegmatites in the basement of the Broken Hill region. The shear zone observation, however, is shown to be unrelated to the pegmatite segregation process and requires the introduction, by elemental exchange, of a more radiogenic strontium, whilst maintaining relatively fixed Rb/Sr ratios. Significant proportions of SiO2, K2O and Ba were removed at the same time. Na2O, Al2O3, TiO2, MgO, P2O5, Zr, Ni, Sc, Y, Nb, Rb remain essentially constant after allowing for volume loss of the removed elements. CaO and Sr increase slightly overall.

K–Ar isochron age of the North Mountain Basalt, Nova Scotia

Ages of five whole rock samples from the North Mountain Basalt, Nova Scotia, determined by Isotopes, Inc. in 1967, were discordant and indicated the effect of initial argon with a high 40Ar/36Ar ratio. Reanalysis of the same samples has yielded a 191 Ma isochron with a low initial 40Ar/36Ar ratio. Although the isochron age is similar to the value previously assigned to the basalt, the result suggests that application of the K–Ar isochron method may improve the precision of K–Ar age determination considerably.

A paleomagnetic and K–Ar study of the Umfraville Gabbro, Ontario

Two anti-parallel remanence directions have been isolated from samples of the Umfraville Gabbro situated in the southern part of the Grenville Province, Ontario. The magnetization yields a pole position at 160°E, [Formula: see text], A95 = 9.0°. Because of the dispersion of remanence directions, however, individual site poles plot over much of the Grenville Track. Regardless of direction and polarity of remanence, all samples unblock within a narrow temperature range of 525–560 °C. It is inferred that the magnetization process has involved slow cooling with both TRM and TCRM components being generated. Although one sample of zircon extracted from a genetically related syenite probably provides an estimate of the time of emplacement of 1160 Ma, the magnetization may be more closely related to the K–Ar age. Whole rock K–Ar ages range from 873 Ma to 1130 Ma, but all data points lie close to a 911 Ma isochron, with an initial 40Ar/36Ar ratio of 805. The isochron age is in close agreement with ages of 36Ar-free samples.

The Newport granite: its age, geological setting, and implications for the geology of northeastern Newfoundland

The undeformed Newport megacrystic granite of northwestern Bonavista Bay postdates nearly all the other granites in the area, many of which are strongly cataclastic. It also cuts a regional swarm of diabase dikes. The Rb–Sr isochron of 332 ± 42 Ma for the Newport overlaps the 300 ± 18 Ma isochron age of the deformed Lockers Bay megacrystic granite which is cut both by the Newport and the diabase dikes. This indicates that the emplacement of the Lockers Bay pluton, its deformation, and the intrusion of the diabase dikes and the Newport granite all took place in the interval 290–320 Ma, that is in late Carboniferous times. Major movements along the Dover Fault line, which truncates the Newport in the southeast, probably occurred subsequent to this interval.