Interpretation of Nd,Sr and Pb isotope data from Archean migmatites in Lofoten-Vesterålen,Norway

Sm-Nd data for the Archean granulite and amphibolite facies migmatites of Langøy and Hinnøy in Vesterålen are presented which indicate that their protoliths formed ～2.6 AE ago. Rubidium and U loss during a granulite facies metamorphism at ～1.8 AE caused serious disturbance of total-rock U-Pb and Rb-Sr systems. Therefore these systems do not provide any precise age information for the granulite facies migmatites. For the amphibolite facies migmatites of Vesterålen both SmNd, RbSr and PbPb total-rock systems give model ages of ～2.6 AE. The results on both granulite and amphibolite facies rocks are thus in agreement. Previous interpretations based on PbPb data, which indicated an age of 3.41 AE for the Archean terrane of Vesterålen, are not valid.One SmNd model age from the granulite facies migmatites at Moskenesøy in Lofoten indicates that the protoliths of these migmatites formed ～2.0 AE ago and are thus not related to the Vesterålen migmatites.     

U–Th–Pb and Rb–Sr systematics of Apollo 17 boulder 7 from the North Massif of the Taurus-Littrow Valley

Portions of highland breccia boulder 7 collected during the Apollo 17 mission were studied using UThPb and RbSr systematics. A RbSr internal isochron age of3.89 ± 0.08b.y. with an initial⁸⁷Sr/⁸⁶Sr of0.69926 ± 0.00008 was obtained for clast 1 (77135,57) (a troctolitic microbreccia). A troctolitic portion of microbreccia clast 77215,37 yielded a UPb internal isochron of3.8 ± 0.2b.y. and an initial²⁰⁶Pb/²⁰⁷Pb of 0.69. These internal isochron age are interpreted as reflecting metamorphic events, probably related to impacts, which reset RbSr and UPb mineral systems of older rocks.Six portions of boulder 7 were analyzed for U, Th, and Pb as whole rocks. Two chemical groups appear to be defined by the U, Th, and Pb concentration data. Chemical group A is characterized by U, Th, and Pb concentrations and²³⁸U/²⁰⁴Pb values which are higher than those of group B. Group A rocks have typical²³²Th/²³⁸U ratios of ～ 3.85, whereas-group B rocks have unusually high Th/U values of ～ 4.1.Whole-rock UPb and PbPb ages are nearly concordant. Two events appear to be reflected in these data — one at ～ 4.4 b.y. and one at ～ 4.5 b.y. The chemical groupings show no correlation with documented ages. The old ages of ～ 4.4 b.y. and ～ 4.5 b.y. may, like the younger ～ 4.0 b.y. ages, be related to basin excavation events.     

Experimental investigation of CaMg exchange between olivine,orthopyroxene, and clinopyroxene: potential for geobarometry

The distribution of Ca and Mg among coexisting olivine, clinopyroxene and orthopyroxene has been studied in a piston-cylinder apparatus in the temperature range 1100–1300°C and pressure range 9–41 kbar. Ca in olivine decreases with increasing pressure and decreasing temperature. The pressure effect is the result of Ca going into the higher-coordination M2 site in clinopyroxene as pressure is increased. For the CaMg exchange reaction between olivine and clinopyroxene, ΔV°=0.249J bar^?1 mole^?1; this is sufficient for pressure estimates accurate to ±3kbar if temperatures of equilibration are independently known. CaMg exchange between olivine and orthopyroxene is not sufficiently pressure dependent to be used as a geobarometer.Application of the olivine-clinopyroxene geobarometer to coarse garnet lherzolites from southern Africa gives P-T results consistent with a continental geotherm. For spinel lherzolites from southwestern United States, the geotherm appears to be displaced to higher temperatures indicating oceanic affinities. Application of the geobarometer to natural systems requires assumptions about activity relationships in clinopyroxene which should be checked by experiment.     

Inherited isotope systems and the source region pre-history of early Caledonian granites in the Dalradian Series of Scotland

RbSr and UPb isotope analyses are reported for two pre-metamorphic Caledonian granites which intrude Dalradian rocks in the Central Highlands of Scotland. These data indicate that the origin of the granitic magmas involved partial fusion of old crustal material.UPb systems of zircon size and magnetic fractions from the Ben Vuirich granite are strongly discordant. However, U/Pb isotopic ratios precisely define a chord which intersects concordia at 514_?7⁺⁶ m.y. and 1316_?25⁺²⁶ m.y. Geological constraints suggest that the lower intersection records the post-F₂, pre-M₃ emplacement age of the granite. The upper intersection reflects the presence of old zircon xenocrysts incorporated into the granite magma without complete isotopic resetting. The ultimate source of these xenocrysts is probably a metamorphic basement complex which formed about 1320 m.y. ago, but the immediate source region of the granites could have been Dalradian sediments derived therefrom.RbSr whole-rock systems of the Ben Vuirich granite are also strongly discordant, although 8 out of 13 data points scatter about an “errorchron” of 564 ± 24 m.y. with an initial⁸⁷Sr/⁸⁶Sr ratio of about 0.716. This is interpreted as a spurious result due to incomplete homogenization of Sr isotopes in the source region during partial fusion. Initial⁸⁷Sr/⁸⁶Sr ratios at the time of emplacement indicated by the zircon data ranged from 0.7173 to 0.7191. Whole-rock samples from the Dunfallandy Hill granite have Rb/Sr ratios 2–3 times higher than those from Ben Vuirich and define a reasonably good isochron age of 491 ± 15 m.y. with an initial⁸⁷Sr/⁸⁶Sr of 0.7185 ± 0.0008. This may date granite emplacement or subsequent resetting of the high Rb/Sr rocks during Caledonian metamorphism. RbSr systematics indicate that the crustal source regions of these and other Caledonian granites separated from the upper mantle at least ca. 800 m.y. ago and probably ca. 1300 m.y. ago, thus confirming the interpretation of the upper intersection age of the zircon UPb data.     

Nd isotopic systematics and chemistry of Central Australian sapphirine granulites: an example of rare earth element mobility

Lower Proterozoic sapphirine-bearing and associated granulites from Central Australia exhibit the greatest range of present-day¹⁴³Nd/¹⁴⁴Nd ratios (∈_Nd(O)= ?26.5 to +112.3) yet reported for rocks believed to be cogenetic. The Nd isotopic data and REE abundances of these rocks demonstrate extreme fractionation of the rare earths during the formation of stratiform CuPbZn sulfide deposits with which they are closely associated. Field relationships, petrography and chemistry of the sapphirine granulites suggest that their protoliths comprised chlorite-rich rocks which were generated by hydrothermal alteration of a range of rock types prior to metamorphism; calculations employing REE abundances of the sapphirine granulites and associated rocks, combined with bulk solid-fluid distribution coefficient data yield high fluid/rock ratios, consistent with a pre-metamorphic hydrothermal origin for the unusual REE patterns. The SmNd data for these rocks define an age of 1760±75Ma, which is significantly younger than the crust formation age of the terrain ( 2070±125Ma) but indistinguishable from the RbSr whole rock age for granulite facies metamorphism (1790±35Ma). These data are interpreted in terms of major hydrothermal fractionation of the rare earths shortly (perhaps tens of millions of years) before granulite facies metamorphism, followed by redistribution of Nd isotopes or small fractionations of the Sm/Nd ratio during the granulite facies event, and possibly also during intense retrogression which reset RbSr whole rock and UPb zircon and monazite systematics at about 1700 Ma.     

Problems of two-pyroxene geothermometry

It is possible to formulate a two-pyroxene (MgFeCa) geothermometer using non-ideal but disordered site models for the pyroxenes. Experimental data on any two of the following three sets are needed: (a) the site occupancy data in orthopyroxene, (b) the site occupancy data in clinopyroxene (both sets of data with varying composition and temperature), and (c) synthetic phase equilibrium data on coexisting pyroxenes. A geothermometer based on site occupancy data is very sensitive to small variations of composition and may be useless for common petrological purposes but immensely useful for studying disequilibrium and kinetic history of the rocks.     

Rubidium-strontium geochronology of a transect of the Chilean Andes between latitudes 45° and 46° S

Plutons of quartz diorite to adamellite composition give Rb-Sr total-rock and biotite calculated ages of 12 to 176 m.y. along a transect of the Chilean Andes between 45° and 46° S. Miocene biotite dates of quartz diorite from the Puerto Aisen area are interpreted as minimum ages related to probable local structural deformation (faulting). Along a north-south traverse of the Argentine Andes between 40° and 44° S, plutons of similar composition to the Chilean Andes give KAr and RbSr total-rock and mineral dates which range from late Paleozoic to Cretaceous. No systematic unidirectional migration of the radiometric ages is observed.     

Mineral age patterns in ca. 3700 my old rocks from West Greenland

KAr,⁴⁰Ar³⁹Ar and RbSr dates are reported for minerals from the ca. 3700 my-old Amîtsoq and Isua gneisses of the Godthaabsfjord area of West Greenland. KAr dates on biotites and hornblendes range from about 1900 to 3500 my, with hornblendes having a much narrower range (ca. 2250–2750 my) than biotites. One biotite from Isua gives an impossibly high KAr date of 4940 my.⁴⁰Ar³⁹Ar mineral dates are in close agreement with conventional KAr dates over the entire range of apparent age values. The presence of minor amounts of excess argon is observed in the hornblendes, but radiogenic and excess argon in the biotites are completely homogenised and cannot be differentiated.Rb-Sr measurements on biotites are closely concordant and show that all biotites were completely open to diffusion of radiogenic⁸⁷Sr at about 1600–1700 my. This is the first proof of a regional thermal event at this time in the Archaean of West Greenland, although similar dates are well known from the Proterozoic belts to the north and south.The evidence suggests that those KAr biotite dates greater than about 2700–2800 my result from excess radiogenic argon incorporated during a thermal event of about this age or, more probably, during the 1600–1700 my Sr isotope homogenisation event. Scatter of mineral dates below about 2700 my could also be due, at least in part, to overprinting by the 1600–1700 my event.KAr mineral dates and an Rb-Sr mineral isochron from a pegmatite associated with the last major rock-forming event in the Godthaabsfjord area, namely the Qo?rqut granite, indicate an age of emplacement of 2580 ± 30 my.     

Isotopic evidence for crustal contamination in the Karroo rhyolites of Swaziland

RbSr and Pb isotopic data are reported for some of the Karroo volcanics from Swaziland. Linear arrays of Karroo data are found in the RbSr and Pb/Pb isochron diagrams. Certain of these linear arrays, if interpreted as isochrons, give ages considerably in excess of the age normally accepted for the Karroo and must be interpreted as “erupted isochrons”. It is argued that these “erupted isochrons” represent mixing lines resulting from contamination of mantle-derived magma by continental crust. It is unlikely that the RbSr “erupted isochron” has any age significance because the Rb/Sr ratios were probably fractionated at the time or eruption, both during the contamination event and during subsequent fractionation. “Erupted isochrons” in the Pb/Pb isochron diagram are much more likely to have time significance because relatively recent U/Pb fractionation events do not significantly affect the Pb isotopic compositions. The Pb/Pb “erupted isochron” has an apparent age of 3260 ± 60 m y. This age is very similar to that of the Kaapvaal craton on which the volcanics rest and it is argued that the “erupted isochron” approximately dates the formation of the Kaapvaal craton.Six Pb isotope analyses of separated feldspar crystals are also reported. Three of these were demonstrably out of isotopic equilibrium with the magma at the time of eruption. This shows that isotopic heterogeneities on the scale of a few millimetres may be preserved between crystals and their host magma.The nature of the crust which contaminated the magma is considered and it seems most likely that the magma was contaminated by heterogeneous crust of uniform age.     

Hydrothermal Mn-deposits of the Franciscan Assemblage,II. Isotope and trace element geochemistry,and implications for hydrothermal convection at spreading centers

Major element, trace element and Sr, Nd, Pb and O isotopic data for a Franciscan Mn-deposit suggest an origin by seafloor hydrothermal circulation. Based onQ-mode factor analysis the cherts and Mn-lenses of the Blue Jay mine formed from a combination of 4 components representing 1 biogenic, 1 hydrothermal, and 2 detrital sources. RbSr, UThPb and O isotopic systematics in the Mn-lenses were affected by input from the hydrothermal circulation of material leached from the underlying basalts. Nd isotopic compositions in both cherts and Mn-lenses are identical and within the range measured for Pacific Ocean water suggesting the REE were not mobilized by hydrothermal activity. Correlation of δ¹⁸O with SiO₂ and MnO₂ in the Mn-lenses implies the lenses formed by simple mixing of hydrothermally derived Mn-oxides with seawater and biogenic silica. δ¹⁸O of the cherts is both uniform and depleted relative to DSDP Jurassic cherts but similar to microquartz-bearing cherts of the Monterey Formation: this suggests that diagenetic activity exerted more control on oxygen isotope compositions then hydrothermal alteration or metamorphism. Finally, a well defined RbSr isochron of158 ± 5Myr was obtained for these cherts and opens the possibility of determining absolute radiometric ages for similar cherts throughout the geologic record.     

Clinopyroxenes in Mesozoic pillow lavas from the French Alps: influence of cooling rate on compositional trends

The clinopyroxene compositions determined in four spilite samples from the French Alps show trends with high Al and Ti contents. Two samples from the center and outer zones of a single pillow have different Al/Ti ratios as well as different evolution trends in the CaFeMg system. Two samples collected from two different pillows in another outcrop also show different Al and Ti contents.These variations can be correlated with texture and grain size of the rocks and can be seen to follow trends predicted from recent cooling rate experiments. Analyses of relict clinopyroxenes from a glaucophane-lawsonite-bearing pillow show that these metastable clinopyroxenes can survive an episode of low-temperature, high-pressure metamorphism.     

A Nd and Sr isotopic study of the Trinity peridotite; implications for mantle evolution

Field evidence indicates that the Trinity peridotite was partially melted during its rise as a part of the upwelling convecting mantle at a spreading center. A SmNd mineral isochron for a plagioclase lherzolite yields an age,T = 427 ± 32 Ma and initialε_Nd = + 10.4 ? 0.4 which is distinctly higher than that expected for typical depleted mantle at this time. This age is interpreted as the time of crystallization of trapped melt in the plagioclase lherzoliteP-T field. This time of crystallization probably represents the time when the massif was incorporated as a part of the oceanic lithosphere. The SmNd model age of the plagioclase lherzolite totalrock isT_CHUR^Nd = 3.4 AE. This suggests that the Trinity peridotite was derived from a mantle that was depleted rather early in earth history. The peridotite contains many generations of pyroxenite dikes and some microgabbro dikes. We report data for two dikes that clearly crosscut the main metamorphic fabric of the peridotite. A microgabbro dike yields a SmNd mineral isochron age ofT = 435 ± 21 Ma andε_Nd = + 6.7 ? 0.3. A pyroxenite dike yields an initialε_Nd = + 7.3 ± 0.4. The initialε_Nd values for the pyroxenite and gabbro dikes are fairly similar to those for the depleted mantle at this time and are distinct from the lherzolite—demonstrating that they are not genetically related. RbSr data do not give any coherent pattern. However, some bounds can be put on initial Sr values ofε_Sr ? ?21 for the plagioclase lherzolite andε_Sr ? ?8.7 for the microgabbro dike. It is plausible that the dikes represent cumulates left behind from island arc magmas that rose through the the oceanic lithosphere within the vicinity of a subduction zone. Major and trace elements and SmNd isotopic data indicate a multiple stage history for the Trinity peridotite; a small melt fraction was extracted from an undepleted source ～ 3.4 AE or more ago to produce the proto-lherzolite; a large fraction of melt (～ 12 to 23%) was extracted from the proto-lherzolite to produce the present rock; the lherzolite was then crosscut by dikes from average depleted mantle ～ 0.44 AE ago. The data are compatible with the depleted mantle source being formed very early in earth history. Although most available data indicate that the depleted upper mantle has been relatively well stirred through time, the Trinity data suggest that very ancient Nd isotopic values are preserved and thus chemical and physical heteorgeneities are sometimes preserved in the depleted source of mid-ocean ridge basalts as well as the oceanic lithosphere which they intrude.     

Mesozoic structural-magmatic pattern and metallogeny of the western part of the Pacific belt

The distribution of magmatism and related endogenous metallogeny within short intervals of geological time displays strong lateral zonal pattern governed by the positions of contemporaneous eugeosynclines, i.e. previous oceanic basins. This pattern includes: (1) the eugeosyncline with ultramafics and mafics, and with Cu, Au, Cr, Pt; (2) the amagmatic back troughs filled by clastic sediments; (3) a zone of granite-granodiorite batholiths with Au, Mo; (4) a zone of diorite-monzonite with PbZn; (5) a zone of standard and LiF granites with Sn, W, Mo; and (6) a zone of alkaline plutons.The zones in (3)–(5) correspond to calc-alkaline volcanism, and the zone in (6) to alkaline volcanism. The zonal pattern is related to the activity along fossil Benioff zones. Great transversal faults displaced structural-magmatic and metallogenic zonality far inside continents. They are interpreted as transform faults. The existance of a zonal pattern is discussed in terms of plate tectonics.     

The relationship between major element chemistry and tectonic environment of basic and intermediate volcanic rocks

This work reports the preliminary results of a study of the relationship between tectonic environment and major element chemistry of volcanic rocks. Using a file of 8400 analyses of rocks of varying ages and geographic distribution it has been found that a simple ternary plot of MgOFeO(total)Al₂O₃ may be used to distinguish five tectonic environments: ocean ridge and floor, ocean island, continental, orogenic, and spreading center island. Subalkaline “basaltic-andesites” (51–56% SiO₂ calculated anhydrous) are used in this study. Alkaline rocks do not generally show the simple patterns of tholeiitic or subalkaline rocks. The ocean island suite is bimodal with one mode tholeiitic and the other alkaline. A bimodal age/chemistry relationship in the continental suite is tentatively related to different conditions during and after active breaking up of continents (rifting and drifting stages).Most of the Archean rocks in our file generally do not fall in the orogenic field. It is unlikely that modern island arc models can be generally applied to Archean greenstone belts. Locally, however, there may be Archean rocks analogous in some respect to modern calc-alkali rocks.     

The effect of initial230Th disequilibrium on young UPb ages: the Makalu case,Himalaya

Comparative UPb dating of zircon, xenotime and monazite from two different samples of the Himalayan “Makalu” granite shows the two U decay series to be in disequilibrium, particularly in monazite. This disequilibrium is due to excess or deficit amounts of radiogenic²⁰⁶Pb which originate from an excess or deficit of²³⁰Th, respectively, occurring initially in the mineral. Such an initial disequilibrium is caused by UTh fractionation between the crystallising mineral and the magma. Therefore, the UPb ages of Th-rich minerals such as monazite (and allanite) have to be corrected for excess²⁰⁶Pb due to excess²³⁰Th, whereas Th-poor minerals such as zircon and xenotime require a correction for a deficit of²⁰⁶Pb due to deficiency of²³⁰Th. The extent of this correction depends on the degree of ThU fractionation and on the age of the rock. For the two monazite populations analysed here, these excess amounts of²⁰⁶Pb were, with reference to the amount of radiogenic²⁰⁶Pb, 8–10% and 15–20% respectively, and less than 1% for zircon and xenotime. The varying degrees of Th enrichment relative to U in monazite show that the ThU partition coefficients for this mineral are not constant within a single granite. Furthermore, for monazite there is evidence for excess amounts of radiogenic²⁰⁷Pb originating from the decay of initial excess²³¹Pa, also enriched during crystal growth.The very low Th/U ratios of 0.196 and 0.167, determined for thetwo whole rocks from which the minerals have been extracted, substantiate the view that granite formation is a fundamental mechanism for ThU fractionation in continental crust.The different ages of 21.9 ± 0.2m.y. and24.0 ± 0.4m.y., obtained by averaging the corrected²³⁸U²⁰⁶Pb ages of the monazites, suggest that the apparently homogeneous Makalu granite was generated over a period of at least 2 m.y.     

The Palung granite (Himalaya); high-resolution UPb systematics in zircon and monazite

UPb analyses of fractions of zircon and monazite (3–8 grains each) and of single zircon grains resolve a lower Ordovician age of 470 ±4m.y. for the Palung granite which occurs in the High Himalayan nappes south of Kathmandu. Its thrusting during the Alpine orogeny under lower greenschist facies conditions did not affect the UPb systems in zircon and monazite. The granite crystallized from a magma which was mainly generated by anatexis of Precambrian continental crust. The magma was heterogeneous with respect to primary ages and/or metamorphic histories of the magma source rocks. This indicates either a derivation from (meta-) sediments or an intense mixing of different crustally derived magmas. The genesis of the Palung granite is possibly related to an orogeny which affected the Indian shield in lower Palaeozoic times. The detected inherited radiogenic lead in the Palung zircons occurs in perfectly homogeneous, transparent crystals; i.e. this radiogenic (“excess”) lead is not related to the presence of old, microscopically visible, overgrown zircon cores. The minimum ages of the inherited lead components range from about 800 to 1700 m.y.     

210Po and 210Pb distributions in ocean water profiles from the Eastern South Pacific

Two ocean profiles from the Peru Basin from regions with different surface productivities were analyzed for total²¹⁰Pb and²⁰¹Po to evaluate the influence of particulates in the water column on their distribution. Comparison with a published²²⁶Ra profile for the region was made. The profile closest to the coast, where upwelling and productivity are high, shows depletion of²¹⁰Pb relative to²²⁶Ra at all depths, with particularly marked excursions from radioactive equilibrium at the surface and in the bottom water.²¹⁰Po appears to be deficient relative to²¹⁰Pb at depth as well. Mean residence times in the deep water, relative to particulate removal from the water column to the sediments, of about 100 years for²¹⁰Pb and about two years for²¹⁰Po are indicated. The profile northwest of the upwelling region shows the²²⁶Ra²¹⁰Pb²¹⁰Po system close to equilibrium at all depths to 1500 m (except for the effect of atmospheric²¹⁰Pb input seen at the surface.     

Interpretation of a discordant KAr age pattern (Capo Vaticano,Calabria)

KAr age determination on whole rocks, biotites, quartz-feldspar separates and pegmatitic muscovites from a small quartz dioritic stock give a complex discordant age pattern. KAr dates from whole rocks and mineral separates define a single 116 my isochron with positive intercept, whereas muscovites from pegmatites fit a 180 my isochron with a probable negative intercept.Both ages are younger than the probable crystallization age of the stock (around 300 my), indicating a complex post-crystallization history. The fit of different mineral phases and whole rocks to a single isochron with positive intercept suggests that a thermal event caused rehomogenization of Ar among different mineral phases.     

RbSr dating of the Yatsushiro granite and gneiss,Kyushu, Japan

RbSr measurements on the Yatsushiro granite and gneiss, which had been considered stratigraphically to be of possible Precambrian age, are reported. The whole rock isochron for the granite gives an age of 352 ± 8 my with a low initial⁸⁷Sr/⁸⁶Sr ratio of 0.7037 ± 0.0006. Data for constituent minerals of the granites are dispersed irregularly around the whole rock isochron (possibly by later tectonic events). For the gneiss, a metamorphic event around 410 my is indicated by the muscovite RbSr ages. The present results do not support the possibility that the Yatsushiro granite and gneiss are Precambrian in age.