John N Ludden

Uranium-lead isotopic ages of plagiogranites from the Troodos ophiolite, Cyprus, and their tectonic significance

Zircon populations from two plagiogranites in the plutonic section of the Troodos ophiolite yield U-Pb dates between 90.3 ±0.7 and 92.4 ±0.7 Ma. Zircons from one of the two samples yield U-Pb dates of 90.3 ±0.7 and 92.1 ±0.7 Ma. This difference is greater than our analytical error, and the zircons are therefore considered to be slightly discordant. Two fractions from another sample are internally and externally concordant and have an average date of 91.6 ±1.4 Ma. This date is adopted as the age of plagiogranite crystallization. The plutonic section of the Troodos ophiolite is complex in that several generations of intrusion can be identified. The plagiogranites occur at the top of the intrusive section and represent fractionated residua of magma chambers related to the lower pillow lavas. They are possibly slightly older than small ultramafic layered complexes that represent the final intrusive episode within the Troodos ophiolite. There is good agreement between our results and the ages of radiolaria which occur in umbers intercalated with the upper pillow lavas of Troodos (Blome and Irwin, 1985). This shows that the 75 to 85 Ma K-Ar dates on the lavas and sheeted dikes (Desmet et al., 1978; Delaloye and Desmet, 1979; Delaloye et al., 1980; Staudigel et al., 1986) represent age resetting unrelated to ophiolite formation. The Troodos U-Pb zircon ages are slightly lower than U-Pb zircon ages from the Samail ophiolite in Oman, which vary between 93.5 and 97.9 Ma and average 95 Ma (Tilton et al., 1981). The small difference in age between the Troodos and the Samail ophiolites and the geochemical similarities of basalts from both complexes to island-arc volcanic rocks imply that a 3000-km axis of rift-related island arcs formed between 91 and 98 Ma. Emplacement of the ophiolites followed during the early Maastrichtian.

Lead geochronology of zircon by LaserProbe-inductively coupled plasma mass spectrometry (LP-ICPMS)

LaserProbe-inductively coupled plasma mass spectrometry (LP-ICPMS) provides a sensitive, fast, and simple means to determine 207Pb206Pb ages in single zircon grains. A Nd:YAG laser is used to irradiate the zircon surface and leaves a cylindrical pit of 30–60 μm, from which the vaporised materials are transported by argon gas to a Fisons-VG PQII+ ICPMS for analysis. No zircon abrasion, cleaning nor chemical separation procedures are required. The accuracy and the limitation of the method were evaluated by analyzing twenty-one zircon samples ranging in age from 2.7 Ga to 1.0 Ga, which have also been dated by the conventional U-Pb thermal ionization mass spectrometry technique (TIMS). The LP-ICPMS 207Pb206Pb ages for zircons with grain size > 60 μm and 207Pb concentration > 3 ppm are within 1% of the TIMS ages. Smaller zircons (≤60 μm) and those with 207Pb concentration < 2 ppm yield inaccurate ages. Operating the LP-ICPMS at conditions that give a compromise between the ideal spatial resolution and instrument sensitivity, the limits of detection were found to be 0.2 ppm for 206Pb, 207Pb, and 208Pb. The precision of the 207Pb206Pb ratio is generally 0.5–6% (1σ) from each sampling pit and is strongly dependent on the lead concentration. However, the precision for the average of the mean ratios from different pits in one grain or several grains of the same population are generally <1.5% (1σ). The results presented here demonstrate that the LP-ICPMS can be used to determine 207Pb206Pb ages of zircons and that reliable ages can be obtained from high quality, limpid zircons with a simple Pb-loss history. Uses of the technique include screening of zircon populations from different rocks in areas of poorly known age relationships and provenance studies of detrital zircons from ancient and modern sedimentary sequences. Other applications include the study of growth zones and of inherited components in complex zircon populations. In comparison with the SHRIMP (Sensitive High Resolution Ion MicroProbe) technique, the LP-ICPMS has the disadvantage of partially destroying the sample and inability to obtain accurate and precise UPb ratio at present. However, LP-ICPMS has better sensitivity, similar precision, but slightly worse spatial resolution for the measurement of 207Pb206Pb relative to the SHRIMP. Low purchasing and operating cost for the LP-ICPMS provide a simple and economic alternative to the SHRIMP method.

Accretion of Archean oceanic plateau fragments in the Abitibi, greenstone belt, Canada

The arc-arc collision model is often applied to the evolution of Archean greenstone belts. This model predicts concordant stratigraphic relations among the various lithologies evolving generally from komatiitic-tholeiitic affinities toward calc-alkalic affinities. New geological data from the Malartic composite block of the Abitibi greenstone belt indicate that this volcanic pile, rather than representing a conformable sequence, is composed of four distinct mafic lithotectonic domains that are overlain by a calc-alkalic sequence. The mafic domains are thought to be examples of accreted Archean oceanic plateau material that were deformed during their collage state. The younger calc-alkalic sequence represents extension-related volcanism that erupted through the ac- creted and deformed plateau material, possibly as a consequence of ridge subduction in a regime of oblique convergence. The relations described here may represent a common feature of Archean terranes.

The tectonic evolution of the Abitibi greenstone belt of Canada

Based on structural, geochemical, sedimentological and geochronological studies, we have formulated a model for the evolution of the late Archaean Abitibi greenstone belt of the Superior Province of Canada. The southern volcanic zone (SVZ) of the belt is dominated by komatiitic to tholeiitic volcanic plateaux and large, bimodal, mafic-felsic volcanic centres. These volcanic rocks were erupted between approximately 2710 Ma and 2700 Ma in a series of rift basins formed as a result of wrench-fault tectonics. The SVZ superimposes an older volcanic terrane which is characterized in the northern volcanic zone (NVZ) of the Abitibi belt and is approximately 2720 Ma or older. The NVZ comprises basaltic to andesitic and dacitic subaqueous massive volcanics which are cored by comagmatic sill complexes and layered mafic-anorthositic plutonic complexes. These volcanics are overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 ±2 Ma. The tectonic model envisages the SVZ to have formed in a series of rift basins which dissected an earlier formed volcanic arc (the NVZ). Analogous rift environments have been postulated for the Hokuroko basin of Japan, the Taupo volcanic zone of New Zealand and the Sumatra and Nicaragua arcs. The difference between rift related ‘submergent’ volcanism in the SVZ and ‘emergent’ volcanism in the NVZ resulted in the contrasting metallogenic styles, the former being characterized by syngenetic massive sulphide deposits, whilst the latter was dominated by epigenetic ‘porphyry-type’ Cu(Au) deposits.

Peeling oceanic crust in subduction zones

Thin slabs of oceanic pillow basalt formed at ridge axes with associated pelagic sediments are common features in ancient accretionary complexes. Estimates of the average thicknesses of scraped-off basaltic sections of oceanic crust in accretionary prisms vary from a few tens of metres to 300 m, with a mean thickness of 80–100 m in the complexes studied. The permeability contrast within the uppermost part of the oceanic crust is an important factor governing the thickness of delaminated oceanic crust. This mechanism is an effective means of separating oceanic crust altered at low temperatures from the remainder of the downgoing slab and thus controlling geochemical budgets in subduction zones.

A three dimensional perspective on the evolution of Archaean crust: LITHOPROBE seismic reflection images in the southwestern Superior province☆

In 1990–1991 the LITHOPROBE project completed 450 km of seismic reflection profiles across the late Archaean crust of the southwestern Superior province. The results define a broad three-fold division of crust: upper crust in the Abitibi greenstone belt is non-reflective and is a 6–8 km veneer of volcanic and plutonic supracrustal rocks, whereas, in the sediment-gneiss dominated Pontiac subprovince, upper crust comprises shallow northwest-dipping turbidite sequences; mid-crust, in both the Abitibi and the Pontiac subprovinces, is interpreted as imbricate sequences of metasedimentary and metaplutonic rocks; lower crust in both subprovinces has a horizontal layer parallel strycture which may represent interleaved mafic-intermediate gneisses. The seismic signature of the northern Abitibi greenstone belt may be represented in an exposed 25 km crustal section in the Kapuskasing stuctural zone. Preliminary tectonic models based on the seismic data are consistent with a plate-tectonic scenario involving oblique subduction and imbrication of sedimentary, plutonic and volcanic sequences. The northern Abitibi supracrustal sequences either represent an allochthon, or overlie an allochthonous underthrust metasedimentary and plutonic sequence which may be equivalent to a metasedimentary subprovince such as the Pontiac or Quetico. Seismic velocities have yet to be defined. However, crustal thicknesses are relatively constant at 35–40 km. The thinnest crust is adjacent to the Grenville Front where Moho is very well defined.

Mobilization of REE during crustal aging in the Troodos Ophiolite, Cyprus

REE data for secondary minerals from low- and high-temperature alteration environments in the Troodos Ophiolite have been determined using a combined mass spectrometry-neutron activation technique. Smectite- and analcime-REE mimic fresh rock profiles and have concentrations ranging from < 1–5 times chondritic values. Celadonite is depleted in LREE and total REE relative to the rocks. Groundmass-replacing chlorite and epidote are similar to, or enriched in, REE relative to fresh rocks. In contrast, fault-related chlorite and void-filling epidote are LREE-enriched with large positive Eu anomalies and yield comparable REE profiles to those of hydrothermal fluids venting at mid-ocean ridges. The zeolites, clinoptilolite and laumontite, are also LREE-enriched with REE contents significantly lower than rock values. Carbonate is enriched in HREE relative to LREE and concentrations vary from 0.2 to 18 times chondritic values. REE data for these minerals indicate that seawater-REE contents are modified by seawater-rock interaction in volcanic sequences and sheeted dykes in oceanic crust during axial hydrothermal alternation and subsequent crustal aging. The absolute and relative abundances of REE in seawater-derived solutions evolve in response to the precipitation of secondary minerals and the prevailing water/rock ratios. Rocks altered at low temperatures will show no change or a decrease in REE content; the LREE/HREE ratio will reflect the proportion of smectite, celadonite and carbonate in the groundmass assemblage. In contrast, rocks altered at high temperatures will show no change or an increase in REE; the LREE/HREE ratio will reflect the proportion of void-filling and groundmass-replacing chlorite and epidote in the rock.

Geologic evolution of the Late Archean Abitibi greenstone belt of Canada

The southern volcanic zone (SVZ) of the Late Archean Abitibi belt of the Superior province of Canada is dominated by komatiitic to tholeiitic volcanic plateaus and large, bimodal, mafic-felsic volcanic centers. These volcanic rocks were erupted between about 2717 and 2700 Ma in a series of rift basins that formed as a result of wrench-fault tectonics. They overlie and juxtapose a volcano-plutonic assemblage characterized in the northern Abitibi belt. The age of the assemblage is about 2720 Ma or older, and it comprises basaltic to andesitic and dacitic subaqueous massive volcanics, cored by comagmatic sills and layered anorthositic complexes and overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 ± 2 Ma. A tectonic model is proposed in which the SVZ formed in a series of rift basins that dissected an earlier formed volcanic arc. Comparisons are made with rift environments that have been postulated for Phanerozoic areas such as the Hokuroko basin of Japan, the Taupo volcanic zone of New Zealand, and the Sumatra and Nicaragua arcs.

Crystal fractionation and partial melting in the petrogenesis of a Proterozoic high-MgO volcanic suite, Ungava, Québec

There is little concensus on the relative importance of crystal fractionation and differential partial melting to the chemical diversity observed within most types of volcanic suites. A resolution to this controversy is best sought in suites containing high MgO lavas such as the Chukotat volcanics of the Proterozoic Cape Smith foldbelt, Ungava, Quebec. The succession of this volcanic suite consists of repetitive sequences, each beginning with olivine-phyric basalt (19-12 wt% MgO), grading upwards to pyroxene-phyric basalt (12-8 wt% MgO) and then, in later sequences, to plagioclase-phyric basalt (7-4 wt% MgO). Only the olivine-phyric basalts have compositions capable of equilibrating with the upper mantle and are believed to represent parental magmas for the suite. The pyroxene-phyric and plagioclase-phyric basalts represent magmas derived from these parents by the crystal fractionation of olivine, with minor chromite, clinopyroxene and plagioclase. The order of extrusion in each volcanic sequence is interpreted to reflect a density effect in which successively lighter, more evolved magmas are erupted as hydrostatic pressure wanes. The pyroxene-phyric basalts appear to have evolved at high levels in the active part of the conduit system as the eruption of their parents was in progress. The plagioclase-phyric basalts may represent residual liquids expelled from isolated reservoirs along the crust-mantle interface during the late stages of volcanic activity.A positive correlation between FeO and MgO in the early, most basic olivine-phyric basalts is interpreted to reflect progressive adiabatic partial melting in the upper mantle. Although this complicates the chemistry, it is not a significant factor in the compositional diversification of the volcanic suite. The preservation of such compositional melting effects, however, suggests that the most basic olivine-phyric basalts represent primitive magmas. The trace element characteristics of these magmas, and their derivatives, indicate that the mantle source for the Chukotat volcanics had experienced a previous melting event.

Rhyolitic volcanism and the geochemical evolution of an Archaean central ring complex: the Blake River Group volcanics of the southern Abitibi belt, Superior province

Abstract The stratigraphic and geochemical relations of the Blake River Group volcanics of the southern Abitibi metavolcanic belt are explained within the context of an Archaean ring complex. Tholeiitic eruptions were limited to the periphery of the complex, whilst the central complex was characterized by voluminous eruptions of calc-alkaline andesites and rhyolites. Relative to the tholeiitic lavas, these andesites are enriched in LIL-elements and are interpreted to represent contaminated tholeiitic magmas. The rhyolites associated with the central complex are high—SiO 2 (75–80% SiO 2 ) and show variable trace element enrichments. The development of such SiO 2 -rich rhyolite liquids is interpreted as taking place within a high-level, zoned magma chamber. The presence of the ore bodies in the Blake River Group is directly related to the rhyolitic volcanics and it is suggested that these ore bodies may, in part, represent magmatic hydrothermal fluids.