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Discussion |
Department of Geological Sciences & Geological Engineering, Queen's University, Kingston, Ontario K7L 3N6, Canada Canada–Nunavut Geoscience Office, 626 Tumiit Building, P.O. Box 2319, Iqaluit, Nunavut, X0 A 0H0, Canada Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Baker Atlas GEOScienceBaker Hughes Building, Stoneywood Park North, Dyce, Aberdeen AB21 7EA, UK
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A. H. Clark & H. A. I. Sandeman write: |
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 Top A. H. Clark &... A. P Nutman, D....References |
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We understand that the editorial handling of Nutman et al.’s paper overlapped with the publication in this journal of our detailed account of the petrogenesis of the Kennack Gneiss, which incorporates an evaluation of the tectonic context of this controversial unit on the basis of an IDTIMS single-zircon date (Sandeman et al. 2000). We consider, none the less, that several aspects of the new contribution require comment. These include Nutman et al.’s interpretations of our earlier documentation of the petrology and age relationships of the Kennack Gneiss (Sandeman et al. 1995), the Man of War Gneiss (Sandeman et al. 1997) and the Porthkerris Plagiogranite (Clark et al. 1998b), as well as our overview of the emplacement history of the Lizard Complex (Clark et al. 1998a), an extended abstract cited but not referenced by Nutman et al. We employ herein the revised Palaeozoic time-scale of McKerrow & van Staal (2000).
All ages are herein recorded with 2
errors (N.B. Nutman et al.’s fig. 2 represents SHRIMP dates with 1 and TIMS dates with 2 errors). The geological units in question are discussed in the sequence employed by Nutman et al.
Man of War Gneiss and Lizard Head Sill.
These latest Cambrian to earliest Ordovician meta-igneous rocks, which make up a considerable part of the Basal Unit of the Lizard Complex, are not central to Nutman et al.’s analysis, but are relevant to the later tectonic history of the unit. Nutman et al. record an ‘unpublished’ U–Pb zircon age of c. 500 Ma which confirms that the Lizard Head Sill ‘granodiorite’ was broadly coeval with the 499Ma Man of War Gneiss (Sandeman et al. 1997). Chemical data, however, indicate that the two felsic units are not interrelated in any simple fashion, despite the fact that the Lizard Head Sill, like the most silicic members of the Man of War Gneiss, is tonalitic, with trondhjemitic affinities (Sandeman et al. 1997). More critically, Nutman et al. demonstrate that the metasedimentary Old Lizard Head Series is at least 500 Ma old and therefore not an integral part of the "ophiolite", despite the MORB-like geochemistry of metabasites intercalated in the clastic succession (Kirby 1979; Sandeman et al. 1997).
More strictly germane to the main theme of the paper under discussion are the 40Ar/39Ar ages (Sandeman et al. 1997) for amphiboles from the Man of War Gneiss. The spectra, although disturbed, are interpreted by Sandeman et al. as indicating cooling at c. 374 Ma, ascribed to amphibolite-grade recrystallization, plausibly at the time of emplacement of the complex. This date, not referred to by Nutman et al., is sensibly identical to their imprecise 379 ± 50 Ma SHRIMP zircon age for metabasaltic Landewednack Hornblende Schist from Pen Olver, only 1.1 km ENE of the Polbream Cove Thrust (Jones 1994) that separates the hornblende schists from the Old Lizard Head Series.
Porthkerris metabasic rocks.
The polydeformed, amphibolite- to granulite-facies metagabbros and metabasalts of the Porthkerris area are interlayered with lherzolite tectonite (e.g. Green 1964b) and are interpreted as representing a transition from the Landewednack to the Traboe Hornblende Schist units, the latter of which widely borders the peridotite. Nutman et al. present a weighted-mean 387 ± 14 Ma 206Pb–238U SHRIMP date for metamorphic zircon in a metagabbro from a location [8050 2300] close to that of a leucogabbro which yielded a slightly disturbed 40Ar/39Ar hornblende age spectrum with a plateau age (84.3% of 39Ar released) of 381 ± 12 Ma (Clark et al. 1998a). These mutually concordant data are interpreted as defining the time of the youngest amphibolite-grade metamorphism experienced by this segment of the (probable) basal unit of the complex.
Traboe Hornblende Schists from west coast.
Nutman et al. record weighted-mean 206Pb/23UU dates of 396 ± 12 Ma and 386 ± 14 Ma for metamorphic zircons from, respectively, amphibolite-facies (Predannack Head) and granulite-facies (Lawarnick Pit) metabasic rocks. However, loss of radiogenic Pb as a result of a Early Carboniferous metamorphic overprint at c. 320 Ma was identified in both cases. This late event, or events, is apparently recorded by our 347 ± 9 Ma hornblende plateau age for a mylonitized Traboe Hornblende Schist member from Polurrian Cove and a pseudo-plateau of 306 ± 5 Ma for actinolite from a greenschist-facies metabasic lens in Old Lizard Head Series clastics at Venton Hill Point (Clark et al. 1998a).
Porthkerris plagiogranite.
Clark et al. (1998b) argue that a 397 ± 2 Ma TIMS zircon date for a plagiogranite from the Porthkerris Quarry defines the age of silicic magmatism contemporaneous with the intraoceanic extensional ductile shearing, mylonitization and amphibolite facies metamorphism documented by Gibbons & Thompson (1991). This Emsian age was therefore proposed as a best estimate for the time of generation of the Lizard Ophiolite.
Nutman et al. record this date, but cast doubt on its significance, stating (p. 819) that they ‘reserve judgement on the "plagiogranite" interpretation of this lithology, until more details on it are published. It could be part of the Kennack gneiss suite’. The textural relationships of the granitoid veins in this area (24 separate bodies were studied petrographically by A.H.C.) are variable: pre- to late-kinematic bodies display metamorphic fabrics with large, unzoned, porphyroclastic or phenoclastic feldspar grains in a decussate-granular feldspar–quartz groundmass, whereas post-kinematic veins exhibit, in many cases, oscillatorily-zoned phenocrysts in a similar matrix. In the former cases, the feldspar composition for both large and matrix grains falls in the range An35Ab62Or3–An28Ab71Or1, whereas the oscillatorily-zoned phenocrysts in the latter exhibit maximum core to rim zoning of An44Ab56–An6Ab92Or2. Microcline occurs only as a subordinate constituent of the locally abundant albite–quartz pegmatitic patches. The sampled Porthkerris granitoid rocks, including the dated sample (Clark et al. 1998b), are therefore tonalitic, with trondhjemitic affinities and, given the geological context, we consider it not unreasonable to refer to them as plagiogranitic. They are certainly petrographically and geochemically distinct from the felsic components of the Kennack Gneiss, which range from markedly peraluminous granodiorites and monzogranites to, particularly on the west coast, marginally metaluminous syenogranites (Sandeman et al. 2000). Alkali-feldspar-poor granitoids similar to the Porthkerris tonalites have not been documented in the Kennack Gneiss suite.
Kennack Gneiss.
Three of the six more precise SHRIMP U–Pb dates reported by Nutman et al. are for felsic components of the Kennack Gneiss, the inferred, weighted mean, ages ranging from 396 ± 20 Ma (Trevenwith) to 384 ± 8 Ma (east of Kennack Sands). The authors were apparently unaware of the documentation by Sandeman et al. (2000), which incorporates the most comprehensive petrological data available for this controversial assemblage, as well as a 376.4 ± 1.7 Ma zircon date for a granitic body from Kennack Sands. Sandeman et al. argue that this Famennian or late Frasnian age records the time of emplacement of volumes of commingling and mixing (hybridizing) granitic and basaltic magmas into the base of the Lizard Peridotite, shortly before c. 3–4 kbar amphibolite-facies metamorphism at c. 365 Ma (40Ar/39Ar cooling ages for metamorphic hornblende: Sandeman et al. 1995). These dates are similar to the Rb–Sr whole-rock isochron age, 369 ± 12 Ma, reported by Styles & Rundle (1984), and overlap within 2s error with Nutman et al.’s three SHRIMP dates. The mode of the latter dataset, however, is markedly older than the TIMS zircon age, and it was presumably this that led Nutman et al. to suggest that the Porthkerris plagiogranite may constitute a member of the Kennack Gneiss suite, an interpretation which we reject. Nutman et al.’s further inference, based on the occurrence of Proterozoic cores in their analysed zircons, that the Kennack Gneiss was emplaced after ‘stacking of the Lizard Peridotite on crust which melted to give the Kennack Gneiss granites’ (p. 809), is entirely in agreement with the conclusions of Sandeman et al. (2000), which drew on a full body of geochemical data, including a single zircon analysis yielding a 207Pb–206Pb age of 1389 Ma.
The origin of the marked difference in the two bodies of U–Pb zircon age data is uncertain, but we see no reason to question our precise TIMS date for this terminal magmatic event in the evolution of the Lizard Complex.
Discussion: generation and emplacement history of the Lizard Complex.
The major contribution of Cook et al. (2000) and Nutman et al. (2001) is the recognition that the Lizard Peridotite does not represent an integral upper mantle substrate of the clearly ophiolitic Crousa Downs troctolite–gabbro–dolerite assemblage, but a fragment of upper-mantle tectonite that was exhumed from c. 15 kbar/1250–1300 °C to < 8 kbar, < 900 °C prior to development of the suite of mafic plutons and dykes. None the less, a major impediment to clarification of the overall evolution of the complex is the absence of significant age constraints on the peridotite itself, other than the presumption that it is younger than the Man of War Gneiss–Lizard Head Sill complex. At the moment, therefore, the chronology is delimited by the time of emplacement of the Porthkerris plagiogranite and of the ductile deformation and largely amphibolite facies metamorphism of its metabasic country rocks. Here, Gibbons & Thompson's (1991) model of intra-oceanic deformation is not incompatible with Nutman et al.’s concept of deformation and metamorphism in the envelope of the peridotite during its exhumation and emplacement. The best estimate of the age of this event remains, we argue, the 397 ± 2 Ma zircon date determined by Clark et al. (1998b) for the plagiogranite. It is significant that Nutman et al., in their figure 2, provide an age of ‘>397Ma’ for peridotite emplacement, although their oldest SHRIMP date for metabasic rocks adjacent to the ultramafic body is 393 ± 12 Ma, and, moreover, they would prefer to assign the plagiogranite to the Kennack Gneiss, the youngest igneous component of the complex. We argue, therefore, that it is likely that all of the ultramafic/mafic rocks of the Goonhilly and Crousa Downs units are Emsian in age.
The subsequent metamorphic and deformational history of the complex remains partially constrained by our zircon date for the Kennack Gneiss at its type-locality, Kennack Sands. We maintain that the commingling magmas invaded the complex in the Famennian or late Frasnian, at 376.4 ± 1.7 Ma. The older, and less precise, SHRIMP dates provided for the Kennack Gneiss by Nutman et al. overlap analytically with the TIMS date, and it is possible that they record ‘gneiss intrusion’ as early as the Emsian. However, this would imply an extremely brief interval between the intra-oceanic events recorded by the Porthkerris plagiogranite and the emplacement of the complex over the Gramscatho flysch succession. We would prefer to accept the wide range of 40Ar/39Ar age plateaux for hornblendes and muscovites in the envelope of the peridotite as evidence of a relatively protracted history of metamorphism and deformation extending at least to 360 ± 10 Ma, as is suggested by the Ar/Ar dates for Old Lizard Head pelitic rocks from Porthallow and Polpeor Coves, as well as for the Man of War and Kennack Gneisses (Clark et al. 1998a). The younger age plateaux yielded by several hornblendes and whole-rock metapelites plausibly record the continued exhumation/cooling and/or metamorphism of the Lizard Complex until 347 ± 9 Ma, or even 305 ± 5 Ma (Clark et al. 1998a), in permissive agreement with the late loss of radiogenic Pb recorded for several samples by Nutman et al.
In conclusion, we: (i) contest the conflation of the Porthkerris plagiogranite and Kennack Gneiss proposed by Nutman et al.; (ii) question the significance of the 384 ± 8 to 396 ± 20 Ma SHRIMP dates determined by those authors for the granitoid members of the Kennack Gneiss; (iii) maintain our proposal that the wider Crousa Downs complex records a history of magmatism and metamorphism extending at least from 397 ± 2 Ma (Porthkerris plagiogranite intrusion) to c. 370 Ma (3–4 kbar amphibolite-facies metamorphism of Kennack Gneiss). This c. 30 million year, Emsian–Famennian, interval is indeed apparently in permissive agreement with the ‘380–400 Ma’, i.e., Frasnian–Emsian, period proposed by Nutman et al.. The latter chronology is, however, anchored at one extreme by our TIMS date for the Porthkerris granitoid rocks, and at the other accomodates the reported 1s error in the youngest SHRIMP dates for the Kennack Gneiss, two units which Nutman et al. suggest could record a single event. The apparent concordance between the two models is therefore illusory: they are ultimately based on entirely differing interpretations of key lithological units in the complex.
3 December 2001
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A. P Nutman, D. H. Green & A. C. Cook reply: |
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TopA. H. Clark &...A. P Nutman, D....References |
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(1) Extent and correlation of pre-400 Ma rocks at the Lizard.
In the attempt to deduce tectonic settings for components of the Lizard Complex, it is important to characterize each of the lithological units and to place both protolith formation and metamorphism (where observed) in a chronological framework. Both the studies by Sandeman et al. (2000) and Clark et al. (1998b) and our own study identify c. 390–400 Ma as an emplacement and recrystallization age for the ultramafic rocks. These studies also show that the last event in the sequence of Lizard igneous rocks, the Kennack Granite/Gneiss is of similar or slightly younger age. The identification of older (pre-400 Ma) rock units is important and our new data, included in the database to our paper (British Library Supplementary Publication No. SUP18165, but not discussed fully in the text of Nutman et al. (2001), is very relevant.
A valuable contribution by Sandeman et al. (1997) was to show that tonalitic gneisses on the Man of War rocks off the southern point of the Lizard have a U–Pb zircon date of 499 Ma. From their significantly greater age, felsic composition and trace element characteristics (e.g., Cook 1999) the Man of War Gneiss reflect a continental or continental margin setting and are not attributable to oceanic lithosphere. Onshore, the Old Lizard Head Series at the southern end of the Lizard consist of diverse micaceous schists and phyllites interlayered with amphibolite, which are intruded by granodiorite sheets, subsequently deformed. In our 2001 paper (p. 811 and fig. 2), we reported dates of c. 500 Ma on two of these granodioritic bodies (primary data are in British Library Supplementary Publication No. SUP18165. Also mentioned, with data given fully in the supplement was zircon geochronology on a thin, conformable biotite schist horizon in Landewednack Hornblende Schists (fig. 6 in Nutman et al. 2001). Such rocks are important in defining ages for crustal rocks underlying the peridotite. The geochronology shows that the two granodioritic bodies [6945 1154] in the Old Lizard Head Series have ages of 490 ± 7 and 488 ± 9 Ma (95% confidence, MSWDs <1.0), similar to Sandeman et al.'s (1997) date on the nearby Man of War Gneiss. The biotite schist layer in the Landewednack Hornblende Schists at Pen Olver [7130 1180] contained zircons with both composition and U–Th age matching those of the granodiorite sheets in the Old Lizard Head Series. The schist also contained older zircons (fig. 6 in Nutman et al. 2001). These older zircon cores had thin overgrowths interpreted as crystallizing in the 379 ± 25 Ma metamorphic event dated by metamorphic zircon in the enclosing Landewednack Hornblende Schist. The zircon populations in these samples are most simply interpreted as dating components of a nearby terrane which includes igneous activity of continental margin type at c. 490–500 Ma (Old Lizard Head Series and Man of War Gneiss). The Landewednack Hornblende Schist must have a protolith (basalts and mafic intrusives of MORB affinity (Cook 1999) with rare interlayered volcano-clastic and clastic sediments) deposited between c. 490 Ma and 400–390 Ma which is the peak metamorphic age of Traboe and Landewednack Hornblende Schists from dating metamorphic zircon (Nutman et al. 2001)
Clark & Sandeman noted in their comment that the granodiorite sheets in the Old Lizard Head Series, although indistinguishable in age from the tonalitic gneisses on the Man of War rocks 100–200 m offshore, were not cogenetic. Conversely, geochemical comparison (Cook 1999; Sandeman et al. 1997) reveals identical trace-element relative abundances, including rare earth elements, and indistinguishable initial 143Nd/144Nd ratios (data of M. Styles presented in Cook 1999). We therefore interpret both the diverse intrusive suite of the Man of War Gneiss and the granodioritic Lizard Head Sill as components of the same continental margin igneous activity, with the Old Lizard Head Series representing metasediments and possibly acid volcanics of similar or older age.
(2) Age of the Kennack Gneiss.
Sandeman et al. (2000) state "`A prerequisite for clarification of the geodynamic significance of the Lizard Complex is the delimitation of tectonothermal events during the c. 21 Ma hiatus between generation of the oceanic lithosphere (at 
397 ± 2 Ma: Clark et al. 1998b) and its emplacement onto the SE Avalonian margin"’ (page 1241). This 21 million year gap is derived from Sandeman et al.’s (2000) U/Pb zircon date of 376 Ma for intrusion of the Kennack Gneiss (following emplacement of the peridotite over crustal rocks) coupled with the Clark et al. (1998b) date of 397 ± 2 Ma for a felsic intrusion in Traboe Hornblende Schists named as a ‘plagiogranite’. On the other hand, Nutman et al. (2001) calculated U/Pb zircon dates of 396 ± 10 Ma, 391 ± 8 Ma and 391 ± 8 Ma for three Kennack Gneiss samples, which within their greater errors, are probably 15–20 million years older than the zircon date Sandeman et al. (2000) calculated for a Kennack Gneiss. This disparity between ages for the Kennack Gneiss was noted by Clark and Sandeman in their comment "`The origin of the marked difference in the two bodies of the U/Pb zircon age data is uncertain, but we see no reason to question our precise TIMS date for this terminal magmatic event"'. We accept their TIMS data but offer an alternative treatment of it, which produces an older calculated date of ca. 390 Ma and takes account of observed variations amongst the single, whole-grain zircons analysed (Sandeman et al. 2000).
(2.1) Comparisons between SHRIMP and TIMS analyses.
Further discussion on the geochronology of the Kennack Gneiss, requires some appreciation of the different analytical techniques used by us and by Sandeman et al. (2000). Our results were obtained by ion microprobe (SHRIMP) in situ analyses of c. 20 µm wide by <1 µm deep domains in zircons (which equates to sample size of c. 0.6 ng of zircon) in polished sections through multiple zircons mounted in an epoxy disc. The very small amount of sample means a small number of ions of Pb and U species are available for measurement, and hence counting statistics dictate enlargement of analytical errors. To circumvent this problem, numerous analyses are made of similar zircon domains, effectively to enlarge the sample size, and hence yield a pooled date with a smaller uncertainty. The SHRIMP in situ analyses do have an advantage in that pre-analysis cathodoluminescence (CL) imaging of the sectioned zircons is used to select apparently homogeneous, single-component domains. It is a characteristic of zircons that complex zoning in igneous zircons and overgrowths, dissolution and regrowth, and variably altered grains can be observed by CL and should be used in choosing areas for analysis. This spatial resolution effectively allows better ‘geological’ sampling (at the c. 25 µm scale). Thus, for a simple one-population and one-event zircon sample, the SHRIMP method often produces a less precise date than TIMS, but in complex zircons, the SHRIMP method will reveal and may resolve different growth zones or populations with sufficient accuracy to give insight into both sources and histories of host rocks.
In cutting edge TIMS analysis, single grains or grain fragments are used, with each sample weighing up to a few microgrammes. These samples are dissolved, and their chemically separated Pb and U are then measured. Because of the much greater sample size, analytical errors on each individual sample will be much smaller than individual SHRIMP measurements on similar zircon. If it is certain that the dissolved zircons are homogeneous in terms of their age and U–Pb isotopic systematics, then the greater precision of the TIMS method will give an accurate age determination of a geological event. However, if the zircons are not homogeneous (e.g. igneous zircons containing inherited older zircon cores from older rocks or zircons having variable radiogenic Pb loss due to damage in the crystal), the data obtained by TIMS can only be regarded as a ‘precise’ average of the age of several components/events recorded by the grain. To avoid this pitfall, some TIMS zircon geochronologists have taken to first making polished mounts of the zircons, then on the basis of CL imaging, selecting the most homogeneous, least structured ‘half’ grains. Sandeman et al. selected whole grains on the basis of external visual appearance, but this is not diagnostic for the presence or absence of internal structure reflecting cores and overgrowths.
(2.2) Reconciling TIMS versus SHRIMP dating of the Kennack Gneiss.
Sandeman et al. (2000) dated four zircons (up to 3 µg each) from a Kennack Gneiss sample. One of these (#1) was markedly discordant, with much older ages, which they rightly regarded as containing Proterozoic inheritance. The other 3 grains (#2, 3 and 4) gave dates of c. 350–400 Ma (Fig. 1). Of these, only one (with by far the largest analytical errors) is statistically indistinguishable from Concordia—the remaining two are discordant. Sandeman et al. (2000) chose a simple linear regression of these data, and obtained a lower concordia intercept of 376.4 ± 1.7 Ma, interpreted as giving the igneous age of the Kennack Gneiss. There are two assumptions implicit in this approach, which Sandeman et al. (2000) did not elaborate on. First, it implies there has been no modern loss of radiogenic Pb from their four grains. However, our multiple SHRIMP analyses on igneous zircons from other Kennack Gneiss samples show domains of younger Pb loss, even in zircon of the same U abundances as those dated by Sandeman et al. (e.g. sites 16.1 and 19.1 of sample 96/517 in table 1 of Nutman et al. 2001). Second, their interpretation of the regression requires that three and perhaps all four of their analysed zircons must contain amounts (large to small) of inherited zircon. In the absence of acknowledged post-magmatic Pb loss, this is necessary to account for the lack of concordance in at least three of the four zircons analysed. Therefore, we suggest that their zircon data must have three components (inheritance, igneous zircon and younger Pb loss), for which a simple linear regression of all four grains is not an appropriate treatment.
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(3) 397 ± 2 Ma plagiogranites?
In our paper we stated (p. 819) that we ‘reserve judgement on the plagiogranite interpretation of this lithology, until more details on it are published. It could be part of the Kennack gneiss suite.’ This was a note of caution, as there was little description of the rock in Clark et al. (1998b) dated at 397 ± 2 Ma. Clark & Sandeman now provide a more detailed petrographic description, although lacking geochemical data. From that description and our detailed observations in the Porthkerris area, we correlate this rock type with segregation veins and patches (neosomes) within the brown hornblende granulites (Traboe Hornblende Schists) showing intense syn-metamorphic deformation on subvertical axial planes in a narrow marginal zone around strongly deformed lherzolite (Green 1964a, b, c). Incipient melting at granulite facies conditions has been noted in this area as well as near the western, subvertical contacts of the recrystallized lherzolite with Traboe Hornblende Schists. Our zircon dates on these metamorphic rocks are 387 ± 7 and 393 ± 6 Ma (Nutman et al., 2001). Thus the coincidence of age of metamorphism in our study with that obtained by Clark & Sandeman is in our opinion, an excellent confirmation that the age of emplacement of the Lizard lherzolite within the Landewednack Hornblende Schists and the Traboe Hornblende Schists is at or very close to 397 Ma. The emplacement of lherzolite, retaining evidence of decompression from c. 1.2–1.5 GPa to <7 GPa, was followed by intrusion of troctolite and gabbro, and then by mafic dykes—all representing aspects of ‘oceanic’ or ‘ophiolitic’ tectonic setting. The dismemberment by thrusting of and within the Landewednack Hornblende Schists and ultra-mafic to mafic complex, carried the ‘oceanic/ophiolitic’ suite over the basement of Man of War Gneiss and Old Lizard Head Series. The Kennack Granite/Gneiss intrusion into the ultramafic rocks accompanied or was only slightly younger than the final mafic dyke suite but implies that the Lizard Complex was by that stage, already in place as an ‘oceanic’ terrane or ‘ophiolite’ overthrust on to a continental margin. The results of our own and the Sandeman & Clark studies are leading to the inference that the ‘oceanic’ setting may have been a Red Sea analogue (cf. Zabargad Lherzolite) and that a time-interval of about 10 million years was all that was required for closure and obduction on to neighbouring continental crust.
27 September 2002
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