Journal of Petrology | Volume 38 | Number 11 | Pages 1461-1487 | 1997
© Oxford University Press 1997
Interlayered Eclogites, Blueschists and Epidote Amphibolites from NE Oman: a Record of Protolith Compositional Control and Limited Fluid Infiltration
1 Department of Earth Sciences, Sultan Qaboos University, P.O. Box 36, Al-Khod PC 123, Oman
2 Department of Geological Sciences, Southern Methodist University, Dallas, TX 75275, USA
3 Department of Geology and Earth Systems Science, Standford University, Stanford, CA 94305, USA
Received October 31, 1996; Revised typescript accepted June 9, 1997
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ABSTRACT |
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 TOP ABSTRACT IntroductionGeological SettingPetrography and Mineral...Whole-Rock Chemical DataStable Isotope DataP-T Conditions and P-T...Causes of InterlayeringComparison with Other High...Appendix a: Representative...References |
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Eclogites, blueschists, clinopyroxene-rich rocks and epidote amphibolites occur interlayered on a millimetre to decimetre scale in a mafic unit interbedded with calcareous and quartz mica shists in NE Oman. These rocks constitute part of the continental margin of Oman subducted during the Cretaceous. Petrological data indicate that they evolved through a clockwise P–T path and attained peak conditions at T
560°C, P >12 kbar, where the mineral assemblages of the blueschists, eclogites and clinopyroxene-rich rocks were all stable. Whole-rock major, trace and rare earth element data indicate that the mineralogical differences between these rock types resulted from the metamorphism of interlayered basaltic flows and tuffs of variable compositions possibly reflecting different degrees of magmatic differentiation. The interlayered epidote amphibolites are chemically similar to the blueschists and appear to have formed from the eclogites and blueschists during unroofing aided by fluid infiltration. Oxygen isotope data for minerals and whole rocks indicate low fluid:rock ratios during metamorphism and unroofing. The data also suggest that different layers may have interacted with fluids of different isotopic signatures throughout their history. These results are similar to data reported for other areas of continent–continent collision or continental crust subduction. KEY WORDS: blueschists; eclogites; epidote amphibolites; fluids; stable isotopes
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Introduction |
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TOPABSTRACTIntroductionGeological SettingPetrography and Mineral...Whole-Rock Chemical DataStable Isotope DataP-T Conditions and P-T...Causes of InterlayeringComparison with Other High...Appendix a: Representative...References |
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Interlayered blueschists, eclogites and/or greenschists are common in several areas such as Cazadero, California (e.g. Oh et al., 1991
), the Shuksan suite, Washington State (Dungan et al., 1983; Owen, 1989), the Tauern Window, Eastern Alps (e.g. Selverstone et al., 1992) and several Cycladic islands (e.g. Okrusch & Bröcker, 1990). This interlayering has been attributed to: (1) differences in the P–T conditions of equilibration in the different layers (e.g. Reinsch, 1979; Ridley, 1984); (2) original differences in the chemical compositions of the layers before metamorphism (e.g. Dungan et al., 1983; Oh et al., 1991); or (3) differences in the extent of retrograde overprinting as a result of variable fluid infiltration (e.g. Schliestedt & Matthews, 1987; Bröcker, 1990; Barrientos & Selverstone, 1993).
Evidence for these alternative hypotheses for any high-pressure, low-temperature (high P/T) metamorphic terrane or outcrop has traditionally been based on petrographic observations combined with whole-rock major and trace element data. Recent investigations have also relied on stable isotope, ion probe and fluid inclusion analysis, and have shed some light on fluid–rock interactions and the mobility of trace elements under high P/T conditions (e.g. Philippot & Selverstone, 1991; Getty & Selverstone, 1994). Integrating these results has led researchers to propose models of fluid flow in subduction zones and areas of continent–continent collision, and the effects of such fluids on magmatism (e.g. Sorensen, 1988; Selverstone et al., 1992; Philippot, 1993; Bebout & Barton, 1993).
Blueschists, eclogites, clinopyroxene-rich rocks and epidote amphibolites are interlayered on a millimetre to decimetre scale in metabasaltic outcrops of eastern Saih Hatat, NE Oman (Fig. 1). Although this area has been the focus of recent petrological and structural investigations (e.g. Le Métour et al., 1986, 1990; Goffé et al., 1988; El-Shazly et al., 1990; Searle et al., 1994), little attention has been paid to the reasons behind this ‘multifacial’ interlayering. In this study, we present petrological, geochemical and preliminary stable isotopic data on these interlayered rocks from two outcrops near the village of As-Sifah in eastern Saih Hatat. Our aim is to (1) provide constraints on the P–T evolution of these rocks; (2) identify the processes responsible for their interlayering, and (3) characterize the nature and amount of fluid interacting with different layers during the various stages of metamorphism. We also compare our results with data from other high P/T terranes in an attempt to relate tectonic setting to protolith bulk-rock compositions, extent of chemical and isotopic equilibration between phases during metamorphism, and fluid flow patterns during and after peak metamorphism.
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Geological Setting |
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TOPABSTRACTIntroductionGeological SettingPetrography and Mineral...Whole-Rock Chemical DataStable Isotope DataP-T Conditions and P-T...Causes of InterlayeringComparison with Other High...Appendix a: Representative...References |
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The continental margin of Oman was the site of major thrusting and folding during the Cretaceous, which culminated in the emplacement of the Semail ophiolite at the end of this period. The Saih Hatat area is a tectonic window that exposes basement and continental shelf units structurally underlying the Semail ophiolite. El-Shazly & Coleman, (1990)
subdivided Saih Hatat into three thrust-bounded tectonometamorphic regions (Fig. 1), each characterized by a set of lithotectonic units metamorphosed under different P–T conditions. The structurally lowest Region III consists of calcareous schists interbedded with quartz mica schists, mafic metavolcanics and minor lenses of Fe-rich metapelites, all of Permian protoliths (Le Métour et al., 1986). These units were subjected to high P–T metamorphism characteristic of the blueschist and eclogite facies, either before (El-Shazly & Lanphere, 1992) or during the emplacement of the ophiolite (e.g. Le Métour et al., 1986, 1990; Goffé et al., 1988; Searle et al., 1994). Three metamorphic zones were mapped in Region III, and define an eastward increase in metamorphic grade (Fig. 1; El-Shazly et al., 1990). Zone A is characterized by crossite epidote schists interlayered on a millimetre to decimetre scale with greenschists; zone B, by garnet glaucophane schists (garnet blueschists); and zone C by eclogites interlayered with garnet blueschists and epidote amphibolites. The extent of retrograde overprinting of these high P/T mafic rocks increases to the west; in zone A, pristine crossite epidote schists constitute only 10% of mafic outcrops, whereas in zone C eclogites+blueschists constitute 60% of similar outcrops. The intensity of deformation increases to the east; in zone A the units are folded into a series of closed to tight, inclined to inverted north-verging folds, whereas in zone C, the folds are nearly recumbent and verge to the ENE, with the metabasaltic unit locally boudinaged into a few megaboudins.
In this study, we focus on two outcrops from the highest grade metamorphic zone C (outcrops 1 and 2; Fig. 1). Outcrop ‘1’, located 2 km from the coast, is the same outcrop from which El-Shazly et al., (1990) collected most of their eclogite and garnet blueschist samples for which they reported peak metamorphic conditions of P >10–12 kbar and T of 500–580°C. Figure 2 is a detailed map of the sloping hill face of this outcrop. Outcrop ‘2’ represents a megaboudin of the mafic unit located 3 km north of As-Sifah along the coast, and consists of coarser-grained eclogites and garnet blueschists, considered by Searle et al., (1994) to have formed at pressures >20 kbar and T 550°C. The lithological and structural characteristics of both outcrops are similar; and include: (1) a layer-parallel foliation with local boudinaging of the eclogite layers, especially along the contact between the mafic unit and the calcareous mica schists, (2) two sets of fracture-filling veins of quartz±haematite±calcite or albite+haematite±calcite±quartz; the first running parallel to the foliation, the second trending NW–SE and dipping to the SW (Fig. 2), both with centimetre-wide greenschist facies alteration halos, and (3) recent east-directed landsliding.
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Petrography and Mineral Chemistry |
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TOPABSTRACTIntroductionGeological SettingPetrography and Mineral...Whole-Rock Chemical DataStable Isotope DataP-T Conditions and P-T...Causes of InterlayeringComparison with Other High...Appendix a: Representative...References |
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Analytical techniques
More than 40 samples representing all rock types in outcrops ‘1’ and ‘2’ (except the metapelites) were examined petrographically; their modes have been given by El-Shazly et al., (1990
, 1994) or are listed in Table 1. One of these samples (labelled ‘slab’; Table 1) consists of blueschists, eclogites and Cpx-rich rocks interlayered on a centimetre scale. Mineral analyses of six eclogite samples (three from each outcrop) were collected on a JEOL JXA-733 electron microprobe with on-line MAGIC-4 or ZAF correction programs, at Stanford and Southern Methodist University. Operating conditions were 15 kV and 15 nA sample current using a focused point beam (1 µm) in both laboratories for all minerals except white micas, which were analysed with a larger beam (3–5 µm). Well-characterized natural and synthetic oxides and silicates were used as standards, and were systematically analysed as unknowns to check for beam current drift.
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Mineral formulae for clinopyroxenes, micas and chlorites were normalized on the basis of a fixed number of cations (see Tables 3 and 4, below) using MINFILE (Afifi & Essene, 1988
). Mole percentages of the end-member components jadeite (Jd), acmite (Ac) and augite (Aug) for clinopyroxenes were calculated following the method described by El-Shazly, (1994). Sodic and sodic–calcic amphibole mineral formulae were normalized to 15 cations less K and 13 cations less Na+K+Ca, respectively, using the program AMPHIBOL (Richard & Clarke, 1990). Garnets (see Table 2) were normalized on the basis of 12 oxygen atoms assuming all Fe as Fe2+. This assumption is justified as the Fe3+ content of analysed garnets (calculated as 2 – Al) is minor and has an insignificant effect on the results of garnet–clinopyroxene geothermometry (<5°C).
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Eclogites
Pristine eclogites constitute
5% of the mafic body in outcrop ‘1’, but are considerably more abundant and better preserved in the coastal outcrop. They occur as layers (locally boudinaged) or fist-sized pods within the mafic or pelitic units and are of two types: granoblastic (e.g. As-92, As-58, C-26, C-85) and foliated (e.g. C-23, As-84, As-85, C-63–8; El-Shazly et al., 1990). In addition to garnet and clinopyroxene, both types of eclogites contain variable amounts of glaucophane, epidote, phengite and quartz, with minor paragonite and biotite (Table 1; El-Shazly et al., 1994).
Garnet is almandine rich (XAlm=0.65–0.7), and is progressively zoned in the granoblastic eclogites and unzoned in the foliated ones. Clinopyroxenes in both eclogite types are chloromelanites according to the classification of Essene & Fyfe, (1967) and contain a maximum jadeite content of 44 mol % (Table 3). Most matrix crystals are zoned from jadeite-rich cores to more acmitic rims, although many crystals show the reverse trend. In the foliated eclogites, clinopyroxene inclusions in garnet become progressively more enriched in jadeite (±augite) and depleted in acmite towards the garnet rims (Table 3; El-Shazly et al., 1990).
Glaucophane porphyroblasts are usually overgrown by winchite or barroisite (Table 4), and appear to have crystallized later than garnet and clinopyroxene. Quartz is ubiquitous in the matrix and as inclusions in garnet, but also occurs in segregations. Phengite is zoned with decreasing Si from core to rim (Table 5). Paragonite has been detected in sample As-59 only, where it is included in garnet. Biotite is a minor phase that occurs along the rims of garnet or phengite in variably retrograded samples (e.g. As-59, As-85, C-84), and is either interleaved with chlorite on a submicroscopic scale or is chloritized (C-84; Tables 5 and 6).
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Most of the granoblastic eclogites are not significantly retrograded, unlike the foliated samples which show variable degrees of overprinting represented by the replacement of garnet with chlorite+epidote+phengite+magnetite±albite along rims and fractures, and of matrix clinopyroxene with barroisite–winchite+albite+magnetite±epidote. In a few samples (e.g. C-84), clinopyroxenes are partially replaced by a mixture of calcite+talc+magnetite+albite. Although retrograded eclogites are not restricted to specific horizons or layers, they are most commonly found close to or along the contact between the mafic unit and the calcareous mica schists (e.g. As-85, C-84; Fig. 2).
Clinopyroxene-rich rocks
These granoblastic to weakly foliated rocks occur as centimetre to decimetre thick layers or boudins within the mafic unit, and are characterized by chloromelanitic clinopyroxene (XJd <0.42; Table 3) being the most abundant phase (35–60% of the rock mode). Sodic amphibole, phengite and rutile are ubiquitous phases, whereas minor amounts of garnet, quartz and epidote occur in a few samples (e.g. C-21, C-64; Table 1) and have the same chemical and textural characteristics as in the eclogites. Intergranular albite, fibrous winchite and, in a few cases, xenoblastic calcite and magnetite replace the clinopyroxene retrogressively, whereas the garnet is replaced by Mg-rich chlorite, epidote±phengite (e.g. C-64, C-63–9; Table 4). These rocks are therefore similar to the ‘omphacitites’ described by Okrusch & Bröcker, (1990) in the Cyclades, except for Cpx being richer in Fe3+ [chloromelanite according to Essene & Fyfe, (1967)].
Garnet blueschists
These rocks consist of glaucophane, epidote, garnet ± phengite ± minor clinopyroxene ± chlorite ±biotite±albite±quartz. Textural and mineral chemical details for these rocks have been given by El-Shazly et al., (1990).
Epidote amphibolites
The epidote amphibolites consist of sodic–calcic and calcic amphiboles, albite, chlorite, epidote±phengite±quartz±biotite, in addition to small amounts of relict glaucophane and garnet. Although these rocks contain albite rather than intermediate plagioclase and may therefore qualify for the definition of ‘greenschist’, they are herein termed ‘epidote amphibolites’ because the amphiboles are predominantly sodic–calcic barroisites [following the classification of Leake, (1978)], which are considered stable under high-pressure epidote amphibolite facies conditions (e.g. Ernst, 1979). Texturally, these rocks appear to have been derived from garnet blueschists and eclogites, as indicated by the occurrence of partially or completely pseudomorphed garnets, clinopyroxenes and sodic amphiboles.
Most epidote amphibolites are fine grained. Barroisite and winchite occur as prismatic or fibrous, randomly oriented or radiating crystals which may preserve relict glaucophane in their cores. The much less abundant actinolite is almost always fibrous, and is most common in veins and segregations. Epidote (pistacite 23–33 mol %; Table 6) usually replaces garnet, and contains inclusions of glaucophane, quartz and rutile. Chlorite is Mg rich (XMg=0.54–0.6; Table 6), and is usually restricted to the garnet pseudomorphs, although it is intergrown with albite in pseudomorphs after sodic(?) amphibole in a few samples. Albite occurs as anhedral intergranular crystals with inclusions of fibrous sodic–calcic amphiboles and titanite. Phengite is zoned from Si-rich cores to less siliceous rims (Table 5). Minor biotite replaces the cores of glaucophane in a few samples (Table 5). Some of this biotite is replaced by chlorite along rims (Tables 5 and 6). In most samples, the Fe-oxide resulting from the replacement of garnet is magnetite (Table 6). Rod-shaped or anhedral Ti-haematite (<3.5 mol % ilmenite; Table 6) also occurs, either as clusters in chlorite after garnet, or intergrown with fibrous sodic–calcic amphiboles that replace clinopyroxene.
Quartz and calcareous mica schists
The quartz mica schists occur as several layers of variable thicknesses and mineralogies in both outcrops (Fig. 2; Table 1). Phengite is ubiquitous, zoned with lower Si at rims (Table 5), whereas paragonite occurs in a few samples (e.g. C-164). Some layers contain intergranular albite+perthitic K-feldspar (e.g. C-37; C-147). Other minor constituents include zoisite (partially replaced by kaolinite; e.g. C-164), intergranular calcite and dolomite. The calcareous mica schists consist of calcite+phengite+paragonite, with small amounts of quartz and minor oxides (usually amorphous Fe-oxides pseudomorphing magnetite and/or haematite).
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Whole-Rock Chemical Data |
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TOPABSTRACTIntroductionGeological SettingPetrography and Mineral...Whole-Rock Chemical DataStable Isotope DataP-T Conditions and P-T...Causes of InterlayeringComparison with Other High...Appendix a: Representative...References |
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Analytical techniques
Samples were ground in a tungsten carbide mill and analysed for major and trace elements by X-ray fluorescence at the University of Tasmania, Australia, and XRAL, Toronto. Detection limits are 0.01% for major elements and 10 p.p.m. or better for trace elements. FeO values were determined by titrimetry following the method of Wilson, (1955)
. Results are reproducible to 0.1%. Two samples analysed at both laboratories showed negligible interlaboratory differences.
Analytical results
Some whole-rock major and trace element data were presented by El-Shazly et al., (1994); new data are listed in Table 7. Standard trace element discriminant diagrams and normalized rare earth element (REE) plots revealed that all samples are broadly tholeiitic with E-MORB (mid-ocean ridge basalt) affinities (El-Shazly et al., 1994). Plots of MgO versus other major element oxides as SiO2, total alkalis, FeO* or the relatively immobile minor and trace constituents P2O5, TiO2 and Zr (Fig. 3), show that the blueschists, Cpx-rich rocks and eclogites are chemically distinct. These diagrams also show broad correlations between the various oxides for all rock types (except the eclogites) which are similar to those shown by some tholeiitic series (e.g. Galapagos spreading centre; Clague et al., 1981). Whereas the blueschists have the highest MgO and lowest SiO2 and FeO* contents (similar to those of primitive tholeiites), the Cpx-rich rocks have intermediate values for these oxides and higher Fe2O3/FeO contents. The eclogites constitute a unique group of rocks that are least oxidized but most enriched in FeO*, TiO2, P2O5 and Zr, and depleted in MgO (Fig. 3). The unique character of these eclogites is further illustrated by their chondrite-normalized REE patterns, which, unlike those for the blueschists and Cpx-rich layers, are concave upwards (Fig. 4). On the other hand, the epidote amphibolites are largely similar to the blueschists; the anomalously high Zr and TiO2 contents of C-166 and C-171 are a consequence of the preservation of a relict eclogite component in these samples (Table 1).
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) and Cpx-rich rocks (
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Stable Isotope Data |
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TOPABSTRACTIntroductionGeological SettingPetrography and Mineral...Whole-Rock Chemical DataStable Isotope DataP-T Conditions and P-T...Causes of InterlayeringComparison with Other High...Appendix a: Representative...References |
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Analytical techniques
Minerals were separated by hand-picking after magnetic concentration using the Frantz isodynamic separator. Grains with inclusions were eliminated and the separates are generally considered 99% pure. Oxygen was extracted from silicates at Southern Methodist University (SMU) for all samples except C-169 and C-171 using ClF3 following the conventional method of Borthwick & Harmon, (1982)
, after the minerals or whole-rock powders were pre-fluorinated at 50 torr for 1–2 min. For samples C-169 and C-171, oxygen extraction was carried out at the University of Tasmania Hobart (UTH) using BrF5 (Clayton & Mayeda, 1963). CO2 from carbonates was released by overnight reaction with H3PO4 at 25°C following the method of McCrea, (1950). Isotopic analyses of the released gases were carried out on a Finnigan MAT Delta-E mass spectrometer at SMU and a series 2-SIRA at UTH. Gases from all samples (except C-171 and C-169) were extracted and analysed at least twice. All isotopic data are expressed in the notation relative to SMOW {
18OA=1000[(18O/16O)A – (18O/16O)SMOW]/(18O/16O)SMOW}, and were reproduced to ±0.2
. Oxygen was routinely extracted from a quartz working standard (SI-18; 18O=10.2±0.2) and analysed to monitor for systematic errors at SMU. Isotopic fractionation between minerals A and B is expressed as 
A–B=A - B 
1000ln 
A–B, where A–B is the fractionation factor.
Analytical results
Mineral and whole-rock data are presented in Table 7 and Fig. 5a and 5b for the different rock types of outcrops ‘1’ and ‘2’, respectively, and in Fig. 5c for the layer-parallel and crosscutting quartz and albite veins. In these figures, the analysed samples were grouped according to rock type and arranged in ‘stratigraphic order’ with samples from the ‘top’ of the section in each outcrop plotted at the far left of the diagrams (see Fig. 2). It should be noted that only whole-rock data for the epidote amphibolites are reported in Table 8 and Fig. 5; minerals were not separated from these samples owing to their fine-grained nature and textural complexity.
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Figure 5a and b shows that the different rock types and layers in both outcrops are characterized by different whole-rock
18O values, with the mafic rocks having values 2–4 higher than average MORB. Based on these whole-rock data and the 18O values for calcite (C-84 and C-87), it is clear that the calcareous and quartz mica schists have the highest 18O values. Figure 5a and b also shows that quartz from the eclogites is isotopically heavier than that from garnet blueschists, a pattern reflected by the whole-rock 18O data. Different layers of the same rock type are also characterized by different whole-rock or mineral separate 18O values, with the highest values usually recorded for layers closest to, or enclosed by the calcareous schists (e.g. compare 18OQz of C-37 with that of C-111 and C-30; Fig. 5a and b). Despite these differences, there is no systematic relationship between the mineral or whole-rock stable isotope data on one hand and the type of eclogite (whether granoblastic or foliated) on the other.
Figure 5a and b also shows that, with the exception of the mineral pair Qz and Cpx in eclogites and Cpx-rich rocks, isotopic fractionation between other coexisting minerals (e.g. glaucophane–phengite–quartz) is characterized by a wide scatter within samples of the same rock type. Despite this scatter, isotopic reversals are rare, recorded only between phengite and quartz in sample C-30 (collected from a layer sandwiched between a quartz vein and calcareous schists; Fig. 2), glaucophane and clinopyroxene in the retrograded eclogite As-85, and ‘phengite’ and clinopyroxene in C-84 (another retrograded eclogite). The last, however, is not a true reversal, as the ‘phengite’ separate included the retrogressively formed talc, a phase expected to be isotopically lighter than the clinopyroxene based on its lower chemical index (Garlick, 1969). On the other hand, in all eclogites (except retrograded sample As-85), the fractionation between quartz and clinopyroxene is constant at 3.75.
Neither the layer-parallel nor the crosscutting quartz veins show systematic differences in 18O values, which are either similar to, or slightly higher than the 18O values of their host rock quartz (Fig. 5a and c). The highest 18O values are recorded for those veins within the calcareous schists (e.g. C-32, Fig. 5c).
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P–T Conditions and P–T Evolution |
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TOPABSTRACTIntroductionGeological SettingPetrography and Mineral...Whole-Rock Chemical DataStable Isotope DataP-T Conditions and P-T...Causes of InterlayeringComparison with Other High...Appendix a: Representative...References |
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El-Shazly et al., (1990)
estimated the minimum peak pressures of metamorphism at 10–12 kbar based on the jadeite content of clinopyroxene coexisting with quartz, and suggested that maximum pressures may be constrained by the breakdown reaction of albite to pure jadeite+quartz. Based on KDFe–Mg values of 9.5–22.1 obtained for coexisting garnet and matrix clinopyroxene in six eclogites or eclogitic blueschists from outcrop ‘1’ (El-Shazly et al., 1990), and petrogenetic grid constraints (e.g. El-Shazly & Liou, 1991), El-Shazly & Lanphere, (1992) concluded that zone C blueschists and eclogites evolved through clockwise P–T paths with peak conditions of 500–560°C, P >10–12 kbar. These P–T conditions are similar to estimates by Goffé et al., (1988) and Le Métour et al., (1986, 1990) for the same rocks, but significantly different from those of Searle et al., (1994), who concluded that eclogites from outcrop ‘2’ were metamorphosed at P of 23±2.5 kbar and T of 540±75°C. Wendt et al., (1993) also favoured P >20 kbar, on the basis of modelling radial cracks in garnet from metapelites exposed a few km south of outcrop 2, although their calculations are equally compatible with P <15 kbar.
The petrological data presented in this study show that eclogites from outcrops ‘1’ and ‘2’ are similar mineralogically and petrographically. Garnet and clinopyroxene from mafic samples from both outcrops have similar compositions (XAlm 0.65, XJd <0.44) and zoning patterns (Tables 2 and 3). Where these two phases are in contact and have the same textural relations, they yield similar KDFe–Mg values in different samples from both outcrops. Garnet cores and their clinopyroxene inclusions therefore yield the highest KDFe–Mg values (20–35) in all samples, whereas garnet and matrix clinopyroxene rim pairs yield the lowest values (10–20; Table 9). Despite this systematic decrease of KDFe–Mg values from the garnet cores to their rims in all samples studied, the wide range of KDFe–Mg values (10–35) casts some doubt on the extent of equilibration of garnet and clinopyroxene. It is therefore worth while to compare KDFe–Mg temperatures with those obtained from isotopic fractionation.
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The large scatter in isotopic fractionation (
) between Qz, Ph and Gln within samples of the same rock type (Fig. 5) suggests that these phases did not equilibrate with the metamorphic fluid at the same temperature in all layers, and are therefore not useful for isotope geothermometry. The Qz–Mgt pair is also not suitable for constraining peak T, because magnetite is a product of the retrograde breakdown of clinopyroxene and/or garnet, and is either out of equilibrium with quartz or equilibrated with it during retrogression. On the other hand, the pair quartz and clinopyroxene seems suitable for isotope geothermometry, as both phases have the same relative modal proportions and are characterized by a constant of 3.75 in most eclogites and Cpx-rich rocks (Tables 1 and 8; Fig. 5).
Application of the empirical calibration of Matthews, (1994) for the Qz–Cpx isotope geothermometer [which is internally consistent with the experimental data of Chiba et al., (1989)], yields temperatures of 450–525°C for all samples from outcrops ‘1’ and ‘2’ except As-85 (Table 10). These temperatures are 30–40°C lower than temperatures calculated using the Ellis & Green, (1979) calibration of the Gt–Cpx Fe–Mg exchange geothermometer, but are significantly higher than those calculated using the Krogh, (1988) calibration [Table 9 and El-Shazly et al., (1990)]. Given that the diffusion coefficient of oxygen in Qz is higher than in Cpx (Farver, 1989; Farver & Yund, 1991), and that Cpx is much more abundant in these eclogites than Qz (Table 1), this mineral pair must have exchanged oxygen during cooling (see Eiler et al., 1993). Moreover, the compositional growth zoning of matrix Cpx makes it likely that it is also isotopically zoned. If so, the observed rimward increase in XJd would have been accompanied by an increase in 18O. Because we extracted oxygen by conventional fluorination of mineral grains, our 18O values for Cpx represent averages of its growth zones, which makes our Qz–Cpx values higher than expected for equilibrated rim pairs. The Qz–Cpx isotope geothermometer therefore only yields minimum temperatures, and the higher values calculated using the Gt–Cpx geothermometer of Ellis & Green, (1979) are herein considered the best estimates of peak T for zone C. The isotopic temperatures obtained for the retrograded eclogite As-85 (650°C; Table 10) are unreasonably high, and are best explained by modification of equilibrium 18O values by fluids that infiltrated this sample during retrogression.
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All of our mineral chemical and isotopic data indicate that outcrops ‘1’ and ‘2’ have had nearly identical P–T histories. We therefore estimate peak metamorphic conditions for zone C at
560°C, 12 kbar<P<15 kbar, based on the results of Gt–Cpx geothermometry, preservation of zoning in garnet, composition of clinopyroxene coexisting with quartz, and the occurrence of albite and lack of jadeite in quartz mica schists. The high pressures (23 kbar) favoured by Wendt et al., (1993) and Searle et al., (1994) for some outcrops in this zone are inconsistent with our data and phase relations in metapelites and quartz mica schists, particularly the occurrence of paragonite and lack of jadeitic clinopyroxene+chloritoid or +kyanite from these two rock types, respectively.
Combining the thermobarometric results listed above with the zoning patterns of garnet and clinopyroxene, the compositions of clinopyroxene inclusions in garnet, and preliminary results of isolated fluid inclusions in Cpx (El-Shazly & Sisson, submitted), leads to the conclusion that all eclogites and clinopyroxene-rich samples evolved through a clockwise P–T path with three distinct stages (Fig. 6). The first stage is one of increasing P and T defined by the crystallization of garnet cores and their inclusions at temperatures between 400 and 485°C and P >7 kbar, and garnet rims at 485–500°C and P >12 kbar. Stage 2 is characterized by a decrease of P, first accompanied by an increase in T to 560°C which resulted in the crystallization of the outermost rims of some garnets (e.g. C-64 and C-63–2; Table 9) and of glaucophane and phengite rims, as well as barroisitic amphiboles rimming glaucophane (stage 2a), then isothermally to 4 or 5 kbar (stage 2b; Fig. 6). The third stage is characterized by a decrease of P and T during which isotopic exchange between clinopyroxene, quartz and the intergranular fluid continued until their re-equilibration at T 450±65°C. Increased access of some layers to fluids during stages 2 and 3 led to the partial or complete replacement of clinopyroxene, garnet and glaucophane by barroisite or winchite+magnetite+albite+epidote+chlorite (e.g. As-85).
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Causes of Interlayering |
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TOPABSTRACTIntroductionGeological SettingPetrography and Mineral...Whole-Rock Chemical DataStable Isotope DataP-T Conditions and P-T...Causes of InterlayeringComparison with Other High...Appendix a: Representative...References |
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Our petrological data and thermobarometric results indicate that the eclogites and Cpx-rich layers of zone C had identical P–T histories. It is also clear that the assemblages of the interlayered blueschists, metapelites and quartz mica schists interbedded with the mafic unit are all stable at the peak P–T conditions estimated for zone C (e.g. El-Shazly et al., 1990
; El-Shazly & Liou, 1991). Along with the lack of any apparent structural breaks between the different layers or rock types (e.g. Fig. 2), these data rule out differences in the P–T conditions as the factor responsible for this interlayering, which may therefore be considered an ‘isofacial’ phenomenon. Accordingly, this interlayering must have been caused by either original differences in protolith chemistry, or variations in the amounts and/or compositions of fluids infiltrating these rocks during various stages of recrystallization.
Effects of protolith chemistry
Although the eclogites, Cpx-rich rocks, blueschists and epidote amphibolites are broadly similar in composition to tholeiites, there are significant chemical differences between these rock types, particularly in the concentrations of MgO, SiO2, FeO*, Fe2O3, Na2O+K2O, TiO2, P2O5 and Zr (Fig. 3). These differences may well be responsible for the observed interlayering, especially if it can be established that they were all inherited from the basaltic protoliths of the different rocks. Most mafic samples from outcrops ‘1’ and ‘2’ show good correlations between MgO and most other oxides (Fig. 3), which are similar to correlations exhibited by a differentiating tholeiitic magma. In Fig. 3, the blueschists have the highest MgO contents and are chemically distinct from the more ‘oxidized’ Cpx-rich rocks characterized by intermediate MgO and FeO* values. On the other hand, the eclogites have the highest FeO* and lowest MgO and Fe2O3/(Fe2O3+FeO) values, but do not fall on the same trend apparently defined by the other rock types (Fig. 3).
Because it is highly unlikely that metamorphism and alteration could result in good correlation patterns between elements with different chemical properties similar to those plotted in Fig. 3, we conclude that the chemical characteristics of the blueschists, eclogites and Cpx-rich layers reflect differences in the chemistry of their volcanic protoliths. Accordingly, the correlation patterns of Fig. 3 probably resulted from magmatic differentiation characterized by Fe enrichment and oxidation commonly observed in tholeiites (e.g. Carmichael & Ghiorso, 1990), whereas the scatter on these diagrams may be the result of sea-floor metamorphism or weathering of some layers before high P–T metamorphism. The blueschists are probably therefore products of metamorphism of the ‘least differentiated’ volcanics, whereas the Cpx-rich layers have an apparently more differentiated protolith with higher Fe2O3/(Fe2O3+FeO). Both rock types show light REE (LREE) enrichment relative to chondrites (Fig. 4b), with the Cpx-rich layers having a higher concentration of LREE compared with the blueschists, consistent with the latter having a ‘less differentiated’ protolith. The slight positive Eu anomaly characteristic of the Cpx-rich rocks may be due to the enrichment of their protoliths in plagioclase.
Although the eclogites are characterized by the lowest MgO and highest FeO*, TiO2, P2O5 and Zr values, and thus appear to have the ‘most differentiated’ protolith, their SiO2 contents and Fe2O3/(Fe2O3+FeO) ratios are much lower than those of the blueschists and Cpx-rich layers. The unique nature of these eclogites is further demonstrated by their enigmatic ‘concave-upward’ chondrite-normalized REE patterns (Fig. 4a). These features cannot be explained by either differentiation of the same tholeiitic magma from which the protoliths of the blueschists and Cpx-rich rocks formed (Fig. 3), or interaction with an LREE-rich fluid during retrogression, as both retrograded (e.g. As-85) and pristine (e.g. As-92) eclogites have similar concave-upward REE patterns (Fig. 4a). A possible explanation for these patterns is that the protolith of the eclogites resulted from mixing an LREE-enriched basaltic magma either with an Fe- and heavy REE (HREE)-enriched immiscible liquid or with an HREE-enriched cumulate (e.g. Shervais et al., 1988).
Effects of fluids and fluid infiltration
Although the interlayering of blueschists, eclogites and Cpx-rich rocks may be explained by metamorphism of layered basaltic flows and tuffs of different compositions, the occurrence of epidote amphibolites interlayered with these rocks cannot, as the epidote amphibolites are chemically similar to the blueschists (Table 7, Fig. 3). Textural relations in the epidote amphibolites indicate that they formed from either the eclogites or the blueschists through the replacement of Gt, Cpx and sodic amphibole by Chl, Ep, sodic–calcic amphibole, Mgt and Ab.
Using ‘representative’ compositions for clinopyroxene, garnet, chlorite, barroisite, glaucophane and winchite (see the Appendix), and end-member compositions for all other phases, two reactions are considered to have contributed to this transformation:
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According to these reactions, the transformation of eclogites and garnet blueschists to epidote amphibolites required the addition of H2O and resulted in the loss of silica, and an increase in volume and volatile content of the rock, but was otherwise isochemical.
Plots of mean oxide weight percent for blueschists or eclogites against the corresponding values for the epidote amphibolites [isocon diagrams of Grant, (1986), e.g. Fig. 7a] show that the blueschist–eclogite to epidote amphibolite transformation is isochemical for all oxides within 1
. However, these diagrams could be misleading because we did not distinguish between epidote amphibolites with a blueschist precursor and those formed from the chemically distinct eclogites (e.g. Fig. 3). Isocon diagrams in which the oxide contents of an individual epidote amphibolite sample are plotted against the corresponding values of its blueschist or eclogite precursor would therefore be more meaningful.
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Sample As-59 is an eclogite from a fist-sized pod enclosed by epidote amphibolites. As-59B is an epidote amphibolite collected
20 cm from As-59, and is characterized by textures which suggest that it was derived from an eclogite similar to As-59. This sample pair is therefore suitable for investigating element mobility associated with the eclogite to epidote amphibolite transformation. Assuming that this transformation involved an increase in volume but no change in mass, then according to Fig. 7b and c, it must have been accompanied by loss of small amounts of SiO2, FeO, TiO2, Zr, Y, Ce, La and Nb, and gain of Al2O3, CaO, MgO, volatile components, Cr and Ni. If Al2O3 is considered to have been immobile during this process (dashed lines, Fig. 7b and c), then this transformation would have also resulted in loss of small amounts of Fe2O3. In either case, the eclogite to epidote amphibolite transformation requires the addition of H2O and results in the removal of silica, consistent with the predictions of reactions (1) and (2). The loss of Zr, Y, Ce and La suggests that these elements were partially leached out from the eclogites by the infiltrating fluids, as they are incompatible with the newly formed chlorite, albite and sodic–calcic amphiboles, and as the amount of epidote formed at this stage was insufficient to accommodate all of the amounts released from the breakdown of garnet and clinopyroxene.
The interaction of mafic layers with a fluid phase before, during and after high P/T metamorphism is also supported by stable isotope data. High whole-rock 18O values for most mafic samples relative to their MORB protolith probably resulted from sea-floor metamorphism or weathering (which also caused major and trace element changes for some layers, manifested by the scatter in Fig. 3) before high P/T metamorphism. During stage 1 of high P/T metamorphism (Fig. 6), the mafic and pelitic units must have produced aqueous fluids by dehydration reactions. These fluids were possibly released to the surrounding calcareous and arenaceous units, trapped as fluid inclusions in eclogitic minerals, or formed quartz segregations. Although we cannot identify the nature or amount of this fluid, fluid:rock ratios were probably low during stage 1, as indicated by the whole-rock isotopic heterogeneity of the outcrops.
During stages 2 and 3, limited amounts of fluid were exchanged between some calcareous and mafic layers, as indicated by the slightly higher whole-rock and mineral 18O values for mafic layers in contact with the calcareous schists (Fig. 5a and b). In addition, several mafic layers were infiltrated by a larger amount of fluid during these stages, and were subsequently partially or completely transformed to epidote amphibolites. The isotopic signature of these infiltrating fluids varied from one layer to another (and over time?). The isotopic reversal between quartz and phengite in sample C-30 suggests that this quartz mica schist layer exchanged with considerable amounts of 18O-enriched fluids over a long period of time, a reasonable conclusion given its proximity to a layer-parallel quartz vein (C-29) and the calcareous schists (Fig. 2). Phengite, which has a higher diffusion coefficient than quartz (e.g. Eiler et al., 1993), exchanged O-isotopes more readily with the infiltrating fluids to become isotopically heavier (Fig. 5). Infiltration of some layers by 18O-rich fluids during stages 2 and 3 is also supported by the high whole-rock 18O value of some epidote amphibolite layers (e.g. C-171) compared with their blueschist and eclogite precursors (Fig. 5, Table 8).
Not all fluids infiltrating the mafic units during stages 2 and 3 were 18O enriched. The variable and smaller values of Qz–Cpx exhibited by some retrograded eclogites (e.g. As-85, Tables 8 and 10) compared with other samples require that either quartz exchanged with an 18O-depleted fluid or Cpx exchanged with an 18O-enriched fluid during retrogression. As the modal abundance of quartz in these eclogites is significantly lower than that of clinopyroxene (Table 1), and its diffusion coefficient is higher, it will exchange more readily with the metamorphic fluid than the coexisting Cpx (e.g. Eiler et al., 1993). It is therefore more likely that these samples exchanged with an 18O-depleted fluid. It should be noted that the lower whole-rock 18O value for As-59B compared with As-59 (Table 8, Fig. 5) suggests that the transformation of some eclogites to epidote amphibolites was indeed effected by ‘infiltration’ of 18O-depleted fluids.
The preservation of layers of blueschists and eclogites, along with the heterogeneity of whole-rock 18O values among different layers in the same outcrop clearly indicate that any fluid flow during stages 1–3 was channelized rather than pervasive, and that the outcrops correspond to the ‘fractured open system with partly open wall rocks’ of Oliver, (1996). Although the sources and initial isotopic compositions of the fluids are unknown, the 18O-depleted fluids may have been produced by dehydration reactions which took place in the mafic and pelitic layers during stage 1 at T <400–500°C (as dehydration at these temperatures is known to produce fluids that are slightly 18O depleted; e.g. Valley, 1986). These fluids may have been trapped in primary fluid inclusions in quartz, clinopyroxene and garnet, then released when some of these inclusions decrepitated during stage 2a (El-Shazly & Sisson, submitted), possibly contributing to the transformation of some eclogites and blueschists to epidote amphibolites (and the removal of SiO2).
On the other hand, the 18O-enriched fluids may have been externally derived, having acquired their 18O signature by exchanging with the isotopically heavy calcareous layers (Table 8, Fig. 5) before infiltrating the interbedded mafic rocks. Because calcareous schists are generally impermeable (e.g. Holness & Graham, 1995), such infiltration must have taken place either along shear zones during stage 2a, or along fractures after considerable uplift during the late stages of cooling of this terrane (stage 3). The latter type of flow ultimately formed the large layer-parallel and crosscutting quartz veins and their greenschist facies alteration halos. In all cases, fluid flow and/or infiltration appear to have occurred over a long period of time spanning parts of the prograde as well as uplift history of this high P/T terrane.
Fluid:rock ratios
The channelized nature of fluid flow dictates that the fluid:rock ratios vary from one layer to another in the same outcrop. Nevertheless, a very crude estimate of the fluid:rock ratio needed to effect the complete transformation of an eclogite to an epidote amphibolite can be obtained by calculating the amount of H2O necessary to cause the 3.23 wt % loss of silica associated with converting a sample such as As-59 to As-59B.
In the epidote amphibolites, phengite rims [Si 3.3 atoms per formula unit (p.f.u.)] are less siliceous than their cores. According to the experiments of Massonne & Schreyer, (1987), phengite crystallization must have occurred during a stage of depressurization and been completed at P >7.5 kbar (at T >450°C). Epidote amphibolite parageneses must have therefore crystallized at P between 7.5 kbar and peak pressures for the terrane during stage 2 (Fig. 6). However, the dissolution of silica which accompanied the transformation of eclogites to epidote amphibolites requires fluids flowing up-temperature, and is therefore considered to have taken place during stage 2a (Fig. 6) at T 550°C and P 8 kbar. Under these conditions, 1 kg of pure H2O can dissolve 6 g of silica according to the equation of Fournier & Potter, (1982). The dissolution of 3.23 g of silica from 100 g of eclogite would therefore require 538.3 g of pure H2O. Given that sample As-59 has a density of 3.3 g/cm3, and that the density of H2O at 550°C and 8 kbar is 0.95g/cm3, then the minimum volumetric fluid:rock ratio needed to effect the eclogite to epidote amphibolite transformation is 18:1. Larger ratios would be expected if the infiltrated fluids contained dissolved silica or if the transformation had taken place without any change in Al2O3 concentration (i.e. calculations would be carried out on a constant Al2O3 basis; dashed line of Fig. 7b).
The fluid:rock ratio estimated above is similar to estimates for shear zones and areas of limited hydrothermal circulation (e.g. Selverstone et al., 1991; Badger, 1993), but exceeds average fluid:rock ratios in regionally metamorphosed terranes. However, it should be remembered that this value is only a crude estimate of the minimum amount of fluid needed to completely convert an eclogite to an epidote amphibolite; the corresponding amount needed for the blueschist to epidote amphibolite transformation is expected to be much smaller given the chemical similarities between these two rock types (Fig. 7a). Because retrograded eclogites constitute only a small fraction of outcrops ‘1’ and ‘2’, the fluid:rock ratios integrated over the entire outcrop will be considerably smaller than 18:1. Moreover, the period over which fluids interacted with these rocks could have been as long as 20 Ma, the time elapsed between an assumed collisional event at 130–120 Ma and cooling of phengite through its closure temperature (El-Shazly & Lanphere, 1992). Accordingly, the time-integrated fluid flux may be small even though the estimated fluid:rock ratios for the amphibolitized eclogite layers and pods are large.
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Comparison with Other High P/T Terranes |
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TOPABSTRACTIntroductionGeological SettingPetrography and Mineral...Whole-Rock Chemical DataStable Isotope DataP-T Conditions and P-T...Causes of InterlayeringComparison with Other High...Appendix a: Representative...References |
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The reasons for interlayering between blueschists, eclogites, Cpx-rich rocks and epidote amphibolites in Saih Hatat are broadly similar to those proposed for multifacial interlayered rocks in other high P/T terranes. Interlayering between blueschists and eclogites in Ward Creek, California (Oh et al., 1991
), blueschists, eclogites and ‘omphacitites’ in Sifnos, Greece (e.g. Schliestedt, 1986), or blueschists and actinolitic greenschists in the Shuksan Suite, Washington State (Owen, 1989), and Ile de Groix, France (Barrientos & Selverstone, 1993) has been attributed to differences in protolith chemistries. Fluid infiltration is considered responsible for the overprinting of some blueschists by greenschists in Sifnos (Schliestedt & Matthews, 1987) and Tinos (Bröcker, 1990), and by chloritic greenschists in Ile de Groix (Barrientos & Selverstone, 1993).
Among these high P–T terranes, the Cycladic islands (Sifnos and Tinos) are similar to Oman. Both terranes represent areas where continental crust was subducted, and are characterized by similar lithologies and mineral assemblages with comparable P–T conditions and paths. However, in Sifnos, the Eocene blueschists and eclogites were partially overprinted by actinolitic greenschists in the Miocene either as a result of infiltration of small amounts of 18O-enriched fluids (e.g. Schliestedt & Matthews, 1987), or because of a thermal event (Avigad, 1993). In Tinos, Bröcker et al., (1993) concluded that there is no evidence for infiltration of large quantities of fluid, and that the overprinting of blueschists by greenschists was effected by the availability of limited amounts of synmetamorphic fluids. In all three occurrences, the stable isotope data indicate that fluid flow took place along open channels with partly open wall rocks according to the classification of Oliver, (1996).
Isotopic data for other eclogites and/or blueschists from areas of continent–continent collision such as western Norway (Agrinier et al., 1985), the Monviso and Sesia zone, Western Alps (Desmons & O'Neil, 1978; Nadeau et al., 1993), and the Tauern Window, Austrian Alps (Getty & Selverstone, 1994), lead to similar conclusions of limited flow of fluids (either internally or externally derived) across layer boundaries, and overall low fluid:rock ratios, corresponding to either the ‘closed’ or ‘open fractured with closed wall rocks’ systems of [Oliver, (1996). In contrast, Bebout & Barton, (1989, 1993) concluded that in Santa Catalina island, where oceanic crust was subducted at the continental margin of western North America, blueschists, amphibolites and ultramafics record patterns of pervasive fluid flow and equilibration of all units with the same fluid. Based on these contrasting results, we conclude that each high P/T metamorphic terrane has its own characteristics which are largely a function of the rock type being subducted and the P–T history of the area, and that fluid flow mechanisms from one terrane cannot be extrapolated to others, in accordance with the conclusions of Getty & Selverstone, (1994).
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Appendix a: Representative Mineral Compositions Used for Balancing Reactions (1) and (2) |
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TOPABSTRACTIntroductionGeological SettingPetrography and Mineral...Whole-Rock Chemical DataStable Isotope DataP-T Conditions and P-T...Causes of InterlayeringComparison with Other High...Appendix a: Representative...References |
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The following (Table A1) ‘representative’ compositions were used for balancing reactions (1) and (2):
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Acknowledgements |
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We thank R. Gregory for discussions, M. Colucci, G. Kuipers, K. Ferguson and S. Balsely for assistance with stable isotope analysis, D. Deuring, T. Tingle and R. Jones for assistance with microprobe analysis, P. Robinson for help with X-ray fluorescence analysis, and M. Al-Belushi for drafting Fig. 2. Detailed reviews by Dr M. Bröcker, Professor J. Selverstone and an anonymous reviewer helped improve the manuscript substantially. A review by Dr S. Sorensen, who disagrees with our interpretations, is acknowledged. We also thank M. Kassim and Dr H. Al-Azri of the Ministry of Petroleum and Minerals, Oman, for logistical support. Financial assistance was provided by US NSF Grants EAR 91–06016 to R. Gregory, El-Shazly and M. Holdaway, and EAR 92–04563 to J. G. Liou.
* Corresponding author. Address correspondence to Department of Earth Sciences, Sultan Qaboos University, P.O. Box 36, Al-Khod PC 123, Oman. Tel.: 968-515404. Fax: 968-513415. e-mail: aley{at}squ.edu.om
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References |
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