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1 Department of Geology, Middlebury College, Middlebury, Vermont 05753, U.S.A.
2 Department of Earth Sciences, University of Maine, Orono, Maine 04469, U.S.A.
Correspondence: * E-mail: dwest{at}middlebury.edu
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ABSTRACT |
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 Top Abstract IntroductionGeologic settingField relationships and...Analytical methodsResultsDiscussionAcknowledgmentsReferences cited |
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Key Words: Metamorphic petrology • analysis • chemical (rock) • analysis • chemical (mineral) • geochronology • ironstone • Maine
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INTRODUCTION |
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TopAbstractIntroductionGeologic settingField relationships and...Analytical methodsResultsDiscussionAcknowledgmentsReferences cited |
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). Mineral assemblages in surrounding meta-pelites indicate these rocks were metamorphosed under low-pressure (below the Al2SiO5 triple point), amphibolite-facies conditions. Although considerable work has been published on phase relations in metamorphosed Fe-formations (see Klein 2005 for a recent summary), much less attention has been given to the more aluminous and manganiferous bulk compositions that are characteristic of the Wilson Cove unit. Additionally, recent work on grunerite+garnet-bearing amphibolites has led to a better understanding of amphibolite-facies phase relationships in the system CaO-FeO-MgO-Al2O3-SiO2-H2O (Zeh et al. 2005); however, this model system is not applicable to the Mn-rich bulk compositions of the Wilson Cove unit. Thus, detailed studies of these metamorphosed Fe- and Mn-rich bulk compositions provide an opportunity to apply the techniques of petrologic mineralogy (Guidotti and Sassi 2002) to the investigation of phase relationships in these unusual bulk compositions.
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In this contribution, we provide insight on the complete geologic history of the metamorphosed Fe- and Mn-rich rocks of the Wilson Cove unit through a multidisciplinary study involving: (1) detailed field and petrographic analysis; (2) mineral chemistry; (3) whole-rock major- and trace-element geochemistry; and (4) U-Pb geochronology. Our approach, long espoused by Charles V. Guidotti, is that there is great value in integrating detailed field and petrographic observations, mineralogy, and mineral chemistry as a function of bulk-rock composition, textural relationships between the minerals, and an understanding of the regional geologic relationships. This integrated approach takes "petrologic mineralogy" (Guidotti and Sassi 2002) a step further by not only providing insight into metamorphic petrogenesis of rocks, but also providing information on the depositional and tectonic setting of the protoliths.
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GEOLOGIC SETTING |
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TopAbstractIntroductionGeologic settingField relationships and...Analytical methodsResultsDiscussionAcknowledgmentsReferences cited |
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). The bedrock geology in this region is complex, having been multiply deformed, metamorphosed, and intruded by several generations of plutons during Silurian-Devonian (Acadian) orogenic activity (Tucker et al. 2001; Hussey and Berry 2002; West et al. 2003; Gerbi and West 2007). The stratified rocks in this region can be divided into several lithotectonic belts based on their ages and internal stratigraphy (Berry and Osberg 1989). Iron-rich rocks of the Wilson Cove Member of the Cushing Formation are located within the Liberty-Orrington belt—an approximately 170 km long northeast-trending belt of metamorphosed Ordovician volcanic and sedimentary rocks. Rocks within this belt have been subjected to multiple phases of ductile deformation and metamorphosed under amphibolite-facies conditions (Guidotti 1989).
Hussey (1971) first described the distinctive Fe-rich rocks of the Wilson Cove Member of the Cushing Formation in the Casco Bay region of southwestern Maine (Fig. 1). These rocks are interpreted to represent the uppermost member of the Ordovician Cushing Formation—a unit dominated by metamorphosed volcanic rocks (Hussey and Berry 2002). The Cushing Formation lies stratigraphically beneath the largely metasedimentary Cape Elizabeth Formation. In the Casco Bay region, the Wilson Cove Member shows considerable lithologic variability and ranges in thickness from 0 to 120 m (Hussey 1971, 1985).
In mapping an area approximately 75 km northeast of the Casco Bay region (Fig. 1), Pankiwskyj (1976) correlated lithologically similar metamorphosed Fe-rich rocks along the Cushing-Cape Elizabeth Formation contact with the Wilson Cove Member of the Cushing Formation, although these rocks were not continuous with those mapped by Hussey (1971). More detailed mapping in this region by West and Peterman (2004) (Fig. 2) has confirmed this correlation and shown the unit to be similarly thin and lithologically variable. In summary, mapping to date has shown that the Wilson Cove Member of the Cushing Formation is a distinctive but lithologically variable unit that is discontinuously exposed along the Cushing-Cape Elizabeth Formation contact for a distance of over 75 km (Fig. 1).
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FIELD RELATIONSHIPS AND PETROGRAPHY |
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TopAbstractIntroductionGeologic settingField relationships and...Analytical methodsResultsDiscussionAcknowledgmentsReferences cited |
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). The stratified rocks in this area have been regionally metamorphosed under amphibolite-facies conditions, and there is no field or petrographic evidence to suggest any pressure or temperature gradients within the immediate study area (Fig. 2). The most common rock type in the Wilson Cove unit is a dark-gray to black, fine- to coarse-grained, moderately to intensely rusty weathering, grunerite + garnet + quartz + biotite ± hornblende gneiss and granofels. Subordinate lithologies include rusty weathering biotite ± garnet schist and biotite-garnet-bearing quartzite. Where not obscured by deep rusty weathering, mineralogical layering (0.5 to 20 cm) is common and characterized by modal variations in garnet, grunerite, quartz, and biotite.
Euhedral to subhedral pale yellow-brown grunerite, up to 1.5 mm in length, is the most common mineral in the unit, but its modal abundance ranges from <5% in quartz-rich samples (Fig. 3a) to >60% in garnet rich gneisses (Fig. 3b). Poikilitic garnet is ubiquitous, but varies considerably in size (up to 2.5 mm in diameter), crystal form (euhedral to anhedral), and modal abundance (2 to 25%). Inclusions within garnet include quartz, grunerite, apatite, ilmenite, graphite, biotite, and in one sample, stilpnomelane and chlorite. Textures suggestive of multiple stages of growth and radiating patterns of inclusions ("wheel spokes") can be found in some garnets. Biotite up to 0.5 mm in length is modally abundant in quartz-rich samples (up to 15%) and is sparse to absent in samples rich in grunerite and garnet. Dark-green subhedral hornblende, up to 2.0 mm in length, is typically found intergrown with grunerite in samples containing moderate amounts of quartz and biotite. Pale-green hedenbergite, up to 3.0 mm in width, was found in association with hornblende in two samples. Subhedral to anhedral fayalite (Fig. 3c), up to 0.3 mm across, constitutes ~5 modal percent of one grunerite-garnet rich sample. Polygonal quartz ± albite aggregates are abundant (up to 30 modal percent) in some samples and absent in the more Fe- and Mn-rich bulk compositions.
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ANALYTICAL METHODS |
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TopAbstractIntroductionGeologic settingField relationships and...Analytical methodsResultsDiscussionAcknowledgmentsReferences cited |
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) prior to analysis. U-Pb isotopic analyses were carried out at the U.S.G.S.-Stanford sensitive high-resolution ion microprobe (SHRIMP) under standard operating conditions during sessions in November 2004 and January 2006. We used zircon standard R33 (419.3 ± 0.4 Ma; Black et al. 2004). Data were reduced with Squid (Ludwig 2001) and Isoplot/Ex (Ludwig 2003). All analytical data are reported in Table 1 and the ages shown in Figures 4b and 4c are reported as 206Pb/238U ages. Although the few ages >1 Ga would normally be calculated from the 207Pb/206Pb ratio, many of these older grains are very low in U, and consequently have a high 207Pb uncertainty.
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) was completed at Acme Analytical Laboratories Ltd. in Vancouver, British Columbia. Major elements were measured using ICP-emission spectrometry and trace elements were measured using ICP-mass spectrometry. Both methods were completed on fused rock powders and thus the negative loss on ignition (L.O.I.) values observed in some samples was due to the oxidation of Fe during the fusion process. FeO contents were determined separately by titration on non-fused samples; Fe2O3 was calculated by difference from the total Fe2O3 determined on the fused samples. These contents, listed separately in Table 2, are not included in the totals because of the difference in weight between fused and non-fused samples.
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peak (method given in Grew et. al. 2008). A minimum of five grains of each mineral were analyzed from a given sample and, with the exception of ilmenite, all major minerals exhibited nearly constant compositions within a sample. Each reported analysis represents the average of five spots on a single grain. Silicate and oxide standards were used, and calibrations were tested by analyzing mineral standards as unknowns. Data were processed using the X-Phi correction of Merlet (1994). |
RESULTS |
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). Cores are rounded to subangular and most show concentric zoning. In addition, cores are much lower in U and have higher Th/U ratios (Table 1). Rims are homogenous in CL imaging. There is strong textural discordance between the cores and rims.
Core ages range from ca. 463 to 2058 Ma, with a strong peak in the late Neoproterozoic-Early Cambrian (Figs. 4b and 4c). Post-SHRIMP imaging revealed that the two youngest core ages (spots 82 m-9b and 82 m-14b, gray ellipses in Fig. 4b) are mixed core-rim analyses and are therefore excluded from further discussion. Zircon rims produced a uniform 206Pb/238U age of 373 ± 4 Ma (2
) with extremely low Th/U ratios (Gerbi and West 2007). All rim analyses are concordant and tightly clustered, implying a single pervasive amphibolite-grade metamorphic event at that time (Gerbi and West 2007).
Whole-rock geochemistry
Standard "Harker-type" bivariate plots of major-element concentrations in the Wilson Cove rocks reveal the whole-rock geochemical variability in the samples analyzed (Fig. 5). SiO2 varies considerably from approximately 35 to 60 wt% and, as to be expected, SiO2 concentrations mirror modal amounts of quartz in individual samples. Concentrations of Al2O3 (5.2 to 12.6 wt%), MgO (1.4–4.2 wt%), CaO (0.7–15.1 wt%), Na2O (0.01–1.09 wt%), and K2O (0.04–2.73 wt%) are below average crustal abundances (all quoted average crustal abundances are from Taylor and McLennan 1985) in most of the individual samples analyzed. In contrast, Fe2O3(t), MnO, and P2O5 concentrations are notably elevated. Fe2O3(t) range between 15.3 to 43.5 wt% with the mean (29.6 wt%) being more than three times above the average crustal abundance (9.1 wt%). FeO determinations (Table 2) indicate that Fe in the Wilson Cove samples is predominantly ferrous. The presence of ilmenite without hematite exsolution, graphite, sparse magnetite, and low apparent Fe2O3 in garnet are consistent with this finding. MnO concentrations show considerable variability between 0.12 to 12.07 wt% with the mean (2.87 wt%) being more than an order of magnitude greater than average MnO crustal abundances (0.18 wt%). Concentrations of P2O5 vary as a function of modal apatite and range from 0.13 to 3.41 wt% (mean = 0.98 wt%). In summary, the major-element chemistry confirms the extreme variability of the Wilson Cove rocks evident in the field and in the mineral assemblages.
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Mineral chemistry
Amphibole.
All the analyzed samples contain grunerite, and sample 82-m also contains calcic amphibole (Table 3). Grunerite XMg [= atomic Mg/(Mg + Fe)] ranges from 0.01 to 0.36. Manganese contents vary from 0.75 atoms per formula unit (apfu) in sample NB-2i to below the limit of detection in sample NB-4b, and thus none of the analyzed grunerite can be classified as manganogrunerite according to Leake et al. (1997). CaO and Al2O3 vary from 0.2 to 0.9 wt% and 0.3 to 1.0 wt%, respectively.
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Garnet.
Garnet compositions vary significantly from sample to sample but are uniform in composition within a given sample (Table 4). Almandine is dominant. Low Fe+3 contents are suggested by formula recalculations assuming 16 cations and 24 O atoms. Garnet rim and core compositions are quite similar within a sample and no systematic variations of Ca, Mn, or XMg are present. Garnets from samples 82-b, NB-1a, and apatite veins in NB-2f have core Y2O3-contents varying from 0.05 to 0.5 wt% with undetectable Y2O3 in the rims. Texturally distinct garnet grains from within an apatite micro-bed in sample NB-2f are virtually identical in composition to garnet grains in the rest of the sample.
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. Biotite is found in all samples except NB-2i. XMg in biotite varies from 0.08 to 0.25, whereas MnO-content varies from below detection in NB-4b to 0.52 wt% in NB-1a. Biotite contains little F, Cl, and Na, but considerable TiO2 (2.66–5.37 wt%). In addition, biotite is noticeably enriched in BaO, which varies from 1.08 to 5.19 wt%.
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Other minerals.
Manganese-rich fayalite, (Fe1.44Mg0.13Mn0.39) SiO4, is present only in NB-2i where it is in textural equilibrium with garnet and grunerite (Fig. 3c). All analyzed samples contain ilmenite except NB-2b, whereas only samples NB-2d and NB-2i contain magnetite. The MnO content of ilmenite varies from 0.29 to 14.35 wt%. All ilmenite grains have 0.1 to 0.2 wt% Nb2O5 and many of the samples contain detectable amounts of ZnO. Magnetite is relatively pure in NB-2d but contains up to 0.59 wt% TiO2, 0.55 wt% MnO, and 0.33 wt% Al2O3 in sample NB-2i. Tourmaline, which is found only in sample NB-4b, is schorl (Na0.43Ca0.24)(Fe3.05 Mg0.20Ti0.09)Al5.68B3.1Si5.86O27(OH)4 with MnO, F, and Cl below detection limits.
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DISCUSSION |
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TopAbstractIntroductionGeologic settingField relationships and...Analytical methodsResultsDiscussionAcknowledgmentsReferences cited |
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Although the Ordovician is known to be one of the greatest periods of ooidal ironstone deposition in Earth history (Van Houten and Arthur 1989), the geochemistry of the Wilson Cove rocks is inconsistent with this type of depositional setting. Specifically, ooidal ironstones are typically characterized by significantly lower SiO2 and MnO, and higher P2O5 concentrations (Mücke and Farshad 2005) than are found in the Wilson Cove samples. Additionally, ooidal ironstones typically contain very little detrital clastic material (Bhattacharyya and Crerar 1993), a component we will argue is abundant in many of the Wilson Cove rocks (see below). Thus, several lines of evidence suggest that the metamorphosed Fe- and Mn-rich rocks of the Wilson Cove unit were not originally deposited as ooidal ironstones.
Distinguishing a hydrogenous from a submarine hydrothermal origin is made possible by applying geochemical discrimination diagrams. The geochemical data from the Wilson Cove rocks plot consistently within the field of submarine-hydrothermal deposits (Fig. 6a) because elements known to be concentrated through hydrogenous processes, e.g., Ni and Co (Usui and Someya 1997), are relatively low in the Wilson Cove rocks. Patterns of rare earth elements (REE) in Fe-Mn deposits have also been used to distinguish between hydrothermal and hydrogenous sources (e.g., Nath et al. 1997). Chondrite-normalized REE patterns of most Wilson Cove samples are relatively consistent and characterized by relative enrichment in light REEs, a slight enrichment in Ce, and a negative Eu anomaly. Although a negative Ce anomaly is characteristic of many hydrothermal Fe-Mn deposits (Elderfield and Greaves 1981), its absence does not preclude a hydrothermal origin; Ce behavior depends on proximity of the hydrothermal source, local redox conditions, and/or contamination by a hydrogenetic or detrital source (Toth 1980; Hein et al. 1994). Overall, most geochemical signatures from the Wilson Cove rocks are supportive of a submarine hydrothermal origin. Specifically, modern submarine volcanogenic exhalations are often rich in both Fe and Mn (e.g., Bonatti et al. 1972; Barrett 1987; Buatier et al. 2004) and the relatively high amounts of As, Ba, and base metals are consistent with a hydrothermal origin (e.g., Price and Pichler 2005; Canet et al. 2005).
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). Positive correlations between elements that are commonly associated with detrital contributions (e.g., Al2O3, TiO2, and Zr; Fig. 7a) are supportive of a detrital component. In addition, the light REE enrichments and negative Eu anomalies present in the Wilson Cove samples (Fig. 6b) are typical of sedimentary material (Taylor and McLennan 1985). The relative contributions of hydrothermal vs. detrital sediment are further illustrated in Figure 7b where the Wilson Cove whole-rock compositions plot on a mixing curve between metalliferous and detrital sediments. Thus, both the lithologic and geochemical variability of the Wilson Cove rocks most likely reflects different proportions of material derived from hydrothermal exhalations and terrestrially derived detritus.
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) constrains the depositional age of the ironstone to latest Middle Ordovician (Llanvirn) or younger. This age is somewhat younger than, but considering analytical uncertainties, is consistent with the age reported for the underlying Peaks Island Member of the Cushing Formation (471 ± 3 Ma, Hussey and Berry 2002). In addition, Middle Ordovician ironstones and hydrothermal sedimentary rocks similar in composition to that of the Wilson Cove are found along strike to the northeast in the Miramichi highlands of northern New Brunswick (Bathurst Mining Camp: Peter and Goodfellow 1996; Peter et al. 2003). Interestingly, these likely correlative rocks are divided into two types in New Brunswick: (1) older (~469 Ma) carbonate-bearing ironstones associated with massive sulfide deposits, and (2) younger (465–469 Ma) Mn-rich ironstones not associated with massive sulfides (Goodfellow et al. 2003). The Wilson Cove geochemistry and U-Pb geochronology, along with the absence of significant sulfides, suggest these rocks correlate with the latter type defined by Goodfellow et al. (2003).
The distribution of detrital zircon core ages, with a dominant Neoproterozoic population and a low density of Grenville ages, suggests that the sediment was derived from peri-Gondwanan material (Fig. 4c). Major peaks in the age distribution of zircon grains shed from pre-Appalachian Laurentia fall in the ranges 900–1200, 1600–1800, and 2400–2600 Ma (e.g., Cawood and Nemchin 2001). In contrast, Gondwanan-derived material contains ages in some or all of the following ranges: 600–800, 1200–1600, and 2000–2300 Ma (e.g., van Staal et al. 2004). A peri-Gondwanan (i.e., Gander) source for rocks of Casco Bay Group is consistent with the interpretations of West et al. (2004), which were based on the isotopic compositions of metamorphosed volcanic rocks just to the east of this study area.
Marine hydrothermal exhalative deposits mixed with terrigenous clastic sediment form in rather restricted tectonic settings and this knowledge, combined with the depositional age and sedimentary provenance information provided by the U-Pb zircon data, place important constraints on the tectonics of the Liberty-Orrington belt in Middle Ordovician time. Fe-Mn rich hydrothermal deposition has been documented in a variety of modern tectonic settings including submarine volcanic arc and backarc rift systems (Usui and Someya 1997), triple junctions (Nath et al. 1997), and continental margin extension (Canet et al. 2005). The Wilson Cove unit lies stratigraphically above a sequence of metamorphosed subaqueous felsic to intermediate volcanic rocks (Hussey and Berry 2002; Levy et al. 2003) and below a thick sequence of metamorphosed volcanogenic sedimentary rocks (Cape Elizabeth Formation). This association is consistent with an active oceanic arc or back-arc environment, with the abundance of terrestrial detritus suggesting proximity to an actively eroding continental source.
As discussed above, meta-ironstones of the Wilson Cove Member of the Cushing Formation are correlative with rocks of similar age and composition in northern New Brunswick, which van Staal et al. (2003) have interpreted to have been deposited in an evolving back-arc basin immediately adjacent to peri-Gondwanan (Ganderian) crust. Additionally, West et al. (2004) suggested that the Liberty-Orrington belt in Maine, which is composed largely of metamorphosed volcanic and volcanogenic sedimentary rocks, represents a southern extension of this Middle Ordovician back-arc basin. The Wilson Cove whole-rock geochemistry and U-Pb geochronology presented here are consistent with the existence of a Middle Ordovician back-arc basin adjacent to a peri-Gondwanan crustal source.
Metamorphism and phase relationships
West et al. (2003), in a detailed study of pelitic rocks approximately 10 km to the east of the present study site, concluded that the area was subjected to regional metamorphism under low-pressure, amphibolite-facies conditions (specifically 550–600 °C and 3–4 kbar based on petrogenetic grid constraints). The mineral assemblages observed in the Wilson Cove unit (e.g., garnet + grunerite) are consistent with these pressure-temperature conditions (Zeh et al. 2005). Although the grunerite + garnet + fayalite assemblage found in the Wilson Cove rocks is only stable in a very narrow stability field in the petrogenetic grid of Zeh et al. (2005), it occurs only in the most manganiferous sample analyzed (NB-2i = 12.1 wt% MnO). Thus, a more precise pressure-temperature estimation based solely on the Wilson Cove mineral assemblages is hindered by components (e.g., MnO, K2O) not considered in available petrogenetic grids for Fe-rich rocks (e.g., the CaO-FeO-MgO-Al2O3-SiO2-H2O petrogenetic grid of Zeh et al. 2005).
U-Pb SHRIMP analysis of metamorphic zircon rims from the Wilson Cove unit (Fig. 4) indicate that this metamorphism took place in Middle to Late Devonian time (373 ± 4 Ma), consistent with the later stages of Acadian orogenesis in the region (West et al. 1995; Gerbi and West 2007). Although textural evidence for polymetamorphism is widespread in the region (Novak and Holdaway 1981; West et al. 2003), our detailed studies of the Wilson Cove unit have revealed little indication of more than one metamorphic event. Equilibrium textures, regular distribution of Fe, Mn, and Mg among minerals (see below), and a single age of metamorphic zircon growth all argue for a close approach to equilibrium during a single metamorphic event. The only evidence to the contrary is the finding of stilpnomelane as small inclusions within garnet porphyroblasts in one sample. The upper stability of stilpnomelane in ironstones is less than 500 °C (Miyano and Klein 1989), and thus its presence in these rocks is likely a relic of lower-grade, greenschist-facies conditions. Whether the stilpnomelane grew during an earlier distinct, greenschist-facies metamorphic event that was subsequently overprinted, or whether it represents a lower-temperature phase that grew during the prograde path, is unclear.
Mg/Fe decreases in the order grunerite > biotite > garnet (Fig. 8a), whereas Mn/Fe decreases in the order garnet > ilmenite > grunerite > biotite (Fig. 8b). Linear Mg/Fe and Mn/Fe compositional relationships between garnet, biotite, grunerite, and to a lesser extent ilmenite (Fig. 8), are consistent with a close approach to chemical equilibrium in the samples. However, Y zoning in garnet indicates that the distribution of at least one component did not achieve equilibrium.
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illustrate the first-order influence of bulk rock Fe-Mn-Mg composition on the compositions of these individual mineral phases. In addition, the compositions of co-existing garnet and grunerite (Fig. 9) vary systematically as a function of whole-rock MnO concentration, a relationship consistent with equilibrium distribution of Fe, Mn, and Mg. As would be expected, Mn is strongly partitioned into garnet, but grunerite also shows noticeable compositional variation as a function of whole-rock Mn-Fe-Mg concentrations.
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ACKNOWLEDGMENTS |
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Footnotes |
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MANUSCRIPT RECEIVED February 2, 2007; MANUSCRIPT ACCEPTED July 12, 2007
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