Journal of Petrology | Volume 38 | Number 1 | Pages 3-34 | 1997
© Oxford University Press 1997
Phengite-hosted LILE Enrichment in Eclogite and Related Rocks: Implications for Fluid-Mediated Mass Transfer in Subduction Zones and Arc Magma Genesis
1 Department of Mineral Sciences NHB-119, National Museum of Natural History, Washington,DC 20560, USA
2 US Geological Survey MS-923, Reston, VA 22094, USA
3 Department of Geology, University of Florida Gainesville, FL 32611, USA
Received March 6, 1996; Revised typescript accepted July 29, 1996
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ABSTRACT |
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 TOP ABSTRACT IntroductionGeologic BackgroundAnalytical MethodsPetrology and GeochemistryDiscussionConclusionsNote added in proofREFERENCES |
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Geochemical differences between island arc basalts (IAB) and ocean-floor basalts (mid-ocean ridge basalts; MORB) suggest that the large-ion lithophile elements (LILE) K, Ba, Rb and Cs are probably mobilized in subduction zone fluids and melts. This study documents LILE enrichment of eclogite, amphibolite, and epidote ± garnet blueschist tectonic blocks and related rocks from melanges of two subduction complexes. The samples are from six localities of the Franciscan Complex, California, and related terranes of Oregon and Baja California, and from the Samana Metamorphic Complex, Samana Peninsula, Dominican Republic. Most Franciscan blocks are MORB-like in their contents of rare earth elements (REE) and high field strength elements (HFSE); in contrast, most Samana blocks show an IAB signature of these elements. The whole-rock K2O contents of both groups range from 1 to 3 wt %; K, Ba, Rb, and Cs are all strongly intercorrelated. Many blocks display K/Ba similar to metasomatized transition zones and rinds at their outer margins. Some transition zones and rinds are enriched in LILE compared with host blocks; others are relatively depleted in these elements. Some LILE-rich blocks contain ‘early’ coarse-grained muscovite that is aligned in the foliation defined by coarse-grained omphacite or amphibole grains. Others display ‘late’ muscovite in veins and as a partial replacement of garnet; many contain both textural types. The muscovite is phengite that contains
3.25–3.55 Si per 11 oxygens, and 0.25–0.50 Mg per 11 oxygens. Lower-Si phengite has a significant paragonite component: Na per 11 oxygens ranges to 0.12. Ba contents of phengite range to over 1 wt % (0.027 per 11 oxygens). Ba in phengite does not covary strongly with either Na or K. Ba contents of phengite increase from some blocks to their transition zones or rinds, or from blocks to their veins. Averaged K/Ba ratios for phengite and host samples define an array which describes other subsamples of the block and other analyzed blocks. Phengite carries essentially all of the LILE in otherwise mafic eclogite, amphibolite, and garnet blueschist blocks that are enriched in these elements compared with MORB. It evidently tracks a distinctive type of LILE metasomatism that attends both high-T and retrograde subduction zone metamorphism. An obvious source for the LILE is a fluid in equilibrium with metasedimentary rocks. High-grade semipelitic schists from subduction complexes and subductable sediment display LILE values that resemble those seen in the most LILE-rich blocks. Modeling of Ba and Ti suggests that 1–40 wt % of phengite added to MORB can produce their observed LILE enrichment. Thus, the release of LILE from such rocks to fluids or melts in very high-T and -P parts of subduction zones probably depends critically on the stability and solubility relations of phengite, which is thought to be stable at pressures as high as 95–110 kbar at T=750–1050°C. KEY WORDS: geochemistry; LT eclogite; mineral chemistry; metasomatism; phengite
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Introduction |
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TOPABSTRACTIntroductionGeologic BackgroundAnalytical MethodsPetrology and GeochemistryDiscussionConclusionsNote added in proofREFERENCES |
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Potassium, Ba, Rb, and Cs typically are enriched in island arc basalts (IAB), and their differentiates, compared with mid-ocean ridge basalts (MORB) (e.g. Kay, 1980
; Perfit et al., 1980a; Gill, 1981). Part of this LILE enrichment is probably due to an added component derived from subducted sediment or altered portions of the slab; therefore, some LILE must be transferred from the subducted slab to the mantle wedge that overlies the subduction zone at depth (e.g. Gill, 1981; Hawkesworth et al., 1993). Some workers conclude that melts are the primary media of mass transfer from the slab to the mantle wedge (e.g. Kay, 1980; Kelemen et al., 1990; Schiano et al., 1995), but others think that fluids may be the pre-eminent agents (e.g. Tatsumi, 1986, 1989). However, most devolatilization of the altered slab and subducted sediment occurs at shallower depths than the source regions of arc magmas (e.g. Anderson et al., 1976; Delany & Helgeson, 1978; Bebout, 1991; Moran et al., 1992). Therefore, mechanisms such as corner flow in the mantle wedge are called upon to bring the ‘subducted component’ to the source region of arc magmas (e.g. Tatsumi, 1986, 1989; Arculus & Powell, 1986).
Rocks that represent source materials of the majority of arc magmas are only rarely entrained as xenoliths in arc volcanic rocks (e.g. Schiano et al., 1995; Ertan & Leeman, 1996), and they are not exposed in subduction complexes. However, high P/T metamorphic terranes preserve evidence for subduction-related fluid–rock interaction. Fluid-mediated mass transfer at model depths of 20–35 km within the Catalina Schist, a Cretaceous subduction complex in southern California, is documented by trace element and isotopic studies of both relatively high-T (>450°C) and low-T (<450°C) subduction zone metamorphic rocks (e.g. Bebout & Barton, 1989, 1993; Sorensen & Grossman, 1989; Bebout et al., 1993). Knowing how K, Ba, Rb, and Cs are mobilized under blueschist to eclogite facies conditions (T 300–700°C, P 8–15 kbar), can help constrain how these elements may be transferred from the slab to the mantle wedge.
Alkali and alkaline earth elements commonly are mobilized by fluids in many metamorphic environments (e.g. Rose & Burt, 1979; Krogh & Brunfelt, 1981; Alt et al., 1986; Glazner, 1988; Roddy et al., 1988; Bednarz & Schmincke, 1989; Barton et al., 1991). Several studies have described K–Ba–Rb–Cs alteration that accompanies lawsonite–blueschist facies subduction zone metamorphism (Platt et al., 1976; Moore & Liou, 1979; Moore et al., 1981; Bebout & Barton, 1993; Tenore Nortrup & Bebout, 1993). Ion-probe data for minerals from subduction zone metamorphic rocks indicate that K, Ba, Rb, and Cs are probably mobilized in fluids along with hydrophilic elements such as B under various P–T conditions during subduction zone metamorphism (Domanik et al., 1993; compare Moran et al., 1992). Furthermore, integrated trace element and isotopic studies of Be and B indicate that these elements, which track a component of subducted sediment, are transferred from subduction zones to source regions of arc magmas on relatively short time scales (e.g. Tera et al., 1986; Ryan & Langmuir, 1988; Morris et al., 1990; Bebout et al., 1993).
This work integrates whole-rock geochemistry with the mineral chemistry of phengite for low-T [LT: classification of Carswell (1990)] eclogite and related rocks from two subduction zone metamorphic terranes, and inteprets the systematics of K, Ba, Rb, and Cs during fluid–rock interaction that attended metamorphism over a range of P–T conditions. Rather paradoxically, LT eclogite is a relatively high-T (450–700°C) metamorphic rock type in subduction complexes (e.g. Schliestedt, 1990). Although LT eclogite is but a small constituent of subduction zone metamorphic terranes such as the Franciscan Complex of coastal California (Bailey et al., 1964), phase equilibrium and thermal modeling results suggest it is a major constituent of steady-state subducted slabs with little shear heating (e.g. Peacock, 1993).
We describe a distinctive style of K–Ba–Rb–Cs alteration that appears to be acquired during subduction zone metamorphism of LT eclogite, garnet blueschist and epidote ± garnet amphibolite facies rocks from Franciscan and related subduction complexes on the west coast of the USA and Mexico, and from the Samana Peninsula, Dominican Republic. The alteration is manifested by phengite, a Si- and Mg-rich muscovite, in the metasomatized rocks, and probably reflects contamination by fluids derived from subducted sediment. We propose that the Ba content of phengite can be used as a geochemical tracer of the aforementioned K–Ba–Rb–Cs alteration, which appears to accompany both ‘high-grade’ and retrograde metamorphism. Because K, Ba, Rb, and Cs reside primarily in phengite in these metamafic rocks, its stability can conceivably exert an important control upon mass transfer of alkalis from the slab to the mantle wedge. An LILE ‘sediment signature’ stored in phengite by subduction zone metasomatism could be effectively decoupled from other geochemical tracers of sediment contamination, notably Sr and Pb.
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Geologic Background |
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TOPABSTRACTIntroductionGeologic BackgroundAnalytical MethodsPetrology and GeochemistryDiscussionConclusionsNote added in proofREFERENCES |
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LT eclogite, epidote ± garnet amphibolite and garnet blueschist blocks
LT eclogite, epidote ± garnet amphibolite, and garnet blueschist occur as isolated blocks in matrices of either metasedimentary rocks (most commonly meta-argillite or semipelitic schist) or of meta-ultramafic rocks (most commonly serpentinite or talc schist) in melanges of circumpacific and Caribbean subduction complexes (Bailey et al., 1964
; Morgan, 1970; Ernst et al., 1970; Coleman & Lanphere, 1971; Hermes, 1973; Platt, 1975; Feininger, 1980; Maresch & Abraham, 1980; Moore, 1986; Draper & Nagle, 1988; Sedlock, 1988; Ross & Sharp, 1988; Baldwin & Harrison, 1989, 1992; Bocchio et al., 1990). Collectively, these rock types are colloquially referred to as ‘high-grade blocks’. Some workers conclude that high-grade blocks are remnants of high-T metamorphic aureoles that formed during the early stages of subduction (e.g. Platt, 1975; Cloos, 1984, 1985). Others interpret them as reworked basement that predates the initiation of subduction and genesis of the subduction complex (e.g. Coleman & Lanphere, 1971; Moore, 1984; Kunugiza et al., 1986; Sharp & Ross, 1987).
Many high-grade blocks are partially encased in a concentrically foliated selvage, or ‘rind’, of schistose meta-ultramafic rock that contains chlorite ± actinolite ± talc ± white mica ± other amphiboles, titanite, rutile, REE-epidote, and zircon (Coleman, 1980; Moore, 1984; Sorensen, 1988; Sorensen & Grossman, 1989, 1993). This rind forms by aqueous-fluid-induced metasomatic reaction between the block and ultramafic rocks during subduction zone metamorphism (Coleman, 1980; Moore, 1984; Cloos, 1986; Sorensen, 1988). An Mg-rich rind is evidence that its host block was in contact with ultramafic rocks during some stage of its metamorphic history, even if the block is not found in a matrix of ultramafic rocks.
High-grade blocks typically show retrograde high-P metamorphic effects such as partial replacement of garnet by lawsonite and chlorite, rimming of barroisitic amphibole by glaucophane–crossite, and rimming of rutile by titanite (e.g. Coleman & Lee, 1963). The presence of a lawsonite–blueschist facies retrograde assemblage indicates that a high-grade block partially recrystallized under hydrous, lower-T (<400–450°C) subduction zone metamorphic conditions during exhumation (Ernst, 1988). Some LT eclogite blocks contain cavities that are lined with euhedral crystals of sodic clinopyroxene, sodic amphibole, aragonite, and titanite, which indicate high PH2O during subduction zone metamorphism (Cloos, 1986). The following sections provide a brief outline of the geologic context of the high-grade blocks analyzed for this study, and describe the samples.
Setting and samples from the Franciscan Complex, California, USA, and related terranes of Oregon, USA, and Baja California, Mexico
High-grade blocks of blueschist, amphibolite, and eclogite are found in the Central Melange Belt of the Franciscan Complex, California Coast Ranges, in Franciscan-correlative rock units in southwestern Oregon, and in a Jurassic to Early Cretaceous subduction complex in Baja California (Fig. 1a). Most of the Central Melange Belt consists of metagraywacke and metashale (e.g. Bailey et al., 1964; Cloos, 1983, 1986). The metashale matrix of the melange was metamorphosed at relatively high pressures and low temperatures (Cloos, 1983; Dalla Torre et al., 1996). Franciscan high-grade blocks have been subjects of extensive petrologic study (Holway, 1904; Switzer, 1945; Borg, 1956; Coleman et al., 1965; Coleman & Lanphere, 1971; Ghent & Coleman, 1973; Brown & Bradshaw, 1979; Coleman, 1980; Moore, 1984; Moore, 1986; Sedlock, 1988; Oh & Liou, 1990; Baldwin & Harrison, 1992; Krogh et al., 1994). They occur primarily within mud-matrix melange, or as ‘float’. In many localities of the Franciscan Complex, outcrops in the immediate area of float blocks consist of metasedimentary rocks (Crawford, 1965; Moore & Blake, 1989). Although Cloos (1986) reported that many high-grade blocks of the Central Melange Belt of the Franciscan Complex are found in a matrix of fine-grained, argillaceous metasedimentary rock (metashale), he noted that most also bear Mg-rich rinds, which indicate a former association with meta-ultramafic rock. At Ring Mountain on the Tiburon Peninsula, Rice et al. (1976) mapped the high-grade blocks in a shale-matrix melange unit, which is intercalated with serpentinite. On the Vizcaino Peninsula, Cedros Island and the San Benito Islands of Baja California, high-grade blocks are found in serpentinite-matrix melanges (Moore, 1986; Sedlock, 1988).
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Samples of 13 high-grade blocks from six localities of the Franciscan Complex and related units were studied (Fig. 1a). These are (from north to south): (1) two blocks from a unit in southwest Oregon that is correlative with the Franciscan Complex (ORE-1, ORE-3: Moore & Blake, 1989
); (2) the ‘Junction Schoolhouse’ eclogite of the Central Melange Belt at Healdsburg, California (GL-14: Moore & Blake, 1989); (3) a block located near Dos Rios, California (DR: Nelson, 1991, 1995); (4) five blocks from Ring Mountain, Tiburon Peninsula, California (T-90-1, T-90-2, T-90-3, T-90-5 and T-90-6: Ransome, 1895; Tallifero, 1943; Dudley, 1972; Rice et al., 1976; Ingersoll et al., 1984; Oh, 1990); (5) a block on the road to Mt Hamilton, Diablo Range, California (GL-16 and MH-90: Cloos, 1986; Moore & Blake, 1989; Nelson, 1991, 1995; Giaramita & Sorensen, 1994); (6) blocks of epidote amphibolite, epidote blueschist and pumpellyite-bearing blueschist (the last is not a high-grade assemblage) from the Jurassic–Early Cretaceous subduction complex exposed on Cedros Island and East San Benito Island, Baja California, Mexico (RF, 687186, 68718: Sedlock, 1988; Baldwin & Harrison, 1989, 1992). Locality (5) was bulldozed in the summer of 1990 during road work; the block existed then only as partly buried, tumbled slabs.
Sets of samples from six individual blocks and their rinds are represented in the ‘Franciscan’ sample group. These are: ORE-1, ORE-3, GL-14, T-90-1, T-90-2, and GL-16/MH-90 (Tables 1 and 2). Other subsampling was guided by heterogeneity within individual blocks. Accordingly, two blocks that consist of centimeter-scale interlayers of eclogite and garnet blueschist (DR and T-90-3), and two other blocks that consist of interlayered eclogite and amphibolite (T-90-1 and MH-90) were subsampled by layer. Samples that were not collected by the first author were obtained from the following researchers: ORE and GL samples, Dr Diane E. Moore, US Geological Survey; RF, 68718, 687186 samples, Dr Suzanne Baldwin, University of Arizona; DR sample, Dr Mark Cloos, University of Texas [for this sample, compare Domanik et al. (1993)].
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Giaramita & Sorensen (1994)
estimated P and T for eclogite samples T-90-1B, T-90-5 and MH-90-1, using the jadeite content of clinopyroxene (Ghent, 1989) and the Fe2+–Mg, garnet–clinopyroxene geothermometer (Ellis & Green, 1979). The results are (T, Pmin): (a) for T-90-1B, 691 ± 84°C, 10.4 ± 2.1 kbar; (b) for T-90-5, 593 ± 63°C, 9.0 ± 1.3 kbar; (c) for MH-90-1, 671 ± 77°C, 10.5 ± 1.5 kbar. Corrections to the garnet–clinopyroxene geothermometer proposed by Krogh (1988) or by Pattison & Newton (1989) lower the T estimates by 50°C.
Setting and samples from the Samana Metamorphic Complex, Samana Peninsula, Dominican Republic
Blocks of eclogite and high-grade blueschist are found in melanges of Cuba and the north coast of the Dominican Republic (Nagle, 1974; Perfit & McCulloch, 1982; Perfit et al., 1982; Joyce, 1985, 1991; Draper & Lewis, 1991). Lower-grade blueschist has been dredged from the inner walls of the Puerto Rico trench (Perfit et al., 1980b). On the Samana Peninsula of the Dominican Republic, high-grade blocks occur in a zone [the Punta Balandra Zone of Joyce (1991)] within the Santa Barbara Schist, which is one of three units of the Samana Metamorphic Complex of Joyce (1991). Here, high-grade blocks have been emplaced into a section dominated by metacarbonate rocks. Most blocks with diameters >0.5 m are found in lenses of talc ± Mg-chlorite schist that locally contain fuchsite, which were presumably derived from ultramafic rock. These lenses range to tens of meters wide. They locally crosscut the regionally developed foliation of surrounding marbles, micaceous marbles, calcareous schists and semipelitic schists (compare Joyce, 1991).
The mineral assemblage of the semipelitic schist indicates regional, greenschist facies P–T conditions: it consists of actinolite, albite, quartz, clinozoisite, chlorite, white mica, calcite, and titanite. Inclusions of lawsonite are present only in albite and clinozoisite porphyroblasts of semipelitic schist (compare Joyce, 1991). This texture suggests that earlier, high P/T, metamorphism in the metasedimentary host rocks of the eclogite-bearing ultramafic rocks was overprinted by greenschist facies conditions. The eclogite blocks contain epidote and sodic amphibole. Except for a few sodic amphibole veins, the textural relations of the epidote blueschist facies minerals are not obviously retrograde, and no indications of retrograde greenschist facies metamorphism of eclogite are present (compare Joyce, 1991). Mean temperatures for the eclogite samples, estimated using the Fe2+–Mg garnet–clinopyroxene geothermometer and the jadeite contents of clinopyroxene, are in the range T = 502–601°C at Pmin = 8.2–9.9 kbar (Ellis & Green, 1979; Ghent, 1989; Giaramita & Sorensen, 1994). The P–T estimates for eclogite, the mineral assemblages and textures of the metasedimentary host rocks, and structural observations that indicate relatively late emplacement of the meta-ultramafic lenses into host rocks suggest that metamorphism of the high-grade blocks did not occur in situ at the regional P–T conditions (compare Joyce, 1991).
Eclogite blocks were sampled east of the town of Samana near Punta Balandra, on the south coast of the Samana Peninsula (Fig. 1b). Eclogite occurs as: (1) boudins 
0.5 m in their longest dimension in a single outcrop of coarse-grained calcite marble along the coast road near Punta Balandra, (2) inclusions 0.5–3 m in diameter, with or without rinds, in semi-concordant lenses of talc and chlorite schist which locally cut the layering-parallel foliation of sequences of metasedimentary rocks, and (3) loose boulders of 
1 m in diameter, found in canyons or on beaches between the ‘marble locality’ and the town of Samana. The loose blocks, many of which display rinds, are presumably derived from outcrops similar to setting (2). Most of the small eclogite lenses and boudins in the marble outcrop [setting (1)] show extensive cataclasis and alteration of clinopyroxene, replacement of clinopyroxene and garnet by carbonate, and development of retrograde epidote blueschist facies assemblages. Samples SAM-9 and SAM-12E are from this locality.
Four eclogite blocks were studied in detail: (1) SS85-22, a block of 0.5 m diameter, from a pod of talc–chlorite schist 10 m wide within a sequence of marble and foliated micaceous marble; (2) SS84-24, a rind-bearing block 2 m in diameter, found in an arroyo; (3) SS84-24/85, a second block similar in size and from the same locality as (2), but which shows extensive calcic alteration; (4) SS85-27, a rind-bearing block of interlayered blueschist and eclogite 3 m in diameter, from the beach near the locality of (2) and (3). Transition zones and rinds from blocks SS84-24, SS84-24/85, and SS85-27 were also collected and analyzed. The rind around block SS85-27 was subsampled as inner (SS85-27B1-IR) and outer (SS85-27B2-OR) rind. Giaramita & Sorensen (1994) reported the following P–T estimates for three of these blocks: for SS84-22, Pmin = 8.2 ± 0.8 kbar, T = 502 ± 42°C; for SS84-24, Pmin = 9.6 ± 1.1 kbar, T = 597 ± 59°C; for SS85-27, Pmin = 9.9 ± 1.0 kbar, T = 601 ± 55°C. Samples R548, R750, and R751 were collected from the Samana Peninsula by Fred Nagle of the University of Miami, and sent to the third author for analysis.
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Analytical Methods |
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TOPABSTRACTIntroductionGeologic BackgroundAnalytical MethodsPetrology and GeochemistryDiscussionConclusionsNote added in proofREFERENCES |
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Most whole-rock samples of blocks and rinds were analyzed for major, minor, and trace elements by both X-ray fluorescence (XRF) and instrumental neutron activation analysis (INAA). Samples R548, R750, R751, SAM-09, and SAM-12E were analyzed by the third author at the Australian National University using XRF and spark-source mass spectrometric methods. All samples weighed between 0.05 and 2 kg. Sample sizes reflect the grain size and amount of material available; subsamples of rinds and of interlayered lithologies were smaller than those of entire rinds and of the host rocks. Splits were ground to <100 mesh in an Al-ceramic SPEX shatterbox. Pure quartz blanks ground in this mill lack significant contamination of any reported element. The XRF analyses and FeO and loss on ignition (LOI) determinations were carried out in the Department of Mineral Sciences, National Museum of Natural History (DMS-NMNH). The INAA analyses were performed at the US Geological Survey, Reston, Virginia. Details of the analytical procedures and additional references for these techniques have been cited by Sorensen & Grossman (1989)
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Electron microprobe analyses of muscovite were performed with the ARL-SEMQ microprobe at DMS-NMNH, using mineral and synthetic standards developed by Jarosewich et al. (1978). Counting time for each element was 20 s. For each grain, a minimum of three analyses were performed adjacent to each other. Throughout each analysis, the sample was moved slightly to minimize volatilization effects upon K; significantly lower total counts for K were observed if this was not done.
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Petrology and Geochemistry |
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TOPABSTRACTIntroductionGeologic BackgroundAnalytical MethodsPetrology and GeochemistryDiscussionConclusionsNote added in proofREFERENCES |
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Mineral assemblages of high-grade blocks
The mineral assemblages of Franciscan and Samana high-grade blocks are typical of eclogite, high-grade blueschist, and garnet + epidote-bearing amphibolite (Table 1). Sodic amphibole and omphacitic clinopyroxene commonly coexist in high-T mineral assemblages of blueschist, and in eclogite. High-grade blocks typically show complex textural relationships that reflect varying degrees of retrogression under epidote blueschist or lawsonite blueschist facies P–T conditions (Table 1). The samples are classified as blocks, altered blocks, transition zones, and rinds (Table 1) based on field relations and thin section study. Altered blocks are regions of a block's interior that show unusually large amounts of sulfide minerals, or of minerals which typically occur as retrograde minerals (e.g. chlorite). Transition zones are areas that are mineralogically distinct from the block's interior, and which separate blocks from rinds (e.g. Moore, 1984
; Moore, 1986; Sorensen, 1988; Sorensen & Grossman, 1993).
Geochemistry of high-grade blocks
Major and minor elements
Most Franciscan and Samana high-grade blocks resemble basalts sampled from the Pacific Ocean floor in their SiO2, Al2O3, TiO2, FeO*, MgO, and CaO contents (Tables 2 and 3, Fig. 2). Values for major elements are plotted against SiO2 because it is the most abundant constituent in all of these rocks, and it tends to show a restricted range of values in fresh ocean-floor basalts (Fig. 2). The recalculated data for blocks are compared with a 3
bracket about the mean of 2109 electron microprobe analyses of ocean-floor glasses (including highly fractionated basalts, but excluding andesitic compositions) from the Pacific Ocean (source of data: Smithsonian Institution Ocean Floor Glass Database; Melson et al., 1976). The 3 ocean-floor glass field in Fig. 2 represents extreme values for compositions of volcanic glasses from the Pacific ocean basin. The field is intended to proxy for the likely compositions of unaltered protoliths of Franciscan high-grade blocks. (This is not an appropriate assumption for the Samana high-grade blocks; see below.)
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, Franciscan Complex, California and Oregon; 
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, Samana Peninsula; 
, Baja California. Fields of major element concentrations for 35 whole-rock analyses of greenstone blocks from the Franciscan Complex (Shervais & Kimbrough, 1987
; MacPherson et al., 1990) show large overlap with the 3 ocean-floor glass field and with data for high-grade blocks. These greenstone blocks are very low-grade metabasaltic rocks that provide an analog for protoliths altered at low-T metamorphic conditions, either on the seafloor or in the subduction complex. Most of these greenstone samples display mineral assemblages found in ‘classic’ spilite (e.g. Coombs, 1974).
Two other reference suites reflect unusual or uncommon protolith types that nonetheless could be subducted, or are represented by subducted materials. One is composed of highly Fe–Ti-enriched ocean-floor gabbro, and the other, of rodingite. A field of data for Fe–Ti-enriched gabbro is defined in Fig. 2 by values from published analyses of such rocks from ODP core 735B, drilled on Leg 118 (Shipboard Scientific Party, 1989) and from samples dredged from the mid-Atlantic ridge near 24°N (Miyashiro & Shido, 1980), blueschist and eclogite from Alaska (Barker et al., 1994), and unpublished data of S. S. Sorensen & J. N. Grossman (1994) for eclogite facies Fe–Ti metagabbro boudins from Monviso and Gruppo di Voltri, western Alps; all 81 analyses were recalculated on an anhydrous basis. The only sample that plots consistently in the field defined by the Fe–Ti reference suite is RF, from Baja California (Table 2, Fig. 2). The rodingite reference suite represents a type of calcic alteration seen in basalt dikes emplaced into serpentinite within ophiolitic terranes. The rodingite field is defined by 19 analyses of eclogite facies metarodingites from Cima di Gagnone, Lepontine Alps (Evans et al., 1981). Two samples from block SS84/85, from the Samana Peninsula, are as rich in Ca and Al as are metarodingites, but also distinctly poorer in both MgO and FeO* contents. This block contains large modal amounts of calcite, tremolite, and zoisite, and also displays veins and pods of these minerals. It may have undergone metasomatic exchange with carbonate rocks during recrystallization.
In contrast to results for the other major elements, many of the high-grade blocks are enriched in Na2O, and particularly in K2O, compared with Pacific ocean-floor glasses; 14 analyses plot at extreme K2O values compared with ‘average’ Pacific MORB values of 0.11 wt % (Fig. 2; BVSP, 1981). Indeed, even the relatively uncommon E-type MORB typically contains <0.6 wt % K2O (Melson et al., 1976). Some Franciscan greenstone blocks are rich in K2O compared with the Pacific ocean-floor glass data (Fig. 2); this feature suggests both they and the suites of high-grade blocks have been affected by K alteration.
Minor and trace elements, exclusive of K, Ba, Rb, and Cs
The abundances and ratios of ‘immobile’ minor and trace elements for Franciscan high-grade blocks suggest that most are derived from MORB-like protoliths; for Samana high-grade blocks, a comparable dataset supports the suggestion of Perfit & McCulloch (1982) and Perfit et al. (1982) that these rocks primarily are metamorphosed island arc volcanic rocks, or sediments derived from them. ‘Spider-diagrams’ of whole-rock minor and trace elements normalized to MORB (Fig. 3a; Pearce, 1982) illustrate that most Franciscan high-grade blocks are little fractionated in REE, high field strength elements (HFSE), and Sc compared with the normalizing values of Pearce (1982), and span an ‘average’ MORB composition. In addition, most plots of REE for Franciscan high-grade blocks have slightly light (L) REE-depleted or flat REE patterns at abundances of 10–30 x chondrite (Fig. 3d). In contrast, most Samana high-grade blocks are depleted in HFSE (Fig. 3b) and enriched in LREE compared with MORB (Fig. 3e). Most samples display distinctly LREE-enriched patterns at REE abundances of 10–50 x chondrite for LREE, 5–20 x chondrite for HREE. The protoliths of Franciscan greenstone blocks are mostly MORB, but include a few examples of IAB, and possible off-axis seamount basalt (MacPherson et al., 1990). One Franciscan greenstone locality is a semi-intact seamount (MacPherson, 1983). The scatter in the data for Franciscan greenstone blocks in Fig. 3c and 3f thus reflects a wide range of possible protoliths.
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A plot of TiO2 abundance vs chondrite-normalized La/Sm highlights the contrast in geochemistry between Franciscan and Samana high-grade blocks (Fig. 4). Nearly all of the Franciscan rocks display TiO2 values >1 wt %, and show little evidence for LREE enrichment (LaCN/SmCN < 1), whereas the Samana rocks display TiO2 values <1 wt % and are LREE enriched. The former characteristics are typical of MORB, the latter, of island arc volcanic rocks (Perfit et al., 1980a
; BVSP, 1981). All but two analyses of Franciscan high-grade blocks lie within fields defined by data for Franciscan greenstone blocks on this plot, which supports the assumption that the two populations sampled a similar protolith assemblage (i.e. subducted ocean-floor rocks).
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Systematics of K, Ba, Rb, and Cs
In addition to K, the elements Ba, Rb and Cs are enriched in many samples of both Franciscan and Samana high-grade blocks compared with MORB (Figs 2 and 5). Because this feature is also seen in the Franciscan greenstone suite (Figs 2 and 5; Shervais & Kimbrough, 1987
; MacPherson et al., 1990), it might merely reflect isochemical high-grade metamorphism of protoliths altered at low-grade conditions. However, the interelement systematics of K, Ba, Rb, and Cs in the high-grade blocks differ from those seen in the greenstone suite (Fig. 5). In Franciscan and Samana high-grade blocks, K, Ba and Rb are strongly intercorrelated; furthermore, data from both localities show overlapping ranges (Fig. 5a,b). In addition, Cs increases with Ba in both groups of high-grade blocks, although values for Samana blocks define a trend of lower Ba/Cs than do most of the data for Franciscan ones (Fig. 5c). In contrast, increases of Ba versus K, Rb, and Cs are, for the most part, not seen in the data for greenstone blocks (Fig. 5d–f). These observations suggest that: (1) LILE enrichment was controlled by different processes in the high-grade blocks compared with Franciscan greenstone blocks, and (2) the enrichment process was probably the same for both the Franciscan and Samana high-grade blocks.
|
, analyses of Franciscan greenstone from MacPherson et al. (1990)
, analyses of Franciscan greenstone from Shervais & Kimbrough (1987)
, a high-K, palagonitized ocean-floor basalt composite from DSDP Hole 417A, analyzed by Staudigel et al. (1981Variations of major, minor and trace elements within interlayered blueschist–eclogite and amphibolite–eclogite blocks
Blocks DR and T-90-3 consist of interlayered garnet blueschist and eclogite, and blocks T-90-1 and MH-90 (GL-16) consist of interlayered garnet amphibolite and eclogite. To evaluate the contributions of each rock type to the whole-rock composition, as well as the partitioning of elements between rock types, analyses (Table 2) were performed on two adjacent, 2–3 cm thick slices of each block. The first slice was analyzed as a bulk sample. The second slice was separated along boundaries between interlayered blueschist–eclogite or amphibolite–eclogite, and each rock type was analyzed separately. Data for the subsamples were normalized to the bulk sample. Additional samples of blocks T-90-1 and MH-90 represent heavily retrograded regions of the blocks, other discrete layers of amphibolite and eclogite, and rinds around the blocks. Normalized values for subsamples that range within 20% of the bulk-rock value are interpreted to be isochemical within the resolution of the analytical methods and means of separation. If element values for subsampled lithologies do not lie within the ±20% ‘envelope’, one layer must be richer in that element than the bulk sample, and the other poorer in it for mass balance to be maintained. Of 36 major, minor and trace elements that were compared in lithologies of the four subsampled blocks, elements that could not be mass-balanced within 20% are fewer than three per block.
Blueschist and eclogite layers in blocks DR and T-90-3 display strong fractionations between alkali elements. In the Dos Rios block, eclogite layers (DR-E1,-E2, and -E3) are richer in K2O, Ba, Rb, and Cs (as well as in Cu and La) compared with adjacent, Sr-rich blueschist layers (DR-B1,-B2; Table 2). In contrast, a blueschist layer in block T-90-3 (T-90-3A BST) is richer in K2O, Ba, Rb, and Cs (as well as in Zn, Co, and U) than an adjacent, Sr-rich eclogite layer (T-90-3A ECL; Table 2). A second blueschist layer in block T-90-3 (T-90-3B) is rich in Sc and FeO, and poor in HFSE, REE, alkalis and alkaline earth elements, Th, and U compared with blueschist layer T-90-3A. Taken together, these data suggest that the distribution of K, Ba, Rb, and Cs within layered blueschist–eclogite blocks is independent of rock type.
Amphibolite and eclogite layers in block T-90-1 are essentially isochemical; only Cu and Au are enriched in eclogite compared with amphibolite. Sample T-90-1C, a strongly retrograded sample of interlayered eclogite and amphibolite, is enriched in LILE compared with less overprinted samples. This links some LILE enrichment to the development of a retrograde blueschist assemblage. Rind sample T-90-1D is enriched in SiO2 and in elements probably gleaned from ultramafic rock (MgO, Ni, Cr, and Co) compared with the bulk sample of interlayered eclogite and amphibolite. It contains much less Al, Sc, Fe, Mn, Zn, HFSE, REE, and K, but much more Ba and Rb than does the host block. Similar LILE-rich rinds are found around blocks from the Shuksan Metamorphic Complex of Misch (1966) and the Catalina Schist (Sorensen & Grossman, 1993).
Two types of amphibolite layers are present in the Mount Hamilton block (MH-90 and GL-16 samples). Both are poorer in Fe2O3 and richer in FeO, K, Ba, Rb, Cs, Sc, and Co than eclogite layers. The first type of amphibolite (MH-90-8, -9; GL-16-4; Table 2) displays a ‘rind-like’ composition, in that it is rich in Mg, Ni, and Cr compared with both the bulk sample (MH-90-1A) and eclogite layers within the block. Middle-REE, Ti, Hf, and Zr are either very enriched or greatly depleted in rind-like amphibolite compared with eclogite layers in the block. Rind-like amphibolite samples are made up almost entirely of hornblende; they lack garnet, Ca epidote, or lawsonite. The presence of Al-rich hornblende instead of actinolite suggests that this is a type of high-T metasomatic rock (compare Sorensen, 1988). A second type of amphibolite (MH-90-1A AMPH; Table 2) contains both garnet and epidote in addition to hornblende. These layers are not particularly Ni and Cr rich, and contain more Mn and Zn than do either the eclogite or the rind-like amphibolite layers. Compared with interlayered eclogite, garnet–epidote amphibolite is lower in both its Ca and Na contents. The eclogite and amphibolite layers of MH-90-GL-16 appear to be much less isochemical in composition than counterparts in the T-90-1 block, but both show evidence for LILE enrichment linked to retrograde and metasomatic assemblages.
Block-to-rind increases of K, Ba, Rb, and Cs
In addition to the observation that rinds can be much richer in LILE than host blocks, four of the eight sets of blocks and rind samples show progressive enrichment in K, Rb, Ba, and Cs from block to transition zone or rind (Table 2). The rind around block SS84-24 (samples SS84-24C, -D) contains about five times the K, Ba, Rb, and Cs seen in the host block. The blueschist transition zones between blocks ORE-1 and ORE-3 and their rinds (samples ORE-1-5 and ORE-3-5, respectively) show similar LILE enrichments. The low-T rind (GL-16-1) around the Mount Hamilton block is enriched in K–Ba–R–Cs compared with eclogite layers GL-16-8 and MH-90-1A ECL. In addition, rind-like amphibolite (GL-16-4, MH-90-8, MH-90-9), garnet–epidote amphibolite (MH-90-1A AMPH) and retrograded eclogite–amphibolite (MH-90-11C) all are richer in LILE compared with the eclogite layers of this block. The transition zones and rinds formed from regions of the block that were probably close to or at its exterior surface at the time of metasomatism. This suggests an infiltrative mechanism for alkali enrichment during rind formation at both low- and high-T conditions.
Textures of muscovite from high-grade blocks
Muscovite textures suggest that it is a primary as well as a retrograde mineral in many LILE-rich, high-grade blocks (Fig. 6). Thin sections of eclogite, blueschist and amphibolite contain coarse-grained muscovite that is aligned with the foliation defined by coarse-grained omphacite or hornblende, sodic–calcic amphibole, or coarse-grained sodic amphibole (Fig. 6a,b). Coarse-grained muscovite occurs as pressure shadows around garnet grains (Fig. 6a). In sample ORE-3-5, coarse muscovite grains are broken and recrystallized around fold hinges (Fig. 6c). Muscovite grains at the margins of ‘early’ hornblende in a garnet amphibolite layer (sample MH-90-1A) apparently co-crystallized with barroisitic hornblende and are overgrown and rimmed by ‘late’ sodic amphibole (Fig. 6d). On the other hand, muscovite is also part of the mineral assemblage of many rinds, which are retrograde features (Table 1; Moore, 1984; Nelson, 1995). Textures such as partial replacement of garnet by muscovite + chlorite are easily assigned to a relatively late stage of recrystallization, especially if muscovite is intergrown with retrograde assemblages of epidote-group minerals, lawsonite, and sodic amphibole (Table 1, Fig. 6e). Veins of muscovite ± chlorite that crosscut layering are also likely to be relatively late-stage or even retrograde features (Fig. 6f).
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Mineral chemistry of muscovite
The phengite formula
The muscovite found in blocks, transition zones, and rinds alike shows significant tschermakitic substitution (Mg,Fe2+Si
AlVIAlIV) and is thus phengitic (Table 4, Fig. 7a). In formulae calculated on an ‘all ferrous’ basis, Fe2+ ranges from 0.2 to 0.5 cations per 11 oxygen formula unit (p.f.u.), and many samples require more Si cations than are available in order to charge-balance the R2+ cation sum (Mg + Fe2+). If one calculates all iron as Fe3+, the data straddle a line that represents the phengite substitution. Indeed, Mg p.f.u. is generally within 0.1 cation of an Fe2+-free muscovite–phengite join (Fig. 7a).
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, SS85-27; 
, T-90-3; 
, MH-90; 
, DR; *, T-90-02; *, ORE-3.To explain the deviation of data from the continuous line in Fig. 7a (which is expressed by the dashed, least-squares line through the data), some Fe2+ is probably present in some phengite analyses. However, the octahedral cation sums based on the ‘all Fe3+’ assumption indicate that most Fe is probably Fe3+. To obtain meaningful octahedral cation sums, AlVI is calculated by assigning sufficient Al to fill the tetrahedral sites, and subtracting this value from total Al p.f.u.; Ti plus a vacancy are assumed to replace two M2+ cations and maintain local charge balance. The sum of octahedral cations (AlVI + Fe3+ + 2Ti + Mg) is 1.98–2.02 for 168 of 210 analyses and 1.97–2.03 for 188 analyses; the total range of values is 1.95–2.05 (Table 4). In contrast, formula calculations based on ‘all Fe2+’ yield octahedral cation sums that range between 2.00 and 2.11, with only 24 values between 2.00 and 2.03. Because these calculations indicate that ‘all Fe3+’ formulae result in a large number of stoichiometric analyses, all further discussions will be based on this assumption.
Interlayer-site sums (K + Na + Ba, p.f.u.) for 210 analyses range from 0.92 to 1.03, with most values between 0.97 and 1.02 (Table 4). Data for phengite from the Dos Rios eclogite–blueschist block show lower interlayer-site sums (average 0.93) than do any other samples. Analyses of phengite from the Dos Rios (DR) block also show high average Si (3.4), Mg and Fe contents, and low Na contents (<0.05 p.f.u.) compared with phengite from the other blueschist and eclogite blocks (Table 4, Fig. 7). Perhaps unanalyzed components such as Li, B, N, or H2O account for the poor stoichiometry of Dos Rios phengite analyses. However, these analyses yield higher analytical totals than those from other samples (Table 4), which would argue against this hypothesis.
Na and Ba substitution in phengite
The Na content of phengite (the paragonite component) generally anticorrelates with its Si content (Figs 7 and 8; e.g. Guidotti et al., 1994). Paragonite is not observed in these samples, but all of them contain either omphacite or sodic amphibole, which would also buffer the Na content of phengite. The Si content of muscovite is thought to increase with pressure (e.g. Massone & Schreyer, 1987). Although Na substitution in phengite is inhibited by crystal-chemical effects (e.g. Guidotti, 1984; Evans & Patrick, 1987) the paragonite component of phengite in an Na-buffered assemblage increases with temperature at P < 20 kbar [Guidotti et al. (1994) and references cited therein]. Some lower-Si phengite grains from these samples show a significant paragonite component: Na p.f.u. ranges to 0.12 in samples T-90-2, T-90-3, and ORE-3; this value corresponds to Xms = 0.86. In an Na-buffered mineral assemblage (one that contains sodic amphibole ± omphacite) this value of Xms is compatible with formation at moderate- to high-T subduction zone metamorphic conditions (T = 450–600°C; Guidotti et al., 1994).
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In most cases, texturally ‘late’ and retrograde phengite grains tend to be less paragonitic than texturally ‘early’ ones. Phengite from the Dos Rios blueschist–eclogite block and amphibolite–eclogite blocks T-90-2 and MH-90 contains less Na at comparable Si values than does phengite from blueschist–eclogite block T-90-3, eclogite 85–27, and blueschist block ORE-3 (Figs 7 and 8). In the last two samples, textural evidence suggests that the low-Na phengite is a late crystallizing phase. Some analyses of phengite from the retrograde transition zone around the ORE-3 block and the rind around the MH-90 block are richer in Si and poorer in Na than those from the host block. In contrast, phengite from the rind around block T-90-2, and phengite in veins that cut this block both show higher Na contents at similar Si contents compared with texturally late fine-grained phengite in the matrix of the block; in this case, only the ‘latest’ retrograde muscovite is Na poor.
The Ba contents of phengite do not strongly correlate with either Na or Si contents, or with the apparent timing of crystallization (Figs 7–9). For example, block ORE-3 contains phengite with less Ba than counterparts in its retrograde transition zone. Both the high- and low-Na phengite in veins and in the rind around block T-90-2 contain more Ba than does late-crystallizing retrograde phengite in the host block (Fig. 9). Finally, in the MH-90 block, phengite from both garnet amphibolite and high-T, rind-like amphibolite contains more Ba than does phengite from eclogite. The apparent lack of crystal-chemical or textural controls of Ba contents indicated by such relationships suggest that if Si and Na substitution in phengite track metamorphic P and T (respectively), then Ba probably tracks the length scales of fluid access to blocks and the relative timing of their alkali metasomatism.
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Zoning within phengite grains
In many samples, cores of phengite grains tend to display more Na and Ti and less Si and Mg than do their rims (Table 4, Figs 7 and 10). Na and Ti appear to vary systematically; core-to-rim zoning of individual grains is roughly collinear with a trend defined by the array of values for a given sample (Fig. 10). Zoning of Ti is probably a P–T effect, because the substitutions of phengite and paragonite components in muscovite reflect metamorphic P–T conditions, and Ti is well correlated with Na content. In contrast, the Ba contents of phengite do not appear to vary systematically from core to rim within grains from a single thin section (Table 4). This suggests that Ba substitution in phengite is not strongly controlled by the P–T conditions of crystallization, or is much influenced by crystal-chemical effects.
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Phengite compositions and the mass balance of whole-rock K and Ba
The K and Ba contents of phengite define four arrays of constant K/Ba ratio in Fig. 11a-c. Whole-rock K/Ba ratios of most samples from the Franciscan Complex and Samana Metamorphic Complex lie along these distribution lines, which indicates that phengite controls the whole-rock ratio of these elements (Fig. 11). The K/Ba of two phengite samples from the Tiburon locality of the Franciscan Complex well describes this ratio in most of other whole-rock samples from this area as well (Fig. 11b). Phengite from one sample of the (Franciscan) Mount Hamilton block displays the same ratio as most whole-rock samples of other parts of the block, irrespective of lithology. One sample from Samana (R548, Table 3; not shown in Fig. 11) displays very low Ba and K2O contents. It contains only small modal amounts of retrograde phengite and chlorite. This sample may represent a ‘devolatilized’ rock that has lost LILE along with its hydrous minerals. Taken together, these observations suggest that phengite is the principal host for both K and Ba in most K-rich high-grade blocks. Furthermore, the good correlation of whole-rock Ba, Rb, and Cs values (Fig. 5) suggests that the two last elements also reside primarily in phengite.
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Discussion |
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TOPABSTRACTIntroductionGeologic BackgroundAnalytical MethodsPetrology and GeochemistryDiscussionConclusionsNote added in proofREFERENCES |
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Sources and sinks for K, Ba, Rb, and Cs in subduction complexes
Five principal petrologic and geochemical features are evident from the data presented above:
(1) Some otherwise MORB-like high-grade blocks from the Franciscan Complex and arc-like high-grade blocks from the Samana Peninsula contain relatively large amounts of K, Ba, Rb, and Cs. However, aside from this LILE enrichment, metamorphism appears to have been largely isochemical.
(2) The systematics of K, Ba, Rb, and Cs distinguish the geochemical signatures of LILE-rich high-grade blocks from those of low-grade Franciscan greenstone blocks, hydrothermally altered MORB, and palagonitized ocean-floor basalt.
(3) Both Franciscan and Samana high-grade blocks contain ‘early’ high-Ba phengite in their high-grade blueschist and eclogite mineral assemblages. ‘Late’ high-Ba phengite crystallized during retrograde metamorphism of the blocks and rind-forming metasomatism.
(4) Phengite is the host mineral for K and Ba (and probably that for Rb and Cs) in the high-grade blocks.
(5) Because the substitution of Ba in phengite does not appear to depend on Na or Si contents, it probably results from variations in fluid composition and distribution rather than the P–T conditions of phengite crystallization. Can these features be ascribed to the protoliths of these rocks?
The high-grade blocks contain far more K, Ba, Rb, and Cs than do most unaltered ocean-floor rocks. Based on comparison with the compositions of Pacific ocean-floor glasses (Melson et al., 1976) and low-grade greenstone blocks from the Franciscan Complex (Shervais & Kimbrough, 1987; MacPherson et al., 1990), the protoliths for most high-grade blocks were probably variably altered N- and E-MORB, IAB, arc-derived sedimentary detritus, and a minor component of off-axis seamount basalt. Despite significant differences among unaltered MORB types, they are not particularly rich in K2O, Ba, Rb, and Cs. Relatively uncommon alkaline basaltic rocks found in some arcs show immobile element characteristics similar to those of calc-alkaline or tholeiitic volcanic rocks from the same arc (e.g. Gill & Whelan, 1989; Bloomer et al., 1989; Lin et al., 1989; Luhr et al., 1989). However, these K-, Ba-, Rb-, and Cs-rich volcanic rocks also tend to be very rich in Sr, which is not a characteristic shown by many of the LILE-rich high-grade blocks. Based on their REE, HFSE, and Sr contents, it seems unlikely that the otherwise MORB-like Franciscan and arc-like Samana blocks all had alkaline igneous protoliths. The LILE signature of enriched high-grade blocks should therefore not be ascribed to igneous processes.
If the LILE-rich signature of the high-grade blocks is not of igneous origin, how is it related to metamorphic environments or metasomatic processes? The ‘end-member’ possibilities are: (1) LILE-rich high-grade blocks were isochemically metamorphosed from protoliths that had been altered on the seafloor before subduction, or at zeolite to prehnite–pumpellyite facies conditions during subduction, and (2) K, Ba, Rb, and Cs were added to high-grade blocks by one or more fluid-mediated metasomatic processes that attended blueschist to LT eclogite facies metamorphism.
We favor the latter explanation for the following reasons. If in situ hydrothermal alteration commonly affects ocean-floor rocks, one should find evidence of it in subduction zone metamorphic rocks (e.g. Sorensen, 1986; MacPherson et al., 1990). However, although Na alteration (spilitization) of ocean-floor rocks is commonly observed, pronounced K enrichment is relatively rare in suites of altered pillow basalts (e.g. Humphris & Thompson, 1978a,1978b; Staudigel et al., 1981; Alt et al., 1986; Ridley et al., 1994). Moreover, such K enrichment accompanies palagonitization, an extremely low-T phenomenon that affects only the topmost few hundred meters of ocean-floor rocks (e.g. Staudigel et al., 1981; Alt et al., 1986; Ridley et al., 1994). If the LILE enrichment seen in the high-grade blocks took place on the seafloor, its presence requires that not only did we randomly sample large numbers of high-grade blocks that were derived from what should be a rare type of protolith along hundreds of kilometers of strike of the Franciscan Complex (as well as the Samana Peninsula), but also that the source of these blocks was only the uppermost few hundred meters of the slab. This seems geologically implausible. Furthermore, the LILE systematics of high-K, palagonitized ocean-floor basalt differ from those of the high-grade blocks (Fig. 5). Although K, Rb, and Cs are greatly enriched in the palagonitized rocks compared with fresh MORB, Ba is not enriched to the same degree (Fig. 5).
The chemical signature of seafloor alteration is present in some low-grade Franciscan greenstone blocks (Fig. 5d-f). However, only 6 of 43 samples have (anhydrous) K2O contents >1.5 wt %, and all of these contain <600 p.p.m. Ba (Shervais & Kimbrough, 1987; MacPherson et al., 1990). Like ocean-floor basalts, this population of prehnite–pumpellyite to lawsonite blueschist facies greenstone blocks seems to show that K–Ba–Rb–Cs enrichment is far less common than is Na enrichment (i.e. classic spilitization). In addition, the Franciscan greenstone blocks that are rich in K compared with MORB also more closely resemble palagonitized ocean-floor basalt in LILE systematics (Fig. 5).
K–Ba–Rb–Cs enrichment of high-grade blocks, transition zones, or rinds can be attributed to crystallization of phengite under garnet blueschist to LT eclogite facies conditions, low-T blueschist retrograde metamorphism, and rind-forming metasomatism. Our data indicate that at least some LILE enrichment of high-grade blocks can be directly linked to a type of subduction zone metasomatism, rather than isochemical high P–T metamorphism of previously altered basalts. These observations do not preclude a multi-stage process in which palagonitized MORB would form phengite at low- to high-T subduction zone metamorphic conditions, with phengite subsequently becoming enriched in Ba by retrograde subduction zone fluids. However, this scenario also seems unlikely on geologic grounds, especially in cases such as block T-90-2, in which retrograde phengite is poorer in Ba than Na-rich, higher-T grains.
If high-grade blocks were enriched in K, Ba, Rb, and Cs by subduction zone metasomatism, what are the most likely sources of these elements in subduction complexes? The K/Ba, Ba/Rb, and Ba/Cs ratios of high-grade blocks are not consistent with a fluid source in equilibrium with fresh or altered mafic or ultramafic rocks. However, subduction complexes typically contain abundant metashale or meta-argillite, which may have K2O contents up to 3.3 wt % (Bailey et al., 1964). High-grade semipelitic metasedimentary rocks from the Catalina Schist and the Shuksan Metamorphic Complex of Misch (1966) contain up to 2.3 wt % K2O, 2150 p.p.m. Ba, 75 p.p.m. Rb, and 2.7 p.p.m. Cs (Sorensen & Grossman, 1989, 1993). Greenschist facies metasedimentary rocks dredged from the Puerto Rico trench (just east of the Samana Peninsula) contain up to 3.0 wt % K2O, 1290 p.p.m. Ba, 48 p.p.m. Rb, and 1.6 p.p.m. Cs (Perfit et al., 1980b, unpublished data). Plank & Langmuir (1993) reported bulk compositions for sediment sections near eight active trenches. These contain 0.67-2.5 wt % K2O, 205–3250 p.p.m. Ba, 20–88 p.p.m. Rb, and 1.5–4.9 p.p.m. Cs. All of these ‘sediment’ values are remarkably similar to the maximum contents of these elements in the high-grade blocks. Geochemical modeling shows that subducted and metamorphosed sediment is an attractive source for K, Ba, Rb, and Cs in subduction zone fluids (e.g. Bebout & Barton, 1993). Furthermore, Sr–Nd isotopic systematics of both Samana and Franciscan eclogites and their metasomatic rinds provide additional evidence that a sediment component can be transferred to mafic rocks during rind-forming metasomatism (Perfit & McCulloch, 1982; Nelson, 1991, 1995).
The behavior of Ba and TiO2 in the high-grade blocks seems to be analogous to that observed elsewhere for B and Be in that Ba appears to be very mobile and TiO2 relatively immobile. Boron and Be act as monitors of subduction-related metamorphism because they are fractionated during hydrous-fluid–rock interaction (Bebout et al., 1993). Because B is significantly more compatible in a fluid phase than Be, metasomatic fluids should impart high B/Be in rocks altered by them. However, B is so easily mobilized in low-T fluids that a geochemical ‘B-signal’ of a sediment component should become less robust at high-T conditions of metamorphism (Moran et al., 1992; Bebout et al., 1993). Micas are likely hosts for B as well as LILE in subducted rocks (Domanik et al., 1993). Because Ba is evidently not as hydrophilic as B under low-T conditions (You et al., 1996), it may be a suitable geochemical monitor of fluid–rock interactions at higher-T conditions, under which most sediment-derived B should have already left the system.
To evaluate whether the LILE enrichments in the high-grade blocks result from open-system metasomatic addition, we modeled their Ba and TiO2 contents using simple mixing between two end-members (MORB: Ba=15 p.p.m., TiO2=1.5 wt %; average phengite: Ba=6793 p.p.m., TiO2=0.24 wt %). The high-grade blocks lie along a trend very similar to that of the MORB–IAV array (Fig. 12a). Both the observed range of values and the trend of Ba/TiO2 seen in the blocks and rinds can be produced by adding 1–40 wt % phengite to a protolith of unaltered MORB. If altered MORB or IAB-like protoliths are considered, less of the phengite component will effect the same result.
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) and high-P metamorphic rocks from the Samana–Puerto Rico Trench complex (
at 1%, 5%, and then 10% increments. (b) compares Ba and K2O data for metamorphic rocks (labeled as in Phengite-hosted enrichment of K, Ba, Rb, and Cs has also been reported for some garnet amphibolite blocks from the Catalina Schist of southern California (Sorensen, 1991
; Sorensen & Grossman, 1993). Mass balance considerations indicate that the LILE in these enriched garnet amphibolite blocks must reside in phengite (Sorensen & Grossman, 1989; Sorensen, 1991). The Catalina phengite contains about 3.3 Si, 0.3 Mg, and 0.09 Na p.f.u. with all Fetotal = Fe3+ (Sorensen, 1989), a composition resembling some reported here (Table 4). The Catalina blocks show a variety of metasomatic effects that are attributed to fluid–rock interaction at T = 640–750°C, P = 8–12 kbar (e.g. Sorensen, 1988; Sorensen & Grossman, 1989; Bebout & Barton, 1993). Oxygen isotope data for the Catalina amphibolite blocks indicate that the high-T metamorphism was attended by fluids in equilibrium with metasedimentary rocks (Bebout & Barton, 1989). Unlike the high-grade blocks discussed in this paper, Catalina garnet amphibolite blocks lack a blueschist facies overprint. Thus, fluid–rock interactions which produce phengite veins during subduction zone metamorphism can take place at temperatures even higher than those estimated for the Franciscan and Samana high-grade blocks.
Implications for the chemistry and genesis of arc magmas
The flux of Ba in trenches and arcs
McCulloch & Gamble (1991) noted that the estimated global flux of Ba into subduction zones is not balanced by that out of volcanic arcs. Perhaps part of the apparent ‘mass balance’ problem of Ba in volcanic arcs results from the presence of phengite in deeply subducted, metamorphosed and altered basaltic rocks. On the other hand, Plank & Langmuir (1993) have shown that local output of Ba in specific arcs is correlated with local sediment input. The high-grade blocks studied here tend to show higher Ba/K2O than do arc volcanic rocks (Fig. 12b). Perhaps the apparent global retention of Ba in subduction zones reflects the presence of deeply subducted phengite, a phase that might serve to both store Ba and buffer the local variations in Ba contents of arc products that reflect varying sediment input. Schmidt (1996) and Domanik & Holloway (1997) have reported new experimental data on phengite that suggest it is stable to T=1000–1050°C, at P=95–110 kbar. As noted by these workers, phengite can potentially store LILE in a hydrous phase at depths as great as 300 km, provided that it is not soluble in lower-P fluids or melts. Knowing how LILE partition into K-hollandite (the high-P breakdown product of phengite) versus fluids or melts is necessary to understand how these LILE may contaminate the upper mantle during subduction.
Are sediment-derived LILE transferred to phengite via metasomatism?
The processes and sources of the characteristic LILE enrichment seen in IAB remain somewhat enigmatic. Our data support the enrichment of K, Ba, Rb, and Cs (particularly with respect to the REE and HFSE) owing to subduction-related metasomatism. This process produces eclogite that is greatly enriched in LILE compared with MORB. In arc volcanic rocks, high Ba/Rb and Cs/Rb are thought to reflect a component of subducted sediment supplied to source regions of arc magmas (e.g. Ben Othman et al., 1989; McCulloch & Gamble, 1991). McCulloch & Gamble (1991) reported average Ba/Rb for oceanic island arc basalts of the Aleutian, Marianas, New Britain, Vanuatu, and Kermadec volcanic arcs that range from 10 to 30. The average Ba content of these suites is 200 p.p.m., and Rb averages 10.7 p.p.m. The Ba/Rb ratios of Franciscan and Samana high-grade blocks also range from 10–30 (Figs 5 and 13), although the blocks contain much more Ba and Rb than most arc volcanic rocks.
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Ben Othman et al. (1989)
pointed out that because Ba/Rb of MORB could resemble that of subducted sediment, a comparison of Ba/Rb and Cs/Rb is a more reliable indicator of a sediment component in arc magmas. All Samana and some Franciscan high-grade blocks lie within a field of Ba/Rb vs Cs/Rb that is also occupied by arc volcanic rocks (Fig. 13). The LILE signature of contamination by sediment-derived fluids during high P–T metamorphism is also distinct from that which accompanies seafloor alteration (Fig. 13). However, the Francisan samples that plot in the ‘sediment-contaminated’ field of Ben Othman et al. (1989) show immobile element characteristics of MORB, not arc volcanic rocks. Subduction-related alkali alteration of metabasite can evidently produce an ‘arc-like’ alkali signature that is decoupled from REE and HFSE abundances in the rock. Furthermore, the data of You et al. (1996) show that increases in LILE ratios such as Ba/Rb and Cs/Rb may result simply from fractionation in moderate- to high-T hydrothermal fluids of complex chemistry.
In summary, this paper shows that metasomatism and consequent phengite crystallization in mafic rocks could play an important role in storing K, Ba, Rb, and Cs derived from fluids that had previously equilibrated with sediments at moderate depths within subduction zones. This may have significant ramifications for forearc volcanism. However, if phengite is stable at great depths within subduction zones, this mineral is also critical to our understanding of element partitioning via dehydration and melting beneath arcs and the compositional variations of IAB. In particular, phengite breakdown might account for the association of high-K basalts that also have unusually high contents of Ba, Rb, and Cs, over deep segments of subduction zones, and the hydrous, LILE-enriched glass inclusions found in a few metasomatized arc xenoliths (Schiano et al., 1995).
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Conclusions |
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TOPABSTRACTIntroductionGeologic BackgroundAnalytical MethodsPetrology and GeochemistryDiscussionConclusionsNote added in proofREFERENCES |
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(1) Some high-grade blocks from Franciscan and Samana melanges have been enriched in the LILE K, Ba, Rb, and Cs via metasomatic processes that accompany subduction zone metamorphism. Some of this enrichment accompanies rind-formation, which is a well-known, subduction-related metasomatic process.
(2) Phengite, an Si-, Mg-, and Fe-rich muscovite stabilized during high P–T metamorphism, carries essentially all the K, Ba, Rb, and Cs in LILE-rich high-grade blocks.
(3) Some Franciscan high-grade blocks have Ba/TiO2, Ba/Rb, and Cs/Rb characteristics that resemble the ‘sediment signature’ reported for island arc basalts. In such blocks, these ratios are controlled by phengite. The sediment signature of the high-grade blocks appears to have been acquired via metasomatism that accompanied subduction zone metamorphism.
(4) The release of the LILE K, Ba, Rb, and Cs from subducted materials to the mantle wedge in the ratios observed in IAV, which are commonly attributed to a component of subducted sediment, may also critically depend on the stability and solubility relations of phengite. Fluids that deposit phengite could not only redistribute a ‘sediment signature’ of LILE within a variety of rock types during subduction zone metamorphism, but potentially store them at great depths within the eclogitic descending slab.
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Note added in proof |
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TOPABSTRACTIntroductionGeologic BackgroundAnalytical MethodsPetrology and GeochemistryDiscussionConclusionsNote added in proofREFERENCES |
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We were recently referred to an excellent summary of the crystal chemistry of Ba-rich micas (Harlow, 1995
), which reported the effective substitution mechanism Ba(Mg,Fe2+)KAlVI for barian phengites from jadeitite and albitite blocks in serpentinite, Motagua Fault Zone, Guatemala. Harlow (1995) concluded that this exchange indicates growth under increasing P/T, and also remarked on the common association of barian phengite with metasomatized rock units. His SEM images reveal barian phengite grains that display spectacular rhythmic zoning in their Ba contents, which range up to 7.26w BaO (0.4 Ba p.f.u.).
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ACKNOWLEDGEMENTS |
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S.S.S. thanks the Sprague and Becker Funds of the Smithsonian Institution for support of this research, and is grateful to G. Draper for supporting her field work in the Dominican Republic, and introducing her to the geology of Samana. Field work by M.R.P. in Hispaniola was supported by NSF INT-9107784, and a grant from the University of Florida Division of Sponsored Research. M.R.P. thanks W. Wertz for help in the field, and S. R. Taylor and P. Oswald-Sealy for laboratory support at ANU. We all thank those who reviewed this manuscript: C. G. Cunningham and B. A. Morgan, III, USGS; W. G. Melson, Smithsonian Institution; and B. Harte and J. Ryan, for the Journal of Petrology. Every one of them contributed many valuable comments that aided us in revising the paper.
* Corresponding author.
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