William M. Lamb 1, Philip E. Brown 2, and John W. Valley 2. 1 Department of Geology, Texas A & M University, College Station, TX 77843, USA. 2 Department of ...
Contrib Mineral Petrol (1991) 107:472483
C o n t r i b u t i o n s tO
Mineralogy and Petrology 9 Springer-Verlag1991
Fluid inclusions in Adirondack granulites: implications for the retrograde P - T path William M. Lamb 1, Philip E. Brown 2, and John W. Valley 2 1 Departmentof Geology,TexasA & M University,CollegeStation, TX 77843, USA 2 Departmentof Geologyand Geophysics,Universityof Wisconsin, Madison, WI 53706, USA Received September 9, 1989 / AcceptedNovember 16, 1990
Abstract. Investigation of fluid inclusions in granitic and calc-silicate gneisses from the Adirondack Mountains, New York, has revealed the presence of various types, including: (1) CO2-rich, (2) mixed H20 - C02 -}-salt and (3) aqueous inclusions with no visible CO2. Many, if not all, of these inclusions were trapped or modified after the peak of granulite facies metamorphism, as shown by textural relations or by the lack of agreement between the composition of the fluids found in some inclusions and the composition of the peak-metamorphic fluid as estimated from mineral equilibria. Many fluid inclusions record conditions attained during retrograde cooling and uplift, with minimum pressures and temperatures of 2 to 3 kbar and 200 to 300 ~ C. The temperatures and pressures derived from the investigation of these inclusions constrain the retrograde P - T path, and the results indicate that a period of cooling with little or no decompression.
Introduction The study of fluid inclusions in metamorphic rocks yields information concerning peak metamorphic pressure-temperature-fluid ( P - T - X ) conditions if inclusions were trapped at the peak of metamorphism, and if they retain their compositions and densities during retrograde (post peak-metamorphic) cooling and uplift. However, many inclusions are trapped or modified after the peak of metamorphism. Information derived from the study of these inclusions constrains the P - T - X conditions of later events. The presence of high density CO2-rich fluid inclusions has been documented in a number of granulite facies terranes (Touret 1971 ; Hollister and Burruss 1976; Hollister et al. 1979; Coolen 1982; Touret and Dietvorst 1983; Hansen et al. 1984; Rudnick et al. 1984; Santosh Offprint requests to: W.M. Lamb
1985, 1986; Schreurs 1985; Touret 1985; Vry and Brown 1986; Lamb et al. 1987). Most, but not all, of these studies conclude that the inclusions accurately preserve peak metamorphic fluids. If so, then the peak metamorphic fluid in these granulite facies samples was CO2-rich, and the density of the CO2 in these inclusions can be used to constrain peak metamorphic pressures or temperatures. Many studies of high grade metamorphic terranes have also attempted to use fluid inclusions to constrain the retrograde P - T path, either from an estimation of the strength of fluid inclusions (Rudnick et al. 1984) or from the characteristics of fluid inclusions interpreted to have formed (or been modified) after the peak of metamorphism (Hollister et al. 1979; Swanenberg 1980; Touret 1985, 1987; Santosh 1985, 1986; Olsen 1987). The presence of CO2-rich fluid inclusions in granulite facies terranes is consistent with at least two of the mechanisms proposed to explain the formation of granulites. These mechanisms are: (1) partial melting (Fyfe 1973); in this case H 2 0 would be partitioned preferentially into the melt and the residual fluid could be enriched in CO2, and (2) CO2 infiltration; in this case CO2 infiltrates the lower crust, diluting H20, and stabilizing the mineral assemblages characteristic of the granulite facies (Newton etal. 1980; Condie etal. 1982; Janardhan etal. 1982; Friend 1985). As an alternative to a CO2-rich fluid phase, fluidabsent conditions could be common during granulite facies metamorphism (Valley et al. 1990). Such conditions could be generated through magmatic processes such as partial melting, metamorphism of dry plutonic rocks, the passage of dry magmas through the lower crust, or an earlier high-temperature metamorphism. In some Adirondack rocks, for example, values of fluid fugacities determined from mineral equilibria are not consistent with the presence of a free, lithostatically pressured, C O - - H fluid phase (Lamb and Valley 1984, 1985; Valley 1985; Lamb et al. 1987). This paper is the second in a series describing fluid inclusions in granulite facies rocks from the Adiron-
473 675 + 50~ C at 7.0 kbar to 775 4-50~ C at 7.6 kbar; the highest pressures and temperatures are located in the central Highlands (Bohlen et al. 1985). The samples investigated in this study are wollastonite-bearing calc-silicate gneisses from the Willsboro and Lewis wollastonite ore deposits (Valley 1985), or magnetite+ilmenitebearing quartz-feldspathic gneisses which include charnockitic gneisses from the Diana and Tupper-Saranac Complexes (Fig. 1). The peak-metamorphic mineral assemblages contained in all samples are listed in Table 1. Some samples, particularly those containing magnetite + ilmenite, contain small amounts of retrograde minerals (calcite + sericite 4- chlorite 4- secondary amphibole 4- secondary biotite).
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0.9 g/cm 3 are common, particularly in the wollastonite-bearing samples which contain a number of CO2-rich inclusions with densities ranging from 1.05 to 1.14 g/cm 3 (Fig. 3). Isochores for CO2 densities of 1.05 to 1.10 g/cm 3 pass through peak metamorphic P - T, and so are consistent with entrapment during granulite facies metamorphism (Fig. 4), although this consistency does not rule out retrograde formation for these inclusions. The highest CO2 densities (>1.10 g/ cm 3) yield pressures up to 1.5 kbar greater than peakmetamorphic pressures at peak-metamorphic temperatures (Fig. 4). Geobarometry based on the density of CO2-rich fluid inclusions must account for the possibility that constituents other than CO2 may be present in the inclusions. Measurements of the final melting of solid CO2 indicated that no more than minor amounts of other fluids are dissolved in the CO2 because most melting temperatures were within _+0.5~ C of the CO2 triple point of -56.6~ C (Lamb et al. 1987). However, a triple point depression of 0.5~ could correspond to as much as approximately 4mo1% CH4 (Burruss 1981). Furthermore, measurements of the CO2 triple point depression will not detect immiscible fluid species such as HzO. The detection of H20 is difficult in fluid inclusions and is generally based on a visual estimate. As much as 20 vol.% H 2 0 may be present in small fluid inclusions and not be detected optically because it preferentially wets the walls of the inclusions (e.g., see Roedder 1972, Fig. 3). If water is present in COz-rich inclusions, then pressure estimates based on the density of pure CO2 will yield values that are too low. For example, 20 vol.% H 2 0 in an inclusion with a CO2 density of 1.10 g/cm 3 is equivalent to 36 tool % H 2 0 and the inclusion will have a bulk density of 1.08 g/cm 3 (Brown and Lamb 1986). At 750 ~ C the pressure inferred from this hypothetical fluid inclusion is ~9.5 kbar, whereas an inclusion with pure CO2 of density 1.10 g/cm 3 will yield a pressure ~7.8 kbar at this temperature. Because undetected H 2 0 may be present, the isochores derived from the investigation of CO2-rich fluid inclusions yield minimum pressures at any given temperature. Furthermore, estimates of H 2 0 fugacities in a large number of Adirondack samples suggest that if fluids were present at the peak of metamorphism, they contained 10 to 30 tool % H 2 0 (Valley et al. 1990). Thus, any peak metamorphic fluids that equilibrated with these granulites cannot have been pure CO2. Biotite and amphibole are widespread in granulites worldwide (though they are modally minor) indicating that peak metamorphic fluid inclusions from many granulites must contain HzO if they contain fluids that are truly representative of peak metamorphic compositions.
M i x e d Ha O -- C 0 2 .fluid inclusions
In order to locate isochores for mixed H20-}-COz-tNaC1 fluids it is necessary to measure CO2 homogeniza-
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Fig. 5. Densities of the CO2-rich phase in mixed H 2 0 + C O 2 + salt inclusions plotted against estimated XCO2 in the inclusions (see text). Densitywas determinedfrom CO2 liquid-vaporhomogenization temperatures. Data from both the wollastonite-bearing (o) and the magnetite+ ilmenite-bearing(x) samples are shown. Also shown, as bars across the top of the diagram, are the range in densities for the CO2-rich inclusions (no visibleH20; Fig. 3)
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6 . 200 ~ C. The generation of these high densities at lower pressures and temperatures through modification of lower density inclusions is not a viable option, given the possible range of lithostatic pressures as compared to the possible pressures within the fluid inclusions.
leozoic events can be used define broadly the timing of fluid inclusion formation. Fluid inclusions from the Adirondacks yield minimum trapping temperatures of 200 to 400 ~ C, and minimum pressures are often as high as 2 to 3.5 kbar (Fig. 6). Clearly, cooling and decompression from peak metamorphic P - T conditions could account for the pressures and temperatures recorded in Adirondack fluid inclusions, but Paleozoic conditions are less well known. The Cambrian Potsdam sandstone lies unconformably on the Adirondack terrane, and therefore, this area must have been uplifted to the surface prior to Cambrian times (see the discussion in Isachsen et al. 1983). Because remnants of Paleozoic sediments are present in the Adirondacks (particularly in the northwest and southeast), at least some portion of the Adirondacks were reburied at this time. However, no evidence for temperatures and pressures as high as 300~ C and 3 kbar has been found in these sediments. For example, clay-mineral diagenesis and apatite fission-track data indicate that maximum burial temperatures were approximately 175~ C for Devonian and Ordovician sediments of the Appalachian Basin in New York, indicating burial depths were of the order of 8 km for rocks immediately south of the Adirondacks (Johnsson 1986). This conclusion is in agreement with the results of 4~ thermochronometry performed on samples collected from the Adirondacks and portions of the Grenville province in Ontario (Heizler and Harrison 1987; Onstott and Peacock 1987;
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Timing of fluid entrapment: Proterozoic versus Paleozoic
Although many of the fluid inclusions examined in this study were trapped or modified after the peak of metamorphism, and so may be used to constrain the P - T X conditions of later events, the actual timing of these later events is not well known. Hollister et al. (1979) assume that all fluid inclusions in the Adirondacks were formed during cooling and uplift after the peak of granulite facies metamorphism. It is important to evaluate this assumption, however, because it may be possible that the fluid inclusions in the Adirondacks were trapped during the Paleozoic orogenies that have affected eastern North America (e.g., Morrison and Valley 1988). A comparison of P - T estimates from fluid inclusions with the estimated P - T of Precambrian and Pa-
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Fig. 8. Selected isochores for CO2-rich inclusions (dashed lines), and for mixed H20-CO2-salt inclusions (solid lines, from Fig. 6).
Isochores for the mixed inclusions with low XCO2 fall within the
stippled region. A simple retrograde P - T path that is initially nearly isothermal (e.g., path D) is not consistent with the presence of high densityCO2-rich inclusions,or with the mixed H20--CO2 inclusions that fall withinthe stippledregion (see text). Theseinclusions require a retrograde P - T path that is initiallynearlyisobaric (e.g., paths A and B), or the path may be relativelycomplex (e.g., path C). Thus, the investigationof fluid inclusionsprovides useful criteria for differentiatingbetween various retrograde paths, and has important tectonic implications(see text)
480
Cosca et al. 1988). Stepwise degassing experiments on hornblende, muscovite, biotite, and K-feldspar show that this area (particularly the Canadian Grenville in Ontario) has not undergone sustained heating at a temperature above 150~ since the Precambrian. Thus, there is no evidence that burial depths during the Phanerozoic were sufficient to produce the high pressures recorded by some fluid inclusions (i.e. ,~2 to 3.5 kbar), and 4~ rules out temperatures as high as those recorded by some inclusions (i.e. ~200 to 400 ~ C). Therefore, the mixed H 2 0 - C O 2 fluid inclusions may be used to constrain the retrograde cooling and uplift P - T path that the rocks followed after the ~ 1.05 Ga granulite facies event. Minimum entrapment temperatures for CO2-rich inclusions are not restrictive because these inclusions were trapped at T > Th, where Th < 31.1 ~ C. Consequently, Phanerozoic formation of these inclusions might seem plausible. However, in some cases the high density CO2rich fluid inclusions have textures indicative of relatively early formation (i.e. prior to some mixed H 2 0 - C O 2 inclusions). Thus, the formation of the CO2-rich inclusions with the highest densities would require pressures and temperatures (e.g., >3 kbar at 200 ~ C; Fig. 8) that are considered to be unlikely for the Phanerozoic. Thus, high density COl-rich inclusions formed prior to the Phanerozoic; however, the same argument cannot be made for those COl-rich inclusions that have the lowest densities (i.e., CO2 densities for approximately 0.5 to 0.8 g/cm3).
The post-metamorphic P - T path Previous workers used mixed H 2 0 - C O 2 fluid inclusions to constrain the P - T conditions of uplift and cooling in the Adirondacks. These inclusions generally contain fluids with XCO2 between 0.3 and 0.5, and minimum entrapment temperatures of 380 ~ C (Henry 1978; Hollister et al. 1979). The isochores for these inclusions lie at P < 3 to 6 kbar at temperatures of 400 to 800 ~ C respectively. These results are consistent with a retrograde P - T path that is convex toward the temperature axis, (e.g., curve D; Fig. 8), and Hollister et al. (1979) favor this type of path for the Adirondacks. A convex path, however, does not explain the high density isochores described in this study that lie at higher pressures. The "high-pressure" isochores are intersected by a retrograde P - T path that is concave toward the temperature axis, in other words a path that involves relatively little decompression during initial cooling (e.g., curve A; Fig. 8). Hereafter, the terms "isobaric" and "isothermal" will be used to describe retrograde paths that are concave and convex to the temperature axis. We adopt this nomenclature to simplify the discussion, not to imply that uplift and cooling must be either strictly isothermal or isobaric. In fact, a number of other, more complicated, retrograde P - T paths might be proposed for the Adirondacks (e.g., curve C, Fig. 8). The investigation of fluid inclusions provides strong evidence for discriminating between these retrograde P - T paths.
Isothermal decompression The nearly isothermal retrograde P - T path (e.g., curve D, Fig. 8) never intersects the isochores for the CO2-rich inclusions with the highest densities, and so the presence of the high density CO2-rich inclusions argues against a retrograde path that is convex to the temperature axis. Furthermore, the isothermal retrograde path is also not consistent with the fluid densities derived from microthermometric examination of the set of mixed H 2 0 CO2 inclusions with low XCO2 (stippled region, Fig. 8). However, it might be argued that these high densities could be generated through post entrapment changes in inclusion volumes or compositions. This increase in inclusion density might be accomplished through a volume decrease in an inclusion that originally had a much lower density. However, consideration of the P - T position of the pertinent isochores shows that this volume decrease (a ductile phenomena) could only occur at the low temperature end of the nearly isothermal retrograde path (at T< ]00 ~ C for curve D, Fig. 8), and it is unlikely that the exterior pressure would exceed the interior pressure in these inclusions by more than about 100 bars, making compression highly unlikely. Consequently, the presence of the H20-rich mixed inclusions are not consistent with an isothermal retrograde P - T path.
Isobaric cooling An isobaric retrograde P - T path (path A, Fig. 8), is consistent with early formation of the most dense CO2rich inclusions, and formation of the H20-rich mixed inclusions at pressures in excess of 2 to 3.5 kbar. These are the same inclusions that are difficult to reconcile with an isothermal retrograde path (e.g., path D, Fig. 8). In fact, it is possible to reconcile all isochores with the isobaric retrograde path because modification (e.g., stretching or leakage) to produce the low density fluid inclusions from inclusions that originally contained relatively high density fluids is likely given the pressures and temperatures traversed by path A. Bohlen (1987) applied the results of mineralogic thermobarometry to constrain the Adirondack retrograde P - T path. This work is based on the presence of relatively small (often 50 to 100 gm) rims on garnet that are often enriched in Fe, Ca, and Mn and depleted in Mg relative to the central portion of the grain. These garnet compositions are interpreted to indicate that the Adirondacks underwent an initial period of cooling that was nearly isobaric. While care is necessary in the interpretation of garnet zoning (Bohlen 1987; Frost and Chacko 1989; Selverstone and Chamberlain 1990), Bohlen's (1987) results are entirely consistent with our interpretation of Adirondack fluid inclusions.
Other retrograde P - T paths Thus far, the discussion of retrograde P - T paths has focused on two cases that represent near end-members.
481 The evidence from fluid inclusions supports the isobaric retrograde P - T path rather than the isothermal path. However, other paths are also consistent with the constraints imposed by investigation of fluid inclusions (Fig. 8). Paths A, B, and C are representative of the different general shapes that can be hypothesized, however, each path could be varied in detail. As will be discussed, we believe that the general shape of the uplift path has important tectonic implications. Retrograde paths A and B on Fig. 8, involve an early period of cooling that is nearly isobaric. Path B shows that the initial period of nearly isobaric cooling may shortened relative to path A, assuming the maximum uncertainty in isochore placement. Path C differs from A and B in that it involves an initial period of nearly isothermal decompression. However, by cooling nearly isobarically at a pressure in excess of approximately 2.5 kbar it is still possible to form the high density CO2-rich fluid inclusions, as well as the H20-rich mixed inclusions. Schenk (1989) proposed a path similar to C for Hercyntan granulite facies rocks in southern Italy. Paths A, B, and C are constrained to cross the highest density isochores at temperatures above the minimum for specific fluid inclusions, and so many paths intermediate between A and C are consistent with the fluid inclusion data. However, paths similar to D are not consistent with these data. To summarize, the early generation of high density CO2-rich inclusions and the H20-rich mixed H20 -- CO2 inclusions provide the most important constraints on the uplift and cooling history of the Adirondacks. The isochores derived from microthermometric study of these Adirondack fluid inclusions may be used to differentiate between various possible retrograde P - T trajectories. Uplift and cooling in the Adirondacks followed a path that lies between curves A and C on Fig. 8.
Tectonic implications Various tectonic models have been proposed to explain granulite facies pressures and temperatures in the Adirondacks (as well as across the entire Grenville province). The Adirondacks were buried to depths of approximately 25 km during the peak of metamorphism, and the area has a normal crustal thickness today, suggesting the crust may have been some 60 to 70 km thick during orogenesis (McLelland and Isachsen 1980). These crustal thicknesses could be generated by collision of two continents in a tectonic setting similar to the modern Himalayan/Tibetan Plateau region (McLelland and Isachsen 1980, 1986; Windley 1986). Heat flow calculations have been used to constrain P - T - t paths following crustal thickening via thrusting or homogeneous thickening (England and Thompson 1984). In general, these calculations suggest that uplift rates in this tectonic setting will be relatively fast compared to loss of heat through conduction. The resulting retrograde P - T path will be nearly isothermal after the maximum temperature has been attained, a conclusion that is not consistent with
path A or B, or with the garnet zoning results of Bohlen (1987). An alternative to crustal thickening through continental collision is the introduction of magmas at levels in the crust both above and below the rocks that are presently exposed in the Adirondacks. In this case, magmas provide the heat necessary to elevate the geothermal gradient and drive metamorphism as well as the material to thicken the crust, The resulting P - T path might involve heating and burial, followed by nearly isobaric cooling and, finally, uplift. Bohlen (1987) argues that the almost total lack of kyanite in the Adirondacks indicates that this terrane did not experience high pressures (i.e., P > peak metamorphic P) during the prograde portion of the P - T path. However, in other areas of the Grenville province the chemical zoning in pyroxenes (Anovitz and Essene 1990) and in garnets (Tuccillo et al. 1990) is thought to result from high pressures during prograde metamorphism. Consequently, it may be necessary to develop a tectonic model that can reconcile high prograde pressures with nearly isobaric cooling that occurs immediately after the peak of metamorphism. One possible scenario involves extension of the crust after it had undergone thickening (Sonder et al. 1987; Harley 1989). Whereas Anovitz and Chase (1990) argue that the best model for the Grenville province is one that involves thrusting to produce thickening, but the model is modified to account for a period of rapid tectonic denudation. A variety of retrograde P - T paths have been described from granulite terranes located around the world (e.g., see the discussion in Harley 1989). This observation suggests that no single tectonic model can account for the formation of all granulites. In the Adirondacks, the evidence from fluid inclusions indicates that initial cooling was nearly isobaric (assuming a smooth retrograde path), and this conclusion is consistent with the investigation of garnet zoning. Regardless of whether the initial retrograde path was isobaric or isothermal, magmatic activity must have been important because it is clear that the Adirondack Highlands have experienced a protracted tectonic history that has involved the emplacement of a variety of igneous rocks (McLelland et al. 1988).
Acknowledgements. We thank M.L. Crawford and Bob Popp for helpful reviews of this paper, and Jean Morrison for many useful discussions, especiallyconcerningthe timing of fluid inclusion entrapment. Thanks also to Steve Bohlen for the use of some of his Adirondack samples. Comments by Bob Bodnar and Bruce Yardley helped to clarifythe manuscript during review. This study was supported by grants from the National Science Foundation (EAR 81-21214, 85-08102, and 88-05470), the Gas Research Institute (5083-260-0852 and 5086-260-1425), and the Center for Energy and Mineral Resourcesat TexasA & M (CEMR-155031).
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