Geology

doi: 10.1130/0091-7613(1988)016<0358:LPFSMI>2.3.CO;2 1988;16;358-361Geology

 Virginia B. Sisson and Lincoln S. Hollister southern AlaskaLow-pressure facies series metamorphism in an accretionary sedimentary prism,  

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Low-pressure facies series metamorphism in an accretionary sedimentary prism, southern Alaska

Virginia B. Sjsson,* Lincoln S. Hollister Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544

ABSTRACT The low-pressure/high-temperature metamorphism of the Chugach metamorphic com-

plex (Alaska) occurred in an ocean-continent convergence zone. To achieve the high tempera-tures at a relatively shallow depth in an accretionary prism, we propose the large-scale transport of heat by fluids, which preheated the metamorphic belt by tectonic focusing of fluids followed by injection of melts, both of which were generated downdip in a shallow subduction zone.

INTRODUCTION The thermal structure associated with re-

gional metamorphism has been attributed to a steady-state geotherm perturbed by overthrust-ing (e.g., Thompson and England, 1984), in-creased heat flow associated with voluminous

*Present address: Department of Geology & Geo-physics, Rice University, Houston, Texas 77251.

calc-alkaline intrusions in island arcs (Miya-shiro, 1961) and in continental settings (Lux et al., 1986), or increased heat flow from continen-tal rifting (Wickham and Oxburgh, 1985). However, there are several occurrences of low-pressure/high-temperature (low-P/high-7) metamorphic suites for which the increased heat near the Earth's surface is not readily explained by these models. Hudson et al. (1979) and Hud-

son and Plafker (1982) described an occurrence of low-iVhigh-r metamorphism (Chugach met-amorphic complex) developed on a regional scale in an accretionary prism at an ocean-continent convergent zone in southern Alaska and, following Marshak and Karig (1977), sug-gested that the anomalous thermal event might have been induced at a ridge-trench-trench triple junction along the continental margin. On the basis of new, detailed, structural and petrologic data from five localities in the region described by Hudson and Plafker (1982) and new 40Ar/ 39Ar data (Sisson et al., 1988), we find that the high temperatures in the ocean-continent con-vergent zone resulted from a combination of heat introduced by extensive horizontal, as well as vertical, transport of fluids followed by felsic

Figure 1. Metamorphic map of Chugach metamorphic complex revised from Hudson and Plafker (1982). Metamorphic zones are defined by first appearance of garnet, cordierite, or sillimanite. Rocks adjacent to complex are upper greenschist facies with chlorite and biotite. Random-dash pattern indicates plutons. Metamorphic temperatures (°C—bold numbers) were estimated by using garnet-biotite equilibria (Ferry and Spear, 1978). Metamorphic pressures (kbar—boxed or circled numbers) were estimated by using garnet-plagioclase-sillimanite-quartz (Ghent et al., 1979; Newton and Haselton, 1981) and garnet-muscovite-biotite-piagioclase (Ghent and Stout, 1981) geobarometers. Barbed lines indicate thrusts. Inset map shows location of complex relative to accreted terranes (Peninsular/Wrangellia, Chugach, and Prince William; adapted from Jones et al., 1984) in southern Alaska which are separated by Border Ranges fault system (BRFS) and Contact fault system (CFS).

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melts, both of which were generated from downdip in the subduction zone and possibly involved subduction of young, hot oceanic crust.

GEOLOGIC CONSTRAINTS The Chugach metamorphic complex (Hud-

son et al., 1979; Hudson and Plafker, 1982; Miller et al., 1984) comprises the eastern part of the Chugach Mountains in southern Alaska (inset, Fig. 1). It is developed in the Maastrich-tian and Campanian Valdez Group which is predominantly clastic argillite and graywacke deposited in a trench or deep-sea fan setting (Nielsen and Zuffa, 1982). On the basis of a seismic refraction study across the Chugach ter-rene, relict oceanic crust may be as little as 10 km below the present erosional level (Fuis et al., 1985; Page et al., 1986). At present, the belt of amphibolite fades rocks is more than 180 km long and averages 25 km in width (Hudson and Plafker, 1982). Parts of the northern and south-ern borders of the complex are truncated by faults (Fig. 1, inset). These separate the Chugach accretionary complex from younger terranes to the south and older terranes to the north.

The core of the complex is migmatitic, and metamorphic grade decreases outward from the core to biotite grade within the area shown in Figure 1. There are both large- and small-scale felsic intrusions throughout the complex; they are most abundant in the core. Anatexis, which occurred locally within the core, generated gran-itic melts that are compositionally distinct from the felsic intrusive rocks.

In this region there are two generations of structures that correlate with the second and third generations of structures observed by Nokleberg et al. (1988) in the Valdez Group along the Richardson Highway. South-verging folds are the first generation recognized. Farther southwest by 600 km, on Kodiak Island, the formation of south-verging folds in a similar set-ting is related to accretion of sediment by under-plating and duplex formation (Sample and Fisher, 1986). The second deformation occurred after accretion of Prince William terrane, be-cause biotite-cordierite grade rocks and plutons occur across the terrane boundary. Metamor-phism and melting overlapped in time with de-velopment of second-generation north-verging folds, steeply dipping cleavage and schistosity, and near-horizontal east-west fold axes and linea-tions. Anatectic melts are concentrated in the fold hinges. This stage of deformation resulted in boudinage and folding of the felsic sills that were intruded parallel to the axial planes of south-verging isoclinal folds.

On the basis of preliminary data of Hudson and Plafker (1982), field relations at five locali-ties studied in detail by Sisson et al. (1988), recent regional mapping of the U.S. Geological Survey, and age determinations, the thermal and tectonic history of the Chugach metamorphic complex is as follows. (1) Thickening of a clas-

tic sediment wedge occurred during accretion after deposition of sediment during the Campa-nian and Maastrichtian (84-66 Ma). This was accompanied by development of south-verging folds and by greenschist metamorphism. (2) These sediments were intruded by felsic sills and plutons prior to 53 Ma (hornblende K-Ar and 40Ar/39Ar dates). Except for weak foliation at their margins, the sills, which are metres to hundreds of metres thick, are unfoliated. (3) During heating to peak metamorphic tem-peratures, the later stages of deformation oc-curred in response to compression of crust weakened on the whole by injected layers of felsic melt. Additional melt then intruded along the present near-vertical foliation. Although the original north-dipping contacts were folded dur-ing this stage, excellent three-dimensional expo-sure allows recognition of the average northerly dip of the sills. The date of the later stage of deformation is constrained by the cooling dates of the sills. (4) Cooling of the plutons and met-amorphic rocks through the biotite Ar retention temperature (350 °C) occurred between 51 and 47 Ma (biotite K-Ar and ^ A r / ^ A r dates: Winkler and Plafker, 1981; G. Plafker, unpub. data; Sisson et al., 1988).

CONDITIONS OF METAMORPHISM The limits of the Chugach metamorphic

complex are defined by disappearance of chlorite and appearance of biotite, cordierite, and/or garnet (Hudson and Plafker, 1982). In the north, the distribution of garnet relative to cordierite may suggest a slightly higher pressure of metamorphism there, possibly related to synmetamorphic thrusting. Andalusite is gener-ally the stable aluminosilicate polymorph. Fibro-lite, rare staurolite, and pseudomorphs of sillimanite after andalusite occur in the central zone of the complex. Overall, metamorphism is developed on a large scale, and temperature in-creases toward the core, but locally high temperatures occur near the contacts of large felsic intrusives (Fig. 1).

Total pressure throughout the complex was between 2 and 3 kbar (200-300 MPa) on the basis of mineralogy (andalusite-sillimanite facies series), geobarometry (garnet-plagioclase-alumi-nosilicate-quartz,garnet-plagioclase-biotite-mus-covite), and isochores of associated C02-rich fluid inclusions that pass through the calculated pressures at appropriate temperatures. Thus, amphibolite facies metamorphism occurred at a depth of about 10 km by heating of rocks from

Temperature (°C)

Figure 2. Schematic pressure-temperature-time path for core of Chugach metamorphic com-plex. Arrows marked A and B show individual pressure-temperature paths during garnet growth in northern hanging wall (A) and southern footwall (B). Ages are from 4 0Ar/3 9Ar data on synmet-amorphic tonalite sill with hornblende (square), biotite (circle), and plagioclase (triangle). Horizontal errors are derived from range in estimated argon blocking temperature. Heavy line is schematic pressure-temperature path. Final part of uplift path (dashed line) is unknown: either uplift began after steady-state geothermal gradients were established at 35 Ma, or uplift began before argon closure in biotite.

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greenschist fades (300-400 °C) to maximum temperatures of 600-650 °C in the migmatitic core. Depth of burial for greenschist metamor-phism is also constrained to be close to 10 km on the basis of the pressure indicated by iso-chores for synmetamorphic, low-salinity fluid inclusions if we assume entrapment at the temperature of greenschist metamorphism (V. Sisson, unpub. data).

Locally, pressure of metamorphism changed during the garnet growth as calculated from garnet zoning profiles by the method of Spear and Selverstone (1983). In some regions, garnets grew during decompression; in other areas, they grew during compression (paths A and B, Fig. 2). It is reasonable to presume that the amphibo-lite facies metamorphism occurred during thrusting.

CAUSE OF INITIAL HIGH THERMAL GRADIENT

Most thermal models of subduction zones as-sume that a steady-state thermal gradient (10 °C/km) in the accretionary wedge will result in low temperatures of 100-200 °C at a depth of 10 km. Thus, a mechanism must be found to raise the ambient temperature by 200 to 300 °C prior to the amphibolite facies meta-morphism. Two possibilities are (1) heating of fluids at depth, followed by injection of hot fluids into the accretionary prism, and (2) heat-ing by conduction during subduction either of hot, young oceanic crust or of a spreading ridge (Marshak and Karig, 1977). These two mecha-nisms may act together: heat may be conducted from young oceanic crust into entrained sedi-ment (Clowes et al., 1987), and this heat may be transported to shallower levels by aqueous fluids liberated by dehydration of layer silicates within the sediment. Other sources of high geothermal gradients, such as conductive heating during bur-ial (Birch et al., 1968), shear heating (Scholz, 1980), heating due to high radiogenic element content, or heating due to overthrusting of hot rocks (Oxburgh and Turcotte, 1974; Crawford and Mark, 1982; Thompson and England, 1984; Hodges and Royden, 1984), can be discounted because the rock properties and geologic setting are not appropriate for these processes.

Progressive metamorphism of sediment as it is subducted could lead to release of water. This process may be similar to channelized advective dewatering documented in modern accretionary prisms (e.g., Moore et al., 1987; Ritger et al., 1987). Some support for this mechanism is the widespread occurrence of quartz veins in the greenschist facies part of the Valdez Group. Wood and Walther (1986) and Yardley (1986) calculated fluid:rock ratios of 6:1 to 1.5:1 for other areas with abundant quartz veins. There is evidence for abundant fluid flow through correl-ative accretionary sediments on Kodiak Island (Myers and Vrolijk, 1985). Either adiabatic or nonadiabatic rapid transport of hot fluids with

fluid:rock ratios of 4:1 to 1:1 could have pro-vided sufficient heat to raise the temperature to that of greenschist facies because heat capacity of aqueous fluids is about four times that of average rock. Low-salinity aqueous fluid inclu-sions observed in several generations of quartz veins from the greenschist region support this model: the first pulse of fluid in each vein was hotter than the country rock, and the fluid temperature cooled following the initial injec-tion (V. Sisson, unpub. data).

The subducted oceanic plate may have been quite young at the time of metamorphism. Pre-liminary thermal modeling (James et al., 1986) shows that subduction of hot oceanic crust that was less than 0.5 m.y. old could have conducted sufficient heat into the overlying sediment within 2 km of the decollement. Hot young oceanic crust would plunge into the mantle at a shallow angle, which would also be consistent with the absence of an island arc in the area at 60-50 Ma (Winkler and Plafker, 1981). How-ever, if subduction of young oceanic crust ceases, there will be a thermal pulse into the overriding plate that may generate the thermal anomaly.

CAUSE OF HIGH-TEMPERATURE METAMORPHISM

The metamorphic conditions suggest that metamorphic field gradients ranged from 35 °C/km on the edge of the complex to 65 °C/km in the core. This range in high metamorphic gradients, combined with the age data indicating rapid cooling (Fig. 2), implies that steady-state conductive heat flow was not achieved during metamorphism. Thus, advective or convective mechanisms of heat transport are likely.

The injected felsic sills and plutons, although not apparently volumetrically abundant, must

have transported sufficient heat into the section to raise geotherms high enough to form region-ally developed andalusite and cordierite. There was not a significantly larger volume of magma that passed through the complex at this time, because there is no eyidence for voluminous volcanism (Winkler and Plafker, 1981). Accord-ing to Lux et al. (1986), several factors will influence the formation of large-scale metamor-phic zones in response to magma injection. For example, shallow-dipping sills will have wider aureoles than would occur around steeply dip-ping cylindrical plutons. In addition, the initial temperature (300-400 °C) of country rock is important to produce regional metamorphism because if the country rock were initially cool, then only narrow contact aureoles would de-velop. Both criteria are met in the Chugach metamorphic complex.

The north-dipping attitudes of the sills suggest that at least some melt originated to the north of the Chugach metamorphic complex. In that the magmas were apparently generated by melting of subducted accretionary wedge sediment (Hudson et al., 1979), it is unlikely that they were generated from directly below within the accretionary prism because known thicknesses of modern accretionary prisms do not exceed about 20 km. This thickness is comparable to the total thickness of the Chugach metamorphic complex when the melts intruded; relict oceanic crust may now be only 10 km below the erosion thickness (Page et al., 1986), and metamorphic conditions imply that intrusion occurred at a depth of 10 km.

SUMMARY AND CONCLUSIONS The Chugach metamorphic complex is a low-

/Vhigh-T suite in an accretionary prism com-posed of tectonically thickened clastic sediment.

a

km Heating by Ascending Fluids

( H 2 0 , C 0 2 , C H 4 ) and/or

Subduction of Young Crust

N

Figure 3. Schematic de- 6 5 - 6 0 Ma velopment of low-P/high-T metamorphism in Chu-gach metamorphic com-plex. a: Initially, Valdez Group sediments are bur-ied, deformed, and heated by either advection of hot fluids or conduction from hot, buoyant, young oce-anic crust; heating oc-curred between 65 and 60 Ma. Deformation involved thrusting and formation of south-verging folds. Hot fluids are represented by mixtures of H20, C02, and CH4 found in quartz veins and pods (arrows and blobs), b: Additional heat-ing is associated with intrusion of felsic mag-mas from downdip source. Continued compression causes local development of north-verging folds. Migmatites (dot pattern) form around felsic intrusions (random-v pattern). There is no horizontal scale because amount of shortening is unknown.

6 0 - 5 0 Ma

Heating by Intrusion of Tonalité

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The increased heat flow near the Earth's surface is attributed to interrelated mechanisms during ocean/continent accretionary processes (Fig. 3). First, the thickened sedimentary pile was appar-ently heated, at least in part, by advection of hot fluids that were generated by dehydration of sed-iment entrained in what was probably a rela-tively shallow subduction zone involving hot, young oceanic crust. This resulted in regional upper greenschist facies metamorphism. The complex was further heated by felsic magmas, probably generated from farther down the sub-duction zone, which intruded parallel to a north-dipping foliation. The ultimate source of heat for generating fluids and melts is unknown, but the role of young oceanic crust seems likely, consid-ering the possible plate-tectonic setting of the Chugach metamorphic complex during the early Tertiary.

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Reviewers' comments

along the Oregon/Washington margin: Geo-logical Society of America Bulletin, v. 98, p. 147-156.

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ACKNOWLEDGMENTS This work was carried out as part of the U.S. Geo-

logical Survey Trans-Alaskan Crustal Transect proj-ect, which provided the majority of the support; partial support was provided by National Science Foundation Grant EAR 83-19249. We thank George Plafker and Warren Nokleberg (regional geology), Tom James (thermal modeling), Walter Kauzman (properties of aqueous fluids), and Tullis Onstott ( ^ A r / ^ A r age data) for discussion; and Jeff Grambling and Charles Guidotti for helpful reviews of the manuscript.

Manuscript received October 5, 1987 Revised manuscript received December 29, 1987 Manuscript accepted January 7,1988

The central theme, that of focusing metamorphic fluids along major structural zones and thus "preheating" the rocks that are later involved in regional metamorphism, is unique and highly innovative.

Jeff Grambling

Provides a nice argument for low-P/high-T metamorphism in a tectonic setting usually associated only with high-P/low-T metamorphism.

Charles Guidotti

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