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Proc. Natl. Acad. Sci. USA Vol. 94, pp. 9532–9537, September 1997

Buoyancy-driven, rapid exhumation of ultrahigh-pressure metamorphosed continental crust

W. G. ERNST*, S. MARUYAMA†, AND S. WALLIS‡

*Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115; †Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro-ku, Tokyo 152, Japan; and ‡Department of Geology and Mineralogy, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan

Contributed by W. G. Ernst, June 24, 1997

ABSTRACT Preservation of ultrahigh-pressure (UHP) coupled to the descending lithosphere, continental indentation minerals formed at depths of 90–125 km require unusual would occur instead (7). conditions. Our model involves underflow of a For the UHP case discussed here involving well-bonded salient (250 ؎ 150 km wide, 90–125 km long) of continental crust plus mantle, entrance of increasing amounts of sialic crust embedded in cold, largely oceanic crust-capped litho- material into the subduction zone enhances the braking effect sphere; loss of leading portions of the high-density oceanic of buoyancy; this in turn results in loss of the high-density lithosphere by slab break-off, as increasing volumes of mi- lithospheric anchor leading the downgoing plate at interme- crocontinental material enter the subduction zone; buoyancy- diate upper mantle depths where the sinking lithosphere is in driven return toward midcrustal levels of a thin (2–15 km extension (8). Slab break-off (9, 10) enhances buoyancy fur- thick), low-density slice; finally, uplift, backfolding, normal ther, and causes the sialic prong—or at least a slice thereof—to faulting, and exposure of the UHP terrane. Sustained over decouple from the descending but faltering lithospheric plate Ϸ20 million years, rapid (Ϸ5mm͞year) exhumation of the and move back up the subduction channel. Exhumation is thin-aspect ratio UHP sialic sheet caught between cooler aided in part by (i) progressive shallowing of the ruptured and hanging-wall plate and refrigerating, downgoing lithosphere now buoyant, rebounding continental-crust-capped litho- sphere, and perhaps more importantly, (ii) due to reduction of allows withdrawal of heat along both its upper and lower the shear force acting along its base due to its increasingly surfaces. The intracratonal position of most UHP complexes ductile behavior as the slab gradually warms with depth in the reflects consumption of an intervening ocean basin and deep upper mantle. Because of continued subduction-induced introduction of a sialic promontory into the subduction zone. refrigeration tectonically beneath the rising UHP complex, UHP metamorphic terranes consist chiefly of transformed, and observed extensional faulting against the overlying, cooler yet relatively low-density continental crust compared with hanging-wall plate, relatively thin slices of UHP terranes displaced mantle material—otherwise such complexes could effectively lose heat along both upper and lower surfaces not return to shallow depths. Relatively rare metabasaltic, during ascent; thus, such complexes may nearly retrace the metagabbroic, and metacherty lithologies retain traces of subduction-zone pressure–temperature (P–T) trajectory dur- phases characteristic of UHP conditions because they are ing decompression (11, 12). massive, virtually impervious to fluids, and nearly anhydrous. Proposed relationships are shown diagrammatically in Fig. In contrast, H2O-rich quartzofeldspathic, gneissose͞ 1, and apply equally well to the exhumation of high-pressure schistose, more permeable metasedimentary and metagra- (HP) and UHP terranes. An upper normal fault and a lower nitic units have backreacted thoroughly, so coesite and other reverse fault bound the thin-aspect-ratio slab. Such shear UHP silicates are exceedingly rare. Because of the initial senses seem required by structural relations, for example, in presence of biogenic carbon, and its especially sluggish trans- the Dora Maira Massif (13–15), and in the Dabie Shan (16). formation rate, UHP paragneisses contain the most abun- Yet another exhumation scenario involves the antithetic fault- dantly preserved crustal diamonds. ing characteristic of some compressional orogens, in which double vergence is produced during end stages of the collision Previous workers (1) have demonstrated that deep subduction and ascent of sialic crust (17). of continental crust is required to explain the generation of Where thin UHP slices are exhumed during continued ultrahigh-pressure (UHP) terranes. An outstanding petrotec- subduction͞refrigeration, the ascending complex will more- or-less follow the prograde metamorphic P–T path in reverse; tonic problem consists of elucidating the manner in which this phenomenon has been documented, for instance, in these complexes have returned to shallow levels while preserv- HP͞UHP terranes of the western Alps and the California ing intact relics of the UHP phase assemblages. The ‘‘two-way Coast Ranges (18–20). For thick, more nearly equidimensional street’’ nature of subduction zones was recognized long ago masses (Ͼ30 km thick?), the ratio of cooling surface to mass (2–4). Briefly, salients or peninsulas of old continental crust, is low, and central portions are likely to remain sufficiently hot thoroughly embedded in chiefly cold, oceanic-crust-capped during decompression for the complete obliteration of all UHP lithospheric plates, descend rapidly, generating the character- relics, and perhaps even for partial melting to ensue; accord- istic UHP mineralogy (5, 6). The UHP slabs will be subducted ingly, such ascending, hot bodies retain none of the precursor to depths where the buoyancy forces tending to drive them UHP mineral assemblages. back upward are exactly balanced by the dynamic forces tending to subduct them still further. In a contrasting type of Buoyancy Forces , where the sialic crust is weak and poorly The densities of unaltered oceanic crust (Ϸ3.0), continental The publication costs of this article were defrayed in part by page charge crust (Ϸ2.7), and mantle materials (Ϸ3.2) increase with ele- payment. This article must therefore be hereby marked ‘‘advertisement’’ in vated pressure, reflecting the progressive transformation of accordance with 18 U.S.C. §1734 solely to indicate this fact. © 1997 by The National Academy of Sciences 0027-8424͞97͞949532-6$2.00͞0 Abbreviations: UHP, ultrahigh pressure; HP, high pressure; P–T, PNAS is available online at http:͞͞www.pnas.org. pressure–temperature.

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will tend to sink. Of course, if the conversion of crustal slices to UHP mineral assemblages is incomplete, continental crust should be even more buoyant than indicated in Table 1, whereas the oceanic crust would be less negatively (even positively) buoyant, depending on the extent of transformation to high-density phases. The several forces acting upon a sheet of subducted sialic crust, illustrated schematically in Fig. 1, may be described as follows. (i) Subduction of a low-density sialic slab occurs provided shear forces caused by underflow (Fs) overcome the combined effects of buoyancy (Fb) and frictional resistance along the upper wall of the subduction channel (Fr). In this case, Fs Ͼ Fb sin⌰ϩFr.(ii) Slab rise—not necessarily the complete section of sialic crust—occurs provided buoyancy is positive, and greater than the combined effects of shearing along its base and resistance to movement along its upper surface. For this situation, Fb sin⌰ϾFsϩFr. Kinematically, the illustrated process is rather similar to the ‘‘slab-extrusion’’ mechanism that Maruyama et al. (23) pro- posed specifically to account for the Dabie Shan UHP rocks of east-central China, as well as tectonic models advanced for the and the Alps (24–27). Explicit in our scenario, however, is body–force propulsion of the UHP metamor- phosed sheet of continental crust back up the subduction zone due to its overall buoyancy, in contrast to the mechanism of compressional extrusion (28–31). But are other observed features of HP and UHP complexes explicable as conse- FIG. 1. Schematic diagram portraying the deep burial and thermal quences of our formation and exhumation mechanism? structure of a subducted microcontinent or continental salient (a), then decompression cooling of a rising slice of UHP quart- Easternmost Java Trench: A Modern Collision Zone zofeldspathic —not necessarily the complete section of sialic crust—accompanying steady-state subduction (b) [after Ernst and Peacock (12)]. During uplift of a thin UHP terrane (thickness some- A modern geologic analogue of the model shown in Fig. 1 is what exaggerated for clarity), cooling of the upper margin of the sheet represented by the eastern portion of the Indonesian arc takes place where it is juxtaposed against the shallower, lower tem- (32–34). The strongly curved portion of the Australian– perature hanging-wall plate; cooling along the lower margin of the Eurasian collisional suture zone between Timor and Seram is sheet takes place where it is juxtaposed against the lower temperature, illustrated in Fig. 2a) (33, 35, 36), with the geographic distri- subducting͞refrigerating plate. Stages depicted are as follows: (a) bution of uplifted HP blueschists indicated. The driving force prior to exhumation of the UHP complex; and (b) during exhumation for wedge ‘‘extrusion’’ is derived from decoupling and accel- of a thin (2–15 km thick) slice of the UHP complex. It is evident that erated sinking of the oceanic-crust-capped lithospheric slab tectonic exhumation of UHP continental slices requires erosive de- diagrammed in Fig. 2b, and the consequent reduction in shear nudation and͞or gravitational collapse as well as the presence of a sialic root still at depth. The resolution of forces acting on the sialic slab stress along the base of the buoyant, ductile sialic material as in stages a and b are discussed in the text. A, asthenosphere; L, it warms within the upper mantle. This lithospheric detach- lithosphere. ment has been documented seismically by Osada and Abe (35) and by Widiyantoro and van der Hilst (37). Loss of the dense, framework- and layer-silicates to chain- and ortho-silicates. leading, oceanic portion of the Australian plate may be partly Typical UHP mineralogic assemblages and computed rock responsible for a shallowing of the angle of northward sub- densities appropriate for burial depths of Ϸ100 km are listed duction, thus aiding in the exhumation of continental crust. in Table 1 (21, 22). Coesite-bearing granitic gneiss is less dense Most importantly, highly ductile, relatively low-density quart- than garnet (or spinel) lherzolite, whereas metabasaltic eclo- zofeldspathic material is sandwiched between dense, relatively gite is more dense than the mantle. Accordingly, deeply slowly deforming, more nearly rigid mantle peridotite consti- subducted UHP sialic crustal sections will remain buoyant tuting both hanging-wall and foot-wall blocks; hence buoyancy- relative to the displaced mantle and will tend to rise, whereas driven ascent of the sialic material as a Ϸ10-km-thick wedge eclogitized oceanic crust will become negatively buoyant and may take place along the subduction channel (the plate junction) with walls acting as stress guides (38). Table 1. Densities of rocks and minerals (21, 22) involved in UHP metamorphism Brief Summary of Several Exhumed Sialic UHP Complexes Mineral Mode, Rock Rock type Mineral density % density A small number of sialic crustal complexes, chiefly located within Eurasia, exhibit rare, scattered effects of UHP recrys- Garnet lherzolite Enstatite 3.21 30 3.24 tallization (20, 39). Well-recognized tracts include the Qin- Diopside 3.25 15 ling–Dabie–Sulu belt of east-central China, the Kokchetav Olivine 3.22 50 Massif of northern Kazakhstan, the Maksyutov Complex of the Pyropic garnet 3.67 5 southern Urals, the Dora Maira Massif of the western Alps, Basaltic eclogite Omphacite 3.34 55 3.74 and the Western Gneiss Region of Norway. Each complex Garnet 4.23 40 contains relict phases indicative of production at mantle Rutile 4.25 5 depths approaching—and in some cases substantially exceed- Granitic gneiss Jadeite 3.28 40 3.03 ing—100 km (40, 41). Mineralogic evidence includes the Coesite 3.01 35 preservation of trace amounts of minerals and assemblages K-feldspar 2.56 15 such as coesite, diamond, K-rich clinopyroxene, pyropic gar- Muscovite 2.85 10 net, and volatile-bearing assemblages including magnesite and Downloaded by guest on September 28, 2021 9534 Geology: Ernst et al. Proc. Natl. Acad. Sci. USA 94 (1997)

FIG. 2. Modern analogue of continental collision and exhumation of a type A blueschist belt, modified after Osada and Abe (35), FIG. 3. Stability fields for blueschists (shaded pattern), eclogites Charlton (33), and Maruyama et al. (36). The plate-tectonic setting and (stippled pattern), and adjacent metamorphic facies (unpatterned) partly exposed HP blueschist belt (shaded pattern) are shown in a. The [after Maruyama et al. (36)]. Light lines provide equilibrium curves for Australian continental crust is being subducted beneath the Timor– several univariant reactions, mid-weight lines indicate metamorphic– Seram segment of the Indonesian suture zone. The interpretive facies boundaries (actually multivariant zones of appreciable P—T cross-section along line N-S is depicted in b. Geophysically docu- width), and heavy lines show generalized P—T trajectories for specific, mented slab break-off is shown as localized at the continent–ocean well-documented HP͞UHP terranes. Light dashed line indicates the crustal interface (reasonable but not essential). The hypothesized minimum melting curve for H2O-saturated granite. P–T trajectories buoyant exhumation of a slice of profoundly subducted continental for exhumation of the preserved UHP relics may be uncommon, and crust (pattern of crosses) is a consequence of the contrasting densities according to our tectonic scheme, would be more tightly constrained of surrounding mantle peridotite above and below, and medial, to retrace the prograde path in reverse (see text for discussion). Facies sandwiched quartzofeldspathic sheet (Table 1). Shown are two pos- abbreviations: BS, blueschist; Zeo, zeolite; PP, prehnite-pumpellyite; sible contributions to the exhumation of UHP crustal rocks: (i) PrA, prehnite-actinolite; GS, greenschist; AP, actinolite-calcic plagio- rebound of the subducted, ruptured lithosphere due to removal of load clase; EA, epidote-amphibolite; AM, amphibolite; HGR, HP granu- through decoupling of the dense, oceanic crust-capped (block pattern) lite; GR, granulite͞pyroxene-hornfels; EC, eclogite; PG, pumpellyite- lithospheric anchor; and (ii) buoyancy-driven return flow back up the glaucophane subfacies; LG, lawsonite-glaucophane subfacies; EG, subduction channel, reflecting increase in ductility and reduction in epidote-glaucophane subfacies. Phase abbreviations: Jd, jadeite; Qz, the relative importance of subduction-related shear forces. RF, reverse quartz; LAb, low albite; HAb, high albite. fault; NF, normal fault. propriate to the granulite and pyroxene-hornfels metamorphic diopside, coesite and dolomite, talc and kyanite, and͞or facies, complete overprinting of the earlier UHP assemblages phengite, ellenbergerite, lawsonite, zoisite, and Na-amphibole would be expected, as is evident from Fig. 3. Although this (42). According to numerous, multi-laboratory thermobaro- recrystallization may not run to completion under very dry metric measurements, attending temperatures were Ϸ700– conditions (44–47), the rare and fragmentary preservation of 900°C at confining pressures approaching 28–40 kbar. Pro- UHP minerals, chiefly as microscopic inclusions in tough, grade (subduction) and retrograde (exhumation) P–T trajec- refractory host minerals (garnet, zircon, clinopyroxene, etc.) tories for several well-studied UHP and somewhat comparable suggests that these UHP phases retrogressed rather thoroughly HP complexes are illustrated in Fig. 3. on return toward the Earth’s surface. A more plausible con- As with other relatively HP metamorphic terranes, these clusion than that involving an adiabat is that complexes remarkably high pressures and moderate temperatures can be retaining traces of UHP phases must have more-or-less re- generated through the profound subduction of coherent tracts traced the prograde subduction P–T path in reverse. of cold, sialic-crust-capped lithosphere, reflecting the fact that Alternatively, because of the apparent short duration of geologic materials are poor thermal conductors (5, 43). Return individual UHP events, on the order of a few million years, it toward the surface is also understandable, based on the is apparent that in at least a few cases, metastable lower buoyancy of UHP continental crust, once it has decoupled pressure precursor mineral assemblages failed to react during from the downgoing slab (6, 38). Because purely adiabatic prograde metamorphism to produce the stable UHP config- decompression would result in the transit of rising subduction uration except in kinetically favorable metamorphic environ- complexes through P–T regimes (700–800°C, 2–10 kbar) ap- ments (44, 48–52). Because H2O catalyzes transformations Downloaded by guest on September 28, 2021 Geology: Ernst et al. Proc. Natl. Acad. Sci. USA 94 (1997) 9535

through a solution-redeposition mechanism, massive, coarse- (iii) Massive metamafic rocks and͞or siliceous schists typi- grained, impermeable, anhydrous protoliths would be ex- cally contain most of the unambiguous relict UHP silicate pected to retain original lower pressure mineral assemblages phases. Metacarbonates carry very rare UHP minerals; schis- temporarily even at ultrahigh pressures. UHP assemblages, tose metapelites and gneissose quartzofeldspathic lithologies therefore, are not likely to be formed if H2O is absent during generally lack UHP silicates (exceedingly rare UHP micro- relatively brief periods of prograde metamorphism, or pre- inclusions in garnet and͞or zircon have been described), but do served if H2O is present during later retrogression. Optimal carry evidence of neoblastic diamond in a few occurrences. conditions are doubtless only imperfectly achieved at best, and (iv) Postmetamorphic products of are present but could be an important reason for the extreme rarity of UHP are not necessarily voluminous. relics. (v) Well-developed coeval calc-alkaline volcanic͞plutonic Summaries of the geologic setting, mineral parageneses, belts are not generally associated with these UHP terranes. tectonic evolution, and age constraints for five UHP terranes (vi) Insofar as known, the lateral dimensions of UHP belts were presented previously (53). Pertinent data and tentative developed within continental crust are relatively modest (100– conclusions are summarized in Table 2 (12, 54). As evident 400 km in length, 8–75 km in breadth). from this compilation, well-studied Eurasian UHP metamor- Circumpacific convergence zones are characterized by wide- phic complexes consist of relatively thin slabs or sheets 2–15 spread blueschists, tectonic melanges, and dismembered km thick; these are composed dominantly of sialic crust, and complexes, reflecting the underflow of thousands of those that retain UHP relics appear to have been exhumed kilometers of oceanic-crust-capped lithosphere, an environ- ment termed type B subduction by Bally (56); in contrast, type rapidly to midcrustal levels—approximately 5 mm͞year— A zones of continental collision typically involve the consump- maintained over an interval of roughly 20 million years. tion of intervening ocean basins of more modest size. Table 3 Cooling from above along the upper, bounding normal fault, lists aspects of the contrasting natures of type A and type B and from below along the lower, bounding reverse fault could subduction-zone assemblages. Metamorphic prograde and ret- account for the preserved UHP mineralogic relics, as dia- rograde P–T paths followed by such terranes during subduction grammed in Fig. 1. Similar geologic relationships have been and subsequent exhumation are topologically similar to one summarized recently for HP blueschist belts worldwide by another, but type A continental collisional complexes retain Maruyama et al. (36). Now we turn to an enumeration of the relics of UHP metamorphism (Pmax ϭ 28–40 kbar) whereas special characteristics of these relatively unusual UHP ter- type B circumpacific-style underflow characteristically gener- ranes. ates only HP belts (Pmax ϭϷ12–15 kbar).

Petrotectonic Features of UHP Metamorphosed Continental Discussion Crust How do these characteristic petrotectonic features of sialic Any exhumation scenario proposed to explain the origin, UHP complexes relate to the dynamic generation͞exhumation plate-tectonic setting, and P–T evolution of UHP metamor- history previously proposed? We discuss them below in the phosed sialic complexes must account for the following general order just enumerated. observations regarding such terranes (42, 55). (i) Mechanical analyses suggest that continental crust sev- (i) They are developed within old (ϭ cold) sialic crust and eral kilometers or more thick may well be subducted if it enters are now confined principally to intracratonal collisional su- a convergent plate junction (17, 57, 58). A favorable geologic tures. environment would involve entrance into the subduction zone (ii) Quartzofeldspathic rocks (ortho- and paragneisses͞ of a narrow salient or prong of continental crust as an integral paraschists, migmatites, and metagranitoids) dominate the part of an old, thermally relaxed, largely oceanic crust-capped section, followed by metapelitic and metacalcareous strata. slab (6). Although very rare, the existence of UHP metagrani- Mafic and ultramafic rocks are volumetrically minor. toids and quartzofeldspathic gneisses (59, 60) demonstrates

Table 2. Metamorphic-tectonic summary of ultrahigh-pressure metamorphic complexes, modified after Ernst Peacock (12) and Beane et al. (54) Qinling–Dabie–Sulu Kokchetav Massif, Maksyutov Doar Maira Massif, Western Gneiss Region, belt, coesite-eclogite unit I Complex, unit 1 lower Venasca Fjordane complex Protolith formation age 1.3–2.9 Ga 2.2–2.3 Ga 1.2 Ga 303 Ma 1.6–1.8 Ga Temperature of UHP metamorphism, °C 750 Ϯ 75 900 Ϯ 75 625 Ϯ 50 725 Ϯ 50 825 Ϯ 75 Depth of UHP metamorphism, km 90–120 125 90 90–120 90–125 Time of UHP metamorphism, Ma 210–220 530–540 375–380 35–40 420–440 Midcrustal annealing, Ma 180–200 515–517 360 Ϯ 5 (?) 15–25 375 Rise time to midcrust, Ma 25 Ϯ 10 20 Ϯ 5 15–25 (?) 20 Ϯ 555Ϯ5 Exhumation rate, mm͞yr 3–4 5–6 3–5 4–5 1–2 Coesite included Relatively abundant Rare Very rare (?) Relatively abundant Very rare Diamond included Very rare Relatively abundant Questionable Absent Very rare Blueschists Present Rare (?) Present Present Absent Areal extent, km 400 ϫ 75 Ϸ100 ϫ 15 120 ϫ 8 225 ϫ 60 350 ϫ 70 Maximum thickness of complex, km 10 5–10 4–6 1–2 10–15 (?) Exhumation rates were estimated by dividing depth of UHP metamorphism by rise time to near-surface of midcrustal levels of Ϸ15 km (20, 52). Downloaded by guest on September 28, 2021 9536 Geology: Ernst et al. Proc. Natl. Acad. Sci. USA 94 (1997)

Table 3. Generalized lithologic comparison of type A and type B subduction-zone tectonic regimes [after Maryuama et al. (36)] Type A (collisional) Type B (circumpacific) UHP MP Protoliths Shallow-marine sediments Platform carbonates Reefal limestones Clastic wedges Multicycle orthoquartzites, First-cycle graywackes, peraluminous shales olistostromes Deep-sea sediments Uncommon Bedded cherts, Mn nodules Igneous rocks Bimodal, basalts ϩ dacites MORBs, seamounts (OIBs) Continental basement Granitic gneiss complexes Absent Ore deposits Kuroko-type (massive Mid-ocean ridge origin sulfides) Petrology Typical maximum pressure, 28–40 Ϸ12–15 kbar Associated calc-alkaline Rare or absent Huge belt Degree of retrogression Nearly complete Incomplete Mantle fragments Garnet lherzolite Spinel, plagioclase lherzolite Coeval paired belts Absent Present MORB, mid-oceanic ridge basalt; OIB, oceanic island basalt.

that such subduction does occur, and that buoyant sialic are sufficient to cause backreaction to the low-pressure poly- material may be carried down to depths of at least 90–125 km. morph on decompression (62). UHP metamorphism is a predictable consequence of this Carbon is present in important concentrations only in process and is independent of lithology. In our model, how- metasedimentary rocks, so it is no surprise that neoblastic ever, the presence of buoyant sialic rocks is an absolutely diamond and diamond pseudomorphs in crustal rocks are essential prerequisite for the subsequent uplift͞exhumation of virtually confined to paragneisses and paraschists; in such the deeply buried terrane. Large masses of eclogitized oceanic occurrences it is isotopically light, indicating a biological crust would remain negatively buoyant, and hence would origin, according to Sobolev and Sobolev (63) and Lennykh et continue descent into the deep mantle. Accordingly, resur- al. (64). Silicates in general tend to transform at faster rates rected UHP complexes are restricted to zones of continental than that which characterizes the diamond to graphite transi- (and microcontinental) collision, the environment of type A tion, so in some occurrences relict diamond may be preserved subduction (56). even where the UHP silicate assemblages have been com- (ii) Lithospheric slabs descend to depths of at least 650–700 pletely obliterated. km along inclined subduction channels, so where underflow (iv) To elucidate possible retrograde P–T trajectories allow- exceeds a few centimeters per year, UHP metamorphism is ing preservation of UHP relics, the exhumation of such inevitable (5). However, oceanic crust transformed to eclogite terranes has been modelled (12) as extensional along the upper facies assemblages is denser than mantle peridotite and of bounding surface of a thin sheet whereas subduction– course, garnet lherzolite is slightly denser than plagioclase- and refrigeration continues along the lower bounding surface (Fig. spinel-bearing analogues, so the plate will continue sinking 1). The process, therefore, does not require the wholesale after phase transformations occur. Only where a sufficiently uplift of a lithospheric plate, or even the full thickness of continental crust. For this reason, postmetamorphic erosional large volume of low-density continental material is subducted debris, while considerable, need not be especially voluminous. will body forces (buoyancy) overcome frictional resistance and (v) Circumpacific subduction is a long-continued process, so permit its ascent after disengagement from the downgoing sufficient time is available for the development and maturation lithosphere. Accordingly, recovered UHP complexes, and of the typical calc-alkaline magmatic plumbing system. In circumpacific HP terranes as well, consist dominantly of sialic contrast, continental collision in many cases involves relatively phase assemblages, possessing an aggregate density less than short-lived underflow of an old, thermally relaxed oceanic that of the mantle material they have dynamically displaced crust-capped lithospheric section; thus an andesitic volcanic during subduction. Metabasaltic͞metagabbroic and͞or meta- arc͞granitic plutonic belt may not have had time to develop on peridotitic complexes cannot represent more than a volumetri- the stable, nonsubducted lithospheric plate because of volu- cally minor portion of HP͞UHP terranes, or the latter would metrically limited generation of calc-alkaline melts. not be sufficiently buoyant to rise back to crustal levels. (vi) UHP sialic complexes possess modest along-strike di- (iii) Foliated rocks of quartzofeldspathic and pelitic bulk- mensions, on the order of 100–400 km, and recovered depths rock compositions characteristically contain appreciable H2O of underflow of about 90–125 km; accordingly, they have Ϫ bound as structural OH in the constituent mineral assem- dimensions compatible with a conjectural promontory or blages, whereas massive cherts, carbonates, and mafic igneous salient on the leading edge of the subducting microcontinental rocks are more nearly anhydrous. The former are relatively fragment or continental margin. more permeable to aqueous fluids than the latter; thus it is In conclusion, the described characteristic petrotectonic possible that the retention of UHP relics is kinetically favored features appear to be compatible with the proposed scenario in anhydrous eclogites, siliceous schists, and some marbles, for the formation, thermal evolution, and ultimate tectonic disfavored in more ‘‘juicy’’ quartzofeldspathic and pelitic exhumation and exposure through erosion and͞or gravita- compositions. The absence of H2O and closed-system recrys- tional collapse of UHP terranes (65–67). However, few min- tallization is corroborated by the anomalously low ␦18O values eralogic, radiometric, tectonic, and geologic constraints are measured in garnet and omphacite from Sulu belt UHP available for such UHP complexes; hence future work may well eclogites by Yui et al. (61); it is also suggested by experimental require revision or abandonment of this model. In particular, rate studies of the transformation of coesite to quartz which the rapid ascent rates of Ϸ5mm͞year averaged over more than indicate that even H2O contents of 400–500 ppm in the silica 20 million years and seemingly required by UHP geochrono- Downloaded by guest on September 28, 2021 Geology: Ernst et al. Proc. Natl. Acad. Sci. USA 94 (1997) 9537

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