Continental Crust Subduction and Ultrahigh Pressure Metamorphism

3.09 Continental Crust Subduction and Ultrahigh Pressure Metamorphism D. Rumble Geophysical Laboratory, Washington, DC, USA J. G. Liou Stanford University, CA, USA and B. M. Jahn National Taiwan University, Taipei, Republic of China 3.09.1 INTRODUCTION 3.09.2 WORLDWIDE DISTRIBUTION AND AGES OF UHP METAMORPHIC BELTS 3.09.3 EXOTIC VERSUS IN SITU ORIGIN OF UHP METAMORPHIC ROCKS 3.09.4 MINERALOGIC INDICATORS OF “SUPER” UHP METAMORPHISM 3.09.5 GEOCHEMISTRY OF UHP METAMORPHISM 3.09.5.1 Chemical Compositions of Eclogites and Ultramafic Rocks 3.09.5.2 Sm–Nd, Lu– Hf, and Rb –Sr Isochron Ages and Nd –Sr Isotope Tracers 3.09.5.2.1 REE partitioning 3.09.5.2.2 Incorrect geochronologic ages 3.09.5.2.3 Coupled Nd- and O-isotopic disequilibrium 3.09.5.3 Comparative Radiogenic Geochemistry of Eclogites from UHP Metamorphic Belts 3.09.6 THE FLUID PHASE OF UHP METAMORPHISM 3.09.6.1 Fluid Immobility during Prograde Metamorphism 3.09.6.2 Slab–Fluid–Mantle Interactions 3.09.6.3 Composition of Fluids 3.09.7 CONCLUSIONS ACKNOWLEDGMENTS REFERENCES

293 294 298 299 300 300 303 304 305 306 307 309 310 311 312 312 312 313

necessary consequences of collision but evidence for them is to be found in the disciplines of petrology, structural geology, and geochemistry, not physiography. Exhumation of subducted, buoyant continental rock is a physically inevitable process but one that is especially important to geologists as it exposes these remarkable probes of the upper mantle for study. Geochemistry is important for understanding continental crust

3.09.1 INTRODUCTION Evidence of continental collision is as accessible as the nearest copy of the “Times Atlas.” A glance at the map shows the northern edge of the Indian subcontinent festooned with mountain chains rising as India crashes northward into Eurasia. The processes of continental crust subduction and ultrahigh pressure (UHP) metamorphism are 293

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Continental Crust Subduction and Ultrahigh Pressure Metamorphism

subduction and UHP metamorphism because it is uniquely capable of providing a geochronological framework for determining the sequence and duration of collision, subduction, and metamorphism (see Chapter 3.08). Geochemistry is equally important in its capacity to identify protolith signatures and to trace them through the complex events along the margins of converging tectonic plates. Petrology, in combination with geochemistry, traces the P– T time path of rocks through subduction and exhumation and provides a complete characterization of the geodynamics of continental collision (see Chapter 3.07). The continuing discovery of new UHP metamorphic belts, since the seminal publications of Chopin (1984) and Smith (1984), has unexpectedly and repeatedly re-invigorated the entire field. The authors of this review thus face a daunting task in presenting a coherent view of rapidly developing and ever-increasing research results on a topic that is global in scope, diverse in scientific origins, and of great importance to many specialists striving to understand the growth of continental plates. A search of the bibliographic database “GeoRef” on the keyword “UHP” found over 200 references since 1995. A number of book length reviews of the field are available: Smith (1988), Carswell (1990), Coleman and Wang (1995b), Cong (1996), Bebout et al. (1996), Hacker and Liou (1998), Ernst and Liou (2000), and Parkinson et al. (2002). In addition, there are thematic issues of Journals focused on UHP metamorphism, including Liou et al. (1995), Schreyer and Stockhert (1997), Liou et al. (1998), Carswell (2000), Liou and Banno (2000), Liou and Carswell (2000), Liou et al. (in press-a), and Liou et al. (in press-b). Godard (2001) has presented a historical review of the study of eclogites; Chopin’s (2003) review is recommended. Studies of UHP metamorphism in relation to the tectonics and geodynamics of continental crust subduction and exhumation are given by Hacker and Peacock (1995), Hacker (1996), O’Brien (2001), Peacock (1996), and Roselle and Engi (2002). The objective of the present chapter is to present a compact introduction to the literature of continental crust subduction and UHP metamorphism and to illustrate recent geochemical research with selected case studies. Primary evidence for the subduction of continental crust is found in the similarity of lithologic successions of UHP metamorphic terranes with stratigraphic sequences in unmetamorphosed sedimentary rocks. Interlayered quartzite, marble, mica schist, and gneiss resemble sedimentary sequences of sandstone, limestone, shale, and graywacke, respectively, deposited along continental margins. Concordant layers of eclogite in quartzite, marble, schist, and biotite gneiss suggest basaltic sills or

lava flows. Geochemistry plays an important role in recognizing Earth surface features in subducted rocks. The low d18O ðd18 O ¼ ½{ð18 O= 16 OÞsample = ð18 O= 16 OÞVSMOW }21 £ 1;000 where VSMOW stands for Vienna standard mean ocean water) and low dD rocks of Dabie –Su-Lu, China, for example, demonstrate that UHP gneisses, eclogites, and quartzites were altered by meteoric water under a cold climate in a geothermal system at the Earth’s surface prior to subduction (Yui et al., 1995; Zheng et al., 1996; Baker et al., 1997; Rumble and Yui, 1998). UHP metamorphism is defined by occurrences of the index minerals coesite and diamond in regional eclogite-facies metamorphic rocks. Coesite is found typically as inclusions, tens of microns in diameter, in metamorphic porphyroblasts of carbonate, epidote, garnet, kyanite, omphacite, zircon, and zoisite. Diamond occurs as micron-sized inclusions in garnet, diopside, kyanite, and zircon. The lower limit of P – T conditions for UHP metamorphism is P . 2:7 GPa and T . 600 8C. Recent reports of relict majoritic garnet and other “super” UHP mineralogical indicators suggest exhumation from even greater depths, where pressures exceed those at the graphite – diamond equilibrium boundary.

3.09.2 WORLDWIDE DISTRIBUTION AND AGES OF UHP METAMORPHIC BELTS Outcrops of UHP metamorphic rocks occur in regional belts located principally in the Eurasian landmass. They are also known to occur in Africa, Greenland, Indonesia, South America, and Antarctica (Figure 1 and Table 1). The UHP belts mark the margins of continental plates that are either colliding presently or did so at some time in the geologic past. Among the common characteristics of UHP occurrences are the following: .

. .

UHP mineralogical records are preserved mainly in pods and slabs of eclogite and garnet peridotite enclosed within quartzofeldspathic gneiss. Coesite and diamond inclusions, however, are recognized in metamorphic porphyroblasts from a growing list of rock types. Especially significant are occurrences of coesite inclusions in zircons extracted from quartzofeldspathic gneisses, a lithology that dominates many UHP belts but otherwise carries no signature of UHP metamorphism (Tabata et al., 1998; Ye et al., 2000b; J.-B. Liu et al., 2001; F. Liu et al., 2001a, 2002; Katayama et al., 2000a). Lithologies are largely continental in chemical and isotopic composition (Jahn, 1998). Exhumed UHP units are now present in the upper continental crust as thin subhorizontal

Figure 1 Worldwide distribution of UHP localities with diamond (filled diamond) and coesite (filled circle) occurrences. Age of UHP metamorphism in millions of years.

Table 1 Worldwide occurrences of coesite-eclogite and diamond-eclogite facies rocks. Locality

UHP index minerals

P – T conditions

Age of UHP metamorphism

References

Sulawesi, Indonesia

Coesite in zircon in quartzite

T : 800 ^ 100 8C; P: . 40 kb

20 – 27 Ma

Parkinson and Katayama (1999)

Dora Maira, Italian Alps

Coesite in pyrope from quartzite

T : .700 8C; P: . 28 kb

35 – 38 Ma

Chopin (1984); Nowlan et al. (2000)

Gittidas, Pakistan Himalaya

Coesite in omphacite from eclogite

T : 690– 750 8C; P: 27 – 29 kb

40 – 50 Ma

O’Brien et al. (2001); Kaneko et al. (2001)

Zermatt-Saas, W. Alps

Coesite, aragonite

T : 600– 630 8C; P: 27 – 29 kb

50 – 70 Ma

Reinecke (1998)

Rhodope, Greece

Diamond, majorite, quartz pseudomorph after coesite

T : 600– 900 8C; P: 31 – 39 kb

75 Ma

Mposkos and Kostopoulos (2001)

Dabieshan-Su-Lu Belt, Eastern China

Coesite in garnet, omphacite, zircon; diamond in garnet

T : 600– 900 8C; P: 27 – 50 kb

220– 240 Ma

Liou et al. (1996); Li et al. (2000); Xu et al. (1992, 2003)

Saxonian Erzegebirge, Germany

Micro-diamond included in garnet, kyanite and zircon from quartz-feldspar gneiss; TiO2 phase with a-PbO2 structure

T : .900 8C; P: . 40 kb

Maksyutov Complex, Ural Mountains

Quartz pseudomorph after coesite; Graphite pseudomorphs after diamond

T : 694– 637 8C; P: 15 – 17 kb (annealed?)

375– 400 Ma

Chesnokov and Popov (1965); Beane et al. (1995); Leech and Ernst (1998)

Massif Central, France

Coesite in garnet from kyanite eclogite

T : 700– 800 8C; P: .28 kb

400– 420 Ma

Lardeaux et al. (2001)

E. Greenland Caledonides

Palisaded quartz pseudomorph after coesite

T : 972 8C; P: 36 kb

390– 439 Ma

Gilotti and Krogh-Ravna (2002)

Western Gneiss Region, Norway

Coesite in clinopyroxene and garnet from eclogite; diamond; majorite.

T : .800 8C; P: . 32 kb

400– 410 Ma

Smith (1984); Dobrzhinetskaya et al. (1995); Cuthbert et al. (2000); van Roermund et al. (2002)

N. Qaidam, Western China

Coesite

T : 624– 735 8C; P: 28 – 35 kb

495 Ma

Yang et al. (2001)

Lanterman Range, Antarctica

Coesite in garnet from eclogite

T : 850 8C; P: .29 kb

500 Ma

Ghiribelli et al. (2002)

Kokchetav Massif, Kazakhstan

Diamond in garnet, clinopyroxene, and zircon from gneiss and metacarbonate; coesite in garnet from whiteschist

T : 800– 1000 8C; P: 40 – 60 kb

530 Ma

Sobolev and Shatsky (1990); Shatsky et al. (1999); Katayama et al., (2001)

Brazil

Coesite

?

630 Ma

Parkinson et al. (2001)

Pan-African Belt, Mali, W. Africa

Coesite in omphacite from marble

T : 690– 750 8C; P: .27 kb

620 Ma

Caby (1994); Jahn et al. (2001a)

340 Ma

Massonne (1999); Hwang et al. (2000)

Worldwide Distribution and Ages of UHP Metamorphic Belts

.

slabs bounded by normal faults on top and reverse faults on the bottom, and sandwiched in between high pressure (HP) or lower-grade metamorphic units (Xue et al., 1996; Kaneko et al., 2000). Coeval island-arc volcanic and plutonic rocks do not occur, whereas post-collisional or latestage A-type granitic plutons are common in some UHP belts (Chen et al., 2002).

The Dora Maira Massif, one of the two type discovery sites of regional metamorphic coesite (Chopin, 1984), consists of Late Paleozoic and older continental basement rocks, metamorphosed under UHP and HP conditions as a result of Mesozoic – Cenozoic convergence of the European and African plates. The HP þ UHP belt is a stack of four tectonic units separated by low-angle faults. These units have been affected by Cenozoic HP metamorphism and later re-equilibration at lower pressures. The UHP rocks consist of dark- and light-colored eclogites, pyrope quartzites with phengite schist, and jadeite-rich rock inclusions, as well as orthogneiss country rocks and undeformed metagranites. Boudins of pyrope quartzite contain UHP relics such as coesite and other UHP minerals (Chopin et al., 1991; Compagnoni et al., 1995). Kyanite-eclogites with relict coesite and coesite pseudomorphs have estimated peak P – T conditions of 3.7 GPa and 790 8C, within the diamond stability field. Eclogites re-equilibrated at lower-P were metamorphosed at 1.5 GPa and 500 8C (Nowlan et al., 2000). Analyses of U –Pb in zircon with a SHRIMP ion microprobe give core ages of 240 –275 Ma and rims of 35 Ma (Gebauer et al., 1997). The western gneiss region (WGR) of Norway, the second type discovery site (Smith, 1984) lies along an Early Paleozoic collisional zone between the Fennoscandian and Greenland plates. The WGR is 300 km long and 150 km wide; it consists of interlayered pelite and migmatite, marble, quartzite, and amphibolite, with tectonic inclusions of gabbro and peridotite. The WGR exhibits mainly amphibolite-facies assemblages, but relics of UHP assemblages do occur. Coesite and coesite pseudomorphs have been found as inclusions in metamorphic porphyroblasts in eclogites, kyanite-eclogites, and adjacent gneissic rocks (Smith, 1984; Wain, 1997; Cuthbert et al., 2000; Wain et al., 2000). Microdiamond grains 20 – 50 mm in diameter have been described in mineral separates from two gneisses (Dobrzhinetskaya et al., 1995) and in a mantle-derived peridotite lens from Bardane (van Roermund et al., 2002). Carswell et al. (in press) provide new petrographic evidence and a review of the latest radiometric age data, and conclude that the UHP metamorphism of

297

eclogites within WGR occurred at 400 –410 Ma, significantly younger than the previous, widely accepted age of 425 Ma. A two-stage exhumation process was suggested: an initial exhumation to ,35 km depth by ,395 Ma at a mean rate of ,10 mm yr21, and subsequent exhumation to 8– 10 km by ,375 Ma at a rate of ,1.3 mm yr21. The recent discoveries of relict majoritic garnets containing exsolution lamellae of pyroxene located in peridotite bodies on the islands of Otrogy and Flemsoy (van Roermund and Drury, 1998, van Roermund et al., 2001) are startling. The presence of relict majoritic garnet implies that the peridotites may have been exhumed from depths greater than 185 km. A UHP terrane complementary to WGR and separated from it by the opening of the Atlantic Ocean is located on the east coast of Greenland (Gilotti and Krogh-Ravna, 2002). The Kokchetav Massif, northern Kazakhstan, is the type discovery site for the UHP diamondeclogite regional metamorphic facies (Sobolev and Shatsky, 1990). Diamonds occur as minute inclusions in garnet, diopside, and zircon porphyroblasts from pyroxene – carbonate – garnet rock, and garnet-biotite gneiss and schist. Inclusions of coesite and coesite pseudomorphs are found in garnet from eclogite and diamondbearing gneiss. The UHP/HP unit extends NW – SE for at least 80 km and is ,17 km wide. The UHP slab is structurally overlain by a weakto low-grade metamorphic unit and is underlain by a low-P metamorphic andalusite – staurolite facies unit (Kaneko et al., 2000). The UHP slab is composed of felsic gneiss with locally abundant eclogite lenses and minor orthogneiss, metacarbonate, and rare garnet peridotite with a range of Proterozoic protolith ages. Radiometric age dating yields 530 ^ 7 Ma for UHP metamorphism for eclogite, biotite gneiss, and diamondiferous metasedimentary rocks (Shatsky et al., 1999; Katayama et al., 2001). The Dabie –Su-Lu terrane, east-central China, occupies a Triassic collision zone between the Sino-Korean and Yangtze plates. The central UHP coesite- and diamond-bearing eclogite belt ðP ¼ 2:7 – 5 GPaÞ is flanked to the north by a migmatite zone (P , 2 GPa), and to the south by a belt of blueschist, epidote amphibolite, and eclogite facies rocks ðP ¼ 0:5 – 1:2 GPaÞ. Prevalent rock types include felsic gneiss, orthogneiss, marble, and quartzite. Protolith ages range from Proterozoic to Ordovician but the majority of the reported ages are Neoproterozoic (Li, 1996; Hacker et al., 2000). Blocks, boudins, and layers of eclogites and garnet peridotities occur as enclaves in gneisses in the UHP unit. The Dabie – Su-Lu UHP rocks show several important features. (i) Widespread occurrences of coesite with hydrous minerals such as talc, zoisite/epidote, and phengite are found in eclogites. (ii) There are several garnet peridotites

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Continental Crust Subduction and Ultrahigh Pressure Metamorphism

of mantle origin; others are of crustal origin, formed as crystal cumulates in layered intrusions (Chavagnac and Jahn, 1996; Zhang et al., 1998, 2000). (iii) Abundant exsolution textures were identified in UHP minerals from garnet peridotite and eclogite. Diamond inclusions in garnet eclogite and garnet peridotite have been reported (Xu et al., 1992, 2003). Some garnet peridotites record much higher pressures than associated coesitebearing eclogites, reaching 5 –6 GPa (Yang et al., 1993; Zhang et al., 2000). Various isotopic age dating methods for UHP minerals and rocks from the Dabie – Su-Lu terrane gives 220 – 240 Ma (Ames et al., 1996; Li et al., 1993; Rowley et al., 1997; Li et al., 2000; Hacker et al., 2000; Ayers et al., 2002; Chavagnac and Jahn, 1996; Chavagnac et al., 2001). The Erzgebirge crystalline complex (ECC), a new example of rocks in the diamond-eclogite facies of regional metamorphism, is situated at the northern margin of the Bohemian Massif and is in fault contact with a low-grade Paleozoic sequence at its margins. The Variscan ECC consists of abundant ortho- and para-gneisses that include numerous lenses of eclogite. The “gneiss-eclogite unit” at the core of this massif contains some UHP rocks with inclusions of coesite in garnets and omphacites from eclogite (Massonne, 2001) and inclusions of microdiamonds in garnet, kyanite, and zircon from gneisses that contain abundant quartz, plagioclase, phengite, and minor rutile and graphite (Massonne, 1999). Thus far, the diamondiferous gneisses have only been found in a 1-km long strip near the eastern shore of the Saidenbach reservoir. In addition to completely preserved, 1 –25 mm diamonds, partially graphitized diamonds and graphite pseudomorphs after diamond occur in the host minerals. The microdiamond inclusions in garnet and zircon are similar to those in the diamondiferous biotite gneiss of the Kokchetav Massif described by Sobolev and Shatsky (1990). Some of these inclusions contain additional fluid-bearing phases including apatite, phengitic mica, and possible fluid inclusions. The morphology of the inclusions and their mineral association suggest that microdiamonds from both the Erzgebirge and the Kokchetav may have crystallized from supercritical fluids under UHP conditions (Stockhert et al., 2001; Hwang et al., 2001a; Dobrzhinetskaya et al., 2001, 2003). Schmadicke et al. (1995) reported Sm –Nd isochrons for garnet –clinopyroxene – whole rock at 360 ^ 7 Ma for eclogite and 353 ^ 6 Ma for garnet pyroxenites from the Erzgebirge UHP rocks. 40Ar/39Ar spectra of phengite from two eclogite samples give plateau ages of 348 ^ 2 Ma and 355 ^ 2 Ma. Recently, diamond-bearing zircons were dated by SHRIMP, and yielded 336^ 2 Ma (Massonne, 2001). The similarity in

the age of garnet peridotites and eclogite enclosed in gneiss suggests a coeval Variscan UHP in situ metamorphism of the ECC around 340 – 360 Ma. Other UHP terranes have partially preserved trace index minerals in strong containers such as zircon or garnet (Figure 1 and Table 1; for a review, see Liou et al., 1998). The Maksyutov Complex contains cuboid graphitized diamond in pelitic schist (Leech and Ernst, 1998). Inclusions of coesite and coesite pseudomorphs occur in the Zermatt-Sass area of the Western Alps, Makbal and Atbashy of the Krghyzstan Tien-Shan, Mali of Western Africa, and Sulawesi of Indonesia (Table 1). The latest recognized UHP terranes include: (i) coesite in the French Massif Central (Lardeaux et al., 2001), (ii) coesite and diamond from the Greek Rhodope metamorphic province (Mposkos and Kostopoulos, 2001), (iii) coesite pseudomorphs in the Northeast Greenland Eclogite Province (Gilotti and Krogh-Ravna, 2002), (iv) coesite in granulite-facies overprinted eclogites of SE Brazil (Parkinson et al., 2001), (v) coesite in Himalayan eclogite from the Upper Kaghan Valley, Pakistan (O’Brien et al., 2001; Kaneko et al., 2003) and coesite from the Tso-Morari crystalline complex of India (Sachan et al., 2003), (vi) coesite inclusions in gneissic rocks from the North Qaidam belt, western China (Yang et al., 2001), and (vii) inclusions of quartz pseudomorphs and minor coesite in garnets of mafic ecolgites from the Lanterman Range in Antarctica (Ghiribelli et al., 2002). Among all UHP terranes, the oldest has been dated at ca. 620 Ma for coesite-bearing rocks from the Pan-African belt in northern Mali (Caby, 1994; Jahn et al., 2001a). 3.09.3 EXOTIC VERSUS IN SITU ORIGIN OF UHP METAMORPHIC ROCKS A typical outcrop in a UHP metamorphic belt consists of a lens of eclogite enveloped in quartzofeldspathic gneisses. Petrographic examination of thin sections reveals that the eclogite carries UHP index mineral assemblages, whereas the gneiss is amphibolite facies and may show retrograde features. Outcrop relations like these are seen worldwide and have inspired a spirited debate between those who argue that the eclogites are exotic tectonic blocks whose metamorphism is unrelated to that of the gneisses and those who advocate in situ UHP metamorphism. According to proponents of the exotic hypothesis the eclogites were exhumed from great depth and interleaved tectonically with their wall rocks following UHP metamorphism; the quartzofeldspathic gneisses were never under UHP conditions. Proponents of the hypothesis of in situ UHP metamorphism hold that both eclogites and

Mineralogic Indicators of ‘‘Super’’ UHP Metamorphism gneisses experienced the same metamorphic history, including UHP conditions, but that the gneisses failed to develop UHP indicator minerals owing to an unfavorable bulk chemical composition, or that evidence of UHP was erased from gneisses by retrogradation during exhumation (see Coleman and Wang, 1995a; Harley and Carswell, 1995; Krogh and Carswell, 1995; Schreyer, 1995; Smith, 1995; Cong and Wang, 1996; Ernst and Peacock, 1996; Liou et al., 1996). The question of the exotic versus the in situ origin of UHP metamorphics has tectonic implications. If UHP terranes are exposed at the surface as dismembered me´langes of continental, oceanic, and mantle slivers, disruptive deformation scenarios are evoked. But if UHP rocks are subducted and exhumed as thin but laterally coherent slabs, appeal must be made to quite different mechanisms. There is a logical connection between the exotic versus in situ discussion and hypotheses of exhumation. In the absence of plausible, buoyancy-driven exhumation processes, it was appealing to suggest penetrative faulting for the emplacement and interleaving of UHP and non-UHP rock units. However, with the advent of physical models of exhumation that are driven by body forces rather than surface tractions, it has become possible to consider the exhumation of coherent slabs as a reasonable working hypothesis (Chemenda et al., 1995; Ernst et al., 1997). The debate about exotic versus in situ origin has inspired much research. Five different types of evidence supports the hypothesis of in situ origin. (i) Field mapping demonstrates that UHP belts contain thin nappes of structurally coherent rock slivers (Dora Maira: Compagnoni et al., 1995; Michard et al., 1995; Dabieshan: Cong et al., 1995; Xue et al., 1996; Kokchetav: Kaneko et al., 2000). (ii) Petrologic and mineralogical studies have found examples of quartz-feldspar gneisses from UHP belts with evidence of UHP metamorphism. A number of systematic searches for UHP indicator minerals preserved as inclusions in zircon have shown that coesite is widespread in UHP gneisses, despite the observation that their matrix mineral assemblages are consistent with amphibolite facies metamorphism. Identification of coesite has been confirmed with micro-Raman spectroscopy. In Dabieshan, Tabata et al. (1998) found coesite in zircon from orthogneisses whose matrix mineral assemblages are amphibolitefacies. Similar results are reported from Su-Lu (Ye et al., 2000b) and in southern Dabie near the mapped boundary between coesite-bearing and coesite-free eclogites (J.-B. Liu et al., 2001). Detailed sampling of two drill cores from pre-pilot holes of the Chinese Continental Scientific Drilling Project (CCSDP) near Donghai, in SuLu, found coesite inclusions in zircons from granitic gneisses extending to depths of 432 m

299

and 1,000 m, respectively (F. Liu et al., 2001a,b, 2002). Similar findings are reported from Kokchetav (Katayama et al., 2000a). (iii) A reevaluation of thermobarometry in quartz –feldspar – garnet – phengite – epidote – biotite – titanite orthogneisses from Dabieshan resulted in revised P – T estimates of 3.6 GPa and 690– 715 8C, well within the UHP regime (Carswell et al., 2000). (iv) Geochronology has shown that UHP eclogites and their wall-rock gneisses have the same thermal history and similar metamorphic ages (Dabieshan and Su-Lu: Ames et al., 1996; Rowley et al., 1997; Hacker et al., 1998; Li et al., 2000; Chavagnac et al., 2001; Ayers et al., 2002; Rumble et al., 2002; Kokchetav: Claoue-Long et al., 1991; Shatsky et al., 1999; Katayama et al., 2001). (v) The presence of a pre-subduction, coldclimate geothermal area outcropping contiguously over hundreds of square kilometers in both Dabieshan and Su-Lu is recorded by unusually low d18O and dD values in minerals of eclogites, quartzites, para- and ortho-gneisses (Yui et al., 1995, 1997; Zheng et al., 1996, 1998a, 1999; Baker et al., 1997; Fu et al., 1999). That the geothermal system affected both UHP rocks and their wall rocks equally suggests that both units experienced the same geologic history, and supports the hypothesis of in situ UHP metamorphism (Rumble, 1998; Rumble and Yui, 1998; Rumble et al., 2002). Analyses of drill cores from a bore hole near Donghai, in Su-Lu, demonstrate that the thickness of the geothermal system was at least 432 m, with epidote separated from amphibolite at that depth having a d 18OVSMOW of 27.9‰ (Zhang and Rumble, unpublished). Geochemical investigations also play a role in assessing exotic versus in situ hypotheses of the origin of UHP metamorphism. The interleaving of eclogites whose protoliths were ocean-floor basalts with continental metasediments would, for example, suggest tectonic juxtaposition rather then in situ UHP metamorphism (Zhai and Cong, 1996). The geochemical evidence is discussed more fully below. The balance of evidence now favors the hypothesis of in situ UHP metamorphism. The shift towards acceptance of an in situ origin has been accelerated by the development of physically plausible models of the subduction and exhumation of continental crust. The current focus of debate has, in fact, shifted towards evaluation of evidence of “super” ultrahigh pressure metamorphism. These topics will be discussed below. 3.09.4 MINERALOGIC INDICATORS OF “SUPER” UHP METAMORPHISM The discovery of coesite in regionally metamorphosed, eclogite-facies rocks by Chopin (1984)

300

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and Smith (1984) was initially greeted with some skepticism. How could high-pressure minerals be preserved metastably, it was asked, during what must have been an arduous ascent from the upper mantle? The discovery of microdiamonds in eclogite-facies gneisses by Sobolev and Shatsky (1990) stimulated even greater skepticism. Validation of the discoveries by in situ Raman spectroscopic measurements has removed any doubts regarding possible mineral misidentification (Nasdala and Massonne, 2000). Mineralogical indicators of UHP metamorphism have been reviewed by Chopin and Sobolev (1995) and Liou et al. (1998). Exsolution features of UHP minerals have been reviewed by Green et al. (2000) and Zhang and Liou (2000). Recent discoveries of new mineralogical evidence of “super” UHP metamorphism have sparked a new wave of controversy. Some of the new findings suggest pressures of 6.0 GPa, or more; some samples may have been exhumed from depths greater than 200 km. We list, below, novel mineralogical indicators that have been discovered recently. Relict majoritic garnet has been found in the WGR (van Roermund et al., 1998, 2001), Dabie – Su-Lu, China (Ye et al., 2000a, and Rhodope, Greece (Mposkos and Kostopoulos, 2001) suggesting an origin at P . 6:0 GPa and depths .185 km. High-P clinoenstatite has been reported in Alpe Arami (Bozhilov et al., 1999) and Dabie – Su-Lu garnet peridotites (Zhang et al., 2002b). Garnet megacrysts with inclusions of exsolved clinopyroxene þ garnet ^ ilmenite interpreted to be majoritic garnet have been recently suggested from the Granulitgebirge, Germany (Massonne and Bautsch, 2002) and Su-Lu of eastern China (Zhang and Liou, in press). Supersilicic clinopyroxene and silica exsolution is present in eclogite and pelitic gneiss from the Kokchetav massif, Kazakhstan (Katayama et al., 2000b). Ogasawara et al. (2002) report coesite exsolution from supersilicic titanite in UHP marble from the Kokchetav Massif, northern Kazakhstan. Nanometer-size inclusions of a-PbO2-type TiO2 have been found in garnet from diamond-eclogite facies gneisses in the Erzgebirge, Germany, indicating P . 4:0 GPa; T . 900 8C (Hwang et al., 2000). Unusual defect microstructures including nanometer-scale twin lamellae in omphacite, kyanite, and spinel and microcleavages in garnet from Su-Lu eclogite are said to indicate rapid exhumation from depth (Hwang et al., 2001b). Given the application of new determinative methods including micro-Raman spectroscopy, electron microscopy, and intense, focused X-ray beams at synchrotron laboratories, it is likely that new discoveries will push the depth of subduction even further.

3.09.5 GEOCHEMISTRY OF UHP METAMORPHISM Eclogites and peridotites have received a disproportionate share of the attention of researchers despite their volumetric insignificance in UHP metamorphic terranes, because they are the principal rock types that bear evidence of UHP metamorphism. An enhanced level of study of mafic and ultramafic rocks is, moreover, justified because of their importance in the interpretation of the tectonic history of UHP terranes. The evaluation of a hypothesis of continental collision initiated by oceanic crust subduction followed by exhumation triggered by break-off of the oceanic slab, for example, is greatly facilitated by distinguishing eclogites and peridotites originating in continental crust from those of oceanic crust or mantle origin. Radiometric dating provides indispensable tests of tectonic models. Examples of both protolith studies and age dating are given in the following sections.

3.09.5.1 Chemical Compositions of Eclogites and Ultramafic Rocks Eclogites are mainly basaltic in composition, but they show a wide range of major and trace element abundances. This suggests multiple origins and heterogeneous sources. In some cases, the data reflect the fact that analyses were made on banded rocks produced by metamorphic segregation, thus biasing the results. Eclogites and ultramafic rocks that recrystallized under UHP metamorphic conditions often have a coarsegrained texture with distinct mineral banding of variable scale. It is often difficult to obtain a truly representative bulk chemical composition of the protolith of a banded eclogite. Fine-grained or homogenous textured eclogites also occur, but they seem to be minor in comparison with heterogeneous textured facies. Eclogites from the Dabie and Su-Lu terranes exhibit a range of SiO2 contents from 36% to 60%, although the majority still have basaltic or gabbroic compositions with SiO2 ¼ 45 – 52% (Figure 2; Jahn, 1998). Higher silica eclogites (SiO2 $ 53%) suggest a more differentiated protolith, while lower silica eclogites (#45%) imply a cumulate protolith or represent a banded eclogite rich in low-SiO2 phases (e.g., garnet, epidote, rutile, etc.). Garnetite is a good example of metamorphic differentiation; it cannot be used to discuss the nature of its protolith. Many of the chemical compositions of eclogites cannot be matched by reasonable magmatic protoliths. Numerous petrological studies have established that the protoliths of eclogites underwent progressive dehydration during prograde metamorphism, and that many were later rehydrated to

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Figure 2 Major element variations of eclogites and associated ultramafic rocks from the Su-Lu and Dabie terranes of east-central China (Jahn, 1998). Eclogites are generally basaltic and quartz-normative, with some showing the cumulate nature of their protoliths. Type I is gneiss-hosted, type II, marble-associated. Type III are members of layered intrusions or associated with ultramafic rocks. Also plotted are data from peridotites (Perid.), orthopyroxenites (Orthopx.), and clinopyroxenites.

some extent during retrograde metamorphism when the rocks were exhumed (Liou et al., 1998). These processes are not isochemical. LIL elements (potassium, rubidium, caesium, strontium, barium), uranium, and to some extent, lanthanum and cerium, can be mobilized in a

fluid phase together with volatile species such as H2O and CO2. In the subduction of oceanic crust, as in the circum-Pacific environment, dehydration of subducted amphibolite and serpentinite produces hydrous fluids which move upward, enriched in these elements, but leaving behind

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niobium and tantalum, and metasomatizing the mantle wedge (Tatsumi and Eggins, 1995). Magmas produced by the melting of such metasomatized mantle would show enrichment in LIL elements, uranium, and probably also La– Ce, but depletion in niobium and tantalum. This is the characteristic geochemical signature of arc magmas formed in subduction zones (see Chapter 3.09). Subduction of continental crust, has a somewhat different chemical consequence than the subduction of oceanic crust, because continental crust is globally less hydrated. This is true particularly of the lower crust, which is composed mainly of anhydrous, granulite facies rocks. The protolith of an eclogite could also be derived from the upper to middle crust. It may occur as a basic dike or enclave, or as a layered intrusion (e.g., Bixiling or Maowu of the Dabieshan; Chavagnac and Jahn, 1996; Zhang et al., 1998) within granitic gneisses, metamorphosed in the greenschist to amphibolite facies. Such mafic rocks are often observed in Precambrian terranes. During continental subduction, the greenschist or amphibolite units would follow a similar route as the oceanic crust, and undergo dehydration reactions. The patterns of elemental loss should be rather similar to that in a subducted oceanic lithosphere. The present abundances of LIL elements, uranium, and REE patterns in eclogites cannot be easily used to argue for or against chemical mobility. The concentration of these elements shows a great deal of variation in basaltic rocks from diverse tectonic settings, hence any difference in their concentration could be related to original differences in the protoliths, and might not be related to gains or losses during metamorphism. Nevertheless, the clearest evidence of elemental loss during eclogitization is supplied by studies of Rb –Sr isotopic systematics. Analyses of eclogites from western Norway (Griffin and Brueckner, 1985) and from the Dabie –Su-Lu terranes (Ames et al., 1996; Jahn, 1998) indicate that many eclogites have highly radiogenic 87Sr/86Sr ratios that are “unsupported” by their Rb/Sr ratios (Figure 3). This is particularly true for eclogites that occur as enclaves within quartzofeldspathic gneisses (Griffin and Brueckner, 1985; Jahn, 1998). The most plausible explanation is that rubidium is preferentially lost relative to strontium during prograde eclogite facies metamorphism. Alternatively, high 87Sr/86Sr ratios may result from the assimilation of continental crust by mafic magmas. By analogy, elements of similar geochemical behavior, such as potassium, caesium, and probably barium, could have been lost together with rubidium during the same process. Retrograde metamorphism is often observed in HP – UHP eclogites during the exhumation of deeply subducted continental crust. Retrograde rehydration of UHP rocks may replace the mobile

Figure 3 Plot of 87Sr/86Sr versus 87Rb/86Sr for UHP eclogites from the Su-Lu and Dabie trerranes. Eclogites are classified as gneiss-hosted, marble associated, from layered intrusions, and as “nondefined” occurrences. Three reference isochrons based on the protolith ages of the Weihai eclogites (1.7 Ga; Jahn et al., 1996), granitic gneisses of the Su-Lu and Dabie terranes (,800 Ma) and the UHP metamorphic event (,220 Ma) are shown for assessment of Rb– Sr isotopic systems in different types of eclogites. Data sources: Jahn (1998), Li et al. (1999), and Ames et al. (1996). Note that the data are highly dispersed and many samples show “unsupported” high Sr isotopic ratios (data left of the 1.7 Ga isochron), implying depletion of Rb and alkali elements during the metamorphic processes.

elements lost during prograde metamorphism. However, the process should not be considered as reversible with respect to prograde metamorphism. In the Dabie orogen, many studies have shown that during the course of continental subduction and exhumation, a hydrous fluid phase was not pervasively present. This is shown by the preservation of the world’s lowest d 18O values (210‰) in high-temperature rocks (Yui et al., 1995; Zheng et al., 1996; Baker et al., 1997; Rumble and Yui, 1998; Zheng et al.,1998; Fu et al., 1999), the presence of the world’s highest 1Ndð1Nd ¼ ½{ð 143 Nd = 144 NdÞ sample =ð143 Nd = 144 NdÞ CHUR }21 £104 ; where CHUR stands for chondritic uniform reservoir) values (þ270) in eclogites of the Weihai area (Jahn et al., 1996) and the general absence of syntectonic granites in the orogen. Eclogites from the European Hercynides and the Alpine chain are known to have an affinity to oceanic basalts (or MORB); whereas those from the Caledonides and the Dabie orogen have an affinity to continental basalts (Figure 4; see Jahn, 1999, and references therein). REE patterns are invariably LREE-enriched in the eclogites of Dabieshan and Su-Lu (Jahn, 1998), but LREEdepleted, MORB-like patterns are abundant in the Hercynian and Alpine chains. Although LREE,

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eclogites contain #30 ppm Nd (grand average ¼ 11.3 ^ 11.4, 1s). The Sm/Nd ratios for the majority of eclogites lie between 0.20 (for alkali basalts, continental basalts, or average continental crust) and 0.36 (for depleted N-MORB). Many rocks of low samarium and neodymium concentrations have Sm/Nd ratios higher than that of depleted N-MORBs. This suggests that the protoliths of these eclogites are probably not common basaltic rocks, but represent garnet-rich cumulate portions of metamorphic assemblages or simply pyroxene cumulates. 3.09.5.2 Sm–Nd, Lu–Hf, and Rb–Sr Isochron Ages and Nd–Sr Isotope Tracers

Figure 4 REE distribution patterns of eclogites from the Chinese Su-Lu, Dabie, and Hong’an terranes. Localities from Su-Lu and Dabie are given in parenthesis. All eclogites are gneiss-hosted except two from Rongcheng (Su-Lu). The common feature is the enrichment in light REE, which contrasts with NMORB but resembles continental mafic rocks (basalt, amphibolite, or basic granulite). Source: Jahn (1998, for Su-Lu and Dabie; unpublished, for Hong’an).

especially lanthanum, might be susceptible to mobilization in the presence of a hydrous fluid, the stable isotope evidence for the absence of such fluid in Dabieshan makes identification of protolith REE patterns credible (Figure 4). Jahn (1999) compiled samarium and neodymium concentration data for 223 analyses of eclogites and associated ultramafic rocks. Samarium concentrations range from ,1 ppm to .15 ppm but most rocks contain # 8 ppm (grand average ¼ 3.1 ^ 2.5, 1s). Neodymium ranges from ,1 ppm to 65 ppm, and most

Geochronology plays a crucial role in our understanding of the subduction and exhumation of the continental crust. When dealing with UHP metamorphic rocks, key questions related to the timing of events include: the ages of the protoliths, the ages of peak and retrograde metamorphism, timing of later thermal disturbances, and episodes of fluid – rock interaction. Rates of exhumation can be estimated if precise age information can be obtained and linked to a specific stage of an orogenic process (see Chapter 3.08). The availability of comprehensive age patterns for an entire metamorphic terrane make it possible to reconstruct its tectonic evolution (e.g., Dabie: Hacker et al., 1998, 2000; WGR: Carswell et al., 2003; Kokchetav: Katayama et al., 2001; Shatsky et al., 1999). Different events can be dated by different chronometers. The most commonly used chronometers for dating UHP events include U –Pb on zircon or monazite, Sm – Nd on garnet, Ar –Ar on white mica or hornblende, and Rb – Sr on biotite or white mica. Lu –Hf has also been used for garnet dating, and it has been argued that this method may be superior to Sm – Nd in some cases (Ducheˆne et al., 1997; de Sigoyer et al., 2000; Scherer et al., 2000). However, until now only a few results have been published. The U – Pb system in zircon and monazite yields a great amount of chronological information on a variety of thermal events. However, as pointed out cogently by Tho¨ni in his excellent summary of geochronological problems of the Alpine metamorphic rocks (Tho¨ni, 2002), even though the zircon and monazite U – Pb chronometers usually provide exact time information, their use is somewhat hampered by the fact that the mechanisms of zircon and monazite (re)-crystallization are still poorly understood in the context of the P – T evolution of metamorphic rocks (but see Chapter 3.07). In HP and UHP metamorphic terranes, zircon may grow during prograde as well as retrograde recrystallization (Tilton et al., 1991; Gebauer, 1996; Tho¨ni, 2002). Metamorphic recrystallization of zircon and resetting of its

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U – Pb systematics could be enhanced by fluids even at moderate temperatures (,600 8C; Rubatto et al., 1999; Liati et al., 2000). Consequently, zircon may yield no clear petrological record of the P – T path, and it may be difficult to link SHRIMP U – Pb ages with a specific stage of metamorphic evolution. The technique of mineral isochrons is based on the in situ isotopic evolution of an equilibrated mineral assemblage. In dating HP –UHP eclogite, only the minerals that are formed in the eclogite facies metamorphism are useful; these include garnet, omphacite, phengitic mica, etc. Presence of relict minerals or retrograde metamorphic products tend to disturb the isochron relationship between different phases and their whole rock (WR). The mineral isochron method not only provides age information but also yields initial isotopic ratios, which are most valuable in tracing protolith evolution (see below). Among the high-grade metamorphic minerals, garnet is known to be the most useful in metamorphic geochronology. Garnet is ubiquitous; it occurs in a wide range of metamorphic and igneous rocks of crustal and mantle origin. In metamorphic petrology, garnet is often used in thermobarometry, and is most useful in the reconstruction of P – T paths. Since the mid-1980s, garnet has become one of the most powerful metamorphic chronometers because of the high rate of the radiogenic growth of its Nd-isotope ratios (Griffin and Brueckner, 1985; Vance and O’Nions, 1990, 1992; Mezger et al., 1992; Li et al., 1993; Chavagnac and Jahn, 1996; Tho¨ni, 2002). As emphasized by Tho¨ni (2002), the major advantage of garnet dating is the ability to extract both microstructural and thermobarometric information. The combination of textural evidence, P – T estimates and age data allows a much better understanding of the evolution of a metamorphic terrane and hence of orogenic processes. Garnet is most useful in Sm –Nd and Lu – Hf dating, but the interpretation of garnet Sm –Nd ages is not always straightforward, because they are influenced by: isotopic disequilibrium and the presence of LREE-enriched inclusions. 3.09.5.2.1

(Rowley et al., 1997; Jahn et al., 2002). The general patterns for WR and minerals observed for the Maowu rocks are very similar to those for the Norwegian eclogites (Griffin and Brueckner, 1985), the diamondiferous eclogites from Yakutia, Siberia (Jerde et al., 1993; Taylor et al., 1996) and the eclogites of the Adula Nappe of the central Alps reported by Bocchio et al. (2000). They are known to represent chemically equilibrated assemblages. The values of clinopyroxene/garnet partition coefficients (K D ¼ concentration of element A in clinopyroxene/concentration of element A in garnet) are shown in Figure 5. The K D’s of the LREE have greater variation than that of HREE, up to three orders of magnitude for lanthanum, but all K D’s decrease regularly from lanthanum (or cerium) to lutetium. Similar patterns have been observed for ecologites from the central Alps (Bocchio et al., 2000). The K D’s are also quite comparable with those obtained in high-pressure (2 –3 GPa) and high temperature (1,300 – 1,470 8C) experiments of Johnson (1994). Consequently, our results strongly indicate that chemical equilibrium was achieved during UHP metamorphism. The only green amphibole shows a REE pattern almost identical to that of the coexisting clinopyroxene, so that the REE K D values between amphibole and clinopyroxene are close to unity. Thus, the amphibole appears to have been transformed entirely from clinopyroxene during retrograde

REE partitioning

Several recent studies of elemental abundances in eclogite minerals have established the value of measuring elemental partitioning between minerals as indicators of mineral equilibrium. These studies mainly used secondary ion mass spectrometry (SIMS) and laser ablation ICP-MS. The data for the Maowu eclogite-pyroxenite body are used to illustrate this point, because the coherent U – Pb and Sm – Nd age data seem to indicate that REE in coexisting clinopyroxene and garnet attained chemical and Nd-isotopic equilibrium

Figure 5 REE partition coefficients (K D) between clinopyroxene and garnet. The data were obtained on the eclogite – pyroxenite complex of Maowu, Dabieshan, China (data source: Jahn et al., in press). K D values for LREE vary by three orders of magnitude, whereas those for HREE by only one order.

Geochemistry of UHP Metamorphism metamorphism and did not achieve equilibrium with coexisting clinopyroxene (see also Barth et al., 2002). The REE K D values for clinopyroxene/garnet show a dependence on the calcium content of the host phases, with K D’s decreasing with decreasing K D(Ca) (Figure 6). This behavior has also been found in eclogite xenoliths (O’Reilly and Griffin, 1995; Harte and Kirkley, 1997), which have much higher equilibrium temperatures of 1,000 – 1,200 8C than the Maowu UHP rocks (700 – 800 8C). The temperature difference appears to have little effect on the K D values, but this view is not shared by Bocchio et al. (2000) on the basis of their analysis of Alpine eclogites. Sassi et al. (2000) also observed that the patterns of REE K D’s for all UHP eclogites from Dabieshan are similar, but the absolute values vary a great deal, from three orders of magnitude for lanthanum to about one order for lutetium. Furthermore, a weak correlation between K D(Zr) and K D(Ca) for both mantle and Dabieshan eclogites was noticed by Harte and Kirkley (1997) and Sassi et al. (2000). 3.09.5.2.2

Incorrect geochronologic ages

Excess Ar in phengitic mica of HP – UHP rocks has been documented frequently and is the cause of aberrant ages (Li et al., 1994, 2000; Giorgis et al., 2000; Jahn et al., 2001a). Incorrect

Figure 6 Selected elemental partition coefficients between clinopyroxene and garnet. The data were obtained on the eclogite-pyroxenite complex of Maowu, Dabieshan, China (data source: Jahn et al., in press). Note that the K D (REE) values are positively correlated with K D(Ca), and Zr –Y, like HREE, are preferentially partitioned in garnet.

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Sm – Nd mineral isochron ages have also been observed, particularly for “low temperature” (#600 8C) HP – UHP metamorphic rocks. The most notorious examples are from the Alps and the Himalayas (Luais et al., 2001; Tho¨ni, 2002). The following discussion considers examples of “correct” versus “incorrect” Sm –Nd ages. The main cause for this chronometric problem is the lack of isotopic equilibrium between garnet and coexisting minerals. Sm – Nd isotopic equilibrium/disequilibrium depends more on temperature than on pressure. The temperature effect is related to the blocking temperature, T B (Dodson, 1973), which does not have a unique value and is influenced by several factors including mineral grain size, rate of cooling, the duration of metamorphic reactions at temperature higher than T B, the presence of a fluid phase and its composition, and penetrative deformation (e.g., Tho¨ni, 2002). The value of T B ranges from 850 8C to 650 8C for the garnet Sm – Nd isotope chronometer (Humphries and Cliff, 1982; Jagoutz, 1988; Mezger et al., 1992; Burton et al., 1995; Zhou and Hensen, 1995; Gu¨nther and Jagoutz, 1997; Tho¨ni, 2002). Garnet out of isotopic equilibrium often has nonequilibrated trace element patterns. In some cases, the Sm/Nd ratios of garnets are even lower than that of WR samples; in other cases, there is little difference between the Sm/Nd ratios of all coexisting minerals, so that they form a cluster in an isochron diagram. In still other cases, garnet and omphacite may have a negative slope isochron relationship. This has been found in eclogites of Tso Morari of the Himalayas and the Sesia zone of the western Alps (Luais et al., 2001), and the Hong’an Block of China (Jahn, unpublished). Porphyroblastic garnets often contain mineral inclusions. Besides the celebrated coesite, many of them are REE-rich phases, such as monazite, epidote, allanite, titanite, apatite, and zircon. Except zircon, all these phases are highly enriched in LREE and have very low Sm/Nd ratios. Thus, a tiny amount of these inclusions could significantly lower the Sm/Nd ratios of garnets, hence affecting isochrons based on Grt – Cpx – WR and other minerals. Inclusions must have undergone the same metamorphic P – T path as garnet, Cpx, and other major phases. If the metamorphic temperature exceeds the T B of the inclusions (moderate T B minerals, such as epidote, apatite, and titanite), the Nd-isotopic compositions of the inclusions can be expected to reach isotopic equilibrium with the host garnet. In this case, a correct isochron age is to be expected, but the spread between parent/ daughter ratios would be reduced, and hence the statistical error of the age increased. This has been most frequently observed in the “high-temperature” eclogites of the Dabie and Su-Lu terranes. On the other hand, mineral

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inclusions of very high T B (e.g., zircon and monazite) may not be easily reset isotopically, with growing garnet producing a disequilibrium isochron relationship. In conclusion, the failure to produce correct Sm – Nd isochron ages is due to isotope disequilibrium between garnet and its coexisting minerals. The disequilibrium results from two processes: (i) garnet preserves the isotopic composition of precursor mineral (plagioclase) because the rate of Nd-isotope exchange is more sluggish than the rate of chemical and mineral phase reconstitution; and (ii) garnet contains high-temperature refractory LREE-rich inclusions, which have never re-equilibrated isotopically with the host garnet, though the garnet may have attained equilibrium with coexisting Cpx and other principal phases. The above considerations do not include open system behavior during retrograde metamorphism under the potentially strong influence of hydrothermal activity. 3.09.5.2.3

Coupled Nd- and O-isotopic disequilibrium

One of the most interesting results in isotopic studies of UHP eclogites is the demonstration of a direct correspondence in equilibrium (or disequilibrium) state between the Sm –Nd and O-isotopic systems in eclogite minerals (Zheng et al., 2002). The state of O-isotope equilibrium may provide a critical test for the validity of Sm – Nd mineral isochron ages. Zheng et al. (2002) analyzed O- and Sm – Nd isotopic compositions of minerals and WRs in six samples from four eclogite outcrops in southern Su-Lu in east-central China (Figure 7). In four of six samples, Nd –Sm mineral isochrons give ages of 208– 226 Ma that agree, within error, with the age of UHP metamorphism as measured by other methods (Ames et al., 1996; Li et al., 1994, 1999). The four samples with consistent Sm – Nd isochron ages also have high temperature 18 O/ 16O fractionations between garnet and omphacite that are in agreement with petrological estimates of 700 –890 8C (Enami et al., 1993; Zhang et al., 1995a,b; Yang et al., 1998). On the other hand, one of two samples from Yakou and the one sample from Yangkou have Sm –Nd isochrons of 280 Ma and 338 Ma and oxygen isotope fractionations between garnet and omphacite that are not in equilibrium under UHP conditions (Zheng et al., 2002). Tie-lines between coexisting garnet and omphacite plotted as 1Nd(t) versus d 18OSMOW are shown in Figure 7 (redrawn from Zheng et al., 2002). In such a plot, values of 1Nd(t) calculated for the age of UHP metamorphism are expected to be identical for minerals experiencing isotopic equilibration under UHP conditions. The differences between values of d 18O for minerals equilibrated

Figure 7 Tie-lines between coexisting garnet (Gt) and omphacite (Omp) in eclogites from the Su-Lu terrane, China, plotted on a 1Nd(t) versus d 18O diagram. Samples identified by names of localities where collected. See text for discussion (after Zheng et al., 2002).

under high temperature, UHP conditions, D 18O omphacite-garnet, should be in the range 0.8 – 1.0‰ with d 18O omphacite . d 18O garnet (Zheng et al., 2002). These conditions are met by two samples from Majiagou and one sample from Yakou (Figure 7). A fourth sample from Qinglongshan also shows the expected isotopic equilibrium features but is not plotted. In contrast, sample Gt-1/Omp-1 from Yakou gives disparate values of 1Nd(t) for an age of 211 Ma and its oxygen isotope fractionation records a sub-UHP temperature of 545 8C. The sample from Yangkou is in blatant oxygen isotope disequilibrium with reversed 18O/16O partitioning, d 18Ogarnet . d 18O omphacite, and unequal values of 1Nd(t) calculated for a UHP age of 220 Ma. The disequilibrium characteristics of the Yakou sample (Gt-1/Omp-1) are attributed to retrograde metamorphic alteration during exhumation whereas the isotope relationships in the Yangkou sample are caused by incomplete mineral reactions during prograde UHP metamorphism (Zheng et al., 2002). Analytical work on Sm – Nd and 18O/16O relationships in coexisting minerals provides an independent criterion for evaluating geochronologic ages as well as insight into the kinetics of isotope disequilibrium. A survey of published data suggests that rates of oxygen self-diffusion in garnet and omphacite are lower than, or close to, those of neodymium diffusion at the same temperatures (Zheng et al., 2002). Consequently, attainment of high-temperature oxygen isotope

Geochemistry of UHP Metamorphism exchange equilibrium by garnet-omphacite mineral pairs indicates that Nd-isotopic compositions may have been homogenized under prograde metamorphic conditions. Preservation of high-temperature oxygen isotope fractionation despite possible retrograde alteration suggests the likelihood that Sm –Nd isotope equilibrium may have been preserved during exhumation from upper mantle depths. The analysis of both radiogenic and stable isotopes in the same samples of minerals and rocks holds the promise of providing more definitive criteria for the interpretation of geochronologic ages (Zheng et al., 2002).

3.09.5.3 Comparative Radiogenic Geochemistry of Eclogites from UHP Metamorphic Belts A comparison of the radiogenic geochemistry of classic UHP belts demonstrates the capabilities and ambiguities of distinguishing oceanic from continental protoliths. (i) The most celebrated UHP rocks, jadeitekyanite or garnet quartzites (also termed whiteschists) of Dora Maira, are shown to have negative 1Nd(t) values from 25 to 28 (Figure 8). This is consistent with their ultimate derivation from middle Proterozoic continental crust as estimated from their TDM ages of 1.3 – 1.8 Ga (Tilton et al., 1989, 1991). Except for these metasediments, the entire population of eclogites from the Alpine chain possesses positive 1Nd(T) values (Figure 8), suggesting that all of them had an origin as subducted oceanic crust, or directly from the depleted upper mantle such as the garnet peridotites of the Alpe Arami (Becker, 1993). This conclusion is largely supported by available geochemical and petrological data (Paquette et al., 1989; Tho¨ni and Jagoutz, 1992; Miller and Tho¨ni, 1995; Dobrzhinestskaya et al., 1996; von Quadt et al., 1997). (ii) In the Hercynian belt, most eclogites possess positive 1Nd(t) values, suggesting a dominantly oceanic derivation (Figure 8; Bernard-Griffiths and Cornichet, 1985; BernardGriffiths et al., 1985; Stosch and Lugmair, 1990; Beard et al., 1992, 1995; Medaris et al., 1995). The few that exhibit negative 1Nd(t) values probably reflect limited crustal contamination (Figure 8; Stosch and Lugmair, 1990; Kalt et al., 1994). A few eclogites from the Bohemian Massif also show negative 1Nd(t) values (23 to 26, Medaris et al., 1995), but these isotopic signatures were interpreted as due to the involvement of oceanic clay in the melting of subducted oceanic lithosphere, and the eclogites were considered to represent high-pressure cumulate of “enriched” basaltic liquid (Medaris et al., 1995). In any case, the oceanic crust origin of the eclogites from the Hercynian belt is clearly demonstrated, and the

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few negative 1Nd(t) values do not imply derivation from deeply subducted continental crust. (iii) Eclogites from the Caledonides have a much more complex history. Two eclogites from NW Scotland (Sanders et al., 1984) and over half of the UHP rocks (including about equal proportions of eclogites and garnet pyroxenites (Griffin and Brueckner, 1980, 1985; Mork and Mearns, 1986; Mearns, 1986; Jamtveit et al., 1991) have negative 1Nd(t) values (down to 210) (Figure 8). It appears that the Norwegian and Scottish eclogites have both oceanic and “continental” affinities. Although compositionally similar to mantle rocks, some, if not all, were emplaced into the continental crust in Precambrian time(s) and have been an integral part of the continental crust since then. This represents the first stage of evolution. Note that many eclogites have 147Sm/144Nd ratios lower than the chondritic values of 0.1967, suggesting that they were emplaced first as LREE-enriched continental basalts, later metamorphosed to amphibolites, and finally became an integral part of the continental crust (Figure 8). This further implies that an ancient crustal segment was subducted to mantle depths and UHP metamorphic assemblages were produced during the Caledonian orogeny. In WGR, eclogites recorded only the Caledonian thermal event of 410 – 440 Ma (Griffin and Brueckner, 1980, 1985; Mork and Mearns, 1986; Mearns, 1986), whereas garnet peridotites and garnet pyroxenites often recorded a Proterozoic event of ,1.7 Ga (Jamtveit et al., 1991). It is possible that the isotopic systems registering an age of 1.7 Ga in the garnet peridotites at mantle P – T conditions have not been erased or homogenized during the Caledonian event. It is equally possible that these garnets were not isotopically equilibrated, hence that the ages of 1.7 – 1.0 Ga have no strict geological significance. Nevertheless, the newly formed garnets from basaltic protoliths appear to have faithfully recorded the Caledonian orogeny at 410– 400 Ma. Thus, the evolution of the European Caledonides is clearly different from that of the Alpine and Hercynian chains of western Europe. (iv) The Dabie and Su-Lu terranes have a diagnostic “continental” isotopic signature. Eclogites occurring as enclaves or blocks in granitic gneisses have low 1Nd(t) values (26 to 220) at the time of peak metamorphism at < 220 – 230 Ma (Figure 9). These values are among the lowest ever recorded in eclogites and are significantly different from those of eclogites from the Alpine and Hercynian chains in western Europe. Moreover, most rocks have 147Sm/144Nd ratios lower than the chondritic value, reflecting their LREE-enriched geochemical characteristics. The isotopic data and geochemical characteristics indicate that the eclogite protoliths of the Dabie

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Figure 8 Plots of 1Nd(0) versus 147Sm/144Nd (left diagrams) and 1Nd(t) versus 147Sm/144Nd (right diagrams) for eclogites and garnet peridotities from the Alpine, Hercynian, and Caledonian belts. Data source: see Appendices of Jahn (1999). For the Alpine belt, except the UHP metasediments of Dora Maira, all the rocks show positive 1Nd(t) values. Most eclogites and garnet peridotites from the Hercynian belt show positive 1Nd(t) values, suggesting their oceanic affinity. Those having negative values have been interpreted as due to crustal contamination by reinjection of crustal rocks (oceanic clays) into the upper mantle. As for the eclogites and garnet peridotites from the European Caledonide, the majority of the rocks show negative 1Nd(t) values, in strong contrast to those of the Alpine and Hercynian chains. These rocks appear to have resided in the continental crust for a lengthy period of time before their UHP metamorphism.

Orogen have a “continental” affinity and could not have been produced from a subducted Tethyan oceanic slab. The very low 1Nd(t) values further require the protoliths of these eclogites to have formed long before the Triassic collision, probably in mid-Proterozoic times. It appears that, in

addition to the UHP metasedimentary rocks of Dora Maira, Dabieshan (including Su-Lu) and WGR are two regions where coesite- and diamond-bearing eclogites and ultramafic rocks show clear evidence for the subduction of ancient and cold continental crustal blocks to mantle depths.

The Fluid Phase of UHP Metamorphism

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Figure 9 1Nd(0) versus 147Sm/144Nd (left diagrams) and 1Nd(t) versus 147Sm/144Nd plots (right diagrams) for eclogites from the Su-Lu and Dabie terranes of China. All types I and II eclogites (gneiss and marble-hosted) are characterized by negative 1Nd(t) values, suggesting their continental affinity. Type III eclogites are associated with ultramafic complexes. The data of the Weihai eclogites are out of the scale, and their 1Nd and 147Sm/144Nd ratios are given in parentheses. Data sources: see appendices of Jahn (1999).

The eclogite-ultramafic suites in the Su-Lu and Dabie terranes have different tectonic origins. The first suite, represented by the Bixiling and Maowu complexes, consists of layered intrusions initially emplaced at crustal levels and later subjected to UHP metamorphism as a result of continental subduction (Okay, 1994; Zhang et al., 1998; Chavagnac and Jahn, 1996; Fan et al., 1996; Liou and Zhang, 1998). The second suite consists of mantle rocks exhumed together with crustal UHP metamorphic rocks; they are considered here as “tectonic enclaves of mantle origin” within granitic gneisses. A probable example are the garnet pyroxenites of Rizhao. Even though these pyroxenites represent igneous cumulate rocks from a mantle-derived liquid, the magmatic differentiation probably occurred within the mantle. The isotopic systematics of the Maowu layered intrusion (Figure 9; 1Nd(t) < 25 to 26, Isr < 0.707 – 0.708; Jahn et al., in press) differ from those of the Bixiling complex. Upper crustal contamination during magma chamber processes is likely to have occurred in the Maowu intrusion (Jahn et al., in press), whereas the Bixiling

complex has undergone an assimilationfractional-crystallization (AFC) process in the lower crust (Chavaganc and Jahn, 1996). In conclusion, basaltic (or gabbroic) eclogites of continental origin are best represented by the Dabie and Su-Lu terranes in east-central China and by the WGR. Their identification provides strong evidence for subduction of large blocks of continental crust. On the other hand, most eclogites from the Alps and Hercynian belts are of oceanic origin and many have a MORB-like affinity. Their occurrence is not sufficient to indicate that these eclogite-bearing gneiss terranes have also been subducted to mantle depths, unless it can be proven that the oceanic rocks were tectonically emplaced in a continental setting, and that the ensemble was subducted and metamorphosed at great depths. 3.09.6 THE FLUID PHASE OF UHP METAMORPHISM Fluids in HP and UHP metamorphic rocks are known to exist as intergranular phases, and, must

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have been presented to facilitate the attainment of mm- to cm-scale cation and stable isotope exchange equilibrium between adjacent mineral grains during prograde metamorphism (Philippot and Rumble, 2000; Scambelluri and Philippot, 2001). Discussion of HP together with UHP fluid – rock interactions is included in this section in order to provide a more complete review of the evidence of fluid behavior during continental collision and subduction. Intergranular fluid phases progressively expelled throughout subduction and metamorphism are not present in sufficient amount, however, to result in the complete obliteration of UHP index minerals and mineral assemblages by retrograde metamorphism during exhumation. Micron-diameter fluid inclusions in matrix and porphyroblastic minerals provide samples of fluids from UHP (De Corte et al., 1998; Fu et al., 2001, 2002; Xiao et al., 2000, 2002) and HP (Nadeau et al., 1993; Giaramita and Sorensen, 1994; Philippot, 1993, 1996; Philippot and Selverstone, 1991; Philippot et al., 1995, 1998; Scambelluri et al., 1997, 1998) metamorphic belts. The composition of the inclusion fluids ranges from concentrated brines to fresh, meteoric water. The oldest generations of fluid inclusions may represent peak metamorphic fluids, and, possibly, record premetamorphic fluids. 3.09.6.1 Fluid Immobility during Prograde Metamorphism Owing to the diversity of protoliths undergoing subduction, there is ample textural, chemical, and isotopic evidence of fluid interactions during HP and UHP metamorphism. Fluid mobility is limited during prograde metamorphism, as indicated by the failure of rocks to attain textural equilibrium. Preservation of gabbroic textures and primary igneous minerals including plagioclase, orthopyroxene, and biotite at the core of a 3-m coesitebearing eclogite block in Su-Lu (Zhang and Liou, 1997) and the occurrence of intergranular coesite (Liou and Zhang, 1996; Ye et al., 1996) suggest a lack of fluids to facilitate equilibration. Similarly, Oberhansli et al. (2002) document cover –basement relationships in the Dabie UHP terrane, which in part preserve a primary unconformity, pillow structures, and mineral fabrics. Of particular interest is the variable P – T overprint of these units, interpreted as due to incomplete or absent mineral reactions. Additional evidence of limited fluid mobility during prograde HP and UHP metamorphism is seen in the failure of dissimilar protoliths to achieve stable isotope equilibrium on mm-, cm-, and m-scales despite extreme metamorphic conditions (Sharp et al., 1993; Rumble and Yui, 1998; Baker et al., 1997; Zheng et al., 1998a,b, 1999, 2003; Fu et al., 1999; Philippot and

Selverstone, 1991; Selverstone et al., 1992; Getty and Selverstone, 1994; Nadeau et al., 1993; Fruh-Green, 1994; Barnicoat and Cartwright, 1997; Miller et al., 2001). The failure to equilibrate is a boon to those seeking to unravel the geological history of HP and UHP rocks, because remnants of premetamorphic stable isotopic signatures diagnostic of protolith origins may persist. Recognition that vestiges of distinctive Earth surface environments and processes have been preserved in HP and UHP metamorphic rocks, despite subduction into the upper mantle, remains a source of astonishment but, more importantly, greatly facilitates the interpretation of the geodynamic cycle of continental collisions (Rumble, 1998). In the European Alps, stable isotopic evidence of ocean floor and ophiolitic hydrothermal alteration has been observed in HP eclogites (Philippot et al., 1998; Barnicoat and Cartwright, 1997; Miller et al., 2001; Scambelluri and Philippot, 2001). A record of Neoproterozoic surface conditions has been found in Chinese UHP rocks including low d 18O and dD values indicating a cold climate (Yui et al., 1995; Zheng et al., 1996, 1998b; Baker et al., 1997; Rumble and Yui, 1998; Fu et al., 1999). Dabieshan marbles carry the high d13C values of carbonates that are typical of the Neoproterozoic (Yui et al., 1997; Baker et al., 1997; Zheng et al., 1998a; Rumble et al., 2000; Shen, 2002). New U –Pb dating and d 18O analyses of zircons from orthogneisses show that a cold-climate, geothermal system covering hundreds of square kilometers existed along the rifted northern margin of the Yangtze craton during the Neoproterozoic, consistent with snowball Earth conditions (Rumble et al., 2002). Limits on fluid mobility are indicated not only by stable isotope data but also by fluid inclusion studies and argon isotope geochemistry. Low salinity, primary fluid inclusions reported from low d 18O eclogites at Qinglongshan, China, may be samples of Neoproterozoic meteoric water that survived subduction and exhumation (Fu, 2002). The presence of excess argon in UHP eclogites from Qinglongshan, China, correlates mineralogically with early-formed phengite and epidote and later barroisite (Li et al., 1994; Giorgis et al., 2000). Furthermore, excess argon is more prevalent in low d 18O eclogites than in less anomalous eclogites, suggesting argon inheritance and retention throughout subduction, metamorphism, and exhumation (D. Giorgis, 2001, personal communication). The juxtaposition of textural and stable isotope evidence of limited fluid mobility during HP and UHP metamorphism with mineralogical evidence of dehydration and decarbonation, forces one to ask how devolatilization was accomplished. Channelized fluid flow has been found in a number of studies in well-exposed regions such

The Fluid Phase of UHP Metamorphism as the WGR (Austrheim, 1987; Jamtveit et al., 1989; Mattey et al., 1994; Van Wick et al., 1996). Extensive pseudotachylite veins may record the initiation by metamorphic devolatilization of earthquake faulting and consequent expulsion of fluids (Austrheim and Boundy, 1994, 1996; cf. Kirby et al., 1996). Localization of subduction related deformation along serpentinized mylonite shear zones records fluid flow while favoring preservation of pre-subduction mantle and low temperature (oceanic) alteration assemblages within undeformed metaperidotite (Fruh-Green et al., 2001). Regional-scale fluid mobility as well as mechanical mixing is seen in me´lange units of the HP Franciscan Schists, Catalina Island, California (Bebout, 1991; Bebout and Barton, 2002). Other writers have commented on a discrepancy between mass-balance calculations of voluminous devolatilization mineral reactions and the absence of outcrop evidence of fluid flow (Barnicoat and Cartwright, 1997; Miller et al., 2001). An appeal is made to channelized fluid pathways along bedding surfaces, unconformities, and shear zones to resolve the discrepancy, but in many regions specific fluid flow aquifers remain unmapped. Rock rheology, deformation history, reactionenhanced, and deformation-enhanced permeability clearly must play an important role in fluid mobility. Equally clearly, there is a need for additional field study of the pathways by which fluids leave the subducting slab during HP and UHP metamorphism (see also Chapter 3.06). 3.09.6.2 Slab–Fluid–Mantle Interactions The foregoing examples of limited fluid mobility during HP and UHP metamorphism caused by subduction raise questions about interactions between slab fluids and the mantle during continental collision. How deeply into the mantle can mineral-bound water be carried by a subducting slab? Over what depth interval is water transferred from slab to mantle? By what mechanism is the transfer of water into the mantle accomplished? Complete answers to these questions cannot as yet be given. Field observations show that assemblages containing hydrous and carbonate minerals persist in coesite- and diamond-eclogite facies rocks in Dabieshan (Liou et al., 1995), Su-Lu (Enami and Zang, 1988; Hirajima et al., 1992; Yang et al., 1993; Zhang et al., 1995a, 2002a,b), Erzgebirge (Massonne, 1999; Hwang et al., 2001a,b; Stockhert et al., 2001) and Kokchetav (Sobolev and Shatsky, 1990; Shatsky et al., 1995; Zhang et al., 1997; Ogasawara et al., 2000; Okamoto et al., 2000), equivalent to depths of 130 km, or deeper (Hwang et al., 2000). Recent detailed studies of UHP belts reveal rare occurrences of

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hydroxyl in clinopyroxene of concentrations up to 3,000 ppm (Katayama and Nakashima, 2003) and OH-topaz (Zhang et al., 2002a) whose P – T stability limits would permit water transport to greater depths. Experimental determination of the stability of hydrous minerals in mineral assemblages of bulk compositions appropriate to oceanic crust as well as metasediments of continental origin show that water may remain stably bound in minerals to depths of 300 km or more (Schmidt and Poli, 1998; Schreyer, 1995). The evidence of fluid immobility in slabs under UHP conditions is inconsistent with a massive release of fluid and its transfer to the mantle in a single, cataclysmic episode. Fluid release could not have been simultaneous over such a wide range of rock types, because local variations in bulk composition would have controlled the timing of devolatilization reactions in response to changing P – T conditions. The overall devolatilization history is determined by specific subduction P – T trajectories and their intersection with the stability limits of local mineral assemblages. Local fluid compositions and amounts of evolved fluids changed episodically as different mineral assemblages reacted in response to changing P – T conditions. Fluid evolution from the slab and transfer into the mantle was distributed over a significant depth interval, small in amount at any given instant, but with a substantial fluid : rock ratio integrated over the duration of devolatilization (Philippot and Rumble, 2000). Existing outcrop-scale studies inevitably emphasize local effects, whereas estimating the overall devolatilization budget of a subducting slab will require regional-scale investigations. Investigating the transfer of slab fluids into the mantle is even more difficult than estimating the integrated devolatilization mass balance of slab fluids. It might be thought that the contact relationships of garnet peridotite slices interleaved with metasediments in UHP metamorphic terranes would record the mechanisms of interaction between slab fluids and mantle during continental collision. Study of peridotites in UHP metamorphic belts shows two groups of rocks: one of pre-subduction age that experienced crustal contamination prior to UHP metamorphism and one that was emplaced in UHP metasediments during subduction (Zhang et al., 2000). Some of the latter group (type A-2; Zhang et al., 2000) retain mantle Nd-, Sr-, and O-isotopic compositions and show little evidence of interaction with slab fluids. Other peridotites of type A have low Sm/Nd ratio (0.2), low 1Nd(t) of 25, and high 87Sr /86Sr (0.708) and may have been metasomatized by crustal fluids derived from the descending slab (Zhang et al., 2000). Searches for additional exposures for study of contacts between mantle

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peridotites and UHP metasediments should be made and geochemical investigations undertaken to strengthen our present understanding. Innovative experiments on the connectivity and composition of fluids in eclogitic and mantle mineral assemblages under UHP and mantle P – T conditions promise to shed light on possible mechanisms of slab– fluid – mantle interaction (Mibe et al., 1998, 1999, 2002a,b; Yoshino et al., 2002). These experiments present data on the interconnectivity of a grain boundary fluid phase, percolation along grain boundaries, and will lead to improved understanding of diffusion through a connected grain boundary fluid phase. 3.09.6.3 Compositions of Fluids New data from both field and experimental studies suggest that high density, super-critical, silicate-rich fluids may play a role in UHP metamorphism. Such fluids are transitional between hydrothermal solutions and silicate melts. They contain higher solute concentrations than aqueous solutions, but higher water contents than silicate magmas (Scambelluri et al., 2001). The fluids would have an enhanced capacity to transport material within a subducting slab and from a slab into an overlying mantle wedge (see discussions by Philippot and Selverstone, 1991; Philippot, 1993; Philippot and Rumble, 2000; Scambelluri and Philippot, 2001). The discovery of microdiamonds possibly coprecipitated with silicate and oxide minerals as daughter crystals from C – O – H-silicate fluids focuses new attention on these discussions. Microdiamonds included in garnet and zircon from garnet-biotite gneiss of the Kokchetav Massif, Kazakhstan, contain C –O – H – N fluid inclusions (De Corte et al., 1998) and nanometric inclusions with stoichiometries of MgCO3, TiO2, BaSO4, ThO2, and Cr2O3 (Dobrzhinetskaya et al., 2001, 2003). Microdiamonds included in garnet porphyroblasts of a quartz – phengite – plagioclase – garnet gneiss from the Erzgebirge, Germany (Massonne, 1999) occur in polyphase assemblages that may include phlogopite, quartz, paragonite, phengite, kyanite, apatite, rutile, and sodic plagioclase (Hwang et al., 2001a,b; Stockhert et al., 2001). A recent finding of microdiamond together with monazite, phlogopite, spinel, and (Ba,Ca,Mg)CO3 as multiphase inclusions in spinel in a megacrystic garnet-websterite pod from the WGR provides additional evidence of diamond growth from infiltrating fluids during deep continental subduction (van Roermund et al., 2002). Experiments show diamond synthesis within the diamond stability field in the presence of a CO2 –H2O fluid at H2O-rich compositions (Akaishi et al., 2001). A sandwich experiment using interlayered carbonate and pelite starting materials by Hermann and

Green (2001) indicates that diamond could be produced when CO2 – H2O – K2O – CaO fluids from carbonates interact with pelites at UHP conditions. Clearly, these novel fluids under extreme P – T conditions present a compelling target for more detailed study. 3.09.7 CONCLUSIONS Understanding of orogenic belts with UHP metamorphic conditions has advanced greatly in the two decades since their discovery in 1984. Coesite- or diamond-eclogite facies rocks are known from every continent except Australia and North America (Figure 1). Ages of UHP metamorphism range from the Neoproterozoic to the Miocene. Realistic physical mechanisms of continental collision, subduction, and exhumation have been proposed. Some long-standing debates concerning outcrop relationships such as that of the in situ versus exotic origin of UHP metamophism appear to be progressing towards a consensus within the research community. Identification of protoliths grows ever more exacting as researchers employ a wide range of geochemical methods. Radiometric ages are being refined by the application of a variety of radiogenic isotope systems. Examples discussed in this chapter emphasize that definitive research results are far more likely to be forthcoming in studies that break new ground in using a multidisciplinary approach, that combines a battery of geochemical and petrological methods. A number of opportunities for exciting and fruitful research have been identified in this review. The study of mineralogical indicators of “super” UHP metamorphism is likely to continue to be productive as more sophisticated methods of identification and imaging are employed. Application of a variety of geochemical tests to the same rock and mineral samples will yield more definitive protolith signatures and will improve the interpretation of geochronological data. Investigating the role of supercritical fluids in the growth of microdiamond crystals should increase our understanding of material transport under mantle conditions. Finally, a synthesis of geochemical data emerging from studies of the subduction of oceanic crust will lead to a better understanding of the impact of continental collisions on mantle geochemistry. ACKNOWLEDGMENTS We are grateful to Roberta Rudnick for her advice, which surely improved this review. Preparation of the review was supported in part by the National Science Foundation, EAR Continental Dynamics program, EAR-0003276 (Rumble) and EAR-0003355 (Liou).

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Treatise on Geochemistry ISBN (set): 0-08-043751-6 Volume 3; (ISBN: 0-08-044338-9); pp. 293–319