Diffusion creep in the upper mantle: an example from the Tanlu Fault, northeastern China

TECTONOPHYSICS ELSEVIER

Tectonophysics 261 (1996) 315-329

Diffusion creep in the upper mantle: an example from the Tanlu Fault, northeastern China John V. Ross a,*, Jean-Claude

C. Mercier ‘, Yigang Xu

’

a Geological Sciences. Uniaersity of British Columbia, Vancouver, B.C. V6T 124, Canada b Department des Sciences de la Terre, Uniu. de la Rochelle, Rue Marillac, F-17000, La Rochelle, France ’ Laboratoire de Petrologie Physique, Uniu. Paris-7 and IPGP, 2 Place Jussieu, F-75252, Paris, France Received 15 May 1995; accepted 25 October 1995

Abstract Ultramafic xenoliths in Neogene alkalic basalts emplaced within the Yitong graben associated with the Tanlu Fault, northeastern China, are mostly variably deformed lherzolites, werhlites and minor pyroxenites. Glass, rich in SiO,, Al,O,, K,O and Na,O, is optically recognised as occurring in veins, triple-point grain junctions, and patches in werhlite and some lherzolite samples. Textural observations also suggest that the glass (melt) was involved in the deformation, and was present prior to sampling by the host basalt. Electron microprobe scans for these glass elements indicate that glass is not only present at triple-point grain junctions and in veins, but also completely along some planar grain boundaries. A complex textural history is recognised. The earliest recognised deformation resulted in a reduction in grain size through granuloblastic and porphyroclastsic, with a subsequent relaxation event involving olivine grain growth. A final deformation event producing further grain-size reduction and a mylonitic mosaic texture. Geothermometry calculations from electron microprobe data indicate that the early event occurred at _ lOOO”C, while the final event occurred between 800 and 750°C; the latter event occurred near the glass transition temperatures calculated on the basis of thermodynamics and chemical composition(s) of the glass. Olivine rheology and textures suggest that the early event was accomplished by grain-size insensitive non-linear flow (dislocation creep), with the second event accomplished by grain-size sensitive linear flow (diffusion creep). These two events are believed to have occurred at different times, depths and temperatures within the upper mantle beneath Yitong.

They are envisaged being related to changing displacements of the Pacific Plate during its subduction beneath the Asiatic Plate in eastern China during Tertiary times that resulted in the activation of the long-lived and historically active Tanlu Fault.

1. Introduction Several field studies of peridotite massifs document the occurrence of shear zones inferred to be asscociated either with localised movement within * Corresponding

author.

0040-1951/96/$15.00 Published SSDI 0040.1951(95)00158-l

by Elsevier Science B.V.

the mantle or with emplacement of mantle slices into their crnstal positions (e.g., Nicolas and Le Pichon, 1980; Van der Wal and Vissers, 1993; Strating et al., 1993). Mantle shear zones have been inferred from studies on alkali-basalt-bearing mantle xenoliths and from observations of their mineralogy, geochemistry and deformation features (e.g., Basu, 1977; Cabanes

316

J. V. Ross et al. / Tectonoph,wics

and Briqueu, 1986; Cabanes and Mercier, 1988; Downes, 1990; Xu et al., 1993). All of these studies have described well-developed mylonitic textures found in lherzolitic and associated composition(s). These xenolith mylonites are believed to have been produced during large finite strain deformation within the mantle prior to being sampled by the host basalt. Suites of mantle xenoliths have been collected from volcanics located within the floor of the Yitong graben in northeastern China. This narrow elongate graben is believed to be tectonically related to the long-lived and historically active Tanlu Fault. Textures and chemical compositions of these mantle xenoliths have recently been described by Xu et al. (1993) who inferred that the eruptions of basaltic lavas have sampled the underlying mantle providing evidence for the existence of (an) intra-lithospheric shear zone(s).

2. Geologic

setting of the Yitong graben

The Yitong “graben and horst” system comprises the northernmost segment of the historically active Tanlu fault of northeastern China (Fig. 1). It trends northeasterly and at its southern and central region is the locus of moderate (M = 5-6) earthquakes estimated to lie along (a) shallow (15-25 km) surface(s) inclined at about 45” to the west (Li Jialing, 1989). This ancient fault initiated in the early Mesozoic as a left-lateral strike-slip fault with some thrust component. At the end of the Mesozoic, the fault developed several localised continental rifts or grabens along strike, which later became filled with Late Cretaceous elastic sediments. Rifting ceased after the Cenozoic and the regional stress field changed from compressional to tensile, resulting in the Tanlu fault becoming an active right-lateral strike-slip fault zone. This evolution is probably the result of changing subduction direction of the West Pacific Plate towards the Eurasian Plate during Cenozoic times (Hilde et al., 1977). Within the IO-25-km-wide Yitong graben (Fig. 1) is a basaltic volcanic complex comprising eight cones; each cone is characterised by a small central eruption. The basalts are dominantly basanite including some alkali-basalt (Fan and Hooper, 1991). Xenoliths reported on in this paper, and previously

261 (I 9961 315-329

(Xu et al., 1993), come from four volcanic cones: Xiaogushan, Dongxiaoshan, Maanshan and Moliqingshan (Fig. 1). The ages of these four basaltic lava xenolith-bearing localities have been determined by the K-Ar method and range from 9.3 to 14.4 million years (Liu, 1987).

3. Micro-structures

and mineral

chemistry

Mantle-derived peridotite xenoliths from Yitong comprise three compositional suites: spine1 lherzolites, pyroxenites and glass-bearing wehrlites and lherzolites. Different volcanic cones exhibit a wide variation of textural and transitional types. Detailed petrographic descriptions (based on the study of more than 100 samples) resulted in the recognition of five main textural types. Illustrations and rationale for their evolution have been given in Xu et al. (1993) and are summarised in Table 1. The textural types 1, 2 and 3 of the spine1 lherzolite suite are similar to those described by Mercier and Nicolas (1975) and Harte (1977). The samples show evidence for involving a gradual reduction in grain size resulting from changing mantle conditions prior to sampling by the host magma (Xu et al., 1993). Texture 4 is granuloblastic, but is characterised by the local absence of orthopyroxene porphyroclasts and the presence of large (1 X 4 mm) olivine grains that are mostly strain free. Some of these large olivines have inclusions of neoblast-size (0.5 mm) orthopyroxene and most are surrounded by small (0.5 mm) orthopyroxene neoblasts. This fourfold fabric development, henceforth referred to as the “coarse-grained” facies, involved a foliation development (SPO-shape preferred orientation). The final stage (texture 4) involved preferential growth of olivine across the previously evolved finer grains. Some 30% of the Yitong lherzolites comprise mylonites that are very similar to the mylonites rarely reported from subcontintental peridotite xenoliths (San Quintin Basu, 1977; Montferrier -Cabanes and Mercier, 1988). These rocks have a porphyroclastic-mylonitic (grain size - 0.5-O. I mm), mosaic and/or ultramylonitic ( < 0.1 mm) texture. The former comprises aligned small porphyroclasts of olivine, orthopyroxene and rare, spindleshaped spine1 that are transected at low angles (<

J.V. Ross et al. / Tectonophysics

0

Quaternary

261 (1996) 315-329

m

317

Volcano

P.R. CHINA

Fig. 1. Maps showing location of Tanlu Fault in northeastern

China and disribution

of volcanic centres within the floor of the Yitong graben.

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J.V. Ross et ul./Tectonophysics

grained” facies contain this glass phase that comprises 2-5s by volume of the total rock sample. This glass predominantly occurs at triple points between olivine and enstatite and/or diopside grains and as veins. The latter are crosscut by veins infilled with fine-grained basalt. Several samples of werhlite and lherzolite exhibit a porphyroclastic or mosaicmylonitic texture where the glass occurs as pressure shadows asssociated with the larger porphyroclastic phases and also in narrow (< 0.5 mm) veins that make variable angles (o-20”) with the foliation. This texture and grain size places these samples within the “fine-grained” facies. Microprobe analysis has shown this glass to be rich in Na, K and Al (Xu et al., 1993; Xu, 1994). On the basis of these compositions and experimental thermodynamic data (melt and glass heat capacities and heat contents; Russell and Nichols, 19921, we have calculated that the glass transition temperature for this glass varies between 700 and 8OO”C, consistent with the estimated equililibrium temperature for the “fine-grained”

20”) by narrow (0. l- 1.O mm> zones of very finegrained ( < 0. I mm) olivines and orthopyroxenes: the so-called S-C mylonites (Fig. 2A and B). The latter group, referred to as mosaic/ultramylonites, consists totally of very fine-grained equant olivine and orthopyroxene. These two textural groups comprise the “fine-gained” facies and in several samples appear to have developed at the expense of, or transect the “coarse-grained” facies. Several mylonitic spine1 lherzolites contain minor plagioclase (An,,). The pyroxenite suite includes both spine1 websterites and clinopyroxenites. The former all have igneous texture similar to those described by Frey and Prinz (1978) and Irving (1980). Clinopyroxenites occur as composite nodules comprising contact(s) with lherzolite; both phases exhibit porhyroclastic textures. A final suite of peridotites is characterised by the presence of Na-K-Al-rich glass (Xu et al., 1993; Lin et al., 1994; Xu, 1994). Most werhlites and a small percentage of lherzolites belonging to the “coarse-

Table 1 Main characteristics No.

of mantle of xenoliths from Yitong, eastern China

Textural type

Llerzolite

Estimated stress A (MPa)

Chemical equilibrium state

Equilibrium temperature (“C)

10-40

yes

900- I050

residu of partial melting of the upper mantle

< 0.01

100-400

no

750-900

highly deformed products of the coarse-grained samples

l-5

_

h

Petrogenesis interpretation

>4 4-l

1

granuloblastic

“Fine-grained” facies 5 mylonitic

Pyroxenite

suite

igneous texture K-rich

Grain size (mm)

suite

“Coarse-grained” facies I coarse 2 porphyroclastic granuloblastic 3 4

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glass-bearing

890-

1000

cumulate

wehrlite

metamorphose texture modified by interaction of K-rich silicate liquid

-

0.1

_

’ Stress estimated geopiezometer of Ross et al. (1980). b Equilibrium temperatures (“C) according to Xu et al. (1993). ’ Glass transition temperatures calculated on basis of glass composition

700-800

and experimental

c

result from interaction between lherzolite and infiltrating K-rich liquid

data (see text).

J.V. Ross et al. / Tectonophysics

mylonitic facies. These textures are visible in the optical microscope (Xu et al., 1993; Xu, 1994; Lin et al., 1994), but when the same samples are subjected to a qualitative wavelength-dispersive element-scan

Fig. 2. (A) Mylonitic lherzolite with relic porphyroclasts (transmitted with the fine-grained recrystallised matrix (C foliation). (B) Mylonitic

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319

of combined abundances of Na-K-Al (glass composition), collected with an electron microprobe, it is found that element concentrations are located, not only at grain triple points and in veins, but com-

light). The porphyroclasts are aligned (S foliation) at a lherzolite with a mosaic texture. Bar scales are 1.0 mm.

small angle

320

J.V. Ross et al./Tectonophyics

pletely along some grain boundaries (Fig. 3A and B). Similar distribution of glass element. compositions have been described by Draper (1992) in spine1 lherzolites from Goldendale, Washington. Detailed textural descriptions and results of electron microprobe analysis of the different textural types and inferrences from the same have been presented in Xu et al. (1993) and Xu (19941, wherein analysis of the different mineral phases is used to establish that chemical equilibrium was achieved amongst the various phases at different times and

261 (1996) 315-329

under different conditions. The spine1 lherzolite xenoliths subdivided on the basis of texture also exhibit systematic differences in equilibrium state and temperature. Analysis of the minerals, olivine, orthopyroxene, clinopyroxene and spinel, within all of the samples comprising the ‘‘coarse-grained” facies indicates that they are all broadly homogeneous and equilibrium assemblages within the limits of accuracy. However, “finegrained’ ’ mylonitic samples show notable compositional variations (zoning) in orthopyroxene and

Fig. 3. Qualitative wavelength-dispersive element scan of combined abundances of Na, K and Al collected with electron microprobe. (A) Sample 3-15 (werhlite), scanned area - 5 mm across. (B) Sample 2-5 IB (mylonitic Iherzolite), scanned area - 5 mm across. Darkest blebs are spinels; dark stipples areas are cpx with melt patches; dark lines are the highest concentrations of Na, K and Al. Double arrow heads point to areas of optically identifiable glass. Arrow heads point to element concentrations inferred to represent the distribution of this glass phase that outlines many of the planar grain boundaries: hence wetting of grain boundaries by a fluid (now glass) of this composition is inferred.

J.V. Ross et al. / Tectonophysics

261 (1996) 315-329

321

Fig. 3 (continued).

clinopyroxene (Xu et al., 1993). Porphyroclast cores have compositions comparable to those of homogeneous grains within the “coarse-grained” facies, whereas their rims have compositions which are very similar to their associated neoblasts. The textural development comprising the two “facies” and their estimated temperatures of equilibrium are detailed in Xu et al. (1993) and are here summarised in Table 1.

4. Upper mantle rheology The rheological behaviour of the upper mantle is dominated by its mineralogy, whose major constituents comprise olivine, orthopyroxene, clinopy-

roxene and garnet. Of these, olivine is the most common and is also likely the weakest of all the phases, such that it is olivine that probably controls upper mantle flow. This conclusion has been shown repeatedly to be the case from both experimental studies (recently summarised by Kirby and Kronenberg, 1987) and from the examination of microstructures in naturally deformed peridotites (Mercier, 1985). These mantle fabrics, produced under natural non-coaxial conditions, are likely to be produced under conditions represented by a thermally activated Arrhenius flow law of the type: . exp

J.V. Ross et al. / Tectonoph,vsics 261 f 19961 315-329

322

where the steady-state (shear) strain rate (E) is dependent on absolute temperature CT), pressure (P>, the grain size (d) and (shear) stress (a), and b is the Burgers vector of olivine. The factors A (a material constant), CL(shear modulus), E * (activation energy), V * (activation volume), m (grain-size exponent) and n (stress exponent) are established experimentally, while the strain rate, temperature, pressure (depth) and grain size are specified. Thus, for a given temperature and grain size, the strain rate is only a function of the stress to the power “12”. The experimentally determined parameters are totally dependent on the mechanism of flow. Extensive lab experiments on many rock-forming minerals have shown that most have large stress exponents with y12 3, when the dominant mechanism is dislocation creep (Kirby and Kronenberg, 1987). The theoretical basis of this flow law for dislocation creep indicates that (Weertman, 1970) n > 1, and the strain rate is grain-size independent (GSI). Characteristic fabric products of dislocation creep are strong lattice and shape preferred orientation (LPO and SPO) comparable to many of the microstructures observed within mantle tectonites (Mercier, 198.5; Nicolas and Christensen, 1987).

Another mode of flow observed to occur in olivine agregates is diffusion creep (e.g., Karat0 et al., 1986). This mechanism is described by the same relationship between stress and strain rate, but the experimentally determined parameters are markedly different. The stress exponent, IZ, is approximately equal to 1 for diffusion creep and thus strain rate is less sensitive to changes in stress than for dislocation creep. Diffusion creep is very sensitive to grain size (m = 2 or 3; Raj, 1982; Karat0 et al., 1986), becoming more efficient with decreasing grain size. Further, the small grains associated with this mechanism have a lattice fabric (LPO) that is not so well developed as that produced in the GSI regime (Karate, 1988). One method of illustrating the relationships and possible conditions wherein the transition from grain-size insensitive creep (GSI) to grain-size sensitive creep (GSS) could occur is by construction of a deformation regime map (Ashby and Verral. 1978; Karat0 et al., 1986; Patterson, 1987; Carter and Tsenn, 1987) wherein the boundary between the two regimes is plotted in temperature-grain-size space, for a given strain rate, by solving the rate equations for dislocation and diffusion creep. The most recent

-.

_I:

lo

0

10-” lsec

A

4.00 -

4.00 -

“Fine-grained” Facies Texture 5

“Fine-grained” Facios Texture 5

z 2 P

/ 1O-1o isec

B

3.00

I? s cl

-

8 -I

3.00 -

$ A 2.00 -

2.00 -

1.00 -

GSI Flow

‘.OO -

0.00

I 600

“Coarse grained” Facies Texture 2 and 3

eO0

I

Id00

Temperature (“C)

“Coarse

1

/

0.00 600

800

gralned” I

Facles Texturn 2 and 3 1000

1 1200

Temperature (“C)

Fig. 4. -Log grain size (D cm)/temperature (“C) deformation maps for “wet” dunk at constant confining pressure of 1 GPa and variable strain rate of (A) lo-‘“/s and (B) IO-“/s. Each map shows the temperature/grain-size fields for the different textures discussed in the text, together with the calculated boundary between the grain-size-sensitive flow (GSS) and grain-size-insensitive flow (GSI).

J.V. Ross et al./Tectonophysics

and accurate experimentally determined parameters for GSI and GSS creep of polycrystalline olivine is from the work of Chopra and Patterson (1981) and Karat0 et al. (1986) on “wet” Anita Bay dunite and sintered dunite aggregates, respectively. “Wet’ ’ data from these two works are preferred because of the tectonic environment from which the mantle samples are believed to have originated. Values for activation volume are poorly constrained (Karat0 and Wu, 1993), but using values of 18.0 cm3/mol for dislocation creep (Ross et al., 1979; Kohlstedt et al., 1980) and 6.0 cm3/mol for diffusion creep (Karat0 et al., 1993) and the creep data of Chopra and Patterson (198 1) and Karat0 et al. (1986), assuming reasonable strain rates for mylonite formation that vary between lo-” and 10-“/s (Pfiffner and Ramsay, 1982) and having due regard for the admonition(s) of Patterson (1987) concerning the extrapolation of laboratory determined creep data, two deformation regime maps of the transition between GSS and GSI creep have been constructed (Fig. 4A and B) to illustrate the possible conditions involved in the transition between these two creep regimes.

5. Discussion Mantle xenoliths sampled during explosive volcanism are believed to be quenched samples because of the rapidity which they and the host basalt are brought to the surface (Mercier, 1979). Hence, it may be rationalised that their chemistry can be used via geothermobarometers to estimate physical conditions within the upper mantle. Three thermometers (Bertrand and Mercier, 1986; Brey and Kohler, 1990; Witt and Seek, 1991) and a geothermobarometer (Garsparik, 1984) were used to estimate the thermal history and last equilibration conditions of “coarsegrained” and ‘‘fine-grained” mylonitic peridotites. In general, these results indicate that the “coarsegrained” facies equilibrated at temperatures of 2 900- 102O”C, and the ‘‘fine-grained” mylonitic faties equilibrated at temperatures of 680-85O’C (Xu et al., 1993). The textures, mineral chemistries and geothermometric results make us believe that “finegrained’ ’ mylonite peridotites are the strong deformation products of “coarse-grained” ones. It is this strong deformation that has modified both the tex-

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tures and chemistries of pre-existing “coarsegrained” samples and converting them to the “finegrained” mylonitic samples. The lowest temperature recorded by mosaic mylonites implies the final thermal state of the latest deformation episode. Existence of two temperature groups, described above, likely requires that the “coarse-grained” and “fine-grained” facies were sampled from two different pressure (depth) regimes, especially since the ‘‘fine-grained” facies includes samples of spinel/plagioclase lherzolites; plagioclase being totally absent within samples comprising the “coarsegrained” spine1 bearing facies. Hence, it is inferred that the higher-temperature “coarse-grained” facies samples equilibrated at depths of 50-60 km, whereas the lower-temperature “fine-grained” facies samples (one contains calcic plagioclase) equilibrated at N 30 km, close to the boundary between the spinel/plagioclase lherzolite fields (8 kbar according to Garsparik, 1984) and thus near the base of the crust (Moho) in Neogene times. The textural development sequence described above and detailed in Xu et al. (1993) indicates that the “coarse-grained” facies involved foliation development during a gradual decrease in grain size, which was terminated by a phase of static grain size increase (stress relaxation). The existence of a mesoscopic olivine porphyroclast lineation @PO), together with strong LPO fabrics (Fig. 5A and B) for this facies indicates that the dislocation creep mechanism was important. In contrast, the “fine-grained” facies includes porphyroclastic-mylonitic and mosaic textures, the latter having been superimposed on members (porphyroclasts) of the previously developed “coarse-grained” facies. The fine-grained samples with a mosaic texture are further characterised by very fine, equant grains with a weak LPO fabric (Fig. 5C), and no macroscopic or SPO fabric. This textural facies exhibits, in kinematic sections (the XZ plane), variably small (5-20”) angles of obliquity between the S and C foliations, which are indicative of sense and magnitude of shear strain (Nicolas and Poirier, 1976; Simpson and Schmid, 1983; Ramsay and Huber, 1987). It is thus believed that these mylonitic textures with small angles of obliquity not only result from non-coaxial shear but also from high values of shear strain within narrow zones. These zones are

J.V. Ross et al. / Tectonophysics

324

261 Cl9961 315-329

WOI Fig. 5. Olivine lattice fabrics from: (A) texture 3, porphyroclasts, sample 2-38; (B) texture 3, neoblasts associated with porphyroclasts in (A), sample 2-38; and (C) texture 5, neoblasts from mosaic mylonite, sample 2-58. Contours are at 1, 2 and 4% per 1% area. Diameter line corresponds to dimensional foliation of specimens and black spot corresponds to mineral stretching lineation. 100 grains measured in each fabric diagram.

believed to be comparable to those narrow zones, commonly tens of metres wide, frequently observed in ophiolite massifs (e.g., Downes, 1990; Strating et al., 1993; Van der Wal and Vissers, 1993). Plots of the measured grain sizes for each of these textural facies at their respective temperatures of equilibrium are shown in Fig. 4A and B. In each of these figures, at different strain rates, it is seen that the porphyroclastic textures 2 and 3 of the “coarsegrained” facies consistently fall in the GSI regime, as their textural SPO fabrics would imply. In contrast, texture 5, of the ‘‘fine-grained” facies (with a weaker developed LPO fabric, Fig. 5C, and poor SPO) lies entirely within the GSS flow regime,

implying that at lower temperature, at both strain rates and high shear strains, there has been a switch in flow mechanism from GSI to GSS creep. One mechanism whereby the transition from nonlinear dislocation creep to linear diffusion creep may occur is by controlled reduction of the grain size, at a temperature low enough to retard grain growth and yet still high enough that grain-boundary diffusion is efficient (White, 1976; White and Knipe, 1978; Rutter and Brodie, 1988). Experiments on polycrystalline olivine at high temperatures and pressures have shown that, during dislocation creep, grain-size control is through dynamic recrystallisation and is inversely related to the differential stress (e.g.,

J.K Ross et al./Tectonophysics

Mercier et al., 1977; Ross et al., 1980). Thus, as the stress increases within the dislocation creep regime, a reduced grain size can be produced by dynamic recrystallisation recovery process that could eventually promote the onset of diffusion creep. This is especially so with a reduction in temperature because of the lower activation energy for diffusion creep with respect to hat of dislocation creep. Probable examples of such a process are: (1) the “superplastic” fluidal textures of Bouillier and Nicolas (1975) and Bouillier and Gueguen (1975) described in mantle xenoliths from South African Kimberlites; and (2) grain-size variations described by Rutter and Brodie (1988) for grain-size reductions in olivine-rich rocks deformed under granulite facies conditions at subcrustal levels; both examples are believed to have occurred within narrow lithospheric shear zones. Experiments by Beeman and Kohlstedt (1993) on sintered olivine plus Na-rich glass and by Cooper and Kohlstedt (1986) on powdered olivine plus basalt mixtures suggest that the presence of a melt phase reduces the strength of the composite material. In these experiments the melt phase was observed after the experiments to occur as glass at grain triple points and not to wet the planar grain boundaries under these conditions. In experiments on fluid-assisted recrystallisation Urai (1983) found that the fluid films were stable during migration, but when the deformation ceased, migration stopped and the film decayed into an array of discrete fluid inclusions. Similar observations, that a dynamically stable fluid film is unstable at rest, have since been made by Spiers et al. (1986) and Urai et al. (1986) in halite and by Ree (1994) in an organic rock analogue. Toriumi and Kuroda (19871, assuming dynamic recrystallisation, concluded in experiments on olivine and albite melt systems that a static fluid distribution would likely be stable below a certain temperature during diffusion creep and at lower temperatures and higher stress during dislocation creep. Most recently, Hirth and Kohlstedt (1995) have described coaxial experiments on the behaviour of olivine aggregates in the diffusion creep regime under melt-free and melt-added conditions. They conclude that the presence of melt enhances the creep rate by more than an order of magnitude when the melt fraction is N 7 vol.% and, with increasing melt fraction, more and more two-grain boundaries are seen to be completely

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wetted under SEM and TEM. They believe that these form in response to anisotropic interfacial energies rather than some dynamic instability. Further, the wetted planar two-grain boundaries provide rapid transport paths for diffusion, that is rate limited by transport along melt-free grain boundaries. Other recent studies on melt topology in partially molten mantle peridotite during ductile deformation have shown that melt distribution occurs at triple junctions as well as the majority of grain boundaries exhibiting melt partly or completely along their length (Jin et al., 1994). They also showed that under static annealing conditions, melt distribution occurs in tubules having an approximately triangular cros section along three grain intersections. This fluid behaviour, under either dybamic or static conditions, is identical to that described above for salt-brine experiments (Urai et al., 1986; Spiers et al., 1986). Hence, the macroand microscale distribution of fluids in mantle rocks is dependent on the state of stress and the operative deformation mechanisms. The presence of fluid-aided recrystallisation during dislocation creep is likely to produce more softening than in a dry mantle rock undergoing deformation. If this fluid film is stable during a switch to diffusion creep, then the kinetics of the latter would likely be increased. We have described the presence of a fluid phase (now glass) observed optically at triple-point grain boundaries and inferred from electron microprobe scans to be completely along some planar grain boundaries, within the “fine-grained” facies, texture 5. The results of the experiments listed above, lead us to suggest that the presence of a fluid phase during the development of the “finegrained” facies has lead to a reduction in the strength of these developing mylonitic rocks, accompanied by an increase in the strain rate. If it can be assumed that the decrease(s) in grain size for the two textural types reflects marked increase(s) in the stress difference(s) that these samples experienced, then, the question is what are the likely scenarios that may cause large stress increases in the mantle; how do these stresses give rise to the formation of shear zones and once established, what process(es) can cause continued strain localisation? The Yitong Graben, comprising the northernmost part of the Tanlu Fault, is interpreted as having a long-lived complex history, being developed initially

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as an oblique-slip fault along which there has been later graben formation. This northerly segment of the Tanlu system is thought to have been inactive since the beginning of early Tertiary times, whilst the southerly segment is presently active (Tian and Du, 1987). All of the local grabens, of which the Yitong graben is the northermost, are small, being several times longer than they are wide (N 0.25-30 km). Rifting often implies that crustal thinning has occurred with concommitant rise in a local mantle diapir. Although debatable (Nicolas et al., 1994) thinning resulting from crustal extension is frequently believed to be initiated very slowly and the local geotherm is initially the same as cold lithosphere, but with time the strain rate will increase as the geotherm increases. This scenario is the one described by Steckler et al. (1988) for the Gulf of Suez, where initial extension rates are very slow during the initial stage of rifting that is followed by a second phase of rapid extension leading to the formation of oceanic crust and rapid rise in the local geotherm. More likely, the Yitong graben, with its eastern margin comprising the Tanlu Fault, is the result of transtension (Aydin and Nur, 1982) across this long-lived oblique-slip fault with minimal extension and lacking in the formation of oceanic crust. These continental movements are thought to be related to change in motion of the northwesterly subducting Pacific Plate beneath the Asiatic Plate (eastem China) since the late Mesozoic (Hilde et al., 1977; Cao and Zhu, 1987). Subsequent lithospheric thinning and crustal extension formed the NNE-oriented Yitong graben system (Tian and Du, 1987; Ma, 1989). A two-stage model for geodynamic evolution is thus envisaged for the upper mantle beneath Yitong. Firstly, regional upwelling (diapirism) of the upper mantle during Mesozoic times involving continuous GSI deformation giving rise to the high-temperature textures comprising the “coarse-grained” facies. This upper mantle diapirism was later spatially associated with large-scale intra-lithospheric shear zone(s) which developed synchronously with the Yitong graben system and the Tanlu Fault on its western margin during Oligocene and Neogene times. Movement along this fault resulted in the juxtaposition of colder hanging-wall rocks against hotter footwall rocks, thus lowering the temperature in the latter to

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about 700°C and giving rise to the textures comprising the “fine-grained” mylonitic facies. Inititiation and localisation of flow in the upper mantle leading to the propagation of the Tanlu Fault and the mylonitic “fine-grained” facies, may have resulted from stress concentration(s) associated with different mantle petrologies (Kirby and Kronenberg, 1987; Ross and Bauer, 1992) but more likely is associated with softening within zones where there has been a grain-size reduction together with localisation of partial melt. This two-fold association would probably result in enhanced strength reduction and increase of strain rate within the zone(s). The latter event can best be thought of as a continuum wherein, (a) surface(s) propagate(s) laterally and upwards, in the manner of crack propagation, such that the tip of the surface would be associated with high stress concentrations giving rise to finer grain sizes with increased propagation and lower temperature. This latest extension event appears to have taken place in the presence of a fluid phase (now glass) whose transition temperature is similar to that of the temperature inferred from compositions of the “finegrained” facies mineral assemblages. This Na-KAl-rich fluid phase, perhaps resulting from partial melting related to uplift associated with extension, preferentially infiltrated the developing shear zone(s) and accompanied the switch to GSS flow. This mechanism then allows further efficient strain localisation via grain boundary diffusion and sliding at higher strain rates (e.g., Kelemen and Dick, 1995). Both mechanisms are enhanced by the presence of a fluid phase at the grain boundaries, and together with the cyclic rise and fall of pore pressure within that fluid resulting from grain boundary sliding, would promote cracking and localised leakage of the fluid. Once the fine grain size at high stress is established in the presence of a fluid, production of high strains within the zone may result in a further switch in the mode of flow, passing from semibrittle to loss of cohesion, perhaps resulting in seismic slip.

6. Conclusions We have described the various textures recognised within mantle xenoliths (that include spine1 lherzolites, pyroxenites and glass-bearing werhlites

J. V. Ross et al. / Tectonophysics

and Iherzolites) collected from Neogene volcanics emplaced in the floor of the Yitong graben that is genetically related to the Tanlu Fault in northeastern China. In a previous work (Xu et al., 1993) we have rationalised that the majority of these xenolithic textures formed during deformation(s) within the upper mantle prior to sampling by the host basalts and are in fact from part(s) of a lithospheric shear zone, probably the depth continuation of the Tanlu Fault. The textural types comprise two main groups, a ‘‘coarse-grained’ ’ and a “ fine-grained’ ’ facies. The former facies includes a four-fold grain size/growth history wherein an early coarse grain size of both olivine and orthopyroxene become progressively smaller involving the development of a porphyroelastic SPO fabric together with neoblasts of both olivine and orthopyroxene, and finally, an increase in the grain size of olivine grains that then contain inclusions of both neoblast phases. In contrast, the “fine-grained” facies comprises textures that are either mylonitic or ultramylonitic involving grain-size reduction of all phases that also include Na-K-Alrich glasses along their grain boundaries and several examples of spinel-plagioclase lherzolites. The above textures and mineral compositions support the conclusion(s) that two different thermodynamic states are evidenced in this Yitong suite; the “coarse-grained” and “fine-grained” facies probably reaching equilibrium at N lOOO”C, or above, at SO-60 km, and _ 750°C at 25-35 km, respectively. We speculate that the earlier of the two facies likely represents low strain associated with regional upper mantle diapirism resulting from subducion. The later mylonitic facies likely results from localisation of high strain into narrow shear zone(s) wherein the deep-seated Tanlu Fault is evolved as an inclined detachment zone along which localised extension occurred so that uplifted hot mantle rocks in the footwall are continuously cooled by being in contact with colder hanging-wall rocks. This latest extensional movement was probably related to changes in motion in the Pacific Plate and could have been the initial stage in the formation of a small ocean basin, but, if so, motion apparently ceased before enough large-scale diapiric flow or any oceanic crust was produced. However, minimal localised diapiric flow likely occurred because the latest deformational phase is accompanied by a Na-

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K-Al-rich fluid phase along the grain boundaries of the “fine-grained” facies and this fluid was probably produced by localised partial melting under subsolidus conditions during the extensional uplift. It is speculated that under these “wet” conditions of high strain, lower temperature(s) and high stress the dislocation creep mode of flow during upper mantle diapirism then switched to the more efficient diffusion mode of flow. The effect of this rheology change is to change the level of force required first to initiate rifting and then to maintain it at much lower levels (Hopper and Buck, 1993). Existence of lithospheric shear zones have long been speculated upon by geophysicists who have observed seismic anistropy as well as electrical conductivity changes across narrow zones. These anisotropic zones have frequently been interpreted as resulting from either extremely well developed olivine fabrics, both SPO and LPO, or as zones of extensive partial melting accompanying movement. This example from Yitong suggests that the anisotropy within this lithospheric shear zone is due to a combination of well-developed olivine fabric, that locally the mantle is “wet” and that these shear zones provide pathways for the migration of mantle fluids.

Acknowledgements The authors wish to thank Drs. Shun Karat0 and Greg Hirth for their constructive comments on an earlier version of this paper that resulted in substantial improvement. The work was supported by NSERC A2134 to JVR.

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