porphyroblast inclusion-trail orientation data eppure

7/30/2019 Porphyroblast Inclusion-trail Orientation Data Eppure

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J . metamorphic Geol., 1992, 10, 295-307

Porphyroblast inclusion-trail orientation data: eppure- -non son girate* !T. H. BELL , S . E . J OHNSON, B . DAVIS, A. FORDE, N. HAYWARD AND C. WILKINSDepartment of Geology, lames Cook University, Townsville, Qld 4811, Australia

A B S T R A C T Extensive examination of large numbers of spatially orientated thin sections of orientated samples fromorogens of all ages around the world has demonstrated that porphyroblasts do not rotate relative togeographical coordinates during highly non-coaxial ductile deformation of the matrix subsequent to theirgrowth. This has been demonstrated for all tectonic environments so far investigated. The work also hasprovided new insights and data on metamorphic, structural and tectonic processes including: (1) theintimate control of deformation partitioning o n metamorphic reactions; (2) solutions to the lack ofcorrelation between lineations that indicate the direction of movement within thrusts and shear zones, andrelative plate motion; and ( 3 ) a possible technique for determining the direction of relative plate motionthat caused orogenesis in ancient orogens.Key words: foliation intersection axes in porphyroblasts; porphyroblast inclusion-trail orientations;relative direction of plate motion.

INTRODUCTIONPasschier et al. (1992) have discussed a series of paperspublished by ou r research group since 1985 and wewelcome this opportunity to respond. We begin with ahistorical perspective, to show our progression indeveloping and testing the concepts related to the lack ofporphyroblast rotation. Also, we provide ne w data oninclusion trails (Si) in porphyroblasts referring to papers inpress, in review and in preparation, to bridge partially theinevitable gap that develops between the currentknowledge of an active research group and its publishedwork. These data were derived from spatially orientatedthin sections cut from orientated hand specimens. We thenaddress each of the statements of Passchier et a l. (1992), inthe order that they were presented to us.

DEVELOPMENT OF THE CONCEPTThe concept that rigid objects such as porphyroblasts maynot rotate relative to geographical coordinates duringductile deformation came from the construction ofstrain-field diagrams (Ramsay, 1962, 1967; Bell, 1981).These two-dimensional diagrams represent graphically thestrain, relative to a pre-deformation grid of squares, withno change in area of each part of the grid.

Strain-field diagrams were constructed that duplicatedthe geometries observed within and around porphyroblastsas well as crenulated and crenulation cleavages. For

* Italian: and yet they have no t rotated, the words reportedlyspoken, after a strenuous performance, by Santaccia, a ladyinformant of il l repute for the Roman poet Belli (1832).

deformation involving a component of bulk shortening,these diagrams suggest to us that the deformationpartitions into zones of nearly coaxial progressiveshortening (including zones of no strain) and generallynon-coaxial progressive shearing (including a componentof shortening). Within the zones of progressive shortening,portions that do not deform internally do not rotaterelative to geographical coordinates, even during anoverall non-coaxial deformation (e.g. fig. 7 in Bell, 1981).Therefore, a strain-field approach suggests directly thatrigid objects do not rotate during progressive bulkinhomogeneous shortening, regardless of whether thedeformation is coaxial or non-coaxial (fig. 1 in Bell, 1985),if they are located within shortening domains.

PORPHYROBLASTS WITH SIMPLEI NC L USI 0N T R A l L SWe began to test this possibility that porphyroblasts do notrotate by measuring inclusion trails in porphyroblasts innon-coaxially deformed rocks. Measurements were madefirst for simple trails, that is a single straight foliation,which may show curvature on the porphyroblast rims. Thiswas done using randomly orientated thin sections frommany samples across an area which suffered twonon-coaxial deformations after porphyroblast growth(fig. 4a,b in Steinhardt, 1989). The pitches of inclusiontrails in the randomly orientated thin sections (fig.4a-Passchier et al. have misread the figure captions,statement 3 ) are statistically distributed in a subhorizontalplane (fig. 4b, Steinhardt, 1989), indicating that thefoliation within the porphyroblasts is subhorizontal acrossthe area studied. Yet the younger deformations have

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a a \

Fig. 1. Steinhardts (1989, fig. 5) orientation data replotted aspoles. Because the foliation preserved within the porphyroblasts isnear horizontal, slight changes in orientatio n produc e largechanges in strike. The lack of variation in orientation of th e S1foliation (preserved as inclusion trails in porphyroblasts) aroundthe large fold is much more obvious when plotted as poles (closedcircles). In fact, the poles to S1 form a tighter cluster than thepoles to the axial plane foliation to the fold (open sq uares). Thepoles to bedding are shown as triangles an d even though the singlepole from one limb differs markedly in orientation from thos e onthe other limb, the corresponding pole to S1 does no t, it fallswithin the cluster of data . That is, there has been no rotation ofthe porphyroblasts during the non-coaxial ductile deformationassociated with folding. Th e plunge of the fold is shown with asolid square.

obliterated totally all remains of this foliation in thematrix. We replotted Steinhardts (1989, fig. 5 ) dataaround a fold as poles (Fig. 1). This shows moredramatically that the foliation preserved within theporphyroblasts has not rotated around the fold (cf.Passchier et a[. , 1992, statement 3). Similar measurementswere then made by Johnson (1990a, fig. 4, 1990b, 1992)that also demonstrated remarkable consistency in theorientation of inclusion trails across two non-coaxiallydeformed regions (Fig. 2).

Of course, measurements of this type may not alwaysgive constant orientations of inclusion trails, because theremay have been strain and rotation of a foliation prior toinclusion within a newly forming porphyroblast. This isdemonstrated in Fig. 3 where five different paths areshown that commonly produce variation in the orientationsof foliations preserved as inclusion trails. In the past,workers seeing variation in inclusion-trail orientations haveassumed that the porphyroblasts have rotated withoutchecking these other possibilities. However, in spite ofthis, Fyson (1980), Steinhardt (1989) and Johnson (1990a,1992) recorded consistently orientated trails over wide

areas in rocks that have been subsequently non-coaxiallyfolded by one or more deformation events.P O R P H Y R O B L A S T S W I T H C O M P L E XIN C L US I 0 N T R A l LSWe progressed to measuring foliations in porphyroblastspreserving more complex inclusion-trail geometriesthrough two or more stages of growth in different tectonicenvironments (Figs 4 & 5; Wilkins, 1991; Davis, 1992).Also, we examined porphyroblasts which contain spiral-shaped trails (Bell & Johnson, 1989), because thestrain-field approach suggested that rotation occurred onlyin rocks which undergo homogeneous progressive simpleshear (Bell, 1985), and such geological environmentsseemed conceptually to be limited to vertical transform-like faults (Bell et al., 1989).

Bell & Johnson (1989) recognized that truncationalfoliations preserved within porphyroblasts which containspiral and other complex geometries tended to havepredominantly near-horizontal or near-vertical orientations(e.g. Fig. 6; figs 3-9, 13 & 16 in Bell & Hayward, 1991).To explain these orientations, they proposed an orogenicmodel involving cycles of horizontal compression andgravitational collapse. They suggested near-vertical folia-tions develop initially against rigid objects, such asporphyroblasts, perpendicular to the direction of bulkcompression throughout all levels of the orogen duringcompressive stages (figs 2% & 26a in Bell & Johnson,1989; see also Bell & Forde, 1992a). During gravitationalcollapse, near-horizontal foliations initially develop againstrigid objects, but only in that portion of the orogen that isaffected by gravitational collapse (figs 25d & 26b in Bell &Johnson, 1989); the transition from predominantly spiral tostaircase-shaped trails appears to occur around 4.5-5 kbarwithin orogen cores, suggesting that horizontal foliationscan develop in these regions to at least twice this depth(Bell & Johnson, 1992). Successive periods of porphyro-blast growth preserve these predominantly horizontallyand vertically orientated truncational foliations as inclusiontrails over extremely wide areas that have been deformedsubsequently many times (Hayward, 1992; Fig. 6) .However, matrix foliations show a large range oforientations due to the effect of folding or reactivation ofpre-existing foliations (Bell, 1986; figs 7 & 8 in Bell &Hayward, 1991; fig. 1 in Bell & Forde, 1992a; Bell &Johnson, 1992; Hayward, 1992; Johnson, 1992).

F O L I A T I O N I N T E R S E C T I O N AXESW I T H I N P O R P H Y R O B L A S T STherefore, the vertical foliation preserved within por-phyroblasts should lie perpendicular to the bulk directionof compression causing orogenesis (Bell & Johnson, 1989).Consequently, the intersection between near-vertical andnear-horizontal foliations preserved within porphyroblastrims by subsequent growth (foliation intersection axes -FIA, previously referred to as microfold axes byHayward, 1990) should lie perpendicular to the bulk


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P O R P H Y R O B L A S T I NCL U S I O N - T R A I L D A T A 297

u*=89N = 125 CM90

@= 172@=67 @= 78 CM92@= 77@=59 c-t:= 122N = 70@@@a@M106 N =61 CM107 N =87 CM112 N = 8 9 CM124 N =44@a@M143 N=66 CM143A N =39N =66 CM162C N =112 CM162D N =123 CM162D@@@FOLD) @@M161 N=38Fig. 2. Rose diagrams of pitch of internal inclusion trails in cordierite porphyroblasts from Coo ma, N S W, Australia, over an area of20 km2 (from Johns on, 1992). The se were measured from two vertical thin sections per rock or ientate d 45" apar t , one of which isperpendicular to the m ap trend of the vertical inclusion trails. A mark ed dominance of horizontal an d vertical foliations is well developed.These foliations formed du ring successive near-horizontal and near-vertical foliation-producing deformations. Th e equivalent foliation inthe matrix has been obliterate d in most rocks by subsequent non-coaxial defo rmatio n, and yet there has been n o rotation of the meanorientation in the rose diagrams away from the horizontal and vertical.

direction of com pression. This orthogonal relationship willbe independent of the direction of thrusting on theshallowly dipping foliations if thrusting is driven bygravitational collapse. For example, above the basaldetachment between the portion of the orogen affected bycollapse and that still affected by compression, in Bell &Johnson's (1989) orogenic mode l, the direction of thrustingis controlled only by gravitational collapse and hence bulktopography. It may therefore, for example, be highlyoblique to the relative direction of plate motion (e.g. Plattet al . , 1989).

Shear along pre-existing com positional layering, or otherstrongly heterogeneous foliations, can rotate any matrixfoliation that developed initially perpendicular to the bulkdirection of compression causing orogeny (e.g. fig. 1 inBell & Forde, 1992a). However, near-vertical foliationsnucleate during horizontal compression against any rigidobject, as the deformation tends to co ncentrate there. Thisfoliation commonly will be preserved in the orientation inwhich it formed against the rim of the rigid object, becauseof the protection from the effects of subsequentreactivation of pre-existing foliations and compositional


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298 T . H. BELL 1 L .

.cPATH 1

*PATH 2

*PATH 3 *PATH 4\

1PATH 5

Fig. 3. Causes of variation in inclusion-trail orientation in porphyroblasts without involving porphyroblast rotation. For all paths , theporphyroblasts grew early in the even t where their boundaries are first shown with solid lines. Path 1 : all porphyroblasts grew early duringdevelopment of the third deformation that produced the E- W foliation. Consequently, they preserve variously orienta ted S1 rails as wellas differentiated S2 . Path 2: porphyroblasts overgrow a weakly folded earlie r foliation after intern al strain within the zones of progressiveshortening but before developm ent of a throughgoing axial-plane cleavage. Subsequen tly, this event was oblitera ted during developmentof the E-W foliation. Path 3: porphyroblasts overgrow an anastomosing foliation early during development of the E- W foliation. Path 4:porphyroblasts overgrow a previously developed differentiated crenulation cleavage a nd cre nulate d cleavage, as well as foliationreactivated during the N- S even t, early during the development of the E- W foliation. Path 5: two generations of porphyroblasts. Thoseindicated with solid lines in the upper diagram grew early during the N-S foliation-producing deformation. Those indicated with dashedlines grew early during developm ent of the E-W foliation in the lower diagram. Th e central layer in the upper diagram deformed byreacthation during the N- S foliation-producing deformation.layering provided by this object (Bell, 1986). Ifgravitational collapse of the orogen follows horizontalshortening (e.g. Bell & Johnson, 1989), near-horizontalfoliations will nucleate against the rim of the rigid object.If it is a porphyroblast, further rim growth occurring earlywithin this event will preserve the vertical foliation, as wellas the curvature associated with shearing on theporphyroblast margin as the near-horizontal foliationdeveloped (Bell & Hayward, 1991), allowing determina-tion of the FIA (Fig. 7; Hayward, 1990; Bell & Forde,1992).

ORIENTATION OF FOLIATIONINTERSECTION A X E SFigure 8(a) shows FIA determined from garnet porphyro-blasts which contain spiral-shaped inclusion trails in theGrampians. Figure 8(b) shows FIA obtained from garnets

containing spiral-shaped inclusion trails where the matrixfoliation bears no relation to the inclusion trails and wrapsaround the porphyroblasts (Bell & Forde, 1992a). TheFIA in both these figures have consistent orientations forover 100km regardless of several subsequent phases ofextensive non-coaxial deformation, which in the Alpsdestroyed all remains in the matrix of the foliationsassociated with porphyroblast development including theirstrain shadows. The consistency of these data along theorogens strongly suggests that FIA preserved in porphyro-blasts have considerable potential for allowing informationto be obtained on the direction of relative plate motionthat caused orogenesis at the time of porphyroblast growth(Bell & Forde, 1992a).DISCUSSIONRotation of porphyroblasts, or indeed any rigid object,relative to geogrpahical coordinates, during non-coaxial


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P O R P H Y R O B L A S T I N C L U S I O N - T R A I L D A T A 299

FieldData Thin Section Data

Fig. 4. The orientations of contoured polesto S3 , S4 & S5 measured in the field, as wellas the orientation of S2, S3 & S4 measuredfrom inclusion trails in porphyroblasts fromnine different rocks. Th e field data an drocks were collected around a macroscopicD5 fold within the Yilgarn of WesternAustralia (W ilkins, 1991).Contoured onequal area net. Lowest (L) and highest (H )contour intervals and maximum den sity (M)are in per cent per 1 % area. Contourintervals are a geom etric progressionbeginning at 92.5% of the maximum (H).The rose d iagrams show the pitch of S2, S3& S4 measured o n 82 different spatiallyorientated vertical thin sections from thosenine rocks. In thin sections S2 has beentotally obliterated from the matrix andthere are very few remains of S3 . In spite ofthe multiple phases of non-coaxialdeformation tha t have caused this, withinporphyroblasts S2 is predominantlyhorizontal, S3 is near vertical and S4 is nearhorizontal in three dimensions (because theinclusion trails were measured from a rangeof thin-section orientations) all the wayaround the D5 fold.

1 8 %I-

L

ductile deformation, is a major component of thestructural geology paradigm. It is taught in textbooks,appears frequently in the literature and is used bystructural geologists world-wide to interpret the kinematicsof deformed rocks. When such a long-standing and firmlyheld component is questioned, considerable opposition isto be expected and is a normal prerequisite to paradigmchange (Kuhn, 1970).

Prior to our work, the idea of porphyroblast rotationwith respect to geographical coordinates, in the formationof spiral-shaped inclusion trails, had never been ques-tioned or rigorously tested in naturally deformed rocks(e.g. Bell & Johnson, 1990). Schonevelds (1979) studywas regarded as providing a model of porphyroblastrotation that explained all of the internal geometries foundwithin garnet porphyroblasts with spiral-shaped inclusiontrails. Many structural geologists believe that histhree-dimensional pattern of Si (figs 14 & 19 inSchoneveld, 1979), and his detailed cross-sectionalgeometries (fig. 18 in Schoneveld, 1979), were producedby serial thin sectioning of single porphyroblasts; this is notso. Schoneveld (1979) stated that his original intention wasto make a three-dimensional picture of the inclusion

geometry from serial sections through natural garnets.However, this turned out to be impossible due to thenon-traceability of individual Si surfaces from one sectionto the next (Schoneveld, 1979, p. 37). Thus, as explainedin Schoneveld (1979), his three-dimensional image (figs 14& 19) and two-dimensional sections (fig. 18) are based o nhis string model, and not on serial sectioning throughnatural garnets.

One type of test of porphyroblast rotation innon-coaxially deformed rocks, i.e. relating spiral trailasymmetry to movement direction on thrusts, was initiatedby Schoneveld (1979). However, he abandoned this,presumably because the shear sense from some or all ofthe spiral-shaped inclusion trails he examined was theopposite to that known to have taken place (which wouldoccur if porphyroblasts had not rotated, e.g. Bell &Johnson, 1990, 1992).

Passchier et al . (1992) have been critical of previouslypublished data, but have produced no new data thatrefutes our explanations and models. Our approach hasbeen to test for porphyroblast rotation by collecting largequantities of data from naturally deformed rocks. Thesedata indicate that porphyroblasts do not rotate during


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300 T. H. B E L L fT A L .

4 3

v 50'1 + + + + + \t7 5

0 1 2 3 k mIuctile deformation, with one exception. Bell & Forde

(1992b) have found an example of an apparently unusualkink fold where the porphyroblasts have been rotated fromthe short to long limbs during deformation that had bothductile and brittle components.

We now address each of the criticisms of Passchier et al .(1992) in detail.

Statement (1)-porphyroblasts do not rotate sincethey are mechanically fixed in the rock by thestrain shadowsPasschier et al . (1992) appear to have developed thisfundamental misconception on their own in order torationalize data indicating lack of rotation. We have neverclaimed this and disagree entirely. Of the nine referencesthey cite, only Johnson (1990a) has ever discussed it. Hedid this in the context of five possible models for lack ofrotation, and made no conclusion as to its applicability.

The strain-field diagrams constructed by Bell (1981) donot require any mechanical fixing of the porphyroblasts bytheir strain shadows to explain lack of rotation. Indeed,strain shadows adjacent to porphyroblasts can be totallydestroyed during reactivation of matrix foliations (Bell,1986), foliation overprinting (fig. 14, Johnson, 1990a;Hayward, 1992), or total destruction of earlier foliations(Bell & Forde, 1992a), and the data indicate that theporphyroblasts have not rotated (Figs 6 & 8).

15 -> 260

Fig. 5. The orientation of L: intersectionlineations preserved in andalusiteporphyroblasts from samples around theCannibal Creek Granite. These foliationintersection lineations (FIA) are shownwithin vertical thin sections cut parallel toth e FIA trend, which was determine d usingthe technique of Hayward (1990). Outsidethe porphyroblasts, which grew early duringthe fourth deformation, these axes havebeen rotated around a horizontal axisthrough angles of up to 60" in a sense that isalways up towards the granite margin, dueto extensive non-coaxial deformation in thematrix subsequent to porphyroblast growth(Davis, 1992). However, within theporphyroblasts they have variably shalloworientations either side of the horizontalindependent of their location with respectto the granite margin. Tha t is, they have no tbeen rotated within the aur eole schists bythe highly non-coaxial strain associated withgranite emplacem ent (Davis, 1992). Th evariation of ti lunge directions within thehorizontal plane pre-dates porphyroblastgrowth and is interpreted to be due t oheterogeneous strain and rotation of L:towards L within a near-horizontal S3 (seeDavis, 1992).

Statement (2)-millipede microstructures areevidence for deformation partition ing intodomains of progressive shortening andprogressive shearingMillipede microstructures occur on all scales and do notrequire the presence of a rigid object to form (Fig. 9; Bell,1981; fig. 2 in Bell & Rubenach, 1980; fig. 12 in Bell &Johnson, 1992). They form early in the deformation whenthe strain is relatively low and the local deformation morecoaxial. They are then obliterated subsequently as thedeformation intensifies and the strain becomes progres-sively more non-coaxial. Consequently, preservation ofsuch geometries requires either that the strain remain lowor that these structures be included within strongermaterial than the matrix, such as within porphyroblasts orzones of silicification.

Millipede geometries are indicators of deformationpartitioning, as described originally by Bell (1981), whichrepresents any transition from zones of low to high strain.Similar partitioning occurs from crenulation hinges todifferentiated crenulation limbs, and has been describedfor a large range of geometries in Bell & Johnson (1992).Millipede geometries occur locally in the cores of garnetporphyroblasts which contain spiral- and other complexlyshaped inclusion-trail geometries, in al l section orienta-tions, just as spiral-shaped geometries occur in all sectionorientations (Bell & Johnson, 1989; Hayward, 1990; Bell


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v 1 n 80 v3 n 91

V17A n 268

v20 n 226

V54A n 153

V60B n 114

V71 n 40

V17B n 173

v22 n 66

v55 n 96

V65 n 92

V72 n 137

v7 n 109

V19A n 93

V48 n 98

v57 n 79

V66 n 135

V115 n 42

v12 n 138

V19B n 102

V51 n 80

V58 n 106

V67 n 43

V125 n 50Fig. 6. Rose diagrams of pitch of truncations or discontinuity surfaces in garnet porphyroblasts from orienta ted rocks collected around theChester Dom e in Vermont, US A (from Hayward, 1991, 1992). For many of these rocks, there are no strain shadows preserved on themargins of porphyroblasts. All truncations were measured in rocks containing a variety of complex inclusion-trail geometries, includingspirals. The measurements were m ade on 4-10 differently striking, vertically dipping thin sections per rock. The samples givegeographical cover over an ar ea of 4000 km2. Even though there have been several non-coaxial deformation ev ents after the bulk ofporphyroblast growth (Hay ward, 1991), the truncations are dominantly nea r horizontal and near vertical and the porphyroblasts have notrotated . Hence the foliations they define must be predominantly subhorizontal and subvertical in three dimensions.


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302 T . H. B E L L Er A L .

F IATRENDRANGE

FI APLUNGERANGE

Fig. 7. Sketch showing modification of Hayward's (1990)technique in order to get trend and plunge of foliation intersectionaxes (FIA). Initially, thre e vertical thin sections a re cut striking60",120"and 180". The switch in spiral or staircase asymmetry islocated when viewing the sections in the sa me sense arou nd avertical axis (anticlockwise in this case). Two vertical thin sectionsare then cut striking 20" apart between those where the switchoccurred enabling the switch in asymmetry to be determ inedwithin 20". A vertical thin section is then cut to bisect this regionenabling the switch in asym metry to be defined within 10". Thisstep is repeated to define it within 5". A vertical slab is then cutstriking parallel to the c ent re of this 5" range. T his vertical slab issuccessively thin sectioned at 60, 2U", 10"an d 5" intervals in asimilar manner to that just described, except th at all sections arecut striking perpendicular to the previously determined tre nd ofthe switch in asy mmetry (FIA), to define the plunge.& Hayward, 1991; Hayward, 1992). That is, thesestructures also lie in sections cut at a very high angle to the'rotation' axis of Rosenfeld (1968, 1970) and Schoneveld(1979), or FIA of Hayward (1990, 1992) and Bell & Forde(1992a). Rosenfeld (1968, 1970) and Schoneveld (1979)considered mistakenly that symmetrical inclusion trails(millipede-shaped trails) occurred only on or close to therotation axis. We agree entirely with Passchier et al . (1992)that many millipede-shaped geometries within garnetporphyroblasts which contain spiral-shaped trails are aproduct of a cut effect in sections orientated subparallel tothe spiral axis or FIA. Commonly, these includethree-quarter millipedes with axial planes oblique to thehorizontal and vertical. However, those millipede ge-ometries mentioned above that lie at a high angle to thisaxis are not a product of a cut effect and indicate adeformation history of locally coaxial progressive bulkinhomogeneous shortening (Bell, 1981).

Statement (3)-porphyroblasts have not rotatedwith respect to each other since they showidentical orientation of Si over a large area,despite later deformationThis statement is supported strongly by the published data(see also Figs 2, 4 & 5) , remarkably so when the timing ofporphyroblast nucleation is taken into account. The

inclusion trails measured by Fyson (1980), Steinhardt(1989) and Johnson (1990a) were preserved in porphyro-blasts that grew early during the deformation event thatwas rotating the external foliation (cf. Bell & Hayward,1991). Consequently, there will have been locally at leastsome foliation rotation prior to porphyroblast nucleationand growth, as shown in Fig. 3 , path 2 (e.g. Bell, 1985;Steinhardt, 1989; Johnson 1990a,b, 1992). Passchier et al.(1992) suggested that Johnson's (1990a) measurementsmay not be valid because the dip was unconstrained. Asstated clearly by Johnson (1990a), and demonstrated inFig. 10, the inclusion trails he found to have consistentstrikes are also consistently near vertical in cross-section;i.e. the porphyroblasts have not rotated.

For simple inclusion trails, distinguishing variation in theorientation of the included foliation due to deformationeffects before incorporation within a porphyroblast (Fig. 3 )from subsequent rotation of the porphyroblast requiresdetermination of the timing of porphyroblast growth. Thiscan be achieved by checking for curvature of the inclusiontrails in the porphyroblast margins at high magnification inseveral differently orientated thin sections (Bell &Hayward, 1991). All examples of variation in theorientation of simple inclusion trails that we haveexamined so far were a product of one of the paths in Fig.5 .

The rotational effects of deformation on foliations priorto incorporation within a porphyroblast , such as thoseshown in Fig. 3 , can be eliminated by working withporphyroblasts that contain at least two stages of growthduring different deformation events. Our experienceindicates that the foliation forming against the earlierformed porphyroblast core is always subhorizontal orsubvertical (Bell & Forde, 1992a) unless it is a reactivatedearlier foliation, which can usually be determined readily(e.g. Bell & Hayward, 1991; Hayward, 1991, 1992). This isshown in Fig. 6 where the foliation truncations arepredominantly horizontal and vertical in three dimensionssince measurements were made from 4-10 vertical thinsections of different strike to construct each rose diagram.Consequently, FIA in porphyroblasts which contain atleast two stages of growth during different deformationevents (Fig. 8) are always near horizontal (e.g. Hayward,1990,1992; Bell & Forde, 1992a). The commonly observedorthogonal relationship of foliations with respect to thehorizontal shown in Figs 2, 4 & 6 (Hayward, 1990, 1991,1992; Bell & Hayward, 1991; Wilkins, 1991; Johnson,1992) cannot be explained by porphyroblast rotation.

Statements (4) and (5)-porphyroblasts do notrotate, relative to geographical coordinates,during duc tile deformationThe above mentioned data strongly support this statement.Clearly, brittle deformation will rotate porphyroblasts, andplate motion will rotate porphyroblasts in one plate versusanother. However, within rocks from the same fold belt inany one plate, foliations and FIA within porphyroblasts ofthe same generation appear to maintain consistent


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Fig. 8. (a) Map of foliation intersection ax es (FIA) from garnet porphyroblasts which contain spiral-shaped inclusion trails in theGrampians of Scotland. These FIA remain consistently orien tated over many tens of kilometres, even though th e rocks were extensivelynon-coaxially deformed a nd folded several times after porphyroblast growth during both the G ramp ian and Caledonian orogenies. Thefolded boundaries of the Appin Group are shown. (b) M ap of the oldest set of FIA trends in the Alps ( N & S boundary of Alps shown bybold lines). Th e garnet porphyroblasts which contain this 15" se t of FIA are wrapped by the matrix foliation. Tha t is, the last stage of th espiral-shaped trails does not extend into the matrix (Bell & Forde, 1992a). These FIA remain very constant in orientation , and ye t th eporphyroblasts generally have no strain shadows preserved a nd there has been m uch subsequent non-coaxial deformation associated withseveral differently orien tated events. Th ese data directly contradict statemen ts ( l ) , (4 ) & (5) of Passchier et al. (1992).

geographical orientations in plan view as well as in section.The probability that several successive deformations will

rotate all porphyroblasts by the same amount for eachsuccessive event is extremely low. Yet, Figs 1, 2, 4-6 & 8show consistently orientated inclusion trails in rocks whichhave been deformed non-coaxially at least twice afterporphyroblast growth. In particular, Figs 2, 4 & 6 showthat the foliations preserved as inclusion trails are paralleland perpendicular to the horizontal. This cannot beexplained by any form of porphyroblast rotation includingthe method suggested by Passchier et al. (1992, fig. 4),involving uniform rotation of all porphyroblasts relative togeographical coordinates by homogeneous progressivesimple shear. For foliations and truncations to maintainhorizontal and vertical attitudes as demonstrated by Figs 2,4 & 6, and FIA to remain subhorizontal (Fig. 8b), thepervasively developed shear zones would always need tobe vertical with horizontal stretching lineations. N o suchpervasive shear zones are developed in the CoomaComplex, the Yilgarn, the Chester Dome or the Alpswhere these data were obtained.

Statement (6)-porphyroblasts in pelit ic rocksnucleate dur ing progressive deformation inmicrolithons, grow unt il they impinge on cleavagelamellae, and stop growing thereClearly, any foliation or truncation may be overgrown by aporphyroblast. What we pre-empted (Bell et af.,1986), but

have now described in some detail, and tested thoroughlywith many differently orientated thin sections per rock, isthat porphyroblasts that overgrow a differentiated foliationonly do so during a subsequent deformation event that liesgenerally at a high angle to the earlier developed foliation(Bell & Hayward, 1991). This has considerable significancefor the interrelationship between deformation andmetamorphism (Bell & Hayward, 1991).Statement (7)-truncations in porphyroblasts areevidence for polyphase deformation andmetamorphismTruncations were described in Bell & Johnson (1989) aszones separating two apparently different-generation setsof inclusion trails, which may be abrupt with the twoinclusion-trail sets meeting at high angles, or relativelycontinuous such that they resemble differentiated crenula-tion cleavages (ranging from stages 3 to 4 of crenulationcleavage development described in Bell & Rubenach,1983).

They have considerable tectonic significance; forexample, they indicate that metamorphic reactions arecommonly episodic (Bell & Hayward, 1991). Suchtruncations or textural unconformities are common inporphyroblasts with spiral-shaped inclusion trails (Bell &Johnson, 1989; Hayward, 1990, 1992; Bell & Hayward,1991) but were ignored by Schoneveld (1979) and Powell& Vernon (1979). In three dimensions they define


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304 T . H. B E L L E T A Lfoliations that are predominantly subhorizontal andsubvertical around the Chester Dome (Fig. 6; Hayward,1992). These well-defined orientation relationships cannotbe explained by periodic growth during a single stage ofdeformat ion/metamorphism while the porphyroblast isrotating (fig. 6 in Passchier et a l . , 1992). Apart from this,equivalent pairs of truncations can be identical or totallydissimilar in character (e.g. Bell & Hayward, 1991;Hayward, 1991, 1992; Bell et a l . , 1992); the model ofPasschier et al. (1992, fig. 6) requires them to be nearidentical. Nor can these orientation relationships be aproduct of chance as they show the same character in theAppalachians, Norway, Scotland and the Himalayas andoccur in rocks without spiral-shaped inclusion trails (Bell& Hayward, 1991).Statement (8)-spiral garnets do not form byrotation during a single phase of deformation, butare an effect of polyphasedeformation/metamorphism and growthWe resampled all of Rosenfelds (1968) and Schonevelds(1979) published locations, used some of Schonevelds ownrocks, and examined the same thin sections that were usedby Powell & Vernon (1979). From these rocks and thinsections we found that smoothly curving spiral-shapedinclusion trails are uncommon; earlier workers justphotographed those with the best spiral-shaped trails.Consequently, w e drew fig. 20 in Bell & Johnson (1989) toemphasize the role and significance of truncations; fig. 6 inBell et al . (1992) shows a much smoother example withcombinations of smoothly curving and truncationalinclusion trails that we constructed from an actual garnet.

The most smoothly curving trails that we have foundcome from the Alps, and these are described by Bell el al .(1992) who wanted to test whether they could have formedby rotation. This investigation provided a simpleexplanation for their formation that did not involverotation (fig. 5 in Bell et a l . , 1992). Also, it revealed thateven the most smoothly curving trails contain truncationalheterogeneities due to space problems with coalescingtrails (figs 3 & 4 in Bell et a l . , 1992). Commonly, these arenot picked up by the eye until the trails are traced on aphotograph while examining the porphyroblast at highmagnification.

As mentioned earlier, Schonevelds three-dimensionalpattern of S, (figs 14 & 19 in Schoneveld, 1979), and hisdetailed cross-sectional geometries (fig. 18 in Schoneveld,1979), were not produced by serial thin sectioning of single

Fig. 9. (a) Photograph of millipede geometry in matrix of schistfrom the Coom a Complex, Australia, with the millipede axialplane shown with a line. Th e millipede has been o verprin ted bytwo weak crenulation eve nts, one near parallel and the oth er nearorthogonal to the millipede axial plane. C learly, the core of themillipede was not rigid and did not consist of stronger materialthan the surrounding m atrix. P lane polarized light. Longaxis =15mm. (b) Outcrop-scale millipede geom etry in quartzitesin New Zeala nd (courtesy of J. Campbell and A. Downing).Horizontal section of massive bed in lower part is 0. 5 m.


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PO R P HY R O BL A S T I N C L U S O N - T R A I L D A T A 305

Fig. 10. Rose diagrams showing attitude offoliations in vertical sections for thr ee rockschosen at random from those for which thestrike of foliations in horizontal thinsections were shown in fig. 4 in Johnson(1990a). Combined with these latter d ata,these rose diagrams show that the internalfoliations preserved within theseporphyroblasts are predominantly nearvertical in three dimensions. Consequently,the porphyroblasts have not rotated duringseveral phases of subsequent non-coaxialdeformation.

porphyroblasts. His original intention was to make a threedimensional picture of the inclusion geometry from serialsections through natural garnets. However, this turned outto be impossible due to the non-traceability of individual S,surfaces from one section to the next (Schoneveld, 1979,p. 37). Consequently, his three-dimensional image (figs 14& 19) and two-dimensional sections (fig. 18) are based onhis string model, and not on serial sectioning throughnatural garnets. Therefore, his three-dimensional patternsof Si do not contain truncations, although they werepresent in porphyroblasts from the rocks that heexamined. He tried to explain only those porphyroblastswith smoothly curving trails, but did photograph some withwell-developed truncations that he had not discerned-forexample, those containing millipedes.

F A B R I C S INDICATIVE OF ROTATION?Simple inclusion trails in elongate porphyroblasts that varyfrom porphyroblast to porphyroblast are not unequivocalindicators of porphyroblast rotation, as shown in Fig. 3.Changes in length-to-width ratio, such that thoseporphyroblasts with the largest ratio appear to be the mostrotated, can be explained simply by variation in theorientation of the foliation within zones of progressiveshortening and the control of this foliation on porphyro-

Fig. 11. Thes e diagrams show the problemsassociated with producing th e SJS,geometries by rotation shown in fig. 5 inZwart & Calon (1977). Th e porphyroblastscannot have grown during rotation as shownin (a) as they develop the wrong inter naltrail geometry. If they grew in two stages asshown in (b) they would have th e rightinclusion-trail geometry b ut th e volumeproblem associated with oppositely directedrotation would produce boudinage forwhich there is no evidence in the matrix.We suggest that the po rphyroblasts haveovergrown crenulations associated with twosuccessive deformation events which lienear orthogonal to one anoth er. If thesample was orienta ted, we predict thatZwart & Calon would find that the axialplanes of the crenulations preserved in theporphyroblasts would be close to th eprojections of horizontal and vertical planesonto the thin section.

blast shape (p. 112 in Bell, 1985 and figs 15, 17 & 18 inBell et al . , 1986). In particular, the only data for rotationpresented by Passchier et al . (1992), reproduced fromZwart & Calon (1977) in Fig. 11, cannot have beenproduced by rotation because the matrix foliation showsno variation in orientation or character between theporphyroblasts. If the porphyroblasts had rotated awayfrom one another by swivelling at their bases as shown inFig. ll(b), the extension between their tops would resultin attenuation and downwards deflection of the foliation.Alternatively, if they swivelled at some other point, therewould be attenuation with inward deflection of thefoliation at one end and compression with outwarddeflection of the foliation at the other.

CONCLUSIONSA rapidly expanding data base indicates that manyfoliations preserved as inclusion trails in porphyroblasts,particularly truncations preserved against earlier cores, areconsistently orientated near vertically and near horizon-tally. Similarly, FIA preserved in the same-generationporphyroblasts are orientated consistently for largedistances along the length of orogens in spite ofsubsequent non-coaxial deformation and refolding. Thesephenomena indicate that the porphyroblasts have not

b


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306 T . H. B E L L ET ALrotated relat ive to geographical coordina tes during ducti ledeformat ion; the data cannot be explained by porphyro-blast rotation.These data, part icularly the consistency of FIApreserved in same-generation p orphyroblasts alo ng theAlps and the Grampians, also suggest that the model fororogenesis proposed by Bell & Johnson (1989) hasconsiderable potential for application to well-studiedmountain belts. This model is schematic and complicationssuch as shift ing orogen cores, staircase trajectorieschanging across folds forming a t relat ively shallow levels inthe crust (e.g. Ha ywa rd, 1991), as well as the overprintingeffects of different orogenies, must be incorporated.However, i t provides a simple explanation for the lack ofcorrelation of stretching l ineations preserved on thrustplanes, with relat ive plate motion observed by Platt et a l .(1989), but their direct relationship to bulk topographythrough gravitat ional collapse (Bell & Johnson, 1989,1992; Bell & Ford e, 1992a). This mod el predicts also that adirect relat ionship should exist between FIA and relat iveplate motion that the consistency of preliminary data fromthe Gram pians and Alps appe ars s t rongly to sup por t (e.g .Bell & Forde, 1992a) .F I N A L S T A T E M E N TIn direct contradiction t o the final statem ent of Passchier etal. (1992), that porphyroblasts should not be used fordetermination of shear sense unt i l they are bet terunderstood, we argue that porphyroblasts provide suchgood cri teria for shear sense along the foliat ion on whichthey formed that they can be used to cross-check otherstructures whose development is less well understood. Inpar ticular , we refer to th e s t ructures developed in manyzones of high st rain where opposi te shear senses f romroutinely used cri teria a re com monly resolved stat ist ically.Even th is degree of rat ionalization does not provide asolution when the conflict in shear sense occurs betweenone type of cr iterion and another in the s am e shear zone;for example, extensional crenulation cleavage in high-st rain zones indicat ing the opposi te shear sense toasymmetric quartz fabrics (e.g. Platt & Behrmann , 1986 ,p. 27) and other conflicts described by Bell & Johnson(1992). Porphyroblasts not only provide the s hea r sense ona part icular foliat ion, but also enable the history of shearsense dur ing the development of several successivefoliations to b e determined (Bel l & Johnson, 1992). Thisapproach will provide new insight into the structuraldevelopmen t of orogenic belts.A C K N O W L E D G E M E N T SWe gratefully acknowledge B. Merry for providing asource for the t i t le to th is paper . T.H.B. would l ike tothank the Aust ral ian Research Counci l (AR C) and the USNational Science Foundation (NSF) for research support .He thanks also J . Campbel l for the photograph in Fig .9(b). S.E.J. would l ike t o t hank t he NSF, t he AR C, Jam esCook Universi ty and the Rothmans Foundat ion for

suppor t ing his research. B.D. thanks the Commonweal thG o v e rn m e n t , A .F . t h a nk s t h e A R C a n d N . H . a n d C . W .thank James Cook Universi ty for suppor t . We al lacknowledge the supe rb faci li t ies provided by Jam es Co okUniversi ty for microstructural research. W e would l ike tothank the Edi tors fo r providing th is oppor tuni ty to respondto a cri t ical appraisal of our work. It gave us theoppor tuni ty to provide som e idea of the detai led work wehave been doing in test ing these concepts and will al lowthe knowledge gained to be m ore rapidly promulgated. W eparticularly thank D. Norris, M. Sandi ford and M.Will iams for cri t ically assessing this manuscript within avery short t ime frame. Finally, we applaud Passchier et a l.(1992) for placing themselves in the onerous posit ion ofthe church in th is debate.

R E F E R E N C E SBell, T. H . , 1981. Foliation development: the contribution,geometry and significance of progressive bulk inhom ogeneou s

shortening. Tectonophysics, 75 , 273-296.Bell, T. H . , 1985. Deformation partitioning and porphyroblastrotation in metamorphic rocks: a radical reinterpretation.Journal of Metamorphic Ge olog y, 3, 109-118.Bell, T. H., 1986. Foliation development and refraction inmetam orphic rocks: reac tivation of e arlier foliations anddecrenulation due to shifting patterns of deformationpartitioning. Journal of Metamorphic Geology, 4, 421-444.Bell, T. H. & Cuff, C., 1989. Dissolution, solution transfer,diffusion versus fluid flow and volume loss duringdeformation/metamorphism. ournal of Metamorphic Geology,Bell, T. H. , Duncan, A. C. & Simmons, J. V., 1989. Deform ationpartitioning, shear zone development and the role ofundeformable objects. Tectonophysics, 158, 163-171.Bell, T. H ., Fleming, P. D. & Rubenach, M. J., 1986.Porphyroblast nucleation, growth and dissolution in regional

metamorphic rocks as a function of deformation partitioningduring foliation developme nt. Journal of Metamorphic Geology,Bell, T. H. & Forde, A . , 1992a. Alignment of foliationintersection axes from porphyroblasts: a record of relative platemotion during Alpine orogenesis. Geology , in review.Bell, T. H. & Forde, A., 1992b. The microstructural processesoperating during folding: porphyroblast rotation versusnon-rotation. Journal of Structural Geo logy , in review.Bell, T. H., Forde, A. & Hayward, N ., 1992. Can smoothlycurving spiral-shaped inclusion trails form without rotation ofthe porphyroblast? Geology , in press.Bell, T. H . & Hayward, N . , 1991. Episodic metamorphicreactions during orogenesis: the control of deformationpartitioning on reaction sites and duration. Journal ofMetamorphic Geology, 9 , 619-640.Bell, T. H. & Johnson, S . E. , 1989. Porphyroblast inclusion trails:

the key to orogenesis. Journal of Metamorphic Geology, 7 ,Bell, T. H . & Johnson, S . E. , 1990. Rotation of rigid objectsduring ductile deformation: well established fact or intuitiveprejudice? Australian Journal of Earth Sciences, 37, 441-446.Bell, T. H. & Johnson, S . E., 1992. Shear sense: a new approachthat resolves conflicts between criteria in metamorphic rocks.Journal of Metamorphic Geology, 10, 99-124.Bell, T. H . & Rubenach, M . J., 1980. Crenulation cleavagedevelopment-evidence for progressive bulk inhomogeneousshortening from millipede microstructures in the RobertsonRiver Metamorphics. Tectonophysics, 68, T9-TI5.Bell, T. H. & Rubenach, M. J. , 1983. Sequential porphyroblast

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P O R P H Y R O B L A S T I N C L U S I 0N - T R A I L D A T A 307growth and crenulation cleavage development during progres-sive deform ation. Tectonophysics,92, 171-194.Davis, B. K ., 1992. Mechanism of emplacement of the CannibalCreek Granite: a new approach to determining the history ofgranite emplacement. Tectonophysics, in review.Dewey, J. F., Pitman, W. C. 111, Ryan, W. B. F. & Bonnin, J. ,1973. Plate tectonics and the evolution of the Alpine systems.Geological Society of America Bulletin, 84 , 137-180.

Fyson, W. K., 1980. Fold fabrics and emplacement of an.Archaengranitoid pluton, Cleft Lake, Northwest Territories. CanadianJournal of Earth Sciences, 17, 325-332.Hayward, N., 990. Determination of early fold axis orientationswithin multiply deformed rocks using porphyroblasts.Tectonophysics, 179, 353-369.Hayward, N., 1991. Orogenic processes, deformation andmineralization history of portions of the Appalachian orogen,USA, based on microstructural analysis. Unpubl. PhD Thesis,James Cook University, 320 pp.Hayward, N ., 1992. Microstructural analysis of the classicalsnowball garnets of south-east Vermont: evidence fornon-rotation. Journal of Metamorphic Geology, 10, in press.Johnson, S. E., 1990a. Lack of porphyroblast rotation in theOtago schists, New Zealand: implications for crenulationdevelopment, folding and deformation partitioning. Journal ofMetamorphic Geology, 8, 13-30.Johnson, S. E., 1990b. Deformation history of the Otago schists,New Zealand, from progressive developed porphyroblast/matrix microstructures: cyclic uplift-collapse oroge nesis and itsimplications. Journal of Structural Ge olog y, 12, 727-746.Johnson, S. E., 1992. Sequential porphy roblast growth duringprogressive deformation and low-P, high-T (LPHT) meta-morphism, Cooma Complex, Australia: the use of microstr-uctural analysis in better understanding deformation andmetamorphic histories. Tectonophysics, in press.Kuhn, T. S., 1970. The Structure of Scientgc Revolutions.University of Chicago Press, 210 pp.Passchier, C. W., Trouw , R. A . J. , Zw art, H. J. & Vissers, R. L.

M., 1992. Porphyroblast rotation: eppur si muoue? Journal ofMetamorphic Geo logy, 10, 283-294.Platt, J. P. & Behrmann, J . H . , 1986. Structures and fabrics in acrustal-scale shear zone. Journal of Structurat Geology, 8,Platt , J. P. , Behrmann , J. H. , Cunn ingham, P. C. , Dewey, J . F. ,Helman, M., Parish, M., Shepley, M. G., Wallis, S. & Weston,P. J. , 1989. Kinematics of the Alpine a rc and the motion history

of Adria. Nature, 337, 158-161.Powell, C. McA. & Vernon, R . H. , 1979. Growth and rotationhistory of garnet porphyroblasts with inclusion spirals.Tectonophysics, 54, 25-43.Ramsay, J. G., 1962. The geometry and mechanics of formationof similar type folds. Journal of Geology , 70,309-327.Ramsay, J. G. , 1967. Folding and Fracturing of Rocks. McGrawHill, New Y ork, 568 pp.Rosenfeld, J . L., 1968. Garnet rotations due to the majorPalaeozoic deformations in Southeast Vermont. In : Studies ofAppalachian Geology (ed. Zen, E. et al . ) , pp. 185-202. WileyInterscience, New Y ork.Rosenfeld, J. L., 1970. Rotate d garn ets in metamorphic rocks.Geological Society of America Special Paper, 129, 102 pp.Schoneveld, C., 1979. Th e geometry an d significance of inclusionpatterns in syntectonic porphyroblasts. Publ. PhD Thesis,University of Leiden, 125 pp.Steinhardt, C. K., 1989. Lack of porphyroblast rotation innon-coaxially deformed schists from Petrel Cove, SouthAustralia, and its implications. Tectonophysics, 158, 127- 140.Wilkins, C. , 1991. Timing of mineralization at Big Bell Gold Mineand the structural history of the Cue district, MurchisonProvince, Yilgarn Craton, Western Australia. Unpubl. PhDThesis, James Cook University, 330 pp.Zwart, H. J. & Calon, T ., 1977. Chloritoid crystals from Curaglia;growth during flattening or pushing aside. Tectonophysics, 39,

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477-486.Received 9 September 1991; revision accepted 12 October 1991.

porphyroblast inclusion-trail orientation data eppure

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