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The Structural Development of the Kanmantoo Schist Based on FIAs from Samples at Petrel Cove and the Strathalbyn Anticline

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The Structural Development of the Kanmantoo Schist Based on FIAs from Samples at Petrel Cove and the Strathalbyn Anticline
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Bianchi, Laura ( author )
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Foliation intersection/inflection axes
Petrel Cove
Quest 2008
SUNY Oswego
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Inclusions within porphyroblasts from Petrel Cove and the Strathalbyn Anticline provide evidence for metamorphic events not previously recognized in the Delamerian orogen because they contain more than five foliations defining at least two foliation intersection/inflection axes (FIAs). At Petrel Cove one FIA is preserved within cordierite. An identical FIA is preserved within staurolite in the Strathalbyn Anticline but another younger FIA is present in andalusite. The latter sequence appears to resolve the problem of apparently synchronous multiple phases of growth of staurolite and andalusite in these rocks (e.g., Adshead-Bell & Bell, 1999). Reactivation has destroyed these foliations or rotated them into parallelism with the bedding, which is why they were not distinguished until measurements of FIAs were made. The FIA succession distinguishes a progression of metamorphic events and further work of this type in the region will provide enough data for the shear senses suggested by this preliminary study to be confirmed.
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Helen Daly Bohmer Quest Best Paper
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Submitted by Shannon Pritting (pritting@oswego.edu) on 2008-10-24.
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THE STRUCTURAL DEVELOPMENT OF THE KANMANTOO SCHIST BASED ON FIAS FROM SAMPLES AT PETREL COVE AND THE STRATHALBYN ANTICLINE Laura Bianchi (Helen Daly Bohmer Quest best paper) Department of Earth Sciences Inclusions within porphyroblasts from Petrel Cove and the Strathalbyn Anticline provide evidence for metamorphic events not previously recognized in the Delamerian orogen b ecause they contain more than five foliations defining at least two foliation intersection/inflection axes (FIAs). At Petrel Cove one FIA is preserved within cordierite. An identical FIA is preserved within staurolite in the Strathalbyn Anticline but another younger FIA is present in andalusite. The latter sequence appears to resolve the problem of a pparently synchronous multiple phases of growth of staurolite and andalus ite in these rocks (e.g., Adshead-Bell & Bell, 1999). Reactivation has destroyed these foliations or rotated them into parallelism with the bedding which is why they were not distinguished until measurements of FIAs were made. The FIA succession distinguishes a progression of metamorphic events and further work of this type in the region will provide enough data for the shear senses suggested by this preliminary study to be confirmed. 1. Introduction The start of orogenesis on the eastern margin of Gondwana in Australia occurred in the Precambrian (e.g., Preiss, 1987; Foden et al, 2002; Foden et al, 2006) with initiation of subduction and commencement of development of the Delamerian orogen (Talbot & Hobbs, 1968; Steinhardt, 1989; Sandiford & Alias, 2002). Although subduction is well documented, the processes associated with its initiation are poorly understood. This lack of understanding results from the probl em of accessing information on what took place as deformation commenced. Multiples phases of deformation result in destruction of earlier phases through reactivation of compositional layering which both decrenulates and/or rotates developing a nd previously developed foliations into parallelism with the bedding (Bell, 1986; Ham & Bell, 2004). Fortunately, porphyroblasts and their inclusion trails pr eserve evidence for the interaction between heating and deformation during orogenesis for all deformation events after the first, even where this evidence h as been entirely destroyed in the matrix because of reactivation of the bedding (Bell et al., 2003; 2004; 2005). Consequently, the measurement of FIAs provides access to bot h structural and metamorphic information that occurred at the commencement of s ubduction and which has been completely obliterated in the matrix. This paper uses this approach to access information on the early stages of orogenesis associated with th e initiation of subduction on the Eastern margin of Gondwana.

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L. Bianchi 62 I. FIA Measurement: A foliation intersection or inflection axis in porphyroblasts (FIA) is measured for a sample by observing the orientation of the switch in inclusion trail asymmetry within porphyroblasts (clockwise or anticlockwise) fro m a series of vertically oriented thin sections observed consistently in the one direction around the compass (Bell et al., 1998). For example, Fig. 1A contains a simple spiral with a clockwise curvature from core to rim in the 040 section but an anticlockwise curvature in the 360 section. This switch in asymmetry takes place across the FIA trend. The principle is expanded in Figs. 1B and 1C. Fig. 1: A FIA trend is measured for a sample using a ll the porphyroblasts intersected by each vertical thin section, the principle of measurement is shown using a 3-D sketch drawn of a simple spiral. B: Sketch illustrating the principal with a fold preserved in an outcrop. The geologists to either side have no idea of the plunge direction. However the geologist in the ce ntre does. C: Precise measurement of FIA made by cutting sections 10 apart and constraining the as ymmetry switch within 10 (Modified from Bell & Newman, 2006).

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The Structural Development of the Kanmantoo Schist 63 II. Regional Geology II. a. Petrel cove: Petrel Cove is located 100 kms south of Adelaide (Fig. 2). The Rosetta Head area (Petrel Cove) in southern Australia form a part of the Kanmantoo Group of supposed Cambrian-age. There is a variety of metamorphic grades represented, from lower greenschist to amphibolite facies. The rocks of the region consist of schists with interbedded meta-sandstone which also c ontain well-bedded porphyroblastic biotite, andalusite, cordierite and chlor ite (Steinhardt, 1989). The schists at Petrel Cove are not quite true pelites because they lack the nor mal percentage of potassium (Sandiford & Alias, 2002). Fig. 2: Map showing location of Petrel Cove, approxima tely 100 km south-southwest of Adelaide (Modified from Adshead-Bell and Bell, 1999). There has been much debate over the number of phases of deformation at Petrel Cove (e.g., Talbot & Hobbs, 1968; Steinhardt, 1989; Sandiford and Alias, 2002). The most recent workers (Sandiford & Alias, 2002) have suggested that there were three phases of deformation in these rocks. The significance of D1 is uncertain because it is only preserved as a foliation parallel to com positional layering. Dolerite dikes and pegmatites were intruded prior to the development of the S2 fabric. During the third deformation, megacrystic feldspar and biotite growth occurred and granite was emplaced. The highest temperatures achieved within the Petrel Cove rocks are between 550600C. The following equations are chemical changes and mineralogical assemblages

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L. Bianchi 64 that have been previously described as occurring throughout the Petrel Cove sequence (Sandiford & Alias, 2002), (1) Chl + Msc Cord + Bt (2) Cord + Msc And + Bt (3) Cord And + Chl (4) And + Cord St (5) Msc + Chl St + Bt + Qtz + H20 but only the first of these was f ound in the rocks described herein. II.b. Strathalbyn anticline: The Strathalbyn Anticline is located approximately 45 km southeast of Adelaide (Fig. 3). These rocks were always somewhat enig matic because their axial plane lay parallel to all the other regional folds of the Adelai de Geosyncline the north and west which had S1 axial plane, yet these folds appear to have at least S3 as their axial plane structure (Fleming & Offler, 1968; Offler & Fleming, 1968). This problem was addressed by Adshead-Bell & Bell (1999) and they showed that the regional folds all formed at the same time, and that the Strathalbyn Anticlin e was reused and modified several times during younger overprinting deformations. Fig. 3: Southern Australia, the Kanmantoo Group and the St rathalbyn Anticline. The Strathalbyn Anticline is approximately 40km east-southeast of Christies B each. Two samples, K21 and K24, were examined for the purpose of this paper. The map above shows that K24 comes from just to the west of the anticlinal hinge whereas K21 comes from the east side but further away from the hinge. (Modified from Jenkins and Sandiford, 1992).

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The Structural Development of the Kanmantoo Schist 65 Around the Strathalbyn Anticline, the mineralogical compositions consist of namely staurolite and andalusite porphyroblasts in a matrix of quartz, biotite, and on occasion muscovite, ilmenite, sillimanite, and pl agioclase. As one moves from west to east, the mineralogy changes from biotite, andalusite, and staurolite to sillimanite (fibrous and prismatic) and migmatites. Th e following reactions have been inferred based on previous works (e.g., Jenkins & Sandiford, 1992; Adshead-Bell & Bell, 1999). (1) Msc + Str + Qtz Al2SiO5 + Bt + H2O (2) Msc + Chl Al2SiO5 + Bt + Qtz + H2O (3) Msc + Chl Str + Bt + Qtz + H2O (4) Grt + Chl Str + Bt + Qtz + H2O (5) Msc + Chl Grt + Bt + H2O III. Sample Setting III.a. Petrel cove: Two samples, named PC2 and PC3, were collect ed from locations 2 and 3, respectively, at Petrel Cove (Figs. 4 & 5). Bedding, S0, usually has a schistosity S1 lying parallel to it. There appears to be at least two generations of cordierite porphyroblast growth within these schists but they contain the same FIA. A distinctive layering of metamorphic/deformational origin oblique to bedding occurs in phyllites, and cordierite/andalusite mica schists (e.g., Ta lbot & Hobbs, 1968). This layering is known as the stripy layering because of its distinctive appearance. The layers alternate from light to dark-gray bands, and the lighter bands are generally much thinner on the scale of mm-cm in size. It lies parallel to S2 which dips shallowly SE. Bedding can be very difficult to observe within an outcrop because of the dominance of the stripy layering and many have previously mistaken this structure for bedding (T. Bell pers. comm., 2007). Bedding, S0, is shown up close in Fig. 5c. The stripy layering in samples PC2 and PC3 consists dominantly of cordie rite porphyroblasts and quartz. These porphyroblasts contain the same FIA as that preserved in porphyroblasts in the adjacent matrix. However, the inclusion trails in porphyroblasts within the stripy layering are commonly truncated by the dominant matrix foliation S2 suggesting that they both predate this foliation and that it may have b een rotated into parallelism with them by reactivation (Fig. 6, Bell et al., 2004). Fig. 7 shows detail of some folds at Petrel Cove. III.b. Strathalbyn anticline: Two samples, K21 and K24, were examined for the purpose of this paper. Fig. 8 shows that K24 comes from just to the west of the anticlinal hinge whereas K21 comes from the east side but further away from the hi nge. Sample K24 contains smoothly-curving sigmoidal-shaped inclusion trials in andalu site and staurolite porphyroblasts which are continuous into the matrix. The matrix, defi ned by slightly elongate quartz grains and aligned biotite grains, shows a flat foliation.

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L. Bianchi 66 Fig. 4: Petrel Cove, sample setting looking southwest.

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The Structural Development of the Kanmantoo Schist 67 a) b) c) Fig. 5: a) Petrel Cove location 2, facing south. S0 dips west across the outcrop and is locally outlined in blue. S2 is parallel to the stripy layering (red), and S3 has formed sub-vertically (yellow); b) Close up of location 2; c) Petrel Cove station PC 3, photo looking south-southwest. S0 is outlined in green. S2 lies perpendicular to bedding and in this outcrop is parallel to the stripy layering. S3 has formed sub-parallel to bedding.

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L. Bianchi 68 Fig. 6: Model of crenulation cleavage (Sn+1) development during folding by buc kling. In this example finescale crenulations of Sn (sub-parallel to S0) form by buckling due to bulk shortening (a, b, c), cleavage development (Sn+1) occurs through pressure solution of long limbs (f, h), and th e geometry of the cleavage orientation is a function of the compet ition between buckling rota tion of the fold limbs S0 (c, e) versus shear on the folded foliation by flexural flow (f, h). Any shear on the cleavage Sn+1 (f, h) only occurs late in the development of the fold. (Modified from Ham & Bell, 2004).

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The Structural Development of the Kanmantoo Schist 69 Fig. 7: Australian $2 coin for scale. Foliations seen within a Petrel Cove outcrop looking SW Folded stripy layering, which lies parallel to S2 (red) and andalusite layering, lie s sub-parallel to sub-vertical S3 (yellow). The shear sense on S3 is right side up (NW side up).

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L. Bianchi 70 Fig. 8: Location of the Kanmantoo Group and Strathal byn Anticline (Modified from Adshead-Bell and Bell, 1999). IV. Structural Setting IV.a. Petrel cove: Petrel Cove is a region of fine-grained meta-psammites and meta-siltstone that has experienced lower greenschist to almandin e amphibolite facies deformation during the Cambrian (Sandiford & Alias, 2002; Talbot & Hobbs, 1968). The S1 schistosity lies parallel to S0. Cordierite porphyroblast growth is mainly controlled by different composition beds (Figs. 5b, 5c, & 7), but thin section work has revealed that cordierite

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The Structural Development of the Kanmantoo Schist 71 growth defines the stripy layers in many locations and that S2 lies parallel to them as well. Biotite grains are oriented randomly inside the matrix. However, where they are more aligned, cordierite porphyroblasts can be easily found. The folded stripy layers (Fig. 7) were suggested by Talbot & Hobbs (1968) to contain andalusite/cordierite assemblages associated with intrus ion of the Rosetta Head Granite At the PC2 sample site, there is an increase in the intensity of S2 (Fig. 9). The number of porphyroblasts also increases within the S2 fabric, to create the white striped layering. At the PC3 sample site, the S2 fabric is perpendicular to bedding while S3 remains parallel to bedding (Figs. 5 & 7). Fig. 9: Closer location of Petrel Cove, showing second deformation event at Petrel Cove (Modified from Steinhardt, 1989).

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L. Bianchi 72 IV.b. Strathalbyn anticline: The Precambrian rocks of the Adelaide Fo ld Belt lie below the Cambrian Kanmantoo Group. The Strathalbyn Anticline can be descr ibed as a dextral and asymmetrical macroscopic fold which lies within the Kanm antoo group and the Adelaide Fold Belt (Adshead-Bell & Bell, 1999; Bell, 1994). Th is fold, formed by a thrusting tectonic setting, left the stratigraphy near the surface re latively stable and untouched. The fold is presumed by some to have formed at the same time as the spiral inclusion trails, assuming that the porphyroblasts didnt rotate (Adshead-Bell & Bell, 1999; Bell, 1994). The structure of the anticline has a near-vertical axial plane and a shallowly-plunging hinge (Adshead-Bell & Bell, 1999). The firs t deformation event left folds preserved which show axial-plane slaty cleavage w ithin the Adelaide Fold Belt; the young crenulation cleavage lies parallel to the ax ial-plane (Adshead-Bell & Bell, 1999). At least five foliations can be seen which alternate from steep to shallow orientations, thus possibly suggesting that these folds formed as late structures because a differentiation crenulation cleavage and bedding that is paralle l to schistosity exist. However, the folds formed in the early part of orogenesis (Adshead-Bell & Bell, 1999). V. Microstructures/FIAs V.a. Petrel cove: Cordierite porphyroblasts generally have symmetrical strain shadows (Steinhardt, 1989). According to Talbot & Hobbs (1968), the massive Rosetta Head porphyritic granite intruded well-bedded schists, whic h contain well preserved bedding, ripple marks, and slump structures. The rocks are broadly folded and contain a well-developed schistosity overprinted by more than one crenulation cleavage with associated asymmetric micro-folds. Sample locations PC2 and PC3 contain only cordierite porphyroblasts (Figs. 10, 11 & 12). The cordierite porphyroblasts are rela tively fresh in PC2. The FIA in PC2 is located at 30. The stripy layering contains cordierite porphyroblasts and quartz and lies sub-parallel to S2. The compositional heterogeneity provided by the stripy layering causes it locally to behave like bedding and r eactivate when suitably oriented relative to the deforming forces and shear sense (e.g Ham & Bell, 2004). The stripy layering/S2 probably formed sub-horizontal and has been rotated NW side up by the S3 shear which is NW side up in both PC2 and PC3. PC3 contains cordierite grains that are altered into muscovite and chlorite (Fig. 12). The FIA lies between 30 and 60. Some cordierite porphyroblasts show NW side up shear indicating a similar relationship to th at of location PC2. Most grew in a steep to flat event with shear sense top to SE. V.b. Strathalbyn anticline: Although many of the matrix foliations that have developed have been subsequently destroyed by reactivation, porphyroblasts and minerals in their strain shadows are preserved. Most of inclusion trails can be traced into the matrix, especially in andalusite, but also can be seen as continuous with staurolite (Fig. 13). Those that are truncated in staurolite commonly show younger andalusite or biotite with muscovite

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The Structural Development of the Kanmantoo Schist 73 Fig. 10: Sample taken from PC2 at 30. Sample shows a ge neral clockwise trend in the inclusion trails which are continuous into the matrix of quartz, biotite, and muscovite

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L. Bianchi 74 Fig. 11: Sample PC3 trending at 30. Cordierite has been overgrown by muscovite, and the inclusion trails are continuous into the matrix. Quartz and biotite grains are aligned preferentially.

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The Structural Development of the Kanmantoo Schist 75 Fig. 12: This figure from sample PC3 with a trend of 60 s hows that the cordierite grain in the center has been overgrown by muscovite, chlorite, and biotite. Quartz grains are preferentially aligned. Muscovite grains are randomly aligned within the cordierite sugge sting that these grains are relatively older than the matrix.

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L. Bianchi 76 Fig 13: Sample K24 from the Strathalbyn Anticline s howing staurolite grains switching from an anticlockwise pattern to a clockwise direction from th e 20 to the 30 section. The FIA lies between these two sections. The inclusion trails in the LH figure coul d be continuous with the matr ix foliation as inferred by the dashed lines. The inclusion trails in the RH figure are definitely truncated by the matrix foliation. that has overgrown the truncational foliati on and, of course, where the trails are continuous with those in the matrix. The andalusite grains are highly poikiloblastic with over 50% of their mass made of inclusions Their trails consist of biotite, quartz, and a little illite and muscovite. The trails are mostly straight across the porphyroblast with slight curvature near the rims, but some are sigmoidal (Figs. 14 & 15). Sample K21 contains two different FIAs (Table 1). The FIA is located at 30 + 10 for staurolite and 55 + 5 for andalusite. Within the 0 and 30 thin section samples, the inclusion trails within st aurolite are generally straight and gently pitching with a slight curve on their edges. Inclusion trails in staurolite in sections to either side

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The Structural Development of the Kanmantoo Schist 77 Table 1: The data collected shows the changes between ACW and CW FIA patterns as well as similarities between the Petrel Cove samples and Stratha lbyn Anticline samples. are steeply pitching and tend to be sigmoidal in shape. Inclusion trails in andalusite tend to be sigmoidal and steeply pitching close to the FIA and straight and gently pitching further away. Sample K24 also contains differently trending FIAs within staurolite versus andalusite. The staurolite FIA trends around 25 whereas the andalusite FIA trends at 45. Both andalusite and staurolite grains contain steeply pitching sigmoidal trails. Inclusion trials in most staurolite grains are truncated while those in andalusite are

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L. Bianchi 78 continuous with the matrix. The inclusion trails show NW side up for flat to steep changes in pitch and top to the SE for steep to flat changes in pitch identical to those observed and remarked on by Adshead-Bell & Bell (1999). Fig. 14: ACW inclusion trails in Andalusite in the 20 section from Strathalbyn for sample K21, switch to CW in the 60 section across the FIA. The FIA trend is controlled by the sub-vertical foliation-forming event (Bell & Bruce, 2006). The strike of this sub-ve rtical foliation therefore lies on the FIA. Steeply pitching inclusion trails swing through the horizontal on the FIA as one crosses the strike of the vertical foliation creating them in 3D. Therefore the FIA lies clos e to 60. The inclusion trails are truncated by the matrix foliation.

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The Structural Development of the Kanmantoo Schist 79 Fig. 15: Shows the switch in asymmetry in staurolite porphyroblasts from ACW in the 20 section to CW in the 40 section across the FIA trend (30) in sample K21 from the Strathalbyn Anticline. The FIA trend is controlled by the sub-vertical foliation-forming event (Bell & Bruc e, 2006). The strike of this subvertical foliation therefore lies on the FIA. Therefor e, steeply pitching inclusion trails swing through the horizontal on the FIA as one crosses the strike of the ve rtical foliation crating them in 3D. This suggests the FIA trend is close to 20 than 40. The inclusion trails in the RH staurolite are truncated. Inclusion trails commonly appear continuos in thin s ections that lie sub-perpendicular to the matrix foliation because they exit he porphyroblast into strain shadow s relative to that foliation (Cihan, 2004).

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L. Bianchi 80 VI. Interpretation VI.a. Petrel cove: The stripy layering that dominates these ro cks in outcrop, and which is commonly quite oblique to bedding, controlled where cordierite porphyroblasts grew in samples PC2 and PC3. Therefore, it is likely that in these rocks this layering formed as a result of fluids emanating from a granite that altered the margins of the fractures along which they escaped (c.f., Alias & Sandiford, 200 2; Talbot & Hobbs 1968). The steeply pitching inclusion trails preserved in these cordierite porphyroblasts curve anticlockwise looking SW at the rims suggesting that the cordierites grew during top to the SE shearing. S2 my have formed sub-horizontally at this time as a sub-horizontally pitching foliation in porphyroblast strain shadows merges with S2 in the matrix. The sub-parallel stripy layering to S2 indicates that the stripy layering developed relatively early. S2 therefore may have been associated with W to E thrusting (Jenkins & Sandiford, 1992). D3 resulted in NW side up shear and the local development of a differentiated S3 in both samples. VI.b. Strathalbyn anticline: The inclusion trails preserved here show NW side up shear on the steep foliations and top the SW shear on sub-horizontal foliations suggesting uplift to the NW and thrusting to the SE. VI.c. Petrel cove versus strathalbyn anticline: FIA 1 in the Strathalbyn Anticline is similarly oriented to with FIA 1 at Petrel Cove suggesting that they formed at the same tim e. FIA 2 in the Strathalbyn Anticline formed subsequently but no porphyroblast growth at this time was seen in the sample from Petrel Cove. Furthermore, the steep to flat changes in inclusion trail geometries seen in FIA 1 at both Petrel Cove and the Strathalbyn Anticline are identical and strongly suggest top to the SW thrusting during orogenesis. Even though the FIA trends changed by some 30 during the new development of FIA 2 in the Strathalbyn Anticline, the shear sense did not change suggesting that thru sting continued to the SE. This is supported by the fact that flat to steep changes in inclusion tail geometry during the development of FIA 1 in both regions were NW side up indicating uplift to the NW and thrust the potential fold gravitational collapsed and thrusting to the SW at this time (Bell & Johnson, 1989; Bell & Newman, 2006). VII. Discussion VII.a. Porphyroblast rotation? A debate over whether porphyroblasts rotate or not has been taking place ever since Bell (1985) proposed that in general they do not. All quantitative data that has been presented on FIA trends indicates that porphyroblasts do not rotate (e.g., Bell & Newman, 2006). The argument as to whether they do rotate has been entirely theoretical and model driven (Fay et al., 2008 in review). Indeed, modellers argued that porphyroblast non-rotation was impossible in a continuous medium. Fay et al (2008)

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The Structural Development of the Kanmantoo Schist 81 have recently demonstrated that this is not the case. Indeed, they have discovered the phenomenon of gyrostasis whereby once an anastomosing or millipede geometry (a general strain developed within heter ogeneous rocks) has been established by porphyroblast growth early during essentially coaxial deformation, all rotation ceases even in progressive bulk inhomogeneous simple shear (Fig. 16)! Although pressure shadows have been described in the past as being the results from rotation, they can also form as a result of gyrostasis, a pr ocess modelled in the form of a mesh-type structure which demonstrates its irregular sh ape as different magnitudes of strain are forced upon it (Fay et al, 2008; Cihan, 2004) Steinhardt (1989) argued that no porphyroblast rotation had occurred at Petrel Cove because the inclusion trails in all porphyroblasts that he measured were essen tially sub-horizontal. Jiang & Williams (2004) inferred that non-rotation of porphy roblasts during non-coaxial deformation is mechanically impossible. However, the results presented herein disagree with his data because they suggest that many porphyroblasts overgrew a steep foliation rather than a horizontal one. One way in which his data could be reconciled with this data is if he cut a preponderance of thin sections striking within 30 of the FIA which trends at 30 in one sample and between 30 and 60 in the other. Fig 16: Figures showing the effects of shearing on porphyroblasts. The millipede geometry demonstrates that there are various levels of stra in being exerted on the mesh squares, yet none of the above illustrations demonstrate rotation. Bulk shortening is the result of strain on porphyroblasts (Modified from Fay et al, 2008 in review).

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L. Bianchi 82 The similar FIA trends determined at Petr el Cove versus FIA 1 in the Strathalbyn Anticline, which lie approximately 50 kms apar t (Fig. 2), suggest that there has been no porphyroblast rotation (Bell, 1981; Fay et al, 2008) particularly knowing that at least 2 locally penetrative deformations have aff ected the rocks in the Strathalbyn Anticline post the development FIA 1. VII.b. Stripy layering: The origin of the white strip es within the rock formations at Petrel Cove has been controversial. Talbot & Hobbs (1968) suggested that they formed in a non-dilatational event and represent an in situ differentiation process with some degree of chemical exchange between the host rock and a solu tion that may have emanated from the adjacent granite. Sandiford & Alias (2002) have suggested that this layering occurs at a shallow angle relative to schistosity and th at the porphyroblasts form augen that are elongate parallel to this foliation. The resu lts presented here provide a solution to both these observations. The stripy layers presumab ly formed due to fluid emanating from a granite which caused alteration to either si de of the fractures in the country rock through which they passed. During subsequent deformation these altered zones were of a bulk composition such that cordierite prefer entially grew within them at the prevailing PT. The deformation that accompanied cordier ite growth would have caused rotation of the stripy layers towards the developing fo liation. Subsequent de formations tended to cause the stripy layering to behave like bedding and thus reactivate (e.g., Fig. 6) and this further rotated any previously form ed foliations and decrenulated any newly developing foliation leaving S2 sub parallel to them (e.g., Ham & Bell, 2004). VII.c. Thrusting : Previous workers (e.g., Jenkins & Sandiford, 1992) have argued that thrusting occurred from the west to east on the western side of the Mount Lofty Ranges (Fig. 2) during the Delamerian orogen. If this was the Precamb rian margin of the southern Australian craton, subduction would have occurred of an oceanic plate to the east under these rocks to the west, and this accords with the direction of thrusting observed by Jenkins & Sandiford (1992). The inclusion trail asymmetry data from the FIAs accords with this both at Petrel Cove and in the Strathalbyn Anticline. VIII. References Alias, G., Sandiford, M. (2002). The P-T record of synchronous magmatism, metamorphism and deformation at Petrel Cove, southern Adelaide fold belt. Journal of Metamorphic Geology 20, 351-363. Ashead-Bell, N. S., and Bell, T. H. (1999). Progressive Development of a macroscopic. Tectonophysics v 306, 121-147. Bell, T. H. (1981). Foliation development: the controls, geometry and significance of prograde bulk inhomogeneous shortening. Tectonophysics 75, 273-296. Bell, T. H. (1985). Deformation partitioning and porphyroblast rotation in metamorphic rocks: A radical reinterpretation. Journal of Metamorphic Geology 3, 109-118. Bell, T. H. (1986). Foliation development and refraction in metamorphic rocks; reactivation of earlier foliations and d ecrenulation due to shifting patterns of deformation partitioning. Journal of Metamorphic Geology 4, 421-444.

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The Structural Development of the Kanmantoo Schist 83 Bell, N. S (1994). The Structural and metamorphic reinterpretation of the Kanmantoo Foldbelt, South Australia. Honours Thesis from James Cook University. Bell, T. H., Ham, A. P., and Hickey, K. A. (2003). Early formed regional antiforms and synforms that fold younger matrix schistosities; their effect on sites of mineral growth. Tectonophysics 367, 253-278. Bell, T. H., Ham, A. P., and Kim, H. S. (2004). Partitioning of deformation along an orogen and its effects on porphyroblast growth during orogenesis, Journal of Structural Geology 26, 825. Bell, T. H., Ham, A. P., Hayward, N., and Hickey, K. A. (2005). On the development of gneiss domes. Australian Journal of Earth Sciences 52, 183-204. Bell, T. H., and Johnson, S. E. (1989). Po rphyroblast inclusion trails; the key to orogenesis. Journal of Metamorphic Geology 7, 279-310. Bell, T. H., and Newman, R. (2006). Appal achian orogenesis: the role of repeated gravitational collapse In:-Styles of Continental Compression, Eds R. Butler and S. Mazzoli, Special Papers f the Geological Society of America 414, 95-118. Bell, T. H., Hickey, K. A.., and Upton, G. J. G. (1998). Distinguishing and correlating multiple phases of metamorphism across a multiply deformed region using the axes of spiral, staircase and sigmoidal inclusion trails in garnet. Journal of Metamorphic Geology 16, 767-794. Cihan, M. (2004). The drawbacks of sectioni ng rocks relative to fabric orientations in the matrix: A case study from the Robertson River Metamorphics (Northern Queensland, Australia). Journal of Structural Geology 26, 2157-2174. Fay, C., Bell, T. H., and Hobbs, B. E. (2008) In Review. Porphyroblast rotation versus non-rotation: Conflict resolution! Fleming, P. D., Offler, R. 1968. Pre-tectonic crystallization in the Mt. Lofty Ranges, South Australia. Geol. Mag. 105, 35659. Foden, J., Elburn, M. A., Turner, S.P., Sandi ford, M., OCallaghan, J., and Mitchell, S. (2002). Journal of Geological Sciences London 159, 601-621. Foden, J., Elburg, M. A., Dougherty-Page, J., and Burtt, A. (2006). Journal of Geology 114, 189-210. Ham, A. P, and Bell, T. H. (2004). Recycling of foliations during folding. Journal of Structural Geology 26, 1989-2009. Jenkins, R. J. F. & Sandiford, M., (1992). Observations on the tectonic evolution of the southern Adelaide fold belt. Tectonophysics, 214, 27-36. Jiang, D., and Williams, P. F. (2004). Reference frame, angular momentum, and porphyroblast. Journal of Structural Geology 26, 2211-2224. Offler, R., Fleming, P. D., (1968). A synthesi s of folding and metamorphism in the Mt. Lofty Ranges, South Australia. J. Geol. Soc. Aust 15, 2456. Preiss, W. V. (1987). Geological Survey of Australia Bulletin 53, 438. Sandiford and Alias, (2002). P-T Record. Journal of Metamorphic Geology 20, 351363. Steinhardt, C. (1989). Lack of porphyroblast rotation in non-coaxially deformed schists from Petrel Cove, South Aust ralia, and its implications. Tectonophysics 158, 127140. Talbot, J. L, and Hobbs, B. E. (1968). The Relationship of Metamorphic Differentiation to other structural features at three localities. Journal of Geology 76, 581-587.