Tectonophysics 587 (2013) 119–132
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Inter-relationships between deformation partitioning, metamorphism and tectonism T.H. Bell ⁎, M.T. Rieuwers, M. Cihan, T.P. Evans, A.P. Ham, P.W. Welch School of Earth Sciences, James Cook University, Townsville, Qld 4811, Australia
a r t i c l e
i n f o
Article history: Received 31 January 2012 Received in revised form 1 June 2012 Accepted 8 June 2012 Available online 20 June 2012 Keywords: PT overstepping FIAs P–T–t paths Core isopleths Foliation development
a b s t r a c t Thrusting from the east loaded the thick Pomfret dome stratigraphic sequence in Vermont to such an extent that by the time the first schistosity had formed it was 20 km deep. This occurred without garnet growth even though rock compositions were ideal for this phase to grow before they reached this depth. The rocks remained at this depth until garnet growth ceased ~ 50 million years later after 5 periods of FIA development (foliation intersection/inflection axes preserved within porphyroblasts). The first phase of the garnet growth in each sample from the Pomfret dome was overstepped in pressure, nucleating well above the incoming phase boundary for this phase at ~7 kbar for whatever FIA set was the first to develop. This was not the case 45 km S in the Chester dome where a thin stratigraphic sequence overlay a basement high of gneiss. Lateral ramping against this basement thinned the thrust sheet preventing overstepping. Frontal ramping to the WNW had the same effect. The pressure did not increase in both regions to ~ 7 kbars until FIA 2. Approximately 50% of the rocks sampled around the Pomfret dome did not grow garnet during FIA 0. PT pseudosections and overstepped garnet phase boundaries indicate that all would have grown garnet if the bulk composition and PT were the only controlling factors. If metastability alone was a factor the other 50% should have grown garnet during the development of FIA 1. They did not, and this pattern was repeated for FIAs 2 and 3. Why, where and when garnet first grew in this PT overstepped environment was recorded by the inclusion trail geometries in each sample; all grew at the start of crenulation-producing events. The variable partitioning of a succession of differently oriented crenulation deformations through the region from FIA to FIA controlled where garnet growth first occurred. Successive FIAs shifted the bulk shortening direction relative to competent rocks, deforming sites previously protected and protecting others. The NW–SE trend of FIA 0 resulted in most deformation pervasively partitioning NE of the competent gneiss beneath the Chester dome and Green Mountains. Consequently, the bulk of porphyroblast growth within the Pomfret dome region occurred at this time. The effects of NW–SE bulk shortening partitioned through similar amounts of gneiss during FIA 1 generating the same percentage of new sites for garnet growth in both regions. As a result, ~ 60% of all garnet growth within the Pomfret region had occurred before the N–S directed bulk horizontal shortening during FIA 2 began, resulting in an increase in bulk competency. This caused deformation to preferentially partition pervasively through the previously more competent Chester region generating ~35% of all garnet cores plus loading through crustal thickening to a depth of ~ 7 kbar. This increase in pressure prior to FIAs 3 and 4 enabled other rocks with different bulk compositions to grow garnet for the first time during these later periods of orogeny. It also resulted in considerable growth on the rims of garnet porphyroblasts that had grown much earlier during the deformation history. © 2012 Elsevier B.V. All rights reserved.
1. Introduction 1.1. Shifting patterns of deformation partitioning It has become apparent that shifting patterns of distributed and pervasive deformation partitioning during orogenesis control intimate relationships between deformation and metamorphism (e.g., Sanislav and Bell, 2011). The role and significance of such partitioning for understanding many aspects of tectonic processes have surfaced through the quantitative study of porphyroblast microstructures across orogens ⁎ Corresponding author. E-mail address:
[email protected] (T.H. Bell). 0040-1951/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2012.06.014
(e.g., Abu Sharib and Bell, 2011) or in rocks that have made their way through orogens (e.g., Bell and Sapkota, in press). Such studies reveal that as deformation partitions through a rock mass, many generations of early-formed foliations are destroyed or reused (Davis, 1995; Davis and Forde, 1994) by later formed ones in the matrix. As each deformation occurs this is repeated over and over and so the extraordinary extent and multiple effects of continued tectonism are generally lost in the rock matrix (Ham and Bell, 2004). However, where porphyroblast growth has occurred, quantitative measurement of foliations trapped within them commonly reveals much of this history plus provides access to the processes that took place (Aerden and Sayab, 2008; Shah et al., 2011). The most studied large-scale region from this perspective is the Vermont Appalachians. Detailed structural (Hickey and Bell,
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2001), microstructural (Ham and Bell, 2004), foliation intersection/ inflection axes preserved within porphyroblasts (FIAs; Bell et al., 1998, 2003), FIA controlled in-situ age dating (Bell and Welch, 2002) and metamorphic (Ham, 2001; Gavin, 2004; Rieuwers, 2010; Welch, 2003) studies made of these rocks is allowing a fully integrated structural and metamorphic approach to understanding their development. Intriguingly, this work suggests that multiple periods of deformation and associated porphyroblast nucleation and growth were differentially partitioned between the Chester and Pomfret domes and that the character of this partitioning changed with time. Fortunately, the same 5-stage FIA succession documented around both domes allows successive periods of porphyroblast growth to be distinguished and correlated between them. In particular, the PT at which garnet nucleated during each FIA stage can be determined for each dome (c.f., Bell and Sapkota, in press; Sanislav and Bell, 2011). The succession of differences that this reveals between the domes suggests that variation in the partitioning of deformation between them with time has tectonic significance. Combined with changes in pressure it provides a solution as to why overstepping of the incoming of the garnet reaction dominates the Pomfret but not the Chester dome and the tectonic relationships that caused it. 1.2. Foliation development and FIAs The three decades of FIA based microstructural data on foliations and porphyroblast growth summarized below provide a background and introduction to the approach and interpretations used herein to resolve the above mentioned relationships. 1. Foliations preserved within porphyroblasts are predominantly sub-horizontal and sub-vertical wherever they have been quantitatively measured (Fig. 10 in Hayward, 1992; Fig. 8 in Aerden, 1995; Fig. 3 in Aerden, 1998; Fig. 10a in Sayab, 2005; Fig. 17 in Bell and Bruce, 2006 from Gavin, 2003; Figs. 10 and 12 in Bell and Newman, 2006; Fig. 7c in Sayab, 2008; Fig. 6c,e in Bell and Sanislav, 2011; Fig. 12e in Shah et al., 2011; Fig. 5b and c in Bell and Sapkota, in press). 2. A FIA trend is controlled by the strike of the steeply dipping foliation (Fig. 1; e.g., Figs. 2, 3, 8a and 17 in Bell and Bruce, 2006). 3. A minimum of 2 and generally 3 or more foliations define each FIA with up to 7 occurring around each FIA in the Chester dome region (Bell et al., 1998). 4. FIA plunges are dominantly gentle (Fig. 11a in Sayab, 2005; Fig. 18 in Bell and Bruce, 2006; Fig. 10 in Bell and Newman, 2006; Fig. 12f in Shah et al., 2011). Point 1 predicts this (Fig. 1a). 5. Vertical foliations in the matrix can be locally reused if not rotated by the effects by intervening orthogonal deformations (e.g., Fig. 14 in Bell and Hayward, 1991; Fig. 5 in Davis and Forde, 1994; Fig. 12 in Davis, 1995; Fig. 15 in Bell and Hickey, 1998). 6. Sub-horizontal foliations in the matrix are commonly locally reused if not rotated by the effects by intervening orthogonal deformations (e.g., Fig. 14 in Bell and Hayward, 1991; Fig. 9c in Bell and Bruce, 2007; Bell and Sanislav, 2011). 7. Porphyroblast growth in non-migmatitic, regionally metamorphosed pelites occurs during crenulation of a previously developed foliation that lies at a high angle to that newly developing (Fig. 14 in Bell and Hayward, 1991; numerous figures in any paper showing porphyroblasts from vertically oriented thin sections, e.g., Figs. 6 and 15 in Aerden, 1994; Fig. 6e and f in Bell and Sanislav, 2011; Fig. 3b and d in Sanislav and Bell, 2011; Fig. 6 in Skrzypek et al., 2011; Fig. 2). 8. No porphyroblast growth occurs during re-use of a pervasively developed foliation because crenulations do not develop (Fig. 14 in Bell and Hayward, 1991). 9. Partitioning of a deformation through coarsely crenulated foliated rocks, but at a finer scale than occurred previously, can cause
A
intersection of steep with sub-horizontal planes
B any movement direction is possible on the sub-horizontal foliation Sub-horizontal foliation Sub-vertical foliation
The FIA trend is controlled only by the intersection of the subvertical and sub-horizontal foliations
Fig. 1. A. Stereographic net showing how the trend of any intersection between steeply and differently-striking gently-dipping planes is controlled by the steeply dipping plane. B. Sketch shows how the FIA trend depends only on the strike of the subvertical foliation and is independent of the direction of movement on the gently dipping foliation.
porphyroblasts to newly nucleate and grow in the same or a later parallel deformation (e.g., Fig. 16 in Bell and Bruce, 2007). 10. One phase of porphyroblast growth over inclusion trails that curve only at the rim is the most common inclusion trail geometry. Sigmoids are the next and staircase and spiral shaped inclusion trails are the least common (Bell and Bruce, 2006; Fig. 14 in Bell and Hayward, 1991). 11. Any inclusion trail orientation across the core can be preserved by a first phase of porphyroblast growth. Consequently, if the crenulation event that promoted porphyroblast growth has a steeply dipping axial plane, the FIA can have any plunge but its trend is not affected; yet intermediate to steep plunges are rare (Fig. 18 in Bell and Bruce, 2006). If the crenulation event has a gently dipping axial plane the FIA can have a trend that does not lie near orthogonal to the direction of bulk shortening (Bell and Bruce, 2006) but such variation is rare and probably takes place around that orthogonal direction (Bell and Sanislav, 2011). 12. For two or more phases of porphyroblast growth, inclusion trails that form against the rims of earlier growth phases are more dominantly sub-vertical or sub-horizontal than their cores (compare Fig. 5b and c in Bell and Sapkota, in press) with the FIA plunges always gentle and trends always directly related to the direction of bulk shortening. 2. The region The Chester dome in SE Vermont contains Proterozoic basement gneisses of the Green Mountain massif unconformably overlain by the Late Proterozoic to Early Cambrian Hoosac Formation (Fig. 3). These rocks are overthrust by a eugeoclinal sequence of Cambrian to Middle Ordovician calcareous, pelitic and semi-pelitic metasedimentary and, mainly mafic, metavolcanic and intrusive rocks of the Rowe–
T.H. Bell et al. / Tectonophysics 587 (2013) 119–132
a IS28b m
IS28a
1.5 a
~428 Ma
b
121
Deformation and metamorphism in the region were considered to have occurred during the Taconic and Acadian Orogenies (Armstrong et al., 1992; Stanley and Ratcliffe, 1985) but porphyroblast growth ranges from b410 to 360 Ma (Bell and Welch, 2002, see below re pre 410 Ma ages; McWilliams et al., 2010). Thus, porphyroblast growth is interpreted to reflect stages in the Acadian orogeny. 3. Deformation history
b
FIA 1 ~420 Ma b Gt grew at IS28b but not at IS28a
a
c 415 to 370 Ma
Alternating
FIAs 2 to 4
d
3.1. Recorded by the matrix The structural history preserved in the matrix across the Chester and Pomfret dome region consists firstly of an early penetrative fabric that lies sub-parallel to bedding (S0). Hickey and Bell (2001) and Ham and Bell (2004) have suggested that this cleavage formed with an upright orientation and that a well developed crenulation cleavage, which developed with a shallow NW-dipping to sub-horizontal attitude, overprints this earlier fabric. This crenulation cleavage was in turn overprinted by a steeply dipping, NNE-striking, crenulation cleavage. These three foliations have been termed S3, S4 and S5, respectively, in accordance with Hayward (1992). Earlier formed foliations, which are preserved in porphyroblasts, have been rotated into parallelism with S0 in the matrix due to reactivation of the bedding (Bell et al., 2003; Ham and Bell, 2004). A late crenulation with a sub-horizontal axial plane, called S6, was developed locally. 3.2. Recorded by the FIAs
b
415 to 370 Ma
a
e
FIA 5 b a
355 Ma
St grew at IS28b Gt grew at IS28a
Fig. 2. Shows schematically how repartitioning of successive sub-vertical and subhorizontal deformations produced foliations and then crenulations through an outcrop (cross-section outlined in grey) that caused nucleation of garnet porphyroblasts in samples of the same bulk composition much later in one location than the other in NW Maine (data in Sanislav and Bell, 2011). a. Early formed sub-horizontal foliation. b. Shows (a) in dashes overprinted by crenulations with a sub-vertical axial plane (solid black) in FIA 1 during which garnet porphyroblasts grew at ‘b’ but not at ‘a’ because the deformation did not partition through ‘a’. c. Development of crenulations and sub-vertical foliation (solid black) misses locations ‘a’ and ‘b’. d. Development of crenulations and sub-horizontal foliation (solid black) misses location ‘a’ and ‘b’. This pattern can be repeated many times as FIAs 2 through 4 develop. e. Crenulations with sub-vertical axial planes (solid black) partition through ‘a’ and ‘b’ during FIA 5. Garnet grows for first time in ‘a’ and staurolite grows in ‘b’.
Moretown (or Rowe–Hawley) lithotectonic unit (Fig. 3). The youngest lithotectonic unit is the thick Silurian to Lower Devonian sequence of the Connecticut Valley Trough (McWilliams et al., 2010), separated from the Cambro-Ordovician rocks to the west by an angular unconformity (Ratcliffe, 1993, 1995a,b; Ratcliffe and Armstrong, 1995). The Pomfret dome, 45 km to the north (Fig. 3) preserves no evidence of a gneissic core. These rocks consist mainly of a much-thickened portion of the Silurian to Lower Devonian part of the stratigraphic succession.
Garnet across the region records a succession of five FIA sets numbered 0 through 4 via consistent changes in FIA trend from core to median to rims of the porphyroblasts (Figs. 14 and 15 in Bell et al., 1998; Fig. 13 in Ham and Bell, 2004). These FIA sets vary dramatically in distribution between the two domes as shown in Fig. 3b,c. So few samples contain FIA sets 0 and 1 in the Chester dome region that they cannot be easily distinguished on the total FIA plot shown in Fig. 3b. In order to allow comparison of these two FIAs between the domes they have been plotted separately on maps and rose diagrams in Fig. 4. The last four FIA sets (1 through 4) were dated in the Chester dome region by Bell and Welch (2002) as forming prior to 424 ± 3, 405 ± 6, 386 ± 6 and 366 ± 4 Ma; recent developments in this dating technique suggest that the reported errors should have been larger, on the order of 10–20 Ma. Certainly, with recent dating of sedimentary zircon from within the youngest rocks in the region at ~410 Ma (McWilliams et al., 2010), the oldest of these ages must be incorrect (see more on this older age below). The matrix foliations have generally only been observed as continuous with foliations preserved as inclusion trails within garnet porphyroblasts containing the most recently formed FIA set 4 that has a NNE–SSW trend. Inclusion trails in garnet porphyroblasts defining the other FIA sets are commonly truncated by the matrix foliations (Bell et al., 1998; Ham, and Bell, 2004). 4. Metamorphic history FIAs enable a different approach to determining metamorphic history and PT paths (e.g., Sanislav and Bell, 2011). Firstly, they quantitatively reveal a succession of periods of porphyroblast growth that can be confirmed by dating using monazite grains trapped as inclusion trails and correlated over large distances. Secondly, in ~ 66% of samples only one FIA is present. Thirdly, the FIA measurement technique requires at least eight vertical thin sections to be cut in different orientations around the compass from each sample. This enables a thorough search for the exact cores of zoned porphyroblasts such as garnet via both the maximum microstructural history preserved as inclusion trails and highest Mn content cores. Consequently, samples can be found in the Chester and Pomfret regions where garnet nucleated for the first time during one of any of the five FIAs that formed in
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72°30`
72°45`
a Pomfret
72°15`
Dome area
V396
N
N
4 1
0 43°45`
10 km
2
Co nn
ect
icu tR ive r
V387b
3
Vermont
0 n= 67
New York
1 New 43°30` Hampshire
Massachusetts
4
3
c
Total FIA
Chester 3 Dome area
N
4
1
0
Ascutney Intrusion 43°15` New Hampshire Sequence
2
Gile Mtn & Northfield Fms Standing Pond Amphibolite Waits River Fm Ordovician
0 n=146
1
Cambrian Proterozoic
4
3
b
Fig. 3. a. Geology map of the Chester and Pomfret dome region. Inset shows their location in NE USA. The locations of samples V387b and V396 are also shown. b and c. Rose diagrams of the distribution of total FIAs for Sets 0 to 4 around the Chester and Pomfret domes respectively. Sets 0 and 1 are difficult to see in the total plot of all data for the Chester dome because of the small numbers of samples containing them. Consequently, they are compared individually with the same FIA from the Pomfret dome in Fig. 4.
the region (e.g., Bell et al., 1998). The composition of these garnet cores depends on the bulk composition of the rock sample and the PT at the time of nucleation. Therefore, by measuring the Ca, Fe and Mn contents of the garnet core (Table 1), and the bulk composition of the rock sample (Tables 2 and 3), the PT at which garnet nucleated can be calculated at each stage in the FIA succession. This allows a PT path to be determined that is independent of assumptions about matrix/porphyroblast rim relationships. The PT of garnet nucleation occurs where Ca, Fe and Mn isopleths intersect tightly (Table 1) on a pseudosection (Figs. 5 and 6) determined for that bulk composition (Tables 2 and 3) using THERMOCALC (Powell et al., 1998; e.g., Sanislav and Bell, 2011). Most significantly, the composition of the core of the porphyroblast remains unaffected by subsequent orogenic history unless the temperature gets high enough for modification of the garnet by volume diffusion. In non-calcareous pelites, the effects of dissolution causing volume loss during cleavage development can be ignored, as the main component that would be lost, SiO2, has no effect on a PT pseudosection. Consequently, the PT of nucleation of a succession of garnet porphyroblast cores that developed at different stages in the FIA history and, therefore, at different stages along the PT and structural path, can be determined. Pseudosection and Ca, Fe and Mn isopleth intersection data for garnet cores relative to FIA sets is shown for the Chester dome in Fig. 5 and for the Pomfret dome in Fig. 6. The error bands shown for each isopleth are the calculated uncertainties generated by THERMOCALC with the error ellipses plotted from the overlapping rhomboidal area and modified to an ellipse as in Powell and Holland (1994), Evans (2004) and Sanislav and Bell (2011). Using eight thin sections per sample to find those garnet porphyroblasts showing the most microstructural
history (maximum curvature or complexity of inclusion trails) and the highest Mn content cores resulted in very tight clustering for the intersection of the isopleths. This allows considerable confidence in the results shown in Figs. 5 and 6 that is strongly supported by a very significant difference between the two domes; the PT of garnet core nucleation is overstepped relative to the incoming reaction on a pseudosection close to the Pomfret dome (Fig. 5) but not at the Chester dome (Fig. 6; see discussion and interpretation of this below). Furthermore, the samples analysed were collected from ~50 square km for each dome (Figs. 3a and 4a,b). Yet for the Pomfret dome the range of P obtained from 10 samples is ~2 kbar including the uncertainty! Fig. 7 combines Figs. 5 and 6 to show the distribution of PT of nucleation for all the samples that have been analysed in this way for each dome. These distributions indicate that the pressure progressively increased to just over 8 kbar before settling back around 7 kbar over a narrow temperature range of 510–560 °C for ~50 Ma around the Chester dome (Fig. 7a). In the Pomfret dome region, over a similar temperature range, the pressure jumped to around 8 kbar before settling back to around 7.2 kbar and remaining there throughout the PT history recorded by porphyroblasts (Fig. 7b). Apart from the progressive versus abrupt jump in pressure they appear similar. 5. The distribution of FIA sets No sample contains all five FIAs and only a third of them contain more than one FIA. Only two of FIA set 0 have been found in the Chester dome whereas the number of FIAs in this set is large around the Pomfret dome (Figs. 3, 4 and 8a). The total number of FIAs in each successive set increases around the Chester dome (except for FIA set 4) and decreases
T.H. Bell et al. / Tectonophysics 587 (2013) 119–132
123
n=2
V205
V261a
V634a V634a V437b V240
FIA 0
V257 V436b V259a
TN
TN MN
MN
a
b
n = 20 n = 21
FIA1
TN
TN
MN
MN
c
n = 19
d
Fig. 4. Maps showing the distribution of FIAs for Set 0 around the Chester (a) and Pomfret (b) domes. A rose diagram of the FIA distribution for each set is shown adjacent to each map. The location of all the samples analysed for PT is also shown (except for V387b and V396 shown in Fig. 3a); it lies at the centre of the sample numbers where no arrow is present or at the arrow tip on both maps. Maps of the distribution of FIAs for Set 1 around the Chester (c) and Pomfret (d) domes. A rose diagram of the FIA distribution for each set is shown adjacent to each map.
around the Pomfret dome as shown in Fig. 8a. The percentage of which FIA is preserved in the garnet core for each sample has a different distribution as shown in Fig. 8b. 6. Interpretation 6.1. FIA succession The same succession of FIA trends in garnet porphyroblasts around both domes (Figs. 3 and 4), which lie 45 km apart, indicates that they went through the same sequence of directions of bulk
orogenic shortening (e.g., Bell et al., 2004) and overall deformation history. This began with overthrusting around 410 Ma from the east by a 6 to 10 km thick thrust sheet immediately after the sediments were deposited (see below; see also McWilliams et al., 2010). FIAs 0 and 1 were the first to develop, followed by FIA 2 (dated at 405 Ma but more likely to be around 395 Ma, see below), FIA 3 (385 Ma) and FIA 4 (360 Ma; Bell and Welch, 2002; age determinations are discussed below). No sample contains all five FIAs and only a third of them contain more than one FIA. In many samples from both domes the growth of garnet occurred for the first time in only one of the five FIA sets in the succession. Thus, samples where
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Table 1 Garnet core analyses used for the isopleth determinations and the FIA set present. Suffix a, b or c indicates which of several samples taken from one outcrop was used in this and subsequent tables. Sample
FIA
XMn
XCa
XFe
AV17 AV36c AV19 AV29 AV26a AV33 V387b V396 AV27 AV138 V436b V634a V240 V257 V261a V205 V653 V259a V437b
0 0 1 1 2 2 3 3 4 4 0 1 2 2 2 3 3 4 4
0.1114 0.0873 0.1975 0.1755 0.1037 0.1824 0.1272 0.1366 0.1262 0.1320 0.1659 0.1421 0.1004 0.0688 0.1937 0.1974 0.0978 0.1656 0.1900
0.2576 0.2345 0.2054 0.2767 0.2578 0.2425 0.1718 0.1730 0.2408 0.2044 0.0933 0.0432 0.1219 0.1885 0.1829 0.2618 0.0602 0.2000 0.2600
0.5790 0.5964 0.5577 0.5106 0.5871 0.5308 0.6469 0.6483 0.5782 0.6044 0.7068 0.7690 0.7002 0.7010 0.5664 0.5312 0.7579 0.5743 0.5100
the garnet cores first grew in only one of FIA 0, FIA 1, FIA 2, FIA 3 or FIA 4 are present in each region. Such samples, plus the eight or more thin sections cut from each, enable a core Ca, Fe and Mn isopleth intersection determination of the PT at the commencement of garnet growth for each FIA. The consistent FIA succession enables correlation of periods of growth and thus a direct comparison of a structurally controlled PT path for each region. The compositions of garnet rims against the matrix give anomalously high pressures between 9.5 and 13.5 kbar (Welch, 2003) compared with the PT of successive cores. The average PT mode of Thermocalc used to determine these assumes that the boundaries of the porphyroblasts and adjacent matrix phases are in equilibrium. The inclusion trails in the rims of porphyroblasts containing FIA sets 0 through 3 are truncated by the matrix foliation. Combined with post FIA 4 reactivation of the matrix foliation during the last ductile deformations to affect these rocks (Bell et al., 1998; Ham and Bell, 2004) it makes achievement of equilibrium unlikely. Consequently, these average PTs are not used herein.
6.2. Temperature development relative to pressure changes Both domes show very little variation in temperature with all garnet porphyroblasts nucleating within 25° of 535 °C (Figs. 5 and 6). There is some concordance between temperature increasing as the pressure increased and decreasing when it decreased.
Table 2 XRF data and FIA set in the garnet core for samples analysed from the Chester region. SAMPLE V436b
V634a V240
V257
V261a V205
V653
FIA SiO2 TiO2 Al2O3 Fe2O3T MnO MgO CaO Na2O K2O P2O5 SO3 LOI SUM
1 61.70 1.09 22.50 7.51 0.13 1.00 0.43 0.90 3.99 0.32 bd na 99.57
2 65.73 0.75 15.54 6.78 0.09 2.14 1.47 2.29 2.88 0.17 bd 1.67 99.51
2 74.11 0.72 12.21 4.66 0.10 1.68 0.68 1.30 2.78 0.14 bd 1.41 99.79
3 4 72.80 52.90 na 1.13 15.30 22.37 4.98 10.51 0.11 0.55 0.47 3.02 0.16 1.38 0.87 1.33 2.96 4.12 0.05 0.15 bd 0.03 na 3.1 97.70 100.5
0 58.19 1.06 21.77 8.58 0.17 1.78 0.55 0.80 4.96 0.16 bd 2.46 100.48
2 54.60 0.93 25.10 9.60 0.12 1.76 0.65 1.86 3.35 0.20 0.02 2.70 100.90
3 61.17 1.15 17.25 8.68 0.12 2.76 0.98 1.16 4.06 0.22 bd 3.20 100.70
V259a
V437b 4 61.00 1.09 17.20 7.80 0.13 3.01 1.12 1.68 3.90 0.16 bd 3.0 100.1
6.3. Pressure development at both domes All garnet porphyroblasts around the Chester dome nucleated at pressures between 3 and 8.4 kbar (Figs. 5 and 7a). Garnet in the sample containing FIA 0 nucleated at 4.5 kbar and, in the sample containing FIA 1, nucleated at 3 kbar. The pressure then jumped to between 6 and 8.4 kbar and remained there for ~40 Ma during the development of the FIAs 2, 3 and 4. Garnet porphyroblasts around the Pomfret dome first nucleated between 6 and 8 kbar for every FIA set indicating that the pressure did not change by more than 2 kbar through the ~50 Ma that deformation and metamorphism were accompanied by porphyroblast growth in this region (Figs. 6 and 7b). 6.4. Significance of contrast in pressure development Why should the pressures be so different during the development of FIAs 0 and 1 between the domes? Why is there no evidence for a progressive increase in pressure around the Pomfret dome? To answer these questions we need to examine below: 1. when and where porphyroblasts nucleate with respect to deformation in schists, 2. where garnet cores nucleated in PT space relative to the incoming boundary for this phase on a pseudosection for each sample, 3. how recently deposited sediments can reach 6.3 or more kbar without garnet porphyroblast growth occurring on the way down when their bulk composition indicates that it should have, 4. how the pattern of deformation partitioning changed with time. 6.5. Porphyroblast nucleation and growth relative to deformation Microstructurally, porphyroblasts grow in pelitic rocks in the hinges of crenulations as soon as the latter begin to form. Porphyroblasts cease to grow when differentiated crenulation cleavages begin to develop along crenulation limbs close to their rims (Spiess and Bell, 1996; Bell and Bruce, 2006, 2007). Growth on the rims of such porphyroblasts does not occur until crenulations begin to form with axial planes near orthogonal to those that accompanied the previous phase of growth. At this stage, the same pattern of growth in crenulation hinges occurs, ceasing when a differentiated cleavage begins to develop (Bell and Hayward, 1991; Bell et al., 2003). This microstructural phenomenon has been confirmed metamorphically by Sanislav and Bell (2011) using samples collected within 1.5 m of each other from a single outcrop of the same bed that have virtually identical bulk compositions (Fig. 2a). After garnet nucleated in one sample (b in Fig. 2b) it did not nucleate in the other (Fig. 2c,d) until 56 Ma later when deformation repartitioned through the outcrop crenulating both locations at the scale of a porphyroblast (a in Fig. 2e). At this time numerous staurolite porphyroblasts grew in the sample where garnet had grown previously (b in Fig. 2e) sample but only one was found in eight differently oriented thin sections cut from the other (a in Fig. 2e). The T and P history was identical but the deformation history was not! 6.6. The pressure of garnet core nucleation relative to its phase boundary in PT space For each sample from the Chester dome (Fig. 5) the Ca, Fe and Mn isopleth intersection lies within the same mineral assemblage preserved as inclusions within the garnet core, with three exceptions; sample V259a, which contains epidote inclusions in garnet but not the matrix, sample V437b, which has inclusions of zoisite in garnet rims but not the core or the matrix, and sample V261a, which contains zoisite inclusions in garnet but not in the matrix. Most isopleth intersections indicate a pressure that lies close to the reaction boundary for the incoming of garnet. However, V261a and V257 have this intersection at a higher pressure than the incoming boundary for this
T.H. Bell et al. / Tectonophysics 587 (2013) 119–132
125
Table 3 XRF data and FIA set in the garnet core for samples analysed from the Pomfret region. Sample
AV17b
AV36c
AV19
AV29
AV26a
AV33
V387b
V396
AV27
AV138
FIA SiO2 TiO2 Al2O3 Fe2O3T MnO MgO CaO Na2O K2O P2O5 SO3 LOI SUM
0 51.38 1.23 22.54 10.07 0.13 4.71 1.01 1.18 5.05 0.26 bd 3.41 100.97
0 67.30 0.84 14.40 5.48 0.12 2.18 3.20 2.08 2.08 0.10 0.02 1.40 99.20
1 66.86 0.97 15.09 7.05 0.18 2.16 0.97 2.10 2.96 0.18 bd 1.72 100.24
1 73.20 0.66 11.92 4.60 0.09 2.11 1.92 2.77 1.38 0.16 0.01 1.19 100.01
2 67.86 0.74 13.97 6.48 0.11 3.21 1.31 1.92 3.18 0.15 bd 1.21 100.14
2 48.73 1.29 25.34 9.65 0.15 4.68 0.71 1.36 5.84 0.14 bd 2.76 100.65
3 63.00 0.99 18.6 7.28 0.05 2.04 0.28 1.18 3.77 0.11 0.02 2.42 99.70
3 60.00 1.09 17.30 10.40 0.20 2.63 0.93 1.22 3.13 0.24 0.01 2.21 99.40
4 63.06 0.88 16.56 7.97 0.14 3.74 0.94 1.07 3.96 0.14 0.01 2.04 100.51
4 54.20 1.04 20.40 8.06 0.13 4.33 2.10 3.08 3.76 0.12 0.05 2.68 100.00
phase (the garnet “in” line, Fig. 5d,e) in PT space relative to the appropriate assemblage from which it formed on the relevant pseudosection. This suggests that this reaction was overstepped in pressure in these samples before garnet grew. Samples V240, V257 and V261a all grew their garnet cores during the development of FIA set 2 during which period the pressure increased significantly. The intersection of isopleths for V240 (6.7 kbar, Fig. 5c) was not overstepped whereas that for V257 (7.1 kbar, Fig. 5d) and V261a (8.2 kbar, Fig. 5e) was by ~0.9 kbar. Yet garnet in sample V653, which formed during FIA 3 at a similar pressure to V261a, did not overstep the garnet in-reaction (Fig. 5f). This suggests that the garnet incoming reactions in samples V257 and V261a, metastably lagged behind the change in pressure because of the speed at which it increased while FIA 2 was developing. Note that up to seven foliations accompanied the development of each FIA set in Vermont (Bell and Newman, 2006) so garnet growth could lag behind in some samples due to the lack of crenulation development until late in that FIA set. During the development of FIAs 3 and 4 the incoming reaction for garnet was not overstepped and the pressure dropped slightly (Figs. 5g,h and 7a). The intersection of the Ca, Fe and Mn isopleths for garnet cores on the pseudosections determined for each sample within 12 km of the centre of the Pomfret dome contrasts strongly with those from the Chester dome. The path into the mineral assemblage for each rock appears to have occurred via the same assemblage without garnet in Fig. 6 based on the phases preserved as inclusions within the porphyroblasts. However, all indicate that garnet nucleated at a higher pressure than its incoming phase boundary (the garnet “in” line, Fig. 6) in PT space. The pressure averages 1.2 kbar higher (from Fig. 7b; V387b collected 20 km WNW of the dome, Fig. 3, is not overstepped). This higher pressure resulted from overstepping of the incoming of the garnet reaction because: 1. garnet porphyroblasts that grew for the first time in FIA set 4 are as overstepped as the cores of those that first grew in FIA sets 0 or 1 (see below), 2. all the samples are tightly clustered in pressure between 6 and 8 kbar suggesting that these rocks reached a depth of ~20 km before porphyroblast nucleation and growth began and stayed there for the ~50 Ma that the succession of FIA sets took to develop, 3. reaction overstepping occurs in all samples analysed near the dome and disappears with distance WNW or S from the dome, 4. reaction overstepping is independent of the low and variable CaO contents of the samples (Table 3). Low CaO contents mean that overstepping is unlikely to have resulted from a change in bulk composition due to carbonate or plagioclase loss through dissolution, solution transfer and volume loss accompanying foliation development (e.g., Bell and Bruce, 2006, 2007). If these rocks had been calc pelites, volume loss during
foliation development could have shifted the bulk composition from that operating at the time of garnet growth and produced apparent rather than real overstepping of the incoming reaction for this phase. 6.7. Pomfret dome overstepping 6.7.1. FIA 0 to FIA 4 overstepping The persistent overstepping of the phase boundary for the first phase of garnet growth in every rock analysed within 12 km of the centre of the Pomfret dome, independent of which FIA was developing when that occurred (Fig. 6), is interpreted to have resulted from the rocks previously reaching a high pressure. For the first phase of garnet growth in any single sample, overstepping occurred because there was no drop in pressure from one period of FIA development to the next. Furthermore, the samples did not diverge dramatically from one another in bulk composition (Table 1) and should have grown garnet at lower pressures. After the first phase of garnet growth, any phases of regrowth in subsequent events would tend to have a composition that was equilibrated to the prevailing PT conditions. 6.7.2. Achieving high pressure before garnet growth began Within pelites, porphyroblasts grow in crenulation hinges (e.g., Fig. 2; Sanislav and Bell, 2011). The initiation of crenulation hinges provides stored strain energy and local microfractures along the crenulated foliation causing porphyroblast growth from material both in situ and dissolving close by due to developing strain gradients (e.g., Bell and Hayward, 1991). The latter lead to strain and geometric softening, cessation of growth and, commonly, cleavage development (Bell and Bruce, 2006, 2007). Consequently, porphyroblasts generally do not grow during the first deformation that produces a cleavage or schistosity because no crenulations develop or the scale at which they form from a bedding fissility is only visible on a scanning electron microscope (e.g., Weber, 1981). However, once the PT and bulk composition are appropriate, porphyroblasts will locally grow wherever a crenulation deformation partitions at an appropriate scale through an outcrop containing a previously developed slaty cleavage or schistosity (e.g., Sanislav and Bell, 2011; compare Fig. 2b and e). For no garnet porphyroblast growth to have occurred as the pressure and temperature increased, rocks with the bulk compositions similar to those in Table 3 had to be taken to ~20 km deep without crenulations developing (see below). Thrust sheets between 8 and 12 km thick can leave the sediments below undeformed (Laubach and Diaz-Tushman, 2009; Rob Butler pers. comm, 2007, 2009). Computer modelling and strain measurement approaches suggest that ~50% bulk shortening of a rock mass can be accommodated without any foliation development (e.g., Cloos, 1947; Dieterich, 1970). Consequently, horizontal bulk shortening and consequent crustal thickening of such a rock pile could take the
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10
10
V436b
a
chl g mu
9
g pl bi mu
st g mu
V634a
b
chl ctd g mu
9
g pl ky mu
chl st g mu
10 9
Pressure(kbar)
8
8
chl g ctd zo mu
8
g pl bi st mu
6 l ch
7
chl g pl bi mu
XFe
XMn
6 5
chl pl zo mu
bi st g pl mu
chl ctd g pl mu
6 g st bi pl mu
5 4
7
g ctd chl pl mu
chl g pl mu
u om lz p g
XFe chl st g pl mu XCa
chl ctd g pl mu chl ctd pl zo mu
g st chl pl mu
7
XMn
ctd chl pl mu
5
ctd chl pl mu
XCa
XMnXFe g pl bi sill mu
3
chl ctd zo mu
chl g st mu
chl g ctd mu
ctd mu
chl g zo mu g st pl mu
V240
cchl zo
chl pl mu
4
4 bi pl g sill mu
3
FI A 0
bi sill g pl mu
3
XCa
FI A 1
FI A 2
chl pl mu bi pl g and mu 2 2 450 470 490 510 530 550 570 590 610 630 650 450 470 490 510 530 550 570 590 610 630 650 450 470 490 510 530 550 570 590 610 630 650 10 10 g chl 10 XFe chl g pl mu zo mu g chl bi zo mu 9 9 9 chl bi g pl mu g pl bi mu g pl mu chl bi g pl zo mu bi g pl mu 8 8 g chl bi 8 g pl pl zo mu st mu XCa 7 g st bi pl mu 7 7 XFe XMn chl bi pl chl bi g pl bi mu XCa X Ca zo mu XMn g chl pl mu 6 XMn 6 pl zo 6 mu XFe bi g st g chl pl bi mu g st pl bi mu pl mu 5 5 5
g pl bi and mu
2
V257
V261a
e
Pressure(kbar)
d
V653
f
k
g bi pl sill mu
chl bi pl mu chl pl zo mu
4
bi g sill pl mu
4
4
3
3
g pl bi chl mu g pl sill bi mu
3
FI A 2
FI A 3
FI A 2
g bi pl and mu g pl and bi mu bi g sill pl 2 2 2 450 470 490 510 530 550 570 590 610 630 650 450 470 490 510 530 550 570 590 610 630 650 450 470 490 510 530 550 570 590 610 630 650
10 chl pl zo g mu
Pressure(kbar)
9 8 chl pl zo mu
bi g pl mu
chl bi pl g mu
bi pl g st mu
bi pl g mu
XCa
i
9
XCa
8
chl bi pl g mu
bi pl g st mu
XMn
st g pl bi mu
6
XMn
chl bi pl mu
4
chl bi pl mu bi pl g sill mu
bi pl g and mu
4
FI A 3 bi pl sill mu
FI A4 bi pl sill mu
2 2 2 450 470 490 510 530 550 570 590 610 630 650 450 470 490 510 530 550 570 590 610 630 650 450
Temperature(°C)
g pl bi sill mu
3
3
3
chl g pl bi mu
XFe
5
5
g pl bi ky mu
chl g pl mu
7
XMn XFe
g pl bi mu
V259a
chl g zo mu
chl bi pl zo mu
6
chl bi pl zo mu
10
V437b
7
bi pl g sill mu
4
chl pl zo g chl bi pl g zo mu XFe
chl g pl zo mu
XCa
5
9
h
8
chl bi pl g zo mu
7 6
10
V205
g
470
490 510 530
Temperature(°C)
g pl bi and mu
FI A 4
550 570 590
610 630 650
Temperature(°C)
Fig. 5. Pressure–temperature pseudosections constructed in the MnNCKFMASH (MnO–Na2O–CaO–K2O–Fe2O3–MgO–Al2O3–SiO2–H2O) system using THERMOCALC plus Ca, Fe and Mn isopleths (1σ-error for each isopleth is based on errors propagated from the thermodynamic data) for all samples analysed around the Chester dome. Error ellipses are plotted as described in Section 4. The bulk rock chemical analyses are presented in Table 2. The medium heavy line marks the garnet “in” line.
sequence below the thrusts to 20 km deep without crenulations developing. McWilliams et al. (2010) recognized that overthrusting of the Gile Mountain Formation from the east was necessary to take the sequence in this region to high pressures immediately after it was deposited. By the time the rocks around Pomfret reached a depth of 20 km they may have only locally developed a sub-vertical foliation and the distribution of FIA sets strongly supports this (see below). The pressure did not drop below 6 kbar throughout the development of FIAs 0 through 4 (Fig. 7b). Consequently, wherever shifting of deformation partitioning from deformation to deformation resulted in crenulations
affecting any portion of rock for the first time, garnet grew at more than 6 kbar and thus was overstepped. The lack of overstepping in sample V387b, which lay 20 km WNW of the Pomfret dome, suggests the overlying thrust sheets thinned to the WNW and supports the interpretation of McWilliams et al. (2010) that thrusting was from the east. 6.8. Chester dome lack of overstepping The very thick sequence of sedimentary and volcanic rocks that host the Pomfret dome thins dramatically southwards to where they overly
T.H. Bell et al. / Tectonophysics 587 (2013) 119–132
a thick competent feldspar gneiss basement that underlies the Chester dome as well as the Green Mountains to the west (Fig. 3). The thick thrust sheets that overrode the Gile Mountain Formation would have thinned to the south due to the development of lateral ramps against this competent, non-bedded, basement topographic high (e.g., Bell, 1991). This is apparent because nucleation of garnet in the Chester region was not significantly overstepped (Fig. 5). That is, dramatic crustal thickening prior to porphyroblast growth did not develop here as it did at Pomfret and so these rocks were not taken to pressures higher than the incoming reaction for garnet for these bulk compositions (Table 1) before this phase could grow. 6.9. FIA distribution Pomfret versus Chester dome — an introduction Although the same succession of FIA trends is present around both domes, the proportion of samples containing each FIA set varies dramatically from dome to dome as well as from set to set within them (Figs. 3, 4 and 8a). This has considerable significance for the relationship between deformation partitioning and metamorphism. A general summary of what is currently known from a large database was presented in Section 1.2. Potential pitfalls in interpretation are presented immediately below. In combination, these provide the necessary background for understanding Sections 6.10 through 6.12. 6.9.1. Young porphyroblast growth over old FIA defining foliations Potentially, shifts in deformation partitioning could leave portions of sub-vertical foliation unrotated by later differently striking events with steep axial planes until all were crenulated around gently dipping axial planes late in the FIA succession. If crenulation hinges formed in the three differently striking foliations generated in Fig. 9a to c, and local microfracture allowing porphyroblast growth was possible in all three, then three differently trending FIAs would be preserved (Fig. 9d). For microfracture to occur, all three foliations would have to lie at high angles to the extension direction for the axial plane of the developing crenulation cleavage (see Section 6.10 for more on this topic). Furthermore, reactivation of any of the three foliations lying oblique to the stretching direction is likely from the commencement of deformation and this would prevent crenulation and thus microfracture development (e.g., Bell et al., 2003). The lengthy successions of FIAs preserved by porphyroblasts, their disparate orientations relative to matrix foliations and the general situation for most orogens that only the inclusion trails defining the youngest FIAs are continuous with matrix foliations suggest that this situation occurs rarely. However, this possibility should be kept in mind as it generally could not be distinguished by monazite dating; the ages of monazite grains in the included foliation would still correlate from FIA set to set unless on some rim growth accompanied porphyroblast growth. 6.9.2. Young porphyroblast growth over old gently-dipping foliations The situation is different for a pre-existing sub-horizontal foliation that survives unaffected through several FIA sets before a crenulation event partitions through that location because the axial plane of the latter will be sub-vertical and define the FIA. Dating monazite grains preserved in that sub-horizontal foliation would provide an older age than the FIA unless on some, rim took place during crenulation initiation prior to porphyroblast growth. Furthermore, foliations form sub-horizontally throughout orogenesis whereas subvertical ones form in different orientations with change in FIA trend. Consequently, reuse of sub-horizontal foliations above or below a portion of earlier foliated rock, at any scale, leaving the latter in its original state, could be a common occurrence. This possibility is significant for this study because the FIAs dated by Bell and Welch (2002) were obtained from Ordovician age samples of the Cram Hill formation between the Chester dome and the Spring Hill synform. Bell and Welch obtained many ages from a garnet porphyroblast in sample V634A that grew during the development of FIA 2. Individual ages of spots in
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monazite inclusions range from 436 to 411 Ma and a mean age for one grain of 424 Ma was obtained (Table 2 in Bell and Welch, 2002). These all pre-date the 410 Ma age for the Gile Mountain Formation (McWilliams et al., 2010). Therefore, they cannot be the age of a pseudo FIA 1 (crenulation hinges from an earlier event that predate and have nothing to do with porphyroblast growth) preserved in this sample as FIA 1 is well developed in this horizon (Figs. 3, 4 and 8). At that time FIA 0 had not been found in the Chester dome region and only 2 examples have been found since (Figs. 2 and 8). The oblique foliation crossing the garnet core in Figs. 5 and 6 in Bell and Welch (2002) is a reactivated foliation that postdates but locally preserves relics of pseudo FIA crenulations with sub-vertical axial planes that are best seen in the back scatter images. Relic hinges of these crenulations cause the gentle dips of chloritoid grains obvious in the backscatter electron microscope images. The monazite grains dated from the porphyroblast core lie in relics of this gently dipping foliation. Therefore, as described above, this foliation can be older than FIA 1. Since so few samples containing FIA 0 have been found in the Chester dome region it could easily be a foliation that formed before FIA 0 and thus be Salinic in age (e.g., Hibbard et al., 2010; van Staal et al., 2009)! Relics of such old monazite grains in the median of sample V634A would bias the age of FIA 2 towards 405 Ma (Table 2 in Bell and Welch, Table 2). It is more likely to be closer to 395 Ma. 6.10. Significance of repartitioning of deformation for metamorphism at the Pomfret dome A third of all the samples from both domes contain more than one FIA set; these are included in Fig. 8a but not in Fig. 8b. Fig. 8b contains only the first formed FIA captured when the cores of the garnet porphyroblasts first grew in a sample. The pressure of garnet core growth was overstepped by an average of 1.2 kbar relative to the garnet-in reaction (Figs. 6 and 7b) for all samples analysed chemically within 12 km of the Pomfret dome (Figs. 3a and 4b); each sample contains one of the five FIA sets in garnet cores. This combination of facts allows significant conclusions to be drawn concerning metamorphic processes if Sections 1.2 and 6.9 are used while examining Fig. 8b. The P overstepped environment meant that each FIA in Fig. 8b resulted from the first deformation event in that sample that developed with a sub-vertical axial plane because the latter controls the FIA trend (Fig. 1). Deformation partitioning leaves portions of an orogen at all scales unaffected by each deformation event. Therefore, the first deformation event with a sub-vertical axial plane in any one sample can have occurred in any of FIA 0, 1, 2, 3 or 4. For porphyroblast growth to occur a pre-existing foliation must be undergoing crenulation and, as shown in Fig. 1, the FIA trend is controlled by the sub-vertical axial plane event (Section 1.2). The microfracture of crenulation hinges that leads to porphyroblast growth (Bell and Hayward, 1991) will only develop if their axial planes lie at a high angle to and thus cause near coaxial deformation of the pre-existing foliation, which can be steeply or gently dipping (e.g., Bell and Bruce, 2006). In events with steeply-dipping axial planes a sub-vertical direction of maximum extension resolves the space problem of bulk shortening (Bell and Sapkota, in press) and microfracture of crenulation hinges forming in gently dipping foliations can readily occur allowing porphyroblast growth independent of the developing FIA trend. During gravitational collapse and spreading along gently dipping foliations towards the earth's surface (e.g., Bell and Sapkota, in press), if crenulation of steeply dipping foliations lying at low angles to the direction of maximum extension was possible, microfracture along the crenulated cleavage would be unlikely. Consequently, porphyroblast growth would not occur during sub-vertical bulk shortening of some orientations of pre-existing steeply dipping foliations but would occur for others. A spatial indication (because the maps for the last 4 FIA sets include some samples with 2 or rarely 3 FIAs) of where a vertical foliation first formed for each FIA set is shown by their distribution in Fig. 4b and
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10 9
10
AV36c
a
9
chl pl g mu
Pressure(kbar)
8
pl g bi mu
8
bi st g pl mu
XMn
g bi pl
XCa 5
bi pl g st mu
XCa 6
6
u sil l m
chl pl g mu bi pl g mu
8
chl pl g bi mu
7
XCa XMn
6
XMn
chl pl mu
5
XFe
bi pl g ky mu
chl bi pl g mu
7
7
AV19
c 9
XFe
chl pl g mu
chl pl g bi mu
10
AV17
b
chl bi pl mu
bi pl g sill mu
bi st pl g mu
XFe
5
bi st pl g 4
4
4
bi st pl g chl bi pl g sill
3 chl pl bi mu
bi pl sill mu
3 chl g pl bi and
FI A 0
chl bi pl mu
bi pl g sill mu
3
FI A 0
bi pl and mu
FI A 1
bi pl g and mu 2 2 2 450 470 490 510 530 550 570 590 610 630 650 450 470 490 510 530 550 570 590 610 630 650 450 470 490 510 530 550 570 590 610 630 650 10 10 10 chl pl g zo mu pl g bi mu chl pl g mu XFe chl pl g bi zo mu chl bi pl g zo mu 9 9 9 XCa bi pl bi pl g mu bi pl g g ky bi pl g st mu ky mu mu X Ca 8 8 8 chl pl bi g mu chl pl zo mu XCa XMn 7 7 7 chl pl bi bi pl bi st pl zo mu chl bi pl zo mu XMn g st g mu chl bi pl g mu XMn XFe 6 6 6 chl pl g bi mu XFe chl pl mu bi pl g ky bi pl g st chl 5 5 5 chl pl bi mu bi pl g sill mu chl bi pl mu chl pl bi mu bi pl g sill mu 4 4 4 chl pl g bi sill bi pl g sill bi pl g and mu bi pl g sill bi pl sill mu 3 3 3 bi pl g chl pl bi and mu g bi pl sill 2 2 2 450 470 490 510 530 550 570 590 610 630 650 450 470 490 510 530 550 570 590 610 630 650 450 470 490 510 530 550 570 590 610 630 650 10 10 10 bi pl g mu chl g bi pl ky mu g zo mu chl pl zo g mu bi g XFe chl chl g mu chl pl g mu 9 9 pl mu 9 XFe pl zo chl zo mu chl pl chl pl g zo mu chl bi pl g zo mu mu g zo bi pl g chl bi pl g mu XCa bi pl g ky mu mu ky mu 8 8 8 chl pl g mu XFe
AV29
AV33
e
AV26a
f
Pressure(kbar)
d
FI A 1
V387b
bi st pl g mu
bi
6 chl pl mu
l lp ch
7
u tm is gb
chl pl g bi mu
bi pl g sill mu
XMn
5
chl pl mu
ch l m bi p u l
3
FI A 3
chl pl
bi pl g
si l l
4
bi st pl g mu
7 chl bi pl 6 zo mu
XCa
bi pl g mu
3
bi g pl st mu
6
bi pl g sill mu
4
chl pl zo mu
AV27
i
XCa XMn
5 4
st
XMn
5
V396
h
chl pl zo mu
7
ch lp lg
Pressure(kbar)
g
FI A 2
FI A 2
bi pl g sill mu
chl bi pl mu
bi pl g sill
bi pl sill mu
3
FI A 4
FI A 3
chl bi pl bi pl g bi g and and mu mu 2 2 2 450 470 490 510 530 550 570 590 610 630 650 450 470 490 510 530 550 570 590 610 630 650 450 470 490 510 530 550 570 590 610 630 650
Temperature(°C)
10
j
AV138
bi pl g mu
chl pl bi g zo mu
9
chl pl bi g mu
Pressure(kbar)
8 7
chl pl bi zo mu
XCa XMn
6
XFe
bi pl g st mu
bi pl g ky mu
l si l lg p bi
u m
5
chl pl bi mu 4
bi pl g sill
3
FI A 4
chl bi pl g and 2 450 470 490 510 530 550 570 590 610 630 650
Temperature(°C)
Temperature(°C)
T.H. Bell et al. / Tectonophysics 587 (2013) 119–132
10
10
Chester dome:garnet core nucleationP–T V653
9
129
Pomfret dome:garnet core nucleation P–T AV19-1.6kb overstepped in P
9 V261a- 1kb over stepped in P
V387b- not overstepped–but sample from 20km west of Pomfret dome
3
AV26a- 1.4kb overstepped in P
8
8
2
V437b
3
4
7
V257- 1kb over stepped in P
6
3
V240
V205
5
4 AV138- 0.9kb overstepped in P
2
AV33- 0.5kb overstepped in P
4 1 1 0 AV29-1kb & ?T overstepped 0 3
7
Pressure(kbar)
Pressure(kbar)
V259a
AV27-1.2 kb overstepped in P AV17-1kb overstepped in P
6 AV36c-1.8kb over stepped in P
5
V396-1.4kb overstepped in P-sample from 10km west of Pomfret dome
0
V436b
4
4 V634a
3
3
a
b 500
520
540
b
a
2 560
580
600
Temperature(°C)
2 500
520
540
560
580
600
Temperature(°C)
Fig. 7. Graphs showing nucleation PT of garnet (error ellipses from Figs. 5 and 6 after the approach used by Evans, 2004) in each sample analysed and the FIA set it belonged to for the Chester (a) and Pomfret (b) domes. An estimation of pressure and temperature overstepping is made relative to the maximum P or T of the garnet “in” line for the mineral assemblage stable for that bulk rock composition.
d for FIAs 0 and 1, and can be found in Fig. 13c,d,e in Ham and Bell (2004) for FIA sets 2, 3 and 4. These figures also show where vertical foliations generated during the development of FIA 0 did not form; in a PT environment where the garnet-in reaction is significantly overstepped, garnet would have locally nucleated and grown wherever the first cleavage or schistosity that had formed in that location was subsequently crenulated (Section 1.2). For example the sub-vertical foliation defining FIA 0 in every location shown in Fig. 4b did not form in most of the locations shown for FIA 1 in Fig. 4d and so on for FIAs 2, 3 and 4 in Fig. 13c,d,e in Ham and Bell (2004). Approximately half of all samples first grew garnet in FIA 0 (Fig. 8b). However, half of the sample sites where garnet did subsequently nucleate and grow for the first time were still available because the PT conditions were overstepped for garnet growth in all rocks analysed for every FIA set (Fig. 6). Garnet should have grown in most if not all of these samples during the development of FIA 0 if pressure, temperature and bulk composition were the only factors involved. Indeed, it should have grown during the development of FIA 1 in the remaining samples and so on during the development of FIAs 2 and 3. Such preservation of sites where garnet did not grow during the development of each of the first 4 FIAs is impossible in a significantly PT overstepped environment for all bulk compositions analysed unless some other factor was involved. That this controlling factor was the development of crenulations is demonstrated by the retention of the foliation that predated the porphyroblast as inclusion trails across the core and the curvature of this foliation at the rim of the core phase of growth that was used to measure the FIA in each sample (Figs. 8 and 9 Ham and Bell, 2004). It provides dramatic further evidence of the necessity of crenulation
development for porphyroblast growth once the PT conditions for the incoming of that phase in a particular bulk composition have been reached (c.f. Sanislav and Bell, 2011).
6.11. Significance of deformation for metamorphism at the Chester dome The pressure increased more gradually during orogenesis at the Chester dome and most samples show no evidence for overstepping. Therefore, it is possible that several foliations developed and were folded, reactivated and rotated into parallelism with the compositional layering as the rocks were taken deeper for which we have no record because no porphyroblast growth was taking place. Indeed, there is monazite age evidence for a gently dipping foliation that developed prior to porphyroblast growth and thus the development of FIA 0. It is the foliation with the 411 to 436 Ma Salinic ages in Ordovician rocks mentioned above (Table 2 in Bell and Welch, 2002). Fig. 8b shows that most porphyroblasts nucleated and grew for the first time much later in the deformation history than at the Pomfret dome. Indeed, after the development of FIAs 0 and 1, only 22% of all garnet bearing samples had grown this phase for first time. Most garnet porphyroblasts (~35%) grew for the first time during FIA 2 when the P and T reached that of the Pomfret dome. There was little change to a slight drop in the PT of garnet nucleation during FIAs 3 and 4 from FIA 2 (Figs. 5 and 7a). Consequently, the situation during the development of FIAs 3 and 4 was somewhat similar to which occurred at the Pomfret dome in that all garnet growth should have been accomplished by the end of FIA 2. This was not the case because although the P and T had reached that needed for garnet growth,
Fig. 6. Pressure–temperature pseudosections constructed in the MnNCKFMASH (MnO–Na2O–CaO–K2O–Fe2O3–MgO–Al2O3–SiO2–H2O) system using THERMOCALC plus Ca, Fe and Mn isopleths for all samples analysed around the Pomfret dome. The bulk rock chemical analyses are presented in Table 3. The medium heavy line marks the garnet “in” line. The 2 samples used for PT work on FIA 3, V396 and V387b, were collected for the Bell et al. (1998) study. V387 lies 20 km WNW of the Pomfret dome (see Fig. 3a) and is not overstepped.
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60
occurred over this period of orogenesis (Fig. 8b) probably because crenulation deformations had to partition through both regions relatively equally! The dominant early growth of garnet during FIAs 0 and
a Pomfret dome Chester dome
50
a Number of FIAs
40
A B
30
C 20
foliation formed at A during FIA 0 10
b
0 FIA 0
FIA 1
FIA 2
FIA 3
A
FIA 4
B %
C
b
Number of FIAs
50
foliation formed at B during FIA 2
40
Chester
c
30 20
Pomfret
A
1
B F0IA
F1IA
F2IA
F3IA
C
F4IA
Fig. 8. a. Histogram showing decrease in the number of FIAs from Set 0 through 4 around the Pomfret dome and the overall increase around the Chester dome. This figure includes all FIAs. b. Histogram showing only the FIA in the core of samples for the Pomfret (solid line) and Chester (Dashed line) domes as a percentage. It does not include subsequently formed differently trending FIAs in porphyroblast medians or rims.
foliation formed at C during FIA 4
d A it only occurred when crenulations began to partition through that sample at the scale of a porphyroblast (e.g., Fig. 2e). 6.12. Partitioning differences and their effect on garnet growth between the Pomfret and Chester domes
B C DB
DA
DC
porphyroblasts grew at D during FIA 4 During FIA 0 horizontal bulk shortening was directed SW–NE. The Chester dome was little affected because it and the Green Mountains to the W and NW are underlain by competent feldspar rich basement gneiss. The result was that deformation was more pervasively partitioned into the Pomfret dome region than the Chester dome with 46% of the first phase of all garnet growth occurring in the former region at this time (Fig. 8b) and only 2% in the latter. During FIA 1 horizontal bulk shortening was directed NW–SE and partitioning of the deformation around the competent basement gneiss was not possible. 20% of the first phase of all garnet growth in either region
Fig. 9. Shows schematically how different generations of sub-vertical foliations could potentially develop with different strikes in different locations through an outcrop containing a sub-horizontal foliation because of the effects of deformation partitioning. a. Deformation during FIA 0 produced a sub-vertical foliation at A (black) but not at B or C. b. Deformation during FIA 2 produced a sub-vertical foliation (black — with a different strike in 3D from at A — grey) at location B but did not occur at A or C. c. Deformation during FIA 4 produced a sub-vertical foliation (black — with a different strike in 3D from at A or B — grey) at location C but not A or B. d. If a crenulation with a subhorizontal axial plane did not affect this outcrop until during the development of FIA 4, 3 differently trending FIAs would form if microfracture and thus porphyroblast growth could occur at DA, DB and DC and preserve FIAs 0, 2 and 4 respectively.
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1 around the Pomfret dome increased the bulk competency of this region. This caused more pervasive deformation to shift into the Chester dome region as well as 20 km WNW to the location of sample V387b (Fig. 3) during the N–S bulk shortening that resulted in FIA 2. This decreased the number of sites where garnet could grow for the first time around the Pomfret dome. However, it generated the most sites (36%, Fig. 8b) for the first phase of garnet growth for any FIA in the Chester dome region (Fig. 8b). This in turn preferentially thickened the crust in Chester dome region as well as that where sample V387b was located taking the pressure up to that of the Pomfret dome (Fig. 7). Due to this pressure increase garnet growth became possible in a larger range of bulk compositions during this FIA and those that followed (compare Table 2 and Fig. 5). 6.13. Tectonic summary Overthrusting by a 6 to 10 km thick sheet from the east, which thinned due to lateral ramping over a gneissic basement high to the south and through frontal ramping to the WNW, impacted significantly on subsequent interplay between deformation and metamorphism. Preferential thickening of this sheet in the Pomfret region due to SW–NE directed bulk horizontal shortening during the first of five periods of change in the direction of relative plate motion (e.g., Bell and Sapkota, in press) took these rocks to 20 km deep (Fig. 6). This led to all subsequent garnet growth being pressure overstepped (Fig. 7b). The rocks at the Chester dome lay in the strain shadows of basement gneiss both below and to the NW at this time and were much less shortened. Consequently, little garnet growth occurred there (Fig. 8b) but with a shift to NW–SE directed horizontal bulk shortening, neither dome was differentially protected by basement gneiss. Therefore, both were similarly affected by deformation with similar amounts of porphyroblast growth (Fig. 8b). A switch to N–S bulk shortening preferentially partitioned deformation into the Chester region because of the increased competency of the Pomfret region resulting from ~60% of all garnet growth having already occurred there (Fig. 8a). The crustal pile thickened to a similar level to that in the Pomfret region with most new garnet nucleating at this time in the Chester region. Subsequent shifts in the direction of bulk shortening produced much more growth in the Chester region because rocks protected earlier in largescale strain shadows became susceptible to further deformation. Furthermore, the increase in pressure prior to FIAs 3 and 4 enabled rocks with different bulk compositions to grow garnet for the first time during these later periods of orogeny as well as considerable growth on the rims of garnets that had grown earlier during the deformation history. Acknowledgements We thank the Australian Research Council for funding much of this research. We thank Paul Karabinos, Cees van Staal and Wally Bothner for discussion and access to material. We thank the referees, Mike Williams and Bob Wintsch for their very helpful reviews. References Abu Sharib, A.S.A.A., Bell, T.H., 2011. Radical changes in bulk shortening directions during orogenesis: significance for progressive development of regional folds and thrusts. Precambrian Research 188, 1–20. Aerden, D.G.A.M., 1994. Kinematics of orogenic collapse in the Variscan Pyrenees deduced from microstructures in porphyroblastic rocks from the Lys–Caillaouas Massif. Tectonophysics 238, 139–160. Aerden, D.G.A.M., 1995. Porphyroblast non-rotation during crustal extension in the Variscan Pyrenees. Journal of Structural Geology 17, 709–726. Aerden, D.G.A.M., 1998. Tectonic evolution of the Montagne Noire and a possible orogenic model for syn-collisional exhumation of deep rocks, Hercynian belt, France. Tectonics 17, 62–79. Aerden, D., Sayab, M., 2008. From Adria- to Africa-driven orogenesis: evidence from porphyroblasts in the Betic Cordillera, Spain. Journal of Structural Geology 30, 1272–1287.
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Armstrong, T.R., Tracy, R.J., Hames, W.E., 1992. Contrasting styles of Taconian, Eastern Acadian and Western Acadian metamorphism, central and western New England. Journal of Metamorphic Geology 10, 415–426. Bell, T.H., 1991. The role of thrusting in the structural development of the Mount Isa Mine and its relevance to exploration in the surrounding region. Economic Geology 86, 1602–1625. Bell, T.H., Bruce, M.D., 2006. The internal inclusion trail geometries preserved within a first phase of porphyroblast growth. Journal of Structural Geology 28, 236–252. Bell, T.H., Bruce, M.D., 2007. Progressive deformation partitioning and deformation history: evidence from millipede structures. Journal of Structural Geology 29, 18–35. Bell, T.H., Hayward, N., 1991. Episodic metamorphic reactions during orogenesis: the control of deformation partitioning on reaction sites and duration. Journal of Metamorphic Geology 9, 619–640. Bell, T.H., Hickey, K.A., 1998. Multiple deformations with successive sub-vertical and sub horizontal axial planes: their impact on geometric development and significance for mineralization and exploration in the Mount Isa region. Economic Geology 93, 1369–1389. Bell, T.H., Newman, R., 2006. Appalachian orogenesis: the role of repeated gravitational collapse. In: Butler, R., Mazzoli, S. (Eds.), Styles of Continental Compression. Geological Society of America Special Papers 414, pp. 95–118. Bell, T.H., Sanislav, I.V., 2011. A deformation partitioning approach to resolving the sequence of fold events and the orientations in which they formed across multiply deformed large-scale regions. Journal of Structural Geology 31, 1206–1217. Bell, T.H., Sapkota, J., in press. Episodic gravitational collapse and migration of the mountain chain during orogenic roll-on in the Himalayas. Journal of Metamorphic Geology 30. Bell, T.H., Welch, P.W., 2002. Prolonged Acadian orogenesis: revelations from FIA controlled monazite dating of foliations in porphyroblasts and matrix. American Journal of Science 302, 549–581. Bell, T.H., Hickey, K.A., Upton, G.J.G., 1998. Distinguishing and correlating multiple phases of metamorphism across a multiply deformed region using the axes of spiral, staircase and sigmoidally curved inclusion trails in garnet. Journal of Metamorphic Geology 16, 767–794. Bell, T.H., Ham, A.P., 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., Kim, H.S., 2004. Partitioning of deformation along an orogen and its effects on porphyroblast growth during orogenesis. Journal of Structural Geology 26, 825–845. Cloos, E., 1947. Oölite deformation in the South Mountain fold, Maryland. Bulletin of the Geological Society of America 58, 843–918. Davis, B.K., 1995. Regional-scale foliation reactivation and re-use during formation of a macroscopic fold in the Robertson River Metamorphics, north Queensland, Australia. Tectonophysics 242, 293–311. Davis, B.K., Forde, A., 1994. Regional slaty cleavage formation and fold axis rotation by reuse and reactivation of pre-existing foliations: the Fiery Creek Slate Belt, north Queensland. Tectonophysics 230, 161–179. Dieterich, J.H., 1970. Computer experiments on mechanics of finite amplitude folds. Canadian Journal of Earth Sciences 7, 467–476. Evans, T.P., 2004. A method for calculating effective bulk composition modification due to crystal fractionation in garnet-bearing schist: implications for isopleth thermobarometry. Journal of Metamorphic Geology 22, 547–557. Gavin, B.P., 2004. The microstructural and metamorphic history preserved within garnet porphyroblasts from southern Vermont and northwestern Massachusetts. Unpublished PhD thesis, James Cook University, 252 pp. Ham, A.P., 2001. Integration of porphyroblast growth with the microstructural and thermal histories during fold development in the Acadian orogeny of the Vermont Appalachians. Unpublished PhD thesis, James Cook University, 319 pp. Ham, A.P., Bell, T.H., 2004. Recycling of foliations during folding. Journal of Structural Geology 26, 1989–2009. Hayward, N., 1992. Microstructural analysis of the classic snowball garnets of southeast Vermont. Evidence for non-rotation. Journal of Metamorphic Geology 10, 567–587. Hibbard, J.P., van Staal, C.R., Rankin, D.W., 2010. Comparative analysis of the geological evolution of the northern and southern Appalachian orogen: Late Ordovician– Permian. In: Tollo, R.P., Bartholomew, M.J., Hibbard, J.P., Karabinos, P.M. (Eds.), From Rodinia to Pangea: The Lithotectonic Record of the Appalachian Region: Geological Society of America Memoir, 206, pp. 51–69. Hickey, K.A., Bell, T.H., 2001. Resolving complexities associated with the timing of macroscopic folds in multiply deformed terrains: the Spring Hill synform, Vermont. Bulletin of the Geological Society of America 113, 1282–1298. Laubach, S.E., Diaz-Tushman, K., 2009. Laurentian palaeostress trajectories and ephemeral fracture permeability, Cambrian Eriboll Formation sandstones west of the Moine Thrust Zone, NW Scotland. Journal of the Geological Society of London 166, 349–362. McWilliams, C.K., Walsh, G.J., Wintsch, R.P., 2010. Silurian-Devonian age and tectonic setting of the Connecticut Valley–Gaspe Trough in Vermont based on U–Pb SHRIMP analyses of detrital zircons. American Journal of Science 310, 325–363. Powell, R., Holland, T.J.B., 1994. Optimal geothermometry and geobarometry. American Mineralogist 79, 120–133. Powell, R., Holland, T.J.B., Worley, B., 1998. Calculating phase diagrams involving solid solutions via non-linear equations, with examples using THERMOCALC. Journal of Metamorphic Geology 16, 577–588. Ratcliffe, N.M., 1993. Bedrock geologic map of the Mount Snow and Readsboro Quadrangles, Bennington and Windham counties, Vermont. US Geological Survey Miscellaneous Investigation Series, Map I-2307, scale 1:24000.
132
T.H. Bell et al. / Tectonophysics 587 (2013) 119–132
Ratcliffe, N.M., 1995a. Digital bedrock geologic map of the Cavendish Quadrangle, Vermont. US Geological Survey Open-File Report 95-203-A. Ratcliffe, N.M., 1995b. Digital bedrock geologic map of the Chester Quadrangle, Vermont. US Geological Survey Open-File Report 95-576-A. Ratcliffe, N.M., Armstrong, T.R., 1995. Preliminary digital bedrock geologic map of the Saxtons River 7.5°x15° Quadrangle Windham and Windsor Counties, Vermont. US Geological Survey Open-File report 95–482. Rieuwers, M. 2010. Integrating microstructure and geothermobarometry in deciphering complex tectonometamorphic histories: examples from the New England Appalachians, USA. Unpublished PhD Thesis, James Cook University, 209 pp. Sanislav, I.V., Bell, T.H., 2011. The inter-relationships between long-lived metamorphism, pluton emplacement and changes in the direction of bulk shortening during orogenesis. Journal of Metamorphic Geology 29, 513–536. Sayab, M., 2005. N-S shortening during orogenesis within the Mt Isa Inlier (NW Queensland, Australia): the preservation of early W–E trending foliations in porphyroblasts revealed by independent 3D measurement techniques. Journal of Structural Geology 27, 1445–1468. Sayab, M., 2008. Correlating multiple deformation events across the Mesoproterozoic NE Australia using foliation intersection axes (FIA) preserved within porphyroblasts. Gondwana Research 13, 331–351. Shah, S.Z., Sayab, M., Aerden, D.G.A.M., Khan, A., 2011. Foliation intersection axes preserved in garnet porphyroblasts from the Swat area, NW Himalaya: a record
of successive crustal shortening directions between the Indian plate and Kohistan–Ladakh Island Arc. Tectonophysics 509, 14–32. Skrzypek, E., Schulmann, K., Stipska, P., Chopin, F., Lehmann, J., Lexa, O., Haloda, J., 2011. Tectono-metamorphic history recorded in garnet porphyroblasts: insights from thermodynamic modeling and electron backscatter diffraction analysis of inclusion trails. Journal of Metamorphic Geology 29, 473–496. Spiess, R., Bell, T.H., 1996. Microstructural controls on sites of metamorphic reaction: a case study of the inter-relationship between deformation and metamorphism. European Journal of Mineralogy 8, 165–186. Stanley, R.S., Ratcliffe, N.M., 1985. Tectonic synthesis of the Taconic orogeny in Western New England. Bulletin of the Geological Society of America 96, 1227–1250. Van Staal, C.R., Whalen, J.B., Valverde-Vaquero, P., Zagorevski, A., Rogers, N., 2009. Precarboniferous, episodic accretion-related, orogenesis along the Laurentian margin of the northern Appalachians. In: Murphy, J.B., Keppie, J.D., Hynes, A.J. (Eds.), Ancient Orogens and Modern Analogues: Geological Society, London, Special Publications, 327, pp. 271–316. Weber, K., 1981. Kinematic and metamorphic aspects of cleavage formation in very low-grade metamorphic slates. Tectonophysics 78, 291–306. Welch, P.W., 2003. Microprobe dating of monazite in relation to porphyroblast growth and thermodynamic modelling P–T paths for rocks affected by prolonged orogenesis. Unpublished PhD Thesis, James Cook University, 271 pp.
