Dear colleagues and friends,

You may have noticed my recent Comment in JSG entitled:
Comment on “Reference frame, angular momentum, and porphyroblast 
rotation” by Dazhi Jiang and Paul F. Williams. Journal of Structural 
Geology 27, 943-1138. (in press)
Jiang and William's "Reply", which was both constructive and 
instructive, prompts me to place some additional comments regarding 
three issues for those interseted in the subject.

Cheers,
Domingo



1.
In my comment, I questioned Dazhi Jiang and Paul Williams's approach of 
treating the matrix as a homogenous fluid or "continuous medium". In my 
opinion, this ignores the effect of matrix heterogeneity and resulting 
partitioning of deformation at the scale of porphyroblasts. J & W 
reject this criticism and reaffirm that rocks may be safely modelled as 
continuous media as long as the scale of observation is much larger 
than individual matrix grains. They write:

"As pointed out in Jiang (2001), a garnet porphyroblast typically 
occupies a volume that would contain w1000 matrix grains, making it 
justifiable to use the matrix vorticity to represent the angular 
velocity of a spherical garnet. "

In other words, J & W regard grain-scale processes (microcracking, 
dissolution, solution transfer, reprecipitation) as irrelevant to the 
development of structures at larger-scales. The viscosity and vorticity 
of the matrix can, according to them, be assumed to be the average of 
the viscosities and vorticities of the constituent mineral grains. In 
my opinion, this viewpoint overlooks the role of small-scale 
instabilities in the formation of macroscopic patterns. A wide variety 
of natural phenomena studied in physics, chemistry, biology etc. 
exhibit small-scale heterogeneity being amplified into regular 
macroscopic patterns. To mention a recent  title on the subject is: 
"Evolution of Spontaneous Structures in Dissipative Continuous Systems" 
by F.H. Busse and S.C. Muller (editors). Lecture Notes in Physics 
series. Springer, Berlin, 1998, 583 pp.). I would argue that similar 
laws apply to the partitioning of deformation in metamorphic rocks. 
Deformation-induced differentiation in microlithons, foliation septae, 
shear bands etc. leads to regular patterns ("microfabrics") which are 
intimately related to the development of porphyroblast inclusion 
trails. Aporphyroblast can approach the size of a single microlithon 
and still show the same basic inclusion trail geometries as larger 
porphyroblasts in the same rock. Although we know that "lack of 
porphyroblast rotation" results from a particular pattern of 
deformation partitioning, we do not fully understand the origin of 
these patterns in physical terms. A physical theory with true 
explanatory power must be based on the grain-scale physical processes 
rather than on uncertain analogies and extrapolations.

2.
Figure 4 in my original "Comment" I showed  3 large garnets with 
spirals defined by a continuously curving single foliation. These 
microstructures were argued to be inconsistent with "non-rotation" 
models by Jiang and Williams (2004). Apart from argueing why this is 
not true in my opinon, I also showed that internal truncation surfaces 
and lines interconnecting inflexion points in Fig. 4 exhibit a certain 
degree of vertical and horizontal preferred orientation. I argued that 
this supports a similar origin as more "truncational" spiral showing 
more pronounced preferred vertcial and horizontal orientations,such as 
described by Hayward (1992), for example. Jiang & Williams replied:

"We lack the imagination to agree with Aerden’s interpretation of his 
fig. 4b–d in terms of straight-line segments and fail to see the 
‘subtle, yet distinctly orthogonal patterns’ (fig. 4b–d)." Further, we 
find it far-fetched to claim that the orthogonal patterns are either 
parallel or perpendicular to the earth’s surface (fig. 4f). Some as 
interpreted are close, but the initial choice of straight-line segments 
is completely subjective."

Here I point out that correct identification of inflexion points and 
truncation surfaces involves little interpretation and that no 
"imagination" is required to agree with geometries that are factual. To 
demonstrate the "objectiveness" of these drawings I have constructed a 
dip-isogon map for one of the spirals (Fig. 4b), which is attached as a 
PDF file. I drew the dip-isogons conveniently by rotating the line 
drawing of inclusion trails 10° increments on the computer screen  and 
each time tracing the tangent points of inclusion-trail lines with the 
horizontal computer-screen raster lines. One can notice the close 
correspondence of the isogon pattern and the originally identified 
truncations and inflexion points. These features are not subjective but 
real, and the same counts for their orientation indicated relative to 
the Earth's surface.





3.
One of my motivations for writing a discussion was to draw attention to 
the fact that proponents of conventional porphyroblast-rotation models 
have never explained or even addressed  vertical and horizontal 
preferred orientations of inclusion trails (VHPOIT) and associated 
truncations in different metamorphic regions, including classic 
"snowball garnet" areas (e.g. Hayward, 1992). Likewise, no explanations 
in terms of "rotation" have been attempted for the highly consistent 
orientations of successive FIA generations (foliation intersection 
axes) preserved within progressive stages of porphyroblast growth in 
metamorphic region. I challenged Jiang and Williams to address this 
data with specific refernce to Hayward (1992). They replied as follows:

"Numerous papers have been published and more may continue to be 
published showing ‘evidence’ that rigid porphyroblasts do not rotate 
with respect to Earth. We admit that we have lost track of, and 
ignored, some of these papers. We have presented our arguments in 
Williams and Jiang (1999), Jiang (2001) and Jiang and Williams (2004). 
In the spirit of our original paper (Jiang and Williams, 2004, p.2212): 
“We do not repeat arguments already presented in the literature”

Jiang and Williams suggest that they (or perhaps other workers 
defending rotation) already discussed the origin of VHPOIT in the 
literature, which is false. The papers they refer to are purely 
theoretical works in which model predictions are not tested by any 
means. Not a single real microstructure can be found in these papers. 
This contrasts sharply with the approach followed by advocates of 
"non-rotation" of integrating detailed microstructural analysis and 
field work with systematic measurements of inclusion-trail 
orientations. A selection of this work is given below in order that no 
one should loose track of the data. So I totally disagree with J & W's 
idea that the "non-rotation" concept is the fruit of a dogma. It is 
based on detailed observation of natural microstructures, field work  
and the collection of 3D orientation data that can be checked by 
independent workers if necessary.


References

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 236, 139-160.
1996 - Aerden, D.G.A.M. The pyrite-type strain fringes from Lourdes, 
France: indicators of Alpine thrust kinematics in the Pyrenees. J. 
Struct. Geol., 18, 75-92.
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.G.A.M., 2004. Correlating deformations in the Iberian Massif 
(Variscan belt) using porphyroblasts; implications for the development 
of the Ibero-Armorican Arc. Journal of Structural Geology, 26, 177-196.
Bell TH, Chen A. 2002. The development of spiral-shaped inclusion 
trails during multiple metamorphism and folding.  Journal of 
Metamorphic Geology 20 (4): 397-412.
Bell TH, Kim HS. Preservation of Acadian deformation and metamorphism 
through intense Alleghanian shearing.  Journal Of Structural Geology 26 
(9): 1591-1613.
Bell, T.H. & Johnson, S.E., 1989. Porphyroblast inclusion trails: the 
key to orogenesis. Journal of Metamorphic Geology 7, 279-310.
Bell, T.H., Hickey, K.A. & Upton, G.J.G., 1998. Distinguishing and 
correlating multiple phases of metamorphism accross a multiply deformed 
region using the axes of spiral, staircase, and sigmoidally curved 
inclusion trails in garnet. J. Metamorphic Geol., 16, 767-794.
Hayward, N., 1992. Microstructural analysis of the classical spiral 
garnet porphyroblasts of south-east Vermont: evidence for non-rotation. 
Journal of Metamorphic Geology 10, 567-587.
Hickey, K.A.and Bell, T.H. , 1999. Behaviour of rigid objects during 
deformation and metamorphism: a test using schists from the Bolton 
syncline, Connecticut, USA. Journal of Metamorphic Geology, 17, 
211-228.
Mares, V.M. 1998. The structural development of the Soldiers Cap Group 
within a portion of the eastern foldbelt of the Mount Isa inlier: a 
succession of near-horizontal and near-vertical deformation events and 
large-scale shearing. Aus. J. Earth Scs. 45, 373-388.
Mohammad Sayab.  Microstructural evidence for N–S shortening in the 
Mount 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.
Stallard, A. and Hickey, K., 2001. Shear zone vs folding origin for 
spiral inclusion trails in the Canton Schist. Journal of Structural 
Geology, 23, 1845-1864.

Dr. Domingo Aerden
Profesor Titular
Departamento de Geodinámica
Universidad de Granada
18002 GRANADA, Spain
Tel. +34 958242825
Fax: +34 958248527
E-mail: [log in to unmask]