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Guacha Corral Shear Zone, Sierra de Cordoba, Argentina


Roberto Weinberg
School of Geosciences, Monash University, Australia




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Guacha Corral Shear Zone, summary of observations of June 17 from Merlo to close to the pass on the west side of the Sierra de Cordoba.

This is essentially a continous sequence of rocks starting at the base of the Sierra with a schist with only incipient evidence of in situ melting and no peritectic minerals. This is followed upwards by an increase in melt fraction to metatexites that preserves original layering between more fertile pelitic layers and unmelted psammitic layers (a and c). Further up this changes into a diatexite with bands of granite. This is intensely folded and sheared and could be interpreted as a metatexite, except that psammite layers are disrupted (b and d) and in a number of places, melanosomes are chaotically distributed in a granitic ground mass. No anhydrous peritectic minerals are present up to this point, and these rocks are interpreted to represent the Conlara Complex. Furher up there is a layer where amphibolites are interlayered with a black psammite where melting is incipient, but the psammites are melted, unlike at the base of the system. After this band, the sequence returns to diatexite and granite, but now with pink garnet in the leucosomes. These anhydrous peritectic minerals are similar to those we observed in the Monteguazu Complex. The granite here is similar in appearance to the one found in the lower diatexites of the Conlara Complex.


Interpreation: there is an upward temperature increase from incipient melting to widespread melting in the presence of water then to biotite dehydration in the upper reaches. The sequence is a continuous one that links the migmatites from the Conlara Complex that crops out west of the shear zone, to the migmatites of the Monteguazu Complex that crops out to the east of the shear zone. They represent a gradual increase in temperature that has been inverted by thrusting on the shear zone and are most likely not two separate migmatization events.


The deformation of the package is characterized by shearing and folding. There is a dominant shear plane that is parallel to the axial plane of folds, and a dominant stretching lineation that is parallel to fold axes (d). At the base, the dominant foliation dips towards 70-90 / 30 and further up the sequence this gradually turns to 130-140/30, while the stretching lineation remains roughly constant plunging towards 60-90, and there is also an intersection lineation plunging towards 180-190. The fact that the stretching lineation and fold axis are parallel suggests a pure shear component during deformation (see Robin and Cruden).


The intensity of shearing varies from protomylonite to ultramylonite (c and e-h). Typically the shear zone is dominated by protomylonites where the fabric of the protolith is still well preserved, with bands of mylonites. Ultramylonites are only a very small fraction of the total width, 1% at the most. These are intensely foliated, banded (e-h), including strongly stretched pegmatitic bands (f), and have widths that vary from 2mm to 20cm. In the wider examples, they include lenses of less sheared, myloniitic pegmatites. Collectively, bands of ultramylonites can appear over a thickness of 1-1.5 m of mylonites, creating a high strain zone (e).


The folded migmatites have axial planar leucosomes (i and j and also m and n for Los Linderos area also in Conlara migmatites). These are also parallel to the shear plane and ultramylonites (k-l).


Interpreation: melting was syn-kinematic. Since the shear zone is interpreted to be of Famatinian age, melting should also be Famatinian. Sample CH19 was collected for dating.


Quartz vein, thrust Quartz vein, thrust
a) Coherent metatexite of interlayered pelitic migmatited and unmelted psammite. This is structurally below the diatexite in (b). b) Diatexite with an isolated, irregular psammite block.
Quartz vein, thrust Quartz vein, thrust
c) Protomylonite of unmelted psammite in contact with a more fertile layer with leucosomes with asymmetry indicative of thrusting to the west (right). Photo parallel to the lineation. d) Fold hinge in diatexite with a block of psammite. Three-dimensional outcrop showing stretching lineation parallel to fold axis.
ultramylonite ultramylonite
e) High strain zone with mylonite above a 1m thick zone dominated by ultramylonite. f) Detail of ultramylonite in (a), white bands are sheared pegmatite dykes.
strain variation ultramylonite
g) Variation in strain and amount of feldspar clasts in mylonite-ultramylonite zone. h) Narrow band of ultramylonite in centre of photograph, separating mylonites with different amounts of strain and apparent truncation of mylonitic foliation above.
folds and leucosomes folds and leucosomes
i) Folds with axial planar leucosomes. Plane of photo perpendicular to to stretching lineation. j) Folds with axial planar leucosomes. Plane of photo perpendicular to to stretching lineation.
folds and ultramylonite folds and ultramylonite
k) Folds capped by an ultramylonite band below pen (dip direction and dip 90/30 or 105/20) with same orientation as axial plane S= 90/30 (dip direction and dip) marked by slightly lighter oblique planes. Plane of photo perpendicular to stretching lineation. l) Ultramylonite band parallel to axial plane of fold.
folds and ultramylonite folds and ultramylonite
m) Train of W-verging folds close to Los Linderos in migmatites without anhydroud peritectic minerals and west of the main Guacha Corral Shear Zone (Conlara Complex). These too have axial planar leucosomes suggestive of anatexis during west-thrusting. n) Same.