The Bazar shear zone
Structure and kinematics
Along the NW-SE branch of the unit, close to the Bazar village, a new ductile high-strain zone has been identified: The Bazar shear zone (BSZ). The thickness of the shear zone is about 600 m from the contact with the Monte Castelo unit (Fig. 2A, B). Within the shear zone, amphibolites are the most common lithology, but some lenses of phyllonitic metasediments and retrogressed ultramafics have been identified. A penetrative foliation (Sm) and lineation (Lm) are developed across the zone, defined by mineral segregation of amphibole, plagioclase and epidote into fine layers. Preferred orientation of amphiboles define a penetrative linear fabric with a general N-S trend (Lm= 7/27N; Fig 2C).
Kinematic criteria are abundant within the shear zone, and show a consistent top - to - the S sense of shear. Shear bands (SC’C’’), asymmetric folds and σ- porphyroclasts, are common structures (Passchier and Trouw, 1996). We have analyzed shear band fabrics (SC’; Lister & Snoke 1984; Blenkinsop & Treloar 1995) to obtain flow vectors (LSC’). This is calculated plotting the S- and C’-plane intersection vectors into a stereoplot, and rotating 90º the intersection vectors within the S planes (Fig. 2D; Gómez Barreiro et al., 2010b). Average LSC’ flow vector is 172/32N (Fig. 2C).
Figure 2. Geological context of the study
(A) Geological map and section (B) of a representative sector of the Bazar shear zone (BSZ). Average fabric (S, Lm, Lsc) data are represented. See the text for a discussion of tectonic contacts. (C) The geometrical calculation of a LSC’ vector is explained (see Gómez Barreiro et al. 2010 for a discussion).
Petrology
There are different types of metabasites in the Bazar shear zone. Mylonitic metabasites are the distinct product of the shear zone. Beside, relics of undeformed or barely deformed metabasites (metagabbros and high temperature amphibolites) could be found at different scales within the mylonites. Those relics correlate well with metabasites types outside the shear zone: Metagabbros and HT-amphibolites.
Undeformed metabasites are more granoblastic, and could preserve gabbroic textures (Fig. 3A, B), mineral abbreviations after Kretz (1983). Brown amphibole (am1), rutile/ilmenite, and plagioclase define the primary mineral assemblage. The am1 shows cloudy inclusions of opaque minerals, interpreted as Fe-Ti exolutions related to the retrogression of pyroxene (Díaz García, 1990). Apart from igneous relics, plagioclase-amphibole mixtures and load-bearing frameworks are the most common microstructural configurations in these rocks (LBF, Handy, 1994). Retrogression under the shear zone conditions is also found, with epidote and green/blue amphibole (am2) as the main secondary phases defining a rough foliation (Fig. 3B).
Figure 3. Principal metabasite types across the Bazar shear zone
Representative photographs under parallel (upper row) and cross polar (lower row) of principal metabasite types across the Bazar shear zone, including mylonite protholiths (A and B) and mylonites. (A) Metagabbro relics with igneous texture partially preserved are found. Igneous phases are not preserved in these samples, and only amphibolite facies assemblage related to HT event, identified in the Bazar unit, and a partial retrogression under mylonitic conditions are found. (B) HT-amphibolite partially retrogressed under mylonitic conditions, in which a shape fabric is recognized. (C) Mylonites of the Bazar shear zone are characterized by an heterogeneous deformation, with the development of protomylonitic to ultramylonitic fabric. Segregation of phases led to the development of mylonitic layers. Sigmoidal prophyroclasts and C’ shear bands show a consistend top-to-the S sense of shear. Porphyroclasts of protholith brown amphibole (am1) are preserved in protomylonitic domains, with abundant microcracks, sharp and straight grain boundaries and evidences of dissolution-precipitation creep. Synkinematic blue/green amphibole (am2) grows at the strain shadows of am1 clasts. See the text for a detailed discussion. Abbreviations: am1 : prekinematic brown amphibole; am2: blue/green synkinematic amphibole, Pl: plagioclase, Ilm: ilmenite, Ttn: titanite, Rt: rutile, Ep: epidote, Qtz: quartz.
Mylonitic metabasites depict a strong nematoblastic fabric and segregation of phases into amphibole and plagioclase/epidote – rich layers (Fig. 3C). Most of amphiboles here show a distinct green/blue color (am2), and plagioclase appears mixed with epidote grains in the matrix. Ti-phase are ilmenite and titanite which lies along the mylonitic foliation. ilmenite is found as inclusions in titanite grains in the most deformed volumes.
Deformation is highly heterogeneous at all scales. It is commonly observed in mylonitic metabasites the existence of two microstructural domains:
1) Protomylonitic domains, where inherited features from undeformed metabasites (Fig. 3 A, B) are preserved, including brownish green amphibole porphyroclasts, and relics of igneous plagioclase porphyroclasts partially transformed into the plagioclase-epidote matrix.
2) Mylonitic/ultramylonitic domains show a strong grain-size reduction, where green/blue amphibole (am2), epidote, quartz and plagioclase are the main constituents, typically segregated into compositional bands. Chlorite is scarce and could be located along some shear bands and late microfractures.
Main mineral phases have been identified and characterized in previous works (e.g. Díaz García, 1990). Microprobe analysis of amphibole and plagioclase were recalculated from Díaz García (1990) (Table VII-7), assuming, in the case of amphiboles, total cations equal to 13, except Ca, Na and K (Leake 1978; Spear and Kimball 1984; Leake et al. 2004; Yavuz 2007). It is found that amphiboles, am1 and am2, are calcic amphiboles with terms varying among magnesiohastingsite and tschermakite, but pargasite, magnesio-hornblende and edenite terms are also identified. Common plagioclase corresponds to albite (An=3.2%; An % range= 7-0.5%).
We have applied semi-quantitative pressure indicators like the Al content in hornblende (e.g. Anderson and Smith 1995) and Ti in Ca-amphibole (e.g. Ernst & Liu, 1998), that suggests a retrograde trend from am1 (~7.5 kbar), to am2 (~6 kbar), and a thermal range between 600º - 500ºC (~ 0.67 TiO2; ~11.85 Al2O3). Variation in colour from brown, in am1 (core), to blue, in am2 (rim), in prophyroclasts (Fig. 3) could be also related to a reduction in Ti content across the crystal, and hence with a decrease in temperature (e.g. Leake, 1965, Stokes et al., 2012).
After those observations a qualitative evolution of the assemblages could be suggested:
(1) Ca-rich plagioclase + Ti-rich brown amphibole (am1) + rutile/ilmenite + quartz
(2) albite + Ti-poor blue/green amphibole (am2) + epidote + ilmenite/titanite + chlorite + quartz
The evolution from (1) to (2) is coherent with a transition from amphibolite to greenschists facies, in a broad sense (Spear, 1993).