Development of crystallographic preferred orientation and microstructure during plastic deformation of natural coarse‐grained quartz veins

Abstract

[1] The microstructure and crystallographic preferred orientation (CPO) of quartz were quantified in 17 samples of natural monomineralic tabular veins. The veins opened and were deformed, up to shear strain g > 15, in a small temperature window (about 25°C) above 500°C, as established by Ti-in-quartz thermometry. The veins filled a set of fractures within the Adamello tonalite (southern Alps, Italy) and localized homogeneous simple shear during postmagmatic cooling. The local (square millimeter scale) and bulk (square centimeter) CPO were investigated by computer-integrated polarization microscopy (CIP) and X-ray texture goniometry. Weakly deformed veins (WDV: g < 1) consist of millimeter-to centimeter-sized crystals with a strong CPO showing a c-axis girdle slightly inclined, mostly with the shear sense, to the foliation (XY) plane and a strong maximum close to the lineation (X). Moderately deformed veins (MDV: 2 < g < 3) consist of elongated nonrecrystallized ribbon grains and most have a CPO showing a strong Y maximum of c axes some with weak extension into a YZ girdle. Strongly deformed veins (SDV: g = 4 to 15) are pervasively to completely recrystallized to fine (34-40 mm grain size) aggregates with a strong CPO similar to that of MDV. The slip systems during plastic deformation were dominantly prism hai with subordinate rhomb and basal hai slip. Recrystallization occurred rather abruptly for 3 < g < 4. In contrast to dislocation creep experiments in quartz (and other minerals), a steady-state recrystallized fabric is achieved at early stages of deformation (g ≈ 4) as there is no evidence, with increasing strain, of strengthening of the CPO, of rotation of the fabric skeleton, or of change in grain size. WDV represent weakly deformed relicts of veins with an initial CPO believed to have developed during crystal growth but unsuitably oriented for prism hai slip during subsequent shear. MDV and SDV appear to derive from veins different from WDV, where the vein crystals grew with orientation favorable for prism hai slip. The relationship between the initial growth CPO and the kinematic framework suggests that veins opened at a temperature close to that at which there is a switch between the activity of prism hci and prism hai slip, with the temperature of growth causing growth of crystals well oriented for slip. The initial CPO of veins, from which quartz mylonites are commonly derived, plays a critical role in the fabric evolution. The strong growth-and strain-induced CPOs of these sheared veins inhibited significant reworking during lower temperature stages of pluton cooling when basal hai slip would have been dominant.

Brittle grain-size reduction of feldspar, phase mixing and strain localization in granitoids at mid-crustal conditions (Pernambuco shear zone, NE Brazil)

“…Overall, the microstructure and CPO suggest T around 500-550 • C (e.g. Stipp et al, 2002;Pennacchioni et al, 2010). This T estimate is somewhat lower than the estimate of Neves et al (2000).…”

Tertiary Pegmatite Dikes of the Central Alps

“…The CPO of the Bergell quartz mylonites is directly comparable with that determined for the quartz mylonites within the Adamello batholith (the largest batholith of the Oligocene Alpine intrusions along the Periadriatic Fault; Fig. 1a), which are also associated with mylonites having stable biotite and feldspar (oligoclase and K-feldspar) in the foliation (Pennacchioni 2005, Pennacchioni et al 2010. Similar mylonites developed under high-grade conditions were observed to occur in other plutons during post-magmatic cooling (e.g., Pennacchioni & Zucchi 2013), including the Novate intrusion (Ciancaleoni & Marquer 2006).…”

“…The {110} pole figure is also slightly asymmetric with the strongest <a> axis maxima rotated synthetically with the sense of shear with respect to the shear plane. This type of CPO is almost identical to the CPO of mylonitic quartz veins associated with mylonites within the Tertiary Adamello pluton (Pennacchioni et al 2010).…”

Transpression and tectonic exhumation in the Heimefrontfjella, western orogenic front of the East African/Antarctic Orogen, revealed by quartz textures of high strain domains

“…glide that was produced the submaximum close to Z (Jessel 1988). In naturally occurring mylonites and experimental studies, such complete girdle textures are associated with lower temperatures than textures with a single point maxima near Y (Stipp et al 2002;Pennacchioni et al 2010).…”