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

摘要

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 .r > 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: r < 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 < -y < 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: -y = 4 to 15) are pervasively to completely recrystallized to fine (34-40 um grain size) aggregates with a strong CPO similar to that of MDV. The slip systems during plastic deformation were dominantly prism (a) with subordinate rhomb and basal (a) slip. Recrystallization occurred rather abruptly for 3 < r < 4. In contrast to dislocation creep experiments in quartz (and other minerals), a steady-state recrystallized fabric is achieved at early stages of deformation (y 7= 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 (a) 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 (a) 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 (c) and prism (a) 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 (a) slip would have been dominant.