Porphyroclast
Porphyroclasts are remnants of relatively large, mostly rounded, resistant crystals in metamorphic rocks . They differ in their grain size by more than an order of magnitude from the constituents of the surrounding, recrystallized matrix . The layers of foliation within the matrix usually gird themselves around the rheologically more resistant porphyroclasts and are deflected by them in their course.
etymology
The term porphyroclast is a word creation derived from the Greek terms πορφύρα porphyra (purple) and κλαστός klastós (broken). What is meant here is the feldspar color in a porphyry . Klastós goes back to κλάω kláo (to break). A clast generally represents a rock fragment, as occurs with sediments.
Constituent minerals
Porphyroclasts usually consist of the minerals feldspar ( alkali feldspar and plagioclase ), garnet , staurolite , tourmaline , sphen , muscovite , hornblende , pyroxene ( orthopyroxene ), dolomite and only rarely quartz .
Occurrence
Porphyroclasts occur in cataclasites and in mylonites formed from granitoids and gneisses .
Typology
In general, porphyroclasts can be separated into simple and complex forms. Simply structured porphyroclasts are single crystals of the minerals listed above, subordinate to crystal aggregates. Most mica fish are simply structured porphyroclasts. The complex porphyroclasts (also referred to as the porphyroclast system, English porphyroclast system ) include the coated porphyroclasts (English mantled porphyroclasts ). Their fine-grained mantle consists of minerals of the same species as the clast.
Mica fish
Covered porphyroclasts
The encased porphyroclasts can be divided into five different types:
- Θ-type
- Φ-type
- σ-type
- δ-type
- complex objects
The Θ-type (theta-Klast) has as the only no wings (English wings ), all other types have wings on. The coat of the Φ-type (Phi-Klast) is characterized by orthorhombic symmetry, the coats of all other types are monoclinic in shape. The mantle of the σ-type (sigma-clast) is very wide directly on the porphyroclast and then ends in two pointed wings. The wings are not on the same level, but are offset from one another like a staircase (English stair-stepping ). In the orthorhombic Φ-type with a slightly thinner coat, the two wings are in the middle at the same height. The jacket of the δ-type (delta-clast) is also quite thin; but the two wings are also twisted. They either run out in the middle or show stair-stepping. The complexly structured objects are similar to the δ-type, but have another pair of wings on the porphyroclast that is reminiscent of the σ-type in its design. They too end either in the middle or offset like a staircase.
Sheathed porphyroclasts usually consist of feldspar in a quartz-feldspar-mica matrix, more rarely of orthopyroxene in peridotites or dolomite in a calcite matrix.
The encased porphyroclasts are considered to be the result of crystal-plastic deformations. In response to the shear movements in the surrounding matrix, dislocation tangles collect in the edges of the porphyroclasts . This has the effect that the edge area of the porphyroclast then recrystallizes as a core-and-mantle structure . The fine-grained and rheologically soft mantle area is finally pulled out into wings (or tails) on both sides of the porphyroclast, which regulate themselves parallel to the mylonitic shape fabric . While the core of the porphyroclast remains rigid or recrystallizes further at its edges and thus shrinks, the expansion and change in shape of the wings continues. The design of the wings can be used as an indicator of the shear sense and as a measure of the rheological properties.
Others
Small boudins can also act as porphyroclasts.
Emergence
Porphyroclasts arise due to the rheological difference in the rock components during deformation. Relatively hard , resistant minerals develop into porphyroclasts, whereas rheologically soft components are sheared and become part of the fine-grained matrix. Matrix minerals react plastically even at lower temperatures, while the porphyroclasts are still deformed in a fracture manner. In the case of quartz, for example, ductile deformation begins at around 270 ° C, whereas feldspar only becomes plastic at around 400–500 ° C. Quite similar with the mica. Biotite behaves ductile from 250 ° C, while muscovite is still stable and is deformed into mica fish.
Porphyroclasts do not always form automatically in the same minerals, since pressure-temperature conditions during deformation and the initial grain size also play a very decisive role. In contrast to porphyroblasts , which grow anew in the course of the metamorphosis , porphyroblasts are, so to speak, fossils of the original rock structure and can provide valuable information on its composition.
use
As already mentioned, the specific geometry of sheathed porphyroclasts enables a statement to be made about the prevailing sense of shear within a shear zone in many cases . Because of their internal asymmetry and their offset wings, good shear sense indicators are therefore sigma clasts, delta clasts with stair-stepping and complex objects with stair-stepping.
Individual evidence
- ↑ SH White: The role of dislocation processes during tectonic deformation with special reference to quartz . Ed .: RJ Strens, The physics and chemistry of minerals and rocks. Wiley, London 1976, pp. 75-91 .
- ^ Cees W. Passchier and Carol Simpson: Porphyroclast systems as kinematic indicators . In: Journal of Structural Geology . tape 8 , 1986, pp. 831-844 .
- ↑ Cees W. Passchier and Rudolph AJ Trouw: Microtectonics . Springer Science & Business Media, 2005, ISBN 978-3-540-29359-0 , pp. 132-141 .