Page 129 - Microtectonics

118 5 · Shear Zones
The mineral composition of inclusions in pseudo- Box 5.1 Misidentification of pseudotachylyte
tachylyte is commonly disproportional to the mineral
composition of the wall rock; quartz and to a lesser ex- Some dark cataclasites and layers or veins filled with dark
minerals such as chlorite or tourmaline resemble pseudo-
tent feldspar are common as inclusions, while Fe-Mg rich
tachylyte in the field and even in thin section. Pseudotachy-
Al-silicates are under-represented (Figs. 5.4a, 5.5, 5.6; lyte differs from these rocks by (1) the sharp boundaries with
Maddock 1986; Maddock et al. 1987; Lin 1994). Micas are the wall rock; (2) the occurrence of injection veins; (3) evi-
rarely present as inclusions. Quartz fragments have an- dence for melting such as a relative scarceness of micas,
gular outlines with numerous internal fractures and fluid pyroxene and hornblende as inclusions in the matrix and the
corrosion of such minerals along vein contacts; (4) presence
inclusion planes while fragments of feldspar, hornblende
of spherulites and devitrification structures and (5) the ab-
or pyroxene tend to be rounded. Where the contact of sence of contemporaneous quartz or calcite veins. Most pseu-
pseudotachylyte and wall rock is a straight fracture, as dotachylyte has a chemical composition almost identical to
along main fault veins, embayments of pseudotachylyte the host rock, while other veins or cataclasite zones will usu-
may exist where micas or amphiboles were in contact ally show a different composition.
with the pseudotachylyte matrix. The matrix of a pseu-
dotachylyte differs from that of cataclasite or breccia in of feldspar, biotite, amphibole or orthopyroxene known
that the smallest size fragments are lacking and isolated as microlites (Lofgren 1971a,b; Toyoshima 1990; Macau-
fragments are contained in a relatively homogeneous dière et al. 1985; Magloughlin 1992; Di Toro and Pen-
matrix (cf. Figs. 5.3, 5.4a, 5.5, 5.6). All these features are nacchioni 2004). Microlites may occur as simple acicular
attributed to preferential dissolution of Fe-Mg Al silicates grains, as skeletal and dendritic shapes, or be arranged
and feldspar in the pseudotachylyte melt (O’Hara 1992; into spherulites (Lofgren 1974; Doherty 1980; Clarke 1990;
Lin 1994). Another typical microstructure of pseudo- Lin 1994, 1998; Di Toro and Pennacchioni 2004). There
tachylyte is the presence of aggregates of small sulphide may be a sequence of increasingly complex shape from
particles in larger quartz fragments. These sulphide drop- acicular, skeletal, dendritic to spherulitic from margin
lets may have formed from a sulphide bearing melt (Mag- to core of a pseudotachylyte vein, which may be due to
loughlin 1992). differences in cooling rate (Lin 1994, 1998). Spherulites
Microstructural and experimental evidence suggests of biotite or feldspar are commonly nucleated on inclu-
that most pseudotachylyte forms through an initial stage sions. Microlites in pseudotachylyte are commonly pow-
of cataclasis so that the melt is actually formed from the dered by fine magnetite grains (Maddock 1998).
crushed rock rather than from the intact wall rock Melting temperature of pseudotachylytes is hard to
(Swanson 1992; Spray 1995; Ray 1999; Fabbri et al. 2000; determine since they do not form by equilibrium melt-
Ray 2004). In some pseudotachylyte main fault veins, ing. Information can be obtained from microlite compo-
cataclasite occurs in isolated pockets along the contact sition, or from the presence or absence of certain miner-
with the wall rock, but not in injection veins (Ermanovics als with different melting points in the matrix, between
et al. 1972; Killick et al. 1988; Magloughlin 1989, 1992; 550–650 °C for micas, through 1100–1500 °C for feldspars
Curewitz and Karson 1999). Hydrated ferromagnesian and pyroxenes, to 1700 °C for dry quartz (Toyoshima 1990;
minerals were preferentially fragmented into a fine- Lin 1994). Estimates on melting temperatures using these
grained cataclasite groundmass with included larger tools range from 750 °C to exceeding 1700 °C. (Wallace
fragments of feldspar and quartz (Allen 1979); subse- 1976; Sibson 1975; Maddock et al. 1987; O’Hara 1992; Lin
quent melting preferentially incorporates this ground- and Shimamoto 1998). Many of the microstructures ob-
mass, leaving clasts of quartz and feldspar, but may also served above have been mimicked in experimental gen-
partly melt these remaining clasts (Magloughlin 1989; eration of pseudotachylyte (Spray 1987, 1988, 1995; Lin
Maddock 1992). There may even be transitional fault rock and Shimamoto 1998).
types from cataclasite (without melt) through catacla-
5.3 site with some melt to pseudotachylyte, which form by 5.3
increasing strain rate and heat production in the fault Mylonite
zone (Spray 1995). Although pseudotachylyte does not
form in porous rocks with a pore fluid, there is chemical 5.3.1
evidence that a minor amount of fluid was present in the Introduction
pre-pseudotachylyte cataclasite phase, which was incor-
porated in the melt (Magloughlin 1992). A mylonite is a foliated and usually lineated rock that
Devitrification features (Lofgren 1971a,b) or struc- shows evidence for strong ductile deformation and nor-
tures formed by growth from a melt are common in the mally contains fabric elements with monoclinic shape
matrix of pseudotachylytes (Maddock 1986; Maddock symmetry (Figs. 5.2, 5.7–5.9, ×Photos 5.8, 5.9a,b; Bell and
et al. 1987; Lin 1994). They are similar to those observed Etheridge 1973; Hobbs et al. 1976; White et al. 1980; Tullis
in obsidian and consist of idiomorphic acicular grains et al. 1982; Hanmer and Passchier 1991). Mylonite is a