Geology Reference
In-Depth Information
Plate 38 Recrystallized Limestones and Metamorphic Carbonates (Marbles)
The original depositional microfacies and early diagenetic textures of limestones are often altered (-> 1) or
destroyed by recrystallization and processes acting during burial diagenesis and thermal history in relation to the
ups and downs of the rocks.
Recrystallized carbonate rocks
(-> 2-3) should not be disregarded in facies analyses, even though deposi-
tional criteria are obliterated or completely destroyed. Recrystallization depends on clay content (> 2 % may
inhibit recrystallization) and can create microspar (see Pl. 6/2) and pseudospar, the latter with crystals measuring
tens to even hundreds of microns in diameter. Burial diagenesis and changes in temperature, pressure, and
volatile content will favor the formation of carbonate rocks with crystal sizes up to several tens of millimeters.
Some of these rocks already correspond to marbles, showing a fine- to coarse-grained granoblastic texture.
Marble
(-> 4-8; Sect. 7.9) is a metamorphic rock consisting predominantly of fine- to coarse-grained, dy-
namically recrystallized calcite and/or dolomite, usually with a granoblastic, crystalline texture. Differentiation
of marbles in thin section requires the description of mineralogical composition (carbonate and non-carbonate
minerals), grain shape, fabric (size and mutual relations of crystals), deformation features like frequency, bend-
ing and orientation of twin lamellae with respect to a reference plane (-> 4), and granulation textures (caused by
mechanical breakage along gliding planes).
Recrystallized limestones
1
Tectonically elongated fusulinid foraminifera (
Quasifusulina
). Low-grade tectonic deformation can remodel the shape of
fossils. The tests were originally deposited parallel to each other and later stretched along their length axis. Fossils of
known geometric parameters deformed in this way are important tools in reconstructing shear processes. Permian: Bükk
Mountains, northern Hungary.
2
Neomorphized limestone.
Note the differences in crystal size and preservation of primarily calcitic micrite clasts (MC)
and the aragonitic dasyclad alga (DA). The skeleton of the alga has been replaced by calcite. Arrows point to twin
lamellae developed within the segmented algal skeleton. The former micritic matrix is altered by aggrading recrystalliza-
tion. Early Carboniferous: Anhui, southern China.
3
Neomorphized recrystallized limestone.
Dark areas are relicts of former micritic grains (1). Former cement crystals in
interparticle (2) and intraparticle pores suffered aggrading neomorphism, indicated by the coarsening of calcite crystals.
The outline of the structure marked by (3) suggests a foraminiferal shell whose central part has been replaced by coarse
calcite. Common diagnostic features of the neomorphic sparite (pseudospar) are crystals of conspicuously different sizes,
predominantly curved interfaces between crystals, and large crystals in association with smaller crystals. Early Carbonif-
erous: Anhui, southern China.
Marbles
4 Inequigranular coarsely crystalline calcite marble with silicate impurities. Xenotopic and poikilotopic fabric (larger cal-
cite crystals enclose smaller silicate mineral crystals: black arrows). White arrows point to an enfacial junction (a joining
point of three intercrystalline boundaries where one of the three angles is 180°). The frequency of this type of triple
junctions is used in distinguishing neomorphic and metamorphic spar and carbonate cement. Note the abundance of thin
twin lamellae indicating deformation at relatively low temperatures (Ferrill 1998). As shown by the varying directions of
the twin lamellae, and strong differences in grain size, the marble is a protomylonite, caused by multiple deformations.
Paleozoic: Roman quarry Tentschach, Carinthia, Austria.
5 Inequigranular medium to coarsely crystalline marble, a product of intensive strain recrystallization. Xenotopic fabric.
Note smaller crystals (size about 0.16 mm, arrows) between larger crystals (maximum size 1.0 mm), indicating grain
diminution. Marble used in Roman buildings. Hergla, Gulf of Hammamet, eastern Tunisia.
6 Equigranular finely crystalline marble consisting of anhedral calcite grains, probably resulting from cataclastic granula-
tion. Arrow points to a former shell now consisting of twinned and bent calcite crystals. Twinning is often indicated by a
conspicuous strippy pattern. Ghosts of fossils (especially shells, echinoderms) may be preserved even in highly metamor-
phic carbonates. Jurassic (Chemtou marble): Roman quarries, Chemtou, Tunisia.
7 Inequigranular coarsely to very coarsely crystalline
marble
. Xenotopic fabric consisting of calcite crystals exhibiting no
crystal faces bounding the mineral grains (anhedral crystal shape). Size between 0.9 and 2.5 mm. Note different directions
of twin lamellae indicating dynamic recrystallization. Mesozoic marble: Sterzing, South Tyrol, northern Italy.
8 Same thin section as -> 7, crossed nicols. Note extensive twin development. Twin planes are closely spaced. The black
arrow points to multiple set twin lamellae, the white arrow to bent and displaced twin lamellae.
-> 1: Kahler 1988