Schlieren are fragile, usually elongate concentrations of mafic material. The origin of schlieren are not always clear, but may be produced by differential magma flow or disaggregation of xenoliths, among other mechanisms. Schlieren are unusual in the Halifax Pluton, and also have rather unusual shapes if my limited experience is any guide. There are several different shape types at this stop, all of which may be related because many seem to be connected on the outcrop and because they are all in close proximity on an outcrop surface only 50 m across. In this small area the regional flow foliation defined by large K-feldspar phenocrysts is disrupted, and K-feldspar-rich clusters are common. Phenocrysts are generally absent from the schlieren at this locality, but biotite is strongly aligned in the schlieren planes. The suggestions in the guidebook for the origin of these schlieren include shear flow around sinking xenoliths or rising bubbles (large ones). For more on these see the arocknid home page.
This is schlieren B in the field trip guidebook. It consists of a biotite-rich mantle with granodiorite inside and outside. The whole schlieren is shaped like a tube or a tight fold, exposed along a shallow angle of section. This schlieren margin is only ~1 cm thick. which is exposed over a considerable width in the left-hand end.
Closeup of the margin of the schlieren seen above. There are several fine-grained zones along the schlieren margin that are comparatively fine-grained.
Photograph of schlieren F in the field trip guidebook. The prominent schlieren define a structure rather like the hinge region of an isoclinal fold. However, complex though less prominent schlieren extend beyond the 'hinge region' some distance into the surrounding rock.
Closeup of the upper left side of the schlieren in the photo above, showing dark, biotite-rich prominent part of the schlieren curving to the right, with thinner, less prominent biotite -rich streaks extending upwards. Note that the K-feldspar phenocrysts are approximately parallel to the schlieren margin.
Spidery "arocknid", E in the field trip guidebook. It is composed of two sprays of thin schlieren that seem to emanate from a single schlieren line that crosses the wet (and dark) pit in the lower center of the photo.
Thick portion of schlieren C in the field trip guidebook. Note the irregular convex surface of the schlieren, the parallel alignment of K-feldspar phenocrysts near the concave surface, and the K-feldspar phenocrysts in the host granodiorite that are nearly perpendicular to the convex margin of the schlieren (top center).
K-feldspar-rich mass in normal foliated granodiorite ~100 m south of the schlieren outcrop, shown above. Part of the view is of wet rock (to the right of the crack) and the rest is dry. These concentrations are common in many areas.
Fine-grained rock inclusion, also ~100 m south of the arocknid outcrop. The trip leader said that there are no mafic inclusions in the entire pluton, and that the dark color of this rock is the result of being finer grained and perhaps slightly more biotite-rich than the surrounding granodiorite. This inclusion obviously has large crystals and crystal clumps that look remarkably like the host granodiorite. That suggests that this inclusion was originally emplaced as a pillow lava into the granodiorite magma chamber, and engulfed crystals in the surrounding magma during emplacement. Alternatively, this inclusion was a liquid at some point similar to the host granodiorite and already crystallizing phenocrysts of its own when it was quenched. In either case, the fact that this inclusion has retained its textural identity suggests to me that it is indeed more mafic than the host granodiorite.
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Schenectady, NY 12308 U.S.A. |