Textural studies

The quartz texture of the SG1

The grain size maxima of quartz in the phases and varieties of the Schellerhau granite massif (Figs. 7.3 and 7.4) help to reconstruct the multiphase intrusion and crystallisation history of the pluton. We focussed in our study on SG1 and SG2 phases of the SGC since these are the only ones well exposed at the surface, whereas the afs-rich SG3 varieties are predominantly represented by a few drill cores and are mainly characterised by secondary (recrystallisation) structures which overprint and remove the primary ones.

Fig. 7.3 The phenocryst portion of textural varieties of the SG1 and SG2, illustrated by black-and-white prints of lacquer peels. Quartz appears black and feldspars grey or white. The phenocryst portion varies between 5 to 40%; the portion of 15% is most frequent and common.

Fig. 7.4 Grain size maxima, shapes and distributions of quartz crystals in the SG1 and SG2. A maximum in the grain size distribution in a magmatic rock indicates a burst of nucleation caused by a high degree of melt undercooling. The texture is contrasted by black-and-white prints of laquer peels. The quartz appears black and the feldspar grey or white. 1 - quartz phenocrysts (1^st generation); 2 - quartz microphenocrysts (2^nd generation);

3a - granophyric quartz in miaroles; 3b - microgranophyric quartz (3^rd generation); 3c - isometric aplitic quartz (3^rd generation); 3d - anhedral quartz (3^rd generation).

The phenocrysts of the early quartz generation (1^st generation) are large, 1.5 to 6 mm idiomorphic hexagonal dipyramidal crystals with hexagonal prism faces (β-quartz). The median grain size of SG1 quartz phenocrysts is 2.3 mm. The large feldspar phenocrysts (up to 2 cm long) are surrounded by microgranophyric intergrowths with quartz. The “typical” SG1 rocks contain a portion of ca. 15 vol.% phenocrysts (5% quartz + 10% feldspar, less than 1%

mica). Phenocryst portions below 12 vol.% and more than 18 vol.%, resp., are rarely represented and occur only in the southern part of the Schellerhau granite (Pöbelknochen and Seifenbusch area, Schilka and Baumann, 1996). The Pöbelknochen (elevation 833 m) represents the uppermost part of the Schellerhau granite massif in the studied vertical profile.

The matrix quartz, we call it later 3^rd generation, exhibits variable shape. The major portion of the granite is characterised by aplitic matrix quartz which contains small crystals (0.02 - 0.2 mm) that are anhedral and more or less isometric. They surround the small euhedral feldspar crystals as clusters or networks or occur as overgrowths on quartz phenocrysts. Crystals (< 0.1 mm) are often in granophyric intergrowths with feldspar. The matrix quartz is in a microgranophyric intergrowth with feldspar especially in the upper 30 m of the cupola region of the locality Pöbelknochen. This texture is superimposed by interconnected miarolitic texture (IMT, Candela and Blevin, 1995). The IMT consists of micropegmatitic miaroles that form 3-dimensional interconnected networks (Fig. 7.4). The size of the druses and miaroles ranges from 0.2 mm to pegmatitic cavities of several cm size. Below 800 m elevation the microgranophyric texture of the matrix quartz continuously changes into aplitic texture and the IMT disappears. In the deeper parts, the IMT occurs only rarely and is developed 2-dimensionally exclusively along early fractures that were filled by residual melt portions (locality Paradies-Fundgrube).

Supported by the small grain size of the miarolitic matrix, another quartz type could be detected at the Pöbelknochen locality. However in contrast to the phenocrysts, these microphenocrysts are euhedral, hexagonal dipyramids of only 0.1 to 0.5 mm size, and are homogeneously distributed within the matrix. They are mantled by matrix quartz, similar to the phenocrysts, and are classified as 2^nd quartz generation. The percentage of this frequent grain generation is difficult to determine. For example, the miarolitic SG1 rocks contain 5 to 7 vol.% of this 2^nd quartz generation. It may be assumed that this generation is represented also within the aplitic type. However, due to the small grain size it is not distinguishable from the aplitic grains in studies of the lacquer peels and thin sections.

Hydrothermal quartz is well distinguishable from the above described primary magmatic quartz. Based on postsolidus recrystallisation textures (metasomatic overprint and blasteses),

visible CL zonation, and dark red-brown luminescence, those metasomatic quartz textures formed by hydrothermal processes could be excluded from this study.

The quartz texture of the SG2

The SG2 exhibits a weak hiatalporphyritic texture (Figs. 7.3 and 7.4) that was described in the past as homophan (Pälchen, 1968) or equigranular (Schilka and Baumann, 1996). However, the grain size distribution shows a significant hiatus between the both maxima at 0.7 mm and 3 mm. The median grain size of the SG2 quartz phenocrysts (1^st generation) is 2.6 mm (the corresponding value in the SG1 for the 1^st quartz generation is 2.3 mm). The most frequent phenocryst portion amounts to 15 vol.% as in SG1 (5 % quartz + 10% feldspar). Variations in comparison to the “normal” type are situated only in the southern part of the granite massif near the locality of Seifenbusch. The mine dumps of the abandoned “Paradies-Fundgrube”

contain SG2 varieties with up to 35 vol.% phenocryst portions (quartz + feldspar), and others with less than 10 vol.%. The role of the 2^nd quartz generation may be similar to that in the SG1. The anhedral matrix quartz (3^rd generation) of the SG2 was feeding the irregular cavities between the subhedral to euhedral feldspar crystals and envelopes the quartz phenocrysts.

The quartz texture of the SG3

The textures of the granite phases succeeding SG1 and SG2, in the following summarized as SG3, are heterogeneous, aberant and show no or only few similarities to the previously described textural types (Fig. 7.5). Numerous leucocratic SG3 phases and varieties, hiatalporphyritic, seriate, equigranular and aplitic ones, are, in general, characterised by an alkali feldspar-rich matrix. Within the matrix composed of euhedral, microcrystalline albite plates (20 - 600 µm) occur micropoikilitic, amoebic and snowball-like quartz phenoblasts of different size (0.3 - 5 mm). These belong to magmatic-hydrothermal transition processes and form a 4^th quartz generation that is grown in a highly viscous crystal mush with a low portion of residual melt in coexistence with fluids exsolved from the melt. Albite has been clearly determined as the liquidus mineral as it forms inclusions in all the other rock-forming minerals. Small isolated K-feldspar phenocrysts have irregular grain boundaries. The quartz crystals contain numerous euhedral albite crystals that also typically occur in the matrix. The irregular grain boundaries of the quartz, surrounding the albite crystals of the matrix, support the idioblastic growth. Partially, the quartz grains show euhedral shape due to occurrence of idioblastic growth creating textures characteristic of highly fractionated Sn-bearing granites (Cobbing et al., 1992; Müller and Behr, 1997; Beskin et al., 1994).

Fig. 7.5 The quartz phenoblasts (4^th generation) of the SG3. a - Porphyroblastic texture of the SG3. Quartz is black and feldspar white; b - SEM-CL image of the idioblastic, micropoikilitic snowball quartz of the SG3. The CL and the growth textures are different from that of the phenocrysts and microphenocrysts. The zoning follows the shape of trigonal α-quartz (combination of the hexagonal prism m and the rhombohedrons r and z).