Zmienność petrograficzna granitów masywu strzegomskiego

Abstract

PETROGRAPHIC DIVERSITY OF GRANITES OF THE STRZEGOM MASSIF (LOWER SILESIA)SummarySeveral kinds of granite rocks may be distinguished among the products of the main stage of consolidation of the Strzegom massif magmas (Fig 1). Their most characteristic features are summarised up in Table 1. The largely preponderant biotite granites are divided into the Kostrza and Chwałków types. The most characteristic features are summarised up in Table 1. The largely preponritic rocks containing dark, iron-rich biotite as the principal mafic constituent. Kostrza granites crystallised under quiet conditions out of a hot, largely liquefied and homogenous magma. The Chwałków granites, the biotite of which is a lighter, more magnesian variety, are much less uniform and several subtypes could be distinguished here. Their variability is connected with the primary heterogeneity of the original material and the limited extent of their metasomatic homogeneisation under anatectic mobilisation conditions. Chwałków magmas have never been strongly liquefied. Aboundant relicts of preanatectic minerals, especially plagioclases and biotites, are still discernible in those rocks. More or less conspicuous traces of late- and postcrystallisational deformations are characteristic for this rock-kind. In places they pass into true orthogneisses. The Graniczna sub-type granites (belonging to the Chwałków type) are rocks transitional in character between the Kostrza and Chwałków types. The two-mica granites are products of a special kind of endocontact alteration which affected varied subtypes of the Chwałków granites along their boundary with wall-rocks. The outermost zone of granite body has been tectonically deformed and undergot subsiquently late-magmatic microclinisation and muscovitisation. Their plagioclases have been also decalcified and homogeneised. All the types and subtypes of granite rocks, defined on the mineralogical-petrographic basis, within the Strzegom massif, have also their own mean quantitative mineral compositions and characteristic extent of their variability which are summarised in Tables 2 - 5 and ilustrated in Fig. 2 - 6. From the less important rock-kinds the fine-grained granites and medium-grained tonalites are also worth to be mentioned here. The fine-grained granites are older rocks, mechanically incorporrated and in part assimilated by the enclosing them Kostrza granites magma. Intense assimilation of those rocks yielded to the formation of strongly porphyritic kinds of the Kostrza granites. The sporadically encountered medium-grained tonalites may be considered to be unaltered relicts of a rock body which has been transformed in the Gola granite magma under granitisation and mobilisation processes.