Enklawy w granicie Tatr Wysokich

Abstract

Enclaves are typical objects found commonly in the granitoid plutons. Different genetical types of enclaves are used as indicators of granitoid magma origin, while their mineralogical-petrological characteristics are thought to be the important carriers of information about the dynamics of the magma chamber, interaction with the envelope rocks, of different chemical and genetic characteristics. Commonly the enclaves are the only indicators of changes in temperature and pressure in the magma chamber, as well as the fluctuations of oxygen fugacity and activity of volatiles. In case of lacking envelope rocks or their restricted presence in the field, the enclaves, especially metamorphic rocks xenoliths, are the key for understanding the character of the metamorphic envelope to the intrusion and processes acting during intrusion. The presence of enclaves in the High Tatra granite was stated in the beginning of the XX century, but their investigations were very restricted. The present author investigations allowed to subdivide the following genetical types of enclaves: xenoliths of metamorphic rocks (including both metapelitic and calc-silicate rocks), surmicaceous rocks, two types of schlieren, enclaves of quartz diorites (hybrid mafic precursors), mafic microgranular enclaves, enclave of apatite melasyenite, enclaves of fine-grained leucogranites. All these enclaves were found inside two petrographical types of High Tatra granite: biotite monzogranite and porphyritic granite. Seven petrographical varieties of metapelitic xenoliths and 6 petrographical varieties of calc-silicate rocks were found in the xenoliths type. The records of metamorphic conditions (P-T paths) in the separate petrographical varieties of rocks differ from each other. Such a diversification is a result of a different metamorphic history of blocks coming from different levels of lithosphere and different time length of their interaction with granitoid magma. Some of the xenoliths, so called stoped bloks, are the evidence of the magmatic stoping process — multistage intrusion of the granitoid magma inside the envelope rocks, causing the crushing both the metamorphic rocks and the products of partial solidification of the earliest granite pulses. Xenoliths documenting that stage of development were sinking in the granitoid magma in the opposite direction to the magma intrusion; some of them — especially the big ones — could explode, enabling the physical contamination of the granitoid magma and formation of the A-type schlieren. Internal parts of such xenoliths conserve both the pre-intrusive mineral assemblages and the age of the pre-intrusive metamorphism (368 Ma) concordant with the migmatization age, revealed in the metamorphic envelope of the Western Tatra Mountains. Xenoliths acted also as the resisters for the flowing magma, causing the lost of its heat and in that way increasing the crystallization rate, lowering diffusion rate and increasing the magma viscosity. As a consequence, the presence of xenoliths could have catalyzed the formation of magmatic layering (B-type schlieren). Enclaves of leucogranites, similar to alaskites found in the Western Tatra Mountains and surmicaceous enclaves were also assumed to be xenoliths. The chemical assimilation of the xenoliths (and country rocks themselves) was very limited. Enclaves of hybrid quartz diorites (341 Ma), are interpreted as mafic precursors for the High Tatra granite magma. Their chemical and isotopic composition pointed out the lower crustal origin. The cumulate origin was suggested for the ultrapotassic apatite melasyenite, containing over 13 vol.% of apatite. Magmatic processes leading to the origin of that rock were related to magma mixing and mingling, in the large time interval 361—345 Ma. The youngest — probably concurrent with granite — are the mafic microgranular enclaves. Small portions of the mafic magma showed the mineral composition and textures typical of mixing/mingling processes acting between magmas differing in chemistry and origin. Simultaneously, their presence could explain the relatively high temperatures of metamorphism (exceeding 800°C) found in some xenoliths. The best explanation for the presence of different types of mafic enclaves in the High Tatra granite is a model of slab break-off. According to that model in a place of the disruption of the subducting southern plate at the depth 48—50 km the place for the mantle plum was formed. The presence of the mantle plum could explain the melting of the upper mantle/lower crust portion and formation of the parent magma for the quartz diorites. Further intensive heat flow could cause melting of the upper crust and as a consequence the formation of the granite magma at depth 20—25 km, intruding further on to the level 12—15 km. The remnants of the mantle-derived component of the granitoid magma are the mafic hybrid portions, which gave rise to the formation of mafic microgranular enclaves. Strong decompression, both predating granitoid magmatism and acting during the granite intrusion is one of typical features of collision-subduction process, which led to lithosphere thickening, slab break-off and sinking of broken slab into the mantle. The influence of the magma mixing/mingling processes to the present-day state of the High Tatra granitoids allows understanding the petrological and geophysical problems, pointing out the complex petrological character of the Tatra granitoid rocks.