Abstrakt: | 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. |