MINERALOGICAL PECULIARITIES OF MELANOCRATIC ENCLAVES IN THE BURGASY QUARTZ SYENITES (WEST TRANSBAIKALIA)

Patrusheva G.N., Tsygankov A.A.

Geological Institute SB RAS, Ulan-Ude, Russia, gerka_85@mail.ru

The mafic microgranular enclaves (ММЕ) often occur in rocks of granite composition, but their origin is different in each specific case. Such inclusions are more often considered as xenoliths of host rocks entrapped by magma just at the level of magmatic chamber formation or in the way of magma movement to the surface. At the same time, these can be fragments of substrate as well, from which acid magmas were melted; products of the same melt early crystallization (autoliths); and finally, dispersed «drops» of recrystallized basite melt that testify to the existence of acid and basite magma hearths (Popov, 1984). The latter one is believed particularly important, as the wide scale granitoid magmatism is impossible without additional thermal effect of mantle magmas.

The mafic inclusions in quartz syenites of the Burgasy massif that is localized in the watershed part of the Ulan-Burgasy Ridge have been studied by us. A particular attention was paid to mineralogical peculiarities that preserved the information on nature of inclusions (particularly, relic minerals) to more extent, as it seems to us, while their chemical composition suffered noticeable changes due to interaction with host rocks (melt).

The Burgasy massif near 100 km² by area consists of three intrusive phases (Reif et al., 1970): 1) subalkaline gabbros; monzogabbros, monzonites and syenites that compose two not large bodies about 4% intrusion surface by area; 2) middle-grained porphyry-like quartz syenites that compose the bulk pluton; 3) middle-grained alaskite granites and granite-porphyries that form two stocks 3.5 кm² by total area.

The mafic intrusions are mainly spread in quartz syenites of the second phase. Their amount varies from several inclusions to 15-20 ones per square meter. Sometimes, accumulations or swarms of inclusions, where the latter ones make up more than 50% of the total rock volume. MME shape is rounded, more rarely elongated or angular with rounded angles. The inclusions of complex “ameba-like” form occur even more rarely.

Two ultimate types associated by transition varieties are clearly distinguished within the inclusions: 1) numerically prevailing grey fine-grained amphibole-plagioclase rocks, not rarely porphyry-like (Pl or Ks) with hypidiomorph-grained microstructure; 2) brown fine grained, sometimes also porphyry-like inclusions with dolerite-like microstructure.

As was mentioned, the rock composition of the Burgasy massif varies from subalkaline gabbros and monzodiorites to leucocratic granites, comprising the range of silica acidity from 51.4 to 75.8 mass.% SiO₂. The mafic inclusions characterized by higher alkalinity (6.5-10.5 mass.%) and wide variations of silicic acidity (54-60 mass.%).

The rocks of each among the three intrusive phases form discrete fields that totally fall into the linear trends that are specific of the single magmatic melt products of differentiation. The variations of SiO₂ contents in MME are overlapped by monzonitoids of the first phase. However, the inclusions are sharply differed from the latter ones by the lower concentrations of TiO₂, FeO*, P₂O₅, Zr, Sr, Ba, Y, partially CaO and Nb, by higher aluminousity and Rb content. The initial composition of inclusions was most probably not preserved. The inclusions with the minimal content of silica ( 54 mass.%) and minimal concentrations of mobile microelements (LILE) are the most similar to it among the studied samples. On the whole, geochemical characteristics of such inclusions are similar to basalts of interpolate type.

Plagioclase, biotite and amphibole with te possible addition of pyroxene, potassium feldspar and quartz are the main rock forming minerals of MME. The accessory minerals are represented by magnetite, sphene, apatite, manganoilmenite (to 8 % MnO) and zircon.

Plagioclase and pyroxene appeared the most informative minerals to clarify the nature of inclusions. The plagioclase composition varies from albite to bytownite (75.4% An). The latter composes the fissure resorbed cores of zonal crystals surrounded by acid rim (22-25.2 % An). The large weakly zoned crystals of plagioclase that consist of main core (48-52 % An) occupying nearly 65-70% of grain area, and oligoclase rim (19-35 % An) are also widespread. Small idiomorphic grains mostly have oligoclase composition (22-27 % An) as well that is similar to the one of zonal crystal marginal parts. Thus, three generations of plagioclase can be distinguished: 1) resorbed cores of labradorium bytownite composition; 2) central parts of idiomorphic porphyry-like impregnations represented by andesine; 3) crystal rims of types1 and 2, and small idiomorphic grains of matrix (oligoclase). This data allow to suggest that the most basic plagioclase (labradorium-bytownite) represents intratelluric crystals being balanced with initial basalt melt. The crystallization of zonal impregnations likely occurred in already other conditions from the partially fractionated melt, and oligoclase of the main mass and rims of zonal grains results from effect of quartz-syenite melt, from which plagioclase of similar composition was crystallized.

Potassium feldspar in the inclusions is represented by large cribrate porphyry-like extracts (to15 mm long) with spotted-perthite structure. Plagioclase phase of perthites contains up to 22 % anorthite component. Porphyry-like impregnations Kfs from host quartz syenites are also characterized by irregular distribution of perthite ingrowths, but calcium lacks in them, and potassium phase contains to 3.5 % ВаО. In addition, interstitional Kfs are present in quartz syenites, where thin ingrowths of albite are subparallely oriented. This Kfs is differed by the lower content of albite component in bulk composition. In MME, such potassium feldspar is lackingт.

The monocline pyroxene is specific of of the first phase rocks and MME. It occurs extremely rarely as relics in hornblende of quartz syenites. Pyroxene forms idiomorphic tabular grains variously replaced by amphibole in monzogabbros and monzonites of the first phase. In the inclusions, pyroxene occurs as relics of irregular form within crystals of magnesial hornblende, or small (< 0.3mm) idiomorphic grains. At the same time, any varieties in composition have not been revealed. Pyroxenes of the first phase rocks and MME form the compact field on the border of salite and augite on the pyroxene tetrahedron.

In the rocks of the massif, including MME, amphibole is represented by the two main varieties:1) magnesial hornblende, sometimes containing relics of pyroxene and 2) actinolite developing on it.

Thus, the morphological peculiarities of ММЕ, their chemical composition, morphology and composition of ore forming minerals – al that indicates their formation from basite melt. Therefore, the mafic inclusions in quartz syenites of the Burgasy massif represent the product of magma (being contrast by composition) mechanical mixture.

The work was carried out with financial support of Russian Foundation for Basic research-Siberia (08-098017), RFBR-MNTI (06-05-72007), Integration projects of SB RAS N6.11 and 6.5.

References

Popov V.S. (1984) Mixture of magmas is important petrogenetic process // Notes of VMO. Issue 1, P.CXIII. p.229-236.

Reif F.G., Bazheev E.D., Logvinenko B.G. et al. (1970) Geological structure and deposits of the Kurba-Itantsy interfluve //Report of the Angyrskaya GSP. Ulan-Ude. 350p.

Tsygankov A.A., Patrusheva G.N. (2007) Mafic inclusions in quartz syenites of the Burgasy intrusion: composition, conditions of formation // Petrology of magmatic and metamorphic complexes. Issue 6. Tomsk. Izd-vo TSU. P.153-159.