ИСТИНА

The formation and evolution of mantle magmas remain a topical problem in geochemistry and petrology. During the past decade, a considerable contribution to this problem was related to the investigation of melt inclusions in minerals, which provided insight into the composition of the parental melts of igneous rocks. Owing to these investigations, extensive new evidence was obtained on the compositions and evolution of melts from subduction zones, island arcs, active continental margins, ocean spreading zones, and mantle plumes generating ocean islands (Portnyagin, Magakyan et al., 1996; Kamenetsky, Crawford et al., 1998). Much less is known about the conditions of generation and compositions of melts related to continental intraplate complexes. In particular, there are only a few determinations for the Cenozoic complexes of Central Asia. This contribution reports the results of an investigation of the primary magmas of basic rocks composing a number of “lava rivers” of considerable dimension in the periphery of the Southern Baikal volcanic belt (SBVB). The lavas of these rivers were very mobile, which allowed their flow for distances up to 175 km from the eruption center (Yarmolyuk, Lebedev et al., 2001; Yarmolyuk, Kozlovskii. et al., 2004). The compositions of these magmas and conditions of their crystallization are of special interest for the characterization of the sources of intraplate magmatism, which has occurred widely in the Central Asia Fold Belt. The SBVB comprises several fields of Late Cenozoic basalts at the southwestern coast of L. Baikal over an area of 350 x 450 km. The volcanic rocks are basalts, trachybasalts, and, occasionally, basanites and basaltic trachyandesites. The magmatic activity of this area was dominated by fissure eruptions of basalts. Crystalline and primary melt inclusions were found in olivine phenocrysts from the basalts. Their compositions correspond to the forsterite mole fraction Fo81-60. The crystalline inclusions are spinel, ilmenite, and titanomagnetite. In terms of chemical composition, the spinel inclusions are chrome spinels with up to 26.2 wt % Cr2O3 and from 3.1 to 4.3 wt % TiO2. The ilmenite inclusions contain up to 45 wt % FeO and 53 wt % TiO2. The titanomagnetite shows high TiO2 contents (26.5 wt %). The primary melt inclusions are randomly distributed in the host mineral, have rounded or elliptical shapes, and vary from 20 to 40 µm in size. The inclusions are partly crystallized and contain residual glass, daughter minerals, and a gas phase. Pyroxene, amphibole, plagioclase, and spinel were identified among the daughter minerals. The daughter clinopyroxene has low SiO2 (39.8 – 41.9 wt %) and very high Al2O3 contents (11.0 – 14.3 wt %). In addition, it is rich in TiO2 (3.6 – 6.5 wt %) and P2O5 (up to 0.9 – 1.2 wt %). Based on the chemical composition, it can be classified as titanaugite. The daughter amphibole contains up to 55 wt % SiO2, up to 20 wt % Al2O3, up to 2.5 wt % K2O, and 2.5 wt % TiO2. The total of components in the amphibole analyses is 1.5-3.0 wt % lower than 100 wt %, which is probably due to the presence of water. The composition of daughter amphibole approaches that of pargasite. The daughter plagioclase of melt inclusions contains 52-55 wt % SiO2, 27-29 wt % Al2O3, 9-11 wt % СаО, and 5-6 wt % Na2O, which corresponds to the labradorite composition. Spinel occurs usually as tiny acicular grains, and its total volume fraction in the inclusions is negligible. Its composition corresponds to chrome spinel with up to 23 wt % Cr2O3. The residual glasses of partly crystallized melt inclusions showed high contents of SiO2 (60 – 66 wt %), Al2O3 (21 – 25 wt %), TiO2 (up to 2 wt %), Na2O (2.7 – 5.3 wt %), K2O (2.5 – 3.5 wt %), and P2O5 (1.2 – 1.9 wt %) and low MgO (0.3-0.4 wt %) and СаО (1.3-2.0 wt %). During heating experiments, the first signs of melting of daughter minerals were observed within the melt inclusions at 1060–1080оС. The extensive melting of daughter phases was observed at temperatures above 1100оС. Clinopyroxene was the last daughter mineral to dissolve at 1160-1170оС. The inclusions were completely homogenized at 1170-1190оС. The compositions of glasses from the homogenized melt inclusions are basaltic with 46.4-49.5 wt % SiO2 and high alkali contents, Na2O + K2O from 3.1 to 8.2 wt %. The Na2O content (2.5 – 5.5 wt % %) is significantly higher than that of K2O (0.7 – 2.7 wt %). Compared with basaltic melts from continental rifts of other regions, the melts in the inclusions are enriched in TiO2 (cf. up to 1.7 and 3.5 wt %, respectively) and P2O5 (up to 2 wt %). The obtained compositional characteristics of melt inclusions are consistent with those observed in similar rocks from other lava rivers that were formed at the margin of the Southern Baikal volcanic belt within the last 3 Ma (Naumov, Portnyagin et al., 2003). The ion microprobe analysis of melt inclusion glasses showed water contents of no higher than a few hundredths of weight percent, which is consistent with the high homogenization temperatures (1170-1190ºC). This result indicates the relatively anhydrous composition of primary magmas for the Dzhida basalts. The contents of other volatile components in the melts are also very low: 0.05 wt % F, 0.03 wt % Cl, and 0.07-0.09 wt % S. Based on the analysis of glasses in the homogenized melt inclusions, two groups of melts were distinguished, with 5.1-7.5 wt % Na2O + K2O (group I) and 3.1-4.7 wt % Na2O + K2O (group II). These groups are also different in Al2O3 and FeO contents. The contents of Al2O3 range from 15.3 to 19.2 wt % in group I and from 13.6 to 15.9 wt % in group II. FeO is 7.8 – 10.0 wt % for group I and 11.1 – 13.4 wt % for group II. Noteworthy are considerable variations in trace element contents in the melt inclusions. There are melts with trace-element characteristics similar to those of the host rocks (group I) and melts with much lower trace element contents (group II). Both the rocks and melt inclusions of the two groups show similar trace element distribution patterns normalized to the primitive mantle composition. The highest differences in trace element contents between group I and II inclusions were observed for Ва (on average, 300 and 130 ppm, respectively), Sr (790 and 350 ppm), Zr (160 and 76 ppm), Nb (40 and 17 ppm), Y (19 and 8 ppm) and V (125 and 55 ppm). The average contents of total REE determined by SIMS are 122 ppm for group I melts, which is similar to a bulk-rock value of 116 ppm, and only 53 ppm for group II melts. The normalized [La/Yb]N values are 8.5-9.1 for all melts from the inclusions and 13.5 for the compositions of rocks. It was shown that the group I melts are similar to ocean island basalts, whereas the group II melts are distinctly depleted in all incompatible trace elements. The glasses of melt inclusions with the lowest contents of trace elements can be regarded as the most primitive melts trapped by olivine during early stages of its crystallization, whereas the melts enriched in incompatible trace elements are more evolved and probably correspond to later stages of rock crystallization. The role of crystallization differentiation in the formation of the Dzhida basalts was evaluated from the behavior of trace elements in the melt inclusions depending on variations in Nb and FeO content in the melts as a function of mg-number. It was found that the majority of trace elements (Zr, Ta, Ti, U, Th, Hf, Y, V, Сu, and REE) are directly correlated with Nb. As can be seen from the FeO–mg-number diagram, the compositions of all melt inclusions also form a single trend with a negative correlation between these parameters, which is probably controlled by the fractionation of olivine and minor pyroxene. There is a decrease in FeO content between mg-values of 0.4 and 0.25, which is explained by the fractionation of oxide phases, spinel, titanomagnetite, and ilmenite. The established regular variations in the contents of various major components and trace elements in melts depending on mg-number and Nb content suggest an important role of crystallization differentiation in the formation of the rocks.