Crystal fractionation in the petrogenesis of an alkali monzodiorite-syenite series: The Oshurkovo plutonic sheeted complex, Transbaikalia, Russia
Journal
Lithos
Journal Volume
64
Journal Issue
3-4
Pages
97-130
Date Issued
2002
Author(s)
Abstract
The Oshurkovo Complex is a plutonic sheeted complex which represents numerous successive magmatic injections into an expanding system of subparallel and subvertical fractures. It comprises a wide range of rock types including alkali monzodiorite, monzonite, plagioclase-bearing and alkali-feldspar syenites, in the proportion of about 70% mafic rocks to 30% syenite. We suggest that the variation within the complex originated mainly by fractional crystallization of a tephrite magma. The mafic rocks are considered as plutonic equivalents of lamprophyres. They exhibit a high abundance of ternary feldspar and apatite, the latter may attain 7-8 vol.% in monzodiorite. Ternary feldspar is also abundant in the syenites. The entire rock series is characterized by high Ba and Sr concentrations in the bulk rock samples (3000-7000 ppm) and in feldspars (up to 1 wt.%). The mafic magma had amphibole at the liquidus at 1010-1030 °C based on amphibole geothermometer. Temperatures as low as this were due to high H2O and P2O5 contents in the melt (up to 4-6 and ∼2 wt.%, respectively). Crystallization of the syenitic magmas began at about 850°C (based on ternary feldspar thermometry). The series was formed at an oxygen fugacity from the NNO to HM buffer, or even higher. The evolution of the alkali monzodiorite-syenite series by fractional crystallization of a tephritic magma is established on the basis of geological, mineralogical, geochemical and Sm-Nd and Rb-Sr isotope data. The geochemical modeling suggests that fractionation of amphibole with subordinate apatite from the tephrite magma leaves about 73 wt.% of the residual monzonite melt. Further extraction of amphibole and plagioclase with minor apatite and Fe-Ti oxides could bring to formation of a syenite residuum. Rb-Sr isotopic analyses of biotite, apatite and whole-rock samples constrain the minimum age of basic intrusions at ca. 130 Ma and that of cross-cutting granite pegmatites at ca. 120 Ma. Hence the entire evolution took place in an interval of ≤ 10 My. Initial 87Sr/86Sr ratios for the mafic rocks range from 0.70511 to 0.70514, and for syenites from 0.70525 to 0.70542. Initial εNd (130 Ma) values for mafic rocks vary from - 1.9 to - 2.4, and for syenites from - 2.9 to - 3.5. In a εNd(T) vs. (87Sr/86Sr)i diagram, all rock types of the complex fall in the enriched portion of the Mantle Array, suggesting their derivation from a metasomatized mantle source. However, the small but distinguishable difference in Sr and Nd isotopic composition between mafic rocks and syenites probably resulted from mild (10-20%) crustal contamination during differentiation. Large negative Nb anomalies are interpreted as a characteristic feature of the source region produced by Precambrian fluid metasomatism above a subduction zone rather than by crustal contamination. © 2002 Elsevier Science B.V. All rights reserved.
Type
journal article