A.N. Zanvilevich

Petrogenesis of syenite-granite suites from the Bryansky Complex (Transbaikalia, Russia): Implications for the origin of A-type granitoid magmas

Abstract Two syenite–granite suites, metaluminous and peralkaline, that form the Bryansky Complex in Transbaikalia, Russia, have been studied with the aim to constrain the existing models of A-type granitoid magma generation. The Bryansky Complex is a large intrusive body of about 1600 km2 emplaced in the central part of the Mongolian–Transbaikalian granitoid belt, which extends for more than 2000 km and is 200–300 km wide. The Belt comprises about 350 A-type granitoid plutons and numerous volcanic fields. U–Pb and Rb–Sr isotope dating revealed that all the intrusive rocks of the Complex and closely associated comendites were emplaced within a narrow time span, 279–285 Ma. The isotope characteristics are rather similar for all main rock types. The metaluminous suite has a (87Sr/86Sr)T value of 0.7050±0.001, eNd(T) from −1.9 to −3.0, and the peralkaline suite has (87Sr/86Sr)T=0.7053±0.0008, eNd(T)=−2.1 and −2.4. Comendites and trachyandesites have similar eNd(T) values (from −2.2 to −3.5), but a slightly higher (87Sr/86Sr)T value of 0.7062±0.0002. The systematic change in chemical and mineralogical composition from syenitic to granitic rocks in both suites and the similar isotopic ratios suggest that the granites were formed by fractional crystallization of the syenite magmas. Several lines of evidence suggest that metaluminous syenite is the parental magma for the whole Bryansky Complex. Study of melt inclusions in quartz phenocrysts from the peralkaline granite and in pyroxene from the nordmarkite indicates that fractional crystallization has resulted in significant F enrichment in the granitic magma (up to 1.5–1.7 wt.%). The syenite magmas crystallized at rather high temperature >940 °C whereas the near-liquidus temperature of the peralkaline granite was lower, 760–790 °C. Very high homogenization temperatures of the melt inclusions in quartz phenocrysts from comendites (1000–1100 °C) suggest that the alkali-rich silicic magma formed at a depth of 50–60 km (?) far exceeding the normal crust thickness. The Sr–Nd isotope data advocate the main role of mantle-derived material in the source region from which the alkali-rich syenitic and granitic magmas were produced.

Crystal fractionation in the petrogenesis of an alkali monzodiorite–syenite series: the Oshurkovo plutonic sheeted complex, Transbaikalia, Russia

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 eNd (130 Ma) values for mafic rocks vary from −1.9 to −2.4, and for syenites from −2.9 to −3.5. In a eNd(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 compositions 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.

Geochemical evolution of Phanerozoic magmatism in Transbaikalia, East Asia: A key constraint on the origin of K‐rich silicic magmas and the process of cratonization

During the Phanerozoic, granitoid magmatism occurred on a vast scale in Transbaikalia and Mongolia in east central Asia. Within this huge and geologically complex region, many hundreds of individual plutons of syenite, quartz syenite, quartz monzonite, and K-rich granite have been intruded within NE-SW oriented belts 200–400 km wide and 2000–3000 km long. Five compositionally distinct, major stages of magmatism are distinguished: (1) Ordovician-Silurian, (2) Devonian, (3) Early Permian I, (4) Early Permian II, and (5) Permo-Triassic. Granites and syenites occur within each suite, but the proportion of syenite increases with time, as does the alkalinity of all magmas. Synplutonic mafic rocks are associated with plutons of all five suites and mafic/felsic magma-mixing textures are common. Laser fluorination oxygen isotope analysis (Sharp, 1990) of titanite from granitoids indicates that within and among plutons of a given suite 18O/16O is fairly constant. This indicates that these magmas probably formed with a high degree of isotopic (and chemical) homogeneity. However, there is a progressive decrease in δ18O of titanite with time from about +6.5 in the earliest suite to about +1.5 in the youngest suite (corresponding to a decrease in magmatic δ18O from ∼+11 to ∼+6). The systematic evolution of geochemical and isotopic composition with time allows us to develop a model explaining the progressive increase in alkalinity (especially K2O) and decrease in δ18O over 250 m.y. of plutonism. These constraints may be satisfied by progressive hybridization of K-rich (alkali) basalt magmas with crustally derived silicic melts, accompanied by fractional crystallization of K-poor phases such as plagioclase and pyroxene (cf. Barker et al., 1975). Such a mechanism could form large volumes of syenitic residual liquids, having the appropriate isotopic composition. Younger suites were in part derived through remelting and hybridization of material formed or intruded during earlier magmatic episodes, leading to a progressive increase in overall alkalinity and decrease in δ18O. The repeated cycles of magmatism involved significant anorogenic crustal growth and a gradual cratonization of the continental crust of East Asia, caused by multiple melting and remelting events and associated large influxes of mantle-derived alkalic mafic magma.

Genesis of Alkaline and Peralkaline Syenite-Granite Series: The Kharitonovo Pluton (Transbaikalia, Russia)

The Kharitonovo pluton is located in the central part of the large Mongolian-Transbaikalian province of Late Paleozoic alkaline syenites and granites. The province stretches for a distance of almost 2000 km and contains over 350 plutons. The Kharitonovo pluton occupies an area of

Peralkaline granitoid magmatism in the Mongolian–Transbaikalian Belt: Evolution, petrogenesis and tectonic significance

230 km.^{2}

A Stable Isotope Study of Anorogenic Magmatism in East Central Asia

It is made up of A-type granitoids forming two successive alkaline and peralkaline syenite-granite series; syenites largely predominate over granites. Rocks of both series are characterized by abundant mesoperthitic feldspar (about 90 vol % in syenites, more than 60% in granites) while plagioclase is almost absent. In the peralkaline series mafic minerals are riebeckite-arfvedsonite, kataphorite and aegirine with secondary annitic biotite. In the alkaline series edenite and iron-rich biotite are the major mafic minerals. In the alkaline series coeval mafic rocks commonly occur in the form of synplutonic composite dikes; they indicate at least two stages of mafic magma injection into the silicic magma chamber. Major and trace element data are used to test various petrogenetic models for the origin of the syenites and granites. Of the alternatives considered, it is most likely that each series resulted from crystal fractionation of syenite magma. Mass balance calculations suggest that in the early alkaline series, crystal fractionation was probably combined with mixing of felsic and mafic magmas, a conclusion also supported by field evidence. The peralkaline series syenite parental magma could have been produced by partial melting (about 20%) of the earlier alkaline syenites. In this respect alkaline syenites can be regarded as parental rocks for the whole pluton. We suggest that the alkaline syenite magma originated via two possible petrogenetic schemes: (1) partial melting of deeply buried crustal rocks or (2) crystal fractionation of hybrid melt produced by mixing of subalkaline basaltic magma (80%) with about 20% of silicic lower crustal melt. In either case some additional input of potassium and possibly other incompatible elements is required in order to achieve the observed composition.

Petrogenesis of A-type granites and origin of vertical zoning in the Katharina pluton, Gebel Mussa (Mt. Moses) area, Sinai, Egypt

The Katherina Ring Complex (KRC) in the central Sinai Peninsula, Egypt, was formed in three consecutive stages: volcanic, subvolcanic, and plutonic. The outer Katherina ring dike, about 30 km in diameter, marks the contour of a paleocaldera. Volcanic ignimbrite extrusions representing the earliest stage of the KRC were followed by emplacement of subvolcanic peralkaline microgranite bodies. The ring dikes are composed mainly of porphyritic quartz monzonite and plagioclase-rich quartz syenite, with less abundant alkali feldspar quartz syenite and peralkaline granite. The central alkaline granite pluton (ca. 210 km2) was emplaced at ∼595 Ma. The quartz monzonite–syenite group is characterized by positive Eu anomalies (Eu/Eu* = 1.1-1.6), which is consistent with its enrichment in accumulated plagioclase crystals (xenocrysts). These features, along with the positive εNd(T) values in quartz monzonite (up to +5.6) suggest that the initial silicic magma was hybridized by plagiclase-rich mafic magma. Mineral geothermometry and melt inclusion studies point to the formation of the silicic magmas at high temperatures, up to 900 to 1000 °C. Oxygen and Sr-Nd isotope data suggest that the source of the magmas was moderately depleted mantle or young juvenile crust. The trend of compositional change from quartz alkali feldspar syenite to alkali feldspar and peralkaline granite is consistent with a fractional crystallization model. A specific feature of the magma differentiation process is that the residual melt separation could occur when the magma was ∼55 percent crystallized (“rigid percolation threshold”) and clusters of crystals formed a rigid skeleton in the magma. At this stage, residual melt flowed pervasively through the pore space. The suggested model of melt separation alleviates the problem of a differentiation process since it does not require crystal settling that seems unrealistic in highly viscous silicic magmas at shallow depth. Chemical and Nd isotopic distinctions between the leucocratic volcanic–subvolcanic rocks (εNd(T) = 4.2-4.6) and the Katherina pluton granite (2.6-3.9) suggest that the silicic magmas of the volcanic-subvolcanic and the plutonic stages were probably produced from different mantle-derived sources.