Dykes composed of basic rocks and granite are formed due to interactions between melts in a wide range of conditions, from contrasting compositions and fluid saturation rates to various tectonic settings and processes at different depths. Textures and petrochemical characteristics of the dykes are thus widely variable. This paper is focused on composite dykes observed in the West Sangilen region in SouthEast Tuva, Russia. The Sangilen wedge is a fragment of the Early Caledonian orogenic structure of the Tuva-Mongolia Massif which evolved in a succession of geodynamic settings, from collision (transpression, 570-480 Ma) to transform faulting (transtension, 480-430 Ma). Intensive tectonic deformation facilitated massive basic-rock and granite magmatism at various layers of the crust and associated heating and metamorphism of the rocks (510-460 Ma). Basic-rock-granite composite dykes were formed in the above-mentioned period in various tectonic settings that controlled conditions of dyke intrusions and their compositions. We distinguish two groups of composite dykes observed on two sites, in the area between the Erzin and Naryn rivers and on the right bank of the Erzin river (Strelka and Erzin Sites, respectively) (Fig. 1). The dykes in both groups originated from one and the same basic-rock melt source. However, mingling of the contrasting melts was carried out by different mechanisms as suggested by the proposed intrusion models. In the area between the Erzin and Naryn rivers (Strelka Site), the host rock of the composite dykes is granite of the Nizhneerzin massif. The mingling dykes are composed of amphibole gabbro and monzogabbro, granosyenite and twofeldspar granite. Contacts between basic and felsic rocks vary from smooth contrasting to complex 'lacerated' flameshaped, and gradual transition zones are present (Fig. 6). The dykes were formed at mesoabyssal or abyssal depths, and the subliquidus heat regime was thus maintained for a long time, and even the smallest portions of the basic-rock melt were consolidated through quite a long period of time. As a consequence, indicators of deformation are lacking in the composite dykes, while transition zones and hybridization are present. On the right bank of the Erzin river (Ersin Site), the dykes cut through migmatite-granite of the Erzin formation in the same-name tectonic zone. Contacts with host rocks are transverse. Melanocratic rocks are represented by smallgrained diorite and quartz diorite, and the felsic composite dykes are composed of medium-and small-grained twofeldspar granite and leukogranite. Transition zones, hornfelsing and contact alterations are absent at contacts of all the types (Fig. 8). The composite dykes of this type intruded and emplaced when the shear zone was subject to extension and fragmentation, which predetermined active intrusion of basic and, possibly, felsic melts through conjugated faults. Crystallization of the melts was rapid, and their potential heat impact on the adjoining rocks was thus excluded, as evidenced by the presence of ...
The paper presents 39Ar/40Ar and UPb (SHRIMP zircon method) geochronological data on minor picrodolerite intrusions from western Mongolia. Picrodolerite magmatism in western Mongolia took place within different age intervals and in different geodynamic settings: accretionary–collisional Ꞓ1-2, ~510 Ma (Üüreg Nuur association, Hayrhan intrusion), intraplate D1, 410–390 Ma (Tsagaan Shuvuut Range, Mor’t Uula intrusion), intraplate D3–C1, 345–360 Ma (Altan Gadas, Tavtyn Hundiyn, and Hu Tsan Bulak intrusions), island-arc C2, 315–335 Ma (Dzahuy, Yaryn Had, and Javhlant intrusions), and intraplate P1, ~270 Ma (Dzaraa Uula intrusion). Petrographic, mineralogical, and geochemical data permit distinguishing derivates of picrobasaltic (12–14 wt.% MgO) (Üüreg Nuur association, Tsagaan Shuvuut, Javhlant, Yaryn Had) and melabasaltic melts (7–10 wt.% MgO) (Mor’t Uula, Altan Gadas, Dzahuy, Dzaraa Uula) among picrodolerite magmatism in western Mongolia. Picritoids in these associations resulted from early olivine fractionation. The Early Devonian picrites and picrodolerites of the Tsagaan Shuvuut Range and the olivine dolerites of the Mor’t Uula intrusion can be assigned to mafic magmatism in the Devonian large igneous province (LIP) in the North Mongolian megablock. The Late Devonian–Early Carboniferous picrodolerite intrusions in the Baruun Huuray zone (Altan Gadas) and Mongolian Altay (Tavtyn Hundiyn) are related to the Tien Shan LIP. Bimodal volcanism on the southern margin of the Hangayn Mts. (Dzaraa Uula), in the eastern segment of the Hanhöhiy Ridge (Dzagday Nuur, Hara Teg), and in the Argalantu trough (Tegshiyn Gol, Muhur Shurgah, and Deed Shurgah massifs) may be related to the Tarim LIP, where they are part of an Early Permian volcanoplutonic association. Carboniferous picrodolerite intrusions in the South Mongolian megablock and the Trans-Altai Gobi formed in subduction-related settings (Dzahuy, Yaryn Had, Javhlant).
We consider the structure, composition, and age of Early Paleozoic intrusive associations of the Bumbat-Hairhan area located in the central part of the island-arc system of the Lake Zone, western Mongolia. There are large polychronous plutons (Bumbat-Hairhan and Gundguzin) and small intrusions in this area. We have established that they are composed of granitoid and gabbroid associations of different lithology, ages, and geodynamic nature. The results of U-Pb dating show that granitoid and basic magmatism took place within 551–465 Ma. Two main stages of intrusive magmatism are recognized, which reflect two stages of the geodynamic evolution of the region: island-arc (551–524 Ma) and accretion–collision (511–465 Ma). At all stages, predominant rocks in the region were diorite–tonalite–plagiogranite and plagiogranite associations of calc-alkaline series.
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