Tectonic position of mingling dykes in accretion-collision system of early Caledonides of West Sangilen (South-East Tuva, Russia)

Abstract

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 South-East 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 oxygonal chips of igneous and host metamorphic rocks, vein pegmatoid intrusions, and composite dykes of the reticulate-cuspate texture with the dominant basic-rock component. The mingling dykes classified in the first group intruded when the Erzin and Kokmolgarga shear zones were formed at the early stage of the tectonic-magmatic evolution of the Sangilen orogen (510–490 Ma). Intrusions of the basic-rock melts were accompanied by the formation of relatively large massifs of the basic composition, i.e. the Erzin and Bayankol gabbro-monzodiorite massifs, as well as by the occurrence of composite dykes that are abundant in the area between the Erzin and Naryn rivers. In the second stage (460–430 Ma), the composite dykes occurred when the orogen was subject to extension along the system of tectonic zones, the Bashkymugur gabbro-monzodiorite massif was emplaced, and fracture-vein structures, including the dykes, were formed.