Crust-forming processes in the Hercynides of the Central Asian Foldbelt

Abstract

The paper reports data on the evolutionary history of magmatism, its conditions, and sources in the process of the development of the Southern Mongolian Hercynides during the pre-accretion, continental-mar- gin, and rifting stages within the time span from the Silurian to Early Permian. The Hercynian continental crust in the southern Mongolian segment of the Central Asian Foldbelt (CAFB) was determined to have grown in the environment of ensimatic island arcs, backarc basins, spreading centers, and oceanic islands or plateaus, with material coming from the depleted and, perhaps, also enriched mantle sources in the open ocean that surrounded the Siberian paleocontinent on the side of the Caledonian margin. This made it possible to recognize the Early- Middle Paleozoic epoch of juvenile crustal growth in CAFB and the corresponding isotopic crustal province with a total area of more than 200 thousand km 2 . The principal differences between the composition and struc- ture of the blocks surrounding the Hercynian regions (Caledonides in the Gobi Altai and Grenwillides in the South Gobi microcontinent) testify that the southern margin of the Caledonian Siberian continent and the Gren- villides of the South Gobi microcontinent had different geological histories and were spatially separated. The structural complex of the Paleoasian ocean, including the terranes of the South Gobi microcontinent, were transformed into a continental block in the latest Devonian-earliest Carboniferous, in relation with accretion processes, folding, metamorphism, and tectonic delamination along the boundaries of structurally heteroge- neous domains. The subsequent recycling of the crust by magmatic processes was related to the development of an active continental margin (ACM). The development of an ACM in the Hercynides resulted from and was a continuation of the motions of the continental and oceanic lithospheric plates, i.e., processes that brought about the Hercynian accretion. The evolution history of the ACM was subdivided into two stages: early (a con- tinental-margin stage proper) and late (rifting stage). The rocks of the early stage were produced at 350-330 Ma and compose a differentiated basalt-andesite-rhyodacite complex and related massifs of the granodiorite-pla- giogranite and banatite (diorite-monzonite-granodiorite) associations. During the rifting stage at 320-290 Ma, a bimodal basalt-comendite-trachyrhyolite association was formed, along with accompanying alkali granite massifs. In the southern Mongolian segment of the Hercynides, the rocks of the rifting stage compose two sub- parallel rift zones: Gobi-Tien Shan, which extends along the boundaries of the South Gobi microcontinent, and the Main Mongolian lineament, which marks the boundaries between the Hercynides and Caledonides in the CAFB. The rift structures are made up of alkali granitoids and normal-alkalinity granitoids, which are atypical of rift zones. Their genesis is thought to have been related to crustal anatexis, a process that was triggered by rift-related magmas at an unusual combination of rifting and ACM tectonic setting. The basic rocks of the rift associations have geochemical signatures atypical of continental rifting. They show Ta and Nb minima and K and Pb maxima, as is typical of rocks generated at convergent plate boundaries. Nevertheless, the broad varia- tions in the concentrations and ratios of some major and incompatible trace elements and in the Sr, Nd, and O isotopic composition of the rift basaltoids allowed us to distinguish their high- and low-Ti varieties, which were produced with the participation of three mantle sources: depleted mantle similar to the source of basalts in mid- oceanic ridges, enriched mantle like the source of basalts in oceanic islands, and the mantle material of the metasomatized mantle wedge. The origin of andesites in the rift zones is explained by the contamination of mantle basaltoid melts with sialic (predominantly sedimentary) material of the continental crust or the assimi- lation of anatectic partial granite melts.