Magma mixing

Mongolia is just south of the Precambrian Siberian Craton, which is now overlain by relatively undeformed early Paleozoic miogeoclinal sedimentary rocks. Areas of Precambrian basement have been mapped in Mongolia, but they are only minor in extent. The continental crust of Mongolia is primarily composed of deformed and variably metamorphosed subduction complexes and continental fragments, accreted to the Siberian Craton during the Paleozoic. These low-grade metasedimentary and metavolcanic rocks were intruded by granitic plutonic complexes of Cambrian to Triassic age, which formed widespread batholiths, especially in central Mongolia.

Crust assembly was complete by the late Permian. Subsequent to modest erosion, conglomerates and other continental sedimentary rocks were deposited in Mesozoic rift basins. Mesozoic sedimentary sequences can be greater than 1000 meters thick and are in angular unconformable contact with the underlying Paleozoic metamorphic and igneous rocks. The famous dinosaur localities of the Gobi Desert are in fluvial deposits of this age. By the early Tertiary, Mongolia was stable and had been eroded to a region of low relief.

From the middle Tertiary to the present, central Mongolia has experienced regional uplift and incision by streams to produce uplands of high elevation and low relief, with altitudes as high as 3000 meters (Khangai Mountains). The region is drained by large rivers in deep alluvial valleys 1-2 km wide. We spent many evenings speculating on the origin of the wide alluvial valleys occupied by the rivers near which we camped, but could never agree on a good explanation of their origin in what otherwise appeared to be a regionally uplifted area.

Along with the uplift, north-south oriented rift zones have developed, locally associated with mafic volcanic activity and thick, extensive, valley-filling basalt flows of Neogene to Quaternary age. The regional uplift, extension, and mafic volcanic activity suggest a mantle plume beneath Mongolia (Windley and Allen, 1993). In western and southern Mongolia, transpression along active strike-slip faults has produced rising, linear mountain ranges and adjacent subsiding basins, usually with internal drainage. Strike-slipfaulting is likely related to indenter tectonics associated with the India-Asia collision to the south.

Although Mongolia has experienced alpine glaciation during Quaternary glacial periods, glacial landforms and deposits are not well developed and lower elevation stream terraces, though present, are not extensive.

The GSA Mongolian Geotrip primarily traversed the area to the north of the Khangai Mountains. We passed through regions of deformed metasedimentary and metavolcanic rocks of Paleozoic age and saw large, well-exposed Paleozoic and early Mesozoic granitic intrusive complexes, which are eroded to highly scenic massifs. We observed the Paleozoic/Mesozoic unconformity along the Selenge River and spent two nights camped on Quaternary basalt flows which had been cut into dramatic river canyons. We spent two nights at Lake Khovsgol, which fills an active rift valley associated with the Lake Baikal rift system of southern Siberia.

- Greg Holden

Bibliography

Komatsu, G., J. M. Dohm, and T. M. Hare, 2004. Hydrogeologic processes of large-scale tectonomagmatic complexes in Mongolia--southern Siberia and on Mars. Geology, v. 32, p. 325-328.

Windley, B.F. and M. B. Allen, 1993. Mongolian plateau: Evidence for a late Cenozoic mantle plume under central Asia. Geology, v.21, p. 295-298.

Dergunov, A. B., ed., 2001. Tectonics, magmatism, and Metallogeny of Mongolia. Routledge, London, 288 p.