GEODYNAMICS AND GRANITOID MAGMATISM OF COLLISION OROGENS

Abstract

The Pamir-Himalayan and Central Asian intracontinental fold belts developed to follow different geodynamic scenarios. In the first case, it was an ultimate version of hard collision, when cratons directly interacted with the Early Precambrian crust and thick lithosphere mantle. The second case was a soft collision, which came to an end in Late Paleozoic-Early Mesozoic time, without reaching the stage when the Siberian, Sino-Korean, and Tarim cratons would be struck. Direct geophysical observations cannot be used for ancient epochs; therefore, an indicator of the thickness of lithosphere of colliding plates and microplates are Sr-Nd isotope characteristics of granitoid batholiths, which display a direct dependence on the average composition and age of the crust, thus indirectly indicating the thickness of a genetically related underlying lithosphere mantle. The upper mantle dynamics has been analyzed for different stages of collision tectogenesis. It has been concluded that at the moment of inversion (the beginning of the early collision stage), a slab is detached and an asthenosphere inlier appears in, the vicinity of the future collision building immediately beneath the Moho discontinuity. As a result, short-term anomalous temperature gradients appear in the lower crust, large-scale melting occurs, and bimodal volcanic series form, which, on the one hand, still retain suprasubduction geochemical labels but, on the other hand, reflect the composition of the lower crust subjected to advanced melting. Then the collision tectogenesis follows the classical scenario of the thickening of the crust and its lithosphere root, covering the end of the early orogeny stage and the late orogeny stage. The time of formation and extent of an orogen depend on the thickness of colliding plates, and the composition of granitoid batholiths is directly correlated with the composition of the geologic environment. The relationship with the mantle, if any, is expressed in specific forms, e.g., in the form of rifts in the orogen foreland during a head-on collision or in the form of feathering strike-slip-pull-apart faults during a tangential collision. The dynamics of development of collision orogens radically changes at the postcollision (taphrogenic) stage. With a greater thickness of the lithosphere root (Pamirs-Himalayas), the density instability causes the lithosphere delamination, and asthenosphere flows move beneath the Moho, thus causing a drastic rise in relief, followed by the orogen's collapse. With a smaller thickness of the lithosphere (collisions of the arc-arc, arc-seamount, arc-microcontinent, and other kinds), there is no delamination, and the orogen's break is due only to gravitation landslides and detachments in the crust. Central Asia is unique for the presence of a large lower mantle plume. Therefore, the processes of the seemingly classical "soft" collision actually led there to the initiation of local plumes beneath folded orogens. A model is proposed for the induced plumes that permits the formation of giant granitoid batholiths or their source areas at the postcollision stage, as well as their specific composition combining plume and collision characteristics.