Linking the P-T evolution of granulites and the geochemistry of ultra-K volcanics from Bohemian Massif and Central Tibet
Abstract
Variscan orogeny is traditionally compared to Himalayan-
Tibetan orogeny thanks to two main reasons: 1) Variscan orogen
originated through progressive amalgamation of Gondwana
derived blocks which formed belt that was subsequently squeezed
between two continental plates - Laurussia and Gondwana. Similar
sequence of events is characteristic for Tibetan-Himalayan orogeny
as well, where previously Gondwana accreted blocks were squeezed
between Asian continent and India. Therefore, both orogens result
from similar reorganization of plates in Paleozoic and Mesozoic-
Cenozoic times, which initiated rapid northward drift of Gondwana
derived blocks and their final collision with northern continental
masses. 2) This process resulted in both cases in formation orogenic
root of double-crust thickness characterized by long lasting
(HP) granulite facies metamorphism in lower crust followed by
isothermal decompression still at granulite facies conditions. In
addition, the granulites in central Tibet were transported by high
potassium lavas, with geochemical and isotopical signature corresponding
to Mg-K granitoids typically associated to HP granulites
in the eastern part of the Variscan belt (the Bohemian Massif in
particular). In both cases, the origin of granulitic lower crust and
associated high potassium lavas are attributed to relamination and
thermal maturation of lower crustal allochthon underthrust below
upper plate crust during collision. Based on above similarities
we argue that the Devonian - Carboniferous evolution of the Bohemian
Massif may be a proxy of Eocene to recent crustal dynamics
of the Tibetan system. To better quantify similarities between
both orogens, we present a set of numerical models which reproduce
the tectonic multistage scenario proposed for the Variscan
tectonics of Bohemian Massif. These models are subsequently
compared with P-T data acquired from the Himalaya and southernTibetan
granulites and xenoliths from central Tibet.