| Author |
Lingzhu Kong, Yingde Jiang, Karel Schulmann, Jian Zhang, Roberto Weinberg, Min Sun, Sheng Wang, Tan Shu, Jun Ning
|
| Abstract |
Structural and metamorphic effects of a Devonian extension-shortening cycle in the NW
Chinese Altai were investigated using combined structural-petrological analysis and zircon and monazite U-Pb
geochronology. Structural observations revealed a ubiquitous, sub-horizontal metamorphic fabric (S1), which
was originally sub-parallel with horizontal bedding (S0) of supracrustal Devonian volcanic-sedimentary basins.
This metamorphic fabric is defined by a sillimanite-bearing migmatitic gneissosity associated with extensional
shear bands and a cordierite-bearing schistosity in the orogenic lower and middle crust, respectively.
Staurolite relics preserved in the S1 fabric are interpreted as relics of older, higher pressure Barrovian-type
metamorphism. These structural-metamorphic features are correlated with decompression and heating
and interpreted as reflecting a crustal-scale extensional phase. S1 was folded to form N-S-oriented upright
antiforms cored by migmatites and locally transposed by steep S2 foliation during a ∼W-E-directed shortening
(in current coordinates). Andalusite and cordierite overgrew the migmatitic S2 foliation attesting to decreasing
pressure associated with vertical extrusion of migmatites in cores of F2 antiforms. New U-Pb age data from
subhorizontally foliated migmatites suggest that the D1 extension started at least before ∼410 Ma. Age data of
syn-D2 granite intrusions and dykes constrain the D2 shortening starting soon after D1 extension and ending
at ∼378 Ma. Combined with regional data, cycles of extensional and contractional tectonic regimes can be
defined, which also show eastward migration along-strike of the Altai range. Such extension-shortening cycles
could result from alternating retreating and advancing subduction, which governed the evolution of the Altai
accretionary system from the Ordovician until the Carboniferous.
|
