P-T-t-D records of Early Palaeozoic Andean-type shortening of a hot active margin: The Dunhuang block in NW China

High-pressure (HP) granulites form either in the domain of the subducted plate during continental collision or in supra-subduction systems where the thermally softened upper plate is shortened and thickened. Such a discrepancy in tectonic setting can be evaluated by metamorphic pressure-temperature-time-deformation (P-T-t-D) paths. In the current study, P-T-t-D paths of Early Palaeozoic HP granulite facies rocks, in the form of metabasic lenses enclosed in migmatitic metapelite, from the Dunhuang block, NW China, are investigated in order to constrain the nature of the HP rocks and shed light on the geodynamic evolution of a modern hot orogenic system in an active margin setting. The rocks show a polyphase evolution characterized by (1) relics of horizontal or gently dipping fabric (S1) preserved in cores of granulite lenses and in garnet porphyroblasts, (2) a N-S trending sub-vertical fabric (S2) preserved in low-strain domains and (3) upright folds (F3) associated with a ubiquitous steep E-W striking axial planar foliation (S3). Garnet in the granulites preserves relics of a prograde mineral assemblage Mia equilibrated at similar to 11.5 kbar and similar to 770-780 degrees C, whereas the matrix granulite assemblage (M1b) from the S1 fabric attained peak pressure at similar to 13.5 kbar and similar to 850 degrees C. The granulites were overprinted at similar to 8-11 kbar and similar to 850-900 degrees C during crustal melting (M2) followed by partial re-equilibration (M3) at similar to 8 kbar and similar to 625 degrees C. A garnet Lu-Hf age of 421.6 +/- 1.2 Ma dates metamorphism M1, while a garnet Sm-Nd age of 385.3 +/- 4.0 Ma reflects M3 cooling of the granulites. The mineral assemblage, M1, of the host migmatitic metapelite formed at similar to 9-12.5 kbar and similar to 760-810 degrees C, partial melting and migmatization (M2) occurred at similar to 7 kbar and similar to 760 degrees C and re-equilibration (M3) at similar to 5-6 kbar and similar to 675 degrees C. A garnet Lu-Hf age of 409.7 +/- 2.3 Ma dates thermal climax (M2) and a garnet Sm-Nd age of 356 +/- 11 Ma constrains M3 for the migmatitic metapelites. The timing of this late phase is also bracketed by an emplacement age of syntectonic granite dated at c. 360 Ma. Decoupling of M1 and M2 P-T evolutions between the mafic granulites and migmatitic metapelites indicates their different positions in the crustal column, while the shared pressure-temperature (P-T) evolution M3 suggests formation of a melange-like association during the late stages of orogeny. The high-pressure event D1-M1 is interpreted as a result of Late Silurian-Early Devonian moderate crustal thickening of a thermally softened and thinned pre-orogenic crust. The high-temperature (HT) reequilibration D2-M2 is interpreted as a result of Mid-Devonian shortening of the previously thickened crust, possibly due to 'Andean-type' underthrusting. The D3-M3 event reflects Late Devonian supra-subduction shortening and continuous erosion of the sub-crustal lithosphere. This tectono-metamorphic sequence of events is explained by polyphased Andean-type deformation of a 'Cascadia-type' active margin, which corresponds to a supra-subduction tectonic switching paradigm.

Automated summary

In this study, the P-T-t-D paths of Early Palaeozoic HP granulite facies rocks from the Dunhuang block in northwest China were investigated to understand the tectonic setting and geodynamic evolution of a modern hot orogenic system. The results revealed the different tectonic environments and metamorphic evolution processes between the granulites and migmatitic metapelites, providing important insights into the dynamics of active margin settings.