Devonian Andean-type convergence in the southern Dunhuang block (NW China): Petro-structural, metamorphic P–T and geochronological constraints

Abstract

Archean to Palaeoproterozoic basement rocks exposed in the Dunhuang block in NW China were affected by Palaeozoic crustal reworking, as constrained by previous zircon U–Pb geochronological investigations. However, relationships between the Palaeozoic metamorphic ages, P–T evolution and deformational history of the region remain ambiguous. In order to address this issue, P–T–t–D paths of paragneisses from the basement of the Hongliuxia belt in the southern Dunhuang block were investigated. Inclusions in garnet and kyanite from the paragneisses are considered as vestiges of Palaeozoic M1 metamorphism corresponding to initiation of the prograde evolution. The earliest continuous metamorphic fabric is an originally steep N–S striking foliation S2. This fabric was reworked by vertical folds F3 associated with the development of a ubiquitous steep, mainly south-dipping, E-W striking axial planar foliation S3. The S2 foliation in paragneisses is mainly associated with Grt–St–Ky–Sil–Bt–Ms–Pl–Qz–Rt assemblages in samples from the western domain and with Grt–Ky–Sil–Bt–Kfs–Pl–Qz–Rt assemblages in samples from the northeastern domain of the Hongliuxia belt. The S3 foliation is associated with Grt–Sil–St–Bt–Ms–Pl–Qz–Ilm assemblages in the western domain and with Grt–Sil–Bt–Ms–Pl–Qz–Kfs–Ilm assemblages in the northeastern domain, followed by growth of chlorite in both domains. Early prograde stage (M1) from 4.0–6.5 kbar and 540–560°C to metamorphic peak (M2a) at 9–10 kbar and ~650–675°C is mainly recorded by paragneisses from the western domain. Subsequent decompression is initially accompanied by heating (M2b) constrained to 6.5–7 kbar and 675–710°C in the western domain, and to 6–6.5 kbar and ~730°C in the northeastern domain, followed by cooling (M3) through 4–6.5 kbar and 550–650°C till late chloritization (late M3). In situ U–Pb dating of monazite combined with monazite trace-element compositions suggests that prograde evolution (M1) most likely started at c. 406 Ma, peak-P conditions (M2a) were reached at 400–394 Ma, decompression associated with heating (M2b) took place at 393–391 Ma, and cooling (M3) during exhumation probably lasted from 380 to 354 Ma. The prograde metamorphism probably reflects burial during underthrusting of neighbouring continental basement (the Alxa block or an equivalent) below the Dunhuang block. This event culminated in pure shear thickening (D2a) of the whole supra-subduction margin followed by minor heating and exhumation (D2b). The D3-M3 event is interpreted as reflecting exhumation during orthogonal shortening of the system, possibly in response to an independent orogenic cycle. Combined with the available regional data, this study reveals the existence of a complex tectono-metamorphic evolution for the Dunhuang block characterized by two distinct orogenic phases with (i) the thickening of a previously thinned arc-back-arc crust recorded in the northern and central belts at 420–410 Ma in the pro-wedge side of the active margin (Sanweishan phase), followed by (ii) the 410–390 Ma thickening in the retro-wedge side (Hongliuxia phase). Such a tectonic evolution of the whole Dunhuang block resembles Andean-type migration of crustal thickening from the convergent front to hinterlands. The D3-M3 event, potentially responsible for the juxtaposition of rocks from different geological occurrences and depths, is seemingly independent from this Andean-type orogenic cycle.