Geochronologic and geochemical evidence for persistence of south-dipping subduction to late Permian time, Langshan area, Inner Mongolia (China): Significance for termination of accretionary orogenesis in the southern Altaids

Abstract

The Langshan area in Inner Mongolia is situated in the southern Altaids between the Beishan suture to the west and the Solonker suture to the east. This paper addresses the poorly known tectonic evolution that led to formation of the terminal Solonker suture. Dating of deformed porphyries and undeformed dolerites and gabbros constrains the timing of the relevant NE-E-striking and north-vergent deformation. Deformed granitic-granodioritic porphyries in this area are characterized by high SiO₂ (65.38-78.00%), low TFe₂O₃ (1.29-5.07%), MgO (0.13-0.63%), and variable K₂O (0.53-4.14%) and Na₂O (2.05-4.62%). All samples have enriched LREE (La/Yb 6-18) and negative Nb anomalies (Nb_N/Th_N0.09-0.48), but different Eu anomalies (Eu* <0.7 or ∼1); these geochemical features can be ascribed to a heterogeneous source in a subduction-related environment. Gabbros and dolerites have 42.33 to 52.03 percent SiO₂. All mafic samples have similar La/Yb ratios of 4 to 6 and negative Nb anomalies (Nb_N/Th_N) ratios of 0.2 to 0.8, suggestive of a subduction-related setting. Two granitic porphyries yielded ²³⁸U/²⁰⁶Pb weighted mean ages of 284.7 ± 2.1 Ma with MSWD of 1.6 and 291.7 ± 2.1 Ma with MSWD of 1.14; these ages are consistent with ²³⁵U/²⁰⁷Pb and ²³⁸U/²⁰⁶Pb concordia ages of 281 ± 17 Ma with MSWD of 0.87 and 289.8 ± 9.2 Ma with MSWD of 0.66. A dolerite yielded concordia ages of 256.2 ± 2.6 Ma with MSWD of 0.44 and 256 ± 2.5 Ma with MSWD 0.45. The ages and geochemistry of the deformed porphyries indicate that in the early Permian there was important deformation and recrystallization in a subduction-related setting. The isotopic and geochemical signatures of all the rocks indicate that they formed during subduction-related conditions. We propose that Langshan was a Permian active continental margin arc built on the edge of the North China Craton by southward subduction, which led to closure of the ocean, concomitant formation of the Solonker suture in the late Permian-early Triassic, and termination of the accretion-subduction orogen of the southern Altaids.