Geochronology and geochemistry of meta-mafic dykes in the Quanji Massif, NW China: Paleoproterozoic evolution of the Tarim Craton and implications for the assembly of the Columbia supercontinent

摘要

The Quanji Massif, located in the northeastern margin of the Tibet Plateau, is interpreted as a fragment of the Tarim Craton. The massif exposes a suite of metamorphosed mafic dykes. LA-ICP-MS U-Pb dating constrains the timing of intrusion of these dykes at ca. 1834 +/- 23 Ma. The dykes display minor variations in major elements, with SiO2 = 46.8-53.4 wt%, MgO = 5.19-8.10 wt%, FeOt = 8.83-15.6 wt%, TiO2 = 0.58-1.78 wt% and Mg-# = 46.3-69.7. Their immobile trace element compositions show a sub-alkali basalt affinity. The positive correlation of TiO2 with FeOt/MgO in these rocks shows an arc tholeiite evolutionary trend. The enrichment of LILE and LREE and depletion in HFSE suggest that the precursor magma was generated in a back-arc environment. Furthermore, these rocks possess (Nb/La)(N) of 0.31-0.52 and (Th/La)(N) of 0.61-1.39, whole rock (Sr-87/Sr-86)(i) values of 0.707598-0.724141, epsilon Nd(t) of -2.9 to +0.3 and ((206)pb/Pb-204)(t) of 16.8148-24.1513, ((207)pb/Pb-204)(t) of 15.3422-16.3630 and ((208)pb/(204)pb)(t) of 36.8415-39.7926. The magmatic zircons yielded epsilon Hf(t) of -3.4 to +6.1 and depleted mantle model ages (T-DM) of 2.01-2.37 Ga. The geochemical and isotopic characteristics of these rocks suggest that their precursor magma was derived from a subduction-related fluid-metasomatized subcontinental lithospheric mantle mixed with a depleted mantle component. We suggest that a prolonged subduction-accretion-collision process along the southeastern margin of the Tarim Craton prevailed at similar to 2.1-1.80 Ga, with local consumption of the Southeast Tarim Archipelagic Ocean during the collision of the Southeast Tarim Block with the Quanji Massif (microcontinent) at around 2.1-1.9 Ga. The final closure of the whole Southeast Tarim Archipelagic Ocean occurred at similar to 1.85-1.82 Ga and collision between the Southeast margin of the Tarim Craton and other unknown continental blocks at 1.82-1.80 Ga, broadly coeval with the amalgamation of the Western and Eastern Blocks of the North China Craton and their assembly in the Columbia supercontinent. (C) 2014 Elsevier B.V. All rights reserved.