Late Devonian-early Permian subduction-accretion of the Zharma-Saur oceanic arc, West Junggar (NW China): Insights from field geology, geochemistry and geochronology

摘要

Migration of arc magmatism is of significant importance for accretionary orogens, but few examples have been documented in ancient orogens. In this paper we report late Paleozoic-early Permian, migrating magmatism of the intra-oceanic Zharma-Saur arc in the Tarbagatay Mountains, West Junggar, NW China, in order to address the general role of a growing arc in accretionary orogenesis, and the debates on the formation and timing of the Kazakhstan orocline in the Central Asian Orogenic Belt (CAOB). Our detailed field work suggests that in the Zharma-Saur arc plutons and dikes intruded and lavas erupted in association with accretionary complexes that were dominated by ocean plate stratigraphy (OPS). Mafic to intermediate magmas have arc-type geochemical signatures such as depleted HFSEs and enrichments in LILEs and LREEs, coupled with high epsilon Nd(t) (+ 5.7 to + 6.8) to low Sr-87/Sr-86 initial values (+0.7035 to +0.7037). The Tarbagatay accreted rocks contain early Paleozoic pillow lavas, which have depleted LILEs, high eNd(t) (+7.4 to +8.2) and high Sr-87/Sr-86 initial values (+0.7041 to +0.7063). These data suggest that the Zharma-Saur arc was an intra-oceanic island arc that developed on accreted OPS material. U-Pb zircon isotopic data (the ZWTB I diorite pluton, 322 +/- 3 Ma; the JLDK diorite pluton, 318 +/- 4 Ma; andesitic lavas, 312 +/- 2 Ma, 310 +/- 4 Ma, 301 +/- 3 Ma) demonstrate that the Zharma-Saur arc magmatism occurred in the late Paleozoic. Integration with published geochronological data on the Zharma-Saur arc leads to a complex model of arc growth with a general southward younging of the arc magmatism from 383 Ma to 262 Ma. These relations imply a southward growth and migration of the magmatic axis of the Zharma-Saur arc that was associated with a short period of ridge subduction. The magmatic activity of the Zharma-Saur arc that probably continued until the early Permian resulted from migrations towards the forearc and backarc, as well as oblique or parallel motion to the trench. These new results not only provide robust evidence for resolving controversies about the Phanerozoic accretionary and continental growth of the Central Asian Orogenic Belt, but also shed light on the migration of magmatisni in accretionary orogens in general.