A Silurian-early Devonian slab window in the southern Central Asian Orogenic Belt: Evidence from high-Mg diorites, adakites and granitoids in the western Central Beishan region, NW China

Rongguo Zheng,Wenjiao Xiao,Jinyi Li,Tairan Wu,Wen Zhang

doi:10.1016/j.jseaes.2016.12.008

Abstract

The Beishan orogenic belt is a key region for deciphering the accretionary processes of the southern Central Asian Orogenic Belt. Here in this paper we present new zircon U–Pb ages, bulk–rock major and trace element, and zircon Hf isotopic data for the Baitoushan, and Bagelengtai plutons in the western Central Beishan region to address the accretionary processes.The Baitoushan pluton consists of quartz diorites, monzonites and K-feldspar granites, with zircon LA–ICP–MS U–Pb ages of 435Ma, 421Ma and 401Ma, respectively. The Baitoushan quartz diorites and quartz monzonites exhibit relatively high MgO contents and Mg# values (63–72), display enrichments in LILEs and LREEs, and exhibit high Ba (585–1415ppm), Sr (416–570ppm) and compatible element (such as Cr and Ni) abundances, which make them akin to typical high-Mg andesites. The Baitoushan quartz diorites and quartz monzonites were probably generated by the interaction of subducted oceanic sediment-derived melts and mantle peridotites. The Baitoushan K-feldspar granites are ascribed to fractionated I-type granites with peraluminous and high-K calc-alkaline characteristics. They exhibit positive εHf(t) values (2.43–7.63) and Mesoproterozoic–Neoproterozoic zircon Hf model ages (0.92–1.60Ga). Those early Devonian granites, including Baitoushan K-feldspar granite and Gongpoquan leucogranites (402Ma), are derived from melting of the mafic lower crust and/or sediments by upwelling of hot asthenospheric mantle. The Bagelengtai granodiorites exhibit similar geochemical signatures with that of typical adakites, with a zircon SHRIMP U-Pb age of 435Ma. They exhibit relatively high Sr (502–628ppm) and Al2O3 (16.40–17.40wt.%) contents, and low MgO (1.02–1.29wt.%), Y (3.37–6.94ppm) and HREEs contents, with relatively high Sr/Y and (La/Yb)N ratios. The Bagelengtai granodiorites were derived from partial melting of subducted young oceanic crust, with significant contributions of subducted sediments, subsequently hybridized by the mantle peridotite.On the basis of our data and combined with previous data, we conclude that a Silurian–early Devonian slab window model accounts for all geological records in the western segment of the Central Beishan. We further propose that magmatic events associated with a slab window played an important role in crustal growth of the Central Asian Orogenic Belt.

Highlights

Other important features are: high-temperature metamorphism closely associated with near-trench plutons created by heat released through a slab window [DeLong et al, 1979; Iwamori, 2000]; and porphyry gold and copper mineralization often associated with adakitic rocks [Sun et al, 2010]
When a ridge subducts under a continental margin, the diverging plates continue to separate creating a slab window that forms between the separating plates [Santosh, Kusky, 2010]
Prominent in one of two slab windows defined in West Junggar are two swarms of late Carboniferous high-Mg hornblende-biotite diorite dykes with sanukitoid chemistry that intruded through the slab window

Summary

Introduction

Cenozoic ridge subduction and the resultant slab windows have been well documented worldwide [Sisson et al, 2003], especially along the western margins of North and South America [Thorkelson, Taylor, 1989]. The key magmatic products are: adakites that include plutonic and volcanic rocks, have intermediate- to highSiO2, high Sr/Y and La/Yb ratios, elevated MgO, Na2O, K2O, Ni and Cr contents, and high LILE and LREE elements.

Results

Conclusion