Abstract
Detailed geochronological, geochemical, and Sr-Nd-Hf isotopic data are presented for early Paleozoic volcanic rocks in the Karadaban area from the northern Altyn region, NW China, with the aim to constrain their petrogenesis and tectonic implications. The Karadaban volcanic rocks show a bimodal distribution in composition, with rhyolite and basalt. The LA-ICP-MS zircon U-Pb age indicates that the volcanic rocks were erupted at 512 Ma. The mafic rocks are calc-alkaline, enriched in light rare earth elements (LREE) and large-ion lithophile elements (LILE; Ba and U) and depleted in high-field strength elements (HFSE; Nb and Ta). These features together with their depleted isotopic signature (initial87Sr/86Sr=0.70413–0.70817,εNdt=2.7to 3.7) suggest that they were likely derived from a depleted mantle source but mixed with crustal components while upwelling. The felsic rocks show an A-type affinity, with high alkalis and Rb/Sr and Ga/Al ratios; enriched in LILE (e.g., Rb, K, Th, U, and REE) and depleted in Ba, Sr, Nb, P, and Ti; and with fractionated REE patterns with strong negative Eu anomalies. The combination of the decoupling ofεNdtvalues (−2.5 to −6.3) andεHftvalues (+5.5 to +14.7) in the setting of subduction indicates that the felsic rocks were generated by partial melting of the juvenile crustal as a result of magma upwelling. The geochemical and Sr-Nd-Hf isotopic characteristics, coupled with regional geology, indicate that the formation of the Karadaban bimodal volcanic rocks involves an extensional regime associated with a subduction-related environment. The rifting of the back arc in response to the retreat of the subducting northern Altyn oceanic lithosphere may account for the Karadaban bimodal volcanic rocks.
Highlights
There was a set of bimodal volcanic rocks discovered in the NE of the North Altyn Tagh, which is helpful for studying the magmatic events and determining the timing of oceaniccontinental subduction
We present U-Pb zircon dating and geochemical and Sr-Nd-Hf isotopic composition for Karadaban bimodal volcanic rocks from the north Altyn region
Major element compositions were measured by wavelength dispersive X-ray fluorescence spectrometry (XRF) on fused glass beads using a Philips PW2404 spectrometer; analytical uncertainties are less than ±1% for major elements
Summary
Bimodal volcanism typically characterizes an extensional environment, which can occur within various tectonic settings, including continental rifts [1, 2], within-plate extensional settings [3,4,5], intraoceanic islands [6], ocean island arcs [7, 8], incipient back-arc depressions [9], mature island/active continental margins [7,8,9], and back arcs [9,10,11,12] In each of these modern tectonic environments, the volcanic activity may give rise to specific features, such as lithological assemblage, geochemical signature, and type of associated ore deposits [12, 13]. Our aims are (1) to constrain petrogenesis and magma sources of bimodal volcanic rocks in the subduction setting and (2) to discuss the early Paleozoic evolution of the Altyn Tagh region
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