Paleozoic mafic-intermediate intrusions (320–287Ma) in the Kalatongke area, southern Altai, NW China: Products of protracted magmatism in a convergent tectonic setting

• Published in: Journal of Asian earth sciences 2018-06, Vol.159, p.294-307
• Main Authors: Qian, Zhuangzhi, Duan, Jun, Li, Chusi, Xu, Gang, Feng, Yanqing, Ren, Meng
• Format: Article
• Language: English
• Subjects: Altai Orogenic Belt; Hf-Sr-Nd isotopes; Mafic rocks; Trace elements; Zircon U-Pb ages
• ISSN: 1367-9120; 1878-5786
• DOI: 10.1016/j.jseaes.2016.12.038

[Display omitted] •The mafic-intermediate intrusions in the Kalatongke area of the southern Altai Orogenic Belt formed at 287–320Ma.•The protracted magmatism in such a small area (4.5×2km) does not support a mantle plume model.•The magmas before crustal contamination had arc-like trace element and Sr-Nd-Hf isotope characteristics.•Temporal correlation is not an effect tool to determine the size of a mantle plume. Numerous small mafic-intermediate intrusions are present in the Kalatongke area in the southern part of the Paleozoic Altai Orogenic Belt, NW China. Previous studies reveal that most of these intrusions were emplaced at ∼287Ma, broadly coeval with the eruption of alkaline flood basalts at ⩽282Ma in the Tarim Craton. The similar ages have led some researchers to believe that the Tarim flood basalts and the Kalatongke mafic-intermediate intrusions are related to the same mantle plume. New and existing geochronological and geochemical data for the mafic-intermediate intrusions in the Kalatongke area together do not support such interpretation. Most of the intrusions in this small area (4.5×2km) were emplaced at ∼287Ma but our new data reveal that older intrusive rocks with zircon U-Pb ages from 290±1.5 to 320±2Ma are also present. The temporal and spatial distribution of these intrusions does not show a hotspot track as expected by the mantle plume model. The intrusive rocks have similar Nd-Hf isotope compositions (εNd=3–9, εHf=15–19) and are all characterized by light REE enrichments relative to heavy REE, plus pronounced negative Nb-Ta anomalies. The oldest intrusion (∼320Ma) also shows negative Zr-Hf anomalies, which are common in arc basalts but absent in continental flood basalts. The results of mixing calculations based on Sr-Nd-Hf isotopes and selected trace elements (Th, Nb and Yb) indicate that the parental magmas for these intrusions were all enriched in Th and depleted in Nb prior to crustal contamination, similar to the magmas generated in a convergent tectonic setting from subduction to post-subduction elsewhere in the world. The results from this study remind us that temporal correlation is not a reliable tool to determine the size of a mantle plume.