Early Paleozoic slab rollback in the north Altun, northwest China; new evidence from mafic intrusions and high-Mg andesites

• Published in: Lithosphere Vol. 10; no. 6; pp. 687 - 707
• Main Authors: Ye Xiantao, Ye Xiantao, Zhang Chuanlin, Zhang Chuanlin, Wang Aiguo, Wang Aiguo, Wu Bin, Wu Bin, Wang Guodong, Wang Guodong
• Format: Journal Article
• Language: English
• Published: Geological Society of America 01-12-2018
• Subjects: absolute age; alkali metals; alkaline earth metals; Altun Mountains; andesites; Asia; Baijianshan China; Cambrian; cathodoluminescence; chain silicates; chemical composition; China; clinopyroxene; Dabanxi Intrusion; dates; Dawan Intrusion; Far East; gabbros; Geochemistry; Geochronology; igneous and metamorphic rocks; igneous rocks; intrusions; isotope ratios; isotopes; Kaladawan China; Kaladawan Formation; Lapeiquan Formation; lithosphere; mafic composition; magnesium; major elements; metals; Nd-144/Nd-143; neodymium; nesosilicates; orogenic belts; orthosilicates; Paleozoic; petrography; Petrology; plate tectonics; plutonic rocks; pyroxene group; radioactive isotopes; rare earths; Rb-87/Sr-86; rhyolites; rollback; rubidium; samarium; silicates; slabs; Sm-147/Nd-144; Sr-87/Sr-86; stable isotopes; strontium; tectonics; U/Pb; volcanic rocks; whole rock; Xinjiang China; zircon; zircon group
• ISSN: 1941-8264
• DOI: 10.1130/L732.1

North Altun, one of the key tectonic units of the Altun orogen, lies at the northern margin of the Qinhai-Tibet Plateau. This belt is marked by high- to ultrahigh-pressure metamorphic rocks, ophiolites, and granitic rocks. To refine the model of subduction processes in the North Altun, we report detailed studies on the petrography, geochronology, and elemental and isotopic geochemistry of the mafic intrusions and andesitic lavas in the Kaladawan area along the northern margin of the Altun. Zircon U-Pb ages of rhyolites from the Lapeiquan Formation indicate that the Lapeiquan volcanic-sedimentary sequences were deposited in the late Cambrian (485-495 Ma), whereas the Dawan gabbros and Dabanxi mafic intrusions were emplaced at ca. 515 Ma and ca. 460 Ma, respectively. The Dawan gabbros generally produce flat rare earth element (REE) patterns ([La/Yb]N = 0.99-1.07) and negative Nb anomalies on primitive mantle-normalized diagrams (Nb/La = 0.13-0.80). These gabbros have positive εNd(t) values (+0.6 to +7.4), indicative of a depleted mantle source. These features reveal that the Dawan gabbros were probably derived from asthenospheric mantle with variable involvement of lithospheric mantle. The andesites from the Lapeiquan Formation are characterized by high Mg# (40-58) and TiO2 contents (0.61-1.21 wt%), sharing the signatures of calc-alkaline high-Mg andesites, and they are complemented by light (L) REE enrichment and strong Nb depletion. Their isotopic compositions show negative εNd(t) values (-3.7 to -5.8). Geochemical data indicate that the high-Mg andesites were produced by interaction between sediment-derived melts and mantle wedge peridotite-derived basaltic melts. The slightly younger Dabanxi gabbros display enriched LREE and depleted high field strength element (HFSE) patterns. Isotopically, they have variable positive whole-rock εNd(t) values (+0.6 to +4.1). Integrating the elemental and Nd isotopic compositions, we suggest that the primitive magmas of the Dabanxi gabbroic intrusion were most likely derived from the mantle wedge that was modified by slab-derived fluids. Combined with previous studies, we conclude that the formation of the Dawan gabbros and Dawan high-Mg andesites in the North Altun was related to asthenospheric upwelling triggered by rollback of the North Altun oceanic slab. We infer that these rocks in the North Altun area might represent part of a previously active arc-back-arc basin system.