Paper Number: 1497
Petrogenesis of Permian intermediate-mafic dikes in the Chinese Altai, NW China: implication for a post-accretion extensional scenario
Cai, K.D.1, Sun, M.2 and Jahn, B.M.3
1Xinjiang Research Centre for Mineral Resources, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China
2Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China
3Department of Geosciences, National Taiwan University, Taipei 106, Taiwan
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The Central Asian Orogenic Belt was a long-lived accretionary orogen and the late Paleozoic is considered to be a critical period for its terminal amalgamation of three tectonic collages. However, the exact timing of amalgamation and geological process of such a huge accretionary orogenic belt are poorly understood. This paper presents new geochronological and geochemical data for Permian intermediate-mafic dikes in the Chinese Altai, a key region between the Mongolian and the Kazakhstan collage systems. Based on the mineral assemblage and petrographic texture, the intermediate-mafic dikes can be categorized as gabbro-norite and quartz diorite. The grabbro-noritic and quartz dioritic dikes yield zircon U-Pb ages of 276.7 ± 2.9 Ma and 273.2 ± 4.3 Ma, respectively. The gabbro-norites are characterized by variable SiO2 (47.1-51.3 wt.%) and MgO (5.33-8.46 wt.%), and moderate Cr (71.2-95.7 ppm) and Ni (80.6-192 ppm) contents. Geochemical modeling indicates that the parental magma was possibly contaminated by 4-12% crustal materials. Zircon εHf(t) (+13.2~+16.7) and whole-rock εNd(t) (+4.9~+6.1) values as well as moderate Sm/Yb ratios (1.75-1.89) imply that the parental magma was likely derived from a depleted mantle source dominated by spine lherzolite. The quartz diorites exhibit higher SiO2 (57.3-58.3 wt.%), slightly lower whole-rock εNd(t) (~+2.5) and zircon εHf(t) (+9.1~+14.4) values, suggesting derivation of the parental magma from mafic lower crust. The quartz diorites have high Y (>39.8 ppm) and HREE (Yb>3.64 ppm) concentrations as well as low Sr/Y (<12) ratios, having geochemical fingerprints of partial melts at crustal depths above the garnet stability field (< 10 kb). Major element compositions of the quartz diorites are comparable to the intermediate liquids generated by ~40% partial melting of alkali-enriched basaltic rocks at conditions of T=1050-1100 °C and P=8 kbar. Such a high geothermal gradient, in response to regional tectono-thermal event, is inferred to be a consequence of intraplating and/or underplating of hot basaltic magmas in an extensional setting, which may shed a light on the ubiquitous tectonic scenario after complete amalgamation of tectonic collages.
This study was financially supported by the Major Basic Research Project of the Ministry of Science and Technology of China (2014CB448000), Xinjiang outstanding youth scientific grant (2013711003) and the Talent Awards to KDC from the China Government under the 1000 Talent Plan.