Geological Magazine, 2009, 146(4): 540-551

Geochemistry of Middle Triassic gabbros from northern Liaoning, North China: origin and tectonic implications

XIAOHUI ZHANG, HONGFU ZHANG, MINGGUO ZHAI, SIMON A. WILDE and LIEWEN XIE

Abstract

The Xiaofangshen mafic stock is a hornblende gabbroic body emplaced in the Faku dome of northern Liaoning within the continental interior of the North China–Mongolian plate. Zircon U–Pb SHRIMP dating yields an emplacement age of 241 ± 6 Ma. These gabbroic rocks exhibit strong enrichment in large ion lithophile elements (e.g. Th, U) and light REE, slightly negative Eu anomalies, and pronounced depletion in high field strength elements (e.g. Nb, Ta, Zr, Ti). They show a relatively narrow range of isotopic compositions with initial 87Sr/86Sr ratios of 0.7053 to 0.7055, Nd(t) values of +0.40 to +0.68 and zircon Hf(T) values from +5.0 to +7.4. These geochemical features suggest that they might have been derived from partial melting of a subduction-related metasomatized lithospheric mantle source, which is tectonically affiliated to the Xing-Meng orogenic belt. Combined with our previous geochronological dating on the predominantly granitic intrusions from the Faku dome, it is inferred that the northern Liaoning block has a tectonic affinity with the Phanerozoic accretionary orogenic belt. This revelation further leads to the proposition that the Chifeng–Kaiyuan fault likely represents the Mesozoic lithospheric boundary between the North China craton and the Xing-Meng orogenic belt in northern Liaoning. The Xiaofangshen gabbros, together with the Triassic mafic–ultramafic cumulates and granulite xenoliths and the Triassic alkaline intrusions within the continental interior of the newly amalgamated North China–Mongolian Plate, constitute an important post-orogenic to within-plate anorogenic magmatic province, in response to the continued magmatic underplating caused by lithospheric delamination and hot asthenosphere upwelling.