Petrogenesis and tectonic implications of granitoids from western North Altun, Northwest China

Abstract

The Altun orogenic belt in northwest China is part of the northern margin of the Tibetan Plateau. The North Altun ophiolitic mélange belt is an important tectonic unit within the Altun orogenic belt that contains voluminous early Paleozoic granitoids. In this study, we report the petrological features, geochemical compositions, and zircon UPb and Hf isotope data of three granitic plutons from the western segment of the North Altun ophiolitic mélange belt. Zircon UPb dating yields magmatic crystallization ages of 499, 493 and 496 Ma for samples of granodiorite, quartz diorite and syenogranite, respectively. The granitoids have metaluminous to weakly peraluminous and medium-K to high-K calc–alkaline characteristics and display relative enrichments in large ion lithophile elements (Rb, Th, U, K) and light rare earth elements (LREE) and relative depletions in Nb, Ta, Sr, P and Ti, suggesting an arc-related origin. The granodiorites (499 Ma) have positive zircon εHf(t) values ranging from +1.87 to +6.59 with two-stage Hf model ages (TDMC) of 1.05 to 1.35 Ga, implying that the granodiorites were derived from juvenile crust. The quartz diorites (493 Ma) have similar Hf isotopic characteristics to the granodiorites (εHf(t) = +2.59 to +6.04, TDMC = 1.08 to 1.30 Ga), indicating derivation from juvenile crust. The syenogranites (496 Ma) have high total REE and K2O contents, and low zircon εHf(t) values (−1.69 to +1.54), suggesting that they were derived mainly from juvenile crust mixed with ancient crustal materials. Combined with data from previous studies, we conclude that magmatism in the North Altun ophiolitic mélange belt can be subdivided into three episodes: Episode 1 (520–470 Ma) granitoids are related to subduction; Episode 2 (460–425 Ma) granitoids formed in a continent–continent collisional setting; and Episode 3 (<420 Ma) granitoids are post-collisional granites. Our results are consistent with south-directed subduction of the North Altun oceanic lithosphere beneath the Central Altun Block during the early Paleozoic (520–460 Ma), which was followed by collision with the Dunhuang Block.