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 melange 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 U Pb and Hf isotope data of three granitic plutons from the western segment of the North Altun ophiolitic melange belt. Zircon U Pb 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 e Hf (t) values ranging from +1.87 to +6.59 with two-stage Hf model ages ( T DMC ) 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 (e Hf ( t ) = +2.59 to +6.04, T DMC  = 1.08 to 1.30 Ga), indicating derivation from juvenile crust. The syenogranites (496 Ma) have high total REE and K 2 O contents, and low zircon e Hf ( 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 melange 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 (