Petrogenesis of the Triassic granitoids from the East Kunlun Orogenic Belt, NW China: Implications for continental crust growth from syn-collisional to post-collisional setting

Abstract The Triassic granitoids are widespread in the eastern section of the East Kunlun Orogenic Belt (EKOB) on the northern Tibetan Plateau. These granitoids well record the evolution of the Paleo-Tethys oceans (named as A'nyemaqen Ocean in the EKOB). Our new zircon U Pb data together with ages in literature show that these granitoids represent long-lasting magmatism from the early (T1, ~ 251–248 Ma), middle (T2, ~ 247–238 Ma) to late (T3, ~ 234–214 Ma) Triassic. The Triassic granitoids display calc-alkaline I-type granite affinities and hybrid mantle-crust geochemical signatures. The T1 granitoids possess andesitic to felsic bulk continental crust (BCC)-like chemical composition (e.g., enriched in Rb, K and Pb, depleted in Nb, Ta, Sr, P and Ti), coupled with high ISr (0.7067–0.7148), negative eNd(t) (−7.32 to −1.66) and negative to positive eHf(t) (−5.11 to 3.59) as well as (Dy/Yb)N = 1.1, suggesting that the T1 granitoids were formed by melting of the subducted A'nyemaqen oceanic crust with terrigenous sediments under the amphibolite facies conditions in a syn-collisional setting. The T2 and T3 granitoids may be originated from a relatively homogeneous source with almost consistent mean values of ISr (0.7136 [T2], 0.7094 [T3]), eNd(t) (−5.83 [T2], −5.97 [T3]) and eHf(t) (−3.52 [T2], −3.58 [T3]). They present garnet signature of adakitic rocks and can be explained by partial melting of the juvenile mafic lower continental crust and mixing with upper crustal components during magma ascent. This process is considered to be associated with post-collisional extension which induced by asthenosphere upwelling and mantle melting, providing heat for mafic lower crust melting to form T2 and T3 granitoids. The T1 granitoids with mantle signatures (e.g. eHf(t) > 0) as well as BCC-like compositions represent a net flux of juvenile dioritic to granitic materials adding to the continental crust, in support of the hypothesis of “continental collision zones are primary sites for net continental crust growth” along the EKOB. The genetic link between T2 and T3 granitoids means the EKOB had transformed to post-collisional setting since the middle Triassic (~ 247 Ma). All these hypotheses are conceptually important for understanding the origin of the juvenile crust and continental crustal growth through magmatism from syn-collisional to post-collisional settings.