Continental crust growth induced by slab breakoff in collisional orogens: Evidence from the Eocene Gangdese granitoids and their mafic enclaves, South Tibet

Abstract

Collisional orogens are traditionally regarded as main regions related to crustal reworking with limited or without crust growth. However, our study on the granitoids and their mafic enclaves of the Ringqênzê plutonic complex, a representative syn-collisional intrusive complex of Gangdese batholith, shows huge mantle contributions and considerable continental crust growth. Zircon U-Pb dating yields Eocene ages of 50.2 ± 0.5 Ma, 46.4 ± 0.5 Ma, 50.6 ± 0.6 Ma and 50.4 ± 0.5 Ma for the quartz diorite, hornblende granodiorite, host pyroxene-bearing granodiorite and its dioritic enclave, respectively. Reverse zoning of plagioclase (spike zone), pyroxene and plagioclase relicts in both of the dioritic enclaves and the granitoids, as well as the mixing trends on plots of major and trace elements, indicate a petrogenesis of magmatic mixing. The relatively low SiO2 (53.43–56.23 wt%) contents, high Mg# values (48–56) and mineral compositions further suggest that precursor magmas of the enclaves are mantle-derived. The dioritic enclaves and granitoids have indistinguishable (87Sr/86Sr)i ratios (0.70460–0.70480) but slightly different εNd(t) values (+1.8 to +3.0 for the dioritic enclaves and +0.2 to +0.6 for the granitoids). The (87Sr/86Sr)i ratios and εNd(t) values plot on the mixing line defined by depleted mantle (DM) and lower crust xenolith from the Southern Lhasa subterrane and further simulation reveals a contribution of >55% from mantle. In the zircon εHf(t) versus δ18O diagram, the dioritic enclaves (εHf(t) = +3.7–+8.5, δ18O = 4.56–7.00‰) and the granitoids (εHf(t) = +1.2–+8.5, δ18O = 5.59–7.21‰) plot around the mixing line defined by zircons from granulite xenolith and a relatively depleted zircon from the dioritic enclaves. Meanwhile, the simple binary mixing calculation based on Hf-O isotopes suggests that the mantle contributions are not <30% (55–65% in average) to the magma of these granitoids. In addition, the synthesized data show the majority of the syn-collisional Gangdese granitoids were also generated by magma mixing processes with similar large quantity of mantle contributions to the Ringqênzê granitoids. Therefore, the extensive and significant mantle contributions in the magma of the collision-related Gangdese granitoids demonstrate considerable continental crust growth during the syn-collisional period. Furthermore, our data combined with previously published data suggest that the ca. 50 Ma samples have highest Ti-in-zircon temperatures (up to 900 °C) and high pyroxene crystallizing temperatures (>1000 °C), whereas other samples show lower Ti-in-zircon temperatures mostly below 750 °C. Because the Gangdese granitoids with unusual high temperatures had been produced dominantly at a very short time (~50 Ma) and the high temperature was related to underplating of mantle materials, we suggest that a slab breakoff was most likely responsible for the formation of the high-temperature magmas and the continental crust growth at the syn-collisional period in collisional orogens.