Early Late Cretaceous (ca. 93Ma) norites and hornblendites in the Milin area, Eastern Gangdese: lithosphere-asthenosphere interaction during slab roll-back and an insight into early Late Cretaceous (ca. 100-80Ma) magmatic "flare-up" in southern Lhasa (Tibet)

At more than 500km in length, the mainly Jurassic-Early Eocene Gangdese batholith is one of the most important constituents of the southern Lhasa sub-block and provides an ideal site for study of Tibetan orogenesis. Recent studies on Gangdese intermediate-felsic intrusive rocks, mainly granites, demonstrate that remarkable crustal growth as well as an early Late Cretaceous (ca. 100-80Ma) magmatic "flare-up" event occurred in southern Tibet. However, the mechanism that drove this magmatic event and its relationship to crustal growth event are not yet clear. Here, we report detailed petrological, geochronological, geochemical and Sr-Nd-Hf isotopic data for recently identified norites and hypersthene-bearing hornblendites in the Milin area, southern Lhasa sub-block. These mafic rocks are dated at early Late Cretaceous (ca. 93Ma), and are characterized by relatively uniform Sr-Nd-Hf isotopic compositions ((⁸⁷Sr/⁸⁶Sr)_i=0.7042 to 0.7047, ε_Nd(t)=+2.9 to +3.6 and ε_Hf(t)_zircon=+10.9 to +17.0), suggesting that they evolved from similar parental magmas. The Milin norites and hornblendites are likely to be the products of mineral fractionation and accumulation from a common parental magma during the early and late stages of the magma evolution, respectively. Thermometric calculations indicate that pyroxenes from the Milin norites have high crystallization temperatures (1240-1349°C). The parental magmatic compositions calculated from pyroxene trace element compositions in the Milin norites show slightly flat to enriched light rare earth element (LREE) patterns ([La/Yb]_N=2.9-3.4; [La/Gd]_N=2.1-2.9) with variable negative Nb anomalies ([Nb/La]_N=0.18-0.81), indicating their dual or hybrid geochemical characteristics. We suggest that their parental magmas may have been generated by the interaction of upwelling asthenospheric and metasomatized lithospheric mantle. Taking into account the spatial and temporal distribution of the Mesozoic magmatic rocks and regional paleomagnetic data, we further suggest that the early-Late Cretaceous magmatic "flare-up" in the southern Lhasa sub-block was also triggered by the asthenospheric upwelling, which resulted from roll-back of subducted Tethyan oceanic slab.