New SHRIMP U-Pb zircon data from the Gangdese batholith: implications for the Transhimalayan magmatic evolution
Abstract
The Gangdese batholith that crops out in the southern Lhasa terrane marks one of the main products of the Transhimalayan magmatism caused by northward subduction of the Neo-Tethyan slab before the India-Asia collision. Here we present new SHRIMP U-Pb zircon results for this batholith, which are combined with published age data to delineate a magmatic duration from ∼104 to 40Ma. Such a prolonged magmatic activity, however, appears to have clustered in two stages, in the Cretaceous (∼104--78 Ma) and Paleogene (∼60--40 Ma), respectively, with a gap during ∼78--60 Ma. The age span of the former stage overlaps with that (∼135--75 Ma) of the granitoids and associated volcanic rocks emplaced in the Nyainqentanglha belt in the northern part of the Lhasa terrane. In comparison with the Nyainqentanglha granitoids that are dominantly S-type and marked by significant involvement of the continental crust in magma generation, the Cretaceous Gangdese granitoids show geochemical affinities with adakites from modern subduction zones. The Gangdese adakites (SiO_2 = 66--69%) are highly depleted in HREE and Y (<10 ppm), so coupled with elevated La/Yb and Sr/Y, and do not display Eu anomalies. Their overall geochemical and isotopic features are consistent with an origin by melting of a garnet-bearing source that is most likely to be the subducting Neo-Tethyan slab. These observations point to a flat subduction that, as in the case of central Andes, can account for the heat required to melt the slab and the widespread nature of the Cretaceous phase of the Transhimalayan magmatism in the Lhasa terrane. In contrast, the younger phase of this arc magmatism was confined to the south, represented by the Paleogene stage of the Gangdese plutons and contemporaneous Linzizong volcanic successions. These magmas, ranging from mafic to felsic compositions typical of the calc-alkaline nature, have Sr-Nd isotope ratios [e.g., ɛNd(T)= +3 to +5] suggesting a prevailing contribution by a juvenile mantle component that we infer to have been caused by an asthenospheric doming due to rollback of the Neo-Tethyan slab and its subsequent breakoff into the deep mantle.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....8864W