Breakup of the northern margin of Gondwana through lithospheric delamination: Evidence from the Tibetan Plateau

The central-southern Tibetan Plateau represents an important part of the northern segment of the Gondwanan supercontinent during the early Paleozoic. Here, we present zircon U-Pb, Lu-Hf isotope, and whole-rock geochemical data from the early Paleozoic Duguer magmatic suite of the central Tibetan Plateau, with a view to gain insights into the geotectonic evolution of the northern margin of Gondwana. A granite gneiss yielded an emplacement age of 453 +/- 2 Ma with negative epsilon(Hf) (t) values (-7.1 to -3.8). Zircon grains in three andesite samples are dated at ca. 475 Ma, whereas basalt and gabbro yielded ages of 483 +/- 3 Ma and 490 +/- 9 Ma, respectively. The andesitic and mafic rocks show epsilon(Hf)(t) values of -6.6 to 2.2 and -9.2 to 1.2, respectively. Geochemically, the granite gneiss is high-K calc-alkaline and shoshonitic, and it belongs to the aluminous A-type granite suite. The andesite rocks display slightly negative Eu, Nb, and Ta anomalies. The mafic rocks geochemically resemble enriched mid-ocean-ridge basalt (E-MORB) with no Nb and Ta depletions. We suggest that the protolith of the granite gneiss was derived from partial melting of ancient continental basement with minor mantle input and fractional crystallization. The andesitic and mafic rocks were derived from low- to medium-degree partial melting of an enriched mantle (garnet peridotite) that was contaminated by minor lower-crustal components. The andesitic and mafic rocks probably represent melts generated by high and low extents of fractional crystallization, respectively. The compression and extension are related, as is evidenced by the modern-day Himalayan orogenic belt. The Pan-African collisional orogeny represents the final assembly of eastern and western Gondwana. We posit that this orogenic event would have generated extension parallel to the orogenic belt, as well as lithospheric extension and thinning (delamination). Therefore, we conclude that the late Cambrian-Late Ordovician Duguer magmatic suite studied here represents the products of lithospheric delamination. The observed Cambrian-Ordovician unconformities on the central-southern Tibetan Plateau can be attributed to extension in a rift basin caused by lithospheric thinning and collapse of the orogenic belt. Lithospheric delamination may have initiated the breakup of the northern margin of Gondwana.