Perovskite and baddeleyite from kimberlitic intrusions in the Tarim large igneous province signal the onset of an end-Carboniferous mantle plume

Several tens of kimberlitic pipes and dykes are exposed in the Wajilitag area in the western Tarim large igneous province. Here we report for the first time secondary ion mass spectrometric U-Pb age data on perovskite and baddeleyite grains in a kimberlitic pipe and a kimberlitic dyke from the Tarim Craton. The perovskite yielded a well-defined intercept age of 299.8 +/- 4.3 Ma, which is consistent with its corresponding concordia and Pb-206/U-238 ages, corrected for the common Pb contribution, of about 300 Ma. The baddeleyite separated from two kimberlitic samples from a dyke display identical concordia U-Pb ages of 300.8 +/- 4.7 Ma and 300.5 +/- 4.4 Ma. Our age data show that the kimberlitic intrusions were emplaced at ca. 300 Ma, rather than in the late Permian as previously regarded. These new ages are slightly older than the eruption ages of Tarim flood basalts (291-273 Ma), offering a critical regional time marker for the onset of Permo-Carboniferous magmatism in the Tarim Craton. Detailed petrographic observations did not reveal any ultrahigh pressure mineral assemblage in the Wajilitag kimberlitic intrusions. Phlogopites from these intrusions show epsilon(Nd)(t) values of +3.7 to +4.2. The baddeleyites which are texturally primary and therefore inferred to have crystallized directly from the kimberlitic magma, yield a range of epsilon(Hf)(t) from +4.8 to +8.7. These results combined with previously reported geochemical data, suggest that the Wajilitag kimberlitic intrusions were most likely derived from a moderately refractory and depleted subcontinental lithosphere mantle, metasomatized by subduction components associated with an early-middle Paleozoic convergent regime. The kimberlitic magma was generated by small-degree partial melting of the lithospheric mantle in response to the impingement of the Tarim mantle plume. Thus, our new geochronological data suggest the arrival of the mantle plume beneath the Tarim lithosphere at least 10 million years before the onset of Tarim flood basalt volcanism. The end-Carboniferous Wajilitag kimberlitic intrusions, the oldest known phase associated with Carboniferous magmatism in the Tarim Craton, signals the initial magmatic pulse triggered by mantle plume impingement. (C) 2012 Elsevier B.V. All rights reserved.