The Himalayan collision zone carbonatites in western Sichuan, SW China: Petrogenesis, mantle source and tectonic implication

Major and trace element compositions, and Sr-Nd-Pb and O-C isotope data for Cenozoic carbonatites (WSC) in western Sichuan, east Tibet, China are presented in this paper. The WSC occur in the eastern Indo-Asian collision zone (EIACZ), and occur as sills or dykes in the contemporaneous syenitic intrusions, which form a 270-km long, NS-trending belt of carbonatite-alkalic complexes, controlled by strike-slip faults that accommodated and adjusted to the collision strain. These carbonatite-alkalic complexes with Cenozoic potassic felsic rocks and calc-alkaline (shoshonitic) lamprophyres form a semi-continuous potassic igneous province with magmatism peaking at similar to 35Ma in the EIACZ. The WSC are characterized by low SiO2 (< 10.22 %), FeO (< 1.20 %) and MgO (< 0.73 %), and a wide range of CaO content (40.7 similar to 55.4 %) that distinguish them from primary magnesiocarbonatites. Overlapping emplacement ages and Sr-Nd isotopic compositions and similar mantle-normalized trace element patterns to spatially associated syenites suggest an origin for the carbonatites by liquid immiscibility. The WSC are extremely enriched in LILE (Sr, Ba) and light REE, but relatively depleted in high-field strength elements (Nb, Ta, P, Zr, Hf, Ti), suggesting a metasomatized mantle source. Their delta O-18(V-SMOW) (6.4 similar to 10.5 parts per thousand) and delta C-13(V-PDB) (-3.9 to -8.5 parts per thousand) values are similar to those of primary, mantle-derived carbonatites, also suggesting a mantle genesis. However, they have extremely low aNd((t)) of -3.2 to -18.7 and relative high (Sr-87/Sr-86)(i) of 0.706020 similar to 0.707923, as well as a wide range of Pb-207/Pb-204 (15.362 similar to 15.679) and Pb-208/Pb-204 ratios (38.083 similar to 39.202), which distinguish them from most carbonatites around the world. Their Sr-Nd, Sr-Pb and Nd-Pb isotopic signatures indicate that some carbonatites underwent Sr-Nd-Pb contamination by crustal materials, but the least-contaminated carbonatites were derived from a transitional source between EMI and EMII components. The T-DM and modeling on Sr-Nd isotopic compositions indicate that the formation of this source probably was related to recycling of oceanic crust and pelagic/terrigenous sediments with various mass ratios. Partial melting of the mantle source was most likely triggered by a Cenozoic asthenospheric mantle diapir related to Indian-Asian continent collision at 65-45Ma. Rising and emplacement of carbonatitic magmas with coeval potassium-rich magmas took place in the tectonic regime of the transition from transpression to transtension at Eocene/Oligocene boundary in the EIACZ. (c) 2006 Elsevier B.V. All rights reserved.