Baddeleyite U-Pb age and Hf isotopes, and constraints on genesis of the Panzhihua carbonatite in SW China

Abstract

The Panzhihua district is well-known for its magmatic V-Ti-magnetite deposit, with plenty of carbonatite intrusions and extrusives (total length: over 20 km), making it one of the world’s largest carbonatitic belts. Mantle-derived xenoliths (dunite, lherzolite and olivine lamprophyre) of different sizes are the most direct and important evidence of magmatic carbonatite. The carbonatite consists mainly of calcite, dolomite, olivine, clinopyroxene, orthopyroxene and biotite, almost without felsic minerals. We performed baddeleyite U-Pb dating on the four largest carbonatite intrusions at Panzhihua, which yielded weighted mean 206Pb/238U ages (from NE to SW: Zhujiabaobao, Jianbao, Gongshan and Lanaqing) of 261.8 ± 3.8 Ma (MSWD = 0.084, n = 28), 263.7 ± 2.3 Ma (MSWD = 0.10, n = 27), 260.6 ± 2.3 Ma (MSWD = 0.27, n = 22), and 262.3 ± 1.8 Ma (MSWD = 0.44, n = 34). Samples of three extrusive carbonatites and one carbonatitic breccia yielded weighted average baddeleyite 206Pb/238U ages of 261.7 ± 3.0 Ma (MSWD = 0.095, n = 33), 263.8 ± 2.5 Ma (MSWD = 0.21, n = 30), 263.2 ± 2.9 Ma (MSWD = 0.13, n = 28), and 259.1 ± 3.6 Ma (MSWD = 0.12, n = 28), respectively. The weighted 206Pb/238U ages of baddeleyites for all intrusions and extrusives are 262.1 ± 1.2 Ma (MSWD = 0.26, n = 111) and 262.4 ± 1.5 Ma (MSWD = 0.18, n = 119), respectively. Baddeleyites from different intrusions and extrusives have varying Hf isotope compositions, with the weighted average εHf(t) of −0.8 ± 0.5 (Zhujiabaobao), −3.2 ± 0.8 (Jianbaobao), −5.5 ± 1.5 (Gongshan) and −11.9 ± 0.5 (Lanaqing), which are largely negative and decrease progressively from northeast to southwest. εHf(t) values of the two extrusive carbonatite and one carbonatitic breccia samples from the southwest of Panzhihua are −14.5 ± 0.7, −15.1 ± 0.8 and −7.8 ± 1.3, and one extrusive sample from the Lanjian ore-block has weighted mean εHf(t) of −7.8 ± 1.3. δ13CVPDB and δ18OV-SMOW values of the Panzhihua carbonatite range, respectively, −13.4 to +0.4 ‰ (avg. −3.7 ‰) and +15.4 to +21.8 ‰ (avg. +19.5 ‰), similar to those of most global carbonatites. εHf(t) of baddeleyites in Panzhihua carbonatite are dominantly negative, and analogous to that of the standard Phalaborwa baddeleyite, indicating that carbonatitic magma may have been derived from partial melting of the metasomatized lithospheric mantle or an enriched asthenospheric component during the Permian intracontinental rifting (Emeishan mantle plume-related). The hyperthermal carbonatitic magma has partially or completely assimilated mantle xenoliths in the chamber and during its ascent, implying it was of high temperature greater than the solidus curve of peridotite (≥1100℃). Assimilation of xenoliths and wallrocks may have altered the Hf isotope compositions and yielded the distinctive alternating dark peridotite and white carbonatite layers around each xenolith. As a newly identified rock type, the Panzhihua may be one of the global largest carbonatite belts, which also enrich the rock sequences of Emeishan large igneous province.