40Ar– 39Ar age, geochemistry and Sr–Nd–Pb isotopes of the Neoproterozoic Lengshuiqing Cu–Ni sulfide-bearing mafic–ultramafic complex, SW China

Abstract

The Lengshuiqing Cu–Ni sulfide deposit of the Yanbian area in Sichuan Province, SW China, is hosted by relatively small mafic–ultramafic bodies that intruded Mesoproterozoic metavolcanic rocks and schists. The mafic–ultramafic complex is mainly composed of the olivine-hornblende pyroxenite and hornblende gabbro units. 40Ar/ 39Ar age, elemental and Sr–Nd–Pb isotopic results of the complex are reported in this paper. 40Ar/ 39Ar dating of hornblende yielded an age of 821 ± 1 Ma for the mafic–ultramafic complex. This age is similar to the SHRIMP U–Pb zircon age of 825 ± 12 Ma for the adjacent Gaojiacun mafic–ultramafic complex. This suggests that the Lengshuiqing mafic–ultramafic complex originated from Neoproterozoic mafic magma. The olivine-hornblende pyroxenite unit is characterized by high MgO contents (24.25–28.08%), Mg # values (74.5–78.0) and compatible elements (Cr = 1179–2336 ppm, Ni = 732–1303 ppm), low ΣREE (19.5–44.6 ppm) and other incompatible element contents. The hornblende gabbro unit has MgO = 7.32–12.51%, Mg # = 55.0–69.3, Cr = 37.7–1131 ppm, Ni = 50.0–243 ppm and ΣREE = 37.2–75.4 ppm. Most of the rocks are moderately enriched in LREE ((La/Yb) N = 2.28–7.67), and significantly depleted in Nb–Ta ((Nb/La) PM = 0.17–0.47; (Ta/La) PM = 0.19–0.58), with slightly moderately positive Eu anomalies ( δ Eu = 0.95–1.65). The trace element feature of the ore-bearing samples is similar to that of the rocks in the complex. The variations in major and trace elements can be well explained by magma crystal fractionation. Isotopically, the rocks in Lengshuiqing mafic–ultramafic complex are characterized by low 87Sr/ 86Sr(i) (0.7039–0.7044), positive ɛ Nd(i) (+2.2 − +4.3) and low radiogenic Pb values ( 206Pb/ 204Pb(i) = 17.382–17.913, 207Pb/ 204Pb(i) = 15.518–15.578, 208Pb/ 204Pb(i) = 37.227–38.019). The ores also have positive ɛ Nd(i) (+2.4 − +5.4), and low radiogenic Pb values ( 206Pb/ 204Pb(i) = 17.256–17.733, 207Pb/ 204Pb(i) = 15.484–15.555, 208Pb/ 204Pb(i) = 36.924–37.665). All the samples exhibit positive ɛ Nd(i) values, suggesting that their parental magmas were derived from a long-term depleted asthenospheric mantle. The relatively large variations in 87Sr/ 86Sr(i) of the ores (0.7012–0.7046) could be ascribed to the interaction between magma and country rocks. A good positive correlation between Nb/La and Nb/Th reveals that crustal assimilation was involved in their genesis. This contamination resulted in the observed depletion of Nb and Ta in rocks and ores. The crustal contamination and fractional crystallization, similar to the process of magmatic assimilation and fractional crystallization (AFC), are thus critical for the formation of the Lengshuiqing Cu–Ni ore deposit. The Neoproterozoic Lengshuiqing Cu–Ni sulfide-bearing mafic–ultramafic intrusions formed in an intraplate continental rift, which was most likely related to a mantle plume underneath the supercontinent Rodinia.