SIMS zircon U–Pb ages, geochemistry and Nd–Hf isotopes of ca. 1.0 Ga mafic dykes and volcanic rocks in the Huili area, SW China: Origin and tectonic significance

Abstract

Late Mesoproterozoic to early Neoproterozoic igneous rocks (ca. 1100Ma to >860Ma) are sparse in the southwestern Yangtze Block, hindering our understanding of South China's position during the late Mesoproterozoic to early Neoproterozoic assembly of the supercontinent Rodinia. We report here Cameca secondary ion mass spectrometry (SIMS) zircon U–Pb ages, geochemistry, and Nd–Hf isotopic data for mafic dykes that intruded in the lower Tianbaoshan Formation, and that of intermediate to felsic volcanic rocks from the upper Tianbaoshan Formation of the Huili Group in southwestern Yangtze Block. The mafic dykes were formed at 1023±6.7Ma. The volcanic rocks were dated at 1025±13Ma and 1021±6.4Ma, contemporaneous to the mafic dykes. The mafic dykes show weakly positive ɛNd(t) values (+0.41 to +1.6) and positive ɛHf(t) values (+7.0 to +10.3). They are characterized by slightly LREE-enriched and HREE-depleted patterns and the trace element patterns with typical depletion of Nb and Ta and slightly enrichment of Th relative to La. The parental magma for the mafic rocks exhibits affinity of low-Ti tholeiitic basaltic magma generated by melting of a depleted asthenospheric mantle source. The primary magma of these mafic rocks likely underwent variable degrees of the fractional crystallization and crustal contamination. The intermediate to felsic volcanic rocks in the Tianbaoshan Formation are characterized by significantly LREE-enriched and slightly HREE-depleted patterns, showing negative Nb–Ta, Eu, Sr, P and Ti anomalies in the primitive mantle-normalized multi-element plot. In addition, the volcanic rocks display variably elevated Ga, Rb, Zr, REE, and Y concentrations, and high Ga/Al ratios (2.31–2.64), consistent with the geochemical characteristics of A2-type granites. The volcanic rocks exhibit negative ɛNd(t) values (−5.1 to −7.1) and variable ɛHf(t) values (−0.67 to +3.9). They give two-stage Nd model ages (TNd2DM) and two-stage Hf model ages (THf2DM) of 1.83–1.99Ga and 1.62–1.91Ga, respectively. The Nd–Hf isotopic signatures indicate that the primitive magma for the Tianbaoshan intermediate to felsic volcanic rocks was probably derived from partial melting of the granulite-facies lower crust driven by underplating of mafic magmas, and underwent extensive fractional crystallization during emplacement. The studied 1.02Ga felsic volcanic rocks and mafic dykes at Huili are interpreted to have formed in an impactogen setting during the initial stage of collision between the Yangtze and Cathaysia blocks prior to the Sibao Orogeny.