The Paleoproterozoic bimodal magmatism in the SW Yangtze block: Implications for initial breakup of the Columbia supercontinent

Abstract

The fragmentation of the Columbia supercontinent is thought to have begun as early as ~1.7 Ga; however, the breakup of its core domain occurred only at ~1.5–1.25 Ga. This study presents the integrated petrological and chemical characteristics of gabbro and granite porphyry intrusions in the Haizi region of China, which record the oldest magmatism associated with the initial breakup of Columbia in the southwestern Yangtze Block. The dating of gabbro and granite porphyry samples using laser ablation–inductively coupled plasma–mass spectrometry zircon UPb geochronology gives crystallization ages of 1754 ± 14 Ma and 1743 ± 4 Ma, respectively. The gabbro is characterized by low La/Ta, Th/Y, (La/Yb)N, and (Dy/Yb)N ratios, negligible NbTa anomalies, moderately depleted Nd isotope signatures (εNd (t) = +2.4 to +5.3), and enriched zircon Hf isotopes (εHf(t) = −1.8 to −7.8), indicating derivation of the primary magma from ambient asthenospheric mantle and complex petrogenetic processes before emplacement. Geochemical modeling indicates a variable but low degree of partial melting (1%–15%) and minor crustal contamination during magma ascent. The gabbro samples have relatively high TiO2 (1.94–3.73 wt%) and Zr (95–271 ppm) contents, typical of rocks formed in an intraplate tectonic setting. The granite porphyry samples display high FeOT/MgO and Ga/Al ratios, high contents of high-field-strength elements, and low CaO–Sr contents, resembling A-type granites. Compared with the gabbros, these porphyries show wider geochemical gaps, more variable ratios between incompatible elements, and lower radiogenic Nd isotopic compositions (εNd(t) = +0.3 to +2.7 and TDM2(Nd) = 2226– 2028 Ma). However, the porphyries have more radiogenic zircon Hf isotopes (εHf(t) = 0 to +3.7 and TDM2(Hf) = 2421–2189 Ma) than those of the gabbros, suggesting their derivation from the partial melting of a Paleoproterozoic crustal source triggered by an upwelling basaltic magma in an intraplate extensional setting. The ~1.75 Ga bimodal magmatism reported in this study, together with the ~1.75–1.65 Ga mafic magmatism documented in other cratonic blocks, suggests a possible spatial linkage between the Yangtze Block, northern Australia, northwestern Laurentia, and southern Siberia in the late Paleoproterozoic Columbia supercontinent.