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

摘要

The fragmentation of the Columbia supercontinent is thought to have begun as early as similar to 1.7 Ga; however, the breakup of its core domain occurred only at similar to 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 U-Pb 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 Nb-Ta anomalies, moderately depleted Nd isotope signatures (epsilon(Nd) (t) = +2.4 to +5.3), and enriched zircon Hf isotopes (epsilon(Hf)(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 radio genic Nd isotopic compositions (epsilon(Nd)(t) = +0.3 to +2.7 and T-DM2(Nd) = 2226-2028 Ma). However, the porphyries have more radiogenic zircon Hf isotopes (epsilon(Hf)(t) = 0 to +3.7 and T-DM2(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 similar to 1.75 Ga bimodal magmatism reported in this study, together with the similar to 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. (C) 2019 Elsevier B.V. All rights reserved.