Wet calc-alkaline magmatic fractionation in the middle-upper crustal sections of continental arc: Insights from Neoproterozoic Nanba intrusive complex, western Yangtze Block, South China

Abstract

Elucidating petrological and geochemical characteristics of continental arc crustal fragments can provide unique insights into the magmatic evolutional path and physicochemical conditions (e.g. P-T-H2O) in continental magmatic arcs. This contribution provides a comprehensive dataset of petrology, mineralogy, geochronology, and geochemistry for Neoproterozoic Nanba intrusive complex in the western Yangtze Block, which stands for the well-exposed arc middle-upper crustal sections of Neoproterozoic continental magmatic arc, in order to evaluate their magmatic source and fractionation process. The Neoproterozoic Nanba intrusive complex, composed of cumulate gabbros, hornblende gabbros, gabbro-diorites, diorites, granodiorites, K-feldspar granites, and monzogranites, were emplaced at ca. 796–790 Ma. The different lithologies from the Nanba intrusive complex display relatively homogeneous Sr-Nd-Hf isotopes, indicating a cogenetic evolutional process. The predominantly depleted isotopic characteristics, together with high values of Ba, Rb/Y, and Ba/La, demonstrate that these rocks mainly originated from subduction fluids-metasomatized mantle source. Petrological and geochemical characteristics as well as hornblende thermobarometer collectively support that the ca. 796–790 Ma Nanba intrusive complex, following the calc-alkaline trend, was dominated by wet cogenetic fractionation of hornblende accompanied by plagioclase and accessory minerals under middle-upper crustal levels (2.4–4.7 kbar). The cumulate gabbros stand for accumulation residues of hornblende + plagioclase + Fe-Ti oxides from fractionated basaltic melts. The high-SiO2 K-feldspar granites and monzogranites display complementary trace elements, which represent residual silicic cumulates and extracted interstitial liquids in the shallower crystal mush reservoir, respectively. In combination with previous studies, we propose that the middle-upper crustal sections of Neoproterozoic continental arc in the western Yangtze Block underwent hornblende-dominated fractionation followed by significant crystal-liquids separation for the genesis of high-SiO2 melts in shallower crustal level. The cogenetic evolution plays a significant role into moulding petrological and geochemical diversities of Neoproterozoic igneous rocks and differentiation process of continental arc crust in the western Yangtze Block.