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Abstract
Early Paleozoic alkaline mafic rocks hosting Fe-(Ti) deposits and intermediate rocks hosting Nb-REE deposits are widely exposed in the North Dabashan area of China; however, their petrogenesis and relationship with regional mineralization remain unclear. In this contribution, a case study of alkaline diorite and dolerite dykes located in the Laogao-Zhenping area of the North Dabashan region is presented, aiming to shed light on the genesis of their parental magmas. Zircon U-Pb dating of the dolerite yields concordia plots and shows a Silurian intrusion age of 433 ± 3 Ma, coinciding with the regional ca. 440–430 Ma alkaline magmatic event. The mafic dykes exhibit enrichment in HFSE and REE, along with positive εNd(t) values (+1.08 to + 4.53), consistent with geochemical signatures of ocean island basalts (OIB). The (Th/Nb)PM (0.55–0.86) and La/Sm (3.05–5.12) ratios show narrow variations, and a Nd-isotopic mixing model of the depleted mantle and average lower crust suggests insignificant involvement of crustal contamination (3 %–7%) during magma ascent. In addition, high Dy/Yb (1.85–3.30), Ba/Rb (19.69–67.26), and low Rb/Sr (0.01–0.07) ratios point to a garnet-facies amphibole lherzolite source for the parental magma. The clinopyroxene thermobarometer constrains the dolerite’s crystallization temperatures between 1093–1224 °C and pressures between 0.4 and 8.3 kbar, suggesting that the mafic alkaline magma ascended from the mantle and emplaced at different upper crustal levels. Fractional crystallization is proposed as a key contributor to regional Fe-(Ti) and Nb-(REE) mineralization, supported by (a) similar trace elemental and Nd isotopic signatures and (b) evolutionary trends in major elements from the alkaline dolerite-basalt, through the diorite-dioritic porphyrite, to the syenite-trachyte assemblages in the Ziyang-Zhushan area.
Published Version
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