Nd and Sr isotopic compositions of igneous rocks from the Lower Yangtze region in eastern China: constraints on sources

Nd and Sr isotopic data are compiled from literatures for Mesozoic igneous rocks in the Lower Yangtze region, eastern China. These Mesozoic igneous rocks have been classified into three associations: Na-enriched (alkaline mafic) association, K-enriched association which includes shoshonitic series and ultrapotassic rocks, and high potassium calc-alkaline (HKCA) association. The HKCA association has been further divided into three geographical sub-groups: the rocks north to, along and south to the Yangtze River.

The Na-enriched mafic rocks show ε_Nd(T) of 1.4 to −9.9 and ⁸⁷Sr/⁸⁶Sr(T) of 0.7047 to 0.7077. The K-enriched rock association shows ε_Nd(T) of −1.8 to −9.3 and ⁸⁷Sr/⁸⁷Sr(T) of 0.7053 to 0.7092. Low SiO₂ in the rocks suggests that they are mantle-derived. These Nd-Sr isotopic variations may represent isotopic feature in the mantle source of these rocks. The Mesozoic mantle of this region is thus enriched to slightly depleted and highly heterogeneous in terms of Sr and Nd isotopic characteristics. The isotopic characters of the Mesozoic mantle of the region can be modeled by mixing between DMM component representing asthenospheric mantle and EM II representing lithospheric mantle. These characters closely resemble those of mafic and ultramafic rocks in the North Dabie terrane north to the region studied. This suggests that processes operating during continental collision and deep subduction in the Dabie orogenic belt might also operate in petrogenesis of mantle-derived rocks of the Lower Yangtze region. However, since the range of initial Sr and Nd isotopic ratios for the mantle-derived rocks in the Lower Yangtze region overlaps that of Mesozoic basalts from the eastern coast of China, it is yet not known if this is a prevalent phenomenon in the Mesozoic basalts from eastern China.

The intermediate and acid rocks of the HKCA association were most likely derived from mixing between the mantle-derived mafic magma and different crustal sources. The HKCA rocks occurring in the area north to the Yangtze River have ε_Nd(T) of −13.5 to −15.5 and ⁸⁷Sr/⁸⁶Sr(T) of 0.7058 to 0.7067. The low ε_Nd(T) and ⁸⁷Sr/⁸⁶Sr(T) suggest that a crustal source of old and depleted in Rb, such as the source of the Mesozoic granites from the Dabie terrain, was involved in the formation of this sub-group of rocks. The HKCA rocks along the Yangtze River show ε_Nd(T) of −7.0 to −16.5 and ⁸⁷Sr/⁸⁶Sr(T) of 0.7061 to 0.7101. Old crustal source, such as Archaean Kongling group and Paleoproterozoic Dongling group rocks, was involved in the formation of these rocks. The granitic rocks of the HKCA association in the area south to the Yangtze River show relatively constant ε_Nd(T) of −4.9 to −6.8 but variable ⁸⁷Sr/⁸⁶Sr(T) of 0.7081 to 0.7104. The ε_Nd(T) values of this sub-group rocks are very similar to those of the Neoproterozoic Shangxi group of low-grade metamorphic rocks and volcanic rocks in the region, suggesting that such a component is dominant in the source of this sub-group rocks. These data further suggest that the basement rocks possessing different Sr, Nd isotopic ratios and Nd model ages existed in different locations of the region. The basement similar to that of the Dabie terrain underlays in the depth of the area north to the Yangtze River. Existence of the old basement similar to the Kongling and Dongling groups in the depth of the area along the Yangtze River is very likely although only a few outcrops of the old metamorphic rocks were found in the area. However, the basement like the Neoproterozoic Shangxi group is dominant in the depth of the area south to the Yangtze River.