Neoproterozoic alkaline intrusive complex in the northwestern Yangtze Block, Micang Mountains region, South China: petrogenesis and tectonic significance

Voluminous Neoproterozoic mafic-ultramafic, felsic, and alkaline intrusions are found in the northern Yangtze Block, South China. Here, we present whole-rock major and trace element, and Sr-Nd isotopic compositions, together with zircon U-Pb ages, for syenite and gabbro samples from the Shuimo-Zhongziyuan alkaline intrusive complex in the Micang Mountains region at the northwestern margin of the Yangtze Block. Zircon U-Pb dating yields crystallization ages for the Na- and K-rich Shuimo syenites of 869 +/- 4 (MSWD=0.85, 2 sigma) and 860 +/- 5Ma (MSWD=0.47, 2 sigma), respectively, and for the Zhongziyuan gabbros of 753 +/- 4Ma (MSWD=0.23, 2 sigma), indicating that the syenites and gabbros represent different stages of magmatism. The syenites include both Na- and K-rich types and have high values of the Rittman index (sigma), and high SiO2 and Na2O + K2O contents. These syenites are enriched in light rare earth elements (LREE) and large-ion lithophile elements (LILE), but depleted in high-field-strength elements (HFSE), with high (La/Yb)(N) values and small negative and positive Eu anomalies (Eu/Eu*=0.74-1.17). In contrast, the gabbros have lower SiO2 and Na2O + K2O contents, are only slightly enriched in LREEs, are enriched in LILE but depleted in HFSEs, and have small negative and positive Eu anomalies (Eu/Eu*=0.86-1.37). The syenites have low initial Sr-87/Sr-86 (0.703340) and (Nd)(t) values (+1.9 to +7.7). The gabbros have relatively high initial Sr-87/Sr-86 (0.703562-0.704933) and positive (Nd)(t) values (+1.6 to +4.5). These data suggest that the syenites and gabbros are isotopically similar and were largely derived from melts of depleted mantle. The syenites underwent significant fractional crystallization and small amounts of crustal contamination during magma evolution. In contrast, the gabbros were formed by partial melting (>15%) of a garnet lherzolite source and might also have experienced crustal assimilation. Taking into account the geochemical signatures and magmatic events, we propose that the Shuimo syenites formed in an intra-arc rifting setting, however, the Zhongziyuan gabbros were most likely produced in a subduction-related, continental margin arc setting during the Neoproterozoic, thus suggesting that the alkaline intrusive complex were formed by the arc-related magmatism in the Micang Mountains.