Neoproterozoic magmatism in the northern margin of the Yangtze Block, China: Implications for slab rollback in a subduction-related setting

摘要

The South Qinling Belt was located in the northern part of the Yangtze Block during the Neoproterozoic. Here we investigate two major magmatic intrusions in the northern and southern part of the belt, the Douling plutons (DLP) and the Fenghuangshan plutons (FHSP), to gain insights into the Neoproterozoic tectonic processes along the northern segment of the Yangtze Block. Zircon grains in the DLP yield weighted mean Pb-206/U-238 ages of 750-709 Ma, whereas those from the FHSP show ages of 749-728 Ma. The new results, together with the published data, reveal that these plutons were emplaced at ca. 759-685 Ma and ca. 802-704 Ma, respectively. Geochemically, the intermediate-felsic rocks from the DLP and FHSP show enrichment of light rare earth elements (LREE) and large-ion lithophile elements (LILE) (Rb, Ba and K), depletion of high field strength elements (HFSE) (Nb, Ta, P and Ti), and low Sr/Y and (La/Yb)(N) values, which are typical features of arc magmatic rocks. The gabbros from the FHSP have high Mg#, and display LREE and LILE (Rb, Ba, K, and Sr) enrichment and depletion of HFSE (Th, U, Nb, Ta, and Ti), suggesting that they were probably formed in a mantle wedge setting. Both DLP and FHSP show arc-related geochemistry modified by slab-derived fluids, and were formed at relatively high-temperature and low-pressure conditions. However, they display distinct Sr-Nd-Hf isotopic compositions. Zircon grains of two samples from the DLP respectively show mean epsilon(Hf)(t) values of -2.6 +/- 0.3 and 0.9 +/- 0.3, whereas zircon grains of four samples from the FHSP show mean epsilon(Hf)(t)values ranging from 7.5 +/- 0.5 to 8.7 +/- 0.4. The DLP shows dominantly negative epsilon(Nd)(t) values and inconsistent initial Sr-87/Sr-86 ratios, whereas FHSP shows relatively constant and positive epsilon(Nd)(t) values (3.69-3.74) but variable initial Sr-87/Sr-86 ratios. The Sr-Nd-Hf isotopic and geochemical features suggest that the enriched mantle and crustal materials were involved