Geochemistry and zircon U–Pb–Hf isotopes of the 780 Ma I-type granites in the western Yangtze Block: petrogenesis and crustal evolution

Neoproterozoic I-type granites could provide vital insights into the crust–mantle interaction and the crustal evolution along the western Yangtze Block, South China. This paper presents new zircon U–Pb ages, bulk-rock geochemistry, and in situ zircon Lu–Hf isotope on the Dalu I-type granites from the southwestern Yangtze Block. Zircon U–Pb dating show the crystallization ages of 781.1 ± 2.8 Ma for granodiorites and 779.8 ± 2.0 Ma for granites, respectively. The Dalu granodiorites are Na-rich, calc-alkaline, metaluminous to slightly peraluminous (A/CNK = 0.94–1.08). Zircons from granodiorite have positive εHf(t) values (+2.16 to +7.39) with crustal model ages of 1.21–1.54 Ga, indicating juvenile mafic lower crust source. The Dalu granites are high-K calc-alkaline, peraluminous rocks. They have variable zircon εHf(t) values (−4.65 to +5.80) with crustal model ages of 1.31–1.97 Ga, suggesting that they were derived from the mature metasediment-derived melts by the mixing of newly formed mafic lower crust-derived melts. The geochemical variations in Dalu pluton is dominated not only by the different source rocks but also by the different melting temperatures. Combining with the geochemistry and isotopic compositions of I-type granitoids and tectonic setting in the western Yangtze Block, we propose that the Dalu I-type granodiorites–granites associations are the magmatic response from different crustal levels, which were induced by the heat anomaly due to the asthenosphere upwelling in the subduction-related setting.

新元古代I型花岗岩（Neoproterozoic I-type granites）可为华南扬子地块西部的壳幔相互作用与地壳演化提供关键研究视角。本文针对扬子地块西南部的达鲁I型花岗岩（Dalu I-type granites），报道了全新的锆石U-Pb年龄、全岩地球化学数据以及原位锆石Lu-Hf同位素数据。锆石U-Pb定年结果显示，花岗闪长岩的结晶年龄为781.1±2.8 Ma，花岗岩的结晶年龄为779.8±2.0 Ma。达鲁花岗闪长岩具有富钠、钙碱性、准铝至弱过铝质的特征（A/CNK值为0.94~1.08），其锆石εHf(t)值为正值（+2.16~+7.39），壳模式年龄介于1.21~1.54 Ga之间，指示源区为新生基性下地壳。达鲁花岗岩属于高钾钙碱性过铝质岩石，其锆石εHf(t)值变化范围较大（-4.65~+5.80），壳模式年龄为1.31~1.97 Ga，表明其由成熟沉积变质岩熔融形成的熔体与新生基性下地壳熔融熔体混合演化而来。达鲁岩体的地球化学分异不仅受控于源岩类型的差异，同时也受熔融温度变化的影响。结合扬子地块西部I型花岗岩类的地球化学、同位素组成及区域构造背景，本文提出达鲁花岗闪长岩-花岗岩组合代表了不同地壳层次的岩浆响应，该岩浆作用由俯冲构造背景下软流圈上涌引发的热异常所驱动。