Chronology, Hf isotopes, geochemistry, and petrogenesis of the magmatic rocks in the Shizishan ore field of Tongling, Anhui Province

Abstract

LA-ICPMS zircon U-Pb dating of intrusive rocks from the Shizishan ore field, Tongling, Anhui Province shows that the crystallization ages of Baimangshan pyroxene diorite, Shizishan and Dongguashan quartz diorite are 139±1.9, 138.0±1.7, and 138.4±1.7 Ma, respectively. These three intrusions are high-potassium, alkali-rich, meta-aluminous, and are characterized by enrichment of light rare earth elements and large-ion lithophile elements, depletion of heavy rare-earth elements (HREE) and high field strength elements, and slightly negative Eu anomalies. In-situ Hf analysis of zircon for these three intrusions shows large ranges of Hf isotope compositions: the ɛ Hf(t) values are ɛ Hf(t)= ∼3.3 to ∼14.4 for Baimangshan, ∼14.1 to ∼23.9 Dongguashan, and ∼14.7 to ∼26.9 for Shizishan. The ɛ Hf(t) values of both the Dongguashan and Shizishan quartz diorites are significantly lower than those of Baimangshan pyroxene diorite. Elemental geochemistry and in-situ zircon Hf isotope analyses indicate that the three intrusions were formed through mixing of granitic magma formed by partial melting of the lower crust and water-rich basic magma derived from the enriched lithospheric mantle. The Dongguashan and Shizishan quartz diorites involved greater crustal components than Baimangshan pyroxene diorite. The Dongguashan and Shizishan quartz diorites have geochemical features similar to the adakite, such as high Sr, low Y, and high Sr/Y ratios. Such geochemical features can be attributed to some fractional crystallization of apatite and sphene, and insignificant plagioclase fractional crystallization after magma mixing. These three intrusions are the products of complex crust-mantle interaction during the regional tectonic regime transition from compression to extension.