皖南绩溪县靠背尖高Ba-Sr花岗闪长斑岩年代学及其成因

The Kaobeijian granodiorite porphyry, outcrop area of 0.7km², is a small stock which intruded into Neoproterozoic-Paleozoic marine sedimentary strata in Jixi County, South Anhui Province. With hornblende grains, this porphyry are composed of SiO₂ 64. 05% ∼69. 64% and total alkalis (K₂O + Na₂O) 6. 1% ∼11. 5%, with A/CNK values 0. 77 ∼0. 98. Their LREE/HREE ratios are high and Eu is a bit depleted. It belongs to I-type granitoid formed in an island-arc environment. The Kaobeijian granodiorite porphyry have ε_Nd(t) values of -3. 69 ∼ -6. 24 and initial ⁸⁷Sr/⁸⁶Sr ratios of 0. 70784 ∼0. 71033, roughly in conformity to the syntectic type granites of southern China. The unusual high Ba-Sr composition comes from Neoproterozoic granodiorites which was enriched of Ba-Sr. The mantle-derived materials likely played an important role in the generation of the magma, letting the S-type magma gradually transformed into I-type magma in mixing process. In addition, lots of felsic-quartz veins and hydrothermal alteration indicate that the stock had been underwent an intensively residual melting and/or hydrothermal fluid activities. And the quartz of granodiorite were transformed into sub-grains under high temperature. All these indicate that pervious ⁴⁰Ar/³⁹Ar age of 134. 3 ± 1.4 Ma for biotite from the Kaobeijian granodiorite porphyry is more likely to document a tectonic-thermal event, rather than the forming age of the rock. SHRIMP zircon U-Pb analyses yield consistent ages of 151. 9 ± 1. IMa, 152. 9 ± 1. IMa and 147. 7 ± 1. 3Ma for three samples of granodiorite, indicate the magma was emplaced in Late Jurassic.