大别造山带研子岗碱性岩体成因及其构造意义:锆石U-Pb年龄和地球化学制约

Petrogenesis and tectonic implication of Yanzigang alkali pluton in the Dabie orogenic belt: Constraints from the zircon U-Pb ages and geochemistry

出露于大别造山带西南部的研子岗碱性杂岩体侵位于元古代和新太古代随县群中,本文采用锆石LA-ICPMS U-Pb定年方法,获得岩体的主体岩性角闪正长岩的岩浆锆石年龄为133±1Ma,代表岩体的结晶年龄,这一年龄也是大别造山带中已知形成时代最早的早白垩世碱性岩体的结晶年龄(单颗粒锆石U-Pb法)。研子岗碱性杂岩体具有碱性岩典型的富碱(K_2O+Na_2O=8.38%～11.26%)、低硅(SiO_2=63.41%～66.51%)的特点。主要矿物为微斜条纹长石,暗色矿物主要为镁角闪石。地球化学特征表明,全部岩石均具有高Ba-Sr花岗岩类高Ba(1230×10^(-6)～4865×10^(-6))、高Sr(583×10^(-6)～2088×10^(-6))和无负Eu异常(Eu/Eu~*=0.97～1.12)等特征,并具有A型花岗岩的部分地球化学特征。岩石的Y/Nb比(0.46～1.09)较低,具洋岛玄武岩的部分地球化学特征,(^(87)Sr/^(86)Sr)_i初始比值(0.70513～0.70543)较低,表明岩体的物源主要来源于幔源。Nd二阶段模式年龄(t_(2DM)=1859～1942Ma)和锆石Hf二阶段模式年龄(t_(DM2)=2130～2330Ma)较老,ε_(Nd)(133)值(-12.5～-11.4) 和ε_(Hf)(133)值(-18.2～15.4)较低。综合分析表明,岩体主要为古老的富集岩石圈地幔物质低程度部分熔融和随后地壳物质轻度AFC(混染和分离结晶作用)过程的产物,源岩熔融的热量主要来源于软流圈物质的上涌底侵作用提供。研子岗岩体形成于碰撞后构造环境,岩体的形成预示着大别造山带早白垩世造山过程的即将结束,板内时期的即将来临。

This paper focuses on Yanzigang alkaline complex in the Southwest of Dabie orogenic belt, which emplaced into the Suixian Group of Proterozoic and New-Archaean era. Using zircon LA-ICPMS U-Pb dating method, we obtained the crystallization age of the amphibole-syenite is 133 +/- 1 Ma, this denotes the crystallization age of this intrusive body, and this is also represents the earliest crystallization age of Early Cretaceous alkali complex. Yanzigang alkaline complex have typical characters of alkaline rocks: high alkali (K(2)O + Na(2)O = 8.38% similar to 11.26%) and low silicate (SiO(2) = 63.41% similar to 66.51%). Main mineral compositions are microperthite and Mg-hornblende. Geochemical features indicate that the alkali rocks have high Ba-Sr granitoids features: high Ba (1230 x 10(-6) similar to 4865 x 10(-6)), high Sr (583 x 10(-6) similar to 2088 x 10(-6)) and no negative Eu anomalies (Eu/Eu* = 0.97 - 1.12), these rocks also have partial geochemical features of A-type granites. The low Y/Nb (0.46 similar to 1.09) indicates the rocks have partial geochemical characteristics of oceanic island basalt, the low ((87)Sr/(86)Sr)(i) value (0.70513 similar to 0.70543) indicates the magma has characteristic of mantle origin. The Nd two-stage model ages range from 1859 similar to 1942Ma and Hf two-stage model ages 2130 similar to 2330Ma, and the epsilon(Nd) (133) value are in the extent of -12.5 to -11.4, and the epsilon(Hf)(133) range from -18.2 to -15.4. Compressive analysis showed that the alkali complex were the product of old enriched mantle low grade partial melting and crustal material low degree assimilation and fractional crystallization subsequently. The heat came from the upwelling of asthenosphere materials. The Yanzigang intrusive body formed in the post-collisional environment, and the emplacement of this intrusive body promised the ending orogenic process in the Dabie orogenic belt of the Early Cretaceous, and the within plate period had been coming.