Petrogenesis of the Early Cretaceous adakitic intrusive rocks in Chuzhou and Guandian, Anhui Province: Constraints from zircon U-Pb geochronology and Sr-Nd-Hf isotopic compositions

Abstract

本文以华北陆块东部早白垩世滁州闪长玢岩和管店石英闪长岩为研究对象，报道了新的全岩地球化学（主量元素、微量元素以及Sr-Nd同位素）以及锆石U-Pb年代学和Hf同位素的综合研究结果，进而约束其岩石成因和构造意义。滁州闪长玢岩中锆石发育振荡环带，管店石英闪长岩中锆石具条痕状吸收的特点，它们均为岩浆成因。锆石LA-ICP-MS U-Pb定年结果显示，滁州和管店闪长质锆石年龄分别为128±1Ma和130±2Ma，表明二者均形成于早白垩世。地球化学特征显示，滁州和管店闪长质岩石具有类似的主量元素和微量元素组成，整体具中等的SiO₂含量（58.55%~60.12%）和富MgO（4.43%~5.10%，Mg^#=57~61）的特征，Na₂O/K₂O比值为1.20~1.47，属于高钾钙碱性系列岩石；富集轻稀土元素和大离子亲石元素（如Rb、Ba），亏损重稀土元素和高场强元素（如Nb、Ta、Zr、Hf），并具不同程度的Eu负异常（δEu=0.85~0.93）。它们具高的Sr（575×10^-6~1035×10^-6）和低的Y含量（10.2×10^-6~15.3×10^-6）以及高的Sr/Y比值（57~69），属于埃达克质岩石。此外，滁州和管店闪长质岩石具有类似的Sr-Nd以及锆石Hf同位素的特征，滁州闪长玢岩的⁸⁷Sr/⁸⁶Sr初始比值介于0.7061~0.7066之间，ε_Nd（t）值为-17.7~-16.9，锆石ε_Hf（t）值变化于-23.1~-19.1之间，而管店石英闪长岩的⁸⁷Sr/⁸⁶Sr初始比值为0.7046~0.7059，ε_Nd（t）值为-18.7~-15.8，锆石ε_Hf（t）值为-25.6~-20.0。综合上述，滁州和管店闪长质岩石均属于高镁埃达克质岩石，起源于拆沉的加厚下地壳部分熔融的熔体与地幔橄榄岩反应的产物，形成于早白垩世华北东部陆块拆沉作用下的伸展背景。;This paper reports new geochemical data involving whole-rock major, trace and Sr-Nd isotopic compositions, and zircon U-Pb ages with Hf isotopic data for the Chuzhou dioritic porphyry and Guandian quartz diorite in the Early Cretaceous, with the purpose of constraining their petrogenesis and tectonic significance. Zircons in the dioritic porphyry display typical oscillatory zoning and in quartz diorite show clear striped absorption in cathodoluminescence images, both of which have magmatic origin. LA-ICP-MS zircon U-Pb dating yield age of 128±1Ma in dioritic porphyry and 130±2Ma in quartz diorite respectively, indicating that they were both emplaced during the Early Cretaceous. The dioritic rocks show intermediate SiO₂ (58.55%~60.12%) and high MgO contents (4.43%~5.10%, Mg^#=57~61) with Na₂O/K₂O=1.20~1.47, which are classified as high-K cala-alkaline series. In addition, they are enriched in LREE and LILE (e.g., Rb and Ba), depleted in HREE and HFSE (e.g., Nb, Ta, Zr and Hf) with negative Eu anomalies (δEu=0.85~0.93), and exhibit high Sr (575×10^-6~1035×10^-6) and low Y contents (10.2×10^-6~15.3×10^-6) with significantly high Sr/Y ratios (57~69), being typical of adakitic rocks. Moreover, the Chuzhou dioritic porphyry has uniform (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7061~0.7066, ε_Nd(t) values of -17.7~-16.9, and ε_Hf(t) values of -23.1~-19.1, and Guandian quartz diorite has similar (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7046~0.7059 and variable ε_Nd(t) (-18.7~-15.8) and ε_Hf(t) (-25.6~-20.0) values. As a consequence, the adakitic intrusive rocks in Chuzhou and Guandian are derived from the partial melting of delaminated lower crust in contamination with mantle peridotite, and we supposed that the rocks are formed in an extension setting during the Early Cretaceous delamination in eastern North China Block.