扬子地块西缘峨山新元古代高分异Ⅰ型花岗岩地球化学特征及岩石成因

Geochemical characteristics and petrogenesis of the Neoproterozoic Eshan highly fractionated Ⅰ-type granites,western Yangtze block

峨山高分异Ⅰ型花岗岩位于扬子地块西缘,是扬子地块新元古代岩浆岩带的重要组成部分,对其源区、成因及其构造背景进行系统研究能够揭示新元古代地壳演化历史。本文对峨山高分异Ⅰ型花岗岩开展了LA-MC-ICP-MS锆石U-Pb定年及岩石地球化学研究。研究结果表明,峨山花岗岩主要由肉红色中粗粒花岗岩和灰白色中粗粒花岗岩组成,它们的形成年龄分别为(746±34)Ma(MSWD=4.2)和(732±30)Ma(MSWD=3.3)。全岩地球化学显示峨山花岗岩具有高SiO_(2)(70.32%~78.41%)、Na_(2)O(3.09%~3.94%)、K_(2)O(5.13%~7.35%)含量,低CaO(0.52%~0.90%)、TiO 2(0.001%~0.025%)、P_(2)O_(5)(0.061%~0.097%)含量,富集K、Rb、Th等元素,亏损Nb、P、Ti等元素的特征,与高分异Ⅰ型花岗岩特征一致。全岩Sr-Nd同位素结果显示,ε_(Nd)(t)=-10.8~-7.5,两阶段Nd模式年龄(T DM2)为2.3~2.0 Ga。结合区域地质研究,本次研究认为峨山高分异Ⅰ型花岗岩是在伸展的构造背景下,地幔或年轻下地壳熔融形成的岩浆底侵由中高钾玄武质岩和黑云母片麻岩组成的古元古代上地壳,经部分熔融产生母岩浆,后经高程度的分离结晶作用而形成的。

The Eshan highly fractionated Ⅰ-type granites,situated in the western margin of the Yangtze block,represent a significant component of the Neoproterozoic magmatic belt within the region.A comprehensive investigation into their magma source,genesis,and tectonic settings can provide insights into the evolutionary trajectory of the Neoproterozoic crust.This study conducts zircon LA-MC-ICP-MS U-Pb dating and geochemical analyses on the Eshan highly fractionated Ⅰ-type granites.The results indicate that these granites predominantly consist of red medium-coarse granite and gray-white medium-coarse granite,formed at approximately 746±34 Ma(MSWD=4.2)and 732±30 Ma(MSWD=3.3),respectively.Geochemical analysis reveals a highly fractionated I-type signature characterized by elevated levels of SiO_(2)(70.32%78.41%),Na_(2)O(3.09%3.94%),and K_(2)O(5.13%7.35%),while exhibiting lower concentrations of CaO(0.52%0.90%),TiO 2(0.001%0.025%),and P_(2)O_(5)(0.061%0.097%).The rocks exhibit enrichment in K,Rb,Th,and depletion in Nb,P,and Ti.Sr-Nd isotope analysis showsε_(Nd)(t)values ranging from-10.8 to-7.5,with two-stage Nd model ages(T DM2)falling within the range of 2.3 to 2.0 Ga.Integrating these findings with regional geological studies,it is proposed that the Eshan highly fractionated I-type granites originated from the underplating of mantle-and/or juvenile mafic lower crust-derived magma that intruded the upper crust,triggering partial melting of Paleoproterozoic medium-to high-potassium basaltic rocks and biotite gneiss under an extensional tectonic regime.Subsequently,these granites were formed through a process of high fractional crystallization.