湘东北地区燕山晚期强过铝质花岗岩的岩石化学特征及构造背景探讨

PETROCHEMICAL FEATURES AND TECTONIC SETTING OF LATE YANSHANIAN STRONGLY PERALU MINOUS GRANITES IN THE NORTHEASTERN PART OF HUNAN PROVINCE

根据岩石化学特征,湘东北地区燕山晚期花岗岩不属于A型花岗岩类,而是强过铝质的花岗岩。这些花岗岩可以分为两类。依据Frost等(2001)的定义,第一类为的镁质的钙碱性-碱钙质过铝花岗岩,属于高钾钙碱性系列;第二类为铁质的钙碱质过铝花岗岩,属于富钾的拉斑系列。第一类强过铝质花岗岩铁质含量低,Al2O3/TiO2比值低(<100);第二类铁质含量高,Al2O3/TiO2比值高(>100)。但大多数岩体的CaO/Na2O比值均小于0.3。这些特征暗示,湘东北地区燕山晚期第一类强过铝质花岗岩熔融温度较高(>875℃),第二类熔融温度较低(<875℃);而强过铝质花岗岩的源区主要为泥岩质成分。这些强过铝质花岗岩形成的大地构造背景是后碰撞阶段的造山崩塌环境,其深部热机制背景可能类似于欧洲海西造山带,形成花岗岩所需的能量来自软流圈地幔的平流热传递,以及基性岩浆对地壳的加热作用。

According to the petrochemical characteristics, the Late Yanshania n granites exposed in the northeastern part of Hunan Province are not the A-type granite, but belong to the strongly peraluminous granite (SPG). Based on the def inition of Frost et al.(2001), these granites can be classified into two groups. The first group is magnesian calc-alkalic to alkalic-calc peraluminous granit e, and belongs to high-potassium calc-alkaline series. Nevertheless, the secon d group is ferroan calc-alkalic peraluminous granite, and belongs to potassium-rich tholeiitic series. The first group is characterized by their lower ferroa n content and lower Al2O3/TiO2 ratio( ), while the second group exhibits higher ferroan content and higher Al2O3/TiO2 ratio(>100). However, most of these pluton s exhibit lower CaO/Na2O ratio( ) in both groups. These features imply that the former group is originated under higher temperature(>875℃), but the latter is o riginated from a relatively “cold”source( 75℃). Meanwhile, these Late Yanshan ian SPGs generally have a pelitic source composition, a geotectonic background s imilar to an orogenic collapse setting during the post-collision stage of the Y anshanian orogeny, and a deep thermal mechanism similar to that of the Hercynian orogen in Europe, while the energy as needed for the generation of granite coul d be due to the advective heat transfer from asthenosphere mantle as well as to the crustal heating by intrusive mafic magma.