北京周口店岩体中钾长石巨晶的特征及成因

花岗岩类岩石中的钾长石巨晶,有的认为是斑晶,有的认为是变斑晶。本文通过对北京周口店岩体中钾长石巨晶的野外产状、显微结构、矿物化学等特征研究,证明巨晶是在岩浆结晶作用的早—中期阶段,直接从不饱和水的熔体中生长而成的斑晶。钾长石巨晶多见于较贫钾长石组分的花岗岩类岩石(石英二长岩-花岗闪长岩)中,其原因可能是,与花岗岩和白岗岩岩浆相比,这种成分的岩浆温度较高,粘度较小,组分扩散较快,钾长石的成核速率较小,而生长速度较大。周口店岩体中钾长石巨晶的大小和含量的变异,可能主要受冷凝梯度dT/dt的控制。

THE CHARACTERISTIC FEATURES AND ORIGIN OF K-FELDSPAR MEGACRYSTS IN THE ZHOUKOUDIAN PLUTON, BEIJING

Conflicting theories for the origin of K-feldspar megacrysts in granitoid rocks are considered that (1) the megacrysts are phenocrysts; and (2) they are porphyroblasts. Based on systematic studies of the mode of occurrence, microstructure, mineralogical chemistry,etc., it is argued that the K-feldspar megacrysts in the Zhoukoudian pluton, Beljing are phenocrysts crystallized directly from a water-undersaturated melt at the early to middle stages of crystallization. The reason for the common occurrence of K-feldspar megacrysts in granitoid rocks poorer in K-feldspar components (in the range from adamellite to granodiorite) may be that the magmas of such composition, as compared with granitic and alaskitic magmas, are high in temperature and low in vi- scosity, with greater diffusive coefficients of the components, smaller nucleation rate and greater growth rate of K-feldspar. The size and content variations of K-feldspar megacrysts in the Zhoukoudian pluton are controlled mainly by the cooling gradient dT/dt.