花岗质岩浆起源和多次岩浆混合的标志:包体——以北京周口店岩体为例

ENCLAVES AS INDICATORS OF THE ORIGIN OF GRANITOID MAGMA AND REPEATER MAGMA MINGLING:AN EXAMPLE FROM THE ZHOUKOUDIAN INTRUSION,BEIJING

北京周口店岩株,主要由两次侵入的石英二长闪长岩和花岗闪长岩组成,其中包体广泛分布。在岩体南缘的关坻变质岩中,零星出露了一种块状的闪长质岩石。关坻闪长岩与主岩体同期侵入的岩浆结晶产物,角闪石质包体的形成是二长辉长岩浆结晶—再平衡的结果,富云包体系花岗闪长岩浆源区的耐熔残余。通过二长辉长岩与花岗闪长岩两种岩浆简单混合可形成细粒石英二长质包体,由二长辉长岩、关坻闪长岩和花岗闪长岩3种岩浆混合形成了微粒闪长质包体。主岩体的成分变异,也与岩浆混合有关。

The Zhoukoudian stock in Beijing consists dominatly of quartz monzodioriteand granodiorite formed by intrusions of two episodes. A few types of enclavesoccur extensively in the pluton. In the Guandi metamorphic complex at the south-ern border of the stock, massive diortic rocks were sporadically exposed. It isbelieved that the Guandi dioritic rocks and the main instrusive mass were resultedfrom crystallization of magmas at the same period; and the main rocks andenclaves in the pluton were produced by repeated mixing of possible three typeof magmas. Among them, amphibolitic enclaves are suppossed to be the result ofcrystallization reequilibrium of monzogabbroic magma. The surmicaceous enclavesare refractory residues from granodioritic magmatic souced. The finegrained quartzmonzonitic enclaves could be by derived by simple mixing of monzogabbroic andgranodioritic magmas; the microgranular dioritic enclaves are the possible productof the mixing of monzogabbroic, dioritic and granodioritic mgmas. Compositional variation of the host body is explained in terms of mingling of monzogabbroicand granodioritic magmas in different proportions as well as the Soret effect afteremplacement.