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浙江桃花岛、青田和山东崂山A型花岗岩是中国东部沿海三个典型的燕山期碱性A型花岗岩体.利用电子探针对这些花岗岩中锆石的内部结构和矿物成分进行了分析,结果显示大部分所观察到的锆石颗粒都由两部分组成.一部分显著富钍,ThO2含量大于1%,最高达10.1%,超过了前人实验获得的锆石中钍的溶解极限值(ThO2=5.5%±2.5%),且ThO2/UO2>2,而另一部分则贫钍,ThO2含量小于1%,但该区域以包含微米级钍石包裹体为特征,并常伴有微小的孔洞.同时进行的初步对比研究发现,铝质A型花岗岩中的锆石ThO2含量一般小于1%,ThO2/UO2<2,并且不含任何钍石包裹体.锆石成因矿物学特征表明,富钍锆石可以看作为碱性A型花岗岩的标志性副矿物之一.锆石的富钍性源于其原始岩浆,预示其深源特点.因此,碱性A型花岗岩中高度富钍锆石形成于深源、高温、富钍岩浆的早期结晶阶段;岩浆分异至晚期出现流体相富集,富钍锆石受到流体作用发生溶解重结晶作用,导致低钍锆石和钍石包裹体共生现象.
Zhejiang Peach Blossom Island, Qingtian and Shandong Laoshan A-type granites are three typical Yanshanian alkaline A-type granites along the east coast of China.The internal structure and mineral composition of zircons in these granites were analyzed by electron probe. Most of the observed zircon grains are composed of two parts, with a significant thorium rich content, a ThO2 content of more than 1% and a maximum of 10.1%, exceeding the dissolution limit of thorium in zircon obtained in previous experiments (ThO2 = 5.5 % ± 2.5%), and ThO2 / UO2> 2, while the other part is thorium-depleted and the ThO2 content is less than 1%, but this region is characterized by inclusions of micrometric thoriated inclusions and is often accompanied by tiny holes. A preliminary comparative study found that the zircon ThO2 content of aluminum type A granites is generally less than 1%, ThO2 / UO2 <2, and does not contain any thorium inclusions. Zircon genetic mineralogical characteristics indicate that thoriated thorium can As one of the typical accessory minerals of alkaline A-type granites, the thorium-rich zircon of zircon originates from its original magma, indicating its deep-source characteristics. Therefore, the highly-thoriated zircons from alkaline A-type granites are formed in deep source , High temperature, rich thorium magma early crystallization stage; magmatic differences to late out Phase enriched fluid, zircon thorium rich fluid takes place by dissolution recrystallization, resulting in low thorium and thorium zircon stone inclusions symbiosis.