The chemical composition of REE-Y-Th-U-rich accessory minerals in peraluminous granites of the Erzgebirge-Fichtelgebirge region, Germany, Part I: The monazite-(Ce)-brabantite solid solution series
The chemical composition of REE-Y-Th-U-rich accessory minerals in peraluminous granites of the Erzgebirge-Fichtelgebirge region, Germany, Part I: The monazite-(Ce)-brabantite solid solution series
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DOI：
10.2138/am-1998-3-409
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发表时间：
1998-04
影响因子：
3.1
通讯作者：
H. Foerster
中科院分区：
文献类型：
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作者：
H. Foerster
德国Erzgebirge-Fichtelgebirge的过铝花岗岩是独居石矿物群中各种成员的东道主，显示出前所未有的成分多样性。Eibenstock s型花岗岩是世界上第三个报道的布拉辉石矿床，也是该矿物首次在花岗岩中发现。发现了许多新的闪辉石-(Ce)，以及一种介于(Ce) -（独居石）和赫顿石之间的独居石族矿物，并将其命名为赫顿质独居石。即使是“普通”的独居石-(Ce)也可能显示出锕系元素和镧系元素浓度的极端范围。含斑长岩和闪长岩-(Ce)的花岗岩为高分化、强过铝质、s型亲和的低温残余熔体，富氟等挥发性成分，缺钍和轻稀土元素。这种高度演化、富含挥发物的成分类似于稀有元素伟晶岩，似乎有利于闪辉石（Ce）和辉长岩的沉淀，但不利于含有大量胡顿质取代的独居石的沉淀。相反，这些矿物优先出现在a型亲和的贫f黑云母和富f锂云母花岗岩中。尽管硅酸盐熔体中铀的含量可能超过钍的含量，但在独居石中铀的替代仍然有限。本文报道的成分数据与岩浆条件下独居石-(Ce)-辉长石固溶体系列的完全混溶相一致。这些花岗岩中含有的独居石几乎涵盖了世界范围内报道的独居石群矿物的整个组成范围，因此花岗岩似乎是研究该矿物群晶体化学的理想岩石。
Abstract Peraluminous granites of the Erzgebirge-Fichtelgebirge, Germany, are hosts of various members of the monazite group of minerals that display an unprecedented compositional diversity. The Eibenstock S-type granite constitutes the third reported occurrence worldwide of brabantite and the first occurrence of this mineral in a granite. Many new occurrences of cheralite-(Ce), as well as a monazite-group mineral intermediate between monazite-( Ce) and huttonite for which the term huttonitic monazite is proposed, were discovered. Even ‘‘common’’ monazite-(Ce) may show extreme ranges of actinide and lanthanide element concentrations. The granites that host brabantite and cheralite-(Ce) are highly differentiated, strongly peraluminous, low-temperature residual melts of S-type affinity, which are rich in fluorine and other volatile constituents but depleted in thorium and the light rare-earth elements. Such highly evolved, volatile-rich compositions resemble rare-element pegmatites and appear favorable for the precipitation of cheralite-(Ce) and brabantite, but not of monazite with large amounts of huttonitic substitution. Instead, these minerals occur preferentially in F-poor biotite and F-rich Li-mica granites of A-type affinity. Irrespective of the level of uranium in silicate melts, which may exceed that of thorium, the substitution of uranium in monazite remains limited. The compositional data reported here are consistent with complete miscibility in the monazite-(Ce)-brabantite solid solution series under magmatic conditions. These granites contain monazites that span almost the entire compositional range reported for monazitegroup minerals worldwide, and therefore granites appear to be ideal rocks in which to study the crystal chemistry of this mineral group in general.