Тип публикации: статья из журнала
Год издания: 2021
Идентификатор DOI: 10.2113/RGG20194055
Ключевые слова: behavior of volatiles, compositional variations of apatite, fennoscandian shield, kola peninsula, layered intrusions, mafic–ultramafic complexes, mineralogical and geochemical crystallization trends, monchepluton layered complex, ree minerals
Аннотация: We have investigated the compositional variations of apatite (Ap) and rare-earth element (REE) minerals in the Moncheplu-ton layered complex on the Kola Peninsula. On the basis of large sets of pertinent analytical data, we have estimated geochemical trends involving major, minor, and trace elements and studied their relation with Показать полностьюthe compositions of rock-forming silicate and oxide minerals. The variations observed in Ap differ considerably from trends reported for other layered intrusions. The composition fields of Ap are not consistent with the variations in the chemical composition of the bulk rocks and their constituent minerals, as determined along the repre-sentative cross sections of the entire complex. The compositional variations of Ap are fairly similar in all units of the complex. Chlorapatite (>6 wt.% Cl) is invariably abundant. There is no relationship between the Cl content of Ap and the degree of magnesium enrichment of the coexisting early magmatic silicates. In the F–Cl–OH diagram, broad fields of ternary solid solution are observed. There are no compositions along the Cl–F axis. The compositions of Ap are notably poor in Cl in the marginal series (the Nyud massif) and correspond to hydroxy-lapatite with a high content of fluorapatite component. Two composition fields of Ap are recognized in the Monchepluton complex: ≤3 wt.% and >6 wt.% Cl; there are, however, extensive overlaps. Two generations of apatite are thus implied. The first nucleated at the early stage of crystallization of H2O-bearing intercumulus melt as a result of substantial increase in the contents of P, F, Cl, and other incompatible components. The following stage of degassing of the crystallizing melt caused a decoupling of Cl and F. Fluorine remained mostly in the melt; in contrast, Cl was partitioned efficiently into an H2O-bearing fluid phase. At the early stage, the apatite incorporated combinations of hydroxylapatite and fluorapatite, with a low content of Cl. At the late stage, chlorapatite crystallized from a Cl-rich fluid, and ferrochlo-ropargasite (4.1 wt.% Cl) formed in the Poaz massif as a result of autometasomatic alteration via reactions of this fluid with plagioclase and pyroxene. The apatite has high Sr contents (up to 4.1 wt.% SrO) in the highly magnesian cumulates of the Dunite block and the massifs of mounts Kumuzh’ya, Nittis, and Travyanaya. This enrichment illustrates the accumulation of Sr in the intercumulus melt, in which Ap was the only Sr-bearing phase in the absence or scarcity of intercumulus plagioclase. The REE contents also increased in the intercumulus melt and led to the formation of monazite-(Ce), REE-bearing Ap, and allanite-(Ce) in the remaining microvolumes of melt. Loveringite and Ap crystallized as coexisting phases in Mt. Sopcha. For the first time in a layered intrusion, an extensive range of compositions is documented in the Ce–La–Nd diagram for the REE-bearing phosphates (monazite and REE-rich apatite), which display a predominant La ↔ Nd substitution at the constant contents of Ce. © 2021, V.S. Sobolev IGM, Siberian Branch of the RAS.
Журнал: Russian Geology and Geophysics
Выпуск журнала: Vol. 62, Is. 4
Номера страниц: 427-444
ISSN журнала: 10687971
Издатель: Publishing House of Siberian Branch of the Russian Academy of Sciences