Kamafugitic rocks intruded the Precambrian basement and Phanerozoic sediments at the northeast border of the Paraná basin as part of the Late Cretaceous Goiás alkaline province (GAP). Plutonic complexes dominate the north of the province, whereas lavas and pyroclastic rocks prevail in the south. The central GAP is characterized by kamafugitic diatremes, which may crop out continuously for up to 850 m and consist of a central breccia body, surrounded and overlain by lava flows and crosscut by dykes. The breccias contain some special spheroidal juvenile fragments—namely, accretionary and armored lapilli, frozen droplets, spinning droplets, and wrapped fragments—whose textural and mineralogical aspects are described in detail. Irregularly shaped tuff pockets that occur within the breccias contain textures and structures similar to those of subaerial surge deposits and formed in confined, high gas to solid+liquid ratio domains in the conduit. Diatreme emplacement affected the country rock through thermal metamorphism, development of columnar jointing, and formation of peperite-like mixtures. There is no evidence of phreatomagmatic activity in the diatremes, and CO2, rather than H2O, seems to have been the major volatile component of the kamafugitic magmas. This finding implies that features such as accretionary lapilli and peperites are not exclusively associated with H2O-dominated processes. 相似文献
At Oricola (Aquila-Abruzzo, Italy) carbonatite is associated with phonolitic foidite tuff. The Oricola carbonatite contains fresh silicate glass of kamafugitic foidite composition which, compared with carbonate, shows similar trace element patterns but lower concentrations. As a whole, the mineralogy of the Oricola rocks matches that of the neighbouring Grotta del Cervo kamafugitic foidite and carbonatitic foidite and is in the range of the Intramountain Ultralkaline Province (IUP) of melilitites and carbonatites of Italy.
The IUP carbonatites and kamafugitic foidites definitely form intra-outcrop conjugate pairs. All these co-eruptive rocks have parallel trace element patterns, namely REE, which implies a dilution–concentration relationship among them but always with higher contents in primary calcite. Based on current textural and compositional criteria, as well as comparable experimental data, we attribute this feature to liquid immiscibility dominant over crystal fractionation at crustal pressure. This relatively late immiscibility phenomenon is superimposed on co-magmatic features shown by inter-outcrop conjugate rock couples. In fact if we consider San Venanzo kamafugite and Polino Ca-carbonatite, or Grotta del Gervo kamafugite and Oricola Ca-carbonatite, we note couple by couple that they are chemical heteromorphs erupted in isolation in different place. The REE distribution is another distinctive feature of these couples and shows a marked crossover at MREE level. A fact we interpret as produced by near mantle-solidus immiscibility. After this early phenomenon the two members of the couple can erupt in a near primary state carrying mantle xenoliths or undergo some evolution including settling out mantle xenoliths and crystals and fractionation and finally exsolve a carbonatitic residuum by immiscibility. 相似文献
This work focuses on the kamafugites from Santo Antônio da Barra, Minas–Goiás Alkaline Province. These rocks contain olivine, clinopyroxene, titanomagnetite, perovskite, leucite (pseudomorphs), kalsilite, nepheline, and phlogopite.The rocks investigated are ultrabasic, with high contents of CaO, FeO, and TiO2, high to moderate contents of Al2O3, alkalis, and P2O5, and low contents of MgO. The alkaline characteristic of the rocks is reflected in TiO2, K2O, and Na2O contents and in the frequent presence of normative nepheline and leucite. K2O contents are not primary since most of the leucite was replaced by analcime.The negative K anomaly verified in the extended incompatible element distribution diagram for kamafugites seems to be mainly related to alteration. Kamafugites are characterized by a marked enrichment in incompatible and large ion lithophile elements together with other typical compatible elements.The Santo Antônio da Barra kamafugites are less enriched in titanium, niobium, zirconium, and REE than the Mata da Corda and most of the Toro-Ankole ones. San Venanzo–Cupaello rocks have much lower titanium contents. 相似文献
We present the results of a U–Pb perovskite age study of kamafugites from Mata da Corda (MC) and Santo Antônio da Barra (SAB), Minas-Goiás alkaline province, Brazil. Perovskite crystals were separated from MC mafurites, ugandites, and cognate pyroxenites, as well as from SAB melilite mafurite. The range of ages of Brazilian kamafugitic samples is 15 Ma. The 206Pb/238U perovskite ages generally cluster into three age groupings: 88–90, 80–81, and 75–76 Ma. The two younger periods of kamafugitic magmatism occur in the MC area, whereas the older samples are from the SAB area. These new age results provide the first robust evidence of a progressive eastward younging of mafic alkaline magmatism, most likely related to a mantle plume hotspot track. 相似文献