Reaction rind formation in the Catalina Schist: Deciphering a history of mechanical mixing and metasomatic alteration

The Evate deposit is a Neoproterozoic (~590 Ma) magnetite-apatite-carbonate body emplaced parallel to foliation of the Monapo granulite complex in NE Mozambique. A complicated history of the deposit is recorded in apatite textures visualized in cathodoluminescence (CL) images. In spite of different solid and fluid inclusions, mineral assemblages, and the CL textures, electron probe microanalyses indicate relatively consistent apatite compositions corresponding to fluorapatite (XF = 0.51–0.73, XOH = 0.21–0.47, XCl = 0.02–0.06) with limited belovite- and cesanite-type substitutions. Laser ablation inductively coupled plasma mass spectrometric analyses show that apatites from unaltered magnetite-forsterite-spinel ores are depleted in Y, REE, Ba, and Sr compared to apatites from carbonate-anhydrite ores. Hydrothermally overprinted apatites with complex patchy domain CL textures are enriched in Y-REE in greenish-grey zones, Fe-U-Th in blue zones, and Mn-Sr-Ba in brown domains. Observed CL-emissions in the Evate apatites result from very subtle variations in REE, Mn, and U contents controlled by the variability of redox conditions. The decreased Th:U ratio in the hydrothermally overprinted apatites reflects the oxidation and partial removal of U4+ from the apatite structure during the interaction with oxidizing aqueous fluids capable of transporting U6+. Flat, LREE (La-Sm)-enriched chondrite-normalized patterns with Eu/Eu* = 0.7–1.4 and Ce/Ce* = 0.9–1.5, together with concentrations of diagnostic trace elements (Sr, Mn, Y, REE) are consistent with apatites from magmatic carbonatites and phoscorites. This study corroborates that the Evate deposit is a post-collisional orogenic carbonatite genetically linked with mafic plutonic rocks intruding the Monapo granulite complex after granulite-facies metamorphism, and later overprinted by intensive hydrothermalism. The Evate apatite is peculiar in retaining its pristine magmatic signature despite the extensive hydrothermal-metasomatic alteration accompanied by dissolution-reprecipitation.

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Trace element and isotopic exchange during acid-basic magma interaction processes

Earth and Environmental Science Transactions of the Royal Society of Edinburgh ◽

10.1017/s0263593300006635 ◽

1996 ◽

Vol 87 (1-2) ◽

pp. 225-232 ◽

Cited By ~ 79

Author(s):

G. Poli ◽

S. Tommasini ◽

A. N. Halliday

Keyword(s):

Dynamic Environment ◽

Basic Magma ◽

Geochemical Characteristics ◽

Relative Mass ◽

Mechanical Mixing ◽

Interaction Processes ◽

Physico Chemical ◽

Mass Fractions ◽

History Of ◽

Static Versus Dynamic

ABSTRACT:Interaction processes between acid and basic magmas are widespread in the Sardinia–Corsica Batholith. The resulting hybrid magmas are extremely variable and can be broadly divided into: (i) microgranular mafic enclaves with geochemical characteristics of both magmatic liquids and cumulates; (ii) basic gabbroic complexes with internal parts mainly formed by cumulates and with interaction zones developing only in the marginal parts; and (iii) basic septa with the form of discrete, lenticular-like bodies often mechanically fragmented in the host rock. Different styles of interaction, ranging from mixing to mingling, have been related to variations in several physicochemical parameterś, such as: (i) the initial contrast in chemical composition, temperature and viscosity; (ii) the relative mass fractions and the physical state of interacting magmas; and (iii) the static versus dynamic environment of interaction.A model is presented for the origin and history of interaction processes between basic and acid magmas based on the geochemical characteristics of hybrid magmas. Physico-chemical processes responsible for the formation of hybrid magmas can be attributed to: (i) fractional crystallisation of basic magma and contamination by acid magma; (ii) loss of the liquid phase from the evolving basic magma by filter pressing processes; (iii) mechanical mixing between basic and acid magmas; and (iv) liquid state isotopic diffusion during the attainment of thermal equilibrium.

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The petrology, structure, and geologic history of the Catalina Schist Terrain, Southern California

Earth-Science Reviews ◽

10.1016/0012-8252(77)90148-9 ◽

1977 ◽

Vol 13 (4) ◽

pp. 390

Keyword(s):

Southern California ◽

Geologic History ◽

History Of ◽

Catalina Schist

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Metasomatism in the Whin Sill of the North of England. Part I: Metasomatism by Lead Vein Solutions

Geological Magazine ◽

10.1017/s0016756800099994 ◽

1929 ◽

Vol 66 (3) ◽

pp. 97-110 ◽

Cited By ~ 15

Author(s):

L. R. Wager

Keyword(s):

Solid Rock ◽

Igneous Rocks ◽

General Mechanism ◽

Subsequent Paper ◽

Slow Process ◽

Metasomatic Alteration ◽

Joint System ◽

Alteration Product ◽

The North ◽

History Of

The products of the metasomatism of igneous rocks are found for the most part only in small quantities since diffusion through a solid rock of the solutions responsible for metasomatic changes is a slow process. In the Whin Sill there are two types of metasomatic alteration product which though insignificant in total amount are nevertheless widely distributed. The first type, which was formed along the walls of an early joint system by the action of juvenile solutions, is part of the history of the cooling down of the Whin Sill and will be described in a subsequent paper. The second type was produced by the action of lead vein solutions. Although unconnected with any stage in the cooling down of the Whin Sill, this type has an important bearing on the general mechanism of replacement and especially on the part played by diffusion, whose effects may be seen in both types as an extended transition between altered and unaltered dolerite.

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