Influence of fluorine on the enrichment of high field strength trace elements in granitic rocks

The geology, geochemistry of Iroko-granites have been studied and reported in this paper. The study area has been described as of Archaean –Early Proterozoic terrain underlain by migmatite-gneiss-quartzite complex with supracustal rocks. Large number of granites which outcrops in Iroko-Ekiti represent a typical occurrence of granitoids sporadically distributed in the basement and are known to belong to the Older Granite suites, which are attributable to the Pan-African Orogeny(750± 150Ma). The rocks occur, mostly as flat and low lying within sparse vegetation. Structures common on and around the outcrops include quartz vein, veinlets, pegmatite dykes which trend North-south, discrete exfoliated surfaces and xenoliths of older rocks. This study reveals that the granites belong to calc-alkalic suites, demonstrate metaluminous nature, and exhibit characteristics of I-type granites. The granite is a distinctive type in that it is relatively highly potassic, has high FeO/(FeO + MgO) ratio, and high average Zr (299.75ppm) concentration with other high field strength elements. The trace elements study implicates pronounced fractional crystallization during evolution of the granites and thus petrogenetically discriminates as Syn-collision provenance.

Download Full-text

Geochemical and Sr and Nd isotopic constraints on the origin of late Proterozoic volcanics and associated tin-bearing granites from the Franklin Mountains, west Texas

Canadian Journal of Earth Sciences ◽

10.1139/e87-081 ◽

1987 ◽

Vol 24 (4) ◽

pp. 830-839 ◽

Cited By ~ 19

Author(s):

David I. Norman ◽

Kent C. Condie ◽

Robert W. Smith ◽

W. F. Thomann

Keyword(s):

Field Strength ◽

Hydrothermal Alteration ◽

High Field ◽

Granitic Rocks ◽

High Field Strength ◽

Late Proterozoic ◽

West Texas ◽

Depleted Mantle ◽

Fluoride Complexes ◽

Back Arc

Late Proterozoic volcanic and granitic rocks from the Franklin Mountains in west Texas define a continuous tholeiitic trend from rhyolite (granite) to trachyte with a small compositional gap between trachyte and basalt. Volcanics and granitic rocks have immobile-element geochemical affinities with volcanics from continental-margin-arc systems and associated back-arc basins. Felsic volcanics and granitic rocks appear to have formed by fractional crystallization of basalt during which REE and high-field-strength elements were stabilized as fluoride complexes and concentrated in late or post-magmatic fluids.A Rb–Sr isochron from nonhematized samples (1064 ± 5 Ma, i = 0.7034) falls within the error of reported U–Pb zircon dates and probably dates a period of hydrothermal alteration. εNd values (0.7–2.5 at 1065 Ma) fall below a depleted mantle growth curve and probably reflect contamination of the fractionating magmas with older continental crust. Changes in the isotopic composition of Nd during hydrothermal alteration may also contribute to the spread in εNd values.

Download Full-text

Immiscibility and the origin of ladder structures, mafic layering, and schlieren in plutons

Geology ◽

10.1130/g47634.1 ◽

2020 ◽

Vol 49 (1) ◽

pp. 86-90

Author(s):

Allen F. Glazner ◽

John M. Bartley ◽

Bryan S. Law

Keyword(s):

Field Strength ◽

High Field ◽

Liquid Immiscibility ◽

Granitic Rocks ◽

Calc Alkaline ◽

High Field Strength ◽

Incompatible Elements ◽

Modal Layering ◽

Crystal Accumulation ◽

Chemical Trends

Abstract Granitic plutons worldwide contain ladder structures (LSs) that consist of nested trough-shaped layers alternating between mafic and felsic compositions. LSs and other forms of modal layering have been attributed to crystal accumulation, but their chemical trends differ greatly from those of cumulates and are discordant with chemical variations of their granitic hosts. Mafic layers reach extreme enrichments in transition metals, high-field-strength elements, and incompatible elements, and are extremely depleted in Si and Al. These geochemical characteristics are difficult to explain by crystal accumulation and conflict with sequences of phase appearance during crystallization. They are characteristic of liquid immiscibility, which is an accepted process in the genesis of tholeiitic and alkalic rocks. We propose that ladder structures and other forms of modal layering are markers of immiscibility in calc-alkaline granitic rocks.

Download Full-text