NOTS ON LAMPROPHYRES

INTRODUCTION

The term lamprophyre was introduced by Gumbel (1874), to describe a group of dark colored dyke rocks from West Germany. In Greek the word “lampros” means bright or glistering. The rock collected from West Germany was bright due to biotitic flakes.

According to IUGS Sub-Commission lamprophyres are defined as follows. “They are distinct group of rocks, which are strongly prophyritic with mafic minerals, typically made up of biotite, amphiboles, pyroxene, feldspars and foids. They commonly occur as dyke or small intrusions showing hydrothermal alteration”.

CLASSIFICATION

Lamprophyres are classified into three groups. 1. Calc- Alkaline lamprophyres. 2. Alkaline lamprophyres, 3 Melilitic lamprophyres. The Calc Alkaline lamprophyres are called ordinary lamprophyres. The common members are Minette, Vogasite, Kersantite and Spessartite. In the alkaline lamprophyres, Sannaite, Camptonite and Monchiquite are grouped. Alnonite, Polzenite are called Melitite group.

MINERALOGY

Mica, amphibole, clinopyroxene, melilite are commonly occurring dark colored mineral. Usually they occur as phenocrysts. In the groundmass alkaline feldspar, plagioclase feldspar, foids, carbonates and mica are found. Petrographically, lamprophyres are showing porphyritic texture, but pan idiomorphic is common. Geochemically, they are enriched in K₂O and Na₂O. But generally they are ultra-potasic. High magnesium and iron are characteristic. Invariably lamprophyres are concentrated with H₂O, CO₂, S, P₂O₅ and Ba. They also have higher LREE. In the field lamprophyres occur as minor hypabyssal intrusions. They also found as sill, dyke, stocks, pipes and volcanic necks. Generally calc alkaline lamprophyres are found as multiple plutonic bodies.

ORIGIN OF LAMPROPHYRES

Lamprophyres are produced from alkaline magmas. These alkaline magmas are derived from lower crystal or upper mantle melting processes. When these alkaline magmas or lavas are contaminated, the lamprophyres are produced. Addition of volatile contents such as H₂O and CO₂, assimilation of alkaline materials may be the main cause for generation of lamprophyric liquids.

Lamporites are a group of alkaline rocks similar to lamprophyres they are mostly sub volcanic and extrusive. They differ from lamprophyres in mineralogy and higher SiO₂ content. They are having low foid content than lamprophyres. Lamporites are having diamonds and are close in association with Kimberlites.

INDIAN OCCURRENCES

In India lamprophyres are associated in coal fields of Raniganj and Bokkaro in the form of dykes. Minette is found Kurnool District of Andhra Pradesh. Comptonite is found in Kishangarh in Rajasthan.   Monchiquite is found in Mt.Girnar where as olivine Vogasite is found in Salem districts.