Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Roger H. Mitchell ¹

Affiliations
1 Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, P7B 5E1, CA

Abstract

The nomenclature of potassic lamprophyres occurring in the Gondwana coalfields is discussed from a mineralogical-genetic viewpoint. The concept of a lamprophyre facies and the mineralogical character of calc-alkaline minette and peralkaline phlogopite sanidine lamproite is described. It is concluded that none of the Gondwana coalfields rocks represent calc-alkaline minettes and that there is no "minette-lamproite" association. The Gondwana potassic rocks, together with similar potassic rocks from Majhgawan and the Mainpur region, are considered to be formed from differentiates of distinct magmas formed by the partial melting of metasomatic veins in the lithospheric mantle, and as such cannot be classified using nomenclatural schemes devised for other petrological provinces. The Gondwana potassic rocks, which are the expression of potassic metasomatized lithospheric magmatism in the northern Singhbhum craton, are considered not to be Kaapvaal craton-type orangeites or Kimberley craton-type lamproites, but might be described as lamproites (var.Damodar). It is recommended that use of the terms: "minette-lamproite association"; kimberlite-clan rocks"; and "lamprophyre clan rocks" be discontinued as they serve no useful petrogenetic purpose.

Keywords

Minette, Lamprophyre, Lamproite, Potassic Rocks, Metasomatism, Craton, Gondwana coalfields.

Full Text

     

Roger H. Mitchell ¹

Affiliations
1 Department of Geology, Lakehead University, Thunder Bay, Ontario P7B 5EI, CA

Abstract

Kimberlites, orangeites and lamproites are rocks derived by the differentiation and crystallization of genetically diverse mantle-Derived magmas. Currently, nomenclatural distinctions between these rock types are made on the basis of mineralogical-Genetic classifications and not simple modal classification schemes. The principle followed in such classification schemes is that the groundmass minerals reflect the fundamental characteristics of the melt from which they crystallized. Classification of most of these rocks cannot be made using bulk rock geochemistry as many contain significant and variable amounts of xenocrystal and xenolith material. It is now well-Known that kimberlites are mineralogically and geochemically distinct from orangeites and lamproites and are probably derived from depleted astheriospheric mantle sources. In contrast, isotopic and experimental petrological data indicate clearly that orangeites, lamproites and other potassic rocks are probably derived from ancient veined metasomatized (enriched) lithospheric mantle. Petrological studies have shown that each lamproite province has unique mineralogical and geochemical characteristics. These differences result from the differing metasomatic histories of the source rocks within individual cratons and accreted mobile belts. Orangeites are considered to represent the expression of potassic magmatism in the Kaapvaal craton and to have similar sources and origins to lamproites. The unique cratonic metasomatic history, perhaps coupled with distinct asthenospheric contributions as the impetus for magma genesis, of each potassic province explains why it is difficult to classify some potassic rocks occurring in the Sao Francisco, Aldan and Dharwar cratons as lamproites, orangeites or karnafugites. As the magmas from whlch these rocks crystallized are the unique expression of potassic magmatism in a particular craton they cannot, by definition, be classified according to schemes devised for potassic magmas in other provinces. Inter-Provincial similarities arise because of the common physicochemical character of the metasomatic process in the lithospheric mantle. The differences reflect variations in the ages and modes of the metasomatic veins coupled with differing degrees of partial melting and/or asthenospheric contributions to the magmas. Any given craton can contain both kimberlites and potassic rocks. Rocks in each potassic province could in theory be given its own terminology, which taken to an extreme could lead to a proliferation of cratonic type-Locality names. Alternatively, all of these diverse magmas might be collectively termed the metasomatized lithospheric mantle magma group (or MLM magmas). The mineralogical-Genetic approach to classification implies that none of the MLM magmas are transitional to kimberlite magmas and that use of the portmanteau term "kimberlite clan rocks" for diverse diamondiferous rocks of different genesis is inappropriate for both scientific and economic purposes. The diamond potential of each province of potassic rocks must assessed independently as this will also reflect the unique geological history of the parental lithospheric mantle. Correct classification of potentially diamondiferouos rocks is essential as this has implications with respect to exploration for, and evaluation of, particular intrusions using heavy mineral indicator suites and/Or geophysical methods.

Keywords

Potassic Magmas, Nomenclature, Kimberlite, Lamproite, Orangeite, Metasomatized Lithospheric Mantle.

Full Text