Abstract

Alkalic rocks are emplaced as conformable plutons in the Precambrian Aravalli rocks. These are generally associated with a gabbro. Petrographically, these coarse-grained alkalic rocks with a porphyroblastic texture contain a plagioclase (An5–30) as the dominant feldspar, a subordinate amount of microcline, an amphibole (magnesioriebeckite), and nepheline-sodalite-cancrinite. The occurrences show a, bilateral symmetry with a medial granitoid type, followed out on both sides by a foliated type merging into the peripheral layered impregnation gneiss. A nepheline-free diorite gneiss is generally present along the junction with country rocks. Foliation inside the alkalic rocks is parallel to the borders, to the foliation of the country rocks, and to foliation inside the inclusions, the inclusions having undergone no apparent rotation. Internal structures of the alkalic gneisses are identical with those of country rocks.

Structural, petrographical, mineralogical, and geochemical evidence indicates that the nepheline gneisses originated by in situ replacement of the gabbro. The process was essentially a type of alkali metasomatism in the course of which sodium, potassium, and alumina were added. The “alkalization” process was a phase of the regional granitization. Both these processes were syntectonic to start with, but their later stages outlasted tectonism.

Long after emplacement, the alkalic rocks along with the country rocks were modified by the superimposition of Delhi structure and metamorphism.

First Page Preview

First page PDF preview
You do not currently have access to this article.