The Laccolites of Cutch and their Relations to the other Igneous Masses of the District
Authors: Blake J. F. 
Published: October 2009

abstract

The Author has observed thirty-two domes of various kinds in Cutch, distributed as follows:—(i) those connected with the northern islands; (ii) those of Wagir; and (iii) those along the northern edge of the mainland. They are divisible into four classes: (
a
) those which are so elongated on the line joining adjacent ones that they seem to be mere modifications of anticlinals, though the supposed anticline is not really continuous; (
b
) those which lie in a line, but are not elongated in that direction, and often in no other; (
c
) those which are related to a fault, which cuts them in half; and (
d
) those which are not in any particular relation to each other, or to any other stratigraphical feature.

The domes vary in degree of perfection: some are irregular, while some have the strata running in concentric circles, the outer and newer strata dipping away from the inner and older. In no less than ten of the thirty-two domes igneous bosses are found occupying the centre, and these are distributed amongst all of the above classes. The Author gives reasons for maintaining that the domes are the results of intrusion of igneous rocks in the form of laccolites, and are not anticlinal folds which have afterwards been affected by cross-folds. The domes are contrasted with igneous peaks which occur in abundance in a different part of the area, usually at a higher horizon of the strata and at a higher level above sea. These are probably volcanic pipes through which the lava was forced and extruded at the surface.

The Author compares the rocks of the bosses with those of the dykes and flows. Both are principally perfectly fresh dolerites, but the former are distinguished by the presence of intergrowths of micropegmatite as the last stage of consolidation, as in the ‘Konga diabases.’ There is also among them a felsite-breccia with micropegmatite developed in the cracks.

He considers that nearly all the igneous rocks of Cutch have been derived from a single magma, which in a solid condition must have contained large crystals of augite, olivine, and ilmenite in a groundmass of lime-felspars, and have been throughout of a basic character. Such a magma originated in more than one centre. One was possibly not far from the Sindree basin, whence lines of weakness diverged. Along these, owing to the thickness of the strata, there was no extrusion at the surface, and laccolite-domes were formed. Where the rock reached higher levels, it spread out into sheets between the domes and aided in the production of synclinals. Another centre was west of Bhuj, where the rock reached the surface without materially disturbing the sedimentary rocks, and formed the so-called ‘stratified traps.’ There were also areas here, especially to the south, where escape was impossible and domes were formed. The Author observes that his conclusions, if correct, may be applied to explain the source of the Deccan trap without eruptive centres. It may have been forced out from innumerable orifices as from a sieve, none of these being so much larger than others as to make a definite centre.