The Geology and Petrography of the Tarbela “Alkaline” Complex

Abstract

The Tarbela complex comprises gabbroic rocks (oldest), dolorites, melteigites, albitites, normal and sodic granites, albite-carbonate rocks/breccia, and carbonatites (youngest), together covering an area of about 1 sq. km. The rocks have been intruded along a fault zone between the Salkhala and Tanawal formation. Some of the gabbroic intrusions display in situ differentiation with one intrusion grading born pyroxenitic outer margin to leucogabbroic/dioritic interior with a core of intrusive albitites. Amongst typical alkaline minerals, sodic pyroboles are restricted to the sodic granite, and nepheline to the melteigite. However, trace elements in albite-carbonate rocks, the high quantity of albite ±carbonate along with consistent presence of zircon, and rutile/sphene in most albitites are indicative of their alkaline affinity. The gabbroic rocks might also be alkaline, as suggested by the abundance of amphiboles (hornblende, kaersutite, bastingsite), low An-content of plagioclase, clinopyroxene composition, the general absence of primary quartz and the possibly high Ti-content reflected in amphibole, sphene and ilmenite. These observations, coupled with the close field association point towards an overall alkaline nature of the complex. The rocks shown considerable degree of alteration (autometasomatism) with widespread development of scapolite, carbonate, amphibole, mica, sphene, rutile, etc. The country rocks have been metasomatised in the vicinity of the intrusions and scapolite, albite, carbonate,? quartz, pyrite have been produced. Some of the albitized sedimentary rocks resembles adinoles. This paper presents a detailed account of the petrography of the complex together with optical details of the minerals and a geological map. The petrogenesis of the complex is yet not clearly understood und hypothetical schemes based on differentiation under variable Pco² and liquid immiscibility are presented.