Hornblende in contact zone of granitic intrusions.

Abstract

The Caledonian intrusions in Northern Kazakhstan, which are situated predominantly among Ordovician altered greenstone volcanogenic strata, greatly affected the country rocks. The intrusions caused amphibolization, injection, metasomatism, and assimilation. This resulted in the formation in the contact zone of a large variety of heterogeneous rocks: hornblendes, amphibolites, gabbro-amphibolites, gabbro-diorites, and quartz diorites. They are related by a gradual transition to the tonalites and granodiorites in the central part of the intrusions. The presence of hornblendes is an important (and in places the only) rock-forming mineral, and is a common characteristic of most of the formations. The accumulation of a Mg variety of hornblende in the exocontact zone is characteristic of the process. The content of hornblende in altered rock does not correspond to the content of colored minerals in initial formations. It is sharply higher. This can be attributed only to the effect of the intrusion. The very abundance of the hornblende, distinctly metasomatic character of its development, and elevated amt. of water in hornblende accompanied by a large amt. of apatite in rocks, indicate the participation of volatile components during contact metamorphism. An elevated amphibolization of hornblendes was evidently caused by a delivery from the magma of a no. of mobile components and, before all, of Mg. The silicate analyses indicate that MgO in some rock varieties in the contact zone (amphibolites and gabbro amphibolites) was increased by 3-4 times, i.e. to 15-19% vs. the initial 3.5-6%. The phenomena, observed in endocontact (for example, corrosion and dissolving of hornblendes), suggest that amphibolization (basification) occurred at the expense of materials which were absorbed by the granitic magma during hybridization. The contact process caused a self-purification of the magma. Therefore, the assimilation by magma of the wall rocks was closely related to its decontamination, i.e., with the removal outside of the intrusion of the excess components of the assimilated rocks. [on SciFinder(R)]