Hornblende Syenite Research Articles

Geology of the Alkaline Precambrian Rocks at Pajarito Mountain, Otero County, New Mexico

Pajarito Mountain is an oval-shaped Laramide dome involving Permian carbonates and red beds with a core of Precambrian rocks. The melanocratic rocks of the core have until recently been thought to be a Tertiary intrusive and perhaps laccolithic. The Permian beds lie on the core rocks unconformably above marked soil profiles of weathering, and radiometric dating shows the Precambrian to be near 1170 m.y. The rocks are predominantly melanocratic types high in amphibole. Fine- to medium-grained syenite is the principal terrane. Locally it is granitized or invaded by a coarser-grained granitoid syenite, and both of these are very locally invaded or converted to hornblende syenite pegmatite. Although hornblende is the predominant amphibole, riebeckite is common, and locally in the pegmatite late-stage aegirine has formed and commonly replaces the hornblende or riebeckite.

Riebeckite syenite from Idamakallu-Racherla, Kurnool dist., Andhra Pradesh

The study of the riebeckite syenite from Idamakallu-Racherla area shows interesting textural and microstructural features. Based on the petrographical and petrochemical evidence, it is surmised that a rock, which was originally a hornblende syenite, has been transformed into riebeckite syenite through soda-metasomatism. The way in which the mineral transformations took place and the secondary minerals have developed is discussed.

A preliminary note on the syenitic rocks form the Osawa-gawa area near Matsumae-mati, Oshima Province, southwestern Hokkaido

The Matsumae district is situated in the southern parts of the Oshima Peninsula in Hokkaido. Acidic and basic igneous rocks occur in the present area as a small mass at some places, that is, Osawa-gawa, Araya-gawa and Matsukura-gawa. The rocks consist mainly of pyroxene-biotite-hornblende diorite, hornblende-biotite granite, hornblende gabbro and pyroxene peridotite. The related minor rocks such as pegmatite, aplite and fine-grained granite are found in the dioritic complex. The writer describes the petrographical characters of the syenodiorite and melanocratic hornblende syenite found in the dioritic mass of the Osawa-gawa, and gives some considerations on their genesis.