Post-orogenic Intrusions Research Articles

Precise U–Pb zircon ages from the Gamitagama greenstone belt, southern Superior Province

Volcanism in the Gamitagama Lake greenstone belt, 50 km south of Wawa, Ontario, on the east shore of Lake Superior, took place [Formula: see text] ago at the same time as similar volcanism in the much larger Abitibi greenstone belt to the east. Two post-orogenic intrusions in the metavolcanic–metasedimentary terrain are coeval at [Formula: see text] and [Formula: see text] and could very well be comagmatic as suggested by Ayres. A granitic sill in a metasedimentary–granitic migmatite terrain that borders the greenstone belt to the southwest has an age of 2615 ± 10 Ma and is much younger than the post-orogenic plutons to the northeast. This migmatite region represents either an uplifted deep zone that remained hot long after volcanism or a region that accumulated heat as a result of its sedimentary cover.

Iron- and manganese-rich minor intrusions emplaced under late-orogenic conditions in the proterozoic of South Greenland

Three small intrusions in Ketilidian gneisses near Julianehaab comprise sheets and veins of olivine-magnetite-grunerite or magnetite-amphibole rocks partly surrounded by garnetiferous hornblende-biotite granitoid rock. The latter “skin” locally widens out into diffuse bodies of fayalite-orthopyroxene-quartz syenite or monzonite and biotite “granite”, which show layering similar to that resulting from gravity settling of crystals. Near the intrusions the country rocks lose their foliation and have been partially melted. Intrusion probably occurred at the close of regional metamorphism ∼ 1,750–1,780 m.y. ago, just prior to emplacement of the rapakivi granite suite of South Greenland. The mafic minerals of the intrusions are markedly enriched in iron and, in the case of olivine, orthopyroxene, grunerite and garnet, in manganese as well: olivine Fa90Te5Fo5; orthopyroxene (inverted pigeonite) Ca2Fe77Mn6Mg15; calciferous amphiboles are typically hastingsitic; biotites generally have Fe/Fe+Mg ratios of 0.8; garnets are almandine-grossularite-spessartine mixtures; essentially pure magnetite is the dominant oxide mineral and ilmenite is only moderately manganiferous. Crystallization of the mafic rocks appears to have followed the trend of the quartz-fayalite-magnetite buffer curve from perhaps ∼800°C to <550°C at pressures, calculated from thermodynamic considerations, of 4 to 5 kb. However, the presence of Mn makes estimates of pressure and temperature uncertain. Comparison with other late- to post-orogenic intrusions—the South Greenland and Finnish rapakivi granite suites, the Labrador “adamellite” complex and the Pikes Peak batholith of Colorado—reveals both similarities and differences, particularly with respect to mineral parageneses, depth of emplacement and manganese enrichment.

The X-ray obliquity of potassic feldspar in the granites of Jämtland, Northern central Sweden

Abstract On the basis of the K-feldspar obliquity the graniroid plutonics in Jämtland can be subdivided into two principal groupings: one comprises rocks of the Svecofennian orogenic-to-early-postorogenic development and the other restricted intrusions of the Ragunda and Sörvik types. In the first group, which includes the Revsund and Rätan granites with associated migmatites, the K-feldspars are close to maximum microcline with obliquities above 0.8. Granitoids belonging to the Ragunda and Sörvik types have K-feldspars which range from mono-clinic through inhomogéneous to triclinic structural states. A large area of low obliquity in the orogenic rocks of northern Jämtland is liable to interpretation as a thermal-influence aureole around the postorogenic intrusions. The effect of rock-blasting on K-feldspar obliquity has been checked in samples obtained from road cuttings.

Stratigraphy and Metamorphism of Late Precambrian Rocks in Central Northeastern Nevada and Adjacent Utah

Stratigraphy and Metamorphism of Late Precambrian Rocks in Central Northeastern Nevada and Adjacent Utah