Ilmari Haapala

Tectonic setting and origin of the Proterozoic rapakivi granites of southeastern Fennoscandia

The 1.65-1.54 Ga rapakivi granites of southeastern Fennoscandia represent the silicic members of a bimodal magmatic association in which the mafic members are tholeiitic diabase dykes and minor gabbroic-anorthositic bodies. They are metaluminous to slightly peraluminous A-type granites and occur as high-level batholiths and stocks in an E-W-trending belt extending from Soviet Karelia to southwestern Finland. The Soviet Karelian granites were emplaced into the contact zone between Archaean craton and Svecofennian juvenile 1.9 Ga-old crust, while the Finnish granites were intruded into the Svecofennian crust

Rapakivi granites and related rocks: an introduction

Abstract This article is an introductory chapter for the proceedings of the last, seventh, meeting of the International Geological Correlation Programme (IGCP) Project 315 “Correlation of Rapakivi Granites and Related Rocks on a Global Scale”, 1991–1996. Based on current knowledge, progress made during the six years of the project, and the seven papers published in this special issue, a synthesis of the age distribution, geotectonic environment, petrogenesis, geochemistry, and metallogeny of the rapakivi granites and related rocks is presented

1700 Ma Shachang Complex, Northeast China; Proterozoic rapakivi granite not associated with Paleoproterozoic orogenic crust

Proterozoic rapakivi granites are found on all continents and are characteristically associated with Paleoproterozoic crustal domains that predate the granites by

Intermittent 1630–1220 Ma magmatism in central Mazatzal province: New geochronologic piercing points and some tectonic implications

The northern Burro Mountains in southwestern New Mexico reveal three distinct, intimately juxtaposed Mesoproterozoic magmatic suites in southern Laurentia. At 1633 Ma, the newly formed Mazatzal crust was intruded by tholeiitic diabase with a depleted-mantle–type Nd isotope composition but with enriched incompatible trace element abundances. A potassic granite-minette suite was emplaced ca. 1460 Ma, and a tholeiitic A-type granite-anorthosite suite intruded ca. 1225–1220 Ma. The diabase-minette-anorthosite sequence and the associated silicic rocks record dominantly juvenile additions to the cratonic margin and imply subcontinental enrichment events ca. 1650 Ma (accretion), prior to 1460 Ma (potassic metasomatism), and ca. 1220 Ma (magmatic underplating). The latter two may have been controlled by a major transcurrent structure along the south margin of Laurentia.

Petrology of nattanen-type granite complexes, northern finland

Abstract The Nattanen-type granites form a group of post-tectonic and post-metamorphic 1.77 Ga-old epizonal multiple granite intrusions in the Granulite Belt—granitic gneiss surroundings in northern Finland. The granite complexes (the Tepasto, Pomovaara and Nattanen stocks and the Riestovaara batholith in Finland and the Juvoaivi stock in the U.S.S.R.) are composed of various slightly peraluminous microcline—plagioclasebiotite granites (monzogranites) that contain sphene and magnetite as typical minor constituents. Geochemical evolution within the complexes is demonstrated by the increase of SiO2, and Rb, and decrease of TiO2, FeO∗, MgO, CaO, Sr, Ba and Zr from the oldest to the youngest granite varieties. Molybdenum mineralization is associated with the latest intrusive phases of the Tepasto and Juvoaivi stocks. The isotope geochemical studies indicate a major Archaean crustal component in the source of the Nattanen stock that is situated within the early Proterozoic Granulite Belt.

Anorogenic Gross Spitzkoppe granite stock in central western Namibia: Part I. Petrology and geochemistry

Abstract The Gross Spitzkoppe is a small (~30 km2) anorogenic granite stock that was emplaced into the Damara Orogenic Belt of central western Namibia in response to mantle plume activity and early Cretaceous rifting of western Gondwanaland. The epizonal Gross Spitzkoppe stock (GSS), penecontemporaneous mafic and felsic dikes in the country rocks, and synplutonic mafic dikes and mafic magmatic inclusions within the GSS indicate bimodal magmatism. The GSS consists of three zonally arranged granitic units: medium-grained biotite granite at the margins, coarse-grained biotite granite, and porphyritic granite at the center of the stock. Late-stage silicic dikes and pegmatites cut the granites of the stock. All the granites are mildly peraluminous, topaz-bearing, high-silica monzogranites that contain extremely Fe-rich biotite (siderophyllite-annite) as the only primary mafic silicate. Common accessory minerals include: topaz, fluorite, magnetite, zircon, monazite, thorite, ilmenite, columbite, and niobian rutile. Miarolitic cavities and pegmatite pockets within the stock suggest that fluid saturation was attained. Rectilinear structures of biotite-rich schlieren along the marginal parts of the stock suggest that magma flow was parallel to the walls of the chamber, whereas turbulent or plume-like flows are indicated by curved, circular, and ladder-dike schlieren. Geochemical variation among the granites can be attributed to fractional crystallization of major and accessory minerals and by accumulation of halogen complexes with rare-earth and high-field-strength elements. The negative, partially irregular, correlation between whole-rock contents of F and Cl suggests that Cl was partially lost to a vapor phase during magmatic degassing. The granites of the Gross Spitzkoppe show clear A-type and within-plate granite characteristics, corresponding to the actual tectonic setting. The granites are interpreted to have formed by partial crustal remelting related to mafic underplating in a continental rift environment

Anorogenic Gross Spitzkoppe granite stock in central western Namibia: Part II. Structures and textures indicating crystallization from undercooled melt

Abstract The anorogenic Cretaceous Gross Spitzkoppe granite stock and associated bimodal dike swarms, are related to continental rifting, which led to separation of Africa from South America. The stock is composed of highly evolved, topaz-bearing, A-type granites. Subhorizontal (≤20m wide) aplite dikes and thinner aplite-pegmatite dikes cut the granites. The contact of the stock and the upper contacts of subhorizontal microgranite-aplite dikes are marked by “stockscheiders” composed of layered aplites and pegmatites. The layering is characterized by rhythmic variations in grain size, grain morphology, and mineral composition. Regular orientation of columnar alkali feldspar crystals, or plumose alkali feldspar-quartz intergrowths and dendritic biotite, indicate unidirectional crystal growth against the heat flow: in the marginal stockscheider toward the stock, in the upper parts of the subhorizontal aplite dikes downward, and in some steeply dipping aplite-pegmatite dikes from both margins inward. These structures and textures are interpreted to result from diffusion-controlled oscillatory nucleation and rapid growth from undercooled melt, coupled with changes in vapor pressure. Crystal growth from undercooled melt, with sparse alkali feldspar megacrysts acting as a substrate for the growing crystals, is indicated also by an irregular dike of orbicular granite cutting the marginal granite of the stock