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U–Pb zircon single grain dating using the LA-MC-ICP-MS technique was utilized to determine the age of the host rocks of the Juomasuo Au–Co–Cu deposit located in the late Archean Kuusamo supracrustal belt. Even though the dated samples have diverse geochemical signatures that imply felsic volcanic and sedimentary precursors, the U–Pb data revealed heterogeneous detrital zircon populations for all samples. The host rocks are thus considered to belong to reworked sedimentary/volcano-sedimentary sequences. The maximun depositional ages of the samples fall in the 2.75–2.65 Ga age window, and no Paleoproterozoic ages were recovered in the examined zircon grains (151 analytical spots in total). In addition, a younger population (2.65–2.60 Ga) of internally featureless, BSE-pale/CL-dark zircon and zircon domains, was found in the mineralized sequences. These homogenized zircon grains resemble zircon formed in postmagmatic solid–state processes, in which zircon is recrystallized in metamorphic–hydrothermal conditions. This metamorphic-hydrothermal event most probably occurred in the original provenance area of the metasedimentary rocks. [ABSTRACT FROM AUTHOR]

The Vaasa batholith, western Finland, is a large, peraluminous granitoid pluton that crystallized at 1.88-1.87 Ga during the culmination of the Svecofennian orogeny. The batholith has gradual contacts, through metatexites and diatexites, with the enveloping metasedimentary rocks of the Bothnian Belt. We present ID-TIMS U-Pb age data on monazite from granitoids and xenoliths of the Vaasa batholith and combine these with published U-Pb zircon ages in order to shed further light on the evolution of the Vaasa batholith. The apparent monazite ages for seven of the examined samples are 1870-1863 Ma, and 1855±3 Ma for one further sample from the southern part of the batholith. Combined with pre-existing data, the monazite ages of the granitoids are 9 to 18 Ma (face values) or 3 to 9 Ma (external errors considered) younger than the U-Pb zircon crystallization ages from respective samples. Our new data suggest slow cooling for the Vaasa batholith - the closure/saturation temperature of the monazite U-Pb system was probably reached in ~10 m.y. after the crystallization of magmatic zircon in the examined rocks. [ABSTRACT FROM AUTHOR]

The anorogenic magmatic rock bodies of the Estonian crystalline basement belong to the Fennoscandian Palaeo-Mesoproterozoic Rapakivi Province and include the huge composite Riga batholith (250 km x 230 km in sub surface area, mostly in NW Latvia), as well as at least five granite stocks (Naissaare, Mlirjamaa and its Kloostri satellite, Taebla, Neeme, and Ereda) and the quartz monzodioritic Abja stock in the Estonian mainland. The Riga batholith contains both mafic and silicic rocks, as several Fennoscandian rapakivi complexes do. The granite stocks appear on geophysical maps as gravity and magnetic minima, and they consist of pink, medium- to coarse- grained, microcline-megacrystic, partly trachytoid biotite (in Märjamaa and Naissaare also with hornblende) granite, locally cut by aplitic and microsyenitic dykes. As an exception, the Märjamaa stock is more differentiated and has an anomalously high magnetic central part composed of hybrid granodiorite with hornblende as the main mafic mineral. The Abja quartz monzodiorite stock is strongly magnetic and consists of a dark grey, massive, medium-grained, partly weakly gneissose rock with abundant accessory apatite and titanomagnetite, and is intersected by veins of fine- to medium-grained, slightly porphyritic plagioclase-microcline granite. The major, REE and other trace element contents of the granitic rocks are close to or overlap those of the typical Finnish rapakivis and are best comparable with the less differentiated granitic phases. The Märjamaa granodiorite and the Abja quartz monzodiorite are enriched in Sr, Ti, and P. In Nd and Pb isotopic composition, the felsic and mafic rocks resemble corresponding rocks of the Finnish rapakivi complexes, indicating an approximately chondritic source for Nd in the mafic rocks of the Riga batholith and the Abja intrusion, and a Palaeoproterozoic (Svecofennian) source for the felsic rocks. The TDM model ages of the felsic rocks range from 1890 to 2100 Ma. [ABSTRACT FROM AUTHOR]

This introductory chapter of the IGCP-426 Special Issue of Precambrian Research presents an overall account of the goals and activities of IGCP-426 (Granite Systems and Proterozoic Lithospheric Processes) in the first 4 years (1998–2001) of the project. The thirteen original articles of the issue are based on papers given in the past meetings of IGCP-426 and are briefly introduced. [Copyright &y& Elsevier]

Presents a geochemistry of Jurassic and Cretaceous plutons in California. Chemical and isotopic character of Jurassic and Cretaceous mafic rocks associated with the granitoid plutons; Major and trace element geochemistry as well as the isotopic composition of granitoid rocks separate from the mafic rocks.