O. Tapani Rämö

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

Petrology of the anorogenic, oxidised Jamon and Musa granites, Amazonian Craton: implications for the genesis of Proterozoic A-type granites

The 1.88 Ga Jamon and Musa granites are magnetite-bearing anorogenic, A-type granites of Paleoproterozoic age. They intrude the Archaean rocks of the Rio Maria Granite–Greenstone Terrain in the eastern part of the Amazonian Craton in northern Brazil. A suite of biotite±amphibole monzogranite to syenogranite, with associated dacite porphyry (DP) and granite porphyry (GP) dykes, dominates in these subalkaline granites that vary from metaluminous to peraluminous and show high FeO/(FeO+MgO) and K2O/Na2O. In spite of their broad geochemical similarities, the Jamon and Musa granites show some significant differences in their REE patterns and in the behaviour of Y. The Jamon granites are related by fractional crystallisation of plagioclase, potassium feldspar, quartz, biotite, magnetite±amphibole±apatite±ilmenite. Geochemical modelling and Nd isotopic data indicate that the Archaean granodiorites, trondhjemites and tonalites of the Rio Maria region are not the source of the Jamon Granite and associated dyke magmas. Archaean quartz diorites, differentiated from the mantle at least 1000 m.y. before the emplacement of the granites, have a composition adequate to generate DP and the hornblende–biotite monzogranite magmas by different degrees of partial melting. A larger extent of amphibole fractionation during the evolution of the Musa pluton can explain some of the observed differences between it and the Jamon pluton. The studied granites crystallised at relatively high fO2 and are anorogenic magnetite-series granites. In this aspect, as well as concerning geochemical characteristics, they display many affinities with the Proterozoic A-type granites of south-western United States. The Jamon and Musa granites differ from the anorthosite–mangerite–charnockite–rapakivi granite suites of north-eastern Canada and from the reduced rapakivi granites of the Fennoscandian Shield in several aspects, probably because of different magmatic sources.

New U-Pb ages from the Wiborg rapakivi area: constraints on the temporal evolution of the rapakivi granite-anorthosite-diabase dyke association of southeastern Finland

Abstract New U-Pb data on zircons, monazites, and baddeleyite suggest that the Wiborg rapakivi batholith and associated mafic rocks in southeastern Finland were emplaced mainly between 1650 and 1625 Ma. The earliest anorogenic magmatism related to the intrusion of the rapakivi granites was the emplacement of some diabase dykes about 1665 Ma ago, while the youngest porphyries intruded the rapakivi granites at 1615 Ma. The process involved three peaks of diabase activity at 1665, 1645, and 1635 Ma and two major granite events at 1640±5 and 1630±5 Ma, the former of which comprises also the intrusion of minor gabbroic-anorthosite bodies. On the whole, the result was the emplacement of at least 105 km3 of rock material over a period of 50 Ma. Within southern Finland, the rapakivi magmatism continued until 1540 Ma by emplacement of the West Finnish intrusions, which combined are as extensive as the Wiborg area. Globally, the Proterozoic rapakivi event probably represents the largest pulse of intracratonic magmatism which occurred during geological history and may be a consequence of rapid growth of continental masses in the Early Proterozoic.

Radiogenic isotopes of the Estonian and Latvian rapakivi granite suites: new data from the concealed Precambrian of the East European Craton

Abstract The Precambrian crystalline bedrock of the Baltic countries is covered under Phanerozoic sedimentary rocks that flank the Fennoscandian Shield in the south. The covered bedrock consists mainly of Palaeoproterozoic medium- to high-grade metamorphic rocks and unmetamorphosed rapakivi granites and related mafic rocks (mainly gabbros and anorthosites). Our UPb zircon data show that small rapakivi granite plutons in Estonia are 1630 Ma old and felsic and mafic rocks from the Riga batholith of Latvia and westernmost Estonia are 1580 Ma old. The Estonian plutons have ϵNd (1630 Ma) values ranging from −0.5 to −2.5. The mafic and felsic rocks of the Riga batholith have ϵNd (1580 Ma) values between +0.3 and −0.6, except for a pervasively altered silicic volcanic rock on the northern flank of the batholith with an ϵNd value of −4.6. Initial 87 Sr 86 Sr ratios of the mafic rocks are of the order of 0.7036 to 0.7037 and conform to the evolution of average subcontinental mantle. The Pb isotopic compositions of the felsic and mafic rocks (including the low-ϵNd prophyry) are relatively radiogenic with single-stage μ-values of the order of 8.2. The isotopic characteristics of the Estonian and Latvian rapakivi granites are similar to those of the classic rapakivi granites of southern Finland. Our data suggest that the felsic rocks of the Estonian and Latvian rapakivi suites were derived from Palaeoproterozoic (near-chondritic Nd, relatively high-U Pb ) protoliths. The data also imply that the lower crust and upper mantle in this area are devoid of a major Archaean component and that the lithosphere may become more juvenile southward from the Fennoscandian Shield.

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

1.88 Ga Oxidized A‐Type Granites of the Rio Maria Region, Eastern Amazonian Craton, Brazil: Positively Anorogenic!

1.88 Ga A‐type granites in the Archean Rio Maria granite‐greenstone terrane of eastern Amazonian craton, Brazil, have much in common with the oxidized ∼1.4 Ga “anorogenic” granites of the western United States. Nd isotopic data on these granites and their Archean country rocks show that (1) the Rio Maria crust was differentiated from depleted mantle at 3.0 Ga, (2) the Paleoproterozoic granites were derived from deep parts of this crust, and (3) the Paleoproterozoic granites postdate their protolith by at least 1 Ga. No convergent processes are known to have affected the eastern Amazonian craton at ∼1.9 Ga and, therefore, there is little doubt as to the anorogenic origin of the oxidized A‐type granites of Rio Maria. Recognition of this genuinely anorogenic Proterozoic granite suite in Amazonia adds cymatogenic flavor to the current debate on the origin of the mid‐Proterozoic “anorogenic” granites of the western United States.

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.

Nd isotopic composition of cratonic rocks in the southern Death Valley region: Evidence for a substantial Archean source component in Mojavia

Thirty Early Proterozoic intermediate to silicic metasedimentary and metaigneous rocks in the southern Death Valley region and vicinity show ϵ Nd values of −1.6 to −6.3 at 1.7 Ga and Nd model ages of 2.1 to 2.6 Ga. These cratonic rocks thus reveal an older signature than so far reported for Nd province 1 of the western United States; as much as 30%–40% of their mass may be Archean crustal material. The Archean component was introduced in the form of sedimentary detritus that was probably subducted and mixed with juvenile material at a convergent margin. Three younger Precambrian rocks associated with the cratonic rocks also have a Nd isotopic composition of province 1 type.

Mineral chemistry constraints on the evolution of the 1.88–1.87 Ga post-kinematic granite plutons in the Central Finland Granitoid Complex

Abstract A suite of post-kinematic, 1.88–1.87 Ga, silicic plutons crosscut 1.89–1.88 Ga synkinematic granitoids in the Central Finland Granitoid Complex (CFGC) in south-central Finland. The plutons range from biotite±hornblende quartz monzonite to syenogranite and include pyroxene- and olivine-bearing varieties. Mineral chemical data on feldspars, biotite, amphibole, pyroxenes, olivine, and oxides of the post-kinematic plutons are presented. The data are interpreted to show that these plutons register (1) a considerable range in pressure from 2–4 kbar (amphibole barometry) to 5–7 kbar (olivine–pyroxene barometry), (2) temperatures mostly reflecting resetting during cooling (450–800°C; QUIlF thermometry), and (3) low fO2 (log fO2 ΔFMQ −0.3 to −1.5; QUIlF equilibria). In particular, plutons with olivine- and pyroxene-bearing margins and amphibole-dominated central parts record progressive oxidation and hydration upon cooling, shifting from the QUIlF equilibrium toward KUIlB. The post-kinematic granites can be considered post-collisional in regard to compressional events in the CFGC and display many of the characteristics of the anorogenic 1.6 Ga rapakivi granites further south. They were presumably derived from a deep and dry crustal source, like the rapakivi granites.

Palaeoproterozoic (1740 Ma) rift-related volcanism in the Hekla Sund region, eastern North Greenland: field occurrence, geochemistry and tectonic setting

Abstract Two Palaeoproterozoic volcanic successions, the Hekla Sund (HS) Formation and the Aage Berthelsen Gletscher (AaB) Gletscher Formation, occur within the Caledonian orogen of eastern North Greenland. They consist mainly of mafic pillow lavas, deformed and metamorphosed under greenschist-facies conditions during the Caledonian orogeny. Zircons from a rhyolitic ignimbrite in the HS Formation have yielded an age of 1740±6 Ma. In both formations the volcanic rocks are intercalated with immature sandstones and conglomerates that accumulated in the vicinity of active fault scarps; a shallow marine, rifted basin is implied. Relative concentrations of the more immobile minor and trace elements (Ti, Zr, Nb, Y and rare-earth elements) in both rock suites were unaffected by the metamorphism. Fractional crystallisation of olivine, clinopyroxene and, probably, plagioclase, as well as assimilation of crustal rocks was involved in the petrogenesis of the HS Formation. The AaB basalts have higher Mg, Ni and Cr and lower concentrations of incompatible elements than the rocks of the HS Formation, and they could be regarded as more primitive products of the same magmatic event. However, marked differences in incompatible trace element ratios in the two suites are unlikely to reflect either differences in fractionation histories or variable contamination, and suggest compositional differences in the mantle source rocks. Basalts from the two formations have distinct eNd values (−4.6 and −4.8 for the HS Formation, −5.9–−5.6 for the AaB Gletscher Formation), which is consistent with this interpretation. The volcanic rocks at HS and AaB Gletscher were erupted shortly after a long period of orogenic activity between 2000 and 1750 Ma ago. Following post-orogenic emplacement of granites at ca. 1750–1740 Ma, uplift and erosion took place, and accumulation of extensive immature sediments occurred simultaneously with formation of the volcanic rocks described in this paper. The magmatism that gave rise to the two formations may have been caused by melting during lithospheric extension.