Abstract
We present new geochronological evidence of latest Palaeoproterozoic-earliest Mesoproterozoic magmatism in the Telsiai Deformation Zone, NW Lithuania. Employing the laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the University of Tasmania, Australia, we demonstrate that a granodiorite, which had previously been considered to belong to the 1850-1820 Ma Kursiai charnockitic batholith, has a U-Pb zircon age of ca 1.62 Ga. The weighted mean Pb-207/Pb-206 age obtained from eight concordant zircon grains is 1625 +/- 6 Ma (MSWD = 0.6), while the upper-intercept concordia age is 1619 +/- 27 Ma (MSWD = 0.56). Like similar ages of granitoids from several small intrusions in Estonia, this once more indicates extensive melting of the crust at that time. We suggest that both the 1.62 Ga magmatism and the emplacement and crystallization of the large Riga anorthosite-mangerite-charnockite-rapakivi batholith in Latvia and western Estonia at 1580 Ma were far-field feedback effects of the formation of the new Palaeoproterozoic continental crust during the Gothian orogeny at the present SW margin of the East European Craton. (Less)
Highlights
Understanding the crustal evolution of the Palaeoproterozoic Svecofennian orogen requires the knowledge of its tectonic structure both in the Baltic/Fennoscandian Shield of Finland and Sweden, and in the Precambrian substratum beneath a ca 2 km thick sedimentary platform cover in the Baltic States on the opposite side of the Baltic Sea
One major difficulty is the strong influence of the Riga anorthosite–mangerite– charnockite–rapakivi granite (= AMCG) batholith dated at ca 1580 Ma (Rämö et al 1996)
We suggest that the Velaiciai-2 drill core (Vlc-2) granodiorite and other similar rocks emplaced at ca 1620 Ma in Lithuania can be interpreted as the earliest phases of the AMCG magmatism that led to the formation of the slightly later Riga batholith at ca 1580 Ma
Summary
Understanding the crustal evolution of the Palaeoproterozoic Svecofennian orogen requires the knowledge of its tectonic structure both in the Baltic/Fennoscandian Shield of Finland and Sweden, and in the Precambrian substratum beneath a ca 2 km thick sedimentary platform cover in the Baltic States on the opposite side of the Baltic Sea. Motuza and co-workers (Motuza et al 2008), this region is occupied by the large Kursiai batholith with a few minor plutons made up of a suite of dominantly charnockitic granitoid rocks (mangerite, enderbite, opdalite, charnockite and granite) Some of these have been dated at ca 1850 to 1820 Ma (Claesson et al 2001; Motuza et al 2008), but new geochronological data suggest that granitoids of similar compositions were emplaced into the Palaeoproterozoic crust much later, at ca 1620 Ma (Vejelyte 2012). They differ from the rest of that batholith in their higher magnetic susceptibilities and produce a strong magnetic anomaly (Motuza et al 2008) None of these granodiorites have been dated as yet, but a vein of coarse-grained granite cutting the charnockite in the Genciai-6 drilling within the TDZ has yielded a 1622 12 Ma 207Pb/206Pb age (GNC-2-1 in Vejelyte 2012). The amphibole-bearing vein (Vd1/2535, Table 1) displays very similar REE and trace element patterns
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