Abstract
Abundant porphyritic granites, including Grt-bearing and Bt-bearing porphyritic granites, and porphyritic potash-feldspar granite (trondhjemite-granitic composition) are widely distributed within the Kovela granitic complex Southern Finland, which associated with monazite-bearing dikes (strong trondhjemite composition). The investigated monazite-bearing dikes are dominated by a quartz + K-feldspar + plagioclase + biotite + garnet + monazite assemblage. The monazite forms complexly zoned subhedral to euhedral crystals variable in size (100 - 1500 μm in diameter) characterized by high Th content. The chemical zoning characterised as: 1) concentric, 2) patchy, and 3) intergrowth-like. Textural evidence suggests that these accessory minerals crystallized at an early magmatic stage, as they are commonly associated with clusters of the observed variations in their chemical composition are largely explained by the huttonite exchange , and subordinately by the cheralite exchange with proportions of huttonite (ThSiO4) and cheralite [CaTh(PO4)2] up to 20.4% and 9.8%, respectively. Textural evidence suggests that these monazites and associated Th-rich minerals (huttonite/thorite) crystallized at an early magmatic stage, rather than metamorphic origin. The total lanthanide and actinide contents in monazite and host dikes are strongly correlated. Mineral compositions applied to calculate P-T crystallization conditions using different approaches reveal a temperature range of 700°C - 820°C and pressure 3 - 6 kbars for the garnet-biotite geothermometry. P-T pseudo-section analyses calculated using THERMOCALC software for the bulk compositions of suitable rock types, constrain the PT conditions of garnet growth equilibration within the range of 5 - 6 kbars and 760°C - 770°C respectively. Empirical calculations and pseudo-section approaches indicate a clockwise P-T path for the rocks of the studied area. 207Pb/206Pb dating of monazite by LA-MC-ICPMS revealed a recrystallization period at around 1860 - 1840 Ma. These ages are related to the tectonic-thermal event associated with the intense crustal melting and intra-orogenic intrusions, constraining the youngest time limit for metamorphic processes in the Kovela granitic complex.
Highlights
The Kovela granitic complex in Finland is part of a series of metamorphic events, which have to be fully understood [1] [2] [3]
Abundant porphyritic granites, including Grt-bearing and Bt-bearing porphyritic granites, and porphyritic potash-feldspar granite are widely distributed within the Kovela granitic complex Southern Finland, which associated with monazite-bearing dikes
This paper presents the results of whole-rock analyses of the monazite-bearing dikes, details of their mineral assemblages, their spatial zonal variation, and micro-analytical study of monazite hosted within monazite-bearing dikes and porphyritic biotite granite from the Kovela granitic complex, southeastern Finland
Summary
The Kovela granitic complex in Finland is part of a series of metamorphic events, which have to be fully understood [1] [2] [3]. The Kovela granitic complex is characterized by well-developed zoned structure consisting of several generations of bedrocks: porphyritic K-feldspar granite and small granodiorite bodies at the center, pyroxene gneiss at the margin and garnet-cordierite gneiss as well marginal pegmatite along the outer contact of the complex (Figure 1(a)). Radioactive (monazite-bearing dikes) and weakly radioactive pegmatite dikes are abundant in the central zone and present in lesser amounts in the surrounding country-rocks. The S dikes are most radioactive in the complex (Figure 1(a)). The monazite-bearing dikes belong to the late orogenic granites on the basis of U-Pb analysis of the Karhukoski monazite, 1801 Ma ± 20 [4]
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