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Crystal Chemistry and Geochronology of Thorium-Rich Monazite from Kovela Granitic Complex, Southern Finland

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DOI: 10.4236/nr.2019.106016    144 Downloads   288 Views

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.

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Al-Ani, T. , Hölttä, P. , Grönholm, S. , Pakkanen, L. and Al-Ansari, N. (2019) Crystal Chemistry and Geochronology of Thorium-Rich Monazite from Kovela Granitic Complex, Southern Finland. Natural Resources, 10, 230-269. doi: 10.4236/nr.2019.106016.

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