U-Pb age evidence for repeated Palaeoproterozoic metamorphism and deformation near the Pajala shear zone in the northern Fennoscandian shield
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Abstract This paper presents U-Pb ages for an extended period of geological evolution between about 1860 and 1740 Ma in the northern part of the Fennoscandian Shield. It involved magmatism, ductile deformation and metamorphism along the border between Sweden and Finland. The study area separates into a western and an eastern domain roughly along a north-south trending structural boundary defined by the western margin of the Pajala shear zone. Approximately 1860-1850 Ma zircon ages from two granitoid rocks record contrasting post-emplacement deformation histories on either side of this boundary. While metamorphic monazite records c. 1850 Ma events in rocks of the western domain, metamorphic monazite and titanite as well as zircon overgrowths in the eastern domain verify deformation and high-grade metamorphism in the 1820-1780 Ma interval. This post-1820 Ma phase is most probably related to shear zones and to nearby intrusions. A titanite age at c. 1740 Ma in the eastern domain is suggested to exemplify a tectonic phase, which elsewhere in northern Fennoscandia is marked by fracturing, generation of pegmatites, and hydrothermal activity.Keywords:
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(1960). Monazite and Cyrtolite Crystals at Day, New York Pegmatite. Rocks & Minerals: Vol. 35, No. 7-8, pp. 328-330.
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Monazite-(Sm) was found in the Annie Claim #3 pod of lepidolite-subtype granitic pegmatite within the Greer Lake intrusion of pegmatitic leucogranite, in the Archean Bird River Subprovince of the Superior Province, in southeastern Manitoba.It occurs as tabular crystals ≤0.4 mm in diameter, associated with manganocolumbite, quartz, albite and lithian muscovite.Monazite-(Sm) is yellowish, translucent, with a white streak, vitreous to greasy luster, and no observed fluorescence.One good cleavage is present, tenacity is brittle, and the fracture is uneven.Monazite-(Sm) has no observable pleochroism; ␣ 1.768 (5),  1.771(3), ␥ > 1.808(3); 2V meas 29(8)°, X = b, Z ٙ c = 9° ( obtuse).It is monoclinic, space group P2 1 /n; a refinement from single-crystal and powder (Gandolfi) X-ray-diffraction data gave a 6.725(1), 6.739( 3), b 6.936(1), 6.951(3), c 6.448(1), 6.462(3) Å,  104.02(1)°, 104.03(4)°,V 291.8(1), 293.6(2)Å 3 , Z = 4; D calc derived from average chemical composition is 5.512 and 5.478 g/cm 3 .The strongest six lines of the (Gandolfi) X-ray-diffraction pattern [d in Å(I)(hkl)] are: 4.647(5)(011), 4.164(8)( 111), 3.492(4)(111,020), 3.264(7)(200), 3.065(10)(120), and 2.857(9)( 112,012).The monazite-(Sm) contains, in wt.%, up to 14.29 Sm 2 O 3 , 13.48 Gd 2 O 3 and 6.28 Nd 2 O 3 , and moderate percentages of the huttonite and brabantite components.The most Sm-rich composition gives (Sm 0.197 Gd 0.179 Ce 0.148 Th 0.125 Ca 0.107 Nd 0.090 La 0.030 Y 0.030 Pr 0.023 Tb 0.017 Zr 0.017 Dy 0.016 Pb 0.016 U 0.002 ) ⌺0.997 (P 0.963 Si 0.044 ) ⌺1.007 O 4 .Adjustment of U and Th of the monazite-(Sm) to their original contents 2.64 Ga ago slightly improves the stoichiometry and proves that all the Pb present is radiogenic.The middle-REE-dominant signature of the Annie Claim #3 monazite-(Sm) is shared with the broadly associated Y(Ta,Nb)O 4 mineral (formanite?) and dysprosian xenotime-(Y); this exotic pattern of REE abundances is possibly generated by selective and differential complexing of REE in the granite-to-pegmatite sequence of solidification.
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The near NS ductile shear zone was first discovered in Proterozoic Bendong granodiorite pluton. It dips toward 240°～280° at the dip angle of 40°～63°. The lineations, which represent the shear direction, plunge to 216°～226° at the plunge angle of 39°～46°. The kinematics of Bendong ductile shear zone is characterized by sinistral-normal shearing, and shows a sliding from NE to SW. Typical granitic mylonites were developed in the ductile shear zone and clearly show a zoning from phyllonite through mylonite to initial mylonite from center to both walls. The discovery of Bendong ductile shear zone and mylonite indicates an important structural event superimposed on Proterozoic Bendong pluton.
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The eastern margin of the Chitradurga schist belt is marked by a NNW-SSE trending sub vertical crustal scale ductile shear zone. The kinematic indicators indicate a predominant sinistral sense of strike-lip movement along the shear zone. Syntectonically emplaced granitic rocks are converted to mylonites and ultramylonites as a result of crystalplastic deformation in the shear zone. In contrast, there are localized zones of brittle failure with attendant functional heat generation exemplified by the development of thin but conspicuous bands and veins of pseudotachylytes, which are emplaced either subparallel with or transgressing the C-planes of the mylonites. From our field and petrographic studies it is interpreted that these two coexisting rock types, namely the mylonite and pseudotachylyte, which are the results of contrasting deformational mechanisms, have generated near synchronously in a progressively developed ductile shear zone. The pseudotachylytes represent the brief interlude of sudden increase in strain rate in an overall ductile regime.
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