Archean polyphase deformation in the Lake Johnston Greenstone Belt area: Implications for the understanding of ore systems of the Yilgarn Craton

摘要

A structural and geochronology study on the 2900 Ma Lake Johnston Greenstone Belt (LJGB) has defined multiple deformation events. Quantitative U-Pb age constraints have been obtained on the ductile deformation, and peak metamorphic assemblages. The D_(1LJ) event is associated with the development of fold-nappes during interpreted NNE-SSW shortening. U-Pb SHRIMP dating on monazite from a dyke folded by the D_(1LJ) event gives an age of 2634 ± 1 Ma, providing an upper age limit on D_(1LJ). This age could have been reset by the D_(2LJ) metamorphic event. A major metamorphic/thermal event post-dates the D_(1LJ) event and is inferred to be the result of elevated temperatures coeval with the emplacement of granitoid intrusions. The D_(2LJ) event is associated with peak metamorphic temperatures, and reflects NNE-SSW to NE-SW shortening associated with dextral shearing. A dyke that intrudes and crosscuts D_(2LJ) fabrics, yield a U-Pb SHRIMP age on monazite at 2629 ± 1 Ma and provides a lower age limit on the D_(2LJ) event. D_(3LJ) is defined by crenulation cleavages developed during E-W shortening. The D_(4LJ) episode is defined by brittle deformation and a series of N- to NE-trending, steep-dipping dextral faults formed during craton-scale shortening under a NE-SW directed far field stress. The brittle D_(5LJ) event is characterized by E-W shortening with dip-slip reverse N-S faults. The new D_(2LJ) deformation age is younger than previously constrained ages for major deformation in the 2900 Ma greenstone belts. Deformation within the LJGB was broadly coeval with the waning stage of formation of gold deposits in adjacent greenstone belts. Most peak magmatic and metamorphic events preserved in the greenstone belts have been constrained to ca. 2640-2650 Ma. The presence of such a high temperature deformation event recorded at ca. 2630 Ma in the LJGB belt has implications for the thermal history, fluid formation, and geodynamic context of the development of these gold systems. The 2630 Ma age correlates with late-stage and post-gold emplacement of pegmatite dykes in surrounding terranes. The LJGB is interpreted to have been a mobile belt associated with high grade metamorphism that was inboard (west) of, and active, during the formation of major world class gold systems in the Eastern Goldfields Terranes. The D_(1LJ) to D_(2LJ) changes in the kinematics of the system (i.e. D_(1LJ) to D_(2LJ)), may correlate with a stress switch linked to the influxes of gold-bearing fluids in adjacent greenstone belts.