Rb-Sr Geochronology and Sr Isotope Systematics of Some Major Lithologies in Chandos Township, Ontario

Abstract

<p>The strontium isotopic composition of three major units in Chandos Township was investigated to determine the age of each unit and attempt to discern some geological enigmas associated with these rocks.</p> <p>The Loon Lake pluton is composed of a monzonite core gradational to a quartz monzonite rim with subordinate isolated outcrops of diorite-gabbro and granodiorite gneiss. Separate Rb-Sr whole rock isochrons were determined for the monzonite (t=1052±21 Ma.; Ri=0.7036±6) and quartz monzonite (t=1071±17 Ma.; Ri=0.7034±5). Since these ages and initial strontium ratios (Ri) are indistinguishable, the data from both units were combined to form a Loon Lake composite isochron (t=1065±13 Ma.; Ri=0.7034±4). The age determined from the composite isochron is interpreted as the time of emplacement of this pluton. The overlap in Ri favours a cogenetic relationship for these two units dominated by fractional crystallization but the extreme Rb enrichment in the quartz monzonites and the significant hiatus in the average o18Owr values (monzonite - 9.5 ‰; quartz monzonite - 11.0 ‰) are difficult to explain by this model. The relatively high range of δ¹⁸Owr values for this pluton (8.8 to 12.0 ‰) and the low Ri reflect a source in the lower crust.</p> <p>The Tallan Lake sill consists of an upper 180 meters of amphibolite which is transitional to a lower 80 meters of well foliated syenite. Strontium isotope results for samples from both horizons were combined in the regression treatment to yield a McIntyre III errorchron age of 1225±50 Ma. Although there is some uncertainty associated with this errorchron age, this data is interpreted to indicate the time of intrusion of the sill. This data is consistent with the contention that these rocks represent a consanguineous sequence which now forms part of a large scale nappe structure. A mantle derivation for this sill is supported by the low Ri (0.7031±7).</p> <p>A detailed strontium isotope study of the Apsley biotite gneiss indicated contrasting systematics between the "sodic" and "potassic" suites. The results from a regression treatment of five "sodic" Apsley gneiss samples indicated an isochron age of 1402±57 Ma. This age is interpreted as an average depositional age for the volcanic and sedimentary components of this unit thus the Apsley gneiss represents the oldest known rocks in this part of the Grenville Province. The low Ri for the sodic samples (0.7022±12) provides additional evidence that the isotopic composition of this unit was not severely affected by Grenville metamorphism. On the other hand, the "potassic" Apsley gneiss shows open system behaviour.</p>