Petrogenesis of two-mica granites, northern Monashee Mountains, British Columbia: Evidence from oxygen, hydrogen, and strontium isotopic compositions

Abstract

Syn- to postkinematic two-mica granite dikes, sills, and sheets intrude late Precambrian sillimanite and K-feldspar-sillimanite zone metasedimentary and metavolcanic rocks in the northern Monashee Mountains of British Columbia. U-Pb ages indicate that two periods of granite emplacement occurred ∼40 m.y. apart, at 100 Ma, concurrent with peak regional metamorphism, and at 63 Ma, which marked the beginning of a period of accelerated cooling in the area. Rare earth element contents, high initial 87 Sr/ 86 Sr ratios, inheritance of Precambrian zircons, and the restriction of most of the peraluminous granites to the highest-grade metamorphic zones are all compatible with derivation of the granitic magmas by partial melting of associated metasedimentary rocks. However, stable isotope studies of whole-rock samples and minerals indicate that the source of the peraluminous granitic magmas could not be simple whole-sale melting of spatially associated pelitic rocks, but likely was Precambrian lower continental crust that must have included metapelites. The disparate ages of the granites in conjunction with what is known about the tectonic evolution of the area indicate that the granites are tectonically displaced from their source areas. Isotopic equilibration temperatures of coexisting muscovite and biotite in the granites are near 400 °C and are lower than temperatures estimated for the same mineral pairs in the metapelitic rocks. This difference in equilibration temperatures is likely the result of differential rates of exchange related to cooling in the presence of fluids in the granites. Although the δD values of muscovites from both the granites and the metapelites indicate equilibration with fluids having a relatively restricted range of δD values near -50, the calculated δ 18 O values of fluids in equilibrium with the granites are ∼2 per mille lower than those in equilibrium with the associated metapelitic rocks. In contrast to the hydrogen isotope systematics in the muscovites, δD values of biotite in the granites and metapelites range from -173 to -91, implying that the biotites were affected by early Tertiary meteoric water when this area was at high latitudes and elevations, concurrent with an episode of tectonic denudation that occurred at ca. 45 Ma.