U-TH-PB MONAZITE EVIDENCE OF PROTEROZOIC METAMORPHISM IN THE BELT-PURCELL SUPERGROUP OF NORTHERN IDAHO

A growing body of Lu-Hf garnet data suggests that metamorphic tectonites correlative with the Belt-Purcell Supergroup exposed on the northern margin of the Idaho batholith experienced Proterozoic metamorphism and deformation. Monazites from three garnet-bearing metapelite samples in the Carpenter Creek and Bathtub Mountain areas of northern Idaho were dated by the Electron Microprobe (EMP) method (RPI). The ages and zoning characteristics varied between the samples, but the U-Th-Pb monazite data confirm that at least two Proterozoic metamorphic events are recorded in the area.

Monazites from the Carpenter Creek location are subhedral to anhedral, contain abundant inclusions, and occur both as inclusions within staurolite porphyroblasts and the matrix. Analyzed grains gave ages ranging from 1.3 to 1.1 Ga with the 1.1 Ga age being dominant. Compositional zoning in monazites from samples in the Bathtub Mountain area demonstrate complex growth relationships. Monazites in one sample are anhedral, contain abundant inclusions, and yield two distinct ages. Monazite grains within garnet cores give ages of ca 1.3 Ga whereas grains in garnet rims and in the matrix generally give ages of 1.1 Ga. A second sample from the Bathtub Mountain area contained anhedral, inclusion-rich monazite grains both within garnet and as matrix minerals. Grains within garnets give an age of 1.3 Ga whereas matrix grains yield a complex age array with clusters at 1.3 Ga, 1.1 Ga, and 90 Ma. In some cases, monazite cores from garnets in this sample produced the older 1.3 Ga age while rims were found to have either ca 1.1 Ga or Cretaceous aged growth.

The occurrence of monazites with distinct 1.3 and 1.1 Ga Proterozoic ages that occur as inclusions in garnet and staurolite as well as in the matrix assemblage, suggests that a polymetamorphic Proterozoic history is preserved within metamorphosed equivalents of the Belt-Purcell Supergroup in northern Idaho. The monazite data supports the Lu-Hf garnet results and requires a revision of views of “Grenville”-aged deformation on the western Laurentian margin or re-evaluation of the correlation of metamorphic tectonites in northern Idaho with the strata along the Laurentian margin.