Titanite has been in use as a U-Pb geochronometer since the early work by Tilton and Gruenefelder (1968), yet the interpretation of many titanite ages remains open to debate. Some workers interpret titanite ages strictly as cooling ages indicating cessation of Pb diffusion during cooling from magmatic or metamorphic processes. In many cases, however, titanite ages can be shown to reflect partial or total resetting, and / or new-growth during secondary events. New growth is especially evident in those cases where mineralogically and isotopically distinct titanite generations coexist within the same rock.
Titanite has been systematically dated in many areas of the western Superior Province. In general, the data yield regionally distinctive age patterns reflecting the local response to orogenic and post-orogenic processes. One of the simplest pattern is that observed, for example, in the Shebandowan greenstone belt where titanite ages fall consistently between 2692 and 2680 Ma, the time span of major plutonic and metamorphic activity in the area. There is no evidence in this region for extensive post-metamorphic resetting or new growth. A variation of this pattern is seen in the Uchi and Berens River subprovinces where titanite yield ages that generally range from 2740 to 2690 Ma, correlating with the age of major magmatic or metamorphic events. In these subprovinces there are only local occurrences of titanite with post-metamorphic ages of 2690 to rarely 2630 Ma, generally in the vicinity of fault systems. A more complex age pattern is observed in the Winnipeg River Subprovince, a metaplutonic domain, locally affected by high-grade metamorphic conditions. At one location >2730 Ma - titanite xenocrysts occur within 2705 Ma new-grown magmatic titanite of a late-kinematic diorite dyke. Two other rocks from the same site contain 2705 Ma metamorphic titanite, overgrown by Û2650 and 2627 Ma secondary titanite generations. Titanite populations with ages as young as 2550 Ma are found in central domains of the Subprovince. These post-metamorphic ages tend to show an erratic distribution and large variations even within restricted areas; hence, these ages are likely to indicate secondary crystallization or resetting rather then slow cooling. Somewhat similar patterns have been reported from the central Wabigoon Subprovince where tonalite gneisses preserve titanite as old as 2913 Ma, which locally coexist with much younger 2630 Ma secondary generations (Davis et al., 1988). Another type of distribution is that observed in mineralized portions of gold deposits such as Hemlo or Val d'Or where titanites ages fall in the range 2670 to 2600 Ma. In general these ages remain coherent within individual parts of deposits, and their relationships to the age of other minerals such as rutile and monazite, indicates that the titanite date distinct metamorphic or hydrothermal events.
Titanite ages across the western Superior Province define patterns that are distinct and characteristic for the orogenic and post-orogenic development of each domain. While some areas maintain age patterns that reflect original magmatic and metamorphic evolutions, other areas show that titanites have undergone various stages of overprinting by younger events. The occurrence of rejuvenated dates correlates in some cases with the persistence of elevated thermal conditions. In many such cases, however, it is possible to show that the ages reflect local disturbances or new growth during hydrothermal events rather than simple closing to diffusion during slow cooling. Titanite ages in complex geological situations such as gold deposits and high-grade terrains can be particular useful in tracking the evolution of hydrothermal systems and of distinct mineral parageneses.
Davis, D.W., Sutcliffe, R.H. & Trowell, N.F., Precambrian Res. 39, 171-191 (1988).
Tilton, G.R. & Gruenenfelder, M.H., Science 159, 1458-1461 (1968).