Resolvendo os problemas tectônicos pela datação de minerais detríticos

quinta-feira, fevereiro 11, 2010

Resolving tectonic problems by dating detrital minerals

Barbara Carrapa

-Author Affiliations
Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA


Geochronology and thermochronology applied to detrital minerals such as zircons, monazites, white micas, and apatites have received increasing attention in the past decade for their unique power to determine the timing of crystallization and multiple tectono-thermal events, with relevance for sediment provenance, tectonic processes, and erosion. Recent breakthroughs in multi-dating (applying different geochronologic and thermochronologic systems to the same detrital grains) allow for unprecedented levels of detail in provenance and tectonic studies of detrital sediments. The common pre-conditions for application of these methods are: (1) the source areas are characterized by rocks with different tectonic histories recorded by distinctive crystallization and cooling ages, and (2) the source rocks contain the selected mineral. Whereas zircons occur in most magmatic, metamorphic, and sedimentary rocks, other minerals, such as apatite, monazite, and white mica, are less abundant. This is why zircon geochronology and thermochronology is a particularly useful approach to detrital studies. In cases where different sources are characterized by the same zircon U-Pb ages, differential metamorphism and/or exhumation may produce distinctive thermochronological ages. It is also important to note that different mineral geochronometers and thermochronometers can only answer specific questions. For example, if we want to determine the provenance of detrital minerals by studying the long history of crystallization of a tectonically complex source region, then U-Pb zircon geochronology is the ideal approach. The main strength of zircons resides in the fact that they are capable of surviving multiple phases of physical and chemical weathering, erosion, and deposition.
The increased use of multicollector-laser ablation-inductively coupled plasma–mass spectrometry (MC-LA-ICPMS) in recent years is a significant advancement in the application of U-Pb geochronology to provenance and tectonic problems, because the technique can efficiently generate a large number of analyses (Gehrels et al., 2008). The method has become a common approach for determining sediment provenance, dispersal patterns, and recycling (Dickinson and Gehrels, 20082009a2009b), timing of tectonic processes such as the onset and kinematic history of mountain building (White et al., 2002DeCelles et al., 2004), maximum depositional age of otherwise undatable sedimentary units by using the youngest age component (Surpless et al., 2006Fildani et al., 2003:DeCelles et al., 2007), and source-sedimentary basin evolution (Rahl et al., 2003;Fildani et al., 2009).
However, if one wants to study the details of metamorphic evolution or multiple tectono-thermal events characterized by a broad range of temperatures (T), which are lower than the closure T for zircons (>900 °C; Dahl, 1997), then a different approach is necessary. The first scenario can be better addressed by zircon secondary ion mass spectrometry (SIMS) analysis (Trail et al., 2007Spandler et al., 2005); however, this technique requires extensive analytical time, rendering it less suitable for detrital studies in which large numbers of analyses are required (on average ∼100 per sample; Vermeesch, 2004). The second scenario necessitates a geo-thermochronological approach involving multi-dating of the same mineral or of different minerals with different “closure” temperatures covering the T-window of interest (Fig. 1), such as U-Pb dating of zircons and monazites (e.g., Hieptas et al., 2010, p. 167 in this issue of Geology), 40Ar/39Ar of white micas, or double and triple dating of zircons and apatites (Rahl et al., 2003Campbell et al., 2005Bernet et al., 2006Carrapa et al., 2009).
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