Abstract
The ability to accurately constrain the secular record of high- and ultra-high pressure metamorphism on Earth is potentially hampered as these rocks are metastable and prone to retrogression, particularly during exhumation. Rutile is among the most widespread and best preserved minerals in high- and ultra-high pressure rocks and a hitherto untested approach is to use mineral inclusions within rutile to record such conditions. In this study, rutiles from three different high- and ultrahigh-pressure massifs have been investigated for inclusions. Rutile is shown to contain inclusions of high-pressure minerals such as omphacite, garnet and high silica phengite, as well as diagnostic ultrahigh-pressure minerals, including the first reported occurrence of exceptionally preserved monomineralic coesite in rutile from the Dora–Maira massif. Chemical comparison of inclusion and matrix phases show that inclusions generally represent peak metamorphic assemblages; although rare prograde phases such as titanite, omphacite and corundum have also been identified implying that rutile grows continuously during prograde burial and traps mineralogic evidence of this evolution. Pressure estimates obtained from mineral inclusions, when used in conjunction with Zr-in-rutile thermometry, can provide additional constraints on the metamorphic conditions of the host rock. This study demonstrates that rutile is an excellent repository for high- and ultra-high pressure minerals and that the study of mineral inclusions in rutile may profoundly change the way we investigate and recover evidence of such events in both detrital populations and partially retrogressed samples.
Highlights
One of the most important and unresolved questions in the evolution of the Earth is the question of when did subductiondriven plate tectonics begin (e.g. Smart et al, 2016)? The magmatic record implies that subduction-driven plate tectonics began in the Archean ca. 3 Ga (e.g. Dhuime et al, 2012), while the metamorphic record, or more precisely the lack of (U)HP-LT rocks in the pre-Neoproterozoic, would suggest that the onset of modern subduction occurred after ca. 800 Ma. (U)HP-LT rocks such as blueschists and UHP eclogites are the best direct lines of evidence geoscientists have for modern ‘cold and steep’ subduction, as opposed to the ‘hot and flat’ style of subduction proposed for the Archean (Stern, 2005)
To offer a way forward with the debate about when plate tectonics began, and to look beyond the Neoproterozoic for evidence of (U)HP-LT events, we have developed a new way of utilising rutile
In order to determine if additional P –T information can be obtained using mineral inclusions in samples where evidence of HP events have been partially obliterated in the matrix, average pressure (Av P ) and average pressure–temperature (Av P T ) calculations were performed on mineral inclusion assemblages using the software THERMOCALC v3.33 and the internally consistent thermodynamic database (Holland and Powell, 1998, recent upgrade)
Summary
One of the most important and unresolved questions in the evolution of the Earth is the question of when did subductiondriven plate tectonics begin (e.g. Smart et al, 2016)? The magmatic record implies that subduction-driven plate tectonics began in the Archean ca. 3 Ga (e.g. Dhuime et al, 2012), while the metamorphic record, or more precisely the lack of (U)HP-LT rocks in the pre-Neoproterozoic, would suggest that the onset of modern subduction occurred after ca. 800 Ma (see Fig. 4 in Stern et al, 2013). (U)HP-LT rocks such as blueschists and UHP eclogites are the best direct lines of evidence geoscientists have for modern ‘cold and steep’ subduction, as opposed to the ‘hot and flat’ style of subduction proposed for the Archean (Stern, 2005). Tains geochemical information from the source rock It has a strong affinity for high field strength elements, such as Nb, Ta and Cr, which can be useful in determining source rock lithology (Zack et al, 2004; Triebold et al, 2007). It can be dated using the U–Pb system This study demonstrates that mineral inclusions can provide additional P –T information when used in conjunction with existing methods, and discusses the potential of using detrital rutile to find evidence of HP-UHP events in eroded source terranes
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