In a pluton, there may be different granitic magma intrusions like monzonite, granite and/or syenite which were intruded in different times at the same place like ballooning mechanism. Is there a way to map each pulse in the field or what do you recommend to distinguish each pulse? how should these bodies be sampled?
Of course you can map these pulses in the field, provided you have good exposure. See publications on Galway Granites, You look for gradational and sharp contacts in the field. You could then follow up with geochem. geochron and min studies.
Hai,
It's an interesting question. I'm working on a similar problem and I think my observations will be of some use for you.
Its always advisable to look for the possible field evidences, like contacts, gradation/variation in the mineral assemblages, etc. However, the preservation of contacts between incremental intrusives (pulses) is most likely to be near the contacts between the earliest intruding granites and their non-granitic wall rocks. It's because earliest intrusions have the greatest contrast between the wall rock and magma temperatures - enhancing the likelihood that they would cool quickly and preserve contacts. However, in an incrementally assembled pluton, early intruding granites become the wall rocks for later intruding granites. Supporting this is the fact that incrementally assembled granite plutons spends significant intervals at or near magmatic temperatures, helping to erase contacts between increments.
Therefore, the best possible way to distinguish the incrementally assembled plutons is by taking advantage of the contrast in the closure temperatures of the U-Pb and Ar-Ar systems.
Hope this helps.
For further information regarding the importance of U-Pb and Ar-Ar systems in distinguishing different pulses, do inbox me at [email protected].
Hi Zehra,
Yes, this is usually possible. In the field you will need to look for compositional and textural differences. Look for both gradational and chilled margins. There may be a difference in xenolith or magmatic abundance (if either are present), which may also be useful to distinguish between different magma pulses. Sometimes the contacts are quite cryptic, so you may need to follow-up with thin-sections and / or geochemistry. Quite often subtle variations can be mapped out at a larger scale and this can help with recognising separate pulses. You should also note fabric development, not just the presence or absence of fabrics, but also fabric orientation and fabric type (e.g. pre-full crystallisation fabrics vs crystal plastic strain fabrics).
I hope these comments are of some use to you. Good luck!
Quentin.
Dear Quentin, Thank you so much for your answer. Following xenoliths is good idea. It may work. Fabric is backbone of my study. Presence, absence or variation of fabrics are very crucial for identifying deformation history. On the other hand, using them for determining different pulses sounds exciting.
Dear Feely, I found some nice webpages and papers about Galway Granite. Thank you for your comment.
Dear Leonid,
The contact between pulses are not clear, and composition of the pulses are not significantly different. It can be easily separated by using thin sections but how should it be sampled? what do you recommend?
Dear Dr. Kumar, Actually I would like to learn different pulses (if there are) before taking samples for geochronology and distinguish them by age. But before analytical method, distinguishing pulses may help for geochrolonigical sampling.
Hi Zehra,
if the rocks differ geochemically, may well serve field measurements using XRF spectrometer (grus or hard rock).
Another field procedure is to estimate the percentages of mafic minerals (i.e., hornblende, biotite, titanite) in the rock. This procedure can help identify contacts between different intrusive phases that otherwise exhibit only subtle compositional differences.
You already have three good answers. I would only add that in hypabyssal granitic intrusions, which are commonly porphyritic, you could look for porphyro-aphanitic dikes with chilled margins, in older intrusions with characteristically different textures. You can also look for inclusions of one rock-type in another, and for breccias with distinctive combinations of clasts.
At the Henderson porphyry Mo deposit, unidirectional crystallization features allowed mine geologists to identify a succession of several successive intrusions. Examples of such unidirectional features include plumose feldspars that branch into the younger of successive intrusions, and aplitic vein dikes with crystals that are consistently terminated into the younger of successive intrusions (See Carten et al. ,1988, Cyclic development of igneous features and their relationship to high-temperature hydrothermal features in the Henderson porphyry molybdenum deposit: Economic Geology, v. 83, p. 266-296.).
Depending on the scale you are working at, having a look at any aeromagnetic imagery that may cover the area could be worthwhile. Often, multi-phase intrusions like you describe can contain variable proportions of magnetite (or other magnetic minerals), and zoning patterns, or cross-cutting relationships visible in magnetic imagery can serve as a proxy for mapping the different intrusive phases.
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