How Widespread is 2.4-2.2 Ga Continental Crust?

NASA ADS: How Widespread is 2.4-2.2 Ga Continental Crust?

8/11/13 1:06 PM

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how widespread is 2.4-2.2 Search ga continental crust?

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How Widespread is 2.4-2.2 Ga Continental Crust? Condie, K. C.; Belousova, E. American Geophysical Union, Fall Meeting 2010, abstract #U32A-01 The known distribution of rocks with ages between 2.4 and 2.2 Ga comprises less than 1% of the continental crust with remnants of orogens recognized at only three locations: 1) the Borborema province in NE Brazil, 2) the Arrowsmith orogen in western Canada, and 3) the Trans-North China orogen. Tectonic setting appears to be continental arc, rift or collisional orogen. Exposed 2.4-2.2 Ga rocks are chiefly syn- to post-tectonic high-K granitoids, with a striking absence or near-absence of TTGs and greenstones. Detrital zircon populations from modern river sediments provide a robust method of sampling the continents. One of the advantages of using detrital zircons from modern river systems is that a variety of biases related to bedrock sampling can be avoided, and river drainages, which sample both primary zircons and recycled zircons may approach a more random sample of the continents. U/Pb zircon ages from over 10K zircons from modern river sediments have relatively few age peaks in the range of 2.4-2.2 Ga, and most of these are from East Asian and Australian rivers. Hf isotope data show that both juvenile and reworked sources are represented in the 2.4-2.2 Ga detrital zircon populations. Less than 3% of source areas (primary + recycled) supplying zircons to major river systems have zircons with ages in the 2.4-2.2 Ga time window. However, if Hf model ages (HfTDM) of the total zircon database reflect crustal extraction ages, about 9% of the continental crust was produced between 2.4 and 2.2 Ga. Although the effects of sampling bias cannot be completely eliminated, available age data from both outcrop and detrital zircons agree that the amount of continental crust surviving in the 2.42.2-Ga time window is very minor. It would appear that either this crust has been selectively recycled into the mantle (but how and why) or that global magmatism and probably plate tectonics slowed down during this interval of time. Keywords: [1115] GEOCHRONOLOGY / Radioisotope geochronology, [1600] GLOBAL CHANGE, [4901] PALEOCEANOGRAPHY / Abrupt/rapid climate change The ADS is Operated by the Smithsonian Astrophysical Observatory under NASA Grant NNX09AB39G http://adsabs.harvard.edu/abs/2010AGUFM.U32A..01C

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NASA ADS: How Widespread is 2.4-2.2 Ga Continental Crust?

http://adsabs.harvard.edu/abs/2010AGUFM.U32A..01C

8/11/13 1:06 PM

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