4. Here are 2 Concordia plots showing zircons separated from rocks in southern I

Question

4. Here are 2 Concordia plots showing zircons separated from rocks in southern India from Santosh et al., 2014. The MSWD line in each of the captions is an expression of the error on the measurements -you can ignore that. The n in the caption tells you how many zircons were measured. Call them rocks "a" and "b". In the space below, tell me everything you can about these rocks. When did they form, how do the ages of the 2 rocks compare, what happened to 0.55 SKC11175 045 035 each of these rocks after it formed, and when did that happen? (25 points) 025 Intercepts at 582 25 & 2634+43 Ma MSWD 2.2, ( 95%-conf.) n 20 0.15 0.05 10 12 14 04s (b) SKC11162 24 035 0.25 Intercepts at 543 24 &2465t56 Ma MSWD-0.91, ( 95%-conf.) 0.15 0.05 10

Explanation / Answer

The rocks 'a' and 'b' show age range between 2634 to 2465 Ma (i.e, Neoarchean age). In both the rocks, only a few analyses lie on or close to the upper intercept that show a nearly perfect discordia. This shows that the zircon has experienced a significant loss of Pb-U, followed by crystallisation during 2634-2465 Ma. The rocks show nearly concordant behaviour with a lowe intercept age of around 550 Ma. In rock 'b', high content of U in zircon accelerated Pb loss, hence causing discordant range of Neoarchean zircon ages. Due to this high amount of Pb loss and the MSWD error in the data, the upper intercept age of the zircons from the rock 'b' is insignificant, so the age of this rock type cannogt be determined.

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