26. Umino, S. and Mochizuki, A., 1996. Emplacement of multiple dykes at the Troodos Paleospreading Axis. Abstracts in 30th International Geological Congress, 2, 238.

Chilled margins, thickness and grain size of sheeted dykes of the Troodos Ophiolite are examined in order to understand conditions of multiple dyke formation under an extensional stress field.

We studied sheeted dykes along N-S transection from Kampos to Milikouri and mapped dyke sections near Kykko and Pedoulas in detail. Most dykes strike NNW to NS, and belong to the same structural domain (Malpas and Williams, 1991). Many dykes are chilled on both margins (two-sided dykes), but some are chilled on only one side (one-sided dykes) and cut by another. In Kykko section, the thickness of dykes ranges from a few centimetres to 3.5 m, and the two-sided dykes have a peak at the range of 10-20 cm. Almost all dykes are simple, and only 10 among 183 dykes exposed are multiple. In Pedoulas section, to the contrary, 21 out of 39 dykes are multiple. The total thickness of the Pedoulas multiple dykes ranges from 30 to 220 cm, while the one-sided dykes mainly consisting of margins of multiple dykes are less than 20 cm thick.

The Pedoulas section is adjacent to the high level gabbros to the east and was located very close to the magma chamber. On the contrary, the Kykko section is located 6 km from the sheeted dyke-gabbro boundary, and must have been at a higher level in the dyke complex. A simple conduction model suggests that 20-cm thick dykes intruded at an ambient temperature of 400 oC would solidify in 16 hours. However, repeated injections of magma as seen in the multiple dykes rase the ambient temperature, and hence prolong the time for the same thick dyke to solidifiy up to several days. Thus, near the magma chamber, injection of magma repeated at intervals of 10 hours to several days can occurr through the same pathway before the previouse dyke obtains enough strength to resist the injection of the new magma pulse, forming a multiple dyke. In contrast, dykes intruded into higher levels at low ambient temperatures solidify in short durations, and obtain the strength to avoid splitting before the next magma pulse arrives. The following dykes would find the pathways along dyke margins where the coupling is weakest, yielding the two-sided dykes as seen in the Kykko section.

 

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