a Magma Differentiation History Recorded in a Single Dyke Apothysis
Abstract
We have studied a single apothysis of a dolerite dyke that provides a novel insight into differentiation processes operating in basaltic magma conduits. The apothysis emanates from a large master dyke and gradually pinches out along a distance of about 11 m. Three continuous profiles across an apothysis (17 cm, 29 cm and 69 cm wide) were studied. Contrary to common thinking, the apothysis has revealed remarkable internal zonation that systematically changes along its strike. As it becomes thicker, a zonation pattern with an inward decrease in compatible components and an increase in incompatible components progressively give way to one with opposite compositional tendencies. Here we argue that such spatial zonation is consistent with predictions of our new concept that attributes compositional trends in mafic dykes to a competitive operation between two petrogenetic processes. These are (a) the filling of dykes with magmas that become increasingly more evolved with time and (b) in situ cumulate growth in these magmas against dyke sidewalls. The two processes have opposite effects on rock geochemistry, the first making dykes more evolved and the second more primitive inwards. The spatial zonation of a dyke apothysis likely reflects a gradual change from rocks mostly produced by quenching of progressively more evolved magmas (the thinnest profile) towards those formed by in situ cumulate growth of these magmas (the thickest profile). This change results from the decreasing extent of magma cooling as a dyke apothysis becomes thicker with proximity to a master dyke. Future studies will attempt to establish if this phenomenon is common in small mafic dykes worldwide.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V34B..01L
- Keywords:
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- 3640 MINERALOGY AND PETROLOGY / Igneous petrology