Geology Reference
In-Depth Information
Figure 9.6
Deformed enclaves at contact between two granite facies (upper
coarse grained granodiorite) and lower, finer-grained enclave-bearing facies in
a sheeted pluton, Peru. Note enclaves are stretched out parallel to the contact.
Secondly, the composition of synplutonic dykes record important changes in
magma type and source region that relate overall to the thermal and chem-
ical evolution of plutonic complexes. Finally, it appears likely that in many
instances, synplutonic dykes are themselves the source of magmatic enclaves
(Figures 9.8 and 9.9). Evidence for this comes in the form of disrupted dykes
that intrude into low crystallinity plutons. Here the hotter dyke magma chills
and forms blobs and pillows that become dispersed within the pluton magma
through a combination of momentum related to dyke intrusion and subsequent
thermal convection. Similar effects may occur where mafic magma is intruded
at the base of silicic magma chambers Experimental evidence shows, by anal-
ogy, how the injection of basaltic liquid into a silicic magma chamber can lead
to instability. Basalt intrusion causes a thermal plume that drags the basaltic
liquid to the top of the chamber through a process called viscous coupling. This
upwards movement causes mingling and mixing of the liquids in localised areas
without influencing the bulk of the chamber. The timing of magma mingling
and mixing interactions during different stages of crystallisation of host and
invading magma are summarised in Figure 9.10.
