Geology Reference
In-Depth Information
trace elements form linear trends on Harker diagrams between two parent end
member compositions. Isotopic data will show similar trends. When collecting
samples for geochemical analysis it is vital therefore to select those rock types
that reflect most closely pristine end member compositions.
9.3 Magma Mingling
Magma mingling describes the process whereby co-eval magmas of different
composition come into contact but do not mix and blend. Instead they retain
their compositional integrity and freeze as discrete, identifiable enclave types
(Figure 9.2). This does not however imply that no exchange of material has
occurred, and a defining characteristic of magma mingling is the ability of
crystals to move from one magma type to another. Key observations include
fluid-fluid contacts showing clear evidence of the higher temperature from (gen-
erally the invading mafic component) having chilled against its felsic host. The
idea that mafic magma can intrude into incompletely solidified granitic magma
and become frozen in a fashion analogous to pillow basalts in water might
sound odd but where temperature contrasts fall in the range of several hundred
degrees or more (Table 9.1) then local quenching is inevitable. The geometry of
magmatic enclaves formed during magma mingling varies considerably, as do
the relationships between enclave margin and host rock. Margins in particular
require careful observation and can vary from sharp, brittle-like, through a range
of cuspate and cauliform types indicative of fluid-fluid mingling, into diffuse,
irregular contacts indicative of blending and mixing.
Enclaves are often associated with schlieren and layering in grantitic plutons
(see Chapter 7) and may cross-cut one another, implying successive episodes
of magma mingling. In such cases it is possible to find enclaves with only a
faint outline remaining (ghost enclaves), possibly the result of partial assim-
ilation into the host magma due to prolonged exposure. Given the complex
rheological nature of congested magmas (Figure 9.1) it is not uncommon to
find evidence for simultaneous synmagmatic fracturing and viscous, fluid-fluid
contacts between magmatic enclaves and host rocks in one small area of out-
crop. What appear to be contradictory field relationships with respect to tim-
ing of intrusion are often displayed in the same magmatic enclave population
(Figure 9.3).
9.3.1 Magma mingling: field example
An example of a pluton containing magmatic enclaves and metasedimentary
xenoliths in close proximity is the Ross of Mull granite, Scotland. Here a zoned
pluton was intruded into Precambrian metasediments of the Moine supergroup.
Petrographical and geochemical analysis have revealed the fine grained mag-
matic enclaves are dioritic in composition. The dioritic enclaves are located
